1. Modeling Low Impact Development Techniques and Detention Basins in SWMM
CEE 6/5460
David Rosenberg

2. Learning Objectives Represent LID methods in SWMM
Size a detention basin and specify a basin outlet
Route runoff through the basin and outlet
Represent basins and outlets in SWMM
Iterate and design to meet outflow requirements
CEE 6/5460 – Water Resources Engineering

3. Representing LID Measures in SWMM
More properties of a subcatchment!! (plan view)
Pervious
Impervious with storage
Impervious with no storage
Before LIDs
Width
After LIDs
Pervious
Impervious with storage
Impervious with no storage
LIDs
Width
CEE 6/5460 – Water Resources Engineering

4. Cross-sectional view: subcatchment layers

5. Available LID Controls in SWMM
LID Type
Surface
Pavement
Soil
Storage
Underdrain
Bio-retention XX O
Porous Pavement
Infiltration Trench
Rain Barrel
Vegetative Swale
CEE 6/5460 – Water Resources Engineering

6. Adding an LID Control Select LID Controls in the Data List Click
Enter properties for a generic control in the LID Control Editor
Tabs represent layers
CEE 6/5460 – Water Resources Engineering

7. Adding an LID Control (cont.)
Add generic control to:
An existing subcatchment; displace an equal non-LID area, OR
Allows mix of LIDs (in parallel)
Adjust width and percent impervious properties
Create a new subcatchment devoted entirely to a single LID practice
CEE 6/5460 – Water Resources Engineering

8. 5. Scale the generic control

9. LID Example
How will the magnitude and timing of the peak runoff from Lundstrum Park change if the city repaves 1700 E with permeable pavement?
Build on the 10/4 example
Assume
Road has same slope as park
All paved surfaces will be permeable
Ignore clogging
No underdrain
There will be curb cuts 4”
12”
Curb
Pavement
Gravel
CEE 6/5460 – Water Resources Engineering

10. Properties of Various Soil Types
Reprinted in Bedient et. al (2002)

11. Example Solution LID Control Editor inputs: LID Usage Editor inputs:
Subcatchment Editor adjustments:
CEE 6/5460 – Water Resources Engineering

12. Hydrograph comparison
CEE 6/5460 – Water Resources Engineering

13. Stormwater Detention Basins
Purpose
reduce post-development runoff rate ≤ pre-development runoff rate
Types
Extended detention basins (“dry” detention basins)
Retention ponds (“wet” detention ponds)
We’ll largely ignore storm water pipe and sewer design for PBL-2 (assume unimpeded flow)
CEE 6/5460 – Water Resources Engineering

14. Extended detention basin
Adapted from Mays (2001), p. 601.
Function: settle pollutants; pass soluble pollutants
Efficiency: poor for detention times < 12 hours; good for times > 24 hours
Maintenance: moderate if designed properly
Improper design: eyesore, mosquito-breeding mudhole
Newer designs incorporate marsh around outlet
Regional facilities serving acres can be aesthetic with low maintenance
CEE 6/5460 – Water Resources Engineering

15. Retention pond Permanent water level
Adapted from Mays (2001), p. 602.
Permanent water level
Function: remove pollutants by settling, chemical, and biological process
Efficiency: excellent if properly designed; poor if bottom becomes anoxic
Maintenance: relatively free after 1st year; periodic cleaning (~ 10 years)
Aesthetic design can make pond asset to community
Excellent as regional facility
CEE 6/5460 – Water Resources Engineering

16. What type of basin is in Logan Dry Canyon above Cliffside Terrace??

17. 1. Size the detention basin
Objective: determine the required storage volume, Vs
CEE 6/5460 – Water Resources Engineering

18. Ex 5: What detention volume is needed to manage runoff from the LDS Church adjacent to Lundstrum Park?
Total Area = 3 ac
Grass landscaped area = 0.28 ac
Everything else is impermeable with no storage
Width = 440 ft
All other site characteristics as for Lundstrum Park
Use the Layton design storm event
CEE 6/5460 – Water Resources Engineering

19. 2. Configure the detention basin
Specify length, width, shape, slopes, etc.
Determine the stage-area-storage relationship for basin
CEE 6/5460 – Water Resources Engineering

20. 3. Specify an Outlet Unmanned Weir Pipe (orifice)
L = weir crest length [L]
A = cross-sectional area of orifice [L2]
Cw = discharge coefficient [fraction]
g = gravitation constant [L/T2]
h = water height above crest [L]
CEE 6/5460 – Water Resources Engineering

21. 3. Specify an Outlet (cont.)
Determine the stage (h) – discharge (Qout) relationship
Relationship for the 2-ft long weir outlet whose crest is 0.75 ft above the basin bottom in Example 6.
CEE 6/5460 – Water Resources Engineering

22. Route flow through the basin & outlet
Objective
Determine outflow from a reach (basin, reservoir, river segment) based on inflow, change in storage, physical conditions, and assumptions
Methods available in SWMM
Method
Numerical Technique(s)
Time step (min)
Use for…
Steady flow
Simple translation; Level Pool method
Any
Preliminary analysis
Uniform, steady flow
Dendritic network
Kinematic wave
Continuity eq.; Simple momentum eq.
5- 15
Channel attenuation
Dynamic wave
Complete 1D Saint Venant flow Eqs. ≤1
Channel storage
Backwater, flow reversal
Entrance/exit losses
Pressurized flow

23. Routing (cont.)
Example 6 uses the Level Pool (Modified Puls) method to route the runoff hydrograph through the basin and weir
What is the resulting peak flow?
What must the designer do next?
CEE 6/5460 – Water Resources Engineering

24. Representing Storage Units in SWMM
Runoff
Top of dam
Surface area
Water Storage
Depth
Outflow
Infiltration
CEE 6/5460 – Water Resources Engineering

25. Weirs and Orafices
Attach as a link between the basin and the next junction
Enter the properties (for Example 6 weir)
CEE 6/5460 – Water Resources Engineering

26. Ex 6: What is the outflow from the proposed detention basin and weir?
What should the designer do next?
CEE 6/5460 – Water Resources Engineering

27. Iterate and design Hydrology (PBL-2): Routing (PBL-2): Design:
Quantify design storm size, frequency, and runoff timing
Routing (PBL-2):
Quantify outflow(s), depth(s), flooded areas
Design:
Basin (size, configuration, and outlet)(PBL-2)
Inlets, pipes, manholes, junctions, channels, etc.
Constraints (PBL-2):
Do outflows meet regulations?
Are costs and aesthetics acceptable? no
Is design acceptable?
yes
DONE
CEE 6/5460 – Water Resources Engineering

28. Recap of Design Steps Design storm and hytegraph
Pre-development runoff hydrograph
Post-development runoff hydrograph
Volume of detention needed
Specify detention basin and outlet
Route post-development runoff hydrograph through basin and outlet
Satisfactory? If no, go back to Step #
CEE 6/5460 – Water Resources Engineering