Tc Calculation confusion 3

[daddymem]

The standard practice where I am is to use TR-55 sheet and shallow concentrated flow when determining time of concentration (paved or unpaved for shallow concentrated flow). My superior is asking that I change to the Upland Method for my models. But he says that the first 50' would still use TR-55 sheet flow. Is this mixing apples and oranges?
The resulting peak flows when going from upland method to TR-55 method are substantially different. Any comments on Upland vs TR-55 and when which should be used? I don't think we'll change what the review engineers are used to (TR-55) but I'd like to know for myself.

TIA!

 

[psmart]

From NEH-4: The upland method is intended for conditions that occur in the headwaters of a watershed, including overland flow, grassed waterways, paved areas, and small upland gullies. Upland method is applicable to areas of 2000 acres or less.

As such, the upland method is usualy used on its own, or perhaps with a segment of channel flow. But adding a segment of sheet flow will (artifically) boost the Tc.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

upland method is commonly used for shallow concentrated flow calculation and there is probably no need to add the 50 feet of sheet flow - unless you can identify on the mapping or in the field where sheet flow actually occurs and then transitions to rills and gullies. However, 2,000 acres would be pushing it in my opinion. Anything over about 10 - 20 acres, you will probably transition out of shallow flow into channelized flow.