[dueckrandy]

Try running the individual items instead of “all”. Sometimes this will give you error messages that tell you what the problem is.

[dueckrandy]

Yes. Do you think that is the problem? Suggestions to resolve?

[dueckrandy]

Well, I would still complain if I had to do truly manual mapping through dozens of crossings . Since you have many of these to do, let me clarify what I meant by “manual mapping”. There is a quick way to map an entire RAS model in GIS by using exported cross sections with WSEL’s in the attributes. You can make spatial changes, extend cross-sections, add “mapping-only” cross sections, etc. Then with a few quick steps, re-map the entire model onto a topo raster. The basic steps are to produce a water surface tin, convert to raster, then compare it to the topo raster using the Raster Calculator tool to make an inundation grid raster. Then raster to poly.

[dueckrandy]

This is a problem you will encounter to varying degrees at almost every road crossing you ever model. As mentioned in the previous response, the problem is the linear interpolation of the water surface between the bounding cross sections. The greater the drop in HGL from u/s to d/s, the more pronounced this problem will be. A potential solution to this is to make manual adjustments to the mapped floodplain in GIS. You can do this by copying the bounding cross sections and moving them inward to roughly the top of slope on either side. Then set the WSEL of both of those cross sections to the model results for the u/s bounding cross section. This assumes that the WSEL passing over the roadway is constant, which I believe is a reasonable approximation for mapping purposes in many cases. Then use GIS tools to re-map the area through the crossing and make the required adjustments. If you want to get really detailed and accurately model the flow over a complicated roadway flowpath, you could build a separate model with additional cross sections over the roadway that excludes any culvert or bridge openings and sends the weir flow amount through those sections. Depending on the complexity of the situation, you can build really elaborate models with lateral weirs, separate calcs, rating curves, etc. From your screenshot, it looks like a pretty simply situation. The accuracy of mapping needed at a smaller roadway crossing is typically not very high, since it does not usually affect flood insurance for anyone. Just my 2 cents.

[dueckrandy]

The only type of boundary condition you can apply to a cross section in the middle of a river element is a rating curve – stage/discharge. This would let you force a defined water surface elevation at any cross section.

If you want to apply a normal depth boundary condition with a slope, you would have to model each reach as a separate river. You could then join the rivers by selecting Edit>Move Points/objects and moving the vertices to overlap where you want to create a junction between rivers.
Hope this helps!

[dueckrandy]

Andreas,

If I understand your question correctly, your goal is to determine the velocity in a pool which dissipates energy downstream of a fast-flowing channel. If that is the case, I don’t think modeling a storage area will benefit you. I think you are on the right track with an inline structure to model the drop. As long as you have adequately spaced cross sections to model the transition between channel and pool, I think you will achieve your goal. If the drop is not large enough to behave like a waterfall or plunge pool (ie. 100% kinetic energy dissipation), then you may need to place ineffective flow areas on the cross sections downstream of the drop.

[dueckrandy]

Sara, that depends on a lot of factors in addition to those you mentioned. If you mean a drainage channel, then it depends a lot on what slopes you are working with. It also depends on your n-value, and your upstream and downstream conditions. Also important are erosive velocity considerations. It is difficult to give you a very meaningful answer without knowing more about those.

[dueckrandy]

Have you entered a maximum headwater elevation that is too low? This is entered in the Bridge Culvert Data editor under HTab Param.

[dueckrandy]

Again, I must say that it is not possible for me to give a definite answer for your case without knowing the details of your project. It really depends on where the water goes when it spills over your levees. For example, does the water travel in a separate path and rejoin the main channel downstream? If so, then you may need to model it as a separate channel with split flow optimization and use lateral structures wherever you suspect water will spill back and forth between the 2 channels.

To be honest, I have never used levees in the final version of a model. In my experience, the main benefit of using levees instead of lateral structures is that they are quick and easy, whereas lateral structures take more time and effort. The other scenario where levees may be preferred is if you are designing a levee that is meant to completely contain flood waters, and you need a quick way to modify your model to try different options.

If you would rather not post some kind of project layout here, perhaps it would be better to discuss via email.

[dueckrandy]

Hi Mathilda,
This sounds like a fun project
It sounds like you have some idea of how the different elements of your model will interact. However, it is very difficult to make suggestions in your case without seeing the layout of the river, levees, and canal.
My only caution based on the info you gave is to be wary of levees. If a levee overtops, HEC-RAS assumes the same water surface elevation on both sides.

[dueckrandy]

If you have more than 2 or 3 cross sections to update, I would highly recommend using the GeoRAS tool for ArcGIS, if you can get access to it. The User Manual for GeoRAS is excellent.
Depending on how good your current model is (ie. is it georeferenced, are the n-values good, etc.), availability of aerial imagery, land use data, and other things, you will have to decide how much detail you want to put into your GeoRAS model. GeoRAS is simply a tool to get your RAS model built and populated.

Due to the hugeness of this kind of task, I can’t give you a detailed outline of steps, but feel free to ask specific questions along the way, and I can try to point you in the right direction.

Hope this helps.

[dueckrandy]

Hello NLund,

There are a few ways to do this. It would help if you described the source of the additional flow, but I will tell you based on the assumption that you have a model of a river working with an existing steady flow file, and there is a a channel or pipe which you are not modeling, which is discharging into the river.
1. Open the Steady Flow file: >Edit>Steady Flow Data…>
2. Find the appropriate River and Reach containing the cross section where you want the additional flow to begin. It is extremely important that you understand how and where the Flow Data changes are taking place both upstream and downstream of the point Q. Once you understand this, you will need to either “Add A Flow Change Location” at the desired cross section, or modify it if it exists already. Then, you will need to add the additional point Q to the existing Q in that cross section and also to every flow change location downstream of that point in the model. In other words, HEC-RAS does not have the option to simply add a point Q – you must specify the cumulative flow at every flow change location in the model.

Hope this helps.

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