WMS:TR-55 Shallow Concentrated Flow Equation: Difference between revisions

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Sheet flow generally occurs in the headwater of streams. The following equation is used to describe sheet flow:
After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined using the equations below, in which average velocity (V) is a function of watercourse slope (s) and type of channel:


: <math>T_t = \frac {0.007(nL)^{0.8}}{(P_2)^{0.5}s^{0.4}}  </math>
For Unpaved Channels: V (ft/s) = 16.1345 (s)<sup>0.5</sup>
For Paved Channels: V (ft/s) = 20.3282 (s)<sup>0.5</sup>
 
Tillage can affect the direction of shallow concentrated flow. Flow may not always be directly down the watershed slope if tillage runs across the slope.  After determining average velocity, WMS uses the following equation to estimate travel time for the shallow concentrated flow segment:
 
: <math>T_t = \frac {L}{3600V}  </math>


where:
where:


: <math>T_t </math> = travel time (hr).
: <math>T_t </math> = travel time (hr).
: <math>n </math> = Manning’s roughness coefficient (see Table 3-1 of the TR-55).
: <math>L </math> = flow path length (ft).
: <math>L </math> = flow path length (ft).
: <math>P_2 </math> = 2year, 24 hour rainfall (in).
: <math>V </math> = average velocity (ft/s).
: <math>s </math> = slope of the hydraulic grade line (ground slope) in ft/ft.
: <math>3600 </math> = conversion factor from seconds to hours.
 
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==
==Related Topics==

Revision as of 22:16, 19 March 2013

After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined using the equations below, in which average velocity (V) is a function of watercourse slope (s) and type of channel:

For Unpaved Channels: V (ft/s) = 16.1345 (s)0.5 For Paved Channels: V (ft/s) = 20.3282 (s)0.5

Tillage can affect the direction of shallow concentrated flow. Flow may not always be directly down the watershed slope if tillage runs across the slope. After determining average velocity, WMS uses the following equation to estimate travel time for the shallow concentrated flow segment:

where:

= travel time (hr).
= flow path length (ft).
= average velocity (ft/s).
= conversion factor from seconds to hours.

Related Topics


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