Lateral Weir Questions

[rabrownie]

I am attempting to model an anthropogenically disturbed canyon that does not exhibit properties fundamental to fluvial systems. The main channel in this canyon was pushed to the side of the wall, and a remant channel existings in the valley center. The main channel recieves all of the flow from usptream as it it tied into the stream network of the entire canyon. The remnant channel recieves oveflows from the main channel. I cannot model the entire valley cross section in these areas, because the remnant channel is lower (by 10 feet) than the main channel, Hec-Ras would push all of the water in the low channel. In order to model this situation I have been using lateral weirs, but I have been getting some weird results.

The first thing I noticed is my velocities through the sections with lateral weirs, even for non-overtopping flows, are very high (30-40ft/s in a creek 15' wide at 3-4%). Adding some interpolated sections helps this.

The flow profiles never converge.

Sometimes there are negative Q's leaving the channel, what does that mean? What is coming over the hill? Doesn't make sense.

Any help or guidance on this would be great, thanks.

[dawid_vancoller]

Have you tried using a levee?

If you put a levee on the highest point in the cross section between the two channels, no flow will occur in the remnant channel until the water level exceeds the elevation of the levee.

[sam74heci]

I've had problems with lateral weirs in the past. I had multiple lateral weirs in a channel and several different sections where the flow input changed. If/when the flow input changed the downstream weirs went nuts with negative flows and the like. I sent my model to the Corps of Engineers as a bug and they confirmed that it was.

here is a link to the thread I posted

http://www.bossintl.com/forums/hec-ras/14239-hec-ras-w-lateral-flows.html

[rabrownie]

Thanks for the levee suggestion. I tried that initially, but this portion of the canyon acts like it's own tributary and has a large influx of water halfway down, so I have to break them up and use the lateral weirs. Even the levees give me unrealistic results because if depths are greater in the overflow channel (because it is lower) it assumes it is carrying most of the flow, which is unrealistic because the main channel should be full and at max capacity, while the lower channel has flows less than 1/ 3 of the main.


SAM74, great reply. That's exactly the problem I have. I am going to try it in 4.0 and see what happens.

[sam74heci]

I don't think they fixed this but I hope I'm wrong. If they have not you should email them your bug. The more they see it the more likely they are to address the problem. Optimizing by hand wasn't very fun. You could always look at the worst case scenario and run your biggest flow numbers from the top of the channel and delete all other flow inputs downstream. Not very realistic as it sound like you are modeling a large basin but storm water models are not necessarily that realistic anyway

[rabrownie]

Thanks again. I am going to submit it because the bug is not fixed in 4.0 (non beta version).

I did a comparison modeling the basin with laterals and then using low levees (placed just at top of bank so flow is forced in the main channel). In comparing the two, they are within 0.25-0.5 ft of predicted WSE at low flows and the differences get as great as 2-3 ft at high flows. These errors (if you can call them that) are from the flow optimation scheme failing to converge over the weirs, so there is this excess head above the weir that would not likely be there if critical depth was reached over it.

[dawid_vancoller]

Do you have a link to download HEC-RAS 4 (non Beta). Only the Beta version is available on the website?