11-1

CHAPTER 11

DRAINAGE DESIGN Introduction 11-3 Pipe Culvert Design 11-4 Pipe Culvert Material Selection 11-5 Table 11-1 Pipe Culvert Material Type Selection and Service Life Guidelines Pipe Sizing 11-7 Figure 11-1 Memorandum from Hydraulics Section, Bridge Design Figure 11-2 Hydraulic Analysis of Culverts, 0-150 Acres Figure 11-3A Hydraulic Analysis of Culverts, 150 Acres or More Figure 11-3B Channel & Mainline Cross Section Worksheet Figure 11-4 Map of Hydrology Regions in South Dakota Figure 11-5 Hydraulic Analysis of Culverts Code Form Overtopping Procedure 11-20 Figure 11-6 Overtopping Pipe Form Figure 11-7A Headwater Depth for RC Pipe Culverts with Inlet Control Figure 11-7B Headwater Depth for RC Arch Pipe Culverts with Inlet Control Figure 11-7C Headwater Depth for CM Pipe Culverts with Inlet Control Figure 11-7D Headwater Depth for CM Arch Pipe Culverts with Inlet Control Figure 11-7E Headwater Depth for Box Culverts with Inlet Control Figure 11-8 Ratio of Discharge to Mean Annual Flood Figure 11-9 Completed Overtopping Pipe Form Figure 11-10 Log Graph Paper Pipe Culvert Installation 11-34 Table 11-2 RC Pipe Maximum Allowable Depth of Cover Table 11-3 CM Pipe Maximum Allowable Depth of Cover Table 11-4 CM Arch Pipe Maximum Allowable Depth of Cover Figure 11-11 Controlled Density Fill Limits between Multiple Pipe Table 11-5A Spacing of Multiple Concrete Pipe Table 11-5B Spacing of Multiple Metal Pipe Table 11-6 Additional Length Needed for Skewed RC Pipe (Feet) Figure 11-12 Lengths of Elbows Figure 11-13 Maximum Deflection for Culvert Joints

11-2

Storm Sewer Design 11-46 Figure 11-14 Full Flow Curves, Circular Pipe, n=0.012 Figure 11-15 Full Flow Curves, Arch Pipe, n=0.012 Figure 11-16 Time of Concentration of Small Drainage Basins Storm Sewer Design Procedure 11-55 Table 11-7 Inlet Spacing Computation Sheet Table 11-8 Values of Runoff Coefficients (C) for use in the Rational Method Table 11-9 Rainfall Intensity – Duration – Frequencies (I) Figure 11-17 Intensity-Duration-Frequency Map Figure 11-18A Rainfall Intensity-Duration-Frequency Curves Figure 11-18B Rainfall Intensity-Duration-Frequency Curves Figure 11-18C Rainfall Intensity-Duration-Frequency Curve Table 11-10 Hydraulic Gradeline Computation Form Detention Ponds 11-71 Roadside Channels 11-72 Table 11-11 Classification of Vegetal Covers as to Degree of Retardance Table 11-12 Permissible Shear Stresses for Lining Materials Table 11-13 Manning’s Roughness Coefficients for Channel Liners Table 11-14 Manning’s Roughness Coefficients for Small Natural Streams

11-3

INTRODUCTION Drainage design should achieve the most effective and economical methods by which runoff water can be passed through and removed from the roadway. The primary objectives should be to (1) provide culvert openings for natural drainage channels, (2) prevent undue accumulation and retention of water upon and adjacent to the roadway, and (3) protect the roadway against storm and subsurface water damage. Responsibilities for drainage design are divided among the Offices of Bridge Design, Road Design and Materials and Surfacing -- based principally on the size of the drainage area and the complexity of the structures involved. The Area offices have basic responsibility for furnishing the survey data and field information essential for drainage design. The Office of Bridge Design is responsible for: • all locations with drainage areas of 1,000 acres (400 hectares) and above • all locations requiring bridges • all concrete box culverts The Office of Road Design is responsible for: • design of most culverts accommodating areas up to 1,000 acres (400 hectares) • design of storm sewer systems The Office of Materials and Surfacing is responsible for design of subsurface drainage. Design of bank and channel protection should be performed by the unit responsible for design of the drainage structure. The Department's Hydraulic Engineer is available to consult with and provide guidance to roadway designers for any unusual circumstances that may arise concerning roadway drainage. Due to the current unavailability of metric nomographs and charts, the Office of Road Design does most of its drainage design in English.

11-4

PIPE CULVERT DESIGN The design of pipe culverts can be broken into two major elements: • Determination of pipe size • Determination of pipe wall thickness (RCP-class or CMP-gauge) and

installation characteristics. Pipe size is a function of drainage area, land use, flood frequency and other measures affecting the volume of surface drainage. Pipe thickness or gauge and type of installation are a function of roadway design characteristics such as fill height. Pipes for entrances and cross roads need not be designed unless a significant drainage area is served. Rock and Wire Bank Protection Gabions Where there is potential for erosion of ditch banks and channels, protection should be provided in the form of rock and wire protection gabions. Any velocities over 10 fps require the standard rock and wire protection gabions at the outlet. Velocities over 15 fps should be investigated to determine if there is any additional protection warranted such as additional bank and channel protection gabions, riprap, cut off wall, etc. Rock and wire gabions at the outlets of pipe culverts should be placed under the end section to a distance two feet (0.6m) from the outlet end of the section. Details of the standard installations are shown in the Department's Standard Plate. Other types of erosion control such as seeding, fertilizing, mulching and channel rip rap are discussed in Chapter 14 – Roadside Development.

11-5

PIPE CULVERT MATERIAL SELECTION The pipe culvert materials allowed for use in South Dakota highways include reinforced concrete (RCP), corrugated metal (CMP) and high density polyethylene (HDPE). The locations and soil conditions where each material is allowed are specified below. The Materials and Surfacing Office will provide a soils report with a list of the corrosivity ratings established by the Natural Resources Conservation Service (NRCS). Any recommendation of culvert type to exclude from use, if any, will be included in the soils report. Soil corrosivity has the most affect on the durability or service life of pipes in SD with CMP being the most susceptible. The NRCS rates of corrosivity and the allowable materials provided in the guidelines have been found by past experience to meet these service life requirements. The durability of CMP is also more susceptible to abrasion from flowing bed material than the other culvert types.

Table 11-1 Pipe Culvert Material Type Selection and Service Life Guidelines

Pipe Culvert Installation Location Service Life Allowable

Pipe Culvert Material

Mainline Cross Pipe – Interstate, US & State Highways 75 years RCP Intersecting Roads and Entrances with Paved Surfacing (AC or PCCP) 50 years RCP, CMP 1

& HDPE 3 Intersecting Roads and Entrances with Non-Paved Surfacing 25 years RCP, CMP 2

& HDPE 3 Storm Sewer 75 years RCP

Critical Locations 4 75 years RCP 1 CMP is allowed in soils with a low to moderate soil corrosivity rating. 2 CMP is allowed in all soils, except where not recommended by the Materials and Surfacing Office. 3 The maximum allowable diameter of HDPE pipe that can be installed is 36”. The end sections for HDPE pipe shall be metal, conform to the type of end section as shown in the plans, and be compatible with the HDPE pipe. 4 Critical locations are considered to be culvert installation sites where premature replacement would involve inordinate expense, severe inconvenience or safety considerations, such as high fills.

11-6

CMP should be specified in the plans at locations where CMP is an allowable material and soils meet the allowable corrosivity rating. HDPE pipe and RCP may be substituted for CMP in intersecting roads and entrances by the Contractor at no additional cost to the State. The designer will need to verify with the County or Township what pipe material substitutions are allowed for pipes installed under their roads that fall outside the State’s highway right-of-way width. Flexible pipe (CMP and HDPE) are not recommended for use in highway mainline, storm sewer and critical locations based on the following disadvantages:

a. Customer Service Survey indicates that a smooth riding roadway is very important to our customers. The flexible pipe requires granular backfill which creates a potential conduit for water to enter the highway embankment and creates potential frost heave problems in shallow embankments. Considerations also need to be made for the availability and cost of the granular backfill. b. Flexible pipes have less structural strength. Much of the pipe strength is dependent on the surrounding backfill support or allowable soil bearing pressure. c. Installation requirements are more stringent and require more inspection.

d. More pipe alignment deflection tends to occur during backfilling due to the lighter weight and greater flexibility of these pipes.

e. The lighter weight and buoyancy of these materials create the potential for the pipe to float when inundated with water, particularly at the end sections. These pipes may require the ends to be weighted or more securely fastened to the ground, such as with concrete headwalls.

f. There are greater limitations on the fill height allowed over the pipe.

The major advantages for the use of concrete pipe are generally opposite of the above disadvantages for flexible pipe. These items are major factors in pipe culvert installations.

11-7

PIPE SIZING The two methods used to determine the design flows for sizing of pipes are: • Hydraulic Analysis Method • Techniques for Estimating Flood Peaks, Volumes and Hydrography on Small

Streams in South Dakota. (U.S. Geological Survey, Water-Resources Investigations 80-80)

Storm Design Frequency

Road Class or Type of Installation

*Appropriate Storm Design Frequency

Interstate 50 Year Arterials 25 Year Collectors 25 Year Local Roads ADT<100 **10 Year Local Roads ADT>100 **25 year Approaches **10 year Urban Areas 1000 acres and Over

100 Year

Under 1000 acres 25 Year Storm Sewers 10 Year Ramps Interstate 50 Year Interstate Storm Sewer 10 Year Arterial 25 Year Arterial Storm Sewer 10 Year

* Where there is development, the 100 year event shall be reviewed to see what the possible impacts are. ** Check 25 year event to verify local road or approach will overtop before mainline.

The design criteria set forth above are guidelines for general application. Normally, they should be used. For the larger drainage areas, an assessment of local conditions may suggest use of a different design frequency at specific locations. With storm sewers, a risk analysis can be made to see if a five-year storm would be the more appropriate storm design frequency. Also, check with local officials to see if local ordinances require different frequencies to be designed for or analyzed.

11-8

Design Flow A design flow computer program utilizing the Hydraulic Analysis Method is used for drainage areas up to 150 acres (60 hectares). The Wyoming State Highway Department computer program for design flows utilizing the Techniques for Estimating Flood Peaks, Volumes and Hydrography on Small Streams in South Dakota is used for design flows for drainage areas from 150 acres (60 hectares) to 1,000 acres (400 hectares). Allowable Headwater (Rural Areas) The following guidelines have been developed in order to provide the designer general parameters to use for designing pipes serving drainage areas up to 1000 acres (400 hectares) based on different site/terrain conditions. These guidelines are to be used in conjunction with the Department’s current hydraulic policies and software programs.

Terrain Recommended Allowable Head* Flat/Cropland Diameter of pipe plus 1.0’ (0.3m) Rolling 1.5 times diameter of pipe Mountainous 2 times diameter of pipe * The allowable head will be site specific. Good engineering judgment should be used in all cases.

In all cases, the existing structure open area shall be a major factor when determining the final selection. Pipe shall be sized to keep the design frequency water elevation at or below the following to prevent water from encroaching on the highway and to prevent overflow to a different drainage basin. The maximum allowable headwater should not exceed the following:

1. One foot (0.3m) below the highway edge of subgrade elevation at the lowest point of the roadway within the drainage basin.

