1. LID Analysis Presented by: The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organization www.lowimpactdevelopment.org Presented by: The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organization www.lowimpactdevelopment.org

2. The Low Impact Development Center, Inc. has met the standards and requirements of the Registered Continuing Education Program. Credit earned on completion of this program will be reported to RCEP at RCEP.net. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by RCEP.

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4. The purpose of this presentation is to provide an overview of different methods for modeling the effect of Low Impact Development on site hydrology. At the end of this presentation, you will be able to: Compare single event vs. continuous modeling Discuss common methods for modeling LID Purpose and Learning Objectives

5. Hydrologic Analysis for LID Necessary to calculate runoff volumes and/or generate hydrographs Used to establish targets and evaluate alternatives Two alternatives: single event or continuous

6. Single-event Simulation Evaluates one storm event, usually assumed to have a 24 hour duration Target storm event based on recurrence interval (e.g. 2-year event, 95 th percentile event) Simulations can be performed using simple equations and spreadsheets (e.g. Direct Determination, Runoff Reduction Method, TR-55, etc.)

7. Continuous Simulation Evaluates system behavior over a long time period (e.g., one year) Uses recorded local precipitation data More accurate consideration of inter-event effects (e.g. evapotranspiration, underdrain discharge, drying) Can be used to estimate annual pollutant loading Simulation requires sophisticated software (e.g. SWMM, SLAMM, etc.)

8. Comparison of Potential Methods for Analyzing Control Measures MethodStrengthsWeaknesses Direct Determination Methodology (Manning’s Eq.) for runoff determination is same as SWMM Models basic hydrologic processes directly (explicit) Simple spreadsheet can be used Direct application of Horton’s method may estimate higher infiltration loss, especially at the beginning of a storm Does not consider flow routing Does not consider antecedent moisture conditions SWMM Method is widely used Can provide complete hydrologic and water quality process dynamics in stormwater analysis Needs a number of site-specific modeling parameters Generally requires more extensive experience and modeling skills

9. Direct Determination Method Single-event Based on physical processes For Federal lands, targets the 95 th percentile storm event Adaptable to any target storm event EPA 841-B-09-001 December 2009 www.epa.gov/owow/nps/lid/section438 Modifications to be published in the forthcoming Volume Based Stormwater Management Guidance

10. Direct Determination Method for Calculating Runoff Volume

11. Example 95 th Percentile Storms City 95 th Percentile Event Rainfall Total (in) City 95 th Percentile Event Rainfall Total (in) Atlanta, GA1.8Kansas City, MO1.7 Baltimore, MD1.6Knoxville, TN1.5 Boston, MA1.5Louisville, KY1.5 Buffalo, NY1.1Minneapolis, MN1.4 Burlington, VT1.1New York, NY1.7 Charleston, WV1.2Salt Lake City, UT0.8 Coeur D’Alene, ID0.7Phoenix, AZ1.0 Cincinnati, OH1.5Portland, OR1.0 Columbus, OH1.3Seattle, WA1.6 Concord, NH1.3Washington, DC1.7 Denver, CO1.1

12. Depression Storage Rainfall held in micro-depressions Stored water eventually evaporates Impervious surfaces: 0.1 inches Pervious surfaces: 0.2 inches

13. Interception Losses Rainfall intercepted on tree leaves, branches and trunks Intercepted rainfall ultimately evaporates For trees “in leaf”: 0.08 in For bare trees: 0.04 in (Xiao et al, 2000)

14. Infiltration Losses Calculated using Horton’s Equation Assumes infiltration rates on compacted soils are reduced by 99 percent (Gregory et al, 2006) HSGTotal Infiltration Losses over 24 hours (in) UndevelopedDeveloped (Compacted) A16.00.16 B9.70.10 C4.40.04 D0.80.01

15. Required Data 95 th percentile rainfall depth Impervious area Undeveloped area Developed pervious area Tree cover Hydrologic soil groups Topography

16. Direct Determination Method Example

17. Land Cover and Soils DA Area (ac) Impervious Area (ac) Pervious Area (ac)Tree Cover (ac)Upstream DA 129.950 270.1413.5256.621.531 357.870 HSG AHSG BHSG C DA uncompactedcompacteduncompactedcompacteduncompactedcompacted 10.320.0011.970.0017.470.00 20.324.810.3117.100.9015.69 341.700.0013.930.001.860.00

18. Calculated Runoff Volume Rainfall Volume (ft 3 ) Depression Storage Volume (ft 3 ) Volume Intercepted (ft 3 ) Infiltration Volume Capacity (ft 3 ) Run-on Volume from Upstream (ft 3 ) Runoff Volume (ft 3 ) 152,205.9021,743.707,610.30719,092.110.00 356,451.4846,013.88388.7755,154.950.00254,893.88 294,095.3442,013.6214,704.772,942,133.150.00 Target Rainfall Depth: 1.4 inches This analysis yields an estimated runoff volume for the target storm, which can be used to size BMPs

19. SWMM (EPA Stormwater Management Model) Capable of single event or continuous simulation Best suited for urban hydrology and water quality simulation Robust conveyance modeling Wide applicability to large and medium watershed hydrology Current version (v. 5) capable of simulating some LID BMPs

20. SWMM Example Permeable pavement Curb bumpouts (bioretention)

21. For each subcatchment, a portion of the runoff is routed to an LID Control

22. LID controls are classified by type (Bio-retention cell, permeable pavement, etc), and can be further customized with specific design details

