WMS:Eagleson Lag Time Equation

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Eagleson’s method (1962) for computing lag time in completely storm-sewered watersheds is given as follows:

T_{LAG} = 0.32 \left ( \frac {L \ast L_{ca}}{ \sqrt {S}} \right )^{0.39}


TLAG = watershed lag time in hours.
L = maximum flow length in miles.
Lca = length to the centroid in miles.
S = weighted average slope of the maximum flow path in ft/mile.

The typical characteristics of watersheds for which the Eagleson method was applied include the following:

  • Areas from 0.22 to 7.5 square miles.
  • L from 1 to 7 miles.
  • Lca from 0.3 to 3 miles.
  • S from 6 to 20 ft/mile.
  • Impervious cover from 33 to 83 percent.

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