2. Top of ditch block. 3. Finished top of approach. 4. Summit of ditch grade. 5. Intersecting Roads: Analyze the appropriate storm design frequency for the

designated Road Class as listed on page 11-5. Interstate, Arterials & Collectors: Max. headwater should meet 1. above.

Local Roads: Max. headwater should be at the edge of driving surface.

11-9

Pipe Sizing The pipe sizing for the Hydraulic Analysis Method is accomplished by following procedures set forth in Highway Engineering Circular No. 5, "Hydraulic Charts for the Selection of Highway Culverts", (Bureau of Public Roads, 1965). The same pipe sizing procedures are used with the Techniques for Estimating Flood Peaks, Volumes and Hydrography on Small Streams in South Dakota method with the addition of consideration for storage and the soil index. Hydraulic Data The size of the drainage area, soils index, and channel slope in feet per mile are received from the Hydraulics Section, Bridge Design Program (Figure 11-1) for each proposed pipe site. Hydraulic Analysis Worksheet After receiving the hydraulic data from Hydraulics, fill out the appropriate worksheet for each proposed culvert site. For areas having 0 - 150 acres (60 hectares), use Figure 11-2. However, when terrain is relatively steep and soil is impermeable use the 150 - 1000 acre (60-400 hectares) design, Figures 11-3A & 11-3B. Areas over 1000 acres (400 hectares) will be handled by the Bridge Program and their data sheet will be sent to you (preliminary and final). The Bridge Program should be contacted when problems exist when sizing pipe from 0 - 1000 acres (400 hectares).

11-10

Figure 11-1 Memorandum from Hydraulics Section, Bridge Design Program

11-11

Figure 11-2 Hydraulic Analysis of Culverts, 0-150 Acres

11-12

Figure 11-3A Hydraulic Analysis of Culverts, 150 Acres or More

11-13

Figure 11-3B Channel & Mainline Cross Section Worksheet

11-14

Hydraulic Analysis Worksheet (continued) After entering Hydraulic data on the worksheets, put the following information on the appropriate worksheets: 0 - 150 acres/0-60 hectares 1. Existing pipe size (Topography notes, old plans) 2. Hydrology Region (See Figure 11-4) 3. Culvert type (CMP or RCP) 4. Culvert length (Estimate from a preliminary pipe section for # 4 - 7) 5. Inlet and outlet elevations 6. Slope of culvert 7. Allowable headwater (Elevation difference from inlet to 1' (0.3 meter) below

subgrade, ditch block elevation, 1' (0.3 meter) below approach, or ditch summit.)

8. Recurrence interval (Usually 25 years) 9. Discharge (If predetermined) 150 - 1000 acres/ 60 - 400 hectares 1. Existing pipe size (Topography notes, old plans) 2. Calculate and record discharges and volumes for 25, 50 and 100 year

recurrence intervals using the equations on the worksheet. (Check the gauged sites in the back of the Techniques for Estimating Flood Peaks, Volumes, and Hydrography on Small Streams in South Dakota for possible predetermined data.)

3. Mannings value for downstream channel. 4. Centerline subgrade elevation. 5. Culvert type (CMP or RCP) 6. Culvert length (Estimate from a preliminary pipe section # 6 - 9)

11-15

7. Inlet and outlet elevations 8. Slope of culvert 9. Allowable headwater (Elevation difference from inlet to 1' (0.3 meter) below

subgrade, ditch block elevation, 1' (0.3 meter) below approach, or ditch summit.

10. Maximum size pipe. (1' (0.3 meter) below shoulder minus inlet elevation, but

usually allowable headwater depth) 11. Downstream channel cross section (Long enough to give a good

representation.) 12. Upstream channel cross sections (One at the inlet and as many upstream as

it takes to describe the storage available) Pipe Design for 0 - 150 Acres/ 0-60 Hectares Use the Culvert Sizing computer program to size the pipes. This program is based

upon Hydraulic Engineering Circular No. 5 "Hydraulic Charts for the Selection of Highway Culverts". This includes a separate coding for reinforced concrete and corrugated metal pipe, both circular and arch pipe, with a variety of inlet types.

To use the Culvert Sizing computer program, fill out the code form (Figure 11-5)

using the completed worksheet for all pipes having less than 150 acres (60 hectares). The first computer run should contain the following fixed data:

1. The hydrology region for the project. (See Figure 11-4) 2. 25 year recurrent interval. 3. 01 in columns 59 & 60 (This will cause the maximum head to be 0.1' (0.03

meter) above top of the pipe). Enter the code sheet's data on a computer member and run the Culvert Sizing

program. Review the output from the program and note the pipe diameter vs. the headwater.

The headwater is the higher of either the inlet control or outlet control results (both circular and arch). The last line could be the selected pipe size but be sure that all considerations are made before a final pipe size is selected.

11-16

Figure 11-4 Map of Hydrology Regions in South Dakota

11-17

Figure 11-5 Hydraulic Analysis of Culverts Code Form

11-18

Normally the headwater should be higher than the top of the pipe, yet the headwater shouldn't exceed the maximum allowable headwater that has been determined on the worksheet. Also, compare the selected pipe size to the in place pipe size. If the design pipe size is less than the existing pipe size, the designer shall review the sizing procedure thoroughly, obtain any additional information from field inspection, area landowners and field notes. In following this procedure, the designer shall consider all data available before a final decision is made including consulting with the designer's supervisor. Check minimum and maximum cover specifications. Review pipe culvert slope and check for need of a downspout. Review the effect that any temporary upstream impoundment may have on the surrounding area. When a review has been made, you may now want to change the allowable headwater in order to get the size that is more suitable or a headwater that won't cause problems in the area. If temporary upstream storage is available, it may be desirable to consider some temporary impoundment. In these cases, routing may be investigated as when designing pipe locations with drainage areas of 150 -1000 acres (60-400 hectares). When the headwater is not causing problems and the pipe sizes are suitable, the designer should check the circular vs. arch (size, cover, number, etc.) and do a cost estimate if there is a possible difference in cost. Before finally recording the new pipe size, check the pipe section for the pipe's length and slope. If they are different, change the computer run and verify the sizes. Check the velocities for all the pipe and write it on the worksheet. Refer to previous discussion in the Rock and Wire Bank Protection section regarding the appropriate erosion protection. Pipe Design for 150 - 1000 Acres/ 60-400 Hectares Use the Wyoming State Highway Department's “Culvert Design System" computer program (CDS) to size pipe for these areas. The CDS's design or review process for drainage culverts is accomplished by routing a hydrograph through the culvert, thereby taking advantage of temporary upstream pond storage. To use the CDS program, create an input member according to the CDS instruction sheets for all areas 150 - 1000 acres (60-400 hectares) using the data on the completed worksheet. Manning numbers must coincide with a downstream cross section station. All three recurrence intervals (25, 50, & 100 year) should be entered with the short output form, but make sure entry one contains the interval you wish to design for (normally 25 years). Both circular and arch should also be designed initially for a comparison.

11-19

The output contains various types of information, but we're mostly interested in the culvert performance summary. This contains an initial size and headwater for the given discharge, volume, allowable head and maximum pipe size. Review the size vs. existing and the headwater at 25 year frequency to see if there needs to be adjustment in the allowable headwater and/or maximum pipe size to rid these problems. If the design pipe size is less than the existing pipe size, the designer shall review the sizing procedure thoroughly, obtain any additional information from field inspection, area landowners and field notes. In following this procedure, the designer shall consider all data available before a final decision is made including consulting with the designer's supervisor. The designer should also analyze if there has been any previous flooding problems or any possible future flooding of nearby areas due to the change of the ditch gradeline or channel change. If possible problems exist, the designer should pay particular attention to the headwater (if dwellings are affected, the 100 year flood should be examined carefully). If the designer wants to see how a particular size of pipe or drainage structure would perform with given data, he may do so with the CDS review option. This is handy for performance evaluation of the in place pipe or structure. Before finally recording new pipe size, check the pipe section for the pipe's length and slope. If they are different, change the input member and verify the sizes. Check the velocities for all the pipe and write it on the worksheet. Refer to previous discussion in the Rock and Wire Bank Protection section regarding the appropriate erosion protection.

11-20

OVERTOPPING PROCEDURE Using the overtopping pipe form (Figure 11-6), list all the mainline pipe that have diameters larger than 24” (600 mm) (Equivalent round for arch) and fill in the data known (use computer runs and worksheet). The overtop elevation is the lower of the following possibilities:

1. Centerline finished elevation (lowest elevation in the area) 2. Top of ditch block 3. Finished top of approach 4. Summit of ditch grade

Overtop headwater is the difference between the overtop elevation and the inlet elevation. 0 - 150 acres/0 - 60 hectares Knowing the headwater depth at overtopping and the design pipe size, the discharge is read from the appropriate nomograph (Figures 11-7A to 11-7E). These nomographs for the various pipe culverts were found in the FHWA Hydraulic Engineering Circular No. 5. For inlet control use scale (2) groove end with headwall for pipe with flared end sections and make sure the value used is HW/D (HW=overtopping headwater in feet, D=diameter in feet). The region codes (Figure 11-4) are related to the region designations in the department’s Drainage Manual (which are used in the following graph readings) by the following table:

Code Region

1 E - 9 2 E -8 3 E -11 4 B - 8 5 B - 9 6 B -12 7 B -13 8 B -14 9 D - 9 10 E - 13 (Use a factor of 0.6) 11 E - 9 (Use a factor of 1.2)

11-21

Figure 11-6 Overtopping Pipe Form

Project: Designer: Date: Sheet____ of ____

Station Pipe Size Acres O.T.

Elev Inlet Elev

O.T. Head Water

25 Year

H.W. Q

50 Year

H.W. Q

100 Year

H.W. Q

Overtop

Year Q

No Overtop

Q100 ELEV

11-22

Figure 11-7A Headwater Depth for RC Pipe Culverts with Inlet Control

11-23

Figure 11-7B Headwater Depth for RC Arch Pipe Culverts with Inlet Control

11-24

Figure 11-7C Headwater Depth for CM Pipe Culverts with Inlet Control

11-25

Figure 11-7D Headwater Depth for CM Arch Pipe Culverts with Inlet Control

11-26

Figure 11-7E Headwater Depth for Box Culverts with Inlet Control

11-27

Dividing the design 25 year discharge by the appropriate ratio of discharge to mean annual flood (see the following table or Figure 11-8) gives the mean annual flood for the drainage area. Recurrence Interval E B D . 25 year 6.85 4.11 3.21 50 year 8.73 5.48 4.03 100 year 10.76 6.78 4.81 Record the mean annual flood value lightly above the acreage on the form. Now divide the overtop discharge by the mean annual flood value to obtain the ratio of discharge to mean annual flood. Using the above table or Figure 11-8, find the ratio on the appropriate line and read the recurrence interval. If the recurrence interval is less than 100 year, record the year and the discharge on the profile sheet of the plans. If the recurrence interval is 100 year or more, record the discharge and elevation for the 100 year flood on the profile sheet of the plans. Figure 11-9 and the following examples provide different scenarios (no overtop, overtop and multiple pipe). 150 - 1000 acres/ 60 - 400 hectares Look at the overtop headwater and compare it to the 25, 50 and 100 year headwaters. If it is less than the 25 year headwater, then there is a problem with the headwater that needs to be corrected. If it is between the 25 and 100 year headwaters, interpolate the year of overtopping and record it in the column under OVERTOP on the form. Using the log graph paper (Figure 11-10), plot the recurrence intervals vs. discharge. Locate the determined year overtop and intersect the line drawn. Record the discharge in the column under OVERTOP on the form. If it is over the 100 year headwater, then record the 100 year discharge and the elevation in the columns under NO OVERTOP on the form. Figure 11-9 shows an example.

