sheet vs shallow flows 1

[CivilTyro]

How do you determine the lengths for sheet and shallow flows in a watershed? Is there a standard rule of thumb or depends on watershed characterstics?

 

[BelspringKen]

The lengths of flows depend on the watershed (or topo lines)

Generally, the max length for sheet flow is considered to be 200' (sometimes up to 300') , but you may find that the reviewing agency will only accept 150' or 100' as the maximum. Then the flow becomes a shallow concentrated flow, and you should be able to determine that length using the contours on your topo map or base map.

 

[psmart]

On site-inspection during a rainfall event is the best way to determine actual sheet flow length. Since this is often impractical, local regulations usually prescribe a maximum of 50-150 feet.

The NRCS has a technical note on this topic at

http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/tech_refs/H&Hpp.html

You may get a better response to your question by posting in the Flood/Storm Engineering forum.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jeffcivil2]

Although the maximum length for sheet flow is in the 300' range, what typically happens when an engineer sees no defined channel is he uses the 300'(for example if your site is just a hill), and then the rest of the actual length is computed as shallow. A jurisdiction where I work has informed us that we can't use sheet flow of more than 25% of shallow flow. I personally think this is a good policy to follow, but you won't see such an outright statement often, because the science is a little iffy.