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

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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

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

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.)

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.)

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

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 December Modifications to be published in the forthcoming Volume Based Stormwater Management Guidance

Direct Determination Method for Calculating Runoff Volume

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

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

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)

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) A B C D

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

Direct Determination Method Example

Land Cover and Soils DA Area (ac) Impervious Area (ac) Pervious Area (ac)Tree Cover (ac)Upstream DA HSG AHSG BHSG C DA uncompactedcompacteduncompactedcompacteduncompactedcompacted

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, , , , , , , , , , , ,942, Target Rainfall Depth: 1.4 inches This analysis yields an estimated runoff volume for the target storm, which can be used to size BMPs

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

SWMM Example Permeable pavement Curb bumpouts (bioretention)

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

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

Results of Single-Event Simulation

Projected Annual Load Reductions

PG LID Model

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

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

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

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

Storm Volume (in) 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% Storm Volume (in) BMP Peak Flow Reduction BMP 2_1BMP 2_1 in series with BMP 6_2

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

SLAMM

PG LID Manual Charts

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

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

VS/VR

Runoff Equation Solution

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

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)

8% BMP Determining LID BMP Size

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

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

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

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

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

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

Exploded View of Lot

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% = 3053 Custom LID CN weighted CN = 62 Site has < 30% imperv area. Composite CN = 61

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

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

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

Thank you for your time. QUESTIONS? Low Impact Development Center, Inc