11-28

Figure 11-8 Ratio of Discharge to Mean Annual Flood

11-29

Project: EXAMPLE Designer: Date: April 11, 2007 Sheet____ of ____

Station Pipe Size

Acre O.T. Elev

Inlet Elev

O.T. Head Water

25 Year

H.W. Q

50 Year

H.W. Q

100 Year

H.W. Q

Overtop

Year Q

No Overtop

Q100 Elev

577+00

36"

7.74 85

579+79 46.9

41.1

5.8

2.8

31.8

9.96 77

52 cfs

44.9

497+65

36"

11.24 96

D Block 496+20 34.0

28.9

5.1

3.3

46.2

Q54

5.69 64 cfs

34.0

33+54

Twin 24"

156

Ent 29+40 99.0

2.6

24.3

2.2

30.3

2.7

34.1

Q90

33 cfs

99.0

156+10

Twin 24"

7.47 80

150+32 23.3

18.0

5.3

2.3

30.8

9.35 70

51 cfs

21.4

Figure 11-9 Completed Overtopping Pipe Form

11-30

Ex Figure 11-10 Log Graph Paper

11-31

Example 1 - No Overtop Within 100 Years 577+00 - 36" RC Pipe 1. Find OVERTOP Q Known: D = 3.0 HW = 5.8 HW/D = 1.93 From Figure 7a ---> Q OVERTOP = 77 cfs 2. Divide Q25 by Appropriate Ratio of Discharge (25 year) Code 6 ---> Region B - 12 ... Ratio = 4.11 Known ---> Q = 31.8 31.8/4.11 = 7.73 (MEAN ANNUAL FLOOD - MAF) 3. Divide Q OVERTOP by MAF 77/7.73 = 9.96 > 6.78 (Ratio of Discharge 100 year) ... OVERTOP beyond 100 year storm 4. Find Q100 = & Elev. for 100 year flood MAF Q100 = (7.73) (6.78) = 52 cfs From Figure 11-7a HW/D = 1.25 HW = (1.25) (3) = 3.75 ELEV100 = 3.75 + 41.1 = 44.9 Example 2 - Overtop < 100 Year Flood 497+65 - 36" RC Pipe 1. Find OVERTOP Q Known: D = 3.0 HW = 5.1 HW/D = 1.70 From Figure 11-7a ---> Q OVERTOP = 64 cfs 2. Divide Q25 by Appropriate Ratio of Discharge (25 year) Code 6 ---> Region B - 12 ... Ratio = 4.11 Known ---> Q25 = 46.2 46.2/4.11 = 11.24 (MEAN ANNUAL FLOOD - MAF) 3. Divide Q OVERTOP by MAF= 64/11.24 = 5.69 < 6.78 Ratio of Discharge (100 yr) ... Flood within 100 year storm 4. Find Year of OVERTOP from Figure 11-8. Known - Ratio of Discharge to Mean Annual Flood at overtopping = 5.69 - Region B ... OVERTOP Year = 54 (Q54 = 64 cfs)

11-32

Example 3 - Multiple Pipe 156+10 - Twin 24" - RC Pipe 1. Find OVERTOP Q Known: D = 2 HW = 5.3 HW/D = 2.65 (Twin) From Figure 11-7a ---> QOVERTOP = 35 x 2 = 70 cfs 2. Divide Q25 by Appropriate Ratio of Discharge (25 year) Code 6 ---> Region B - 12 ... Ratio = 4.11 Known Q = 30.8 <--(2 X computer run) 30.8/4.11 = 7.49 (Mean Annual Flood - MAF) 3. Divide QOVERTOP by MAF 70/7.49 = 9.35 > 6.78 Ratio of Discharge (100 year) ... OVERTOP Beyond 100 Year Storm 4. Find Q100 & Elev. for 100 Year Storm Q100 = (7.49) (6.78) = 51 cfs From Figure 11-7a HW/D = 1.78 HW = (1.78) (2) = 3.56 ELEV100 = 3.56+18.0 = 21.6

11-33

Summary When all pipes have overtopping information completed, the designer should highlight the information that goes on the plans so the draftsman can pick it off accurately and put on the profile sheets of the plans. For Overtop 25 - 100 yr. Q yr. = ______CFS (m3/s) EL. = ______ For Overtop 100 yr. + Q 100 = ______CFS (m3/s) EL. = ______ For sites over 1000 acres (400 hectares) the following format is used. The examples also show how to handle sites with optional structures. When there are several optional structures for a site, the worst case data should be shown on the plan sheet. In the case of 25 yr and 100 yr. high water elevations, the highest one from the several options should be shown. If there is overtopping at the crossing, use the lowest overtopping frequency. Normal crossing without overtopping. Q25 = 210 CFS (m3/s) EL. = 73.2 Q100 = 395 CFS (m3/s) EL. = 74.1 In this case another option had a Q100 high water elevation of 74.0 Crossing with overtopping. Q25 = 210 CFS (m3/s) EL. = 73.2 QOT = Q65 = 300 CFS (m3/s) EL. = 73.7 Q100 = 395 CFS (m3/s) EL. = 73.9

In this case another option had an overtopping frequency of QOT = Q70 = 310 CFS (m3/s), and the EL. (100 Yr) is with overtopping.

11-34

PIPE CULVERT INSTALLATION General Typical Pipe Culvert Sizes English(in) Metric(mm) 18 450 24 600 30 750 36 900 42 1050 48 1200 54 1350 60 1500 66 1650 72 1800 78 1950 84 2100 Minimum Pipe Size Size Type Use 12” CMP Outlets from Type D Drop Inlets at Bridges 12” RCP Type D Drop Inlet connecting pipe below mainline at Bridges 18” RCP Storm Sewer (12” may be used in special cases, and must be approved by the Chief Road Design Engineer) 18" CMP Approaches & Minor Intersecting Roads 24" RCP Mainline Cross Pipe & Major Intersecting Roads The 24” RCP minimum mainline pipe size was determined based on maintenance issues involving difficult cleanout, siltation, snow and freezing. After the size of a pipe culvert has been determined there are several other characteristics to be determined. These are as follows: • For REINFORCED CONCRETE PIPE: Class of pipe Method of installation Maximum grade is 6% (Downspouts will be used on grades exceeding 6%) • For CORRUGATED METAL PIPE: Gauge of pipe Maximum grade is 8% (Downspouts will be used on grades exceeding 8%) All of these characteristics vary with the depth of the fill over the pipe.

11-35

Reinforced Concrete Pipe Strength requirements for concrete pipe culverts are shown below in Table 11-2 in terms of maximum allowable cover for various diameters and classes of pipe. The minimum subgrade cover over concrete pipe shall be the greater of 12” or 1/8 the pipe inside rise. In rare cases where the subgrade cover cannot be obtained and only under rigid PCC pavements, the cover can be measured from the top of the rigid pavement as long as there is a minimum of 9” of compacted granular fill between top of pipe and bottom of pavement slab. Class 2 pipe with normal backfill is the standard design. Any other combination of pipe class and backfill must be clearly identified on the plans. For depths requiring greater than Class 5 pipe, special classes of pipe are available (Class 4000 D, 5000 D, and 6000 D). These sites will have to be designed on a site by site basis. When class of pipe required is 4 or higher, step down the class of pipe as the cover decreases. Go as low as Class 2 only if the lengths at the end are more than 16’. As a guideline, do not step down from Class 3 to Class 2 and use only one or the other. Examples of typical pipe class stepping are 4-3 or 4-2 and 5-4-3 or 5-4-2. Class of pipe under railroad tracks require special considerations. Typically, Class 5 pipe is used for a distance of 25’ to 30’ (8-10 m) either side of centerline of the tracks or to the ROW line, whichever is less. However, the Department’s Railroad engineer should be contacted for each specific case.

Table 11-2 RC Pipe Maximum Allowable Depth of Cover

RCP Round RCP Arch Diam. Class 2 Class 3 Class 4 Class 5 Eq. Diam. Class 2 Class 3 Class 4 Class 5

12” 10’ 14’ 22’ 33’ 18” 8’ 11’ 17’ 25’ 18” 11’ 15’ 22’ 33’ 24” 8’ 11’ 16’ 25’ 24” 10’ 14’ 22’ 33’ 30” 8’ 11’ 17’ 25’ 30” 10’ 14’ 22’ 33’ 36” 8’ 11’ 17’ 26’ 36” 10’ 14’ 21’ 32’ 42” 8’ 11’ 17’ 26’ 42” 10’ 14’ 21’ 32’ 48” 8’ 11’ 17’ 26’ 48” 10’ 14’ 21’ 32’ 54” 8’ 11’ 17’ 26’ 54” 10’ 13’ 21’ 32’ 60” 8’ 11’ 17’ 26’ 60” 9’ 13’ 20’ 31’ 72” 8’ 11’ 17’ 26’ 66” 9’ 13’ 20’ 31’ 84” 7’ 11’ 17’ 26’ 72” 9’ 13’ 20’ 31’ 96” 7’ 11’ 17’ 26’ 78” 9’ 13’ 20’ 31’ 108” 7’ 10’ 17’ 26’ 84” 9’ 13’ 20’ 31’ 120” 7’ 10’ 16’ 26’ 90” 9’ 13’ 20’ 31’ 132” 7’ 10’ 17’ 26’ 96” 9’ 12’ 20’ 31’ 102” 8’ 12’ 19’ 31’ 108” 8’ 12’ 19’ 30’

11-36

Corrugated Metal Pipe The thickness or gauge & corrugation of the pipe varies with the height of cover above the top of the culvert. The following tables give the gauge & corrugation of corrugated metal pipes required under various heights of cover. Pipe size, gauge, corrugation, and maximum allowable depth of cover are shown in Tables 11-3 & 11-4. The minimum subgrade cover over corrugated metal pipe required for normal highway live loads (H20 or H25) varies from 12 to 24 inches and depends on the type and size of pipe. See the following tables. In rare cases where the required subgrade cover cannot be obtained and only under rigid PCC pavements, the cover can be measured from the top of the rigid pavement. The gauge & corrugation selected for each pipe, or segment of pipe, must be specified on the particular cross section, pipe estimate and the plan profile sheets of the construction plans.

Table 11-3 CM Pipe Maximum Allowable Depth of Cover

MAXIMUM DEPTH OF COVER (feet) 2-2/3 x ½ Corrugation 3 x 1 Corrugation 5 x 1 Corrugation Thickness, in. 0.064 0.079 0.109 0.064 0.079 0.109 0.138 0.168 0.064 0.079 0.109 0.138 0.168

Gauge 16 14 12 16 14 12 10 8 16 14 12 10 8 Diameter

12” 213 15” 170 18” 142 24” 106 133 30” 85 106 149 36” 71 88 124 42” 60 76 106 48” 93 61 76 54 68 54” 54 68 95 48 60 84 60” 48 61 85 43 54 76 66” 44 55 78 39 49 69 72” 51 71 92 45 63 81 78” 47 66 84 41 58 75 84” 43 61 78 38 54 70 90” 40 57 73 36 50 65 96” 53 69 84 47 61 75 102” 50 64 79 44 57 70 108” 47 61 75 42 54 66 114” 45 58 71 40 51 63 120” 55 67 49 60

CM pipes 96” & less shall use 12” minimum subgrade cover. CM pipes greater than 96” shall use 18” minimum subgrade cover.

