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1100 - Drainage Design Procedures

Published: January 21, 2022

1101 Estimating Design Discharge

1101.1 General

 

The rational method and the USGS regression equations require the determination of drainage basin characteristics such as the contributing drainage area. Use a suitable topographic map or contours generated from LiDAR data collected through OSIP to develop the drainage area.

C1101.1

In order to design highway drainage facilities properly a reasonable estimate must be made of the required design and check storm discharges. Factors affecting discharge are duration, intensity and frequency of the rainfall as well as the contributing drainage area size, shape, slope and land use.

Historically, for drainage areas over 100 acres, 7.5 minute USGS Quadrangles were used. For smaller drainage areas, or where discharges were calculated using the rational method, lesser scale maps were utilized. 

Other methods that use GIS such as USGS StreamStats are acceptable.

Make an evaluation of the land use throughout the drainage area. Consider changes in land use within the drainage area which will occur before the anticipated date of project construction.

Verify drainage areas automatically delineated by software.

Do not assume probable land use changes beyond the start of construction when determining design discharges. It is the responsibility of the local permitting/zoning agency to ensure proper land and water management techniques are utilized. These techniques will minimize the adverse effects of a change in land use.

1101.2 Procedures

1101.2.1 Rational Method

Use this method for drainage areas up to a maximum of 100 acres where no well-defined natural channel exists, and sheet flow prevails.

The design discharge Q is obtained from the Rational Equation:

Q = CiA

Where:

Q =    Discharge in cubic feet per second 

C =    Coefficient of runoff

 i =    Average rainfall intensity in inches per hour, for a given storm frequency and for a duration equal to the time of concentration.

A =    Drainage area in acres

C1101.2.1

The rational method is an empirical approach used for estimating the discharge at a point of concentration for small drainage areas. It assumes that the storm duration equals the time of concentration. The time of concentration is used with the appropriate IDF curve to find the design intensity.

The time of concentration is the time for the runoff to flow from the most remote point of the drainage area to a point of concentration.

Time of concentration is designated by tc and is the summation of the time of overland flow to, the time of shallow concentrated flow ts and the time of pipe or open channel flow td.

tc = to + ts + td

The point of concentration could be a catch basin, inlet or a location in a roadway ditch when checking for shear protection and depth of flow.

The time of overland flow may be obtained from Figure 1101-1, a similar overland flow chart, or from the equation:

to1.8(1.1-C)L1⁄2
     S1⁄3 

Where:

to = Time of overland flow in minutes

C = Runoff coefficient

L = Distance to most remote location in drainage area in feet

S = Overland slope (percent) 

This equation and Figure 1101-1 assume a homogeneous drainage area. Where the overland flow area is composed of segments with varying cover and/or slopes, the summation of the time of concentration for each segment will tend to over-estimate the overland flow time, to. In this case it may be more appropriate to use an average runoff coefficient C and an average ground slope in the Overland Flow Chart.

The overland flow equation is from the US. Department of Transportation publication, Federal Aviation Administration: Advisory Circular 150/5320-5B, Airport Drainage (1970). The equation was developed from airport drainage data collected by the U.S. Army Corps of Engineers and is best suited for small drainage areas with fairly homogeneous surfaces. 

Overland flow maintains a uniform depth across the sloping surface. It is often referred to as sheet flow.

The velocity of shallow concentrated flow can be estimated using the following relationship:

V = 3.3ks1⁄2

Where:

V = Velocity in fps

k = Intercept coefficient (see Table 1101-1)

s = Overland slope (percent)

Sheet flow is assumed to occur for no more than 300 feet after which water tends to concentrate in rills and then gullies of increasing proportion. This type of flow is classified as shallow concentrated flow. For 300 feet assume a homogeneous drainage area.

Table 1101-1

Types of Surface k
Forest with heavy ground litter 0.076
Min. tillage cultivated; woodland 0.152
Short grass pasture 0.213
Cultivated straight row 0.274
Poor grass; untilled 0.305
Grassed waterways 0.457
Unpaved area; bare soil 0.491
Paved area 0.619

Shallow concentrated flow generally empties into pipe systems, drainage ditches, or natural channels. The velocity of flow in an open channel or pipe can be estimated using the Manning's equation.

The travel time for both shallow concentrated flow and open channel or pipe flow is calculated as follows:

ts  or t   L   
                60V

Where:

ts = Travel time for shallow concentrated flow in minutes

td = Travel time for open channel or pipe flow in minutes

L = Flow length in feet 

V = Velocity in fps

Where a contributing drainage area has its steepest slope and/or highest C value in the sub-area nearest the point of concentration, the rational method discharge for this sub-area may be greater than if the entire contributing drainage area is considered. Consider the maximum runoff rate for a sub-area only if it is greater than that for the entire area.

Shallow concentrated flow velocity equation and Table 1101-1 are referenced from FHWA publication HDS-2 (2002) section 2.6.2.2

1101.2.2 Runoff Coefficient

The recommended values for the runoff coefficient C for various contributing surfaces are shown in Table 1101-2. Where two values are shown, the higher value ordinarily applies to the steeper slopes.

C1101.2.2

The runoff coefficient is a dimensionless decimal value that estimates the percentage of rainfall that becomes runoff.

Table 1101-2

Types of Surface C
Pavement & paved shoulders 0.9
Berms and slopes 4:1 or flatter 0.5
Berms and slopes steeper than 4:1 0.7
Contributing area  
Residential (single family) 0.3-0.5
Residential (multi-family) 0.4-0.7
Woods 0.3
Cultivated 0.3-0.6

See FHWA publication HDS-2 (2002) section 5.3.3 for additional runoff coefficient values. 

The total area contributing flow to a given point usually consists of surfaces having varying land cover and therefore requires a weighted runoff coefficient.

See FHWA publication HDS-2 (2002) section 5.3.2 for weighted runoff coefficient determination.

1001.2.3 Rainfall Intensity

\The average rainfall intensity i in inches per hour may be obtained from the Intensity-Duration-Frequency curves shown on Figure 1101-2. Each set of curves applies to a specific geographic area, A, B, C, or D as shown on the Rainfall Intensity Zone Map. Some political subdivisions may have developed curves for their specific area similar to Figure 1101-2. These curves may be based on a much longer period of record and provide more reliable information. Any local curves proposed by the designer require approval from OHE prior to incorporating that information in the drainage calculations.

C1101.2.3

The geographic areas were established from an analysis of rainfall records obtained from Weather Bureau stations in Ohio.

1102 Open Water Carriers

1102.1 General

 

Design open water carriers with a minimum slope of 0.50% with a recommended absolute minimum of 0.25%. Lesser slopes can be used on large width open water carriers.

Maintain a constant slope wherever possible and provide an adequate outfall with positive drainage. Perpetuate existing drainage patterns as much as practicable.

Avoid capturing an existing stream with the roadside ditch. If this is necessary, design the ditch in accordance to Section 1102.2.4.

C1102.1

Open water carriers generally provide the most economical means for collecting and conveying surface water from the roadway. The required capacity of a water carrier involves a determination of the velocity and depth of flow for a given discharge.

Standard ditches are open water carriers. Large width open water carriers are channels.

1102.2 Types of Open Water Carriers


1102.2.1 Normal Ditches

A ditch is normal when:

  1. The centerline is parallel to the edge of the pavement.
  2. The flowline is a uniform distance below the edge of the pavement.

Refer to LD1, Section 307 for more information on normal ditch shape, size and placement for common, safety, clear zone and barrier grading.

1102.2.2 Special Ditches

Refer to LD1, Section 307 for more information on where special ditches are required.

1102.2.3 Median Ditches

A median ditch is defined by location and have the same shape and capacity features as normal ditches.

 

1102.2.4 Channel Relocations

Design Channel relocations or ditch stream captures as follows:

  1. Use the design year frequency specified in Section 1004.2.
  2. Design all channel relocations to prevent erosion.
  3. Whenever possible, limit channel relocations to the downstream end of the proposed culverts.
  4. Perpetuate the existing cross-section including a two-stage channel if evident. See Figure 1101-2 for a graphical representation of the major channel features.
  5. Perpetuate the existing channel as closely as possible in regard to existing geomorphic conditions; channel slope and length, velocity, depth of flow, channel sinuosity, energy dissipation, etc. 
  6. Duplicate the existing hydraulic properties for the bankfull design frequency. Meet the flood clearance criteria given in Section 1005.

C1102.2.4

Avoid major channel relocations

Information on the design of relocated channels can be found in the USDA, Natural Resources Conservation Service publication, Stream Corridor Restoration: Principles, Practices and Processes (National Engineering Handbook 653). The principals given in this publication utilize idealized channel geometry.

A two-stage channel is comprised of two distinct areas. The first of these is a meandering bankfull width that carries the channel-forming discharge. The second area is the flood plain width.

Establish the existing channel geometry and physical characteristics from reference reaches and idealized geometry. Select the reference reaches from stable channel reaches close to the relocated section or in locations with similar watershed and valley conditions.

1102.2.5 Channel Linings and Bank Stabilization

Use soil bioengineering to stabilize banks for channel relocations or ditch stream captures.

Specify native plant species.

C1102.2.5

Bank stabilization using bioengineering is covered in the previously referenced USDA publication as well as the AASHTO Model Drainage Manual and the USDA, Natural Resources Conservation Service publication, Engineering Field Handbook, Chapter 16, Streambank and Shoreline Protection, part 650. The design procedures and methods for determining the effectiveness of the traditional channel linings are covered in the FHWA publication, Design of Roadside Channels with Flexible Linings (Hydraulic Engineering Circular No. 15).

1102.3 Ditch Design Criteria

1102.3.1 Design Frequency

Determine the depth of flow and the shear stress based on the following frequencies:

ADT Depth of Flow Design (years) Shear Stress Design (years)
≤3000 5 2
>3000 10 5

C1102.3.1

 

Where a flexible ditch lining is required for calculated stresses exceeding the allowable for seed, the minimum width of the lining is 7.5 feet. Additional required width is in increments of 3.5 feet. The installed width of all ditch linings is centered on the flow line of the ditch.

The depth of flow is limited to an elevation 1 foot below the edge of pavement for the design discharge. The depth of flow in toe of slope ditches is further limited such that the design year discharge does not overtop the ditch bank.

1102.3.2 Ditch Protection

The shear stress for the five-year frequency storm must not exceed the values shown in Table 1102-1 for the various flexible linings.

Table 1102-1

Permanent Protection
Protective Lining Allowable Shear Stress (lbs./ft2)
Seed (659) 0.40
Sodding, Ditch Protection (660) 1.0
Temporary Protection
Item 670 Ditch Erosion Protection Mat Type __  
B 1.50
C 2.0
E 2.25
G 1.75

The temporary linings will reach a value of 1.0 lbs./ft2 upon vegetation establishment. Use the temporary lining shear stress values in Table 1102-1 on a temporary basis of 6 months or less.

Calculate the actual shear stress by the following equation:

τac = 62.4DS

Where:

D = Water surface depth ft 

S = Channel slope ft/ft

τac = Actual shear stress lbs./ft2

If the calculated shear stress exceeds that shown in table 1102-1 then use the following permanent shear stress values within the stated limitations:

  1. Seeding and Erosion Control with Turf Reinforcing Mat, SS836, where the ditch slope is 10% or less. Allowable shear stress for each type is as follows:
Turf Reinforcing Mat Shear Stress
Type Allowable Shear Stress (lbs./ft2)
1 3
2 4
3 5
4 6
  1. Type B, C or D Rock Channel Protection may be used to line the ditch if the nearest point of the lining is outside the design clear zone or located behind guardrail or barrier. The actual shear stress is based upon the parameters of the channel slope and depth of flow for the 5-year discharge. The shear equation is valid for discharges less than 50 cfs with slopes less than 10%. Allowable shear stress for each type is as follows:
Rock Channel Protection Shear Stress
Type Allowable Shear Stress (lbs./ft2)
B 6
C 4
D 2
  1. Type B or C RCP may be utilized for lining ditches on profile grades from 10%- 25% that carry flow from the end of a cut section down to the valley floor. Use HEC-15 procedures with a safety factor of 1.5 for steep gradient channels. Contact OHE for further guidance of RCP usage for 5-year discharges greater than or equal to 50 cfs.
  2. Tied concrete block mat protection, Item 601, may be used for slopes and channels. Provide for slopes that are 2:1 or flatter. Provide for channels when side slopes are 2:1 or flatter and profile grades are 25% or less. The matting may be used within the clear zone when the top of the blocks are flush with the finished grade. Install per the manufacturer recommendations. The allowable shear stress for each type is 12 lbs/ft2. Specify Type 1 underlayment as the standard option. Provide Type 2 Underlayment in areas where establishing vegetation is difficult, such as, areas with poor soils, flumes on steep slopes, or areas subjected to constant flow.
  3. Articulating concrete block revetment system, Item 601, may be used for slopes and channels with 2:1 or flatter side slopes. The revetment may be used within the clear zone when the top of the blocks are flush with the finished grade. Install per the manufacturer recommendations. The allowable shear stress for each type is as follows:
Articulating Concrete Block Revetment System Shear Stress
Type Allowable Shear Stress (lbs./ft2)
1 17
2 20
3 23
  1. Consider a concrete lining only as a last resort. Contact OHE, before using a concrete lining.

7.5 feet is the common commercially available width for flexible ditch lining.

1102.3.3 Roughness

Suggested values for Manning’s Roughness Coefficient n for the hydraulic analysis of various types of open water carrier linings are listed in Table 1102-2.

Table 1102-2

Manning's Roughness Coefficient
Type of Lining n
Bare Earth 0.02
Seeded 0.03
Sod 0.04
Item 670 0.04
Concrete 0.015
Bituminous 0.015
Grouted Riprap 0.02
Tied Concrete Block 0.03
Rock Channel Protection 0.06 for ditches
0.04 for large channels

 

1102.3.4 Catch Basin Types

CB-4, CB-5 and CB-8 basins are suitable for the standard roadside designs covered in LD1. The bases can be expanded to accommodate larger diameter conduits by specifying SCD CB-4A , 5A, 8A.

The bar spacing can be decreased for safety reasons, by specifying Grate E for CB-4 and Grate B for CB-5. Provide 150 feet of Item 670, Ditch Erosion Protection, upstream of all CB-4, CB-5 and CB-8 basins, regardless of velocity.

The following catch basin types are generally recommended based on the size and shape of the ditch.

  1. CB-4 for depressed medians wider than 40 feet.
  2. CB-5 for 40-foot radius roadside or median ditches. Use Grate B where pedestrian traffic may be expected.
  3. CB-8 for 20-foot radius roadside or depressed medians 40 feet or less in width.
  4. CB-2-2-A in trapezoidal ditches where the basin is in a rural area. Locate the basin outside of the design clear zone or behind guardrail. The capacity of the side inlet window, for unsubmerged conditions, may be determined by the standard weir equation:

Q = CLH3/2

Where C is a weir coefficient, generally 3.0, L is the length of opening in feet, H is the distance from the bottom of the window to the surface of the design flow in feet. The catch basin grate is considered as an access point for the storm sewer and its capacity to admit flow is ignored for continuous grades.

  1. Use a CB-2-2-B basin where minor, non-clogging flows are involved such as yard sections and the small triangular area created by the guardrail treatment for a depressed median at bridge terminals. Provide CB-2-3 through CB-2-6 basins where a larger base is required to accommodate conduits greater than 21 inches in span or sewer junctions, or where a CB-2-2-B will not provide adequate access to the sewer.
  2. In urban areas use Standard Side Ditch Inlets to drain small areas of trapped water behind curbs and/or between driveways.

For lower ADT highways consider using CB-5, CB-2-2-A, within the safety limitations as discussed in Section D above, and CB-2-2-B. Where additional capacity is required use CB-4.

For catch basin details refer to the Hydraulic Standard Construction Drawings.

C1102.3.4

The tilt built into the basin top provides a self-cleaning feature when the basins are used on continuous grades and the wide bar spacing minimizes clogging possibilities, thereby resulting in an efficient design.

1102.3.5 Calculated Catch Basin Spacing

Provide catch basins to intercept flow from open water carriers when the depth of flow or shear exceeds the maximum allowable for the design storm for all highway classifications.

When the calculated depth of flow or shear exceeds the maximum allowable at the checkpoint in the ditch, a catch basin or ditch lining will be required. However, the capacity of the catch basin may be less than the capacity of the ditch and thereby control the catch basin spacing. Figure 1102-1 is used to check the capacity of a catch basin grate in a sump. To use Figure 1102-1, double the calculated discharge at the ditch checkpoint to compensate for possible partial clogging of the grate. 

In cut sections, carry the accumulated ditch flow as far as the capacity, allowable depth, or shear of flow will permit. The first catch basin in the roadside or median ditch will determine the need for a storm sewer system required for the remainder of the cut. Extend shear control as far as inexpensive flexible ditch linings will permit. 

When locating ditch catch basins, provide positive outlets for underdrains and access to longitudinal sewer systems.

C1102.3.5

CB-4, CB-5 and CB-8, include an earth dike. The dike is approximately 12 inches above the flowline of the grate, immediately downstream from the catch basin and serves to block the flow on continuous grades and create a sump condition.

1102.3.6 Arbitrary Maximum Catch Basin Spacing

Catch basins are required at the low point of all sags. Omit the earth dike shown on the Standard Construction Drawings when used in a sag. The maximum distance between catch basins in depressed medians in fill sections is as follows:

Depressed Median Catch Basin Spacing (Fill Sections)
Median Width (ft) Desirable Spacing (ft) Maximum Spacing (ft)
84 1250 1500
60 1000 1250
40 800 1000

Where underdrains are utilized, place catch basins at a maximum spacing of 1000 feet to provide a positive outlet for the underdrains.

 

1103 Pavement Drainage

1103.1 General

 

Refer to the LD1 for pavement cross-slope design criteria.

When curb or barrier is provided, determine the proper type of pavement inlet or catch basin to control the spread of water into the traveled lane. Maximize the allowable spread without exceeding the allowable depth of flow at the face of curb or barrier.

Reduce the need for bridge scuppers by intercepting the flow prior to the bridge.

C1103.1

When paved shoulders are provided, the drainage cost can be decreased due to the large volume of flow that can be carried on the pavement shoulder.

Additional information concerning pavement drainage can be obtained from the FHWA Hydraulic Engineering Circular No. 22, Urban Drainage Design Manual.

