HEC-RAS - Removing Junctions-need advice

[davidpro]

This project is my first formal submittal to FEMA so I'm not sure how they handle this. When I run check-ras it gives me a message describing how it is inappropriate to model a junction unless certain criteria are met (my reaches don't meet the criteria). It says the reaches should be modeled separately. The check-ras help file then goes into details about how to remove the junction from your model.

The instructions include deleting the junction, and adding the boundary conditions to the upstream reaches that are now broken up from the main stream. It says to use the previous model's WSE as the boundary condition (b.c.) for the main channel upstream reach, and the previous model's EG slope with normal depth option for the tributary.

My question is, as a reviewer with a local agency or FEMA, how do you justify these b.c.'s? Can you just say the b.c. came from an original model with a junction? It seems like Check-RAS is just telling you a trick to get the same output from a geometry file w/o a junction, which would mean that I should submit my original model with the junction.

Has anyone had any experience with this? I know most people are not fans of check-ras, but my submittal forms list it as a requirement and a previous project in the same area used it so I know my local agency reviewer will want to see the reports. Any help greatly appreciated. Thanks!

 

[Rye1]

The way you are describing is the correct method...delete the junction and use the downstream reach as your starting ws. This is just like if you were going to model a trib to a larger river. Find the WSE for the larger river where your trib comes in and use these WSEs for the starting WSEs for your trib.

Generally, your starting WSEs are not critical unless you have a bridge or culvert just upstream. I've reviewed models where the engineer has chosen critical depth as a starting starting WSE and within a few cross-sections, the error is corrected. If your geometry (terrain data, bridges, culverts) is keyed correctly and your manning's values are reasonable, your model will be okay. You may have to make some tweaks but after these two things, you will not see a lot of change.

In your documentation, state the bc (starting WSEs) came from the downstream reach.

Just get through the Check-RAS part and show some printouts so the reviewer will see that you ran the program.

Spend some time on your narrative. Explain any assumptions, document your manning's "n" values, reference VT Chow.

Robert Billings

http://www.newrivereng.com

 

[davidpro]

Thanks for the advice, that seems like it should make sense to the reviewer.

We are submitting for a CLOMR now, so we have our own proposed maps of new floodplain/way limits. Once that is approved and the project is built and we submit for a LOMR, how does FEMA go about getting the actual map digitized and revised? I assume they have a mapping partner that takes a digital file and they draw in the new limits. How does that process work?

 

[Rye1]

Well, FEMA's guidelines and specs for submitting mapping products including CLOMRs and LOMRs generally try to have the submitter complete the mapping. The problem FEMA and the mapping partners are running into is that they cannot ask for something that is not required on the MT-2 forms. However, there is languange in the guidelines that require the submitter to atleast be as accurate as the existing FIS.

That being said, was is generally happening is this, you will submit your CLOMR, expect a letter saying that they need more info, if they don't need more info they will also send a letter a "clean ACK" letter. Once the review starts they'll look at the technial data and if that is okay, they will approve the CLOMR. CLOMR approval is less stringent that LOMR approval.

When you submit the LOMR, they will review your data and may ask you to change some stuff. But when it comes down to line weights and how you have drawn your floodplains, it is just generally better for them to correct that with their own mapping partners.

Robert Billings

http://www.newrivereng.com