11-37

Table 11-4 CM Arch Pipe Maximum Allowable Depth of Cover

MAXIMUM DEPTH OF COVER (feet) 2-2/3 x ½ Corrugation 3 x 1 & 5 x 1 Corrugation

Thickness, in. 0.064 0.079 0.109 0.079 0.109 0.138 Gauge 16 14 12 14 12 10

Equiv. Diameter 15” 6 18” 6 24” 6 30” 6 36” 6 42” 6 48” 6 54” 10 60” 10 66” 10 72” 8 78” 8 84” 8 90” 8 96” 8 102” 8 108” 8 114” 8 120” 8

CM Arch pipes 84” & less shall use 18” minimum subgrade cover. CM Arch pipes greater than 84” shall use 24” minimum subgrade cover. The maximum and minimum depth of cover for CM Arch Pipe is controlled by the allowable soil bearing pressure at the pipe corners. The above depths are based on a typical South Dakota allowable soil bearing pressure of 2000 psf. Increasing the pipe wall thickness or gauge will not increase the maximum allowable cover.

11-38

Multiple Pipe Multiple pipes should be installed with the recommended spacing shown in Tables 11-5A and 11-5B. The recommended pipe spacing is based on 20’ between edges of pipe barrels for constructability. Pipe spacing may be reduced when channel width or other topography dictate. The minimum spacing without considering control density fill is based on 12’ between edges of pipe end sections. When the required pipe spacing is less than this minimum, the smallest allowable pipe spacing with controlled density fill up to the pipe haunches should be provided as shown in Figure 11-11. Pipe spacing with controlled density fill is based on either 0’ or 1’ between pipe end sections and depends on the type of end section. The 1’ between pipe end sections is provided for end sections that do not flare and are the same width as the pipe barrel. The 0’ between pipe end sections for CMP sloped and safety ends assumes the end sections are placed as close together as possible by overlapping the perpendicular flare of the end sections (dimension “A” from standard plates 450.37 & 450.38). The pipe spacing and quantity of controlled density fill per foot of pipe length is provided in Tables 11-5A and 11-5B. Controlled density fill between metal pipes will require the pipes to be anchored down to resist buoyancy forces and prevent them from floating up. When using controlled density fill for metal pipes, a plan note shall be included in the plans stating that anchoring methods are determined by the Contractor and incidental to other pipe bid items.

Figure 11-11 Controlled Density Fill Limits between Multiple Pipe

11-39

Table 11-5A Spacing of Multiple Concrete Pipe

C. TO C. SPACING OF MULTIPLE PIPE Recommended Without CDF

Minimum Without Considering CDF With CDF Pipe

Type End

Section Pipe Size

20’ between pipe barrels

12’ between pipe end sections

1’ between pipe end sections

0’ between pipe end sections

Controlled Density Fill

(CDF)

(CuYd/Ft) 18” 22’ 14’ 2.9’ 0.050 Safety 24” 23’ 14’ 3.5’ 0.071 24” 23’ 14’ 3.5’ 0.071 30” 23’ 15’ 4.1’ 0.096 36” 24’ 16’ 4.7’ 0.124 42” 24’ 16’ 5.3’ 0.154 48” 25’ 17’ 5.8’ 0.179 54” 25’ 17’ 6.4’ 0.215

Sloped

60” 26’ 18’ 7.0’ 0.254 18” 22’ 15’ 3.4’ 0.067 24” 23’ 16’ 4.5’ 0.117 30” 23’ 18’ 5.6’ 0.181 36” 24’ 19’ 6.7’ 0.259 42” 24’ 19’ 7.3’ 0.312 48” 25’ 20’ 7.8’ 0.358 54” 25’ 20’ 8.4’ 0.416 60” 26’ 21’ 8.8’ 0.454 66” 27’ 21’ 9.4’ 0.516 72” 27’ 22’ 10.0’ 0.580 78” 28’ 23’ 10.6’ 0.648 84” 28’ 23’ 11.1’ 0.703

Flared

90” 29’ 24’ 12.1’ 0.842 96” 30’ 22’ 10.5’ 0.535 108” 31’ 23’ 11.7’ 0.656

RCP

Sectional 120” 32’ 24’ 12.8’ 0.768

Safety 18” 22’ 15’ 3.3’ 0.038 24” 23’ 15’ 3.9’ 0.041 30” 24’ 16’ 4.6’ 0.056 36” 24’ 16’ 5.4’ 0.071 Sloped

42” 25’ 17’ 6.0’ 0.083 18” 22’ 15’ 3.4’ 0.040 24” 23’ 17’ 4.6’ 0.062 30” 24’ 18’ 5.7’ 0.095 36” 24’ 19’ 6.8’ 0.128 42” 25’ 19’ 7.3’ 0.142 48” 26’ 20’ 7.8’ 0.157 54” 26’ 20’ 8.4’ 0.180 60” 27’ 21’ 9.0’ 0.196 72” 29’ 23’ 11.2’ 0.314

Flared

84” 30’ 25’ 13.3’ 0.423 96” 32’ 24’ 12.7’ 0.287 108” 33’ 25’ 14.2’ 0.332 120” 35’ 27’ 15.7’ 0.395

RCP ARCH

Sectional

132” 36’ 28’ 16.7’ 0.428

11-40

Table 11-5B Spacing of Multiple Metal Pipe

C. TO C. SPACING OF MULTIPLE PIPE

Recommended Without CDF

Minimum Without Considering CDF With CDF Pipe

Type End

Section Pipe Size

20’ between pipe barrels

12’ between pipe end sections

0’ between pipe end sections

Controlled Density Fill

(CDF)

(CuYd/Ft) 18” 22’ 15’ 2.7’ 0.042 24” 22’ 16’ 3.2’ 0.060 30” 23’ 17’ 4.0’ 0.094 36” 23’ 18’ 4.5’ 0.119 42” 24’ 19’ 5.3’ 0.165 48” 24’ 19’ 5.8’ 0.197 54” 25’ 20’ 6.3’ 0.230

Safety & Sloped

60” 25’ 20’ 6.8’ 0.266 18” 22’ 16’ 4.3’ 0.087 24” 22’ 18’ 5.7’ 0.153 30” 23’ 19’ 7.0’ 0.233 36” 23’ 20’ 8.3’ 0.330 42” 24’ 22’ 9.7’ 0.451 48” 24’ 22’ 10.5’ 0.545 54” 25’ 23’ 11.5’ 0.664 60” 25’ 24’ 12.5’ 0.794 66” 26’ 25’ 13.0’ 0.884 72” 26’ 25’ 13.5’ 0.976 78” 27’ 26’ 14.0’ 1.071

CMP

Flared

84” 27’ 26’ 14.5’ 1.167 18” 22’ 16’ 2.9’ 0.021 24” 22’ 16’ 3.5’ 0.031 30” 23’ 17’ 4.4’ 0.048 36” 24’ 18’ 5.0’ 0.060 42” 24’ 19’ 5.9’ 0.085 48” 25’ 20’ 6.6’ 0.105 54” 25’ 20’ 7.2’ 0.204 60” 26’ 21’ 7.8’ 0.238

Safety & Sloped

72” 27’ 22’ 8.8’ 0.248 18” 22’ 16’ 4.2’ 0.041 24” 22’ 17’ 5.5’ 0.071 30” 23’ 19’ 6.7’ 0.106 36” 24’ 20’ 8.3’ 0.160 42” 24’ 21’ 9.3’ 0.204 48” 25’ 22’ 10.2’ 0.250 54” 25’ 23’ 11.5’ 0.476 60” 26’ 24’ 12.5’ 0.568 66” 26’ 25’ 13.5’ 0.665

CMP ARCH

Flared

72” 27’ 26’ 14.1’ 0.636

11-41

Placement Inlets and outlets are placed on the natural drainage channels, if possible. A cross section of the placement site is drawn for each entrance or intersecting road pipe culvert. Pipe culverts 36" (900 mm) diameter or greater should be extended to clear zone or beyond. See Chapter 18 – Plans Assembly, for pipe illustrations. Pipes should not be installed through permanent maintenance crossovers on the Interstate. On other divided highways with depressed medians an effort should be made to minimize the number of drainage pipes installed through median crossovers when practical. Median drainage approaching a crossover should be removed with a pipe across the mainline highway. This will provide for safer crossover inslopes. Culvert Length Measurement Culvert lengths are measured along the culvert flow line. To avoid the need for cutting sections, the design length should be in increments of two feet (0.1m). Typical rounding should be as follows: when the measured length is between 80.6 and 82.5 feet, use a design length of 82 feet. When an installation requires that a section be cut (such as a storm sewer installation), the pipe will be measured from the drop inlets inside wall to inside wall and payment of pipe shall be rounded up to the nearest two feet in all cases. For an installation that is skewed, it is best to plot the culvert in its actual position and scale the required length. See the following paragraphs on skewed installations for additional length of pipe to add to skewed culvert installations. End sections are not considered a part of the culvert length. They are measured and paid for by the number of end sections installed.

11-42

Skewed Installations Wherever practicable, pipe culvert installations should be designed to conform as closely as possible to the natural drainage channels. The degree of skew is measured as the angle between the pipe installation and a line perpendicular to the highway centerline. A culvert angle is described in terms of which end is forward -- left-hand forward (LHF) or right-hand forward (RHF). For example, if the left end of the culvert is ahead of the line perpendicular to the centerline, and the angle is 15 degrees, the installation would be described as "15o left-hand forward (LHF)." The degree of skew shall be to the nearest 1o. Approval from the Chief Road Design Engineer is needed when the degree of skew exceeds 30o.

When a pipe culvert is skewed more than 5o, draw a special cross section of its placement site. The special cross section shall contain the following information: pipe skew, pipe length, and inlet and outlet station, offset & elevation. The measured culvert length needs to be increased by adding the length shown in the following Table 11-6 to each end of the pipe. This length should be added before the pipe length is rounded up to the nearest two feet. The additional length is required to provide a 3:1 inslope to the top of the end of the end section from the point where the pipe originally hits the inslope along the pipe centerline.

Table 11-6 Additional Length Needed for Skewed RC Pipe (Feet)

Round Arch Skew Skew

Pipe Diameter or Equiv.

