Download the entire Volume 3 Criteria Manual - Urban Drainage ...
Download the entire Volume 3 Criteria Manual - Urban Drainage ... Download the entire Volume 3 Criteria Manual - Urban Drainage ...
Calculating the WQCV and Volume Reduction Chapter 3 1.0 0.9 Imperviousness Reduction Factor (IRF), K 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Area-weighted Imperviousness of Disconnected Portion (%) = UIA/(UIA+RPA) f/I = 0.5 f/I = 1.0 f/I = 1.5 f/I = 2.0 Figure 3-9. Conveyance-based Imperviousness Reduction Factor Once K is known for a given storm event, the following equation can be used to calculate the effective imperviousness for that event: DCIA + (K ∙ UIA) I Effective (%) = DCIA + UIA + RPA + SPA ∙ 100 Equation 3-4 Where: DCIA UIA RPA SPA = directly connected impervious area = unconnected impervious area = receiving pervious area = separate pervious area 3-18 Urban Drainage and Flood Control District August 2011 Urban Storm Drainage Criteria Manual Volume 3
Chapter 3 Calculating the WQCV and Volume Reduction Imperviousness Reduction Factor (IRF), K 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Note: When the total depth of the storm event is less than the WQCV and the full WQCV is provided for a sub-basin, K = 0. 0.0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Area-weighted Imperviousness of Disconnected Portion (%) = UIA/(UIA+RPA) f/I = 0.5 f/I = 1.0 f/I = 1.5 f/I = 2.0 Figure 3-10. Storage-based Imperviousness Reduction Factor Four basic steps can be used to determine effective imperviousness when parameters including UIA, DCIA, RPA, SPA, WQCV, f and I are known. For clarity, these steps are accompanied by an example using a sub-watershed with a conveyance-based approach (i.e., no WQCV) with UIA = 0.25 acres, DCIA = 0.25 acres, RPA = 0.25 acres, SPA = 0.25 acres, f = 1.0 inch/hour and I = 0.5 inch/hour. 1. Calculate the area-weighted imperviousness of the disconnected portion. The disconnected portion of the sub-watershed consists of the UIA and the RPA. The area weighted imperviousness is calculated as UIA/(UIA+RPA). For the example, UIA + RPA = 0.25 + 0.25 = 0.50 acres. The area-weighted imperviousness of this area = 0.25/0.50 = 0.50 or 50%. 2. Calculate f/I based on the rainfall intensity for the design storm and the infiltration rate for the given RPA soil type. In this example, the 1-hour intensity is given as 0.5 inch/hour in the problem statement, and the infiltration rate is specified as 1 inch/hour. For this example, based on Table 3-3, the 1.0 inch/hour infiltration rate specified in the problem statement would roughly correspond to a sandy loam soil type for a conveyance-based BMP. For the example, f/I = 1.0/0.5 = 2.0. August 2011 Urban Drainage and Flood Control District 3-19 Urban Storm Drainage Criteria Manual Volume 3
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Calculating <strong>the</strong> WQCV and <strong>Volume</strong> Reduction Chapter 3<br />
1.0<br />
0.9<br />
Imperviousness Reduction Factor (IRF), K<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0.0<br />
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%<br />
Area-weighted Imperviousness of Disconnected Portion (%) = UIA/(UIA+RPA)<br />
f/I = 0.5 f/I = 1.0 f/I = 1.5 f/I = 2.0<br />
Figure 3-9. Conveyance-based Imperviousness Reduction Factor<br />
Once K is known for a given storm event, <strong>the</strong> following equation can be used to calculate <strong>the</strong> effective<br />
imperviousness for that event:<br />
DCIA + (K ∙ UIA)<br />
I Effective (%) = <br />
DCIA + UIA + RPA + SPA ∙ 100 Equation 3-4<br />
Where:<br />
DCIA<br />
UIA<br />
RPA<br />
SPA<br />
= directly connected impervious area<br />
= unconnected impervious area<br />
= receiving pervious area<br />
= separate pervious area<br />
3-18 <strong>Urban</strong> <strong>Drainage</strong> and Flood Control District August 2011<br />
<strong>Urban</strong> Storm <strong>Drainage</strong> <strong>Criteria</strong> <strong>Manual</strong> <strong>Volume</strong> 3
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