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By Holly Piza, PE
By Holly Piza, PE
The Four Step Process
USDCM Volume 3 History and Fundamentals
Common themes:
Education and training to a broad range of participants from public and private sectors.
Incentives including limiting liability for failures, decreased imperviousness or decreased WQCV and / or detention requirements when LID BMPs are implemented.
Partner with local governments to showcase demonstration projects.
Watershed/Master Planning Level• SWMM Modeling• DCIA Level 0, 1, and 2 Curves or CUHP “D” and “R”
Curves where,D=ADCIA/AIMP and R=ARPA/APERV
Site Planning Level• SWMM Modeling Using Cascading Planes• UDFCD Impervious Reduction Factor (IRF) charts or
spreadsheet
UIA
UIA = Unconnected Impervious AreaDCIA = Directly Connected Impervious Area
RPA = Receiving Pervious AreaSPA = Separate Pervious Area
Effective Imperviousness Adjustments for Level 1 MDCIA(Impervious surfaces are designed to drain over grass buffers or other pervious surfaces before reaching a
conveyance system.)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Effe
ctiv
e Im
perv
ious
ness
Total Imperviousness
Directly Connected 2-year Level 1 10-year Level 1 100-year Level 1
Effective Imperviousness Adjustments for Level 2 MDCIAImpervious surfaces are designed to drain over grass buffers or other pervious surfaces before reaching a pervious
conveyance system.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Effe
ctiv
e Im
perv
ious
ness
Total Imperviousness
Directly Connected 2-year Level 2 10-year Level 2 100-year Level 2
Site Level Designs
Targeted runoff eventsTypes of BMPs
• Storage-based• Conveyance-based Grass Buffers
Grass Swales
BioretentionGreen RoofsWet PondsDry PondsPermeable Pavement
Conveyance-based BMPs:
Storage-based BMPs:
),(),( rrd A
IfFctA
PFFctK
),,(P
WQCVAAPFFctK dr
d
K = Imperviousness reduction factorFd = Pervious area infiltration loss (in)f = Pervious area infiltration rate (in/hr) corresponding to saturated hydraulic conductivityP = Design rainfall depth (in)I = Rainfall intensity (in/hr)Ar = RPA/UIAAd = RPA
(Guo et al. Incentive Index Developed to Evaluate Stormwater low Impact Designs. American Society of Civil Engineers (ASCE) Journal of Environmental Engineers, Vol. 136, Issue 12)
Storage-based Imperviousness Reduction Factor
Imperviousness Reduction Factor Final Equation:
Soil TypeConveyance
-based1
Storage-based12-
hours24-
hours40-
hours(in/hr) (in/hr) (in/hr) (in/hr)
Sand 5.85 5.04 4.91 4.85Loamy Sand 1.92 1.40 1.31 1.27Sandy Loam 1.04 0.64 0.56 0.52Silt Loam 0.83 0.46 0.39 0.35Loam 0.43 0.24 0.20 0.18Sandy Clay Loam 0.34 0.16 0.13 0.11
Silty Clay Loam 0.27 0.13 0.10 0.08Clay Loam 0.26 0.13 0.10 0.08Silty Clay 0.18 0.08 0.06 0.05Sandy Clay 0.16 0.08 0.06 0.05Clay 0.12 0.05 0.04 0.03
1 Values for conveyance-based BMPs are based on a 2-hour duration.
EXAMPLE—COLORADO GREEN BUSINESS CENTER
Sub-basin A: Area = 1.15 acres
DCIA = 0%, UIA = 49%, SPA = 13%, RPA = 38%
INPUT & f/I CALCULATIONS
K FACTORS & EFFECTIVE IMPERVIOUSNESS CALCULATION
10- AND 100-YEAR CALCULATIONS—COMPARE EMPIRICAL VOLUMES FROM
ITOTAL WITH IEFFECTIVE
Site 2: Assisted Living Facility in Denver
Conventional Site Plan
Objective: Minimize depth of on-site flood control storage requirements
79% ImperviousHSG Type D soils
Site 2: Assisted Living Facility in Denver
LID approach
Permeable Pavement (PICP)Rain GardensExpanded Tree Grates with
bioretention media
9 subwatersheds
Site 2: Assisted Living Facility in Denver
LID approach
Increasing tree grates and replacing conventional pavement with permeable pavement reduces imperviousness from 79% to 61.4%
Routing impervious runoff into pervious area reduced this to 60.5% (for the 100-year event)
Had the site had HSG A soils effective imperviousness would be between 33 and 57%
What if?
Calculate %impervious based on a user defined event (e.g., NRCS 2-year, 24-hour storm)
