Need HEC-RAS Help
Need HEC-RAS Help
(OP)
When entering steady flow data for the different reaches, should you enter the "total" flow or should you enter the flow that will contribute to that reach upstream?
For example: If you have two reaches that are joined (one upstream (#100) and one downstream(#99)).
(#100)=181.00 cfs
(#99)=251.00 cfs (181.00cfs + 70cfs) (251.00cfs is the total flow going through #99 while 70cfs is the flow contributing only to #99)
Which flow values should you input into the steady flow data window?
181 and 251
or
181 and 70??????
Sorry if this is confusing... I couldn't think of any other way to describe it.
For example: If you have two reaches that are joined (one upstream (#100) and one downstream(#99)).
(#100)=181.00 cfs
(#99)=251.00 cfs (181.00cfs + 70cfs) (251.00cfs is the total flow going through #99 while 70cfs is the flow contributing only to #99)
Which flow values should you input into the steady flow data window?
181 and 251
or
181 and 70??????
Sorry if this is confusing... I couldn't think of any other way to describe it.





RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
How do you know where your water surface elevation "should be"?
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
I work for an engineering firm and I am conducting a master drainage plan for a certain area.
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
You will often find older roads flooded in a 100-year storm event in some places. Mathematical modeling has advanced tremendously since the advent of computers, so if the road/bridge/etc. was built say in the 1950's, it's quite conceivable that it wasn't built to the flood specifications of today.
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
Failing that, have someone review your files.
Where in your model are the above-expected water surface elevations? Throughout the model? Only at the downstream part? If so, check your downstream boundary conditions, particularly downstream slope: this, under steady, subcritical flow, is how HEC-RAS calculates the Normal Depth to set the downstream depth. Calculations are performed from downstream to upstream, so if you've underestimated your downstream slope, then you'll be over estimating your downstream water depth.
RE: Need HEC-RAS Help
I am sorry if this seems like a stupid question. I have had to teach myself how to use the program and I don't find it very user-friendly.
RE: Need HEC-RAS Help
Check your boundary conditions at your junctions. If it's upstream of a particular junction, that ought to isolate your problem.
Check your cross-sections, too. A narrow cross-section/steep bottom slope causes water to speed up and consequently more of the energy to be transfered to velocity head and the water surface elevation to decrease. Conversely, a wide cross-section/flat bottom slope causes the velocity (and consequently velocity head) to drop, with the energy transfered to potential energy via an increase in depth.
You should also check your error messages. If you require additional cross-sections, you can add them by interpolation to save time. Just watch the Manning's co-efficients, because if you have those vary across the cross-section, it will interpolate those too. (If you have insufficient cross-sections, you may find your head loss over-estimated between cross-sections.)
You should also check your flows. If you have an error of a decimal place somewhere, for instance, that isn't carried through downstream, that could account for your problems.
Good luck.
RE: Need HEC-RAS Help
First of all...THANK YOU so much for helping me out!!!
I looked at the Hydraulics Manual and it is much better than the user's manual.
It wouldn't let me check or put in and input for boundary conditions at the downstream junctions. I added a normal depth at the upstream boundary conditions and nothing seemed to change.
The cross-sections that I have were given by the surveyor and are pretty accurate to what is in the field.
I am getting no error messages...the program runs the whole way through.
I check my flows and they are put in correctly. I got my flows from HEC-HMS.
Now, I am having a hard time trying to figure out if I should use subcritical or supercritical flow. When I run the program with subcritical flow, that is when I get the outrageous water surface level. When I run the program with supercritical flow, then water surface elevation seems to be at a reasonable level. I know that the difference in subcritcal and supercritical flow has to deal with velocity.... I guess I am just confused on which one I should choose. Are you familiar with the differences between the subcritical and supercritical flows...or know when to use which one in a particular situation?
RE: Need HEC-RAS Help
Even if the program runs the whole way through, you still get error messages and warning messages. You can view them by clicking the second to last button from the right (immediately to the left of the button with the letters DSS on it).
