WMS:TR-55 Channel 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:
Open channels are assumed to begin where surveyed cross section information has been obtained, where channels are visible on aerial photographs, or where blue lines (indicating streams) appear on United States Geological Survey (USGS) quadrangle sheets. Manning’s equation or water surface profile information can be used to estimate average flow velocity. Average flow velocity is usually determined for bankfull elevation.


: <math>T_t = \frac {0.007(nL)^{0.8}}{(P_2)^{0.5}s^{0.4}}  </math>
Manning's Equation is:
 
: <math>V = \frac {1.49 r^{2/3}s^{1/2}}{n}  </math>


where:
where:


: <math>T_t</math> = travel time (hr).
: <math>V</math> = average velocity (ft/s).
: <math>n</math> = Manning’s roughness coefficient (see Table 3-1 of the TR-55).
: <math>r</math> = hydraulic radius (ft) and is equal to a/p<sub>w</sub>
: <math>L</math> = flow path length (ft).
:: <math>a</math> = cross sectional flow area (ft<sup>2</sup>)
: <math>P_2</math> = 2year, 24 hour rainfall (in).
:: <math>p_w</math> = wetted perimeter (ft)
: <math>s</math> = slope of the hydraulic grade line (ground slope) in ft/ft.
: <math>s</math> = slope of the hydraulic grade line (channel slope, ft/ft)
: <math>n</math> = Manning’s roughness coefficient for open channel flow.
 
Manning’s n values for open channel flow can be obtained from standard textbooks such as Chow (1959) or Linsley et al. (1982). After average velocity is computed using Manning's equation, the travel time T<sub>t</sub> for the channel segment can be estimated using the following equation:
 
: <math>T_t = \frac {L}{3600V} </math>


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.
where:


Generally the sheet flow equation should not be used for lengths greater than 300 feet.
: <math>T_t </math> = travel time (hr).
: <math>L </math> = flow path length (ft).
: <math>V </math> = average velocity (ft/s).
: <math>3600 </math> = conversion factor from seconds to hours.


==Related Topics==
==Related Topics==

Revision as of 22:31, 19 March 2013

Open channels are assumed to begin where surveyed cross section information has been obtained, where channels are visible on aerial photographs, or where blue lines (indicating streams) appear on United States Geological Survey (USGS) quadrangle sheets. Manning’s equation or water surface profile information can be used to estimate average flow velocity. Average flow velocity is usually determined for bankfull elevation.

Manning's Equation is:

where:

= average velocity (ft/s).
= hydraulic radius (ft) and is equal to a/pw
= cross sectional flow area (ft2)
= wetted perimeter (ft)
= slope of the hydraulic grade line (channel slope, ft/ft)
= Manning’s roughness coefficient for open channel flow.

Manning’s n values for open channel flow can be obtained from standard textbooks such as Chow (1959) or Linsley et al. (1982). After average velocity is computed using Manning's equation, the travel time Tt for the channel segment can be estimated using the following equation:

where:

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

Related Topics


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