1103.2 Design Frequency

 

Locate pavement inlets or catch basins to limit the spread of flow on the traveled lane to those shown in Table 1103-1. Base the design on the following frequencies:

Facility Design (years)
Freeways 10
High Volume Highways (Over 6000 ADT) 5
All other highways 2

C1103.2

 

For underpasses or other depressed roadways where ponded water can be removed only through the storm sewer system, check the spread for a 50-year storm on Freeways and other high volume highways as defined above. Use a 25-year storm on other multiple lane highways. Ponding is permitted to cover all but one through lane of a multiple lane roadway.

The depth of flow or ponding at the curb cannot exceed 1 inch below the top of the curb for the design storm discharge regardless of the type of highway. A maximum depth of 6 inches is permitted where a barrier is provided.

These criteria are intended for sag locations with no outlet except through the storm sewer system. Examples include sag locations with barrier wall, underpasses, or other depressed cut sections without an alternative outlet.

Typically, these criteria do not apply to 2-lane or other curbed roadway facilities where water can overtop the curb. Contact OHE if encountered.

Table 1103-1

Facility Allowable
Pavement Spread* (ft)
Freeway 0

High Volume Highways
(Over 6000 ADT)

≥ 45 mph                      
< 45 mph           2 lanes
4 lanes


 

4
6
8

All other highways

2 lanes
≥ 4 lanes

 

6
8

 * Pavement spread applies only to the through lane and assumes a 12 ft lane width.

The speeds listed in the manual are design speeds.

Where lanes are less than the standard 12 ft lane width, reduce the allowable spread an equal amount. Therefore, 11 ft lanes on All other Highways with 2 lanes will have an allowable spread of 5 ft instead of 6 ft.

In some instances, using the legal speed instead of the design speed will result in a more practical pavement spread design. Contact OHE if encountered.

If design requirements cannot be met, contact OHE for guidance in a Performance Based Practical Design.

PBPD focuses on performance improvements that benefit both project and system needs rather than strict adherence to published standards. Standards are not abandoned but all factors are considered to produce a balanced decision that does not compromise safety.

1103.3 Estimating Design Discharge

 

Estimate runoff contributing to curbed pavements by the rational method, as explained in Sections 1101.2.1, 1101.2.2 and 1101.2.3

The time of concentration tc is the actual time of concentration calculated according to Section 1101.2.2 with an absolute minimum time of 10 minutes.

C1103.3

Contact OHE when the contributing drainage area is difficult to determine, and the calculations indicate the need for more basins than existing or the required spacing between basins is less than or equal to 100 feet.

The profile and cross section of the roadway may need to be modified in order to obtain a reasonable basin spacing by using a rolling gutter profile. If the geometrics cannot be revised, a contributing drainage area will need to be assumed. Use the entire contributing drainage area for the storm sewer design.

1103.4 Capacity of Pavement Gutters

 

Use the following equation to determine flow capacity for a standard curb and straight pavement slope: 

Standard curb equation

Where:

Q = Discharge (cfs)

Z = 1/Sx

n = Manning’s Coefficient of Roughness (Table 1102-2)

S = Longitudinal pavement slope (ft/ft)

Y = Depth of flow in gutter section at curb (ft)

C1103.4

The longitudinal slope can vary on the approach to the inlet or catch basin, especially in a sag. When flatter grades are located at a sump, using the flatter slope will underestimate the overall gutter capacity and result in overestimated spread values. Examine the approach lengths of the grades to determine an average slope. If one of the grades has a much longer approach length, use this most predominant slope.

On curbed facilities, design sag vertical curves to prevent inadequate drainage near the bottom. This can be achieved by providing a minimum longitudinal slope of 0.3 percent at the two points 50 ft from the bottom. This yields a maximum value of K = 167 for the vertical curve, which is typically called the drainage maximum.

Use the following equations to determine flow capacity for a composite gutter section:

Q equations

QTotal = Q1 - Q2 + Q3

Where:

Q1, Q2, Q3 = Discharge in each triangular segment (cfs) 

Z = 1/Sx, use Sx(1) for Q1 and Q2, use Sx(2) for Q3

n(1) & n(2) = Manning’s Roughness Coefficient (Table 1102-2)

S = Longitudinal pavement slope (ft/ft)

Y = Depth of flow in gutter section at curb (ft)

Y1 = Depth of flow at gutter/pavement interface     (point of slope change) (ft)

When the longitudinal slope varies along the gutter, use the average or most prominent slope for the analysis.

Composite Gutter Section: In most cases, the top width of the water surface in a pavement gutter far exceeds the height of the curb. The hydraulic radius does not accurately describe the gutter cross section in this situation, thereby requiring a modification to the Manning’s equation to analyze the gutter flow.

Flow capacity for composite gutters

1103.5 Bypass for Continuous Pavement Grades

 

Add the flow bypassing an inlet or catch basin to the total flow of the adjacent downstream gutter section.

C1103.5

Flow bypasses inlets and catch basins as inflow capacity is exceeded. Capacity depends on geometry and catchment characteristics.

For greater efficiency, size inlets to bypass a minimum of 10% to 15% of the design discharge. Use this criteria to determine the type or length of inlet to be used in a given location. It is not intended to establish the required spacing. The most efficient design maintains the allowable spread on continuous grades and at the sag.

1103.5.1 Curb and Barrier Opening Inlets

Avoid the use of curb opening inlets where bicycle traffic is anticipated.

When barrier inlets are placed on continuous grades, the window opening is the primary design feature with the grate considered as a factor of safety only. Locate the grate on the side of the barrier with the largest flow.

 

1103.5.2 Grate or Combination Grate and Curb Opening Catch Basin

Calculate flow intercepted over the outside edge of the grate using the following equations:

Flow intercepted over the outside edge of the grate

Where:

Qa = Flow outside the width of the grate (cfs); grate is assumed to capture 100% of flow within the grate width 

La = Length required for 100% capture of flow bypassing grate (ft)

L = Length of grate (ft)

A = Depression at edge of grate edge nearest the pavement centerline (ft)

Y2 = Depth at the edge of grate edge nearest the pavement based on the flow (without depression added) (ft) 

Q = Total Flow in section just before the catchment (cfs)

Qi =    Flow intercepted over side of grate (cfs)

The total flow bypassing the catchment and efficiency is calculated using the following equations:

Flow bypassing the catchment and efficiency

Where:

Qb = Bypass Flow carried to next catchment (cfs)

E = Efficiency of catchment (%)

Curb opening inlets hydraulic capacity can be enhanced by use of a local depression at the face of curb.

C1103.5.2

Pavement catch basins in this category are considered to intercept all flow over the grate when used on continuous grades. The curb opening of a combination catch basin on a continuous grade will admit some flow; however, the additional capacity is not accounted for.

A portion of the flow outside of the edge of the grate will also be intercepted, the amount varying with the depth of flow Y2 along the edge of the grate. This flow is calculated as a window opening at the edge of the grate with an opening depth of Y2 with a length equal to the grate length.

Composite Gutter Section with Grate:

Composite gutter section with grate

The same equations can be used for curb inlets to determine the required window opening length. Use Y in lieu of Y2 in the equations, which is at the face of the curb.

Section 1103.9 Slotted Drains and Trench Drains are designed with the same equations

Likewise, grate hydraulic capacity can be increased by use of a local depression at the face of the grate closest to the centerline of the roadway. 

Use the local depression values shown in Table 1103-2 for various pavement drainage structures. See Figure 1103-1 for local depression location and values for various pavement drainage structures.

Table 1103-2

Basin/Inlet Type Normal Pavement Slope Depressed Shoulder / Gutter
A (inches) A (inches)
CB-3/3A 0.5 0
CB-6 0.5 0.5
I-2/2A 2 0.5
I-3B/3C/3D-Grate Side 2 N/A
I-3B/3C-Window Only Side 2 N/A

The local depression is not the same depression identified in the standard construction drawings for catch basin grates.

For inlet and catch basin details refer to the Hydraulic Standard Construction Drawings.

1103.6 Grate Catch Basins and Curb / Barrier Opening Inlets in Pavement Sags

 

The spread in the sag is determined from the depth of flow at the edge of grate and includes the total flow contributed from each side of the sag vertical curve reaching the inlet or catch basin.

Provide a flanking inlet or catch basin on both sides of the pavement sag on Freeways. Place them where the grade elevation is approximately 0.20 feet higher than the low point. On barrier sections; place 3 barrier inlets. On curbed sections; place flanking 6 foot inlets or CB-3As with a CB-3 at the low point.

When barrier inlets are placed in pavement sags, locate the grate on the side with the greatest flow. The grate is considered the primary design feature with the window as the factor of safety with the local depression at the front face of the grate per 1103.5.2.

C1103.6


The spread does not need to be checked any closer than 25 to 50 feet on either side of the sag due to the flatness of the profile.

Three inlets or catch basins in a sag can only be justified on the basis of need for other highway classifications.

The capacity of the grates is based on weir flow over the edge of the grate, up to a depth of 0.4 feet. For greater depths, the total area of grate opening is considered, with no deduction made for possible clogging. When evaluating the spread in a depressed sag for a 25-year or 50-year event, the capacity of the window is considered a factor of safety.

A CB-6 catch basin may be used along curbed roadways and medians provided that the grate capacity is not exceeded.

The capacity of the grates is based on the depth of ponding around the grates.

For inlet and catch basin details refer to the Hydraulic Standard Construction Drawings.

1103.7 Arbitrary Inlets and Basins

 

Place inlets or catch basins arbitrarily upstream of all intersections, bridges and pedestrian ramps. When justified, locate inlets or catch basins a minimum of 10 feet off drive aprons, intersection return radii, pedestrian ramps or curb termini.

C1103.7

Minimal flows at these locations do not justify arbitrary placement.

Consider grading out local minimal sag locations along curb radii at side roads to avoid ponding near curb ramps and reduce the number of inlets required.

1103.8 Bridge Drainage

1103.8.1 Bridge Deck Drainage

Design a minimum longitudinal grade of 0.3% for the bridge deck surface when using concrete parapets.

Minimize or eliminate the use of scuppers.

Calculate the allowable spread of flow using procedures described above.

C1103.8.1

 

The fraction of flow captured by the scupper can be determined by the following equation:

Fraction of flow captured by the scupper

Where:

E = Scupper efficiency

W = Width of scupper (ft)

T = Total width of spread (ft)

The scupper bypass flow can be determined by the following equation:

Scupper bypass flow

Where:

Qb = Bypass discharge (cfs)

Q = Total discharge in gutter (cfs)

E = Scupper efficiency

On flatter longitudinal slopes, scuppers will intercept a portion of flow slightly wider than the width of the scupper (side flow), while on steeper longitudinal slopes, a portion of the flow in the gutter section occupied by the scupper (frontal flow) may splash over the grate. Assuming side capture and splash over are negligible, the frontal flow ratio is considered equal to the inlet efficiency.

Scupper bypass flow can also be determined with the spreadsheet found at the OHE Design Resources web page or similar.

Locate scuppers inside the fascia beam unless the parapet and beam spacing make this impractical.

Design scuppers with vertical drops or nearly vertical drops when feasible. If a scupper pan is required, angle the pan as steeply as possible. 

Design an uncollected / free fall per SCD GSD-1-96. When SCD GSD-1-96 will not physically fit due to the parapet, beam line or deck overhang, substitute heavy duty cast iron deck drains as currently manufactured by Neenah or equal. If a drainage collection system is required. meet the following:

  1. System is sloped greater than or equal to 15 degrees.
  2. Bends have a minimum radius of 18 inches.
  3. Bends have angles greater than 90 degrees.
  4. Cleanout plugs are easily and safely accessible.
  5. Include drainage collection when using finger joints or sliding plates. Provide a neoprene drainage trough under finger joints. Show the necessary deck drainage outlet locations on the preliminary structure site plan. Include this information in the STS.

Place scuppers with drainage collection systems close to the substructure unit which drains them. Place uncollected / free fall scupper downspouts as far from any part of the substructure as possible.

Information concerning bridge deck drainage can be obtained from the Federal Highway Administration Hydraulic Engineering Circular No. 21, Design of Bridge Deck Drainage. Software utilizing methods outlined in HEC-21 are also acceptable for scupper analysis.

1103.8.2 Erosion Control at Bridge Ends

Provide curb from the end of the parapet to a basin or flume in order to collect and carry bridge deck drainage that flows off the ends of the bridge in accordance to the following:

  1. Flow less than 0.75 ft3/s for bridges without MSE walls – Provide a flume, as shown on SCD DM-4.1. Locate the flume beyond the limits of the Bridge Terminal assembly.
     
  2. Flow greater than 0.75 ft3/s for bridges without MSE walls - Provide a CB-3A Catch Basin located off the approach slab and outside the curb height taper length. Locate the basin beyond the limits of the Bridge Terminal assembly. Provide a Type F, broken back conduit per Figure 1104-1 for an outlet down the embankment slope and armor the outlet to prevent erosion.

    For bypass flow greater than 0.5 ft3/s, provide a flume downstream from the basin at the end of the curb per SCD DM-4.1.

C1103.8.2

For catch basin details refer to the Hydraulic Standard Construction Drawings.

  1. Bridges with MSE Walls – Provide a barrier at the approach slab with a standard barrier inlet. Locate the inlet outside of the MSE wall soil reinforcement and the barrier transition.

Place Item 670, Slope Erosion Protection, on all bridge approach embankment corner cones beginning at the edge of the crushed aggregate or concrete slope protection.

The barrier inlet is placed outside of the MSE wall to protect against the loss of the MSE wall select granular backfill.

1103.9 Slotted Drains and Trench Drains

 

Slotted drains and trench drain systems are susceptible to clogging and are not recommended where significant sediment or debris load is present.

Locate slotted drains and trench drains longitudinally with the edge of pavement. Keep the drain and any surrounding concrete outside of the travelled way. Locate trench drains at the end of commercial drives to intercept large flows before entering the travelled way.

C1103.9

Slotted and trench drains are used to capture sheet flow in areas where curb is not present. They collect and direct flow to a catch basin such as in a gore area.

For slotted drain and catch basin details refer to the Hydraulic Standard Construction Drawings.

Outlet the slotted and trench drains to a CB-6 catch basin. Provide a CB-6 at a minimum 100 ft. interval for slotted drains and 200 ft. interval for trench drains.

Slotted drains and trench drains intercept flow capacity is best represented by Federal Highway Administration Hydraulic Engineering Circular No. 22, Urban Drainage Design Manual, 3rd Edition equations 4-22a, 4-32, and 4-33 for Curb-Opening Inlets.

On grade:

On grade equation

Where:

LT = Total length of drain required (ft)

KT = 0.6 (unitless, US Customary)

Q = Total discharge in gutter (cfs)

SL = Longitudinal slope (ft/ft)

Sx = Pavement cross slope (ft/ft)

n = Manning's coefficient (unitless)

In a Sag Condition*: 

For a flow depth < .2 ft (Weir Flow)

Weir flow equation

For flow depth: d > .2 ft (Orifice Flow)

Orifice flow equation

Where:

d = Flow depth at edge of gutter (ft)

g = 32.2 (fps)

W = Width of drain (ft)

*Assuming 50% clogging, a safety factor of 2 times the Total Length of Drain Required is recommended.

Refer to SCD DM-1.3 for slotted drain details. Include Plan Note D120 when using slotted drain.

Specify SS839 and SS939 when using trench drain.

Including CB-6 catch basins facilitates future cleanout of the slotted and trench drain systems.

1104 Storm and Sanitary Sewers

1104.1 Storm Sewers

 

Size storm sewer systems to carry the current flow from areas naturally contributing to the highway or from intercepting existing storm sewers.

Following the local drainage criteria and standards is not required on ODOT owned and maintained drainage assets. Storm sewer systems may be oversized at the request of a local government entity to carry flow from areas beyond those considered highway responsibility or increased flows from anticipated development with the approval of OHE. The additional cost to construct the increased sized storm sewer system are the responsibility of the local government. The project funding participation is determined as a percentage of the total cost of the affected plan items. 

Where proposed highway storm sewers or ditches interfere with existing private drains carrying treated or untreated sanitary flow, submit the names and addresses of the affected property owners to the District Right-of-Way permit office with the following:

  1. PID
  2. County - Route - Section
  3. Latitude and Longitude
  4. Size and pipe type or swell size carrying the discharge

The District Right-of-Way permit office will check if a permit has been issued. If a permit has been issued the designer will provide an unrestricted continuance of the discharge. An inspection well will be furnished at the Right-of-Way line for all pipe discharges. If a permit has not been issued, then the District Right-of-Way permit office will pursue a discharge permit. If a discharge permit cannot be granted, then add Plan Note D111 to the General Notes.

C1104.1

Storm sewer systems are designed to collect and carry storm water runoff from the first pavement or ditch inlet or catch basin to the predetermined outlet. Long cut sections often result in the need for longitudinal trunk sewers to accept the flow from a series of inlets or catch basins. Perpetuate existing drainage patterns as much as practical. Consider the possibility of actionable damage for the diversion of substantial volumes of flow. Long fill sections requiring median or pavement drains may best be served by transverse sewers that outlet independently at the toe of fill ditch.

For examples of storm sewer detail sheets, reference Sample Plan Sheets 1312-3 thru 1312-5, maintained by the Office of CADD and Mapping. These provide a useful resource for preparation of hydraulic plans in terms of layout and content.

For inlet and catch basin details refer to the at Hydraulic Standard Construction Drawings.

Do not change the Type of conduit for a short run of which would ordinarily require a different type.

On high fill embankment with transverse drainage, terminate the Type B conduit at a point approximately 10 feet from the embankment slope and a concrete collar provided, per SCD DM-1.1, to connect the Type B and a Type F conduit. Provide Type F conduit, 707.05 Type C or 707.21 for the pipe used for the bend at the top and bottom of the embankment. A detail is provided in Figure 1104-1.

The C&MS specifies the allowable pipe shape and material. Base storm sewer designs on smooth interior round pipe. The choice of the material type is determined by the designer based on durability requirements. 

When extending existing Type B & C conduits, the extensions must match the existing material in kind unless the durability is not adequate to satisfy the 75-year service life.

The length of conduit paid for is the actual number of linear feet measured from center-to-center of small structures. No deduction is made for catch basins, inlets or manholes that are 6 feet or less across, measured in the direction of flow. Conduits placed on slopes steeper than 3:1 or with beveled or skewed ends are measured along the invert.

Changes to grade may occur at existing manholes due to proposed work. With a decrease in grade of not more than 6 inches or an increase in grade of not more than 12 inches, the existing structure should be Adjusted to Grade. Where grade elevation changes are greater, the existing structure should be Reconstructed to Grade.

