1. Stormwater Conveyance Design 1
John Reimer
City of Madison
Engineering Department

2. Hydrologic Cycle

3. Stormwater Runoff
Water falls as rain, snow, or ice. Most seeps into ground.
If ground is saturated, frozen, or has paved surfaces, water flows & is called stormwater runoff.
Stormwater Flows over surfaces such as roads, driveways and parking lots.

4. Where does Stormwater Go?
Stormwater flows to storm drains along streets.
It may carry soil, pet waste, oil, pesticides, & other pollutants with it.
This polluted runoff goes to streams & lakes untreated.

5. Stormwater Quantity Impacts
Impervious surfaces cause higher runoff volume
Increased flooding potential
High velocity stormwater runoff causes stream erosion
Stream erosion causes habitat and property loss
Changes to streambed morphology
Spring Street, Madison, WI 2008

6. Stormwater Quantity Impacts
Stable Channel Downcutting Widening Sedimentation Stable, Entrenched
Pre- to
Post-Development

7. Growth and Development
Urbanization happens…
Madison, WI
1867
2013

8. Stormwater and Urbanization
Arrow lengths indicate increase/decrease
Urbanization Runoff
Hydrograph
Post-development
Increases Peak
Increase Volume
Decreases Time to Peak
Discharge
Pre-development
Time

9. Flow Hydrograph
Area under hydrograph represents volume of surface runoff
Qpeak

10. Separate Storm Sewer
Street Runoff
Inlet
Pipe Flow
Combined Sewer

11. Stormwater Management
Stormwater Runoff
Urbanization
Stormwater Controls
Design and Review Focus

12. Methods of Calculating Runoff
Statistical analysis of streamflow records
Regional methods
Transfer methods
Rational Method
Tabular Method (TR-55)
Computer Models

13. Statistical Analysis of Streamflow Records

14. Regional Methods Q100=48*A0.660*S0.349*I0.172
Correlation of a dependent variable, usually a discharge of specified recurrence interval, with causative or physically related factors such as watershed area and stream slope for a specified area
Assumption:
Underlying assumption in a regional method is that the watershed to which it is being applied is one of or similar to those waterhsed that were used to develop the method
Procedure:
Watershed parameters such as area, stream slope, and percent imperiousness are determined and quantified using appropriate data sources. These parameters are then substituted into the regional method equations and flows of specified recurrence interval are computed
Example:
Q100: 100-year recurrence interval discharge
A: Drainage Area
S : Channel Slope
I: Impervious area
Q100=48*A0.660*S0.349*I0.172

15. Transfer Method
A discharge of specified recurrence interval for tributary area of known size and runoff characteristics is used to estimate a discharge of the same recurrence interval for a larger or smaller watershed having similar runoff characteristics.

16. Rational Method Qp = CiA
C = runoff coefficient also called the “coefficient of imperviousness”
i = rainfall intensity (in/hr)
A = drainage area (acres)
(tc = time of concentration < rainfall duration)

17. C-Typical Values (by surface)
Forested
Asphalt
Brick
Concrete
Shingle roof
Lawns, well drained (sandy soil)
up to 2% slope
2% to 7% slopes
over 7% slope
Lawns, poor drainage (clay soil)
up to 2% slope
2% to 7% slopes
over 7% slope
Driveways, walkways
Ref: Civil Engr. Ref. Manual, 6th ed., Michael Lindeburg, PE, Professional Publications, Inc. ISBN:

18. C-Typical Values (by use)
Farmland or Pasture
Unimproved
Parks or Cemeteries
Railroad Yard
Playgrounds (not asph or conc)
Business Districts
neighborhood
city (downtown)
Residential
single family
multi-plexes, detached
multi-plexes attached
suburban
apartments, condominiums
Industrial
light
heavy
Ref: Civil Engr. Ref. Manual, 6th ed., Michael Lindeburg, PE, Professional Publications, Inc. ISBN:

19. Rational Coefficient C
Must be weighted if you have different area types within the drainage area
Drainage area = 8 acres:
2 acres; C=0.35 (residential suburban)
6 acres; C=0.2 (undeveloped-unimproved)
Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24

