cee5734_week4_2011

cee5734_week4_2011 - CEE 5734 Urban Hydrology and...

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Glenn E. Moglen Department of Civil & Environmental Engineering Virginia Tech The SCS Tabular Method and Determination of Detention Storage CEE 5734 – Urban Hydrology and Stormwater Management Week 4
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Questions Review of Week 3 Correction: “ARC” (not “AMC”) Today SCS Tabular Method Type II Storm Discussion Detention Basin Sizing Today’s Agenda
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Non-homogeneous sub-areas (differing geology, development, curve number, etc.) Need full hydrograph, not just peak Need hydrographs at one or more internal points within a watershed. SCS Tabular Method – Why do we use it?
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When NOT to use Tabular Method
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Q p : peak discharge [ft 3 /s] q u : unit peak discharge [ft 3 /(s-in-mi 2 )] A : drainage area [mi 2 ] Q : runoff [in] F p p u p F Q A q Q = Comparison – Graphical Peak Discharge vs. Tabular Method Q t : peak discharge [ft 3 /s] at time, t q t : unit peak discharge [ft 3 /(s-in-mi 2 )] at time, t A : drainage area [mi 2 ] Q : runoff [in] Q A q Q t t =
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Examining a sheet of q t values Comment on: q t ordinates, hydrograph time increments, travel time increments, T c values, I a /P values, visible trends in the q t ordinates.
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Rounding “Rules” (Time) Round T c and T t separately to the nearest table value and sum. Round T c down and T t up to the nearest table value and sum Round T c up and T t down to the nearest table value and sum From these three alternatives, choose the pair of rounded T c and T t values whose sum is closest to the sum of the actual T c and T t . If two rounding methods produce sums equally close to the actual sum, use the combination in which rounded T c is closest to actual
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cee5734_week4_2011 - CEE 5734 Urban Hydrology and...

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