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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
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
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)
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
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
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
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
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
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