Just a small caution with respect to one of Mr. McRae's comments, namely "...sheet flow would turn into shallow concentrated flow (after 300 feet of sheet flow, using TR 55 rules of thumb)." 300 feet is rather long for sheet flow except under the most favorable conditions (e.g., VERY smooth surface).

NRCS recognizes this, despite what is written in TR55. For example, Hydrology Technical Note No. N4 says: "The maximum flow length of 300' with a most likely length of 100' should be used in overland flow computations for unpaved areas." Further, USDA/SCS in Pennsylvania (Bulletin PA210-0-5, 1990) indicates that the maximum for unpaved areas should be 150', with most likely length being 50 to 100 feet.

Jeffrey M. Rice, P.Eng.
jrice@URBAN-SYSTEMS.COM