I have used the TR-55 method quite a few times for mostly large scale commercial and industrial developments in the Northeast.  I generally limit sheet flow to less than 100 feet.  If the application is for parking areas or curbed asphalt pavement, it is unlikely that the sheet flow distance will be more than 50 feet.  Sheet flow, by definition, must be less than an inch deep. Utilizing 300' for sheet flow will give you seriously undervalued runoff amounts.

I use TR-55 a lot.

Sheet flow should be reserved for natural ground cover.

Nothing in the category of improved or man made.

The shorter the length the more conservative the design.

I will always use 200 for the max. length.

I will use a shorter length for a slope greater than 5% or if the time of concentration is very low.

Good luck.

Try to stay away from rules of thumb.  Look at site maps, topo maps, etc, for this particular project.  Get out and walk the site.  Eventually you will develop your own feel for what makes sense.

A BALLFIELD, SUCH AS A BASEBALL DIAMOND, MAY APPROACH 200'

NRCS (formerly Soil Conservation Service) hydrology uses the Manning-Kinematic equation for the solution of overland flow. For the sheet flow segment, the maximum length is 300', with a most likely length of 100' on unpaved areas. For paved areas, lengths may be longer than 100' until flow concentrates and begins to be channelized. Keep in mind that for sheet flow, the mean depth is 0.002' for paved areas, and 0.02' for vegetated areas. With those nominal depths, the length of sheet flow will seldom exceed 100'-200'.

McCuen has an estimate for the length of sheet flow.  It is given in his book Hydrologic Analysis and Design.  In English units it is L = 100 sqrt(S)/n.  This is to be used with the iterative time of travel equation only and the peculiar result is that in a given location the time of travel depends only on the idf curve you use.  For example, in my area the sheet flow time of travel is 6.94 min for the 10-yr storm regardless of L, surface roughness, or slope.

I was always under the impression, for highway drainage analysis, that the sheet flow length is equal to the distance from the high point to the first inlet.  In some cases for existing drainage systems, that length can approach 300 feet.  Now that I am reading the posts in this thread I think I may be wrong.  Anyone have any input on this as it applies to highway drainage?

I HAVE USED TR-55 QUITE FREQUENTLY AT WORK, FOR USE IN DRAINAGE ANALYSIS OF SUBDIVISIONS, AND COMMERICIAL SITES. mOST OF THE WORK I DO IS IN WESTCHESTER, NEW YORK. IT HAS BEEN ENCOURAGED BY THE TOWN AND DEP TO USE 150' AS THE VALUE FOR SHEET FLOW. ALTHOUGH 300' IS ALLOWABLE, TAKING CONSERVATIVE VALUES IS NEVER A BAD IDEA.

Here in new jersey we are only allowed to use 150' for maximum sheet flow. Your best bet for clculating tc is to use a dot nomograph. I have never had a drainage analysis come back when I used this figure.

Hi!
To really get a feel for Tc,  educate yourself as to how Tc is determined.  Read do not glance at the TR55 manual. It's a good starting point.  Vist sites during storm events and observe to get a feel as to waht is going on, especially take note as to existing soil conditions and cover variables (wet or dry soil, tall or just mowed grass, etc..)

Then see what the industry is doing.  Ask yourself if you agree or disagree with there methods and why!  Run some sample comps and check in the field for results, after all this is how the mathematics of hydrology is built upon.  Use common sense, and by all means if everything else fails, do what they tell you to do, the way they tell you to do it, unless you can prove them wrong!