End Section

Type 5o 10o 15o 20o 25o 30o 35o 40o 45o 5o 10o 15o 20o 25o 30o 35o 40o 45o

18” Safety 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 24” Sloped 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.9 1.7 0.0 0.0 0.0 0.0 0.0 0.3 0.8 1.3 2.0 30” Sloped 0.0 0.0 0.0 0.0 0.0 0.0 0.5 1.3 2.2 0.0 0.0 0.0 0.0 0.1 0.5 1.1 1.7 2.6 36” Flared 1.3 1.7 2.1 2.6 3.2 3.9 4.7 5.7 6.9 0.3 0.7 1.1 1.5 2.1 2.7 3.5 4.4 5.4 42” Flared 1.5 1.9 2.3 2.9 3.5 4.2 5.1 6.1 7.4 0.3 0.7 1.1 1.6 2.2 2.9 3.6 4.6 5.7 48” Flared 1.6 2.1 2.6 3.1 3.8 4.6 5.6 6.8 8.2 0.4 0.8 1.2 1.7 2.3 3.0 3.8 4.8 6.0 54” Flared 3.1 3.6 4.1 4.8 5.5 6.4 7.4 8.7 10.3 0.4 0.8 1.3 1.9 2.5 3.2 4.1 5.1 6.3 60” Flared 3.2 3.6 4.2 4.9 5.6 6.5 7.6 8.8 10.4 0.4 0.9 1.4 2.0 2.6 3.4 4.3 5.3 6.6 66” Flared 5.1 5.6 6.3 7.0 7.9 9.0 10.3 11.8 13.7 72” Flared 4.7 5.3 6.0 6.8 7.7 8.8 10.1 11.7 13.7 3.0 3.6 4.3 5.0 5.9 6.9 8.1 9.5 11.278” Flared 5.4 6.0 6.7 7.6 8.6 9.8 11.3 13.1 15.3 84” Flared 7.0 7.7 8.4 9.4 10.5 11.8 13.4 15.4 17.8 4.1 4.8 5.6 6.6 7.6 8.9 10.4 12.2 14.490” Flared 7.7 8.4 9.2 10.2 11.4 12.8 14.5 16.5 19.1 96” Sloped 4.2 4.8 5.6 6.5 7.6 8.9 10.6 12.5 15.0 3.2 3.8 4.6 5.5 6.6 7.9 9.4 11.2 13.4108” Sloped 3.5 4.2 5.1 6.2 7.5 9.0 10.9 13.3 16.2 3.7 4.5 5.4 6.4 7.6 9.1 10.8 12.9 15.5120” Sloped 6.6 7.4 8.3 9.5 11.0 12.8 14.9 17.6 20.9 3.9 4.8 5.8 6.9 8.2 9.8 11.8 14.1 16.9

11-43

Downspouts Some general criteria needed for designing downspouts are as follows: • Cross pipe downspouts shall be installed with RC Pipe on upstream end, followed by a

concrete to metal pipe transition section. CM pipe elbow, straight CM pipe, CM pipe elbow, straight CM pipe and CM pipe end section. Approach pipe downspouts shall be installed with all CM pipe.

• Use 1 percent grade for sizing pipe on upstream end. Review of different gradients for

alternate sizing of pipes can be done. • Elbows used shall be with 2½° increments. 1° increments will be allowed in special

situations. • The length of CM pipe installed shall not include the length of the CM pipe elbows. The

CM pipe elbows shall be paid for seperately. See Figure 11-12 for laying lengths of elbows.

• Length of downstream end of downspout shall be a minimum of 5 X diameter of

designed pipe. • Recommended size of downspout installations should not exceed 36" (900 mm)

diameter due to constructability. Options would be to install multiple pipe or install the larger pipe on a steeper grade with energy dissipater sections at the outlet end of the pipe. If larger pipe with energy dissipaters is viewed as an option, this must be coordinated with the Hydraulics Office on a per-site basis. Larger than 36” (900 mm) diameter pipe may be used for downspout applications depending on site conditions, constructability and cost comparisons. Constructability and site conditions should be reviewed with Field Staff during the design process for concurrence.

• Maintain a minimum of 1 foot of cover beneath subgrade and inslope. (See Chapter 18

– Plans Assembly for illustration, required note and measurement.) • Use bank and channel protection baskets (or some other type of erosion protection) at

the outlet end. End Treatment See Chapter 10 – Roadside Safety for the appropriate end treatment to use for both reinforced concrete and corrugated metal pipe.

11-44

Figure 11-12 Lengths of Elbows

11-45

Figure 11-13 Maximum Deflection for Culvert Joints

11-46

STORM SEWER DESIGN Criteria The design of storm sewer systems like the design of pipe culverts considers factors such as pipe size, runoff and roughness coefficient. In addition, factors such as time of concentration, type of inlet and inlet spacing are considered. Sewers are designed for the entire area contributing runoff. Design Storm Frequency The design storm frequency for storm sewers is 10 years. A risk analysis can be made to see if a 5-year storm would be the more appropriate storm design frequency. Also, check with local officials to see if local ordinances require different frequencies to be designed for or analyzed (for example Rapid City requests that an analysis should be made on the storm sewer system for 100 year storm frequency). Maximum Allowable Spread The width gutter flow spreads out on the pavement during a rainfall event is considered the spread (T). During the design storm event the maximum allowable spread is the greater of either 8’ or the shoulder width. This 8’ width is based on water spreading over one-half of a traffic lane and a 2’ gutter where there are no shoulders involved. The encroachment of water on the traffic lane is reduced by the shoulder width to a point of no encroachment with an 8’ or wider shoulder. The spread width may be adjusted if assessment of the costs vs. risks warrant it. If the above spread requirement results in very close drop inlet spacing of 100’ or less then alternate drainage interceptors could be considered. This may include allowing the spread to cover the outside lanes of a roadway with 4 lanes or more depending on the project conditions. Roughness Coefficients Concrete Pipe - n = 0.012 Corrugated Metal Pipe - n = 0.024 Pavement / Gutter - n = 0.013 Maximum and Minimum Grades Maximum and minimum grades are determined by the velocities produced. Maximum velocities are variable depending on the location. Velocities of 40 fps (12 mps) are not harmful under certain conditions. Hydraulic grade line may determine maximum allowable grade. When these velocities do get higher energy dissipaters may be used. The minimum allowable velocity is 2 fps (0.6 mps). A lesser velocity will cause silting. However, a minimum grade of 0.5% should be used whenever possible.

11-47

Minimum Pipe Sizes Generally, the minimum pipe size is 18" (450 mm). Pipe sizes increase by 3" (75 mm) increments although the Department usually increases storm sewer size by 6" (150 mm). In special cases, 12" (300 mm) pipe may be used if approved by the Chief Road Design Engineer. Figures 11-14 and 11-15 on the following pages show capacities of pipes flowing full and can be used for a quick reference. Minimum Clearances It is desirable to provide a minimum of 1' (0.3 meter) of cover between the top of the pipe and the top of the subgrade. Also, provide a minimum clearance of 1 foot (0.3 meter) between storm sewers and other underground facilities, such as a sanitary sewer. Time of Concentration The time of concentration (tc) is the time required for water to flow from the hydraulically most distant point of the drainage area to the inlet. Minimum time of concentration is 15 minutes. Large drainage areas may have a time of concentration greater than 15 minutes. Refer to Figure 11-16 to obtain the time of concentration for such areas. Pipe to Inlet Connections Pipes should connect with inlets on the flat side of the walls and not at the corners. Pipe bends and/or a larger drop inlet may be needed for the pipe to fit into one of the sides of the inlet, and to provide a connection away from the inlet corners. This is to prevent compromising the structural integrity of the drop inlet if the pipe were to connect in or near the corner of a wall of the drop inlet. Joints Joints should have mastic seals were conditions warrant such as areas of rock, high water table, or required by local ordinances (for example- Sioux Falls).

11-48

Storm Sewers near Water Lines Storm sewers should be located at least 10 feet horizontally from or at least 18 inches vertically below the bottom of any existing or proposed water mains. When it is not possible to obtain this horizontal or vertical separation, watertight joints will be required for the storm sewer pipe to a distance of 10 feet on each side of the water main. Drop inlets that do not meet these separation requirements will also need to be made watertight. The approved storm sewer pipe joints for these locations include either o-ring rubber gasket concrete pipe, such as Cretex CX-4 (or equivalent), or ungasketed concrete pipe with joints sealed using preformed butyl rubber sealant and encased with a minimum 2-foot wide by 6-inch thick concrete collar reinforced with wire mesh. Concrete arch pipe joints are also approved to be sealed with mastic and a bentonite seal such as Waterstop-RX (or equivalent). This special allowance for arch pipe is based on the unavailability of gasketed concrete arch pipe and the lower tendency for this pipe to shift after installation. One method for making drop inlets watertight is to install a bentonite seal along all joints. The typical joints are located where the pipes enter or exit the drop inlet and where the walls meet the base if cast separately. These requirements are provided by the SD Department of Environment and Natural Resources (DENR) and can be found on their web site at http://denr.sd.gov/des/sw/documents/DesignCriteriaManual.pdf. The approved pipe joints not listed on their web site were specified in a letter from DENR based on their testing. Note that this criteria does not apply to individual service water lines as these are typically seamless lines. Where there is minimal separation from water mains or individual service lines, it may be appropriate to insulate the water line to prevent damage from freezing conditions. Determination whether insulation is needed or not should be discussed with the local governing agency on a case by case basis based on separation distance and depth of waterline. Insulation is usually done by installing a rigid foam insulation barrier between the two utilities.

11-49

Figure 11-14 Full Flow Curves, Circular Pipe, n=0.012

11-50

Figure 11-15 Full Flow Curves, Arch Pipe, n=0.012

11-51

Figure 11-16 Time of Concentration of Small Drainage Basins

11-52

Inlet Types The different types of inlets used by the Department are found in the South Dakota Standard Plates. Those typically used are and their typical application are:

1. Grate Opening

• Type B Frame & Grate (curve Vane)-used most of the time • Type C Frame & Grate-used as an area drain, typically behind the c&g • Type D Frame & Grate-used on bridge approach slabs.

2. Curb Opening-used mostly in sag conditions

• Standard (5'-0"/1500 mm) Type S Storm Sewer Inlet • Standard (10'-0"/3000 mm) Type S Storm Sewer Inlet

3. Miscellaneous

• Type L Median Drain-used as a median drain • Type M Median Drain-used as a median drain • Slotted Reinforced Concrete Pipe-used as a median drain (Not found in South

Dakota Standard Plates) • 24" (600 mm) Concrete Median Drain-used as a median drain • Slotted Vane (Not found in South Dakota Standard Plates) • Slotted Corrugated Metal Pipe-used in median crossovers, sometimes used in

curb and gutter sections where large volumes of water need to be picked up Sumps in Drop Inlets Sumps in drop inlets may be provided when requested by the City and where the City is responsible for maintenance of the storm sewer. The sumps are created by constructing the drop inlet floor 1’ below the pipe flowline elevation. Sumps are provided in order to collect debris.

11-53

Spacing of Inlets Maximum spacing of drop inlets or manholes in order to maintain storm sewer systems are as follows: Pipe Diameter Max Spacing to Inlet or Manhole <48" (1200 mm) 300' (100 m) >48" 600' (200 m) Placement of inlets is described later in the procedure for designing storm sewers. Storm Sewer Grades Storm sewer line gradients should be generally similar to the roadway grade. The same size of pipe will run until the cumulative discharge attains the pipe capacity. When an abrupt reduction in gradient is encountered, an increase of more than one pipe size larger may be required. When increasing the size of pipe, two alternatives are available for design at the junction: • Align the pipe inverts (bottom of pipe) with a continuous flow line, or • Align the inside top of the pipe (soffit) with an abrupt drop in the flow line. Each alternative has advantages and disadvantages. The hydraulic characteristics generally are better when the tops of the pipes are aligned. Also, this approach is better where there is a problem with minimum allowable cover over the pipes. On the other hand, there may be situations in relatively flat terrain where it is necessary to conserve the elevation of the flow line. Under these conditions it may be better to avoid the abrupt drops by aligning the pipe inverts at the junction. Where, for various reasons, gradients must be minimized, the flow lines of concrete arch pipe consecutively increased in size may be aligned more effectively than round pipe. If possible, a slight drop in the flow line is even preferred. A hydraulic gradient is the line of elevations to which the water would rise in successive piezometer tubes along a storm sewer run. Differences in elevations for the water surfaces in the successive tubes represent the friction loss for that length of storm sewer. The friction slope is the slope of the line between the water surfaces.