23. Results of Single-Event Simulation

24. Projected Annual Load Reductions

25. PG LID Model

26. Source Node Gross Pollutant Trap Buffer Strip Vegetated Swale Infiltration Dry & Wet Detention Pond Wetlands

29. HSPF LAND SIMULATION – Unit-Area Output by Landuse – BMP Evaluation Method Existing Flow & Pollutant Loads Simulated Flow/Water Quality Improvement Cost/Benefit Assessment of LID design BMP DESIGN – Site Level Design – SITE-LEVEL LAND/BMP ROUTING Simulated Surface Runoff

30. Overflow Spillway Bottom Orifice Evapotranspiration Infiltration Outflow: Inflow: Modified Flow & Water Quality From Land Surface Storage BMP Class A: Storage/Detention Underdrain Outflow

31. The Interface Landuse Menu BMP Menu click-and-drag 1 edit attributes 2 connect objects 3

32. Storm Volume (in)0.1800.3800.4200.7901.2602.0802.390 Peak Flow Reduction 2_190.3%94.7%98.0%90.9%82.1%69.3%45.7% Peak Flow Reduction 6_289.7%95.3%98.3%94.4%91.1%88.8%64.0% General Assessment of BMP Effectiveness 0% 20% 40% 60% 80% 100% 0.00.51.01.52.02.5 Storm Volume (in) BMP Peak Flow Reduction BMP 2_1BMP 2_1 in series with BMP 6_2

35. SLAMM DeveloperDr. Robert Pitt, U of Alabama; John Voorhees RainfallContinuous Watershed Size10 to 100+ acre Drainage Areas Land UsesResidential, Commercial, Industrial, Highway, Institutional, and other Urban Source AreasRoofs, Sidewalks, Parking, Landscaped, Streets, Driveways, Alleys, etc. Primary UseRunoff Quantity and Quality Application to LIDInfiltration, Wet Ponds, Porous Pavement, Street Sweeping, Biofiltration, Vegetated Swales, Other Urban Control Device

36. SLAMM

37. PG LID Manual Charts

38. National LID Manual Technique DeveloperUS EPA; Prince George’s County RainfallSingle Event Watershed Size Small Sites Primary UseEstimates retention and detention requirement to meet quantity and peak flow goals Application to LID Applies to any BMP with retention storage: bioretention, infiltration, porous pavement, swales, and planters

39. National LID Manual Techniques Based on NRCS methods Uses peak storm event Nomographs that reflect graphical peak discharge method

41. VS/VR

43. Runoff Equation Solution

44. Maintaining Pre-Development Runoff Volume Existing CN: 63 Proposed CN: 73 Required Retention Storage Volume = (0.30in)(1ft/12in)(6.5 ac) = 4.5 ac-ft

45. Maintaining Pre-Development Runoff Volume Existing CN: 63 Proposed CN: 73 Required Retention Storage Volume =(0.50in)(1ft/12in)(6.5 ac) = 0.27 ac-ft Slide 45 Naval Workshop on Low Impact Development (LID)

46. 8% BMP Determining LID BMP Size

47. Pre-Development Conditions Woodland Attributes Runoff amounts low and delayed Stable hydrology Habitat undisturbed CN- woods in good condition

48. Soils Map Analysis Hydrologic Soils Groups D soils - CN = 77 C soils - CN = 70 B soils - CN = 55 A soils - CN = 30

49. Given: 50 acre tract Zoned 1/2 acre residential Environmental constraints present (wetlands, steep slopes, tree conservation) Conventional Calculations 25% of site C soils = 875 58% of site B soils = 1595 17% of site A soils = 255 weighted CN = 54.5

51. Developed Conditions - Conventional SWM Design Conventional SWM Design Concepts Pipe and pond conveyance system Connected flowpaths Mass grade to one collection point

52. Determining CN Values Conventional Calculations 25% of site C soils = 1000 58% of site B soils = 2030 17% of site A soils = 459 From TR55 (table 2-2a: weighted CN = 69.8

53. Developed Condition - Conventional SWM Design Stormdrain Calculations Q 10 = C I 10 A Q 10 =.38 * 5.88 * 2 Q 10 = 4.47cfs DA = 1.9ac

54. Exploded View of Lot

55. Closer look at lot reveals that the density is lower than typical 1/2 acre zoning used in TR55 CN values (20% impervious) In this case: 30% of woods are preserved Average impervious area =15% Developed Condition CN = Impervious Connected = 5% - 98 Impervious Unconnected= 10% 98 Open Space (good cond.)= 55% 61 Woods (good cond)= 30%63 735+1373+945= 3053 Custom LID CN weighted CN = 62 Site has < 30% imperv area. Composite CN = 61

56. LID Post Development Conditions LID Components On-lot SWM BMP’s Multifunctional landscaping integration Open-section roadways Disconnected flowpaths Grading refinement

58. LID Post Development with Drainage Divides LID Site Layout Concepts Pre-existing drainage divides preserved No net runoff Storm drainage infrastructure reduced Development potential maintained

59. Post Development Peak Flow – LID SWM Integration A B C D E F Peak Flow Rates*: A = 1.18cfs B = 0.65cfs C = 0.39cfs D = 0.41cfs E = 0.45cfs F = 0.45cfs Total = 4.09cfs DA = 2.47ac * No net Runoff- All runoff volume is contained in the bioretention facilities

61. Thank you for your time. QUESTIONS? Low Impact Development Center, Inc. www.lowimpactdevelopment.org 301.982.5559