The purpose of a storm drainage system is to collect and transport storm water runoff from the highway to an outlet. The complete system consists of the curb and gutter, inlet structures, lateral and trunk line pipes, and junctions and manholes. The design process for storm drainage systems usually follows the basic steps of planning/data collection, hydrologic/hydraulic design, and outfall analysis. The procedure presented herein will be directed toward non-computer analysis. The pavement and inlet design may be accomplished by a computer program which follows the procedures of HEC 22(7). GEOPAK Drainage is an acceptable automated analysis process for storm drainage system design.
Information gathered during the pre-design study and field surveys that is of particular relevance to the storm drainage system should be assembled for design reference. Planning includes the identification of controls and criteria which must be considered in accomplishing the design. This would include:
(a) Design Frequency
Roadway inlet location, capacities and gutter spread is to be analyzed using a standard rainfall intensity of 4.0 inches/hour. The storm drain pipe system is to be designed using a Q10 discharge with a minimum time of concentration of 10 minutes assuming 100% pick up at each inlet.
In sag areas where relief by curb overflow is not provided the system standard design level (Q25-Q50) is to be used for analysis to insure traffic flow is not interrupted.
(b) Gutter
Gutter Grade
A minimum gutter gradient of 0.20 percent(0.30 desirable) shall be utilized. When lesser slopes are encountered, the gutter shall be warped to provide the minimum slope. A continuous inlet system such as a slotted or trench drain may be used in sag or low gradient gutter sections.
(c) Inlets
The standard inlet for curb and gutter is a combination grate and curb opening (std. no. 840.01 of Roadway Standard Drawings-(17). Use of other type inlets for curb sections require project specific approval.
Standard grated drop inlets shall be used in roadway ditches, non-curbed shoulders and other off roadway locations. Grates of 2 inch or less(small dimension opening) shall be used in areas subject to pedestrian traffic. Traffic bearing grates are to be used for drop inlets within 4 feet of a permanent or temporary travel lane.
The following specific criteria shall be followed in inlet analysis.
- On grades, the curb opening can be ignored in determining inlet capacity. The grate efficiency shall be assumed to equal a parallel bar grate.
- Inlet capacity at sags shall allow for debris blockage by providing twice the required computed opening.
- Inlet spacing shall be sufficient to limit spread to no more than half of a through lane during a 4.0 inch per hour rain storm.
- When the typical section includes a full shoulder or parking lane, no encroachment into the travel lane will be allowed.
- Depth in gutter shall not exceed 5 inches for design flow.
- While there is no maximum spacing for inlets, no trunk line pipe should extend more than 500 feet without access. An exception is made for median and side ditch systems where 700 feet is an acceptable upper limit.
- Pipe systems shall not decrease in size in the downstream direction.
- Provide 0.5 foot minimum from hydraulic grade line to top of inlet grate or junction.
(d) Pipe System
Storm drain pipes shall be concrete unless a site limitation such as grade or corrosive conditions dictate the use of an alternate material. The minimum pipe size to serve a single inlet is 12 inches. For more than one inlet, or a length of more than 100 feet, a 15 inch pipe is the minimum size.
When differing size pipes enter and exit a junction the desired practice is to match the crowns of the pipes.
Storm drainage system design is a two phase process involving first a selection of the required surface inlets, followed by the design of a subsurface pipe system to serve the surface pickups. Automated design systems such as GEOPAK Drainage provide an advanced tool for storm drainage design. However, the following basic design procedure is applicable and can be used for non-automated design, or as a guide to the designer in understanding the analysis process so that he can better interpret the output from an automated design. A similar design procedure is presented in HEC-22(7).
(a) Inlets
(1) Prior to commencing the hydrologic/hydraulic analysis of the surface system a layout of locations requiring inlets should be developed on a set of plans. This would include sag points, upstream of intersections, upgrade of superelevation rollovers, and at locations required to junction back-of-the curb pickups.
(2) With the above noted locations determined, the next step is to analyze the runoff and spread along the roadway to establish additional required inlet locations to meet spread and depth criteria. The hydrologic method used shall generally be the rational formula and will follow the guidance of Chapter VI (Hydrology). The general procedure as outlined in Chapter V (Drainage Plans) shall be used to confirm drainage boundaries, flow paths, outlet conditions and other project special design features.
The design is to be documented on a form similar to Appendix I (Sheet 1 of 7)
. The inlets should be numbered in a logical ascending order and their location referenced to a project station.
(b) Pipe System
(1) The next step is the layout of a pipe system to provide a connecting route of flow from the inlet(s) to the proper outlet point(s).
(2) Sizing of the individual pipes is now accomplished. The following procedure involves a run through the system from beginning to end with selection of pipe sizes by utilizing Mannings' flow capacity equation, with the limitations on maximum pipe capacity presented in Appendix I, sheet 6.Sizing of most systems by this procedure is generally sufficient.