11-54

The storm sewer run will not be under pressure if it is placed on a calculated friction slope corresponding to a certain quantity of water, cross section, and roughness factor -- and the surface of flow (hydraulic gradient) will be parallel to the top of the pipe. This is the desirable condition. There may be reason to place the storm sewer run on a slope less than the friction slope. In that case, the hydraulic gradient would be steeper than the slope of the storm sewer run. Depending on the elevation of the hydraulic gradient at the downstream end of the run, it is possible to have the hydraulic gradient rise above the top of the pipe, creating pressure on the storm sewer system until the hydraulic gradient at some point upstream is once again at or below the top of the pipe. Computation of the hydraulic grade line of a storm sewer run will not be necessary where (1) the slope and the pipe sizes are chosen so that the slope is equal to or greater than the friction slope, (2) the top surfaces of successive pipes are aligned at changes in size (rather than flow lines being aligned), and (3) the surface of the water at the point of discharge does not rise above the top of the outlet. The pipe will not operate under pressure in these cases, and the slope of the water surface under capacity discharge will approximately parallel the slope of the pipe invert. However, in cases where different sized pipe inverts are placed on the same grade, causing the smaller pipe to discharge against head, or when it is desired to check the storm sewer system against larger-than-design floods, it will be necessary to compute the hydraulic grade of the entire storm system. Begin with the tailwater elevation at the storm sewer outfall and progress upward the length of the storm sewer. For every run, compute the friction loss and plot the elevation of the total head at each manhole and inlet. If the hydraulic grade line does not rise above the top of any manhole or above an inlet entrance, the storm sewer system is satisfactory. If it rises above these points, blowouts will occur through manholes and inlets. Pipe sizes or gradients can be increased as necessary to eliminate such blowouts. A hydraulic gradient must have an original base elevation above the outlet tailwater elevation. Any backwater effects of a significant tailwater elevation must be considered carefully (See end of the section for Storm Sewer Design Procedure to compute Hydraulic Grade Line.)

11-55

STORM SEWER DESIGN PROCEDURE The following procedure shows the basic method for designing storm sewer systems using HEC-12 and HYDRA computer programs and hand calculations. Road Design Office currently uses InRoads Storm & Sanitary Software to design storm sewer. This software electronically automates many of the following steps but follows a similar procedure. See the Storm & Sanitary Workflow document for using this software. 1. K factors should be NO greater than 167. (See Chapter 6 - Vertical Alignment) 2. Obtain the following:

• Aerial photos of proposed storm sewer area • Contours of proposed area or field furnished outlines of contributing areas • Plans of existing storm sewer

3. Place all inlets that are necessary due to geometric features as listed below, to

prevent drainage from flowing across the highway and side streets.

• All low points • Median breaks • Intersections • Crosswalks • Side streets

4. Inlet Spacing Computations In order to determine the location of the inlets for a given project, information such

as layout or plan sheets suitable for outlining drainage areas, road profiles, typical cross sections, superelevation diagrams, and contour maps are necessary. The inlet spacing computation sheet (Table 11-7) should be used to document the computations. A step by step procedure follows:

Step 1 Complete the blanks on the top of the sheet to identify the job by

route, date and your initials. A set of check initials should also be added.

Step 2 Mark on the plan the location of inlets which are necessary even

without considering specific drainage areas. Step 3 Start at one end of the job, at one high point and work towards the

low point, then space from other high points back to the same low point.

11-56

Step 4 Select a trial drainage area for the first inlet. Use the drainage area maps for the project.

Step 5 Col 1 Describe the location of the proposed inlet by Col 2 station and offset, left or right in Column 1 Col 3 Identify the curb type and the cross slope of the spread area

in the Remarks Column 19. A sketch of the cross section should be provided in the open area of the computation sheet.

Step 6 Col 4 Place the length of the drainage area under Col 5 under Col. 4. If the Shape is relatively rectangular, place

width in Col. 5. If the shape is irregular, skip these columns. Step 7 Col 6 Compute the drainage area in square feet by either

planimetering an irregular area or using Col 4 times Col 5 if the shape is rectangular. Divide the square feet by 43,560 to get the drainage area in acres, then enter the acres in Col 6.

Step 8 Col 7 Select a C value from tables or compute a weighted value

based on area and cover type and enter in Col 7. Example: Determine the coefficient of runoff (Table 11-8).

A drainage area that is 75% flat commercial (C=0.8) and 25% concrete roadway (C=0.9) would have a composite "C" value of:

0.75(0.8)+0.25(0.9)=0.825

11-57

Table 11-7 Inlet Spacing Computation Sheet

LOCATION RUNOFF DISCHARGE GUTTER DISCHARGE ALLOWABLE SPREAD

INLET DISCHARGE

Inlet No.

Station Offset From CL

Leng h of

D.A.

(ft)

Width of

D.A.

(ft)

Drainage Area "A"

(acres)

Runoff Coeff.

"C"

Time of

Conc. "Tc"

(min)

Rainfall Intensity

"I"

(in/hr)

Q=CIA

(cfs)

Long, Grade "So"

(ft/ft)

Previous Runby

(cfs)

Total Gutter Flow

(cfs)

Depth "d"

(ft)

SpreadWidth

"T"

(ft)

Grate Type

Intercept "Q"

(cfs)

Run-by "Q"

(cfs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

11-58

Table 11-8 Values of Runoff Coefficients (C) for use in the Rational Method

TYPE OF SURFACE RUNOFF

COEFFICIENT (C) 1

Rural Areas

Concrete or Asphalt Pavement 0.8-0.9

Gravel Roadways or Shoulders 0.4-0.6

Bare Earth 0.2-0.9

Steep Grassed Areas (2:1) 0.5-0.7

Turf Meadows 0.1-0.4

Forested Areas 0.1-0.3

Cultivated Fields 0.2-0.4

Urban Areas

Flat Residential, With About 30 Percent of Area Impervious 0.40

Flat Residential, With About 60 percent of Area Impervious 0.55

Moderately Steep Residential, With About 50 Percent of Area Impervious 0.65

Moderately Steep Built Up Area, With About 70 Percent of Area Impervious 0.80

Flat Commercial, With About 90 Percent of Area Impervious 0.80

1 For flat slopes or permeable soil, use the lower values. For steep slopes or

impermeable soil, use the higher values.

Step 9 Col 8 Compute a time of concentration for the first inlet. This will be the

travel time from the hydraulically most remote point in the drainage area to the inlet. Use the minimum of time concentration of 15 minutes unless using Figure 11-16 gives a value greater than 15 minutes. Enter this value in Col. 8.

Step 10 Col 9 Using available Intensity-Duration-Frequency (IDF) curves or table,

select a rainfall intensity for the duration (same as tc) and frequency of the design storm. Enter in Col 9. (See Table 11-9 or Figures 11-17 through 11-18C). For example, the value of I, rain intensity, for an inlet on a project in Rapid City with a maximum time of concentration (duration) of 15 minutes using a 10 year return period (Recommended Minimum Design Frequency for Storm Sewer) is 3.7 inches per hour.

11-59

Table 11-9 Rainfall Intensity - Duration - Frequencies (I) - (Inches per Hour) REGION

RETURN PERIOD

DURATION (minutes)

SIO

UX

CIT

Y,

IA

MO

OR

HEA

D,

MN

MIL

ES C

ITY,

M

T

VALE

NTI

NE,

N

E

BIS

MA

RC

K,

ND

HU

RO

N,

SD

PIER

RE,

SD

RA

PID

CIT

Y,

SD

YAN

KTO

N,

SD

5 6.0 5.0 5.5 5.4 5.7 5.4 5.6 4.6 5.2 10 4.8 4.0 3.7 4.4 4.6 4.2 4.3 3.8 4.2 15 4.1 3.4 2.8 3.8 3.8 3.4 3.5 3.2 3.6 20 3.6 3.0 2.3 3.3 3.3 3.0 3.1 2.6 3.2 25 3.2 2.7 1.9 2.9 2.8 2.6 2.7 2.3 2.8

5 Year

30 2.9 2.4 1.7 2.6 2.5 2.4 2.4 2.1 2.6 5 7.0 5.8 7.0 6.4 6.9 6.2 6.5 5.5 5.9 10 5.6 4.8 4.7 5.3 5.6 5.0 5.0 4.5 4.8 15 4.8 4.1 3.5 4.5 4.7 4.2 4.1 3.7 4.1 20 4.3 3.7 2.9 4.0 4.0 3.6 3.5 3.2 3.6 25 3.8 3.3 2.4 3.5 3.4 3.1 3.0 2.7 3.3

10 Year

30 3.4 3.0 2.1 3.2 3.1 2.8 2.7 2.5 3.0 5 8.0 6.6 8.9 7.5 8.5 7.4 7.5 6.6 6.6 10 6.5 5.5 5.9 6.4 7.0 5.9 5.8 5.5 5.5 15 5.6 4.8 4.5 5.5 5.8 4.9 4.8 4.6 4.8 20 5.0 4.3 3.7 4.8 5.0 4.3 4.1 3.9 4.3 25 4.5 3.8 3.0 4.2 4.3 3.7 3.5 3.4 3.9

25 Year

30 4.1 3.5 2.6 3.9 3.8 3.4 3.2 3.1 3.6 5 8.9 7.3 10.4 8.4 9.6 8.2 8.4 7.5 7.3 10 7.3 6.1 7.0 7.1 8.0 6.8 6.4 6.1 6.0 15 6.3 5.3 5.3 6.2 6.5 5.6 5.3 5.1 5.4 20 5.7 4.8 4.3 5.5 5.6 4.8 4.5 4.5 4.8 25 5.0 4.3 3.5 4.8 4.8 4.2 3.9 3.9 4.4

50 Year

30 4.6 4.0 3.0 4.4 4.3 3.8 3.5 3.5 4.0 5 9.7 8.0 11.5 9.5 10.1 9.0 9.0 8.2 8.0 10 8.0 6.8 7.8 8.1 8.6 7.3 7.0 6.8 6.6 15 7.0 5.9 5.9 7.0 7.3 6.2 5.8 5.8 5.9 20 6.2 5.3 4.7 6.2 6.3 5.3 5.0 5.0 5.3 25 5.6 4.8 3.9 5.5 5.5 4.7 4.3 4.3 4.8

100 Year

30 5.1 4.5 3.3 5.0 4.9 4.2 3.9 3.9 4.5

11-60

Figure 11-17 Intensity-Duration-Frequency Map

11-61

Figure 11-18A Rainfall Intensity-Duration-Frequency Curves (See Map at Figure 11-17)

11-62

Figure 11-18B Rainfall Intensity-Duration-Frequency Curves (See Map at Figure 11-17)

11-63

Step 11 Col 10 Multiply Col 6 X Col 7 X Col 9. Enter in Col 10.

Step 12 Col 13 For the first

inlet in a series enter Col 10 in Col 13 since no previous runby (bypass) has occurred yet.

