WMS:TR-55 Shallow Concentrated Flow Equation

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

$T_{t}={\frac {0.007(nL)^{0.8}}{(P_{2})^{0.5}s^{0.4}}}$

where:

T_t – 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.

Related Topics

• Overview of Map Data Equations