An example of varying conduit type conditions in a single run: Use Type B conduit for the entire transverse storm sewer run that is draining an earth median catch basin in an embankment section under the pavement and then to a ditch outlet.

1104.2 Storm Sewer Design Considerations

1104.2.1 Depth

Keep a storm sewer system as shallow as possible following these guidelines:

  1. For rigid pipe; provide a minimum height of cover of 15 inches to the pavement surface including a minimum cover of 9 inches to the top of the subgrade. Where the pipe is not under pavement, provide a minimum height of cover of 18 inches to the finished grade.
  2. For flexible pipe; provide a minimum height of cover of 24 inches to the pavement surface including a minimum cover of 12 inches to the top of the subgrade. Where the pipe is not under pavement, provide a minimum height of cover of 24 inches to the finished grade.
  3. For extra strength pipe; provide a minimum height of cover of 10 inches to the pavement surface including a minimum cover of 4 inches to the top of the subgrade. Where the pipe is not under pavement, provide a minimum height of cover of 4 inches to the finished grade. If the minimum cover cannot be provided, check with OHE to determine the required extra strength.
  4. Provide adequate depth to permit the use of precast inlets, catch basins and manholes. Refer to the Hydraulic Standard Construction Drawings for this information. Consider the sewer pipe thickness. No part of the pipe can extend into the precast top section.
  5. Provide adequate depth to avoid interference with existing utilities.
  6. Provide adequate depth to create a positive outlet for underdrains with the underdrain outlet generally 12 inches above the flow line of the outlet structure with 6 inches as a minimum.

C1104.2.1

See Glossary of Terms for the definition of cover and height of cover.

  1. Provide sufficient slope to maintain a recommended minimum velocity of 3 feet per second, for self-cleansing. This velocity is calculated using the just full Manning’s Equation.
  2. Match the crown of a smaller upstream pipe in a longitudinal trunk sewer to the crown of the adjacent downstream pipe.

The cleanout velocity is a recommendation for both design and existing conditions. Avoid extensive alteration of the storm sewer to meet this recommendation.

1104.2.2 Longitudinal Location

1104.2.2.1 Under Pavement

Longitudinal sewers are not permitted under the pavement of a limited or controlled access facility. Minimize the length of transverse sewers under pavements, with the objective of not placing manholes in the pavement. 

For other facilities, locate storm sewers outside the limits of the pavement. However, in locations where this would create conflicts with existing utilities the storm sewer may be located under the pavement. Avoid placing manholes in vehicle wheel-paths or within an intersection. Place the center of the manhole in the center of the lane when feasible. 

Provide premium joints on the storm sewer where an out-to-out clearance of 5 feet cannot be provided between parallel storm and sanitary sewers.

Submit exceptions to the above in the early stages of the design to OHE for review and approval.

1104.2.2.2 Under Paved Shoulder

The above applies to paved shoulder areas, unless the cost of any other possible location is prohibitive.

 

C1104.2.2.1

When placing manholes in the pavement of a limited access facility cannot be avoided, bolt down and bury the lid under the surface pavement layers.

1104.2.3 Access

For storm sewers under 36 inches in diameter located under or near the edge of pavement, provide access at intervals up to 300 feet maximum. For sewers sized 36 to 60 inches provide manholes spaced every 500 feet maximum and for larger sewers provide manholes spaced every 750 to 1000 feet maximum.

For manhole, inlet and catch basin details refer to the Hydraulic Standard Construction Drawings.

C1104.2.3

Most standard inlets and catch basins provide satisfactory access to small diameter shallow sewers. They can also be used where changes in pipe size or minor horizontal/vertical changes in alignment occur. Larger changes may require manholes.

It may be necessary to locate longitudinal trunk sewers away from the curb to provide for a utility strip between the curb and the sidewalk and to avoid a conflict with the underdrains. This will require properly spaced manholes in the sewer line.

1104.2.4 Rock Excavation

If it is known that bedrock will be encountered in the excavation for storm sewer installation, relocate the storm sewer. If bedrock cannot be avoided, separate the quantities of the storm sewer in rock and include Item 611, As Per Plan, in the plans.

 

1104.3 Storm Sewer Design Criteria

1104.3.1 Design Frequency

Size all storm sewers using open channel, just full capacity design to flow just full for a 10-year frequency storm. The size is determined by working downstream from the first sewer run. It is acceptable to use a discharge of a more frequent occurrence if consistent with local criteria or to avoid extensive replacement of an existing downstream drainage system.

C1104.3.1

Just full is the depth of flow for maximum discharge. Just full capacity design assumes a free water surface at a depth of 93.8% of the pipe diameter for circular conduits. Maximum flow and velocity are considered to occur at this depth.

This design methodology provides a conservative margin of safety by providing additional headroom due to increased pipe diameters.

1104.3.2 Hydraulic Grade Line

Determine the elevation of the hydraulic grade line at the upper end of each sewer run using a 25-year frequency. Start at the storm sewer system outlet and work upstream. It is acceptable to use a hydraulic grade line of a more frequent occurrence if consistent with local criteria and / or to avoid extensive replacement of an existing downstream drainage system.

The starting elevation for the hydraulic grade line determination is the higher of either: the downstream tailwater channel water surface elevation or (dc+D)/2 at the system outlet as explained in Section 1105.6.1.

Use the same intensity i in the Rational Equation Q = CiA to determine the check discharge for all sewer runs as that calculated for the last, or downstream run, in a continuous sewer system.

C1104.3.2

Ordinarily, the hydraulic grade line is above the top of the pipe, causing the system to operate under pressure. If, however, any run in the system does not flow full, (pipe slope steeper than the friction slope) the hydraulic grade line will follow the friction slope until it reaches the normal depth of flow in the steep run. From that point, the hydraulic grade line will coincide with the normal depth of flow until it reaches a run flatter than the friction slope for that run.

These criteria are not intended to lower existing high-water elevations.

The check discharge is the 25-year frequency.

The hydraulic grade line must not exceed the following: 

  1. 12 inches below the near edge of pavement for sections without curb.
  2. The elevation of a curb opening inlet or grate elevation of a pavement catch basin, as shown on the SCD.

Hydraulic grade line requirement A is for ditch sections and B is for curbed sections.

Use a 50-year frequency check storm discharge to determine the elevation of the hydraulic grade line for freeways having depressed sags drained by storm sewers.

One directional lane of a multiple lane highway or one-half of a lane on a 2-lane highway must be passable when the sewer system is discharging the check storm.

The 50-year frequency is based on Code of Federal Regulation 23 CFR 650.115(a)(2) requirements.

These criteria are intended for sag locations with no outlet except through the storm sewer system. Examples include sag locations with barrier wall, underpasses, or other depressed cut sections without an alternative outlet.

Typically, these criteria do not apply to 2-lane or other curbed roadway facilities where water can overtop the curb. Contact OHE if encountered.

1104.3.3 Runoff Coefficient

Determine the runoff coefficient per Section 1101.2.2.

1104.3.4 Time of Concentration

Determine the time of concentration as explained in Section 1101.2.1. Use a minimum time of concentration of 15 minutes to the first ditch catch basin and 10 minutes to the first pavement inlet. Use the actual calculated time of concentration when values greater than these minimums occur.

 

1104.3.5 Pipe Roughness Coefficient

Use a Manning’s n of 0.015 for sewers 60 inches in diameter and under, and 0.013 for larger sewers. The typical n value for smooth pipe, concrete, vitrified clay, bituminous lined corrugated steel or thermoplastic is 0.012.

C1104.3.5

The increased n values are recommended to compensate for minor head losses incurred at catch basins, inlets and manholes located in a storm sewer system.

1104.3.6 Minimum Pipe Size

Use a minimum pipe diameter of 15 inches for Freeways and Freeway ramps and 12 inches for other highways.

1104.3.7 Maximum Slope

The maximum slope is 4:1 H:V or the slope that produces a velocity exceeding 10 ft/s. Provide drop structures for energy dissipation when slopes or velocities exceed the allowable limits.

For storm sewers along embankment slopes that exceed 3:1 H:V, designate as Type F, Broken Back per Figure 1104-1.

C1104.3.6

Where an existing storm sewer is to remain in service, it is not necessary to replace hydraulically adequate pipes to meet these criteria.

1104.3.8 Outlet Velocity Protection

Provide outlet velocity protection for all Storm Sewers with an outlet velocity greater than five feet per second. 

Provide rock channel protection for erosion control per Figure 1002-4.

Provide a filter with the RCP. Use a geotextile fabric filter when not under water. Use an aggregate filter when the RCP is under water. The cost of the filter is included in the unit bid price for Item 601, Rock Channel Protection with Filter.

C1104.3.8

A filter is provided with the RCP to prevent soil piping through the rock. Aggregate filter is specified for placement under water as the fabric filter is buoyant and may cause difficulty during installation. Use aggregate filter for RCP placed under the OHWM.

1104.4 Storm Sewer Hydraulic Design Procedure

 

Provide storm sewer computations. Tabulate the calculations for lateral connections to the longitudinal trunk sewer separately from the trunk sewer calculations.

Software is available at the OHE Hydraulic Software and Design Resources web page and can be used for these calculations. OpenRoads SUDA may also be used for these calculations. Other software packages may be utilized with approval from OHE.

C1104.4

With the layout suggested in Section 1104.3, start with the upper catch basin or inlet and determine the value of CA for the contributing flow (CA is the product of the weighted coefficient of runoff and the drainage area). Next, determine the time of concentration for the first area and the corresponding rainfall intensity i from the proper curve shown on Figure 1101-2. The design discharge Q to use to determine the required size of the first sewer from MH No. 1 to MH No. 2 is the product of CA x i. At manhole No. 2, determine the value of CA for the additional area contributing at that point and add to the CA for MH No. 1.

Compute the time of flow in the storm sewer from MH No.1 to MH No. 2 in minutes and add to the time of concentration at MH No. 1. Check the time of concentration for the area contributing to MH No. 2, and use the larger of the two as the duration for the new value of rainfall intensity for computing the design flow from MH No. 2 to MH No. 3.

1104.5 Combined Sanitary Sewer Separation

When the Combined Sanitary Authority is under court order to address frequent overflow of the sanitary system due to storm sewer impacts, when feasible, provide an exclusive outfall for the storm sewer. Coordination with the Local is required. While adherence to Local drainage standards is not applicable for ODOT owned and maintained drainage assets it may be possible for the Department to incorporate the needs of the local entity subject to review and approval of OHE.

The Department will fund storm sewer conduit and drainage structures to provide positive drainage of the roadway when a separation is feasible. Conduit and structures required for sanitary sewer are funded by the Local. All conduit located outside of the Department owned right-of-way is also funded by the Local.

 

 

1104.6 Sanitary Sewers

 

Specify joints in accordance with C&MS 706.11 for circular concrete pipe or 706.12 for clay pipe. Permissible thermoplastic pipe may also be specified.

Discharges of treated sanitary flow from abutting property into highway drainage systems are only permitted if the discharge is authorized by the Local Health Department and have a R/W permit.

1004.6.1 Manholes

Specify all new manholes for sanitary sewer lines per the Hydraulic Standard Construction Drawings.

C1104.6

Obtain and follow local sanitary sewer building codes.

1105 Roadway Criteria

1105.1 General

 

Check the design with a single cell round pipe as a first choice. In cases where the required cover or discharge rules out a round pipe, select a shape that reduces the vertical requirements while maintaining the hydraulic capacity. Consider the following shapes in order of minimum cost to increasing cost: single-cell elliptical concrete, metal pipe-arch, prefabricated box culvert or three-sided structure. For justification of multiple cell culverts, see Section 1105.6.2.

Do not place culverts on skews in excess of 45° or as further limited in Section 1008.

Do not locate the upstream invert below the natural channel unless the culvert has a depressed inlet, a paved depressed approach apron, or an improved inlet.

Maintain the existing upstream and downstream hydraulics for the design flows when replacing a culvert. In cases where these parameters must be modified, evaluate any upstream and downstream impacts.

Perpetuate existing drainage patterns such as: depth of flow, direction of flow and overbank flow to the maximum extent achievable. Diversion of substantial volumes of flow requires regulatory consideration and possible actionable damage.

Label the elevation of the OHWM for jurisdictional waterways on the Culvert Detail Sheet for all culverts.

C1105.1

A culvert generally carries a natural stream under the highway embankment. The culvert horizontal and vertical alignment should approximate that of the natural channel.

Culvert design with the best hydraulic performance and least environmental impacts occurs when the roadway alignment is normal to the flow in the channel and is located on a relatively straight and stable section of the channel.

Roadway alignment needs to be considered early in the design process to provide optimum culvert design.

For examples of culvert detail sheets, reference Sample Plan Sheets Section 1312 – Drainage Details, maintained by the Office of CADD and Mapping. These provide a useful resource for preparation of hydraulic plans in terms of layout and content.

1105.2 Stream Protection

 

Stream protection is provided using the following practices and is only applicable to culverts within the Waters of the United States:

  • Bankfull discharge design
  • Depressed culvert inverts
  • Paved depressed approach aprons
  • Flood plain culverts

Water quantity treatment post construction BMP can be provided using stream grade control structures and is only applicable to culverts within the Waters of the United States. See Section 1111.1 and Section 1111.3.

For existing culvert replacements, inspect the channel for erosion that has caused undercutting or downcutting at the inlet of the culvert. At locations with evidence of undercutting or downcutting, provide a concrete apron according to Section 1106.3 at the inlet and outlet of the culvert to restore previous stream elevations and provide stream protection.

C1105.2

Stream protection practices are provided to improve stream channel stability. Erosion of the stream channel can migrate upstream and downstream without proper protection at the structure.

The use of each stream protection practice is limited based on project specific conditions.

Culverts within the Waters of the United States require stream protection to meet waterway permit conditions. Only culverts within the Waters of the United States can also get credit for water quantity treatment post construction BMP's by use of grade control structures. The requirements for post construction BMP’s are described in Section 1111.1. The need for water quantity treatment is described in Section 1111.3 along with the post construction BMP’s that meet water quantity treatment.

Only the project areas that drain to a grade control structure will receive treatment credit. If the treatment provided by a grade control structure does not meet the required percentage of treatment, provide additional water quantity treatment in areas not draining to the grade control structure for the remaining amount required. 

Other water quantity treatment post construction BMP’s 

  • Extended Detention (Section 1113.3)
  • Retention Basin (Section 1113.4)
  • Bioretention Cell (Section 1113.5)
  • Infiltration Methods (Section 1113.6)
  • Constructed Wetlands (Section 1113.7)

See Sections 1111 through 1113 for further information concerning the above water quantity treatment post construction BMP’s.

The above water quantity treatment post construction BMP’s may be utilized within available right-of-way or right-of-way being obtained for roadway, however, project type, site constraints or limitations, will not exempt the project from providing water quantity treatment post construction BMP’s.

1105.2.1 Bankfull Discharge Design

Provide Bankfull Discharge Design for all culverts conveying intermittent and perennial streams with the following exception:

  • The culvert is a replacement structure and an individual waterway permit is not required.

If multiple culverts are required to meet design and cover requirements depress only one culvert. See section 1105.6.2 for additional information

Use the following design steps when performing a bankfull discharge design:

  1. Determine the bankfull discharge using USGS report 2005-5153, Bankfull Characteristics of Ohio Streams and Their Relnoteation to Peak Streamflows. Use the regression equation that utilizes USGS map-based explanatory variables.
  2. Determine the culvert size from traditional culvert hydraulic design.
  3. Depress the culvert invert according to Section 1105.2.2.
  4. Determine the depth of flow for the bankfull discharge based on field-obtained stream cross-sections and the use of a standard step-backwater water-surface profile model such as HEC-RAS or the use of other software capable of calculating depth of flow based on Manning’s equation at locations 25 feet before the culvert inlet, at the culvert, and 25 feet beyond the culvert outlet.
  5. Determine the depth of flow for the post-developed channel using the bankfull discharge at the same locations identified in Step 4 through use of a standard step-backwater water-surface profile model such as HEC-RAS or the use of other software capable of calculating depth of flow based on Manning’s equation. The cross section at the culvert will reflect the geometry of the culvert.
  6. Compare the depth of flow from step 4 to step 5. Adjust the culvert dimensions until the post-developed condition flow depth (Step 5) is approximately equal to the pre-developed flow depth (Step 4).
  7. Add flood plain culverts if required. See section 1105.2.4.
  8. Determine if the culvert meets the required hydraulic design controls. Upsize the culvert as required

C1105.2.1

Culverts utilizing Bankfull Discharge Design are required to convey the bankfull discharge with minimum change in the stream energy for the adjoining channel sections when compared to the existing conditions.

In some cases when an individual waterway permit is required, this exemption may not apply because of minimization or mitigation requirements from the regulatory agencies. OES will coordinate this need with the DEC or designer on a case by case basis.

The proposed culvert will minimize the impact to the stream channel by closely matching the existing depth of flow with the proposed depth of flow for the bankfull discharge in order to facilitate passage of aquatic organisms.

Information on stream flow regime can be found by contacting the DEC or consulting the Ecological Survey Report located on ODOT EnviroNet.

1105.2.2 Depressed Culvert Inverts

Provide depressed inverts for all culverts designed to convey the Bankfull Discharge Design with the following exceptions:

  • The culvert is located on bedrock
  • The culvert slope exceeds 1%

Verify that the culvert meets the required hydraulic design controls realizing that the portion of the culvert depressed will eventually fill with natural substrates. Upsize the culvert as required.

End treatments for culverts with depressed inverts consist of Item 601, Riprap, 6” Reinforced Concrete Slab, with a cutoff wall on both inlet and outlet ends. For details see Hydraulic Standard Construction Drawing DM-1.1.

Depress the culvert invert per Table 1105-1:

Table 1105-1

Type A Conduit Invert
Pipe Diameter or Rise (inches) Depression (inches)
< 36 None
36 - 60 6
66 - 120 12
126 - 180 18
186 - 252 24
> 252 30

Modifications to the standard headwalls are not necessary for the depression depths noted above.

C1105.2.2

Depressed culvert inverts will produce a natural channel bottom within the culvert. The natural channel bottom provides a substrate for passage of migratory species.

The depressed culvert will fill naturally, such that the channel bed in the culvert will be continuous with the adjacent channel sections.

When feasible, install culverts at the existing streambed slope to allow for the natural movement of bedload and aquatic organisms.

1105.2.3 Paved Depressed Approach Aprons

The dimensions of the slab are site specific. Limit the downslope of the apron to a maximum of 2:1. Include a 3-foot length of paving along the natural channel slope prior to the drop-down. Provide a cut -off wall at the upstream end.

