Dam Breach HEC RAS vs HEC HMS 1

[DuckEngineer]

I have been asked to get up to speed on dam breach analysis for permitting. Most of the work is low hazard, relatively short (under 10' high) dams. I don't have any real background in HMS or RAS. My general understanding is that HMS provides the hydrology and hydrographs, while RAS provides the analysis of structures and elevations. The question is whether the dam breach analysis that can be done in HMS is on a par with what can be done in RAS? I have a choice between HMS training and RAS training and this is the likely tie-breaker. Thanks, all.

 

[damsafeengineer]

I have had training in both HMS and RAS for dam breach analysis. I recommend that you stick with HEC-RAS for your dam breach analysis. HMS provides a simplistic breach routing. You can't put roads, bridges, downstream structures, culverts, etc. in HMS. Since your post indicated that you may be doing several breach analyses, then you will encounter these things and therefore time and training in RAS will be well spent. HMS is quicker and easier to set up, but very limited as I pointed out above. RAS will take a little longer but give you many more options and flexibility, and it does a better job of the unsteady flow routing (IMHO). Since you are doing many small structures, you probably won't need to set up complex geometry so a RAS project may not take that much longer than HMS would. I use HMS for the hydrology/hydraulics and then switch to RAS for breach analysis and downstream routing.

 

[francesca]

I would take the HMS training. RAS is pretty easy to figure out on your own if you read the Hydraulic Reference Manual (not the Manual). For dam breach analysis WMS is nice as it does HEC calculations in a GIS wrapper program.

 

[damsafeengineer]

I forgot to mention before that it is important to check with your State's dam safety program. They may require one particular program over another, or they may have recommendations as to which program should be used in which situations. The State of Washington's dam safety program has some good guidance. They even have a spreadsheet dam breach program that they allow to be used for small dams (ie. to do a quick check and see if a failure of that 20 foot high dam would flood that house trailer 500 feet downstream). Their website with all of the technical notes can be found at:

http://www.ecy.wa.gov/programs/wr/dams/guide.html

 

[digger95]

Have you had any experience or thoughts about FLDWAV by the National Weather Service. Illinois DNR allows HEC-RAS, HMS and FLDWAV.

I have used -RAS for Floodplain Modeling/BFE determination, however have not performed Dam Break Analysis before...

Based on reading thorough HEC-RAS Manual and State Guidelines, the requirements appear pretty straight forward.

Any pitfalls to look out for? (small dam Class II - Primary Spillway - 50 yr event, Total Spillway design flood 100-year event, earthen impoundment)

Thanks in advance.