Thanks for your input TerryScan, but I found Technical Note No. N4 to be quite vague for estimating sheet flow lengths (i.e. "The maximum flow length of 300' with a most likely length of 100' should be used in overland flow computations for unpaved areas"). I was hoping for something a bit more concrete that provided a way to actually calculate sheet flow length.
The NRCS has summarized a number of additional research papers at ftp://ftp.wcc.nrcs.usda.gov/support/water/hydrology/Sheet_Flow_References.doc
The prevailing guideline is that sheet flow depth must not exceed 1/10 foot. (At that point it becomes shallow concentrated flow or channel flow.) However, this still tends to require on-site inspection. From the NRCS paper:
"With all the variability across the country with respect to soil, land use, climate, geomorphology, etc, there is no substitute for investigating sheet flow characteristics of the watershed in the field."
You can also find this document under "H&H Papers" at
The final determination of sheet flow length generally comes down to a discussion with the reviewing agency. Because of this subjectivity, some engineers and agencies are opting for other Tc procedres with more objective parameters, such as the Upland Method or Curve Number method.
These procedures are discussed in NEH-4 Chapter 15, pages 6-9 available at
Since sheet flow occurs over very short distances, if at all, the best, absolute reference is.... your eyes! Just go out to the site and look around for evidence of flowing water of any kind. Take a camera with you. Make sure you can accurately pace off distances or bring a long tape if you have to.
Otherwise, follow NRCS tech note 4 and expect sheet flow to be much less than 100 ft - also, the Note states that it is assuming a certain minimum amount of rainfall to occur in order for sheet flow to occur. You may not have any surface flow accoring to their Note for storms less than 2yr storm depth. Check it out carefully.
