Need HEC-RAS Help 4

[Jessica2424]

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.

 

[sam74]

HECRAS doesn't add flows for you. You will have to input 181 and 251 at the appropriate sections.

 

[Jessica2424]

Are you positive...because when I input the flows as 181 and 251, my water surface elevation is about 5 ft. above where it should be. If I put the flos in as 181 and 70 then it gives me reasonable water surface elevations.

 

[francesca]

Sam is correct.

How do you know where your water surface elevation "should be"?

 

[sam74]

Yes, I am positive. Take a look at your cross sections report sheet and see for yourself. It will show you what flows your sections are experiencing. How do you know what reasonable flood elevations are? Do you have a FIS or another previous study? What are you using for topography to obtain your section information?

 

[Jessica2424]

OK. I put the flows in like you said. The water in my watershed is draining into a huge lake. The water elevation that the program gives me is about 5 ft over my roads and over my railroad tracks. This can't be right. Can someone give me an idea on how to model this lake and everything flowing into it? I know that this is probably a very easy question, but I have been dealing with HEC-HMS and this program for about 5 weeks now and it is driving me crazy. I am desperate for help.

 

[sam74]

Were you able to look at the cross section report and assure yourself that your flow inputs were correct? What does the lake outfall into? Is there a dam controlling the water surface elevation? There probably has already been a flood elevation established at some time. If your in the US look up on the FEMA website for a Flood Insurance Study (FIS) and flood map on the lake and there might even be a study already done on the channel your trying to model. Use the hopefully already established flood elevation of the lake as the boundary condition on your last cross section to assume a worst case scenario. Also, where did you get your topo (quad map, field survey, gis)?

 

[Jessica2424]

Where can I look at the cross-section report? I got all of my information from a surveyor.

I work for an engineering firm and I am conducting a master drainage plan for a certain area.

 

[sam74]

I had to look, it's actually called a profile summary table. You can get to it under the view tab or it's the third icon from the right. It will show you the reach/sta/prof/ with the next column being Q which should be a close match to your flow inputs at the appropriate river station or cross section as I had refered to it earlier.

 

[Jessica2424]

It is the exact Q input value that I had.

 

[francesca]

You can also view the entire model in 3D by clicking the "View Cross Sections" button after running the model. (It's the button to the right of the HD button in HEC-RAS.) You can rotate about the horizontal and vertical axes. The longitudinal profile plot of the water surface elevation (next button to the right) is also useful.

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.

 

[Jessica2424]

Am I the only person who thinks that HEC-HMS and HEC-RAS are EXTREMELY frustrating. I am still having a huge prolblem with my water surface elevation that HEC-RAS is giving me. It is showing that my water surface elevation in my watershed is about 10ft. over roads and high railroads. This can't be right. Any suggestions on what I can check. This program is driving me crazy!!!!!

 

[francesca]

Check out FEMA's CHECK-RAS.

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.

 

[Jessica2424]

It is actually mainly at my upstream part. I tried to enter the downstream slope in the reach boundary conditions and it says that I can't set the slope for junctions. I have junctions where my reaches combine and flow together downstream. I have everthing ultimately flowing into a lake (I modeled this as a storage area).

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.

 

[francesca]

Did you read the Hydraulic Reference Manual? (As opposed to the Manual.) I found the Hydraulic Reference Manual much more useful, and also the Help within the program is excellent.

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.

 

[Jessica2424]

Francesca,

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?

 

[francesca]

Jessica,

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:

Reach lengths across the junction are entered in the junction editor, rather than in the cross section data. This allows for the lengths across very complicated confluences (i.e., flow splits) to be accommodated. In the cross section data, the reach lengths for the last cross section of each reach should be left blank or set to zero.

In this example the energy equation will be used to compute the water surface profile through the junction. If the momentum equation is selected, then an angle can be entered for one or more of the reaches flowing into or out of a junction. The momentum equation is set up to account for the angle of the flow entering the junction.

Good luck,
Francesca

 

[TerryScan]

You mentioned that a lake plays a part in your study. How do you currently have the lake modeled?

 

[Jessica2424]

TerryScan,

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.

 

[Jessica2424]

Francesca,

I am still working on your comments.... thanks for the input once again. I am sure that I will have more questions to come.