WMS:TR-55 Shallow Concentrated Flow Equation

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After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined using the equations below, in which average velocity (V) is a function of watercourse slope (s) and type of channel:

For Unpaved Channels: V (ft/s) = 16.1345 (s)_{0.5}
For Paved Channels: V (ft/s) = 20.3282 (s)_{0.5}

Tillage can affect the direction of shallow concentrated flow. Flow may not always be directly down the watershed slope if tillage runs across the slope. After determining average velocity, WMS uses the following equation to estimate travel time for the shallow concentrated flow segment:

T_t = \frac {L}{3600V}


Tt = Travel time (hr).
L = Flow path length (ft).
V = Average velocity (ft/s).
3600 = Conversion factor from seconds to hours.

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