# WMS:Tulsa District Lag Time Equation

The Tulsa District of the US Army Corps of Engineers has developed the following family of equations for computing Snyders lag time:

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

where:

TLAG = watershed lag time in hours.
Ct = 1.42 for natural watersheds in rural areas of central and northeastern Oklahoma.
Ct = 0.92 for the same type areas that are 50% urbanized.
Ct = 0.59 for the same type areas that are 100% urbanized.
L = watershed maximum flow distance length in miles.
S = slope of the maximum flow distance path in ft/mile.
Lca = length to centroid.

The range of watershed characteristics for which these equations apply include:

• Sizes ranged from 0.5 to just over 500 square miles.
• Slopes ranged from 4 to 90 feet per mile.
• Lengths ranged from 1 to 80 miles.
• Length to centroid ranged from 1 to 60 miles.

In addition to developing an equation for lag time, the Tulsa district developed the following relationship for the peak flow rate which can be used in the second equation to solve for Snyder’s peaking coefficient.

$q_p = 380 \ast T_{LAG} {}^{-0.92}$
$C_p = \frac {q_p \ast T_{LAG}}{640}$