20. TR-55 Approach (See previous lecture for detail calculations)
Curve Number Approach
Soil Group A, B, C, D from coarse grain to fine grain
Land Cover
Hydrologic Condition
Poor-Higher runoff
Good-Lower runoff
Antecedent Moisture Condition
Cropping practice
Agriculture
Download WinTR-55

21. Computer Models Storm Water Management Model (SWMM) Developed by EPA
Rainfall-runoff simulation model used for single event or long-term simulations
The routing portion of SWMM transports runoff through a conveyance system of pipes, channels, storage, pumps, and regulators

22. Stormwater Flow in Street
The hydraulic capacity of a pavement cross section is the quantity of stormwater flow that does not exceed an allowable interference criteria for width of spread or depth at the curb
Design approach is to selct a location for analysis and to determine the width T and depth d of flows
Manning equation, with modification, is used. Because T may be 40 times the depth d, the hydraulic radius R can be set equal to the depth d. When integrated across the width of flow, the resulting modified equation is:
Q = K(Z/n)So1/2d8/3
K=0.375 metric or 0.56 English
n=Manning’s coefficient
Q=Stormwater flow
Z=Reciprocal of the cross slope T/d
So=Longitudinal grade
D=Stormwater flow depth

23. Catchbasins and Inlets
Admit runoff to sewer system
Should be designed not just placed
Different types for different applications
Inlet
Catchbasin

24. Detail of Inlet

25. Inlet Design Orifice Weir Inlet computations involve three elements:
Quantity of stormwater aproaching inlet
Quantity of stormwater intercepted by the inlet
Quantity of stormwater that bypasses inlet and is carried over
Orifice
Weir

26. Detail of Catch Basin

27. Storm Sewer Design (Inlet and Manholes)
Inlet Spacing
Is needed and depends on street slope, crown, permissible spread, and bypass flows
Manhole Spacing
Needed at junctions of pipes, grade changes, and alignment changes
Commonly every 300’ to 400’

28. Storm Sewer Design (Energy Considerations)
Energy grade line is assumed to be parallel to the sewer slope
Generally slopes of the sewer are kept to a minimum sufficient to maintain proper flow situations. Storm slopes are frequently parallel to the ground surface
Transition through junctions/manholes are made as streamline as possible in order to minimize turbulence

29. Storm Sewer Design (Minimum & Maximum Velocities)
Velocities must be kept above the minimum scour velocity in order to prevent deposition of solids in the sewer
For storm sewers, the minimum velocities should be 2 ft/sec or greater
Maximum velocities at full flow should generally be less than 15 ft/sec in order to prevent excessive erosion of the pipe material

30. Storm Sewer Design (Pipe Materials)
Various shapes are available:
Corrugated Metal Pipe (CMP)
Concrete (RCP)
PVC
High Density Polyethylene (HDPE)

31. Pipe Friction Losses
Major head loss in a storm sewer collection system is that caused by friction along the periphery of the pipe
Most apply Manning’s formula:
Darcy-Weisbach equation (only used when a pipe is flowing under pressure):
Sf=V2n2/2.22 R4/3
hf=SfL
hf=f(L/D)(V2/2g)

32. Minor Losses
Occur whenever turbulence, other than friction, is introduced into the system.
Can occur at bends, entrances, junctions, manholes, outlets, and transitions
Can be determined by using energy equations or can be estimated by using simpler methods as discussed in previous lectures with gravity sanitary sewer design

33. Minor Head Loss (Energy Equation)
Q1V1
Q2V2

34. Minor Head Loss (Derived from Bernoulli and Darcy-Wisbach Equations)
General Form
No Change in Pipe Sizes
Manhole Losses
Flow
Bend Losses
Flow f
Angle Less than 40o

35. Types of Flow Control
Based on a variety of laboratory tests and field experience, two basic types of flow control have been defined for culverts:
Inlet control
Inlet control occurs when the culvert barrel is capable of conveying more flow than the inlet will accept. Upstream water surface elevation and inlet geometry are the primary factors influencing culvert capacity
Outlet Control
Outlet control occurs when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept. All the geometric and hydraulic characteristics of the culvert play a role in determining culvert capacity.

