Overland Flow Time of Concentration

The overland flow time of concentration, tov, can be computed using the equation

$t_{ov}=\frac{L_{slp}}{3600*v_{ov}}$ 2:1.3.3

where $L_{slp}$ is the subbasin slope length (m), $v_{ov}$ is the overland flow velocity (m s$^{-1}$) and 3600 is a unit conversion factor.

The overland flow velocity can be estimated from Manning’s equation by considering a strip 1 meter wide down the sloping surface:

$v_{ov}=\frac{q_{ov}^{0.4}*slp^{0.3}}{n^{0.6}}$ 2:1.3.4

where $q_{ov}$ is the average overland flow rate ($m^3 s^{-1}$), $slp$ is the average slope in the subbasin (m $m^{-1}$), and $n$ is Manning’s roughness coefficient for the subbasin. Assuming an average flow rate of 6.35 mm/hr and converting units

$v_{ov}=\frac{0.005*L_{slp}^{0.4}*slp^{0.3}}{n^{0.6}}$ 2:1.3.5

Substituting equation 2:1.3.5 into equation 2:1.3.3 gives

$t_{ov}=\frac{L_{slp}^{0.6}*n^{0.6}}{18*slp^{0.3}}$ 2:1.3.6