Contributing area above a Tc flow path.

[TOFDPWENG]

I'm reviewing stormwater calculations and have a Time of Concentration question.

The engineer shows the start of a Tc path mid-slope on a hill at the edge of a woodline. My comment is that there is contributing area above this point and the Tc path should start at the top of the slope. The engineer's argument is that starting at the top of the slope results in a faster Tc than starting mid-slope so the longer time should be used. Is that correct?

My thinking is that the upstream area would "catch up" with the flows and accelerate them. In this case, the distance is not a lot so the flow would be shallow concentrated flow from the higher start where the lower start would be modeled sheet flow still.

If that is correct, I am not sure how to explain that clearly.

If I am not correct, please explain it to me.

Thanks in advance.

 

[psmart]

The Tc path should start at the "most hydrologically remote point" in the subcatchment. This is not necessesarly the highest point in the subcatchment.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TOFDPWENG]

Hi Peter,

Yes, that is correct, but that isn't the question. I can explain it better.

Let's say we have determined the most hydrologically remote point of any given sub-catchment and developed a Tc flow path. We have 50 feet of light underbrush woods at 10% slope and 100 feet of short dense grass at 0.5% slope.

Case 1: It's 50 feet of light underbrush woods at 10% slope, then 100 feet dense grass (unpaved) at 0.5% slope for a Tc of 7.6 minutes. Length = 150 feet

Case 2: Same sub-catchment and flow path, but we choose to ignore the first 50 feet from above and start the Tc at the edge of the woods instead, so we have 50 feet of dense grass at 0.5% slope, then 50 feet of dense grass (unpaved) at 0.5% slope for a Tc of 9.9 minutes. Length = 100 feet.

Case 2 is mathematically longer, but it ignores 50 feet of upstream woods along this flow path. Doesn't the Tc have to start at the highest point of any given path(which may or may not be the highest point in the sub-catchment)?

 

[psmart]

The most hydrologically remote point is the one with the longest travel TIME. In your case, this may not take account of other paths that may be longer and/or from a higher elevation. It's the longest travel TIME that counts, since this determines when runoff from all portions of the subcat will reach the point of collection.

Peter Smart
HydroCAD Software

http://www.hydrocad.net