In general, limit drop-down entrances to 4 feet, or one pipe diameter or rise, whichever is greater. 

C1105.2.3

In many cases, the hydraulic operation of a culvert can be improved by depressing the flowline at the entrance below the channel flowline. The drop-down will alleviate a minimum cover condition, provide for additional headwater depth, and decrease the culvert outlet velocity by reducing the culvert slope. The abrupt change in natural channel slope is affected with a short length of concrete paving to prevent downcutting of the stream.

The Federal Highway Administration has conducted extensive research and studies of paved depressed approach aprons, and recommended design procedures are included in Hydraulic Design Series No. 5, Hydraulic Design of Highway Culverts.

1105.2.4 Flood Plain Culverts

Determine the width of flow for the bankfull discharge from section 1105.2.1. Provide flood plain culverts when the flood plain width is greater than two times the width produced by the bankfull discharge. Place culvert inverts at the water surface elevation that is generated by the bankfull discharge depth. Locate the flood plain culverts within the flood plain at a location well beyond the single culvert. Furnish a minimum of two flood plain culverts. See Figure 1102-2.

Flood plain culverts are not hydraulically designed or accounted for in the hydraulic design of the single culvert. Use Figure 1002-1, Other column, to determine the required diameter. Approximate the line and grade of the natural flood plain.

C1105.2.4

For all new bankfull culvert installations, consider the use of flood plain culverts. In wide flood plains, the installation of a new single culvert constricts the flow of water at the entrance section. The concentrated outflow from the culvert can initiate downstream channel degradation. Flood plain culverts can be used to minimize the effects of this new concentrated discharge by spreading the discharge throughout the flood plain or flood prone area on the outlet side of the culvert.

1105.2.5 Outlet Velocity Control

Provide velocity control for all culverts with an outlet velocity greater than five feet per second.

Provide rock channel protection for erosion control per Figure 1002-4.

Provide a filter with the RCP. Use a geotextile fabric filter when not under water. Use an aggregate filter when the RCP is under water. The cost of the filter is included in the unit bid price for Item 601, Rock Channel Protection with Filter.

Provide the following energy dissipators when the velocity exceeds the limits for RCP per Figure 1002-4:

  • Riprap Basin
  • Ring Chambers

Contact OHE prior to using an energy dissipator.

Ring chambers may be specified at the outlet end of the pipe as an internal energy dissipator. If the outlet velocity for a corrugated pipe is less than 20 feet per second while the outlet velocity for a smooth pipe requires a ring chamber, the corrugated pipe may be specified exclusively.

C1105.2.5

The use of rock channel protection or energy dissipators does not constitute water quantity treatment for post-construction BMP purposes.

A filter is specified with RCP to prevent soil piping through the rock.

A riprap basin is the most cost-effective energy dissipator.

1105.3 Types of Culvert Flow

 

 

C1105.3

Laboratory tests sponsored by the FHWA have established two general types of culvert flow:

Flow with inlet control
Flow with outlet control

Nomographs have been prepared for use in the determination of culvert headwater for the appropriate control. 

Under inlet control, the headwater HWI is directly related to the cross-sectional area of the culvert barrel and the inlet geometry. Under outlet control, the headwater HWO is further influenced by tailwater depth in the outlet channel and the slope, length and roughness of the culvert barrel. As shown in Figure 1105-1, culverts operate with a free water surface if the headwater is equal to or less than 1.2D, and with a submerged entrance if the headwater is greater than 1.2D, where D is the diameter or rise of the pipe.

1105.4 Design Procedure

1105.4.1 General

The design of a culvert involves a determination of the appropriate design and check discharges. The process begins with a delineation of the drainage area, in acres, on a suitable topographic map.

The design discharge Q for most culvert drainage areas is obtained by procedures described in Section 1003.1.2. Use the Rational method to obtain the discharge from small and other unusual drainage areas as noted in Section 1101.2.1.

A representative cross-section of the embankment at the proposed culvert site, along with a profile of the natural stream or ground line, will be required to determine the approximate length and slope of the culvert.

1105.4.2 Hydraulic Analysis

Provide a culvert hydraulic analysis that determines the controlling headwater for each culvert type being considered for a given location. Include supporting data for the required review submissions.

Hydraulic analysis of culverts may be performed using the FHWA Design Series No. 5, Hydraulic Design of Highway Culverts. Computer programs such as FHWA HY-8 or software developed by ODOT, CDSS, is available at the OHE Hydraulic Software and Design Resources web page and can be used for these calculations.

For replacement projects, perform an analysis of the existing structure. Use the same analysis method when comparing the existing and proposed structures. For bridge replacements, the acceptable method of hydraulic analysis is HEC-RAS.

 

1105.5 Design Criteria

1105.5.1 Design Frequency

Use the design frequency as stated in Section 1004.2.
Perform a Flood Hazard Evaluation using a check discharge based on the 100-year flood frequency for all culverts as noted in Section 1005.2.1.

1105.5.2 Maximum Allowable Headwater

See Section 1006.

1105.5.3 Method Used to Estimate Storm Discharge

See Sections 1003 and 1101.

1105.5.4 Scale of Topographic Mapping Used to Delineate Contributing Drainage Areas

See Section 1101.1.

1105.5.5 Manning’s Roughness Coefficient

The Manning’s Roughness Coefficient n values for corrugated metal pipe are given in Figure 1105-2. The n value for all smooth flow pipe is 0.012. Use a weighted Manning’s n for bankfull designed culverts.

 

1105.5.6 Entrance Loss Coefficient

Use the Entrance Loss Coefficient ke found in Table 1105-2 or Appendix D of Federal Highway Hydraulic Design Series No. 5, Hydraulic Design of Roadway Culverts.

Table 1105-2

Type A Conduit
Entrance Loss Coefficient ke
Type of Pipe Headwall Type
  Full Half None
Concrete, Vitrified
(thick wall) *
0.2 0.2 0.2
Corrugated Metal
(thin wall)
0.25** 0.9 0.9

* groove end entrance
** beveled entrance

C1105.5.6

Design plastic conduits without a welded bell inlet as a corrugated metal conduit. Design plastic conduits with a welded bell inlet as a concrete conduit. Use a Manning’s n value of 0.012 for plastic conduit in both cases.

A welded bell can be added to plastic pipe to improve hydraulics.

1105.5.7 Contacts with County Engineer

Contact the County Engineer at the beginning of the design process to review the proposed location, horizontal and vertical alignment and to determine ditch cleanout grades. Use the County Engineer Approval Form, Appendix B to document the approval.

C1105.5.7

Ohio Revised Code, Section 6131.631

1105.6 Special Considerations

 

 

C1105.6

The following are special conditions that will be encountered in the hydraulic design of culverts that warrant clarification.

1105.6.1 Tailwater

When there is no downstream influence, calculate the tailwater by determining the normal depth of flow in the outlet channel when the culvert is discharging the design flow. 

When there is influence from a backwater condition downstream, calculate the tailwater by the following:

Where the drainage areas of the culvert and receiving watercourse are nearly equal, assume concurrent flood peaks.

Where there is a significant, but not excessive, difference in the drainage area of the culvert and receiving stream, use the following design procedure and size the culvert using the combination that results in the highest headwater:

  1. Compute the culvert headwater using the proper design frequency for the culvert and the next  lower frequency per 1004.2 for the receiving stream water surface elevation to determine the culvert tailwater elevation. For example, a 25-year culvert and a 10-year stream.
  2. Use the lower frequency for the culvert and the proper design frequency for the receiving stream to determine the culvert tailwater elevation. For example, use 10-year frequency for the culvert and 25-year for the stream.

Where the drainage area of a culvert is substantially less than the receiving watercourse, ~100 times, the effect of the receiving watercourse generally may be disregarded.

In some locations, a high tailwater will control the operation of a culvert to such an extent that a substantial increase in pipe size will be required for a negligible decrease in the headwater elevation. For this case, size the culvert based on a tailwater elevation using the equation:

Culvert size based on tailwater elevation

Where:

TW = Tailwater (ft)

dc = Critical depth at culvert outlet (ft)

D = Rise of culvert (ft)

C1105.6.1

Tailwater at a culvert outlet can greatly affect the size of culvert required at a specific site. A proper evaluation of the outlet channel must be made so that a reasonable estimate of the tailwater can be calculated.

A determination of the normal depth of flow in the outlet channel, when the culvert is discharging the design flow, normally establishes the culvert tailwater. An examination of the downstream channel may, make known a temporary or permanent obstruction that will control the operation of the culvert. In some cases, the culvert will outlet within the backwater of a river or other fluctuating water surface that could control the tailwater elevation.

1105.6.2 Multiple Cell Culverts

A single-cell culvert is preferred, but at times site conditions or design considerations may create the need for multiple cells. For these cases, it is desirable to limit the number of cells to two.

C1105.6.2

When aligned with a relatively straight channel, multiple cells will operate satisfactory. However, a bend in the immediate upstream channel may cause the inside cell to collect debris during normal periods of flow and can substantially reduce the capacity of the culvert.

1105.6.3 Improved Side and Slope Taper Inlets

Consider improved inlets attached to the entrance end of the culvert to reduce headwater or culvert size. See section 1105.2.3. If additional improvement is needed.

Consider the following two general types of inlets in the following order:

  1. Side-taper - A tapered end section from a round to an oval shape for a pipe, or a square to a rectangular shape for a prefabricated box. The length of the taper section is usually made 1.5 times the diameter or rise of the culvert.
  2. Slope-taper - A combination of side-taper preceded by a drop in the culvert flow line. The drop can be similar to a paved drop-down entrance, see section 1105.2.3, or a more sophisticated reinforced concrete drop provided by a formed cast-in-place section with vertical sides.

The savings in culvert cost must justify the additional cost of the improved inlet.

C1105.6.3

Culverts on relatively steep slopes and under inlet control can see a reduction in the culvert size by furnishing an improved inlet.

The improved inlet has the advantage of admitting more flow and thereby tending to fill the culvert barrel and reduce the culvert outlet velocity.

The Federal Highway Administration has conducted extensive research and studies of improved inlets, and recommended design procedures are included in Hydraulic Engineering Circular No. 13, Hydraulic Design of Improved Inlets for Culverts.

1106 End Treatments

1106.1 General

 

Provide headwalls, or other approved end finishes, at the open ends of all Type A, B and C conduits. Provided headwalls for Type D conduits greater than 24 inches in diameter or rise. Headwalls are not recommended for Type E and F conduits.

Figures 1106-2 and 1106-3 show typical end details for a concrete box culvert without guardrail attachment.

Special end treatments may be required per the LD1, Section 602.6. Details are available from the OHE. Justification for the use of this type of end treatment must accompany the request for details.

1106.1.1 End Treatment Grading

Project the prevailing embankment slope to the back edge of the top of the headwall to establish the required culvert length. When the roadway foreslopes are flatter than 2:1, provide a 2:1 slope from the back edge of the top of the headwall to a minimum of 1 foot, with 2 feet preferred, above the top of the culvert. See Figure 1106-1 for details. Warp the embankment slope on each side of the conduit to fit the prevailing slope. Unless guardrail is provided, maintain the design clear zone grading width, per LD1, Section 601, before the embankment slope changes to 2:1.
Provide clear zone grading at culverts only when the requirements of LD1, Section 307.2.1, are met.
Warp the prevailing embankment slope on either side of a skewed culvert to provide equivalent soil loading and proper side support of the pipe. This is especially true for flexible pipes with large skews and/or large diameters.

C1106.1

The selection of headwall type is based on safety and economics.

1106.2 Headwall Types

1106.2.1 Half-Height Headwalls

Use standard half-height headwalls HW-2.1 and HW-2.2 detailed in the Bridge Standard Construction Drawings for round, elliptical, or pipe arch culverts where clear zone grading is provided. Place half-height headwalls perpendicular to the end of the conduit to eliminate the need for a skew cut. Miter-cut the exposed half of conduits having a diameter or rise greater than or equal to 126 inches to fit the embankment slope. Show miter-cut end sections on the Culvert Detail Sheet.

Payment for half-height headwalls is based on cubic yards of Item 602, Concrete Masonry. Base the quantity of concrete masonry provided in the plans on the pipe alternate requiring the largest quantity of concrete masonry.

C1106.2.1

Masonry quantities for standard half-height headwalls may be obtained from the standard construction drawings.

1106.2.2 Full-Height Headwalls

Use full-height headwall HW-1.1 detailed in the Bridge Standard Construction Drawings where a significant reduction in culvert length can be achieved with foreslopes flatter than 2:1, where right-of-way limits the culvert length and at the entrance end of round pipes when the savings in the reduced size and length of the conduit will offset the additional cost of the headwall.

Provide full-height headwalls for all prefabricated box culverts and three-sided structures. Design headwalls per Section 300 of the BDM. Refer to Bridge Plan Insert Sheet BCHW.

Include appropriate plan notes from Section 600 of the BDM in the project plans.

Perform an investigation of the supporting foundation material to estimate the bearing capacity of the material. Submit the foundation report with the Stage 1 review.

Armor the inlet wingwall footings of full-height headwalls with Type B rock channel protection, with filter, to prevent scour.

C1106.2.2

The use of a full-height headwall will most likely apply where corrugated steel pipe is specified due to cover or size requirements and the bevel provided for the full-height headwall will substantially reduce the entrance loss.

1106.3 Concrete Apron

Provide a reinforced concrete riprap cutoff wall, as shown on Hydraulic Standard Construction Drawing DM-1.1 when the depth of the rock channel protection, including the 6 inch granular filter, exceeds the depth of the headwall.

Provide concrete riprap per Section 1105.2.3, at the inlet end of the culvert where the existing culvert has been undercut. Concrete riprap is not necessary at the inlet of culverts with full height headwalls that have a footing toe extending 3.5 feet or more below proposed channel grade.

 

1107 Bridge Hydraulics

1107.1 General

 

Submit hydraulic design calculations, H&H reports, scour evaluations and flood hazard evaluations with the STS.

C1107.1

Bridge structural design requirements are found in the BDM.

1107.2 Hydrology and Hydraulics Report

 

Provide a small-scale area plan showing the location of all cross sections used for the hydraulic analysis and an accurate waterway alignment at least 500 feet upstream and downstream of the structure for a 1D model. A 2D model requires the distance to be 500 feet or 2x the floodplain width, whichever is greater. Include the alignment of the proposed and existing roadways taken from ground survey.

Include a profile following the centerline of the roadway to compute the overflow section. Extend the profile along the approach fill to an elevation well above high water.

C1107.2

To properly assess the impacts of the proposed structure, extend the waterway alignments upstream and downstream to the point where a convergence of the proposed and existing water surface profile occurs in the model.

If bridges or large culverts are located within 1000 feet upstream or downstream of the proposed bridge, show stream cross sections including the structure and roadway profiles of the overflow sections of the structures.

The upstream and downstream structures may be used as a guide in establishing the waterway requirements of the proposed structure.

1107.2.1 Analysis

The H&H analysis is performed using the design year as defined in section 1004.2 along with the 100-year and 500-year frequencies.

A 1D step backwater analysis software such as HEC-RAS-1D is adequate for most applications. The simplified assumptions used in 1D models create limitations in complex hydraulic situations where 2D modeling software such as HEC-RAS-2D or SRH-2D may be better suited. Contact OHE prior to using 2D modeling. If a 2D analysis is authorized, a 1D analysis may still be required.

Include the following items in the H&H analysis:

  1. Hydrology calculations or source of the discharges used in the analysis. Include the drainage area in square miles.
  2. Input and output data including electronic program files of computations for existing and proposed conditions. If using HEC-RAS, refer to the HEC-RAS Help Applications Guide for the multiple plans file structure. 
  3. Provide enough fully bounded cross sections to properly model the existing and proposed conditions. A minimum of one cross section in close proximity to each face of the structure along with two additional cross sections upstream and downstream outside of the
    expansion/contraction zone are required.
  4. Color photographs of the bridge opening and the upstream and downstream channel.

C1107.2.1

2D models provide more realistic and detailed information on key variables such as velocity and water surface elevation. A 2D model is useful in locations where a 1D flow model cannot adequately describe the flow regime. Such locations may include wide floodplains with large flows into the overbanks, projects where bends and confluences are located near the area of interest, locations with multiple channels, multiple bridges and/or bridge openings, bridges with skewed abutments to the direction of flow, and bridges operating under pressure flow for the design year frequency.

Consider the importance of the structure, based on roadway facility type and ADT, before 2D modeling is pursued.

When performing a step backwater analysis of the floodplain in a FEMA SFHA that has had a detailed hydraulic analysis; obtain the Current Effective FIS model to use as the basis of the model creation process.

Fully bounded cross sections provide sufficient width and elevation to contain all depths of flow produced in the HEC-RAS analysis. Where the model produces a vertical wall to contain the flow, additional ground survey or LiDAR data may be used to supplement the cross section extents. Cross sections with expansive floodplains containing slow shallow flow may be truncated. Use engineering judgement to determine cross sectional widths.

Perform an evaluation of the channel stability for bridge replacements where the existing crossing exhibits continual maintenance issues with sediment aggradation, degradation, or debris accumulation. Maintain channel continuity to the upstream and downstream conditions through the structure by providing a channel similar to that which is naturally occurring. Design the channel cross-section through the bridge to match the bankfull properties of the stream as much as practical. The use of a two-stage channel may be required to convey both the bankfull discharge and the design and check year discharges. Provide the same methodology for new bridges. Coordinate with OHE for additional guidance prior to design.

Avoid over-widening the channel at the structure. This condition may reduce channel velocities leading to maintenance issues such as aggradation and debris accumulation where they had not occurred before.

1107.2.2 Narrative

Include the following in the narrative:

  1. The rationale used to determine the proposed structure size and type by an analysis of design alternatives. Include a comparison of the existing and proposed design year and 100-year headwater elevations and velocities in tabular form.
  2. Compare existing and proposed waterway opening along with the structure low cord elevations and design and check year flood clearances in tabular form.
  3. A statement as to whether the structure is located within a FEMA SFHA. Identify the FIRM showing the project location. Include relevant excerpts from the FIS where applicable. If within an FIS, describe the hydraulic model creation process using standard FEMA naming terminology.
  4. High water data from local residents and observed high water marks including their locations if available.
  5. Approximate Flood Peak Discharge Frequency of roadway overtopping.
  6. A Flood Hazard Evaluation per 1005.2.
  7. Capital costs and risk as part of the discussion. Risk is defined as the consequences attributable to a flood plain encroachment.
  8. Description of the bridge deck drainage. Indicate how the surface water will be collected and discharged. Include any scupper catch basin locations.

