TR-55 Sheet Flow limit
TR-55 Sheet Flow limit
(OP)
We have been using sheet flow of various lengths up to the maximum length of 300' per TR-55 for various projects. During a recent review, a question came up regarding the use of the 150-200' sheet flow lengths and the reviewer notes that the NRCS now limits the 1ength to 100'. We use HydroCad which allows up to 300 feet for sheet flow computations. We are not aware of any local agency or any other reviewer that uses the 100 foot limit for sheet flow. The state highway drainage manual also uses 300 foot as the limit for sheet flow. Thoughts?





RE: TR-55 Sheet Flow limit
"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"
Some time ago regulators in my state performed informal field observations during storm events and have stated that the 300 ft. length is only achieved in ideal conditions on smooth surfaces (perfectly paved parking lot.)
RE: TR-55 Sheet Flow limit
For further details see "References on time of concentration with respect to sheet flow" published by NRCS. A PDF version is available at www.hydrocad.net/pdf/SheetFlowRefs.pdf
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: TR-55 Sheet Flow limit
I honestly hate the TR-55 method for Tc. If you've got a 10 acre watershed and one corner of it has 100 feet of sheet flow through dense forest, but the rest of it is paved, do you really think that little forested corner is having that big of an impact on your runoff hydrograph? Obviously not. And if that's the case, then you're making a judgment call to abandon the method situationally based purely on your own judgment. And if that's the case, then what good is the method? Why not just use your own judgment for the Tc on every basin?
I much prefer other methods that use mean basin slope and land cover.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: TR-55 Sheet Flow limit
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: TR-55 Sheet Flow limit
Robert Billings
www.newrivereng.com
RE: TR-55 Sheet Flow limit
It seems odd that one allows a longer sheet flow in rougher land covers. Surface irregularities tend to cause concentration. (As reflected in the NEH cited above).
RE: TR-55 Sheet Flow limit
Yes, in NJ, the DEP and DOT mandate a maximum of 100 sheet flow length. This is bunk! This is just a way to make sure the calculations are conservative enough. We should be able to do better than that.
RE: TR-55 Sheet Flow limit
The paved area should be analyzed separately, then analyze the total composite area, then use the highest peak flow of the two. There are two conditions to evaluate, with two separate Tc's.
RE: TR-55 Sheet Flow limit
I vastly prefer the CN method Peter references, and if not that, then some other direct methods such as Kirpich, the "USBR Design of Small Dams" (1973) equation, FAA, Izzard, etc. Any method where an assumption dominates a parameter is a bad method, and one cannot possibly know every sheet flow condition for every 100 foot stretch in a large watershed.
But as long as the reviewers demand it, I'll keep using it. I just check it internally vs other methods to make sure I believe what the method is telling me.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: TR-55 Sheet Flow limit
When characterizing watersheds, you either model the whole area with a composite CN, or you break the area up and model each sub area with CNs characteristic of that area, then lag and sum the hydrographs of both appropriately to create a composite hydrograph that includes the flow from both sub areas.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: TR-55 Sheet Flow limit
This is incorrect. I think this is why you have confusion when thinking about what is the correct Tc for a catchment with a pervious area draining over an impervious area to the common catchment outfall.
If you were designing an inlet or culvert and you did not do this test, and just did a composite calculation for the catchment, there is a chance that the smaller downstream impervious could have a higher peak flow. You would miss that if you didn't do this test, and you would not have the max peak to correctly size your stormwater appurtenance.
RE: TR-55 Sheet Flow limit
RE: TR-55 Sheet Flow limit
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: TR-55 Sheet Flow limit
One thing to be aware of regarding the NRCS method in this directly connected impervious scenario, with a larger pervious area over a smaller impervious area, is to not use a weighted average CN number. This is due to the computation of Ia using the weighted average CN.
This produces an erroneously high average initial abstraction value for the composite basin, that includes the impervious area. The modeler will not see how, at times, the smaller impervious area is dominating. In certain previous/impervious basin cover ratios virtually all of the runoff is coming from the smaller impervious area.
RE: TR-55 Sheet Flow limit
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com