LID Analysis

Presented by:

The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organizationwww.lowimpactdevelopment.org

Presented by:

The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organizationwww.lowimpactdevelopment.org

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.

COPYRIGHT MATERIALS

This educational activity is protected by U.S. and International copyright laws. Reproduction, distribution, display, and use of the educational activity without written permission of the

presenter is prohibited.

© Low Impact Development Center, 2012

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, 95th 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

Method Strengths Weaknesses

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 95th

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

Direct Determination Method for Calculating Runoff Volume

Used to estimate runoff volume for a single, 24-hour storm event

Example 95th Percentile StormsCity 95th Percentile Event

Rainfall Total (in)City 95th Percentile Event

Rainfall Total (in)

Atlanta, GA 1.8 Kansas City, MO 1.7

Baltimore, MD 1.6 Knoxville, TN 1.5

Boston, MA 1.5 Louisville, KY 1.5

Buffalo, NY 1.1 Minneapolis, MN 1.4

Burlington, VT 1.1 New York, NY 1.7

Charleston, WV 1.2 Salt Lake City, UT 0.8

Coeur D’Alene, ID 0.7 Phoenix, AZ 1.0

Cincinnati, OH 1.5 Portland, OR 1.0

Columbus, OH 1.3 Seattle, WA 1.6

Concord, NH 1.3 Washington, DC 1.7

Denver, CO 1.1

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)

HSG Total Infiltration Losses over 24 hours (in)

Undeveloped

Developed (Compacted)

A 16.0 0.16

B 9.7 0.10

C 4.4 0.04

D 0.8 0.01

Required Data

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

Direct Determination Method Example

Land Cover and Soils

DAArea (ac)

Impervious Area (ac)

Pervious Area (ac)

Tree Cover (ac) Upstream DA

1 29.95 0 29.95 29.95  

2 70.14 13.52 56.62 1.53 1

3 57.87 0 57.87 57.87  

 HSG A HSG B HSG C

DAuncompact

ed compacteduncompact

ed compacted uncompact

ed compacted

1 0.32 0.00 11.97 0.00 17.47 0.00

2 0.32 4.81 0.31 17.10 0.90 15.69

3 41.70 0.00 13.93 0.00 1.86 0.00

Calculated Runoff Volume

Rainfall Volume (ft3)

Depression Storage

Volume (ft3)

Volume Intercepted

(ft3)

Infiltration Volume Capacity

(ft3)

Run-on Volume

from Upstream

(ft3)Runoff

Volume (ft3)152,205.90 21,743.70 7,610.30 719,092.11 0.00 0.00356,451.48 46,013.88 388.77 55,154.95 0.00 254,893.88294,095.34 42,013.62 14,704.77 2,942,133.15 0.00 0.00

Target Rainfall Depth: 1.4 inches

This analysis yields an estimated runoff volume for the target storm, which can be used to size BMPs

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 ExamplePermeable 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

0 5 10 15 20 25 30 3502468

101214

Catchment 1 Bioretention

Total Inflow in/hr -------- Bottom Infil in/hr -------- Surface Runoff in/hr -------- Drain Outflow in/hr --------

Time (hr)

Flo

w (

in/h

r)

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.80.9

1

Catchment 2 - Permeable Pavement

Total Inflow in/hr --------

Bottom Infil in/hr --------

Surface Runoff in/hr --------

Drain Outflow in/hr --------

Time (hr)

Flo

w (

in/h

r)

0 5 10 15 20 25 30 350

5

10

15

Catchment 3 - Biore-tention

Total Inflow in/hr -------- Bottom Infil in/hr -------- Surface Runoff in/hr -------- Drain Outflow in/hr --------

Time (in/hr)

Flo

w (

in/h

r)

0 5 10 15 20 25 30 350

2

4

6

8

10

12

14

16

Catchment 4 - Bioretention

Total Inflow in/hr -------- Bottom Infil in/hr -------- Surface Runoff in/hr -------- Drain Outflow in/hr --------

Time (hr)

Flo

w (

in/h

r)

Projected Annual Load Reductions

Existing Proposed0

20

40

60

80

100

120

140

Annual Runoff Volume

Runoff

(in

ches)

Existing Proposed0

500

1000

1500

2000

2500

3000

3500

Annual Total Suspended Solids

TSS (

lbs)

Existing Proposed0

5

10

15

20

25

30

35

40

45

Annual Total Nitrogen

TN

(lb

s)

Existing Proposed0

1

2

3

4

5

6

7

8

9

Annual Total PhosphorusT

P (

lbs)