C1107.2.2

The Narrative is a written discussion of the hydraulic adequacy for both the design year and 100-year frequency discharges.

Take headwater elevations away from the face of the structure just beyond the effects of the contraction on the water surface profile.

Maintain the existing waterway opening size as much as possible. Limit reductions to 20 percent.

Standard FEMA model naming terminology includes: Current Effective, Duplicate Effective, Corrected Effective, Existing Conditions or Pre-project, Proposed Conditions or Post-project models.

1107.3 Bridge Rock Channel Protection

 

Provide RCP for bridges over waterways at the following locations:

  1. The entire spill-through slope
  2. Front side of abutments and wingwalls
  3. Corner cones

Use Table 1107-1 to determine the Type of RCP to use:

C1107.3

It is more economical to provide bank protection during the initial construction in order to minimize future maintenance.

Table 1107-1

Channel Mean Velocity (ft/s) RCP Type Thickness (inch)
0 - 8 C 24
8 - 10 B 30
Above 10 A 36

Contact OHE when Velocities exceed 12 ft/s.

Special circumstances such as protection on the stream bank located on the outside of a curve or where ice flow is problematic may require greater rock thickness. 

Show the locations, length, and the top of slope elevations for the RCP on the Structure Site Plan. Show the RCP in greater detail in the roadway section in conjunction with the channel plans.

Table 1107-1 is based on AASHTO (2014) Drainage Manual Volume 1 - Policy, Chapter 17 equations 17B-1 and 17B-2 to calculate the required RCP D50. The D50 size corresponds to RCP Type A, B or C in the C&MS. The equations have average water velocity and depth as variables. For simplicity the table uses only velocity to provide RCP Type determination for most common water depths.

1107.4 Bridges Over Outlet Controlled Waterbodies

 

When replacing a bridge, match or exceed the hydraulic opening of the existing bridge. Maintain the roadway profile and low bridge cord elevation to existing as much as possible.

Where sizing of the bridge is controlled by navigational clearance, provide a cross sectional opening and low bridge chord elevation meeting the navigational clearance requirements.

For new bridges on new alignments that do not require navigational sizing contact OHE for guidance.

C1107.4

Bridges conveying waterbodies that have controlled outlets or spillways require a different design process due to the impacts of these downstream features. Use of USGS StreamStats to obtain discharge flow rates is not applicable due to the influence of the downstream structure. A hydrologic and hydraulic investigation is performed as part of the waterbody design to develop stage, storage, discharge and water pool elevations created by the downstream structure. The water management data must be obtained from the owner to aid in the bridge design and the information must be shown on the structure site plans per the BDM.

The bridge opening serves as an equalizing connection between the two sides of the waterbody, which has a water pool elevation generated by the flow capacity of the downstream structure in combination with the topography of the reservoir.

1108 Miscellaneous Drainage

1108.1 Farm Drain Crossings

Where it is necessary to continue an existing farm drain crossing under the highway, use Type B Conduit, one size larger than the existing farm drain within the right-of-way limits.

 

 

1108.2 Farm Drain Outlets

 

Terminate existing farm drains that outlet through the backslope of the roadway ditch with a minimum length of 10 feet of equivalent size Type F Conduit. Use one size larger Type F Conduit when existing farm drains are plastic.

Place the outlet invert of the Type F Conduit a minimum of 6 inches, with 12 inches being desirable, above the ditch flow line.

Provide an Erosion Control Pad as shown on Hydraulic Standard Construction Drawing DM-1.1.

C1108.2

To allow for possible sedimentation of the roadway ditch, the farm drain outlet is placed above the ditch invert.

The farm drain pipe can be place with a minimum slope of 0.

1109 Notice of Intent

1109.1 General

 

Submit a Notice of Intent for all projects where Total Earth Disturbing Activity is one acre or more, except Routine Maintenance Projects, as defined by Section 1109.2. The Total EDA acreage includes the combination of Project EDA and Contractor EDA.

EDA is defined as any activity that exposes bare ground or an erodible material to storm water as well as anywhere that Item 659, Seeding, or Item 660, Sodding, is being furnished. Project EDA is EDA that occurs within the project construction limits. Contractor EDA is EDA from support activity sources such as field offices, batch plants, borrow/waste pits, and temporary access routes. Project EDA is determined based on the project design, while Contractor EDA is estimated. 

Non-contiguous portions of projects sold under one contract, such as multiple culvert replacements or Part1/Part 2 projects, may be treated as separate projects for the purposes of submitting an NOI if the project sites are located ¼ mile or more apart and the areas between the activities are not being disturbed. If each site is below the Total EDA threshold of one acre, no post-construction BMP or NOI is required. If one or more individual sites meet the Total EDA threshold, an NOI is required for those sites that meet or exceed the Total EDA threshold. The NOI application must reflect the Total EDA for all project sites that meet or exceed the threshold. Provide post-construction BMPs only at the individual project sites that exceed the Project EDA threshold as described in Section 1111.

Disturbed areas that drain into a combined sewer do not require coverage under Ohio EPA’s construction general permit, and therefore are not included towards meeting the Total EDA threshold of one acre. If a project has some disturbed area that drains to a combined sewer system and some disturbed area that drains to a storm water system, only disturbed areas that drain to a storm water system are EDA when determining the need for coverage under the construction general permit or the need for a post-construction BMP. Coordinate with the agency responsible for the receiving treatment plant for construction activities that drain into a combined sewer. Consider the local agency’s temporary erosion and sediment control requirements for construction activities that drain into a combined sewer.

Prepare a Project Site Plan as required by LD3, Section 1308 for all projects that require an NOI or post construction BMPs.

C1109.1

An NOI is an application requesting coverage under Ohio EPA’s National Pollutant Discharge Elimination System (NPDES) general permit for storm water discharges from construction activities (OHC000005). The applicant(s) must certify their intention to comply with the NPDES construction general permit by submitting an NOI. The construction general permit requires specific documentation of site conditions, temporary erosion and sediment controls, post-construction storm water best management practices (BMPs), good housekeeping practices, and other requirements depending on the site.

Example for EDA: an area where pavement is being removed to the sub-grade is considered earth disturbing activity, but bridge deck construction or repair is not considered earth disturbing activity since there is no erodible material under the bridge deck.

The Contractor EDA can be estimated using the NOI Acreage Calculation Form Figure 1109-1.

When the combined Project EDA and estimated Contractor EDA are just less than one acre, the project designer may choose to increase the estimated Contractor EDA to avoid the possibility of the project disturbing one acre or more without coverage under Ohio EPA’s construction general permit.

1109.2 Routine Maintenance Project

 

For the purposes of applying for coverage under Ohio EPA’s construction general permit, submitting an NOI, a Routine Maintenance Project is one in which all of the Project Earth Disturbing Activities are routine maintenance activities that do not change the purpose, line and grade, or the hydraulic capacity of the facility and involve Total EDA of less than five acres. Routine Maintenance Projects do not require permit coverage and therefore do not require an NOI. If a project includes disturbance from both routine maintenance activities and construction activities, then the project, as a whole, cannot be considered a Routine Maintenance Project and all earth disturbed area must be included in determining the requirement for an NOI. Permanent erosion control items are included in the plans, if required.

Projects with five or more acres of Total EDA cannot be classified as Routine Maintenance Projects.

The following activities are considered routine maintenance activities:

  • Bridge Repair and Replacement – repair or replace bridge abutments, approach, and deck and associated grading
  • Fence Repair and Replacement – repairing or replacing existing fencing and/or posts
  • Guardrail Repair and Replacement – repairing or replacing with minor grading work to create proper grade for end assemblies where previous guardrail existed
  • Noise Wall Repair – repairing or replacing existing noise wall
  • Sign Maintenance – repairing or replacing traffic signs and posts
  • Lighting Maintenance
  • Loop Detector Repairs – repairing loop detectors in existing pavement
  • Signal Installation and Maintenance – installing, repairing or replacing traffic signals and poles where previous signals existed
  • Pothole Filling
  • Tree/Brush Removal
  • Linear Grading – reshaping of graded shoulders to establish proper drainage away from pavement
  • Berm Repair or Topsoil placement along shoulders – placing berm material or topsoil on shoulders adjacent to pavement to eliminate drop-offs
  • Ditch Cleanout – maintaining or restoring original flow line and cross-section only
  • Culvert Replacement – replacing a culvert with same line, grade and hydraulic capacity; must be within parameters of the USAC Nationwide Permit #3
  • Culvert Repair or Lining – repairing or lining existing culvert maintaining same line, grade and hydraulic capacity, must be within parameters of the USAC Nationwide Permit #3
  • Curb Repairs – repairing existing curbing along a roadway
  • Utility Repairs – repairs to existing utilities, and associated grading or pavement replacement
  • Sidewalk – replacement of existing sidewalk without other drainage or roadway improvements
  • Land slide repairs – includes grading and repairing roadway features affected by the slide
  • Unpaved/Gravel Roadway or Shoulder Maintenance – dragging, blading, grading, adding aggregate, etc. to an existing unpaved/gravel roadway. This includes paving of an existing gravel road or shoulder in order to stabilize the roadway surface
  • Full Depth Pavement Repair or Replacement – repairs to existing roadway with no changes to the purpose, horizontal alignment, or hydraulic capacity of the roadway. Full depth pavement replacement is considered a routine maintenance activity if no additional impervious area is added outside of the existing edge of the paved roadway

Post-construction storm water best management practices are not required for Routine Maintenance Projects. For projects in which all of the Project EDA is associated with routine maintenance activities, but the Total EDA is equal to or greater than 5 acres, an NOI is required. However, for some of these projects, such as larger land slide repairs or linear grading, post-construction BMPs may not be necessary. Coordinate with OHE and Ohio EPA to determine whether post-construction BMPs will be required for these projects.

Submit an NOI for routine maintenance projects that have all the following criteria:

  • Earth disturbance within 200 feet of Waters of the United States
  • Earth disturbance associated with landslide repair, mitigation, bridge repair, or bridge replacement
  • Total EDA is equal to or greater than 1 acre

Post-construction BMPs are not required for these routine maintenance projects that are within 200 feet of Waters of the United States.

C1109.2

40 CFR 122.26(b)(15)(i) indicates that “construction activities” (which require an NOI) do not include routine maintenance that is performed to maintain the original line and grade, hydraulic capacity, or original purpose of the facility. While the federal language does not include an acreage limitation on routine maintenance activities, Ohio EPA added the limitation that routine maintenance projects (that do not require an NOI) must be limited to projects that have a Total EDA of less than 5 acres.

Ohio EPA’s routine maintenance exclusion for construction activity permitting can be found here: Storm Water Program

1109.3 Watershed Specific NOI Requirements

 

Watershed-specific requirements exist for the Big Darby Creek watershed in Table 1109-1 and portions of the Olentangy River watershed in Table 1109-2. These watersheds are identified by their Hydrologic Unit Code. Coordinate projects in the following watersheds with OHE:

C1109.3

Ohio EPA’s construction general permit includes additional requirements for projects located in certain designated watersheds.

Table 1109-1

Big Darby Creek Watersheds with Additional Permit Requirements
HUC-10 Watershed Name
0506000119 Headwaters Big Darby Creek
0506000120 Little Darby Creek
0506000121 Worthington Ditch - Big Darby Creek
0506000122 Hellbranch Run - Big Darby Creek

Table 1109-2

Olentangy River Watersheds with Additional Permit Requirements  
HUC-10 Watershed Name
050600010901 Shaw Creek
050600010902 Headwaters Whetstone Creek
050600010903 Claypool Run - Whetstone Creek
050600010904 Delaware Run - Olentangy River
050600010905 Deep Run - Olentangy River
050600010906 Rush Run - Olentangy River *

* only the portion north of IR-270

Projects located in the Big Darby Creek watershed must meet the standard permit requirements as well as the following additional requirements described in Appendix A of Ohio EPA’s construction general permit:

  • Sediment settling pond sizing, for temporary erosion and sediment control, that is larger than normally required
  • Quarterly sampling of all concentrated runoff from active construction sites following a rainfall event, ensuring that the effluent TSS concentration is no greater than 45 mg/L
  • Riparian setback mitigation for riparian zone impacts outside of the existing ODOT right-of-way
  • Groundwater recharge mitigation for impacts outside of the existing ODOT right-of-way

See Ohio EPA’s construction general permit for detailed requirements of the above bullets.

In the Big Darby Creek watershed, linear transportation projects which are caused solely by correcting safety related issues, mandates of modern design requirements and/or resulting from other mitigation activities are exempt from riparian setback mitigation and groundwater recharge mitigation if less than one acre of total new right-of-way is associated with the project.

Projects located in the portions of the Olentangy River watershed shown in Table 1109-2 must meet the standard permit requirements as well as the following additional requirement described in Appendix B of Ohio EPA’s construction general permit.

  • Riparian setback mitigation for riparian zone impacts outside of the existing ODOT right-of-way

For projects in the watersheds listed in Table 1109-1 and Table 1109-2, provide groundwater recharge calculations, riparian setback mitigation calculations, and temporary sediment basin sizing calculations and locations to OHE with the BMP submittals as outlined in Section 1112.2. Groundwater recharge calculations and riparian setback calculations are based on impacts outside the existing ODOT right-of-way. Determine the riparian setback limits according to Ohio EPA’s construction general permit and identify the riparian setback limits on the Project Site Plan.

Determine mitigation for groundwater recharge and riparian setback through coordination between the District and OHE prior to the BMP submittal outlined in Section 1112.2. The District and OHE must coordinate with Ohio EPA as to any mitigation proposals prior to submittal of the NOI application.

Determine soil types required for groundwater recharge calculations using the NRCS Web Soil Survey website.

While sediment basin locations are typically provided by the Contractor, designers of projects being developed in the watersheds listed in Table 1109-1 and Table 1109-2 must identify locations of sediment basins with capacities required for these watersheds. Show the locations and calculations for sediment basins on the Project Site Plan. Additional temporary erosion and sediment control features will be added to the Storm Water Pollution Prevention Plan by the Contractor.

Submit the NOI, Project Site Plan, proposed mitigation and supplemental calculations to the Ohio EPA at least two months prior to plan package submittal to ensure that there are no delays. 

A map of HUC boundaries can be found at ODOT’s TIMS website. Click HUC – Stream Order to view boundaries.

1110 Temporary Sediment and Erosion Control

1110.1 General

 

Provide temporary sediment and erosion control on all projects that have Earth Disturbing Activities. As outlined in SS832, projects fall into four different scenarios associated with temporary sediment and erosion controls.

Scenario A: No EDA, No NOI

Scenario B: EDA > 0, No NOI

Scenario D: NOI required due to contractor activities

Scenario F: EDA > 0, NOI required

Include SS832 on all projects. 

Provide Item 832, Erosion Control, on all projects with EDA (Scenarios B, D, and F).

Provide Item 832, Storm Water Pollution Prevention Plan, Item 832, Storm Water Pollution Prevention Inspections, and Item 832, Storm Water Pollution Prevention Inspection Software, on projects with Project EDA > 0 that require an NOI, Scenario F.

Projects that have potential environmental impacts to habitat, species or with specific local requirements may also be required to submit an NOI and prepare a SWPPP as determined by the District Environmental Coordinator.

C1110.1

SWPPP requirements are outlined in SS832.

1110.2 Cost Estimate for Temporary Sediment and Erosion Control

For all projects that require Item 832, Erosion Control, furnish a dollar amount to be encumbered in the final plan package. Use the Item 832 Erosion Control Estimator spreadsheet to estimate this amount. The dollar amount for Item 832, Erosion Control, is used for both the quantity and the total fields.

The units for Item 832, Storm Water Pollution Prevention Plan, Item 832, Storm Water Pollution Prevention Inspections, and Item 832, Storm Water Pollution Prevention Inspection Software, are each lump sum.

 

1111 Post-Construction Storm Water Structural Best Management Practices

1111.1 General

 

For ODOT projects, submit any proposed alternative post-construction BMP designs that are not found in Section 1113 to OHE. A review and approval of the alternative BMP by OHE and Ohio EPA is required. Local-Let Local Public Agency projects may use an alternative post-construction BMP criterion with Ohio EPA approval.

Locate BMPs so that they are protected in accordance with the LD1.

C1111.1

Post-Construction Storm Water Best Management Practices (BMPs) are provided for long term management of storm water runoff quality and quantity so that a receiving stream’s physical, chemical and biological characteristics are protected, and stream functions are maintained.

Ohio EPA’s construction general permit includes requirements for post-construction BMPs on most projects that meet the disturbance threshold for an NOI. The construction general permit allows roadway projects administered by public entities, such as ODOT, to follow the criteria in this manual as an alternative to the specific post-construction BMP requirements in the permit. Many of the post-construction BMP design criteria in this manual are consistent with Ohio EPA’s permit, but some criteria have been tailored to fit linear roadway construction as opposed to standard site development.

Local entities with local post-construction guidance may have more restrictive language regarding selection and use of BMPs as compared to the Department. Storm water discharge from ODOT right-of-way is not subject to local storm water requirements. While the local entity cannot force the Department to use their standards, it may be possible for the Department to incorporate the needs of the local entity subject to review and approval of OHE.

1111.2 Project Thresholds for Post-Construction BMP

 

Projects that do not require an NOI per Section 1109 do not require post-construction BMPs. Since Routine Maintenance Projects do not require an NOI, they do not require post-construction BMPs. For projects that do require an NOI, the requirement for post-construction BMPs is based on the Project EDA. While the requirement for an NOI is based on Total EDA, the requirement for post-construction BMP treatment is only based on Project EDA (Total EDA – Contractor EDA). Contractor EDA is stabilized after construction to match existing conditions.

The following types of projects do not require post-construction BMPs.

  • Project EDA < 1 acre
  • Routine Maintenance Projects as defined in Section 1109.2
  • Projects including only earth disturbance from utility line, fence, guardrail, or noise wall installation

Provide post-construction BMPs for all projects with Project EDA ≥ 1 acre except those listed above.

For projects requiring post-construction BMPs, evaluate the following items:

  • Need for Water Quantity and Quality Treatment vs. only Water Quality Treatment (Section 1111.3)
  • Project Type – Redevelopment or New Construction (Section 1111.6)
  • If New Construction, calculate the Treatment Percent (Section 1111.7)
  • Applicable BMP to be implemented (Section 1113)

All projects, including Local Public Agency projects, ODOT-let and Local-Let, are required to provide post-construction BMPs as indicated in this section. Coordinate with the LPA when a project requires post-construction BMPs outside ODOT right-of-way. Inform the LPA of maintenance responsibilities associated with post-construction BMPs.

