Dear fellow HEC-RAS users:
I am reviewing a drainage design that involves a reach of about 3000 feet of open channel, with a large (7-foot) concrete pipe running a few feet below the channel. The idea would be that the pipe would convey flow during most storms, and the surface channel would only be used to convey flow during larger storms. It is not certain at this time whether or not there will be any catch basins in this reach connecting the surface channel to the pipe below. At the upstream end of this reach is a railroad culvert with open channel upstream. At the downstream end of this reach is a roadway culvert with open channel downstream.
I want to test the design by creating a HEC-RAS model. I am mulling over a couple of options:
1) Model the pipe and channel as a long culvert or bridge. The main problem I see with this is maintaining the same numerical flow "split" between the channel and pipe through the entire reach (assuming there are no catch basins), plus using the weir flow routine for the surface channel flows doesn't sound very accurate.
2) Model them as just a channel with an obstructed area. Again, I see a problem maintaining the same numerical flow split between the channel and pipe.
3) Model them as two separate reaches with a diversion upstream and a junction downstream. It looks like this would require a number of iterative runs to balance heads and discharges.
I would appreciate any thoughts, preferences or warnings you may have on modeling this type of system. Thanks for your input.
Chris Hauser, P.E.
FP&S, Inc.
chris.hauser@fpsengineering.com
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