HEC- Ras: Water leaves riverbed

Question

HEC- Ras: Water leaves riverbed

manholze (Civil/Environmental)

(OP)

6 Jun 16 06:24

Hello everybody
I'm new in ths Forum and this is my first post. So if I forget to follow any Forum rules or something, please tell me.

Now to my Problem. We are doing a river rehabilitation/ renaturation. In HEC- Ras we implemented the future crosssections of the river. The Problem we have, is that if we do a stady flow Simulation we get water covered areas beside the riverbed (look at attached photo). The Areas which are cover with water and do not belong to the riverbed are always lower than the riverbed, which is probably the reason for Problem. So my question: is it possible to have lower floodplains in HEC-Ras, without them be covered by water?

Thanks for your help
have a nice day!

cvg (Civil/Environmental) · Answer 1 · 2016-06-06 19:10:46

well, either modify your cross section to the limits of the channel, make this an ineffective flow area or accept the fact that you have a floodplain that you were not aware of.

nkrigPE (Civil/Environmental) · Answer 2 · 2016-06-07 02:05:41

HEC-RAS fills the cross section by elevation starting at the bottom and considers the entire input cross section. You could mirror that cross section over and over and you would get water in the valleys in each segment.

As stated above, you either need to end your cross section at the top of bank or it an ineffective flow area.

As an aside, it's pretty obvious your channel won't convey whatever storm events your are modeling. I would model the top of bank as a lateral structure so that you can determine how much water is cresting the bank. From the look of you essentially have parallel channels.

gbam (Civil/Environmental) · Answer 3 · 2016-06-07 14:07:54

manholze - cvg mentioned one technique while nkrigPE mentioned another. If your typical river section does not have the capacity to convey your design flow rate then you may want to evaluate split flow and model the parallel channel as a separate reach. a quick method to determine if the channel has capacity is to place a levee at the top of bank ground elevation (STA 4.5 +/-) for each cross section. Flow will be contained within the levee until the levee is over-topped. If the levee elevation is over-topped then you can add the lateral weirs to evaluate the split flow, etc... Hope this helps you out.

manholze (Civil/Environmental) · Answer 4 · 2016-06-08 14:50:08

RE: HEC- Ras: Water leaves riverbed

manholze (Civil/Environmental)

(OP)

8 Jun 16 14:50

Thanks, you could helb me! :)

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HEC- Ras: Water leaves riverbed