[Chris G.]

That is very weird. I’ve never heard of that, especially after the simulation runs just fine. I suggest you package up your plan and send HEC a bug report on that one. The error suggests that you don’t have initial elevations for your storage areas input into your Initial Conditions Tab in the unsteady flow editor. However, I wouldn’t expect the model to run at all if these were missing, so I’m a bit stumpped on this one.

[Chris G.]

Tim-

Yes, unfortunately you have to have everything in the same projection/coordinate system/datum for it to work in HEC-RAS.

[Chris G.]

Yes

[Chris G.]

You may find that you have very different land conditions in one or both of your overbanks. For example, for one of the overbanks, you could have riparian forest next to the main channel and farmland further out in the overbank. Two very different n-values. You can also have different n-values within the main channel, although RAS has autmated methods for compositing main channel n values into one single n value, under certain circumstances. To put in more than the standard “3” n-vlaues, go to options in your cross section editor, and select “Horizontal Variation in n Values”. Then fill in the new column called n Value, that will show up in your Cross Section Coordinates table.

[Chris G.]

This is a very broad and basic question Sami. Give the manuals a thorough read through. This will help you out tremendously.

[Chris G.]

Yes, you have to select computation settings.

[Chris G.]

Or simply how long you want to run the simulation for. But your flow data set needs to be at least as large as your simulation time window.

[Chris G.]

Nope, the program will do it for you based on your defined start time in the unsteady flow analysis window and the time increment you select in the flow editor.

[Chris G.]

I would be very cautious using RAS for this. You may be able to get an answer, but you’re breaking a major “rule” for RAS modeling in doing so. 10% is about the steepest you want to go when modeling a reach in RAS. 2V:1H is a 200% slope! Sounds like a spillway to me. Plus at slopes that steep, air entrainment, bulking of flow, cross-waves all play a major role in the ultimate water surface profile. If you’re looking for accuracy here, you’d need to go with a physical model. Boundary layer theory is a method for computing the water surface profile, but at slopes that steep, I’d still be suspicious. Plus, it won’t do a hydraulic jump for you. If you’re jump is down on a shallower part of teh chute, you may be able to use RAS for it. But RAS will only show a jump from one cross section to the next. i.e. you won’t be able to have a jump span multiple cross sections. At 0.1 m spacing, I thinnk that may be a problem for you.

[Chris G.]

You absolutely can model in and out of a storage area using two lateral structures in unsteady flow RAS. Both lateral structures will be placed in the creek. Their tailwater locations will be the storage area. Flow will be able to go both directions over both lateral structures. If it flows from the storage area to creek, it will be listed as negative flow. I’ve done this many times…it works. The figure below is a crude example of the simplest way to do this. You could also model the offline area as a separate reach, also connected to the main creek by two lateral structures.

[Chris G.]

To get the live reservoir storage, you’ll have to set up a spreadsheet. Make a RAS table that includes the cum. volume for a wide range of steady flow profiles (so that you have a wide range of reservoir elevations). Then copy and paste those results to a spreadsheet. There you can remove cross sections that are not part of the reservoir, and volumes that are not part of the live storage.

To get sediment that has accumulated in the reservoir, go to the sediment spatial plot and view the mass bed change (tons) for the final output sediment profile.

Check out this post to learn more about the different sediment output variables:

http://hecrasmodel.blogspot.com/2009/03/sediment-output-variables-what-do-they.html

[Chris G.]

If you have a storage area you must enter an initial starting elevation. It could be as simple as the invert elevation (minimum elevation) of your storage area to replicate a completely dry stroage area, if you want. If you only have one river and reach, an initial flow value at the upstream end of your reach is all you need.

[Chris G.]

The bridge scour routines in RAS have notoriously been buggy over the years. In an ideal world, run-time errors should never occur. If you input something incorrectly, error-catching scripts should avoid the runtime errors and give you a nice message telling you what you did wrong. You should send a bug report to HEC so that they can fix that.

[Chris G.]

If you are okay using a vertical lift gate to simulate these features, then RAS will take into account the submergence. However, when you set up your gate as a “user defined” gate, you put in a family of rating curves that are independant of tailwater. It’s a limitation, Otherwise you’d have to have a family of family of rating curves :). One way around this is to use Rules as your gate boundary condition. You should be able to put in multiple rating curves, for a range of tailwater elevations, and direct RAS which one to use under a variety of conditions. Using Rules is tricky, requires some understanding of writing code/scripts, and has very little documentation to go with it. But this is an option if you have the time/patience to figure it out.

[Chris G.]

Even though you see 0.1 and 0.3 in the unsteady examples, those values are only there to be used if that geometry file is used in steady flow. Those values are ignored in unsteady flow. If you want to include contraction and expansion coefficients in unsteady flow, you must enter them separately. You can find that table in the geometry schematic, under the “Tables” menu item, “Contraction/Expansion Coefficients (Unsteady Flow)”.

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