Step 13 Col 11 Determine

the gutter slope at the inlet from the profile grade -

Figure 11-18C Rainfall Intensity – Duration - Frequency Curve (See Map Figure 11-17)

Check effect of superelevation. Enter in Col 11. Step 14 Col 15 Using HEC-12 or the available computer model, determine the

spread T and calculate the depth d at the curb by multiplying T times the cross slope(s). Composite curb section and curbed vane grate are commonly used. Compare with the allowable depth and spread as determined by the design criteria. If Col. 14 is less than the curb height and Col. 15 is near the allowable spread, continue on to Step 15. If the depth or the spread are greater than allowable, it will be necessary to reduce the drainage area upstream from the grate and repeat Steps 5 through 15. If the spread is substantially less than the allowable spread, the drainage area should be increased and Steps 5 through 15 should be repeated. Continue these repetitions until Columns 14 and 15 are near the allowable depth and spread, then proceed to Step 15.

11-64

Step 15 Col 16 Select the grate type and enter in Col. 16. Step 16 Col 17 Calculate the Q intercepted (Qi) by the grate and inter in Col 17.

Utilize the nomographs, charts, etc. from HEC-12 or the computer model available for these purposes.

Step 17 Col 18 Calculate the runby by subtracting Col 17 from Col 10 and enter into

Col 18. Step 18 Col 18 Proceed to the next inlet down grade. Select an area below the prior

inlet as a trial. Repeat Steps 5 through 8 considering only the area between the inlets.

Step 19 Col 8 Compute a time of concentration for the following inlet based on the

area between the two inlets. Step 20 Col 9 Determine the intensity (from the IDF curve) based on the time of

concentration determined in Step 19 and enter it in Col 9. Step 21 Col 10 Determine the discharge from this area by multiplying Col 6 X Col 7 X

Col 9. Enter the discharge in Col 10. Step 22 Col 13 Determine total gutter flow by adding Col 10 and Col 12. Col 12 is

the same as Col 18 from the previous line. Step 23 Col 15 Determine T based on total gutter flow in Col 13 by using HEC-12 or

the available computer model. If T in Col 15 exceeds the allowable spread, reduce the area and repeat Steps 18-23. If T in Col 15 is substantially less than the allowable spread increase the area and repeat Steps 18-23.

Step 24 Col 16 Select grate type. Step 25 Col 17 Determine Q1, by using HEC-12 or the computer model available and

enter in Col 17. Step 26 Col 18 Calculate the runby by subtracting Col 17 from Col 10. This

completes the design for this inlet. Step 27 Go back to Step 18 and repeat Steps 18 through 26 for each

subsequent inlet. If the drainage area and weighted C value are similar between each inlet, the values from the previous grate location can be reused. If they are significantly different recomputation will be required.

11-65

5. Inlets in Sag Locations

• Use Type S inlets (curb-opening) at low points. Typically use 10’ openings. Type B inlets (combination) may be used when the drainage is 1 cfs or less.

• Find the low point of a curve with the following equation:

L x G1 Distance to low point from PC = G1-G2

• Find the allowable depth of ponding (d). d = T(Sx) For 8’ spread, the allowable depth would equal 8' x .02 = .16'

• Find the capacity of the inlet based on the allowable depth of ponding.

A curb-opening inlet is in weir flow when d+a < h, and orifice flow when di > 1.4h. weir eq: Qi = Cw(L + 1.8W)d1.5 where: Cw = 2.3, W = gutter width, L = length of opening orifice eq: Qi = CoA(2gdo)0.5 where: Co = 0.67 do = di - h/2 A = hL di = d + a Standard Inlet parameters: Type S Type B Curb opening height (h) = 5.75” 4.5” Length of Opening (L) = 10’ or 5’ 2.96’ Grate Width = NA 1.48’ Depression from Sx,(a) = 4.4” 2.5” Depression from gutter = 3.4” 0.5” (Local depression input in HEC-12) Example: Find capacity of a 10’ Type S inlet with 8’ spread. Qi = Cw(L + 1.8W)d1.5 Qi = 2.3(10 + 1.8 x 2)0.16 1.5 Qi = 2 cfs intercepted

11-66

Notes: Depression (a) typically has no effect on our inlets in sag locations, since they are rarely in orifice flow with the recommended spreads. Orifice flow will be used more with area and median drains.

Depression does have a large impact on the intercepted flow capacity of Type S

inlets (curb-opening) when placed on a longitudinal road grade (on-grade). The capacity of a Type B inlet is based on inflow through the grate only and

ignores the curb-opening when located in either a sag or on-grade, unless the water depth in a sag puts it in orifice flow.

6. Flanker Inlets Where significant ponding can occur, in locations such as underpasses and in sag vertical curves in depressed sections, it is good engineering practice to place flanking inlets at each side of the inlet in the low point. K = L where L = curve length A A = Algebraic difference in Grades Make sure K < 167. Flankers should be offset a distance x = (200 Kd)1/2 Where d = allowable depth of ponding When inlet locations are known, connect the inlets with pipe in a manner that uses the least number of crosspipe. 7. Determine the subgrade elevations at each inlet location and the length of pipe

between each inlet the storm sewer is going to flow. These elevations will be used in the PIP command.

8. Enter known data into HYDRA program. Common commands used in the

HYDRA program are as follows: Command Parameters JOB - Job Title - job title REM - Remark - any alpha/numeric MAP - Map Scale - scale PSZ - Pipe size in inches PDA - Pipe Data - n, min diam, ideal depth, min cover, min vel, min

slope

11-67

SWI - 2 = storm (Rational Only) RAI - Rainfall Data - time, intensity, time, intensity,... NEW - Start New lateral - lateral name PNC - Pipe-Node Connection-unode, dnode, bdn, angle, nodtyp,

bench STO - Storm Flow - acres, runoff coefficient, min time to inlet (15

minimum) FLO - Miscellaneous Flow PIP - Circular Pipe - length, grup, grdn, invup, invdn HOL - Hold lateral - number REC - Recall Lateral - number See "HYDRA" for additional commands and command definitions. 9. In the PIP Command, the invup and invdn parameters may be left blank for an

initial run. HYDRA will set flow line elevations using criteria set in the PDA command, and taking into account any upstream requirements. After the initial run the designer will insert desired invup and invdn elevations as described in the storm sewer grade section in this chapter.

10. The designer will check that when designing storm sewer that the size of pipe

increases as the flow increases. It is undesirable to install for example an 18" (450 mm) pipe, then 24" (600 mm) and return back to an 18" (450 mm) with an increase of flow. The designer shall then adjust the flow lines so that the storm sewer does increase as the flow increases and if impossible remain with the larger size pipe rather than downsizing.

11. ENERGY GRADE LINE (EGL) / HYDRAULIC GRADE LINE (HGL) DESIGN

PROCEDURE The HYDRA computer program is recommended for design of storm drains and

will include a hydraulic grade line analysis and a pressure flow simulation. A step by step procedure is given to manually compute the hydraulic grade line. The hydraulic gradeline computation form in Table 11-10 is used to list data obtained from following this procedure.

HGL is defined as a line representing the potential energy of the water flow. In

closed pipes, it is also called the pressure line. EGL is a line representing the total energy (potential plus kinetic) of the water flow. The difference between the HGL and EGL is the velocity head.

If the hydraulic grade line is above the pipe crown at the next upstream manhole,

pressure flow calculations are indicated; if it is below the pipe crown, then open channel flow calculations should be used at the upstream manhole. The process is repeated throughout the storm drain system. If all HGL exceeds an inlet

11-68

elevation, then adjustments to the trial design must be made to lower the water surface elevation.

Step 1 Identify the station for the junction immediately upstream of the

outflow pipe. HGL computations begin at the outfall and are worked upstream taking each junction into consideration.

Step 2 Identify the tailwater elevation if the outlet will be submerged during the design storm, otherwise refer to the tailwater discussion presented earlier.

Step 3 Identify the diameter (D°) of the outflow pipe. Step 4 Identify the design discharge (Q) for the outflow pipe. Step 5 Identify the length (L) of the outflow pipe. Step 6 Identify the outlet velocity of flow (V). Step 7 Compute the velocity head, V2

°/2g, of the outflow pipe. Step 8 Compute the exit loss, H°. Step 9 Compute the friction slope (SF°) in ft/ft of the outflow pipe. NOTE:

Assumes full flow conditions. Step 10 Compute the friction loss (Hf) by multiplying the length (L) in Step 5

by the friction slope (SF°) in Step 9. On curved alignments, calculate curve losses by using the formula Hc = (0.0033) (delta) (V2

°/2g), where delta = angle of curvature in degrees, and add to the friction loss.

Step 11 Compute the initial head loss coefficient, K° based on relative

manhole size. Step 12 Compute the correction factor for pipe diameter, CD. Step 13 Compute the correction factor for flow depth, Cd. Note this factor is

only significant in cases where the d/D° ratio is less than 3.2. Step 14 Compute the correction factor for relative flow, Cq. Step 15 Compute the correction factor for plunging flow, Cp. The correction factor is only applied when h>d. Step 16 Select the correction factor for benching, Cbp.

11-69

Step 17 Compute the value of K. Step 18 Compute the value of the total manhole loss, K(V2

°/2g). Step 19 If the tail water submerges the outlet end of the pipe, add Step 2

(TW elevation) and Step 7 (Velocity head) to get the energy grade line (EGL) at the outlet end of the pipe. If the pipe is flowing full, but the tailwater is low, the EGL will be determined by adding the velocity head to (dc+D)/2.

Step 20 Once the EGL has been determined at the outlet end of the pipe, the

losses (friction loss from Step 9, bend losses, etc.) in the pipe between the outlet end and the inlet end of the pipe reach are determined and added to the EGL determined at the outlet end to obtain the EGL at the inlet end.

Step 21 Losses in the manhole as determined in Step 18 are added to the

EGL at the upstream end of the downstream pipe and this becomes the EGL for the downstream end of the upstream pipe.

Step 22 Continue to determine the EGL through the system adding losses as

they occur. Step 23 Determine the hydraulic grade line (HGL) throughout the system.

This is done by subtracting the velocity head at any point from the EGL line at that point. Check to make certain that the HGL is below the level of the allowable high water at that point. If the HGL is above the finished grade elevation, water will exit the system (potentially blow the manhole lid) at this point for the design flow.

NOTE: The above procedure applies to pipes that are flowing full, as should be

the condition for design of new systems. If a part full flow condition exists, the EGL is located one velocity head above the water surface.

11-70

Table 11-10 Hydraulic Gradeline Computation Form

STA

TW

Do

Qo

Lo

Vo

Vo

2/2g Ho

SFo

Hf

Ko

CD

Cd

CQ

CP

CB

K

K(Vo

2/2g) EG Lo 2+7

EGL1 10+18+19

HGL EGL-7

ELEV

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)

11-71

DETENTION PONDS When the discharge of a drainage system is regulated by the community or the channel characteristics, the flow can be reduced by detention ponds. Basic tools required for detention pond design are: 1. Inflow hydrograph 2. Stage-discharge relationship 3. Stage-storage relationship There are several different facilities that can be used as detention ponds. The most simple and cost effective is an earth basin. A small culvert acts as an outlet structure, while an emergency spillway is provided for larger storms. Natural terrain may be incorporated into basin design.

An example of the detention pond design is given on pages Ex-1 through EX-21 of the Urban Drainage Design Manual (Publication No. FHWA-HI-89-035). The computer program SCS-TR-55 is available for the design of detention basins.

