Modeling Low Impact Development Techniques and Detention Basins in SWMM

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

2. Learning Objectives • Represent LID methods in SWMM • Sizea 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) Width Width Impervious with no storage Impervious with storage LIDs Impervious with no storage Impervious with storage Pervious Pervious Before LIDs After LIDs CEE 6/5460 – Water Resources Engineering

4. Cross-sectional view:subcatchment layers

5. Available LID Controls in SWMM 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” 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 • 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 100-200 acres can be aesthetic with low maintenance Adapted from Mays (2001), p. 601. CEE 6/5460 – Water Resources Engineering

15. Retention pond • 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 Adapted from Mays (2001), p. 602. 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 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

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 StorageUnits 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): 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? Is design acceptable? no 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