As an aside, this "detail" is extremely critical in modeling.

In my experience there is typically an order of magnitude difference between velocities in the sheet flow regime as compared to shallow flow, therefore what you pick for a length is the prime determinant in what time of concentration you end up with. The shortened overland path is one of the key impacts of development. Simplifying an entire watershed into a single "representative" path, and treating the transition from overland flow to shallow as a hard divide rather than a continuim are both significant simplifications, which seems to raise this one variable to the level of a "black knob" that should be considered carefully.

Bill Heatherman
bheather@OPKANSAS.ORG