Shallow concentrated flow - TR-55 vs. NEH-4/HEC 19 1

[bencraddock]

I've been comparing the figures in TR-55 Fig. 3.1 and NEH-4 Fig. 15.2 (also in HEC 19 Fig. 52) for determining shallow concentrated flow. It looks like the TR-55 figure uses the same data as the NEH-4/HEC 19 figures but the TR-55 doesn't include all the ground cover conditions that NEH-4/HEC 19 does. Does anyone know why TR-55 limits the ground cover options to just "paved" and "unpaved"?

Thanks,
Ben

 

[psmart]

I agree, the underlying equations are the same. The difference in recommended coefficients seems to be due to somewhat different applications:

TR-55 uses this equation specifically for "Shallow Concentrated Flow", and typically combines it with a segment of sheet flow to handle the lower velocity in uppermost part of the drainage area. (This sheet flow segment is often the largest contributor to the overall Tc for the drainage area.)

NEH-4, on the other hand, refers to this as the "Upland Method", and recommends it for a wider range of conditions, including overland flow, grassed waterways, paved areas, and small upland gullies. Since it was intended to cover the flow from the entire upland area (including any sheet flow) lower coefficient values are needed to handle the lower flow depths and wider range of surface conditions.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[bencraddock]

Thanks for your information answer Peter,

Let me take this a bit further then...Since the Upland Method already includes a sheet flow component, then if it were used to determine the shallow concentrated flow component for the TR-55 method and then combined with the sheet flow component, it would yield an artificially increased Tc? (Since you're double counting the sheet flow). The PA DEP E&S manual allows this to be done and I'm wondering if it is an acceptable method.

Additionally it also seems like if one used the Upland Method (as intended by NEH-4) to find the shallow conc. flow by adding the times of multiple segments of differing ground cover, that a longer Tc would result since the "sheet flow component" is being factored in multiple times for multiple segments. Is that an inherent weakness of using the Upland method which is why the TR-55 doesn't recommend it?

 

[psmart]

NEH-4 page 15-7 DOES recommend using multiple flow segments for the upland method:

"The travel time for each type of upland flow can be computed using equation 15.1. The summation of these travel times will equal the Tc in the upland or subwatershed..."

However, I tend to agree with your concern about possible double-counting if combined with the sheet flow procedure. When using a segment of TR-55 sheet flow, I would stick with the paved/unpaved options listed in TR-55. In short, use each method as it was intended and don't mix them up.


Peter Smart
HydroCAD Software

http://www.hydrocad.net