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

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

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

Cross-sectional view: subcatchment layers

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

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

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

5. Scale the generic control

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

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

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

Hydrograph comparison CEE 6/5460 – Water Resources Engineering

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

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 100-200 acres can be aesthetic with low maintenance CEE 6/5460 – Water Resources Engineering

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

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

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

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

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

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

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

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

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

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

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

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

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

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