PG LID Model

Source Node

Gross Pollutant Trap

Buffer Strip

Vegetated Swale

Vegetated Swale

InfiltrationDry & Wet Detention Pond

Wetlands

HSPF LAND SIMULATION

– Unit-Area Output by Landuse –

BMP Evaluation MethodExisting Flow & Pollutant Loads

Simulated Flow/Water Quality Improvement Cost/Benefit Assessment of LID design

0

50

100

150

200

250

2/20/99 6/20/99 10/20/99 2/20/00 6/20/00 10/20/00

Time

Flo

w (

cfs)

0

1

2

3

4

5

6

7

8

9

10

Tota

l Rai

nfal

l (in

)

Total Rainfall (in) Modeled Flow

BMP DESIGN– Site Level Design –

SITE-LEVEL LAND/BMP ROUTINGSimulatedSurface 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 Interfa

ceLanduse

Menu

BMP Menu

click-and-drag

1

edit attributes

2

connect objects

3

Storm Volume (in) 0.180 0.380 0.420 0.790 1.260 2.080 2.390

Peak Flow Reduction 2_1 90.3% 94.7% 98.0% 90.9% 82.1% 69.3% 45.7%

Peak Flow Reduction 6_2 89.7% 95.3% 98.3% 94.4% 91.1% 88.8% 64.0%

General Assessment of BMP Effectiveness

0%

20%

40%

60%

80%

100%

0.0 0.5 1.0 1.5 2.0 2.5

Storm Volume (in)

BM

P P

ea

k F

low

Re

du

cti

on

BMP 2_1 BMP 2_1 in series with BMP 6_2

Storm Volume (in) 0.180 0.380 0.420 0.790 1.260 2.080 2.390

Peak Flow Reduction 2_1 96.6% 81.4% 78.8% 57.4% 42.1% 18.9% 13.7%

Peak Flow Reduction 4_2 96.8% 94.6% 93.9% 78.4% 61.0% 53.2% 28.5%

General Assessment of BMP Effectiveness

0%

20%

40%

60%

80%

100%

0.0 0.5 1.0 1.5 2.0 2.5

Storm Volume (in)

BM

P P

eak

Flo

w R

edu

ctio

n

BMP 2_1 BMP 2_1 in series with BMP 4_2

Storm Volume (in) 0.180 0.380 0.420 0.790 1.260 2.080 2.390

Total Load from Site (lb): 0.142 0.269 0.303 0.287 0.489 1.379 0.628

Total load after BMP 4_2 (lb): 0.025 0.104 0.168 0.197 0.370 1.264 0.552

Lost or Trapped (lb): 0.117 0.165 0.135 0.090 0.119 0.114 0.075

Total Nitrogen Removal (%) 82.14% 61.23% 44.50% 31.27% 24.40% 8.30% 12.00%

General Assessment of BMP Effectiveness

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.18 0.38 0.42 0.79 1.26 2.08 2.39

Storm Volume (in)

Nit

rog

en L

oad

(lb

)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Per

cen

t R

edu

ctio

n

Total Load from Site (lb):

Total load after BMP 4_2 (lb):

% Removal after BMP 2_1

% Removal after BMP 4_2

SLAMMDeveloper Dr. Robert Pitt, U of Alabama; John

Voorhees

Rainfall Continuous

Watershed Size 10 to 100+ acre Drainage Areas

Land Uses Residential, Commercial, Industrial, Highway, Institutional, and other Urban

Source Areas Roofs, Sidewalks, Parking, Landscaped, Streets, Driveways, Alleys, etc.

Primary Use Runoff Quantity and Quality

Application to LID Infiltration, Wet Ponds, Porous Pavement, Street Sweeping, Biofiltration, Vegetated Swales, Other Urban Control Device

SLAMM

PG LID Manual Charts

National LID Manual Technique

Developer US EPA; Prince George’s County

Rainfall Single Event

Watershed Size

Small Sites

Primary Use Estimates 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

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 Calculations25% of site C soils = 87558% of site B soils = 159517% of site A soils = 255weighted 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 Calculations25% of site C soils = 100058% of site B soils = 203017% of site A soils = 459From TR55 (table 2-2a:weighted CN = 69.8

Developed Condition - Conventional SWM Design

Stormdrain Calculations

Q10 = C I10 A

Q10 = .38 * 5.88 * 2

Q10 = 4.47cfs

DA = 1.9ac

Exploded View of Lot

Closer look at lotreveals 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 preservedAverage impervious area =15%

Developed Condition CN =

Impervious Connected = 5% - 98Impervious Unconnected= 10% 98Open Space (good cond.)= 55% 61Woods (good cond) = 30%63

735+1373+945= 3053

Custom LID CNweighted 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 = 1.18cfsB = 0.65cfsC = 0.39cfsD = 0.41cfsE = 0.45cfsF = 0.45cfsTotal = 4.09cfsDA = 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.www.lowimpactdevelopment.org

301.982.5559