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:

T_t = \frac {0.007(nL)^{0.8}}{(P_2)^{0.5}s^{0.4}}


Tt = travel time (hr).
n = Manning’s roughness coefficient (see Table 3-1 of the TR-55 manual).
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 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 for eastern US locations.

Generally the sheet flow equation should not be used for lengths greater than 300 feet. This simplified form of the Manning’s kinematic solution is based on the following:

  1. Shallow steady uniform flow,
  2. Constant intensity of rainfall excess (that part of a rain available for runoff),
  3. Rainfall duration of 24 hours, and
  4. Minor effect of infiltration on travel time.

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