[Chris G.]

It depends. If your drop will be highly submerged (tailwater depth is appreciably higher than the brink elevation) then just model it with cross sections. One upstream of the brink, and one downstream. Otherwise, you might try using an inline structure.

[Chris G.]

Sounds like the flow is being delivered to the bypass channel at the third cross section from the top. By default, I believe RAS does not allow you to connect to the upstream-most cross section, but you might double check this. You can always put 1 or 2 “dummy” cross sections at the top of your bypass channel to overcome this limitation.

[Chris G.]

Make sure that the reach is indeed connected to the storage area. This may assist you:
http://hecrasmodel.blogspot.com/2010/08/how-to-use-storage-area-to-define.html

This post discusses connection of a reservoir to a reach, but the way to connect the reach to a storage area is the same. You have to “move” the last vertex of the reach line into the storage area for RAS to recognize the connection.

[Chris G.]

Mike-

The properties input boxes where you enter this information is greyed out (disabled). Not sure why. Waiting to hear back form HEC on this one. In the meantime, you might explore editing (or creating) the world file for you image. Perhaps that will get you where you need to be in the meantime. Wikipedia actually has a decent writeup on worldfiles. http://en.wikipedia.org/wiki/World_file

Chris G.
@RASModel

[Chris G.]

You’ll probably find more success doing all of your pre-processing in ArcGIS. Then use the HEC-GeoRAS extension to bring everything into HEC-RAS, already georeferenced.

Chris G.
@RASModel

[Chris G.]

You should be able to use a constant discharge for your upstream boundary. And that will be a more appropriate setup. However, if your upstream boundary is far enough upstream that it is not “felt” by the tides, then I suppose it could work. The instabilities you are noticing when using a flow hydrograph for the upstream boundary is probably due to very low stages (these tend to make the model go unstable). Adding a pilot channel, increasing Manning’s n values, or adding more base flow can all help provide stability during low flows.

And you can still get reverse flow through the culvert, even with an upstream flow hydrograph boundary condition.

Good luck.
Chris

[Chris G.]

Adam-

Traditional convention is to assume that if water gets that high, there will be sufficient debris to effectively block the open area through the truss. Also, making that assumption provides a conservative result. Therefore many modelers go ahead an consider it blocked. However, you could certainly make the case that if you don’t expect a lot of debris, it will remain open. In which case you’d ignore the truss work in your station elevation points and maybe decrease your weir coefficient slightly to account for the energy loss of the truss sections. Wither way, pretty subjective. I’d suggest running both, and try a range of weir coefficients and see how it affects your results. The objectives of the study, and how much risk the client is willing to take usually will dictate which way you model this.

Why can’t all bridges be single span slabs?!?!

Good luck!
Chris
@RASModel

[Chris G.]

If you have a hydraulic jump upstream of a culvert, that would imply that just upstream of the jump, you have supercritical flow. By definition, you cannot have a backwater extend upstream through supercritical flow. Therefore, you cannot have a rise upstream of the jump. If this is a natural river/stream, I wonder whether you really have supercritical flow, perhaps your n values are too low. In any case. If the rise 900 feet upstream (assuming your mean just downstream of the hydraulic jump, where the flow is subcritical) decreases as your culvert size decreases, something is wrong with how the model is set up, or something else is changing as well, when the culvert size decreases. This is all assuming I correctly understand your problem.

[Chris G.]

Mike-

Conservatively high n values are used when studying flood stages and flood inundation. Conservatively low n values are used when studying velocities and shear stresses. There are obviously many other types of studies, but if you ask the question, “if I increase the Manning’s n value, will it make the thing I’m concerned with better or worse?”, that will usually give you the answer. Sometimes it’s not that clear cut though. For example, increasing n values in the upstream reach may provide higher flood depths, but will also attenuate the flood wave faster. This may result in lower discharges and consequently lower stages further downstream (especially if you have a long reach with a lot of available storage. It’s always good to run three cases in a sensitivity analysis-conservatively high, conservatively low, and best guess n values. Then evaluate each solution. You may eventually combine results of the different sensitivity runs for design conditions.

Chris G.
@RASModel

[Chris G.]

There are so many potential causes for this error. However, it is common for that error to happen when you have an abnormally large amount of deposition in a cross section. This can occur if that cross section is much larger than adjacent cross sections, if ineffective flow areas and mobile bed limits are not properly defined, or if there is just something preventing sediment from passing through that cross section. Check your cross sections. Make sure you don’t have a sudden decrease in velocity where the unrealistic change is happening. If so, you may need to better define contraction and expansion through that reach by including and/or adjusting your ineffective flow areas.

Chris
@RASModel

[Chris G.]

There’s too much to include in a reply here. Try reading through the manuals where they discuss lateral structures. Also search on “lateral structures” in The RAS Solution. There are a number of posts that discuss this topic.

[Chris G.]

Sounds like a fairly complex system. I recommend running in unsteady flow. The split flow optimization works ok for simple flow splits, but when you model gets complex, it’s harder to optimize in steady flow. Unsteady flow automatically optimizes.

[Chris G.]

Not that I’m aware of. There are no procedures in the HECRASController that call the RASMapper. Sorry.

[Chris G.]

Unfortunately, you can’t add a culvert over a junction. Perhaps a lateral structure could serve as both the flow split and a culvert.

[Chris G.]

Check out this post on The RAS Solution:
http://hecrasmodel.blogspot.com/2014/05/controlling-hec-ras.html

Good Luck
Chris G.

@RASModel

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