MTD Sizing Issues
Runoff Reduction Method
L = P × Pj × Rv ×C × A × 2.72 12
• Estimates annual pollutant load exported in
stormwater runoff from small urban
catchments
Runoff Reduction Method
L = P × Pj × Rv ×C × A × 2.72 12
L (lbs/yr) = total post-dev. pollutant load
P (in) = average annual rainfall depth
= 43 in. (VA)
Pj
= fraction of rainfall events producing
runoff = 0.9
Runoff Reduction Method
L = P × Pj × Rv ×C × A × 2.72 12
Rv = volumetric runoff coefficient
C (mg/L) = flow-weighted event mean
concentration (EMC) of TP = 0.26
A (acres) = area of development site
Runoff Reduction Method
L = P × Pj × Rvcomposite × C × A × 2.72
12
Rvcomposite = Weighted runoff coefficient
Rvcomposite = ( RvI × % I ) + ( RvT × %T ) + ( RvF × % F )
Where:
RvI = Runoff coefficient for Impervious cover (0.95)
RvT = Managed Turf/Disturbed soils
RvF = Forest/Open Space
Treatment Volume & BMP Sizing
TvBMP =
( P × Rvcomposite × A)
12
TvBMP = Treatment Volume from contributing
drainage area to BMP + remaining runoff from
upstream practices
P = 90th Percentile rainfall depth = 1”
Rvcomposite = Weighted runoff coefficient
A = Contributing drainage area to BMP
Design Rainfall-90% Rule
Washington Reagan Airport
Tv is in units
of volume
Precipitation Depth (inches)
Tv =P*Rv*A
6
5
4
90th Percentile rainfall
depth =1”
3
2
1
0
0%
10%
20%
30%
40%
50%
60%
70%
Precipitation Event Percentile
80%
90%
100%
Runoff Reduction Spreadsheet-MTD and
Treatment Volume
Enter Credit Area
Calculates Treatment Volume
If Treatment Train Treatment Volume > TP removal load ?
VRRM_NewDev_Compliance Spreadsheet_v2 9_2011_07232015 (2).xlsm
Sizing by Volume
•Volume based practices store
treatment volume and treat at or
below design flow rate
•Provides pretreatment
•Improved Efficiency
Water Quality Treatment Volume
Peak Flow Rate
qpTv = qu × A × Qa
qpTv = Treatment Volume peak discharge (cfs)
qu = unit peak discharge (cfs/mi2/in
A = drainage area (mi2)
Qa = runoff volume (watershed inches), equal to
Tv/A
*Volumetric conversion to an intensity - 1” in 24 hr.
Assumes a NRCS type II rainfall distribution
Water Quality Treatment Volume
Peak Flow Rate
1000
CN =
10 + 5 P + 10Qa − 10(Qa 2 + 1.25QaP ) 0.5
[
Where:
CN = Curve Number
P = Rainfall (inches), 1.0” in Virginia
Qa = Runoff volume (watershed
inches), equal to Tv/drainage area
]
qp = quAQ
Find qu on chart (type II)
• Use Ia/P ratio
and tc value
to find qu
• For CN of 98
Ia/P=.041
Extrapolate
C0,C1, C2
values
From NRCS
TR-55 to
generate curve
Continuous Model Simulation
Method – Another Approach
Washington
Wisconsin
Continuous Model Simulation
Method – Another Approach
1 acre of pavement, 90% of runoff treated at 100 gpm
Water Quality Treatment Volume –
Other States
• New Jersey uses 1.25 inch/2 hour
• Georgia-1.2 inches of rainfall
• South Carolina 1.8 in/24 hour (1 year)
• Impervious cover only
• Offline only (volume only)
• Impervious vs pervious land cover type (Rv)
Design Flow and Performance