[Chris G.]

Please download and read The Corps of Engineers EM 1110-2-1603, Hydraulic Design of Spillways. http://publications.usace.army.mil/publications/eng-manuals/EM_1110-2-1603_sec/toc.htm

Here you will see how to apply the turbulent boundary layer development energy loss to computation of a water surface profile on a spillway crest and chute. In addition, you will see how to incorporate pier end waves, air entrainment and bulking, as well as a cavitation index to your computations.

[Chris G.]

Ah…got it. Sorry about the confusion. I just tried it on a flume example I have (I applied 0.5 loss coefficient to the entire reach), and it definately works. Perhaps your velocity head is very small, so the minor losses are too small to detect in the output?

[Chris G.]

For a RAS project that only has one river/reach, you would have, at a minimum, one upstream boundary condition, and one downstream boudnary condition. The upstream is typically a flow hydrograph, but could be flow hydrograph, stage hydrograph, or stage/flow hydrograph. The downstream boundary condition could be stage hydrograph, flow hydrograph, stage/flow hydrograph, rating curve, or normal depth. Typically I see either stage hydrograph or normal depth as the downstream boundary. Sometimes a rating curve. And yes, the initial conditions is typically the first time-step flow (from your upstream boundary flow hydrograph).

That’s it.

[Chris G.]

A stage hydrograph presents a time series of stages. A flow hydrograph presents a time series of discharges.

[Chris G.]

You can always try a different initial flow distribution. See what RAS is heading towards while going through the optimization. If it looks like the optimization runs are converging, but just run out of iteration attempts before it could get there, then use the flow distribution of the last iteration attempt, and then rerun RAS. If the optimization runs are diverging, or going unstable (i.e. solution errors are oscillating out of control, rather than minimizing for each iteration), then you may just have a geometry that is not conducive to split flow optimization. It could be that one of the channels is so much smaller than the other, or the ground points on one of the splits are so high, that the channel wants to go dry. In that case, perhaps a pilot channel would help.

If nothing seems to work, you can always use a stepdown scheme hotstart to let RAS determine the distribution of flow. http://hecrasmodel.blogspot.com/2010/12/how-to-create-hotstart-file-in-hec-ras.html

good luck

[Chris G.]

k values are not meant for minor losses. They are an alternative to n values. The advantage of using a k value is that RAS then computes an n value that is a function of (not only k but also) depth. Please read chapter 3 of the hydraulic reference manual for more information.

Frankly, I don’t hear of people using k values that much in RAS, so I wouldn’t be suprised if there are some bugs in the software when trying to use them.

[Chris G.]

sounds like you have storage area connections that are not fully connected. Go to the geometry window, click on the storage area connections button, and then make sure that each storage area connection you have is connected on both it’s upstream and downstream sides to either a storage area or a cross section. Hope this helps.

[Chris G.]

If you are indeed only using the rating curves, then the problem lies there. It’s very simple, RAS takes the computed flow at the structure and uses the curve(s) to get the upstream water surface elevation. That’s it. You could even do some hand calculations to verify your rating curves. Not sure if this has anything to do with it, but be careful using the gates with low-level outlets, since the user-defined curves are NOT tailwater dependant (i.e. no way to account for high tailwater (submerged ocnditions).

[Chris G.]

No…not by that figure alone. However, you have a 20% slope. Maximum slope suggested in the HEC-RAS user’s manual is 10%. Is this a spillway? In that case, I suggest using boundary layer theory instead of RAS.

[Chris G.]

The application of ineffective flow areas around bridges always has been and always will be very subjective. Every modeler has their own preferance. What you are trying to do, more than anything else, is maintain flow distribution consistency. In other words, if on the upstream side of the bridge you have 10% of the flow in the left overbank, 75% of the flow in the main channel, and 15% of the flow in the right overbank, then you want to have about 10%, 75%, and 15% on the downstream side of the bridge. In unsteady flow modeling, numerical stability becomes and issue as well. Using permanent ineffective flow areas can help to minimize the shock of a sudden change of storage volume to conveyance volume. How do you justify permanent? If the roadway embankment slope is shallow enough, I usually use non-permanent. If it is abrupt, or even near vertical, you could justify permanent.

Vertical position of the ineffective flow triggers is also very subjective and up to the preferances of individual modelers. Again, make sure you have flow consistency (if the upstream triggers turn off, the downstream triggers should trun off at the same time/profile). Personally, I like to put my upstream triggers about 0.5 ft above the high chord on the upstream side, and about 0.5 ft below the high chord on the downstream side. Then I check flow consistency and adjust from there.

[Chris G.]

In steady flow, yes, you have to have contraction and expansion coefficients. If you have supercritical flow, you’ll want to reduce those coefficients by an order of magnitude (example, if you are uinsg 0.1 and 0.3 for subcritical flow, then you should use 0.01 and 0.03 for supercritical flow.

When using unsteady flow, momentum changes are captured in the St. Venant equation of conservation of momentum, so you do not have to explicitly add in contraction and expansion coefficients. There is, however, a separate table for unsteady contraction and expansion coefficients, that you can use (but it is optional) if you feel you need more energy loss from contraction and/or expansion.

[Chris G.]

Hi ggdm. When using lids in steady flow, I believe RAS uses the energy equation to solve for energy from one cross section to the next (just like with regular cross sections). If the answer you are getting does not meet what you believe to be the true capacity of the tunnel, your only recourse is to alter the n value as a calibration knob. Keep in mind that RAS, even with lids, is not the best tool for modling tunnels or pipe flow.

[Chris G.]

You do not have to have a river upstream of the storage area. However, to prevent the reach below the storage area from going dry, you may need to put in a lateral inflow to the storage area or the cross section just downstream of the storage area.

When the flow reaches the next downstream storage area, the water level will go up based on the elevation volume curve you enter. Just make sure that elevation volume curve extends to a high enough elevation to contain all of the volume that enters that storage area.

Good luck.

[Chris G.]

Hi Albert-

you need to have discrete streams and their compliments of flow lines and bank lines upstream, between, and downstream of the reservoirs. To connect the stream centerlines to the storage areas, read up on this post in the HEC-RAS Bloggery:

http://hecrasmodel.blogspot.com/2010/08/how-to-use-storage-area-to-define.html

Good luck.

Chris

[Chris G.]

Hi Nils-

If the lake is causing a backwater into the river upstream, then you’ll have to know something about that relationship if you want to omit the lake and delta from the model. Normal Depth will not work as a downstream boundary in your case. If you are in steady flow, you’ll need a single water surface elevation for your downstream boundary. If your area of interest in your model is far enough upstream, away from your downstream boundary, you can be quite liberal with how you select your downstream boundary water surface. Any errors associated with that selection will work themselves out as the computations progress upstream (as long as the backwater from the lake does not extend up to your area of interest. Check here for an article written about this: http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html

For your second question, I don’t see it as a problem at all. Eeach scenario will be a different plan with its own flow file. A single geometry file will be shared by all of the plans.

Best of luck!

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