From Part 630 Hydrology National Engineering Handbook, Chapter 15:
"Typically, sheet flow occurs for no more than 100 feet before transitioning to shallow concentrated flow (Merkel 2001)." and "Kibler and Aron (1982) and others indicated the maximum sheet flow length is less than 100 feet. To support the sheet flow limit of 100 feet, Merkel (2001) reviewed a number of technical papers on sheet flow. McCuen and Spiess (1995) indicated that use of flow length as the limiting variable in the equation 15–8 could lead to less accurate designs, and proposed that the limitation should instead be based on:
L = 100 * S^.5/n (eq. 15–9) (length, slope, mannings n)
Table 15-2 Maximum sheet flow lengths using the McCuen-Spiess limitation criterion
Cover type, n values,Slope(ft/ft),Length(ft)
Range ,0.13, 0.01, 77
Grass ,0.41, 0.01, 24
Woods ,0.80, 0.01, 12.5
Range ,0.13, 0.05, 172
Grass ,0.41, 0.05, 55
Woods ,0.80, 0.05, 28"

Sheet FLOW время поверхностной концетрации