11-72

ROADSIDE CHANNELS SDDOT uses the procedures set forth in Hydraulic Engineering Circular No. 15 (HEC 15) for the design of roadside channels with flexible linings. This manual covers channel lining design for discharges of 50 cfs (1.42 m3/s) or less. The use of the procedures in HEC 11 is recommended for the design of erosion control for channels with design flows in excess of 50 cfs. Typical roadside ditch sections vary between projects with most standard ditch configurations found in the cross section chapter. Proposed channels that are not within the highway ditch section typically have dimensions with a 10' bottom and 5:1 side slopes. Hydraulic conditions in a drainage channel can become erosive even at fairly mild highway grades. As a result, these channels often require stabilization against erosion. The following computation procedures are used to determine areas of erosion problems. Shear Stress Computations Designers shall analyze drainage locations throughout the project with potential erosion problems. The types of locations to consider are as follows:

- Existing erosion areas - Ditch grades steeper than 4% - Drainage areas greater than 10 acres - Roadway embankments that transition from a ditch cut to a fill - Toe of fills that result in a v-ditch - Narrow channel sections (steep sided v-ditches)

In addition to the above suggested locations, sometimes even small concentrated flows in a channel should be considered depending on the channel grade and configuration. Shear stress in a channel is the tractive force caused by water flowing in the channel. A channel is unstable when the flow-induced shear stress exceeds the permissible shear stress of the channel liner material. The maximum shear stress on the channel bed occurs at the maximum water depth and is computed with the following equation: τ = γ d S where: τ = shear stress (psf) γ = unit weight of water (62.4 pcf) d = maximum depth of flow (ft) S = channel slope (ft/ft) The maximum water depth can be computed by trial and error using Manning’s equation for uniform channel flow.

11-73

The Road Design Office has developed spreadsheets to compute the normal water depth, shear stress and water velocity at the required locations for a SDDOT 20’ standard ditch, flat bottom ditch, or v-ditch. Shear stress shall be analyzed for a 10-yr flood event, and the discharges shall be computed using the same methods required for culvert design, which varies based on drainage area size. Drainage areas less than 150 acres require that the discharge be computed from the small pipe program by inputing the ditch drainage area and a 10-yr event. Shear stress computations shall use a Mannings ‘n’ value of 0.035 for vegetative channels, even though actual ‘n’ values may vary from this. The Designer shall provide these spreadsheets or similar information to the Road Design Erosion Control Staff prior to scheduling a Final Design Inspection for their consideration to install permanent erosion control measures. The computer program “Design of Roadside Channels with Flexible Linings”, which follows procedures in HEC 15, is also available to compute shear stress and other channel parameters for trapezoidal and triangular channels. However, this program cannot model the SDDOT standard ditch section. This program requires the following input: 1. Discharge (cfs) 2. Channel Slope (ft/ft) (m/m) 3. Channel Bottom Width (ft) (m) 4. Left Side Slope 5. Right Side Slope 6. Channel Lining Type Permanent Linings 1. Vegetation (need retardance type, A-E) see Table 11-

11 2. Rock Riprap (average diameter) 3. Concrete (n = 0.013) Temporary Linings 4. Paper Net 5. Jute Net 6. Curled Wood Net 7. Fiberglass Roving-Single 8. Fiberglass Roving-Double 9. Synthetic Mat Special Linings 10. Composite (need low flow lining, side slope lining, low-

flow channel depth, low flow channel perimeter.

11-74

The program will give the following output:

1 Permissible shear 2. Calculated shear 3. Hydraulic radius 4. Manning coefficient 5. Normal depth 6. Velocity

Channel Slope The channel bottom slope is generally dictated by the roadway profile, and therefore is usually fixed. If channel stability conditions warrant and available linings are not sufficient, it may be feasible to reduce the channel gradient slightly relative to the roadway profile. For channels outside the roadway right-of-way, the slope may be adjusted slightly. Channel slope is one of the major parameters in determining shear stress. For a given design discharge, the shear stress in the channel with a subcritial slope is smaller than one with supercritical slope. Roadside channels with gradients in excess of about 2 percent will flow in a supercritical state. Most flexible lining materials are suitable for protecting channel gradients of up to 10 percent. Riprap and wire-enclosed riprap are more suitable for protecting very steep channels with gradients in excess of 10 percent. Channel Bends Flow around a bend in an open channel induces centrifugal forces because of the change in flow direction. This results in a superelevation of the water surface. The water surface is higher at the outside of the bend than at the inside of the bend. This superelevation can be estimated by the equation: ∆d = V2 T = Superelevation of the water surface G Rc Where: V = mean velocity T = surface width of the channel G = gravitational acceleration Rc = mean radius of the bend

11-75

Freeboard The freeboard of a channel is the vertical distance from the water surface to the top of the channel at design condition. The importance of this factor depends on the consequence of overflow of the channel bank. At a minimum the freeboard should be sufficient to prevent waves or fluctuations in water surface from overflowing sides. In a permanent roadway channel, about one-half foot of freeboard should be adequate, and for temporary channels, no freeboard is necessary. Steep gradient channels should have a freeboard height equal to the flow depth. This allows for large variations to occur in flow depth for steep channels caused by waves, splashing and surging. Lining materials should extend to the freeboard elevation. The Designer shall provide the freeboard elevations to the Erosion Control Designer at the critical locations.

11-76

Table 11-11 Classification of Vegetal Covers as to Degree of Retardance

Retardance Class Cover Condition

A Weeping lovegrass ………….. Yellow bluestem Ischaemum ……..

Excellent stand, tall (average 30”) (76 cm) Excellent stand, tall (average 36”) (91 cm)

B

Kudzu ……………………………. Bermuda grass ………………………… Native grass mixture (little bluestem, bluestem, blue gamma, and other long and short midwest grasses) ………………………. Weeping lovegrass …………. Lespedeza sericea …………. Alfalfa ……………………………. Weeping lovegrass …………….. Kudzu ……………………… Blue gamma …………………

Very dense growth, uncut Good stand, tall (average 12”) (30 cm) Good stand, unmowed Good stand, tall (average 24”) (61 cm) Good stand, not woody, tall (average 19”) (8 cm) Good stand, uncut (average 11”) (28 cm) Good stand, unmowed (average 13”) (33 cm) Dense growth, uncut Good stand, uncut (average 13”) (28 cm)

C

Crabgrass …………….. Bermuda grass ………….. Common lespedeza ……… Grass-legume mixture – Summer (orchard grass, redtop, Italian ryegrass, and common lespedza) ………. Centipedegrass …….. Kentucky bluegrass ……

Fair stand, uncut (10 to 48”) (25 to 120 cm) Good stand, mowed (average 6”) (15 cm) Good stand, uncut (average 11”) (28 cm) Good stand, headed (6 to 8 inches) (15 to 20 cm) Very dense cover (average 6 inches) (15 cm) Good stand, headed (6 to 12 inches) (15 to 30 cm)

D

Bermuda grass ………… Common lespedeza ………. Buffalo bluegrass ………… Grass-legume mixture – Fall, spring (orchard grass, redtop, Italian ryegrass, and common lespedeza) …….. Lespedeza sericea …….

Good stand, cut to 2.5 inch height (6 cm) Excellent stand, uncut (average 4.5” (11 cm) Good stand, uncut (3 to 6 inches) (8 to 15 cm) Good stand, uncut (4 to 5 inches) (10 to 13 cm) After cutting to 2” height (5cm). Very good stand before cutting.

E Bermuda grass …………… Bermuda grass ……………

Good stand, cut to 1.5 inch height (4cm) Burned stubble

NOTE: Covers classified have been tested in experimental channels. Covers were green and generally uniform. (ref.: Table from Handbook of Channel Design for Soil and Water Conservation SCS-TP-61 revised June 1964) NOTE: Most of the grasses listed in the above table are not allowed in South Dakota DOT seed mixtures. Typical vegetative grasses used in South Dakota highway ditches have a Retardance Class of B.

11-77

Table 11-12 Permissible Shear Stresses for Lining Materials

PERMISSIBLE UNIT SHEAR STRESS

LINING CATEGORY LINING TYPE (lb/ft2) (Kg/m2) Temporary Woven Paper Net 0.15 0.73 Jute Net 0.45 2.20 Fiberglass Roving Single 0.60 2.93 Double 0.85 4.15 Straw With Net 1.45 7.08 Curled Wood Mat 1.55 7.57 Synthetic Mat 2.00 9.76 Vegetative Class A 3.70 18.06 Class B 2.10 10.25 Class C 1.00 4.88 Class D 0.60 2.93 Class E 0.35 1.71 Gravel Riprap 1-inch 0.40 1.95 2-inch 0.80 3.91 Rock Riprap 6-inch 2.50 12.21 12-inch 5.00 24.41 Bare Soil Non-cohesive See

HEC 15 See

HEC 15 Cohesive See

HEC 15 See

HEC 15

11-78

Table 11-13 Manning’s Roughness Coefficients for Channel Liners

Lining Category

Lining Type

n – value1 Depth Ranges

0-0.5 ft 0.5.20 ft >2.0 ft (0-15 cm) (15-60 cm) (>60 cm)

Rigid Concrete 0.015 0.013 0.013 Grouted Riprap 0.040 0.030 0.028 Stone Masonry 0.042 0.032 0.030 Soil Cement 0.025 0.022 0.020 Asphalt 0.018 0.016 0.016 Unlined Bare Soil 0.023 0.020 0.020 Rock Cut 0.045 0.035 0.025 Temporary Woven Paper Net 0.016 0.015 0.015 Jute Net 0.028 0.022 0.019 Fiberglass Roving 0.028 0.021 0.019 Straw With Net 0.065 0.033 0.025 Curled Wood Mat 0.066 0.035 0.028 Synthetic Mat 0.036 0.025 0.021 Gravel Riprap 1-inch (2.5 cm) D50 0.044 0.033 0.030 2-inch (5 cm) D50 0.066 0.041 0.034 Rock Riprap 6-inch (15 cm) D50 0.104 0.069 0.035 12-inch (30 cm) D50 -- 0.078 0.040

Table 11-14 Manning’s Roughness Coefficients for Small Natural Stream Channels Manning ‘n’ Natural Stream Channels: 1. Fairly regular section: a. Some grass and weeds, little or no brush 0.03-0.035 b. Dense growth of weeds, depth of flow much greater than weeds 0.035-0.05 c. Some weeds, light brush on banks 0.035-0.05 d. Some weeds, heavy brush on banks 0.05-0.07 e. Some weeds, dense willows on banks 0.06-0.08 f. For trees within channel, with branches submerged at high stage, increase all above values by 0.01-0.02 2. Irregular sections, with pools, slight channel meander; increase values given above about 0.01-0.02 3. Mountain streams, no vegetation in channels, banks usually steep, trees and brush along banks submerged at high stage:

a. Bottom of gravel, cobbles, and few boulders 0.04-0.05 b. Bottom of cobbles, with large boulders 0.05-0.07 Flood Plains (adjacent to natural streams): 1. Pasture, no brush: a. Short grass 0.03--0.04 b. High grass 0.035-0.05 2. Cultivated areas: a. No crop 0.03-0.04 b. Mature crops 0.035-0.05 3. Heavy weeds, scattered brush 0.05-0.07 4. Light brush and trees 0.05-0.08 5. Medium to dense brush 0.08-0.10 6. Dense willows, summer, not bent over by current 0.15-0.20