WMS:TR-55 Channel Flow Equation

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:

$V={\frac {1.49r^{2/3}s^{1/2}}{n}}$

where:

$V$ = average velocity (ft/s).

$r$ = hydraulic radius (ft) and is equal to $a/p_{w}$

$a$ = cross sectional flow area (ft²)

$p_{w}$ = wetted perimeter (ft)

$s$ = slope of the hydraulic grade line (channel slope, ft/ft)

$n$ = 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_t for the channel segment can be estimated using the following equation:

$T_{t}={\frac {L}{3600V}}$

where:

$T_{t}$ = travel time (hr).

$L$ = flow path length (ft).

$V$ = average velocity (ft/s).

$3600$ = conversion factor from seconds to hours.

Related Topics

• Overview of Map Data Equations