Your error/warning messages will tell you if there was no subcritical answer (in which case it would default to critical depth and indicate a dashed red line on your cross-section plot). You want to start with subcritical flow, as this is the highest water surface elevation.
If your error/warning messages indicate that critical depth was used extensively in your model, you still might not need to run subcritical flow (or preferably, mixed flow). I had this problem once with a small stream that had a well-defined channel, but the channel walls were heavily vegetated. By using my channel bottom Manning's coefficient on the channel walls too, I was overestimating the velocity in the channel. When I changed my Manning's coefficient on the channel walls, I no longer had the critical depth warning messages.
Of course, if your problem is high water surface elevations, then it's unlikely that you've underestimated your Manning's coefficients, but perhaps overestimated them.
Describe the area where you're having trouble and tell me what Manning's coefficients you've used. (Did you get them from the Hydraulic Reference Manual or another source?)
You're right that you can't specify boundary conditions for junctions, but in the Geometry Editor you can specify a junction width and an entry angle. An error I made initially with my junctions was stationing my minor stream channel right up to where it meets the major stream channel. In fact, under flood conditions, the water will be flowing on the overbanks and your entering stream should be stationed with zero where its waters meet with the water flowing on the overbanks of the main channel. You then enter a junction width to account for the width of the main channel.
In the image above, Stream 1-2 is the north-south stream; Stream 2-3 is the east-west stream. If you use the Energy equation you don't need to specify the entry angle, but if you use the Momentum equation you do.
The HEC-RAS help screen says the following about editing junctions:
Good luck,
Francesca
RE: Need HEC-RAS Help
RE: Need HEC-RAS Help
I had the lake modeled as a storage area, but I didn't know exactly what values to use for the SA. I ended up taking the lake out.
My watershed drains into Lake Pontchartrain. I am not really sure on how to model this situation.
RE: Need HEC-RAS Help
I am still working on your comments.... thanks for the input once again. I am sure that I will have more questions to come.
RE: Need HEC-RAS Help
h
if there is already a study in your location it will give you a realistic water surface elevation for your downstream condition (lake). Your stream may have already been studied as well.
This weblink will take you directly to where you can get a small flood map of your area if a study has been done.
htt
If you find your location has been studied we often print a copy and scan as a tiff file for use as a raster reference into Microstation. Find the correct scale and overlay on your surveyed sections for a rough look at how it compares with FEMAs study.
RE: Need HEC-RAS Help
I see what you are saying about modeling the junctions, about how you need to put the reach lengths in the junction editor versus the cross-section editor (of the last cross-section near the junction). My problem with that is that I have a culvert that connects one reach to the other and then flows donstream together. This culvert allows water to flow under railroad tracks into the main ditch on the other side of the tracks. The main ditch then flows downstream and comes into contact with another culvert with the same situation as the first.
The manual says that in order for me to model a culvert I have to have a 2 cross-section, one before the culvert and one after. Well, I have made the cross-section length before the culvert long enough to include the culvert length and I have made the cross-section after the culvert 1ft long, then it goes into the junction to combine with the main ditch. Am I doing this wrong?
RE: Need HEC-RAS Help
I'm confused about your "cross-section lengths" -- do you mean your downstream reach lengths? Or your cross-sections?
You need a normal cross-section before the culvert and a normal one after. If your culvert is particularly long, especially if you have flow over the top, then you can include cross-sections over in between. (I've never done this myself, but I read it can be done.)
You might want to check your culvert modeling assumptions. Do you have pressure flow checked? Also check your culvert entry loss coefficients. On one of our projects we were getting results we didn't believe from the culvert modeling in HEC-RAS and so we checked our output by doing a Hydraflow detention basin analysis on the culvert. HEC-RAS was definitely more conservative, and our justification for going with the Hydraflow in this instance was that HEC-RAS is a one-dimensional model, while Hydraflow does stage-storage calculations.