With regard to sheet flow legnth, McCuen recommends that the sheet flow length be given by L n/sqrt(S) = 100 ft. This is for use in the iterative equation for time of travel that contains L n/sqrt(S) in the equation. This approach has some appeal because intuitively, it seems that the length of sheet flow should increase with slope and decrease with surface roughness.

The effect of using this is that the time of travel for sheet flow ends up being a function only of the IDF curve. This means that the sheet flow time of travel does not depend on surface roughness or slope. McCuen's paper "Policy Considerations in Time of Concentration Estimation," (J. of Floodplain Management, Vol.2, No. 1, July 2000) makes for interesting reading.

Warren Campbell
Warren.Campbell@HSVCITY.COM