WMS:TR-55 Sheet 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).

$P_{2}$ = 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 NOAA Atlas 2 or NOAA Atlas 14 maps for many US locations or from the map in appendix B-4 of the TR-55 reference manual [1] for eastern US locations.

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

Related Topics

• Overview of Map Data Equations