36. Inlet Control
The flow passes through critical depth just downstream of the culvert entrance and the flow in the barrel is supercritical.
The submergence of the outlet end of the culvert does not assure outlet control. In this case, the flow just downstream of the inlet is supercritical and a hydraulic jump forms in the culvert barrel.
The inlet end is submerged and the outlet end flows freely. Again, the flow is supercritical and the barrel flows partly full over its length.
A hydraulic jump will form in the barrel. The median inlet provides ventilation of the culvert barrel. If the barrel were not ventilated, sub-atmospheric pressures could develop which might create an unstable condition during which the barrel would alternate between full flow and partly full flow.

37. Outlet Control
Classic full flow condition, with both inlet and outlet submerged. The barrel is in pressure flow throughout its length.
Outlet submerged with the inlet unsubmerged. For this case, the headwater is shallow so that the inlet crown is exposed as the flow contracts into the culvert
Entrance submerged to such a degree that the culvert flows full throughout its entire length while the exit is unsubmerged. This is a rare condition, it requires an extremely high headwater to maintain full barrel flow with no tailwater. Outlet velocities are unusually high under this condition
The culvert entrance is submerged by the headwater and the outlet end flows freely with a low tailwater. For this condition, the barrel flows partly full over at least part of its length (subcritical flow) and the flow passes through critical depth just upstream of the outlet
With neither the inlet nor the outlet end of the culvert submerged. The barrel flows partly full over its entire length, and the flow profile is subcritical

38. Culvert Design Modeling
Computer Model (HY 8)
Developed by the Federal Highway Administration (FHWA)
Interactive culvert analysis program to compute the culvert hydraulics and water surface profiles for various pipe shapes
Users can analyze inlet and outlet control for full and partially full culverts, analyze tailwater, analyze flow over the roadway embankment, and balance flows through multiple culverts

39. Storm Sewer Design (Design Storm Frequency)
Pipe Sizing
Should base the pipe sizing so they just flow full at the design storm frequency
Normally the design for the residential areas storm piping is based on a 10 year frequency event
Normally the design for detention pond overtopping is based on a 100 year frequency event
Always analyze the final design to determine the effect of larger storms

40. Procedure for Storm Sewer Design
Hydrology
Delineate Watersheds
Determine quantity of stormwater runoff
Select rainfall intensity and recurrence interval
Apply Method
Rational Method, TR-55, etc.

41. Procedure for Storm Sewer Design
Storm Sewer Layout
Sewer location should be determined by evaluation of construction, maintenance, replacement costs, and utility conflicts
Sewer depth should be defined by hydraulics, structural loading, and utility conflict considerations

42. Procedure for Storm Sewer Design
Computations
Use hydrologic calculations to determine hydraulic conditions are satisfied (i.e. does the pipe convey the proper amount of water from the contributing area)
Verify inlet flows are achieved and at low points
Calculate pipe flows (Manning’s Equation)

43. Case Study: Storm Sewer Design Procedure
Final Grading Plan with Flow Directions

44. Case Study: Storm Sewer Design Procedure
Delineation of Basins
Identify discharge points
Determine drainage patterns based upon grading plan for each phase
Identify off-site drainage and drainage from undisturbed areas
Delineate drainage subbasins for each phase
Determine area of each subbasin including bypass drainage

45. Case Study: Storm Sewer Design Procedure
Delineation of Basins
Schematic Layout

46. Case Study: Storm Sewer Design Procedure
Design Criteria:
Quantity Determination: Rational Method
Storm Frequency: 10 Year Recurrence Interval
Friction Flow Formula: Manning (n=0.013 for RCP)
Minimum Velocity: 3 feet/second
Minimum Pipe Diameter: 12 inch per code
Materials: Reinforced Concrete Pipe

47. Case Study: Storm Sewer Design Procedure
Computations
From Manning’s
Q=1.49/n A R2/3 S1/2
Q=CiA
Reach
Rational Method
Peak Discharge
Pipe Diameter
1-0
2.4 cfs
12 in
1-1
3.0 cfs
1-2
4.1 cfs
15 in
1-3
11.6 cfs
21 in
1-4
25.0 cfs
24 in
1-5
1-6
38.2 cfs
1-7
43.0 cfs
30 in

48. Questions