C1111.2

As described in Section 1109, EDA is defined as any activity that exposes bare ground or an erodible material to storm water as well as anywhere that Item 659, Seeding, or Item 660, Sodding, is being provided. Contractor EDA is generally outside of the ODOT right-of-way and therefore is unable to be addressed by post-construction BMPs.

Projects may have a Total EDA ≥ 1 acre but a Project EDA < 1 acre. For these types of projects, an NOI is required because the Total EDA threshold is met, but a post-construction BMP is not required because the Project EDA threshold is not met.

Projects that include construction activities only associated with utility line, fence, guardrail, or noise wall installation do not require post construction BMPs. These types of projects may require an NOI if the Total EDA threshold is met, but not a post-construction BMP.

1111.3 Water Quality and Water Quantity Treatment

 

Post-construction storm water treatment is divided into two categories: water quality treatment and water quantity treatment. Projects exceeding the minimum thresholds in Section 1111.2 must address water quality and potentially water quantity treatment in the post-construction BMP.

BMPs to address water quantity are not required for projects that meet any of the following criteria:

  • Redevelopment projects as defined in Section 1111.6.1.
  • New Construction Projects as defined in Section 1111.6.2 where less than 1 acre of new impervious area is created in new permanent right-of-way area being acquired for the project.

C1111.3

Water quality treatment is providing for reduction of pollutants from storm water runoff before leaving the site. Water quantity treatment is reducing the volume or peak flow rate of storm water runoff in order to protect the receiving stream’s physical characteristics.

  • Portions of New Construction Projects, as defined in Section 1111.6.2, which discharge from ODOT right-of-way, directly to a large river or to a lake and where the development area is less than 5 percent of the watershed area upstream of the development site, unless known water quality problems exist in the receiving waters. Only the project areas that drain from ODOT right-of-way to a large river or lake will be excluded from the requirement to provide quantity treatment. If portions of a project discharge to smaller waterbodies, quantity treatment may still be required for those portions.

Do not subdivide projects into multiple NOIs for the sole purpose of attempting to reduce post-construction treatment requirements.
BMPs that treat water quality and water quantity include: 

  • Detention Basin
  • Retention Basin, also called Wet Extended Detention Basin in Ohio EPA permit
  • Bioretention Cell
  • Infiltration Trench
  • Infiltration Basin
  • Constructed Wetlands

BMPs that treat only water quality include:

  • Manufactured Systems
  • Vegetated Biofilter
  • Vegetated Filter Strip

BMPs that treat only water quantity and must be paired with a water quality BMP include:

  • Stream grade control structures, within Waters of the U.S.
  • Underground Extended Detention

If there is a question regarding the stream classification, contact OHE. A map of stream classifications can be found at ODOT’s TIMS website. Click the HUC – Stream Order tab to view stream layers.

A large river has a drainage area >100 square miles or is fourth order or greater.

ODOT’s BMPs are divided into two categories of treatment because Ohio EPA’s General Construction Permit (OHC000005) states “Discharge rate is considered to have a negligible impact if the permittee can demonstrate that one of the following three conditions exist:

  1. The entire WQV is recharge to groundwater;
  2. The larger common plan of development or sale will create less than one acre of impervious surface;
  3. The storm water drainage system of the development discharges directly into a large river with drainage area equal to 100 square miles or larger upstream of the development site or to a like where the development area is less than 5 percent of the watershed area, unless a TMDL has identified water quality problems into the receiving surface waters of the state.”

For ODOT projects, if discharge rate has a negligible impact (as defined in this document), then water quantity treatment is not required.

1111.4 Water Quality Volume

 

Use the water quality volume to determine sizing for the following BMPs:

  • Detention Basin
  • Retention Basin
  • Infiltration Trench
  • Infiltration Basin
  • Constructed Wetlands

Use the following equation to calculate the water quality volume:

Water quality volume equation

Where:

WQV = Water Quality Volume (acre-feet)

RV = Volumetric Runoff Coefficient: 0.05 + 0.9 * i

i = impervious area divided by the total area (within the BMP drainage area)

P = Precipitation (0.90 inches)

A = Contributing Drainage Area to the BMP (acres) 

Treat all areas within existing ODOT right-of-way as impervious when determining the impervious area within the BMP drainage area.

C1111.4

The water quality volume calculation is used to define the amount of storm water runoff from any given storm that should be captured and treated in order to remove a majority of storm water pollutants on an average annual basis.

Ohio EPA determined that the WQV precipitation depth of 0.90 inches is the appropriate depth for sizing BMPs in order to achieve an estimated 80 percent reduction in total suspended solids (TSS) on an average annual basis based on long-term, historic Ohio rainfall data.

All areas within existing ODOT right-of-way are treated as impervious because ODOT and Ohio EPA acknowledged that roadway construction generally compacts soils, even outside of impervious areas. Therefore, Ohio EPA requires that ODOT consider existing right-of-way impervious in BMP design as a conservative approach to avoid under sizing BMPs.

While existing right-of-way is treated as impervious, pervious areas in newly acquired right-of-way for a project are not considered impervious for BMP calculations.

1111.5 Water Quality Flow

 

Use Water Quality Flow to determine sizing for manufactured systems and vegetated biofilters.

The WQF is calculated based on the rational method as described in Section 1101.2.2.

Q=CiA

Where:

Q = Discharge in cubic feet per second 

C = Coefficient of runoff

i = Average rainfall intensity in inches per hour, for a given storm frequency and for a duration equal to the time of concentration.

A = Drainage area in acres

The C value used for the WQF calculation must be consistent with the rational method and Table 1101-2. Treat all areas within existing ODOT right-of-way as impervious with a C value of 0.90 when determining the appropriate C value.

The rainfall intensity i for the WQF calculation is different for the design of manufactured systems compared to vegetated biofilters.

C1111.5

The coefficient of runoff C used in the WQF equation is not the same as the volumetric runoff coefficient RV used in the WQV calculation.

1111.5.1 Rainfall Intensity for Manufactured Systems

The process for determining the rainfall intensity for manufactured systems is similar to the process in Section 1101.2.2. Calculate the time of concentration from the most remote point of the drainage area to the manufactured system. Then, use that time of concentration to determine the appropriate water quality intensity according to the Duration vs. Intensity Table in Figure 1111-2.

C1111.5.1

The Duration v. Intensity Table in Figure 1111-2 is taken from Ohio EPA’s Construction General Permit (OHC000005)

1111.5.2 Rainfall Intensity for Vegetated Biofilters

Use the rainfall intensity of 0.65 in/hr for sizing of vegetated biofilters.

C1111.5.2

The typical length, percent of the drainage area that is grass-covered, and contribution from off-site runoff leads to high average time of concentrations. This, combined with conservative requirements in Section 1113.2.2 makes 0.65 in/hr an appropriate intensity for sizing of vegetated biofilters.

1111.6 Project Type - Redevelopment and New Construction

1111.6.1 Redevelopment Projects

Redevelopment projects include:

  • Projects constrained entirely within existing right-of-way
  • Projects that do not add new impervious area in new permanent right-of-way

While all areas within existing ODOT right-of-way may not be covered by impervious surfaces, the area within existing ODOT right-of-way is considered impervious area for the purpose of post-construction BMP design considerations. Therefore, consider all area within existing right-of-way to be impervious when performing post-construction BMP calculations.

1111.6.2 New Construction Projects

Projects that add new impervious area inside new permanent right-of-way are considered new construction projects.
New construction projects allow for the reduction of treatment requirements based on the amount of new impervious area relative to the existing impervious area within the Project EDA, see Section 1111.7. Consider all area within existing ODOT right-of-way to be impervious for post construction BMP calculations.

1111.6.3 Pedestrian Facilities and Shared Use Paths

For Redevelopment Projects or New Construction Projects that include Project EDA only associated with pedestrian facilities and shared use paths, with no Project EDA from planned roadway improvements, narrow Vegetated Filter Strips are an acceptable post-construction BMP per Section 1113.2.1. For these projects, quantity treatment per Section 1111.3 is not required.

 

1111.7 Treatment Requirements for Projects

 

The amount of treatment required for a project to meet the post-construction BMP treatment requirements is based on the Project EDA and the weighted average for new and existing impervious area. 

Use a Treatment Percentage (T%) of 20% for redevelopment projects.

Determine the Treatment Percent for New Construction projects using the following equation:

Treatment percent for new construction projects

Where:

T% = Treatment percent (Percentage)

Aix = Project EDA that is inside the existing right-of-way

Ain = The new impervious area inside new permanent right-of-way minus any impervious area that is removed inside new permanent right-of-way.

All Project EDA within existing ODOT right-of-way is included in the Aix value. All area within existing ODOT right-of-way, whether impervious or pervious, is considered to be impervious for post-construction BMP calculations.

Provide post-construction treatment area equal to: Project EDA * T%.

Area draining to a post-construction BMP will earn treatment credit equal to the amount of ODOT right-of-way area treated by the BMP.

The treatment credit, the ODOT right-of-way area treated by BMPs, must be equal to or greater than the treatment requirement (Project EDA * T%) for the project.
Size the BMP based on the entire contributing drainage area, offsite and on-site, to the BMP.

Credit for water quality and water quantity treatment is only applied to the portion of the contributing drainage area within ODOT right-of-way. Include any offsite contributing drainage area in the BMP calculations for sizing purposes. Do not include the offsite area in the determination of treatment credit.

For projects with multiple distinct stream crossings that do not immediately share a common confluence downstream, provide post-construction BMP treatment proportional to the amount of Project EDA tributary to each stream.

If there is an existing post-construction BMP that treats runoff from the project site, and the BMP is sized appropriately to manage runoff from T% of the Project EDA, then additional BMPs are not required to meet post-construction treatment requirements. Include the existing post-construction BMP in the Project Site Plan. Include calculations demonstrating the BMP’s capacity to manage runoff from the project site as well as any other existing sources of runoff into the BMP in the BMP submittal described in Section 1112.2.

C1111.7

All areas within existing ODOT right-of-way (Aix) are treated as impervious because ODOT and Ohio EPA acknowledged that roadway construction generally compacts soils, even outside of impervious areas. Therefore, Ohio EPA requires that ODOT consider existing right-of-way impervious in BMP design as a conservative approach to avoid under sizing BMPs.

While existing right-of-way is treated as impervious, pervious areas in newly acquired right-of-way for a project are not considered impervious for BMP calculations.

Example: A vegetated biofilter that has offsite contributing drainage area of one acre and on-site contributing drainage area of two acres (total drainage area of three acres) would result in a treatment credit of two acres. The vegetated biofilter must be sized for the total contributing drainage area of three acres. Multiple areas of a project may provide treatment to meet the treatment requirement. If the total area requiring treatment in this example was four acres, another vegetated biofilter with a minimum of two acres of on-site drainage area would be needed to meet the treatment requirements.

Example: A large new roadway project is constructed and 100% of the project EDA drains to a post-construction BMP. If a future portion of this roadway is redeveloped, and that area already drains to an existing BMP, no new BMPs would be required to meet post-construction treatment requirements.

Example: A large highway redevelopment project (100 acres) is constructed and 20% of the project EDA (20 acres) drains to various post-construction BMPs. A future redevelopment project has a project EDA of 10 acres within the original 100-acre project. The treatment requirement for the future project is 2 acres. If at least 2 acres of the future project drains to existing post-construction BMPs, then no new BMPs would be required to meet post-construction treatment requirements. If the future project is planned for a section of the roadway where BMPs were not implemented in the original project, then new BMPs are required that ensure a minimum of 2 acres of the future project drain to a BMP.

1112 BMP Selection & Submittals

1112.1 BMP Selection

 

Base selection of BMP on providing maximum runoff treatment while minimizing impacts to the remaining project design features, including utilities and right-of-way. In addition, each BMP option comes with unique maintenance requirements. 

Obtain approval from Ohio EPA to use alternative BMPs not listed in Section 1113. Alternative methods will be approved or denied on a case-by-case basis if the alternative methods are demonstrated to sufficiently protect the overall integrity of the receiving streams and the watershed. For curbed roadways, total contributing drainage areas to sumps or intersections that are less than or equal to 0.25 acres as shown in Figure 1112-1 do not require a BMP. Note that these exceptions are unique circumstances. Provide BMP as necessary for all other project features.

For projects where the drainage sheet flows off the roadway and continues outside existing or proposed right-of-way, do not channelize flow for the sole purpose of providing a post-construction BMP. Treatment is not required for areas where sheet flow off the roadway continues to sheet flow outside ODOT right-of-way. Document areas where this occurs in the post-construction BMP calculations and identified on the Project Site Plan.

For projects where portions of the disturbed area sheet flows outside ODOT right-of-way, calculate the treatment requirement area as follows:

(Project EDA – Sheet Flow Area) * T%

Where:

Project EDA = as defined in Section 1109.1

Sheet Flow Area = Area within the Project EDA that sheet flows outside ODOT right-of-way (acres)

T% = Treatment percent as defined in Section 1111.7

If a BMP can fit in an area that sheet flows outside of ODOT right-of-way, such as a vegetated filter strip, the project may install a BMP in that area and receive treatment credit. However, if a BMP is installed in an area that sheet flows outside of ODOT right-of-way, that area must not be excluded from the Project EDA in determining the required treatment area.

Design criteria for all BMP are available in Section 1113. A flow chart to determine BMP treatment requirements is provided in Figure 1111-1.

C1112.1

Contact the Office of Maintenance Administration for detailed BMP maintenance information.

1112.2 BMP Submittals

 

Consider BMPs early in the design process to allow for right-of-way and utility coordination as well as evaluation with respect to waterway permitting issues.

For PDP projects characterized as Paths 4 and 5, provide a description of the planned BMPs to be used for the project in the Preliminary Engineering Phase. Submit final BMP design during Stage 1 plan development as identified in later tasks of the Preliminary Engineering Phase. Further refinement may be needed within the Environmental Engineering Phase.

For projects categorized as Paths 1-3, it is unlikely a conceptual BMP task will be needed. Include BMPs in the Environmental Engineering Phase and potentially the Final Engineering Phase of the PDP.

Submit the BMP final design during Stage 1 to OHE. Include the following information:

  • Estimated Project Earth Disturbed Area
  • Treatment Percent Calculation as well as Treatment Requirement Area
  • BMPs selected for use
  • Drainage area mapping for post-construction BMPs that show the total contributing drainage area and the amount of contributing drainage area within ODOT right-of-way.
  • Plan sheets showing locations of post-construction BMPs
  • Calculations for each BMP. See section BMP Toolbox
  • Explanation for any area that is not treated such as environmental commitment, total parcel takes, environmental resource impact, sheet flow runoff, etc.

Identify the final locations and EDA treatment credit of each individual post-construction BMP in the Project Site Plan as described in Section 1308 of the LD3. If applicable, provide cross-references to sheets showing post-construction BMP details on the Project Site Plan.

C1112.2

The following design resources are available on the OHE Post Construction Storm Water BMP web page.

  • Post-Construction BMP Design Review Checklist
  • BMP Calculation Spreadsheet
  • Post-Construction BMP Design Examples
  • Post-Construction BMP Training Workshop Slides

1113 BMP Toolbox

1113.1 Manufactured Systems

 

Supplemental Specifications 895 and 995 cover the material and performance criteria for these devices. Place manufactured systems in an off-line configuration with manholes to allow for routine maintenance procedures. See Figure 1113-1.

Use the following procedure for design of manufactured systems:

  1. Determine the total contributing drainage area.
  2. Calculate the WQF according to Section 1111.5.
  3. Provide a No. 3 Manhole, With ___” Base ID and ___” Weir where flow is to be diverted to the off-line manufactured system according to Table 1113-1 and 1113-2 and the calculated WQF.

Table 1113-1

Manufactured Systems
Type WQF (cfs) No. 3 Manhole Base ID (inches) 611-Type B Conduit Diameter (inches)
1 1 84 12
2 2 90 15
3 3 96 18
4 6 108 24

Table 1113-2

Reserved Area for Manufactured System
Type W (ft) L (ft) 611-Type B Total Conduit Length (ft) Weir Height (inches)
1 15 30 20 6
2 20 32 30 8
3 25 33 40 9
4 25 37 40 12
  1. Furnish two lengths of 611, Type B Conduit placed perpendicular to the inflowing sewer See Table 1113-2 for the total length required.
  2. Reserve an area, as measured from the centerline of the No. 3 Manhole, according to Table 1113-2. If this area is not attainable, contact OHE for further guidance. Confirm the area is void of all utilities and is accessible for routine cleanout and maintenance.

For manufactured systems located along a roadway with a legal speed limit over 45 mph, locate the area for the manufactured system outside all paved areas.

For manufactured systems located along a roadway with a legal speed limit of 45 mph and less, it is preferred to locate the area for the manufactured system outside paved areas. If it is not feasible to locate the area outside of the paved area, select another BMP or contact OHE for further coordination.

When a manufactured system is connected to a storm sewer with a depth exceeding 10 feet, contact OHE.

Manufactured systems are typically not suited for treatment of flows in large trunk sewers. As indicated in Table 1113-1, do not provide manufactured systems on sewers that are carrying a water quality flow greater than 6 cfs. The water quality flow calculation is based on the entire contributing drainage area to the storm sewer.

Add Item 895, Manufactured Water Quality Structure, Type__, to the plans when using a manufactured system.

Label the location and EDA treatment credit on the Project Site Plan for each manufactured system on the project.

C1113.1

Manufactured systems consist of underground structures that treat the WQF by removing particulate matter through settlement or filtration.

1113.2 Vegetation Based BMP

1113.2.1 Vegetated Filter Strip

The Vegetated Filter Strip consists of the grassed portion of the graded shoulder and the grassed foreslope. The VFS must be void of gullies or concentrated flow. The water flow is characterized as overland flow throughout the grass.

The minimum VFS required is defined in Table 1113-3 below. The VFS can start at the end of the graded shoulder or at any point on the slope. Areas such as pavement, graded shoulder, or any grass slope that drain to a VFS receive treatment credit including the VFS area.

Table 1113-3

Maximum Pavement Width (ft) Maximum Slope (H:V) Minimum Filter Strip Width (ft)
22 3:1 15
24 3:1 17
26 3:1 18.5
28 3:1 20.5
30 3:1 22
32 3:1 24
34 3:1 25
48 6:1 25

Measure the VFS width down the grass slope starting at the grass and ending at the inside edge of the ditch bottom.

