WMS:TR-55 Sheet Flow Equation

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Sheet flow generally occurs in the headwater of streams. The following equation is used to describe sheet flow:


File:WMSEqation132.png


where:

Tt – travel time (hr).
n = Manning’s roughness coefficient (see Table 3-1 of the TR-55).
L – flow path length (ft).
P2 – 2year, 24 hour rainfall (in).
s – slope of the hydraulic grade line (ground slope) in ft/ft.

2 year, 24 hour rainfall values can be determined from the map in appendix B-4 of the TR-55 reference manual for eastern US locations and from the NOAA Atlas 2 maps for the western US locations.

Generally the sheet flow equation should not be used for lengths greater than 300 feet.

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