# 2:1.3.1.1 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

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