Do not include any area associated with concentrated flows that outlet to a VFS in the treatment credit.

C1113.2.1

A Vegetated Filter Strip is a BMP that filters storm water through vegetation.

Vegetated filter strip performance and design criteria were supported by research conducted by Ohio University. The research titled Vegetated Biofilter for Post Construction Storm Water Management for Linear Transportation Projects by Gayle F. Mitchell, R. Guy Riefler, and Andrew Russ for the Ohio Department of Transportation Innovation, Research, and Implementation Section and the United States Department of Transportation Federal Highway Administration State Job Number 134349 May 2010. While the title reads “Vegetated Biofilter,” it in fact supports vegetated filter strips, and not the Department’s vegetated biofilter BMP.

For projects that include EDA only associated with pedestrian facilities and shared use paths, with no EDA from planned roadway improvements, widths of the VFS can be narrower than those in Table 1113-3. Vegetated Filter Strips are an acceptable post-construction BMP for these projects when the following criteria are met:

  • The minimum VFS width is equal to the width of the contributing impervious area
  • The maximum slope of the VFS is 3:1
  • All runoff must be sheet flow, with no concentrated flows to the VFS    

Similarly, to standard VFS, treatment credit for narrow VFS is given to the impervious area draining to the filter strip as well as the area of the filter strip itself.

Projects that have EDA from a combination of pedestrian facilities or shared use path as well as roadway improvements may not utilize VFS narrower than those shown in Table 1113-3 without project-specific permission from Ohio EPA.

Label the station range and location, the VFS width, and the EDA treatment credit on the Project Site Plan for each VFS provided on the project.

Add 4” of Item 659, Topsoil, to the grass portion of the shoulder and foreslope of the VFS

Add Item 670, Slope Erosion Protection, to the plans when using VFS.

Narrow VFS Example 1: A project includes the addition of 4-foot wide sidewalk along a road to the extent that the project EDA is greater than 1 acre, but no roadway improvements are included. That project may incorporate 4-foot wide Vegetated Filter Strip collecting runoff from the sidewalk in order to meet its post-construction treatment requirements.

Narrow VFS Example 2: A project includes the addition of a 10-foot wide bike path, but no roadway improvements are included in the project. The project may incorporate 10-foot wide Vegetated Filter Strip collecting runoff from the bike path in order to meet its post-construction treatment requirements.

1113.2.2 Vegetated Biofilter

The Vegetated Biofilter consists of the grassed portion of the graded shoulder, grassed foreslope, and flat grassed ditch. The purpose of the VBF is to allow runoff to spread out and move slowly through a shallow, flat, and grassed conveyance. VBF must be void of rills, gullies, or visible erosion on the grassed foreslope of the ditch as well as in the bottom of the ditch.

When widening existing ditches, consider the following before purchasing new right-of-way:

  • A steeper ditch foreslope
  • A steeper ditch backslope
  • Reducing the bench width to a minimum of 4 ft.

Consider soil conditions and safety issues prior to making any of the above changes to the existing slopes or benches.

Changes to existing ditches may be regulated through waterway permits since ditches may be considered streams or wetlands. Avoid or minimize all impacts to existing streams and wetlands to the maximum extent practicable. To determine if the proposed ditch will impact an existing stream or wetland, contact the District Environmental Coordinator.

For projects utilizing the VBF, provide a ditch width using the Enhanced Bankfull Width or the standard ditch width to provide water quality treatment. Use the following steps to determine the ditch width:

  1. Determine Enhanced Bankfull Width: 

The EBW is the width in a trapezoidal ditch for which the following criteria are met: 

C1113.2.2

If the Vegetated Filter Strips will not provide the required treatment, consider using a Vegetated Biofilter.

A VBF is a BMP that filters storm water through vegetation and potential infiltration.

  • The minimum EBW is 4 ft.
  • The depth of flow for the water quality flow rate (WQF) is less than or equal to 4 inches
  • The velocity of flow for the water quality flow rate (WQF) is less than or equal to 1 ft/sec

Use the water quality flow rate (WQF) per Section 1111.5.

Use Manning’s Equation to determine the depth and velocity of flow:

Manning’s Equation:

Manning's equation

Where:

The minimum EBW width of 4 ft. is based on the ability to construct and maintain a flat bottom ditch. Narrower ditches are less likely to maintain a flat ditch bottom.

The 4-inch depth limitation is based on keeping the depth of flow within the height of grass to promote filtering.

The 1 ft/sec velocity limitation is based on limiting the velocity to avoid grass bending over and reducing filtering capacity.

Q = flow rate (cfs)

n = Manning’s Roughness Coefficient (0.15)

A = Cross section area of flow (ft2)

R = Hydraulic Radius (ft) (Area/Wetted Perimeter)

S = Longitudinal Slope of ditch (ft/ft)

There is not a direct calculation to determine EBW. Use a trial and error method to determine a width for which the depth and velocity criteria are met for the WQF, assuming open channel flow. The EBW is in whole numbers only, no half-foot increments. 

The enhanced bankfull width corresponds to the dimension of the bottom width of the trapezoidal ditch.

  1. Determine Standard Ditch Width:

Determine the size of the trapezoidal ditch that would typically be specified for the project without accounting for water quality treatment using typical roadway design practices.

Use the bottom width dimension of the trapezoidal ditch. Ignore any rounding lengths associated with the trapezoidal ditch.

  1. Determine the VBF ditch width required for water quality treatment as described below: 
     
    1. If the EBW is less than or equal to the standard ditch width, use the standard ditch width.
       
    2. If the EBW is greater than the standard ditch width, use the EBW.

The EBW can be calculated at multiple locations along its length. This would allow the width to be reduced where there is less tributary area, such as at the upstream end of the ditch. However, use the entire contributing drainage area to the location in the ditch being evaluated to determine the EBW.

Recalculate the EBW at points where concentrated offsite runoff is accepted.

Treatment credit for VBF is given to: 

  1. Areas within the project limits that sheet flow off the roadway into a grassed shoulder, grassed foreslope, and then into a grassed trapezoidal ditch sized as described above. Tributary areas to a VBF that do not meet this criterion, i.e. drainage from concentrated flow or outside project limits, must be included in the determination of the EBW, but do not receive treatment credit.
  2. The area of the defined VBF including the shoulder, foreslope, ditch bottom, and backslope within the permanent right-of-way

A value of 0.15 for the Manning’s Roughness Coefficient is required because flow depths within the height of grass (4 inches or less) have Manning’s Roughness values more consistent with overland flow rather than open channel flow.

Make sure that rock or other impervious soil layers will not prevent grass from being established at the invert of the flowline. A minimum of 1-foot separation is required between the invert of a VBF and bedrock. If the velocity is such that rock channel protection, reinforced concrete mats, or SS836 are required, that section of the ditch cannot be used as a VBF.

Bedrock is solid in-place rock typically underlying soil and exhibiting structure, such as bedding and jointing.

Use of VBF with grassed foreslopes steeper than 3:1 must be coordinated with the District Maintenance department to confirm that maintenance is feasible.

Constriction points in the enhanced bankfull width at drive pipes or other drainage related features are acceptable. Transition back to the calculated width immediately following the constriction point.

Label the station range and location, bottom width, and EDA treatment credit on the Project Site Plan for each VBF provided on the project.

Add 4” of Item 659, Topsoil, to the grass portion of the shoulder and foreslope of the VBF.

Add Item 670, Ditch Erosion Protection, to the plans when using VBF. Size the width of ditch erosion protection consistent with Section 1102.3.1, using the width for the 5-year frequency storm. 7.5 feet is the minimum width of lining. Additional required width is in increments of 3.5 feet.

Maintaining grass foreslopes and ditch bottoms may be difficult on steeper slopes, especially behind guard rail. Often, areas behind guard rail are allowed to grow brush instead of grass, which does not meet the grass requirement for a VBF.

1113.3 Extended Detention

Extended detention is a method that captures storm water during rain events and slowly releases the captured volume over a period of time. The WQV is used to determine the storage volume of the detention basin. The WQV is discharged over a 48-hour time frame. Increase the WQV by 20% when sizing the BMP to allow for sediment storage. Detention can be either above or below ground. Use detention basins that are above ground when feasible. However, when project site parameters dictate, an underground system may be considered.

Due to the safety considerations and potential impacts to the drainage system, the use of extended detention BMPs requires approval from OHE. Provide submittals according to Section 1112.2. Do not locate extended detention BMPs with more than one foot of ponding water in the clear zone without prior approval from the Office of Roadway Engineering.

 

1113.3.1 Detention Basin

Use the following procedure for design of a detention basin:

  1. Calculate the WQV per Section 1111.4.
  2. Calculate the Design Check Peak Discharge per Section 1113.3.3.
  3. Increase the calculated WQV by 20% to determine the required size of the detention basin.
  4. Provide a forebay that is 10% of the WQV, if feasible. The forebay volume is part of the required volume and is not an additional volume requirement.
  5. Provide a micropool that is 10% of the WQV, if feasible. The micropool volume is part of the required volume and is not an additional volume requirement.
  6. Size the water quality basin outlet structure for proper discharge of the WQV and for proper discharge of events up to the design check discharge according to Section 1113.3.1.1. Consider the water surface elevations created by the basin in the design of the upstream drainage system.
  7. Provide anti-seep collars for the outlet pipe according to Section 1113.3.1.2.

The following criteria apply when designing a detention basin:

  1. Use side slopes of 4:1 maximum.
  2. Consider vehicle access to the basin for periodic maintenance.
  3. Do not locate on uncompacted fill, steep slopes of 2:1 or more, or where infiltrating ground water could adversely impact slope stability.
  4. Vegetate the sides of the basin with Item 670, Slope Erosion Protection.
  5. Embankment work to create the impoundment will be constructed and paid for as Item 203, Embankment, Using Natural Soils, 703.16 A.
  6. Provide gravel pack protection at the outlet structure. Refer to Hydraulic Standard Construction Drawing WQ-1.1.
  7. Place channel protection of RCP or Tied Concrete Block Mat at the entrance of the basin to minimize erosion and sediment resuspension.
  8. Provide a Water Quality Basin, Detention per section 1113.3.1.1.
  9. Label the location and EDA treatment credit on the Project Site Plan for each extended detention basin on the project.

C1113.3.1

A detention basin is a partially dry pond that detains storm water for quality and quantity treatment.

In Ohio EPA’s Technical Memo for Sediment Storage Design for Post-Construction Practices, they state: “the purpose of the sediment storage volume is to avoid reduced treatment effectiveness and maintenance frequency as sediment accumulated within the practice (WEF/ASCE, 2012)”.

1113.3.1.1 Water Quality Basin and 25-Year Overflow Weir

Provide an outlet structure that fully drains the WQV in 48 hours or more. No more than 50% of the WQV may be released from the detention basin in less than one-third the drain time, which is equal to 16 hours.

The outlet structure consists of a catch basin with a perforated riser pipe on the inlet side and a conduit on the outlet side. The perforated riser pipe is used for flow control to achieve the required discharge time. A gravel envelope surrounds the perforated riser pipe along the inlet side of the catch basin to prevent blockage of the orifice holes in the pipe. The catch basin and riser pipe are paid for as Item 611, Water Quality Basin, Detention.

Details of a perforated riser pipe outlet structure can be found on Hydraulic Standard Construction Drawing WQ-1.1.

The equation for a single orifice is:

Single orifice equation

Where:

A = Area of orifice (ft2)

H = Head on orifice as measured to the centerline of the orifice (ft)

C = Orifice coefficient

 

C1113.3.1.1

A detention basin is a partially dry pond that detains storm water for quality and quantity treatment.

In Ohio EPA’s Technical Memo for Sediment Storage Design for Post-Construction Practices, they state: “the purpose of the sediment storage volume is to avoid reduced treatment effectiveness and maintenance frequency as sediment accumulated within the practice (WEF/ASCE, 2012)”.

Table 1113-4

Orifice Coefficient Guidance
C Description
0.66 Use for thin materials where the thickness is equal to or less than the orifice diameter.
0.88 Use when the material is thicker than the orifice diameter.

The design check discharge must be able to bypass the perforated riser pipe outlet structure and overflow through the catch basin grate into the discharge conduit. Convey the full design check discharge through the catch basin and discharge conduit without overtopping the detention basin. Determine the design check discharge per 1113.3.3.

In order to protect the detention basin from uncontrolled overtopping and berm erosion, provide an overflow weir sized to convey a minimum of the 25-year peak flow rate. Protect the weir from erosion.

Table 1113-4 Orifice Coefficient values from CALTRANS, Storm Water Quality Handbooks, Project Planning and Design Guide, September 2002.

1113.3.1.2 Anti-Seep Collar Design

Provide anti-seep collars on conduits through earth fills where water is being detained. The following criteria apply to anti-seep collars:

  1. Provide a minimum of 2 collars per outlet conduit. Increase the seepage length along the conduit by a minimum of 15%. This percentage is based on the length of the pipe in the saturation zone.
  2. Place anti-seep collars equally within the saturation zone. Place one collar at the end of the saturation zone. In cases where the spacing limit will not allow this, place at least one collar within the saturation zone.
  3. Maximum collar spacing is 14 times the minimum projection above the pipe, but not more than 25 feet. The minimum collar spacing is 5 times the minimum projection, but not less than 10 feet.
  4. Extend the collar dimensions a minimum of 2 feet in all directions around the outside of the conduit, measured perpendicular to the conduit. Center the anti-seep collars around the conduit.
  5. The top of collar must not be less than 6 inches below, measured normal to, the finished ground line.
  6. All anti-seep collars and their connections must be watertight.
  7. Minimum thickness is 6 inches.
  8. Payment for the collar is made using Item 602, Concrete Masonry. Refer to Hydraulic Standard Construction Drawing WQ-1.2.

The design procedure for anti-seep collars is as follows:

  1. Determine the length of the conduit within the saturated zone. The assumed normal saturation zone can be determined by projecting a line through the embankment, with a 4:1 (H:V) slope, from the point where the water elevation at the 10-year design storm meets the upstream slope to a point where it intersects the invert of the conduit. This line, referred to as the “phreatic line”, represents the upper surface of the zone of saturation within the embankment, see Figure 1113-7. The 10-year storm pool elevation is the phreatic line starting elevation.

L_S=Y(Z+4)(1+S/(0.25-S))

Where:

Ls = Length of the conduit in the saturated zone (feet)

Y = Depth of the water at the spillway crest, 10-year frequency storm water surface elevation (feet)

Z = Slope of the upstream face of the embankment (Z feet horizontal to 1 foot vertical)

S = Slope of the conduit (feet per foot)

  1. Determine the required seepage length increase.

∆L= 0.15LS

  1. Choose a collar height and width that is at least 4 feet larger than the outside diameter of the conduit (minimum projection of 2 feet from all sides of the conduit). Give collar sizes in one-foot increments. 

P = W - D

Where:

P = Projection of collar (feet)

W = Height or width of collar (feet)

D = Inside diameter of conduit

  1. Determine the total number of collars required. The collar size can be increased to reduce the number of collars. Alternatively, the collar size can be decreased by providing more collars. In any case, increase the seepage length by a minimum of 15%.

Number of collars required = ∆LS ⁄ P

 

1113.3.2 Underground Detention

  1. Confirm the Hydraulic Grade Line design of the storm sewer will pass through the structure and meet the requirements of Section 1104.3.2
  2. Provide an outlet structure that fully drains the WQV in 48 hours or more. Release no more than 50% of the WQV from the detention basin in less than one-third the drain time, which is equal to 16 hours.
  3. Locate access to the conduits for periodic maintenance so that traffic impacts are minimized. 
  4. If practical, provide pretreatment of the storm water with vegetation.
  5. Payment for the conduit is Item 611 ____” Conduit, Type____, for underground detention.
  6. Label the location and EDA treatment credit on the Project Site Plan for each underground detention on the project.

1113.3.3 Design Check Discharge

Use a design check discharge with the frequency of a 10-year event. Use the entire drainage area that contributes to the BMP to calculate the design check discharge.

 

1113.4 Retention Basin

A retention basin is a wet pond that has a minimum water surface elevation between storms that is defined as the permanent pool. Above the permanent pool is a detention pool that provides storage for 100% of the WQV and drains in 24 hours or more. The detention volume above the permanent pool is called the Extended Detention Volume. The full storage water depth is typically between 3-6 feet and the volume is less than 15 Ac-ft. The permanent pool is sized to provide storage for 100% of the WQV. A retention basin may be considered for large tributaries, but it may require a large amount of space.

Use the following procedure for design of the retention basin:

  1. Calculate the WQV per Section 1111.4.
  2. Calculate the Design Check Peak Discharge per Section 1113.3.3.
  3. If feasible, provide a forebay that is 10% of the total storage volume. The forebay volume is part of the required volume and is not an additional volume requirement. 
  4. Size the water quality basin for proper discharge of the WQV and the 25-year overflow weir for proper discharge of events up to the 25-year frequency according to Section 1113.4.1. Consider the water surface elevations created by the basin in the design of the upstream drainage system.
  5. Provide anti-seep collars for the outlet pipe according to Section 1113.3.1.2.

The following criteria apply when designing a retention basin:

  1. Place channel protection (RCP or Tied Concrete Block Mat) at the entrance of the basin to minimize erosion and sediment resuspension. 
  2. Use side slopes no steeper than 4:1.
  3. Use a length to width ratio of at least 3:1 to prevent short-circuiting.
  4. Furnish a trash rack at the outlet structure.
  5. Compact the underlying soils to prevent infiltration of the permanent pool or use an impervious liner.
  6. Consider vehicle access to the basin for periodic maintenance.
  7. Retention basin must be greater than 10,000 feet from a municipal airport runway. 
  8. Vegetate the sides of the basin with Item 670, Slope Erosion Protection.
  9. Embankment work to create the impoundment will be constructed and paid for as Item 203, Embankment, Using Natural Soils, 703.16.A.
  10. Furnish a Water Quality Basin, Retention per 1113.4.1.
  11. Label the location and EDA treatment credit on the Project Site Plan for each retention basin on the project.

 

1113.4.1 Water Quality Basin and 25-Year Overflow Weir

A retention basin outlet structure is designed similar to the outlet structure for a detention basin. The difference is that the EDV, which for a retention basin is equal to 100% of the WQV, must discharged out of the basin in 24 hours or more. No more than 50% of the EDV can be released from the retention basin in less than one-third of that 24 hour drain time, which is equal to 8 hours. The outlet structures are of a similar type, except the openings will be set at a high enough elevation to maintain at least 100% of the WQV in the permanent pool. The catch basin and riser pipe are paid for as Item 611, Water Quality Basin, Retention.

