Flow optimisation over lateral structure does not reduce downstream flow

[Naomi Moran]

I am trying to model water flooding over one bank of a watercourse and calculate how much this reduces the flow downstream (Water that has left the channel will not flow back in). I have added a lateral weir and have set the tailwater connection to ‘out of the system’. However, although the model gives a value for flow over the lateral structure, the downstream flow does not decrease.
I tried to include ‘flow optimization’, but after clicking on ‘Optimization’ in the Lateral Structure Editor window, all tabs are greyed out except the ‘Lateral Structures’ tab; however, this tab has one row with columns ‘Junction’ and ‘Optimize’. I can click on ‘optimize’ and get a tick in the box, but upon running the model it makes no difference to the results.

Can you tell me what is wrong? I’m using HEC-RAS 6.0.0 Beta 3 Dec 2020.

Many thanks.

[Luis Partida]

First off, you may want to use RAS 6.1 as the beta has been outdated.

Flow optimization using lateral structures takes a little more effort, i will try and explain. Your tailwater condition should not be out of system. Instead connect it to a theoretical SA and then turn optimization on. Assuming RAS converges, you must then go into the results file and you will see a reduction in flow rates, you must then create a new flow file with these adjusted flow rates, turn optimization off and then rerun your simulation. Hope this helps

[Naomi Moran]

Thanks Luis! I have a 1D model: is it possible to add a storage area?

[Luis Partida]

Yea good point, steady state doesnt need it, ignore that part of the explanation

[Naomi Moran]

Luis,
Many thanks for your help, and sorry for the delay in my reply.

I have downloaded HEC-RAS version 6.1 and tried it out with my lateral weir.

In the results table I get a flow over the weir (Q Weir). The tailwater connection is set to ‘Out of the System’; however, the discharge in the channel downstream of the weir is not reduced.

Also, for one profile in my flow plan, the flow over the weir is much greater than the flow rate entered in the plan: the total flow rate for the profile is 1.62 m3/s but the flow leaving over the weir is apparently 3.17 m3/s, 196% of the total flow in the cross-section. How can this be?

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