The sheet flow procedure was originally limited to a maximum length of 300 feet, although many agencies use a lower limit, sometimes as little as 50 feet, so you need to check your stormwater regulations.

BTW, it doesn't matter which runoff procedure you use (Rational method, SCS, etc) the issue of the sheet flow length is the same.

Peter Smart
HydroCAD Software
www.hydrocad.net

depending on the methodology, "rational method" time of concentration may not include any limitation to length of flow path (sheet flow or otherwise).

I was not suggesting a limit to the overall flow length, only to the portion that is evaluated as sheet flow.

Unless the surface is a perfect plane, sheet flow cannot persist for more then 50-300 feet. At some point the flow depth will start to vary and it will transition to a different flow regime. The sheet flow procedure is also limited to a flow depth of 1/10 foot.

See the National Engineering Handbook for complete details, including the length limitation.
https://directives.sc.egov.usda.gov/OpenNonWebCont...

Peter Smart
HydroCAD Software
www.hydrocad.net

while I agree with you, some methods do not distinguish the flow regime when calculating the Tc.
for example, see the method below which does not consider sheet flow at all:

Understood. There are various procedures that do not restrict the flow length. But the OP was specifically asking about "sheet flow", which has definite limitations on flow length, as indicated in my reply and referenced documents.

Peter Smart
HydroCAD Software
www.hydrocad.net

the OP question was whether he could use the rational method. if he uses the method I posted, than yes he can, if he uses the NRCS method, than no he must follow the requirements for sheet flow