While a check of the system by development of a hydraulic grade line requires minimum additional design time when utilizing an automated design process such as GEOPAK Drainage, a manual procedure can be very time consuming. Therefore, the engineer must evaluate and justify the need for a hydraulic grade line check of a system on a case by case basis. Conditions that may warrant undertaking this additional design analysis are:
- Systems with outlets that are subject to high tailwater conditions.
- Systems that transition from a steep to flat gradient.
- Systems on flat gradient that have substantial junction and/or bend
losses.
Pipe System Design Procedure
Reference Appendix I, sheet 2, for initial system design documentation.
Items 1 - 2. These are inlet numbers corresponding to inlet computation sheet.
Item 3. Total drainage area served by the section of pipe.
Item 4. Sum of the incremental portions of the drainage area and corresponding runoff coefficients.
Item 5. Length of the pipe run between study points.
Item 6. Time of concentration for portion of drainage
area in-flowing at beginning end of pipe.
Item 7. Flow time for first pipe equals inlet time.
Subsequent sections are a sum of the time of concentration of the previous reach (min. tc = 10 minutes) plus time of flow in subject pipe.
Item 8. Larger value from Items 6 and 7. Use 10 minutes as minimum value. For times greater than 30 minutes, a flood hydrograph or other routing procedures is recommended.
Item 9. Design storm rainfall intensity for duration equal to design time.
Item 10. Design discharge for pipe reach.
Item 11. Invert elevation of pipe inlet.
Item 12. Invert elevation of pipe outlet.
Item 13. Invert slope of pipe.
Item 14. Diameter of pipe. This size is to be selected
utilizing Mannings’ full flow capacity equation. Q = 0.46/n (D2.67)(So0.5)
A nomograph solution for this equation is provided in Appendix I, sheet 3.
The capacity utilized for design cannot exceed the values contained in the table - Appendix I, sheet 6
Item 15. Velocity based on design discharge and selected pipe size (can use charts Appendix I).
Item 16. Remarks.
Hydraulic Grade Line Development Procedure
A Hydraulic grade line will provide the potential elevation, under design conditions, to which water will rise in the various inlets and junctions. This can serve as a check for potential unacceptable outflow or pressure problem areas within the system dictating a change in the system design.
Reference, Appendix I, sheet 7, for tabulation of the procedure.
Item 1. The inlet number or junction location immediately
upstream of the outlet.
Item 2. Water surface elevation at outlet or 0.8D + invert elevation of the outflow pipe, whichever is greater.
Item 3. Diameter (Do) of outflow pipe.
Item 4. Design discharge (Qo) for the outflow pipe.
Item 5. The length (Lo) of the outflow pipe.
Item 6. Friction loss (Hf) for full pipe flow. Loss due to flow in the pipe can be computed by multiplying pipe length (Lo) x friction slope (Sf). Friction slope can be determined from pipe flow charts or by using the formula:
Sf = [Q/K]2
K = 1.486/n (AR0.67)
Appendix I, sheet 5 provides values of (K) for various pipe sizes.
Item 7. Contraction loss (Hc). Loss due to contraction
of flow at inlet of outflow pipe. Computed by the formula:
Hc = 0.25 (Vo2/2g)
Where: Vo = Flow velocity in outlet pipe(full flow)
Item 8. Expansion loss (He). Loss due
to expansion of flow into the junction. Use expansion loss from primary inflow line.
He = 0.35 (Vi2/2g)
Where: Vi = Flow velocity in inlet pipe(full flow)
Item 9. Bend loss (Hb) loss due to change
in direction of flow. Use change in angle of primary flow line.
Hb = K (Vi2/2g)
90 degrees K = 0.70 40 degrees K = 0.38
80 degrees K = 0.66 30 degrees K = 0.28
70 degrees K = 0.61 25 degrees K = 0.22
60 degrees K = 0.55 20 degrees K = 0.16
50 degrees K = 0.47 15 degrees K = 0.10
Item 10. Total losses (Ht), sum of friction, contraction,
expansion, and bend losses.
Item 11. Inlet water surface elevation. This is the potential water surface elevation within the inlet or junction.
Item 12. Inlet rim elevation or top of junction. The water surface elevation is to be a minimum of 0.5 feet below this elevation. If not, the pipe size should be increased or other measures taken to reduce the water level.
Item 13. Remarks. Repeat the procedure for the upstream
junction and plot the potential water surface elevation if above the crown elevation of the outlet pipe.
The storm drainage system design must include an evaluation of the downstream receiving channel or system to determine its adequacy. This evaluation should address:
NC DOT
NC DOH
NC GOV
