TR-55 Tc Question

[jmccabe001]

A basic question: when adding a channel flow segment for computing the time of concentration how does one decide what flow depth to use (or wetted perimeter and hydraulic radius)? I'm using HydroCAD. My difficulty is that the flow depth depends on the flow which is what I'm trying to figure out in the first place. I could assume a depth but how would I know if this depth yields the right velocity giving the right channel flow time to compute the Tc. Does anyone understand what I'm talking about.

And while we're at it - let's say I get it right for the 2-yr storm, it seems incorrect to assume that for all the other storms the depth will be the same (greater return period gives bigger flows which in turn gives a differant velocity and thus a differant flow time).

Now in the end the answer may be that the difference in channel flow time is insignificant compared to the overall Tc and that I can neglect it, but I am unsure.

 

[KatieTX]

Iterate.

 

[cvg]

you can iterate, or you can assume a conservative depth and just move on. probably doesn't matter as the peak flows will probably not be all that sensitive to minor variations in depth and velocity. you can confirm that by running a sensitivity analysis.

 

[psmart]

In the subcatchment summary, HydroCAD reports the channel capacity for each segment of channel flow. You can use this to adjust the flow depth to achieve the desired flow.

But as others have mentioned (1) the velocity typically doesn't vary that much with flow depth, and (2) the channel-flow travel time is usually small in comparison to the sheet and shallow-concentrated flow. If the channel-flow travel time IS significant, you may want to use a separate reach routing anyway.



Peter Smart
HydroCAD Software

http://www.hydrocad.net