[cameron]

Are the units in the DSS data set the same as the HEC-RAS model? The values you have may be the same units, but does DSS think they are the same units?

Section 7.4 of the DSS users manual discusses how to set the units.

[cameron]

Can you share your files?

[cameron]

You could just model them as a single structure or do lidded cross-sections.

If the wse does not reach the low chord of the bridge or pipe crossing then you could just model it as cross-sections.

[cameron]

The easier answer is just to rename the snapshot file and reopen Mapper.

For instance when it is created it will be called myproject.p01.snapshot.hdf. Just rename it to myproject.p01.hdf and it will open in Mapper. If you already have a solution file because you are just rerunning the model, you will have to delete it

[cameron]

This has happened to me a few times. Is the proposed terrain name similar to the existing terrain name? Have you tried closing mapper and updating the .rasmap file manually?

[cameron]

You need to change the hydrograph and detailed output interval

[cameron]

The simple, quick, and cheap answer would be to just use the discharge from FEMA as it is approved and you don’t have to do anything.

How much does the drainage area change from the FEMA discharge to your study area? If it is small then the FEMA discharge probably won’t change much. If you have a tributary coming into your project area and would change the discharge significantly, then I would think about doing an analysis to come up with better discharge.

A rule of thumb I have used in the past is having flow changes when the drainage area changes by 10%, at confluences, FEMA discharge locations, detention basins/dams, and sometimes major roads.

Since your study is in an Zone A, another option you may also look at just doing regression analysis (StreamStats) as it is simple and FEMA approves that. It would need to be checked against the FEMA discharge downstream though as you wouldn’t want it to be higher than downstream.

[cameron]

First question – I believe it does it for the entire network

Second question – I think this to myself every single time I start to optimize the model and fix the cells manually. I would email HEC-RAS and tell them to add this to Mapper.

Third – I look at a few things when it comes to these instabilities:
1. Where are they occurring. If they are outside of my area of interest I focus less on fixing them
2. What is the volume error. If the volume error is above a certain threshold, then I try to fix them. This threshold varies from project to project.
3. When are they occurring. If they happen before or after the peak and won’t have an impact on the max floodplain (if that is what I care about), then I may not fix them.
4. How often they occur for certain cells. I will check to see how many iteration issues occur for each grid cell and if it is above a certain threshold, i will fix them. This can help speed up the runs significantly.

It is also important to check the total volume error in deciding if further adjustments are needed.

[cameron]

This is where engineering judgement comes into play. For Q10, does it really over top or does it just say that since it is using the energy grade elevation and not water surface elevation. If it is not really overtopping, you could then either switch the program to use the wse instead of energy or you could adjust the minimum weir elevation to be above the Q10 elevation it is using.

Basically (as long as you are comfortable with it), you want the Q5 and Q10 to use the same method which should fix your crossing problem. Remember, the program is just doing what you tell it to do and you need to decide if what it is doing is correct or not.

I do find it interesting that the proposed bridge design has pressure flow at Q5. Are there not design requirements that show you must have freeboard or be able to pass the Q50/Q100?

[cameron]

One thing you also always want to check is how the results compare to the diffusion wave solver and a 1D calculation.

I have noticed a few times that the full momentum solver water surface results seem to go up when I go to smaller cells and some times give unrealistic answers (feet higher) when checked against larger grid cells, 1D results, or the diffusion wave solver.

[cameron]

Your computational time (time step) is to large. Follow the guidance in the manual that discusses selecting the correct time step based on the Courant criteria.

[cameron]

what bridge method (energy, pressure, etc..) is being used for the Q5 and Q10? It is important to note that the default is for the HEC-RAS to use the energy grade water surface elevation at bridges and not the actual water surface elevation.

[cameron]

You need to create a raster/tin from the points and then combine that with mosaic to new raster. You may have to add breaklines with the tin to get it to look correct before combining the rasters.

[cameron]

Delete the output files and any folders for the stored results, then rerun the model.

[cameron]

Do the cross-sections look like they came in the correct coordinate system? You could check this in the geometry editor or mapper and see if the coordinates match what you had in GIS.

What is the projection of the 3D XS Cutlines in GIS? Is it in the correct projection?

Have you tried just creating the model in Mapper?

[Author]

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