WMS:Denver Lag Time Equation

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The Denver Lag Time Equation was developed by the Denver Area Urban Drainage and Flood Control District (Wright-McLaughlin Engineers, 1975)

T_{LAG} = C_t \ast \left ( \frac {L \ast L_{ca}}{ \sqrt {S}} \right )^{0.48}

where:

TLAG = watershed lag time in hours.
Ct = time to peak coefficient.
L = length along the stream from the study point to the upstream limits of the basin in miles.
Lca} = length along the stream from the study point to a point along the stream adjacent to the centroid of the basin in miles.
S = weighted average slope of the basin from the study point to the upstream limits of the basin in feet per foot.

The percent impervious (Ia) must already be defined in one of the Loss methods used for HEC-1.

This equation was developed for small urban watersheds (less than 5 square miles) with mild slopes. The peaking coefficient can be computed from the percent impervious (Ia) using the following equations:

C_t = -0.00371 \; I_a^{} + 0.146
C_t = 0.000023  \; I_a^2 - 0.002241 \; I_a + 0.146
C_t = 0.0000033 \; I_a^2 - 0.000801 \; I_a + 0.12

The Denver method used a peaking parameter P and the relationships below to compute the peaking coefficient Cp.

P = 0.002450 \; I_a^2 - 0.0120 \; I_a + 2.16
P = -0.00091 \; I_a^2 + 0.228 \; I_a - 2.06
C_p = P \ast C_t \ast A^{0.15}

where:

Ct= coefficient as defined by the previous three equations.
P = peaking parameter.
A = basin area in square miles.


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