Details of the outlet structure can be found on Hydraulic Standard Construction Drawing WQ-1.1.

The design check discharge must bypass the perforated riser pipe outlet structure and overflow through the catch basin grate into the discharge conduit. Convey the full design check discharge through the catch basin and discharge conduit without overtopping the retention basin. Determine the design check discharge per 1113.3.3.

In order to protect the detention basin from uncontrolled overtopping and berm erosion, furnish an overflow weir sized to convey a minimum of the 25-year peak flow rate. Protect the weir from erosion.

C1113.4.1

A hydrograph curve for the outlet is required to calculate the discharge time of the WQV. The discharge time must correspond to the minimum drain time of 24 hours with no more than 50% of the EDV being released from the retention basin in less than one-third of that 24 hour drain time.

Generally, it is easier to model the outlet structure and discharge time using software such as Pond Pak or HydroCAD to develop the hydrograph rather than producing the stage/storage/discharge calculations by hand.

1113.5 Bioretention Cell

 

Furnish Item 659 Seeding and Mulching for the vegetation of the Bioretention Cell. Cover this area with Item 671, Erosion Control Mat. Do not include any Item 659, Commercial Fertilizer, or Item 659, Lime, in the Bioretention Cell. Other shrubs or plantings may be provided in the Bioretention Cell with permission of OHE.

The water table or bedrock must be at least 1 foot below the invert of the bioretention cell, which is the excavated depth.

A bioretention cell is sized to treat the WQV by allowing that volume of runoff to percolate through the bioretention planting soil. Storm water runoff greater than the WQV is allowed to bypass treatment through an overflow structure. Treatment credit is given to the total area within the right-of-way draining to the most downstream part of the bioretention cell.

There are two configurations of bioretention cells:

  • Level bioretention cell in an open area with grassed side slopes. See Figure 1113-5.
  • Sloped bioretention cell within a grassed ditch. See Figure 1113-5.

1113.5.1 Level Bioretention Cell in an Open Area with Grassed Side Slopes

Pretreat the storm water prior to entering the bioretention cell by one of the following methods:

  1. For sheet flows from impervious areas, the runoff must flow through a minimum of 5 feet, preferably 15 feet, of grassed filter strip with side slopes no steeper than 3:1.
  2. For concentrated flows from a pipe, open channel, or curb cut, the runoff must flow through either a grassed swale at least 20 feet in length or a forebay sized to capture 10% of the WQV.

Provide a raised catch basin per Figure 1113-5 to allow the design check discharge to bypass the bioretention cell. Determine the design check discharge per Section 1113.3.3. Place the overflow outlet in the raised catch basin 12 inches above the surface elevation of the bioretention cell. Locate the raised catch basin outside of the clear zone.

C1113.5

A Bioretention Cell consists of a depressed area that allows shallow ponding and treatment of storm water runoff by evapotranspiration and filtration through an engineered soil (bioretention planting soil). As storm water runoff percolates through the bioretention planting soil, sediment and other pollutants are filtered. An underlying perforated underdrain captures the treated storm water runoff and carries it to an outlet. Vegetation assists in maintaining ongoing performance of bioretention cells.

1113.5.2 Sloped Bioretention Cell within a Grassed Ditch

Pretreat the storm water prior to entering the bioretention cell by one of the following methods:

  1. For sheet flows from impervious areas, the runoff must flow through a minimum of 5 feet, preferably 15 feet of grassed filter strip with side slopes no steeper than 3:1.
  2. For concentrated flows from a pipe, open channel, or curb cut, the runoff must flow through either a grassed swale at least 20 feet in length or a forebay sized to capture 10% of the WQV.

Provide an earth dike covered with item 601 Tied Concrete Block Mat with Type 2 Underlayment per Figure 1113-5 to allow the design check discharge to bypass the bioretention cell. Determine the design check discharge per Section 1102 for the appropriate design storm of the ditch. The dike must be 1V:6H or flatter and pond water to a maximum depth of 12 inches.

C1113.5.2

A Bioretention Cell consists of a depressed area that allows shallow ponding and treatment of storm water runoff by evapotranspiration and filtration through an engineered soil (bioretention planting soil). As storm water runoff percolates through the bioretention planting soil, sediment and other pollutants are filtered. An underlying perforated underdrain captures the treated storm water runoff and carries it to an outlet. Vegetation assists in maintaining ongoing performance of bioretention cells.

Install a dike at every 1 foot of elevation drop along the longitudinal slope of a linear bioretention cell.

Example: If a ditch line is at a 1% slope, install a dike every 100 feet along its length to promote temporary ponding and filtration through the bioretention planting soil.

1113.5.3 Bioretention Cell Design Procedure

Use the following procedure for the design of a bioretention cell:

  1. Determine the total impervious tributary area to the bioretention cell: ATRIB,IMP. Include impervious area within and outside of the right-of-way; treatment credit is only given to the area within the right-of-way. Consider all area within existing right-of-way to be impervious, even if the area is grassed.
  2. The minimum bioretention cell surface area is 5% of the total impervious tributary area.

ABIO = ATRIB,IMP x 5%

  1. Choose one of the two configurations of bioretention cells and follow the appropriate pretreatment and overflow requirements described in Section 1113.5.1 and 1113.5.2.
  2. Limit the maximum depth to 12 inches measured from the final grade of the bioretention cell to the outlet riser pipe, raised catch basin, weir, or check dam.
  3. In addition to the pretreatment required where concentrated flow enters the bioretention cell, limit the incoming velocity to 1 fps or less for the WQF to protect the bioretention cell from erosion. Assume an intensity of 0.65 in/hr and calculate the WQF per Section 1111.5 at the point of concentrated flow. Increase the pipe size, widen the open channel, increase the curb opening to the bioretention cell, or provide energy dissipation to limit the velocity to 1 fps or less. For Curb Cuts, assume all the WQV is captured by the curb opening and use the height of the curb and the opening width to calculate the area. 
  4. Do not place a bioretention cell where the required hydraulic design flows (i.e. 2-year event, 5-year event, 10-year event, or higher) have an Allowable Shear Stress higher than 1 psf or velocity higher than 5 fps
  5. Provide the bioretention cell layers as shown in Figure 1113-5.
    1. Bioretention Planting Soil Layer: Provide 30 inches of bioretention planting soil. See Plan Note W101. When planting shrubs or trees, extend the bioretention planting soil layer at least 4 inches below the lowest root ball.
    2. Filter Layer: Provide 3 inches of Fine Aggregate per C&MS 703.20 directly below the bioretention planting soil layer. Provide 3 inches of Coarse Aggregate size No. 78 per C&MS 703.20 directly below the Fine Aggregate layer.
    3. Gravel Layer for Underdrain: Provide 12 inches of Coarse Aggregate size No. 57 per C&MS 703.20 directly below the No. 78 aggregate layer. Provide a minimum of 3 inches of No. 57 aggregate above and below any underdrain pipes.
  6. For the bioretention planting soil, specify 10% excess planting mix volume to account for expected settling of the uncompacted soil. Show final expected soil elevations on the plans but allow contractor to place bioretention planting soil 3 inches above elevations shown on plans, as described in Plan Note W101.
  7. Provide one 4-inch diameter perforated PVC pipe underdrain per C&MS 605 along the length of the bioretention cell. Include one underdrain at the center for widths 20 feet or smaller. For all other widths calculate the number of underdrains required by dividing the width by 20 and rounding up to the next whole number. Space these underdrains equally around the center with a minimum distance of 5 feet from the outside edge.
  8. Provide a 4-inch diameter PVC observation well/cleanout port in accordance to Figure 1113-5 for every run of underdrain at an interval of 100 feet.
  9. Outlet the underdrain by combining all underdrains into a single type C Item 611 pipe. Provide this pipe with a positive outlet either into a drainage structure that is part of the drainage design or on a slope with Item 611, Precast Concrete Outlet. Show underdrain connection to outlet in the plans. While the underdrains will likely be 3 feet, 9 inches below the surface of the bioretention cell, the designer may choose to raise the underdrain outlet at the point of discharge. The underdrain discharge invert must be a minimum of 2 feet below the final surface elevation of the bioretention planting soil to allow the top 2 feet of the bioretention cell to drain freely. The designer may choose to raise the underdrain at the point of discharge to either hold back internal water storage to increase potential pollutant treatment in the bioretention cell, or in order to allow positive discharge of the underdrain into a downstream catch basin or outlet that has a depth less than 3 feet, 9 inches.
  10. For bioretention cells planted with grass, include temporary erosion control mat Type A, B, C, or E per C&MS 671 with either straw mulch or compost per plan over the surface of all bioretention planting soil. Specify the mat type on the plan sheets.
  11. For non-grass bioretention cells that include shrubs or trees, include a 3-inch layer of wood fiber mulch per C&MS 659.15 above the bioretention planting soil.
  12. Label the location and EDA treatment credit on the Project Site Plan for each bioretention cell on the project.
  13. PAY ITEMS:

203, Excavation As Per Plan (cu yd)
601, Bioretention Cell (cu yd)
601, Tied Concrete Block Mat (sq yd)
605, Underdrain As Per Plan, (includes observation wells, fittings, and couplers as specified) (each)
611, Outlet Pipe (ft)
659, Seeding and Mulching (sq yd)
671, Erosion Control Mats As Per Plan (sq yd)

C1113.5.3

The 5% sizing criterion and other design constraints are based on Ohio EPA’s Technical Memo: Bioretention Filter Bed Area and Ponding Depth Revision.

1113.6 Infiltration

 

Typically, infiltration practices are only suitable when Hydrologic Soil Group HGS Type A soils or, in some cases, HSG Type B soils exist.

Infiltration methods require an extensive investigation of the existing soils and geology to guarantee success. Begin the investigation with a preliminary soil evaluation of the project site early in the design process. In-situ testing is not anticipated during the preliminary evaluation process.

Use available soil and geology data found in the Soil and Water Conservation maps, United States Geological Survey, adjacent projects, or estimations from a geotechnical engineer.

National Resources Conservation Service’s Web Soil Survey website may also provide soil and geology information.
Material property tables for infiltration, permeability, and porosity have been provided for the preliminary evaluation. See Table 1113-5.

If the preliminary evaluation yields favorable results, perform a more detailed evaluation. The detailed evaluation will require a geotechnical investigation of the underlying soils and geology. Soil borings must be performed to a maximum depth of 20 feet, or refusal with samples taken every 5 feet for laboratory testing. The number and location of soil borings must correspond with the approximate size, as determined in the preliminary evaluation, of the infiltration BMP and should be recommended by the geotechnical engineer. 

If the detailed evaluation yields favorable results, the ground water depth must be verified. The geotechnical engineer will provide the seasonal high ground water depth. In some cases, observation wells may be installed, and static water levels may be observed over a dry and wet season for verification.

Test the infiltration and permeability rate of the soil in the detailed soil evaluation at the discretion of the geotechnical engineer. In some cases, in-situ testing at the proposed location of the infiltration BMP may be required.

The following criteria apply to infiltration methods and must be met to be considered a feasible alternative:

  1. Design using the WQV as per Section 1111.4.
  2. Do not place infiltration BMP where snow may be stored.
  3. The appropriate soil type must be present:
    1. Infiltration, the rate at which water enters into the soil from the surface, must be greater than 0.50 in/hr and no greater than 2.4 in/hr. 
    2. Soils must have less than 30% clay or 40% of clay and silt combined.
  4. The invert of the structure must be at least 4 feet above the seasonal high-water table and any impervious layer.
  5. Infiltration techniques are not suitable on fill soil, compacted soil, or slopes steeper than 4:1. Consider the long-term impacts upon hillside stability if applicable.
  6. Provide pretreatment to remove large debris, trash and suspended sediment to extend the service life. An example of pretreatment includes providing vegetated ditches prior to flow entering the infiltration facility.

C1113.6

Infiltration techniques treat storm water through the interaction of a filtering substrate that consists of soil, sand, or gravel. This technique discharges the treated storm water into the ground water rather than into surface waters.

1113.6.1 Infiltration Trench

The WQV must fully drain from the aggregate into the in-situ soil in 48 hours or less.

Design of an infiltration trench must follow the criteria in the Ohio EPA’s Rainwater and Land Development Manual.

The following criteria apply when designing an infiltration trench:

  1. Provide a 6 inch layer of Coarse Aggregate No. 57 or 67 conforming to C&MS 703.20 per C&MS 601.10 on the top of the trench.
  2. Provide Coarse Aggregate No. 1 or 2 conforming to C&MS 703.20 within the infiltration trench.
  3. Provide pretreatment using vegetation to safeguard the longevity of the infiltration trench.
  4. Provide an observation well to facilitate ground water level inspection.
  5. Locate the infiltration trench at least 1,000 feet from any municipal water supply well and at least 100 feet from any private well, septic tank, or field tile drains.
  6. Keep the bottom of the trench below the frost line of 2.5 feet
  7. Include an infiltration trench as Item 601 – Infiltration Trench.
  8. Label the location and EDA treatment credit on the Project Site Plan for each infiltration trench on the project.

C1113.6.1

An infiltration trench is an excavated trench that has been lined with a geotextile fabric and backfilled with aggregate. The storm water is filtered through the aggregate and is stored within the pore volume of the backfill material. It is allowed to percolate through the sides and bottom of the trench.

1113.6.2 Infiltration Basin

Design the Infiltration Basin to store the WQV. Depending on the soil permeability, an infiltration basin may be used to treat from 5 to 50 acres. The WQV must fully drain into the in-situ soil in 24 hours or less.

Use the following procedure for the design of an infiltration basin:

  1. Calculate the WQV per Section 1111.4.
  2. Determine the invert area of the infiltration basin using the following equation:

Invert area of the infiltration basin

Where:

A = area of invert of the basin (acres)
WQV = Water Quality Volume (see section 1111.4) (acre-feet)

S.F. = Safety Factor of 1.5

k = Infiltration Rate (in/hr) (table 1113-5)

t = Drawdown time of 24 hours

Table 1113-5

NRCS Soil Type (from soil maps) HSG Class Rate (k) (in/hr)
Sand A 8.0
Loamy Sand A 2.0
Sandy Loam B 1.0
Loam B 0.5
Silt Loam C 0.25
Sandy Clay Loam C 0.15
Clay Loam & Silty Clay Loam D < 0.09
Clays D < 0.05

Infiltration Rate (k): From Urban Runoff Quality Management WEF Manual of Practice No. 23, 1998, published jointly by the WEF and ASCE, chapter five.

  1. Use a length to width ratio of 3:1.
  2. Determine the required depth of the infiltration basin using following equation:

Required depth of infiltration basin

Where:

A = area of invert of the basin (acres)

WQV = Water Quality Volume (acre-feet)

D = Required depth of the basin (feet)

  1. Allow for 1-foot (min) freeboard above the WQV.
  2. Calculate the Design Check Peak Discharge per Section 1113.3.3.
  3. Provide bypass or overflow for the design check discharge.

The following criteria apply when designing an infiltration basin:

  1. Use an energy dissipater at the inlet.
  2. Vegetate the sides of the basin with Item 670, Slope Erosion Protection.
  3. Provide a 6-inch layer of Coarse Aggregate No. 57 or 67 conforming to C&MS 703.20 per C&MS 601.10 on the bottom of the basin.
  4. Use side slopes of 4:1 maximum.
  5. Consider vehicle access to the basin for periodic maintenance.
  6. Locate basin at least 1,000 feet from any municipal water supply well and at least 100 feet from any private well, septic tank, or drain field.
  7. Provide 10 feet or less width between 4-inch underdrain laterals, if used in the design.
  8. Do not locate the basin where infiltrating ground water may adversely impact slope stability.
  9. Place the invert of underdrains in the basin below the frost line of 2.5 feet.
  10. Embankment work to create the impoundment will be constructed and paid for as Item 203, Embankment, Using Natural Soils, 703.16.A.
  11. Label the location and EDA treatment credit on the Project Site Plan for each infiltration basin on the project.

C1113.6.2

An infiltration basin is an open surface pond that uses infiltration into the ground as the release mechanism.

1113.7 Constructed Wetlands

 

Size the wetland to provide storage for the WQV for a time frame of at least 24 hours, above the permanent pool, while providing a bypass or overflow for larger design check discharge. See section 1113.3.3. Maintain the water depth by an outlet structure capable of providing the required water depth with the provision of a 1-foot freeboard.

The following criteria apply when designing a Constructed Wetland:

  1. Do not place on a steep or unstable slope or at a location, which could induce short-term or long-term instability.
  2. Constructed Wetlands must be greater than 10,000 feet from a municipal airport runway.
  3. Base flow must be present to maintain the constant water depth, such as ground water.
  4. Provide a forebay that is 7% of the total required volume at a depth between 3-6 feet to settle out sediments.
  5. Use side slopes of 4:1 (max).
  6. Consider access for maintenance to the forebay and the outlet structure.
  7. Vegetate the sides and bottom with grass
  8. Provide an impervious liner. Use a compacted clay bottom or a geotextile fabric to prevent infiltration of the storm water.
  9. Use a length to width ratio of 3:1 (min) to prevent short-circuiting.
  10. Label the location and EDA treatment credit on the Project Site Plan for each constructed wetland on the project.

C1113.7

Constructed Wetlands treat storm water through bio-retention. They are depressed, heavily planted areas that are designed to maintain a dry weather flow depth ranging between 0.5 to 2 feet. The surface area required for a wetland is usually quite large due to the limited allowable depth. The area is usually on the magnitude of 1% of the entire drainage area. They are designed in a similar manner as a retention basin.

1113.8 Stream Grade Control

Stream grade control structures are structures installed on the upstream and downstream end of a culvert at a stream crossing to promote stream protection. They provide a grade control in a stream to prevent downcutting of the stream bed. 

The following are Stream Grade Control structures:

  • Concrete aprons shown in Section 1106.3.
  • Three sided culverts with paved Inverts 
  • Three sided culverts with bed rock inverts 

Stream grade control structures provide quantity treatment, but not quality treatment. Therefore, pair stream grade control structures with a post-construction BMP that provides quality treatment. Only those portions of a project within existing and/or new permanent right-of-way that drain to a stream grade control structure receive quantity treatment credit. Stream grade control structures are only an appropriate post-construction BMP when installed within a Waters of the United States and associated with sites that acquire a permit from the Army Corps of Engineers for stream impacts.

Label the location and EDA treatment credit on the Project Site Plan for each stream grade control structure on the project.