WMS:FHWA Shallow Concentrated Flow

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After 300 feet, sheet flow usually turns to shallow concentrated flow. The following equation, based entirely on the length and slope of the arc, is used to compute the travel time for the shallow concentrated segment of flow:

 T_t = \frac {L}{60k \sqrt {S}}


Tt = travel time for open channel flow segments.
L = Length of flow segment.
k = intercept coefficient (values are given in Table 3-3 of the FHWA HEC 22 manual and are repeated in the table below).
S = slope of the ground surface as a percent.

Intercept coefficients for velocity vs slope relationships
Land Cover / Flow Regime k
Forest with heavy ground litter; hay meadow 0.076
Trash fallow or minimum tillage cultivation; contour or strip cropped; woodland 0.152
Short grass pasture 0.213
Cultivated straight row 0.274
Nearly bare and untilled; alluvial fans in western mountain regions 0.305
Grassed waterways 0.457
Unpaved 0.491
Paved area; small upland gullies 0.619

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