Modeling houses in floodplain

[gopher669]

I have a channel running through a residential subdivision. Just wondering how you model houses in the overbanks.

I've seen two separate ways. One is using a little higher n value of 0.07 in the overbanks and using ineffective flow areas (for the storage, but essentially no conveyance). The other way bumps up the overbank n to 0.10, but with no ineffective areas. Your thoughts?

 

[Ryb01]

I would model the houses one of two ways. Code in the houses using the obstruction option or physically code the shell of the houses into the geometry of the cross sections.

 

[gopher669]

It's residential subdivision with approximately 200 houses. Not really an option to model them as blocked obstructions.

It would either require me to have cross-sections every 40-50', which would be overkill. Or it would have blocked obstruction throughout the entire overbank, which would eliminate any sort of storage.

 

[gbam]

gopher - is there a perimeter wall around the subdivision? If yes, then block out entire subdivision. If no then you are back to the over-bank modeling technique as described above.

 

[cvg]

if you are trying to model an underwater subdivision with associated "storage" between the houses, HECRAS (which is what I assume you are using) is perhaps the wrong tool for the job. You might want to consider a two-dimensional analysis. You may also want to ignore the minor storage effect and use Gbam's recommendation for the perimeter wall as you apparently do not meet the typical floodplain regulations (houses above the floodplain) anyway.

 

[beej67]

Just curious - where's this project at? I haven't encountered any areas where they let you put houses in the flood plain, and have only had to deal with that sort of thing when a FIRM is revised to put old houses in it.



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[gopher669]

This is an old existing residential neighborhood that had a lot of flooding problems. The Corps put in a detention basin upstream which relieved some of the problems. A consultant completed the LOMR for that project.

Recently, the City recently went in and upsized some box culverts which were shown by the previous RAS model to be causing a restriction in the channel. This consultant (different from the previous one) took the previous model, and inserted the revised culverts into that model. However, they also removed a lot of the previous ineffective flow areas for the housing areas.

I'm now working on the LOMR for that last piece. I want to keep the model as close to the one that was used to get the original LOMR, which used n=.07 with ineffective for the overbanks in housing areas. But in a model I received from FEMA for a completely separate project, they used an n=.10 with no ineffective areas. Which is what prompted the original question.

 

[beej67]

Interesting stuff. I ran into some similar issues on an existing residential area in Louisiana that just got reclassified. Didn't get to the point where I had to do any modeling myself though, just advised a client on what was going on with it. In our case, the municipality acknowledged that some box culverts were causing a restriction, but refused to upgrade them because removing the restriction would increase flooding downstream of the culvert, whereas my client was upstream of the culvert. Politics were involved, the previous consultant who did the mapping appears to have screwed up to begin with, and the thing's currently unresolved.

Let us know how yours goes.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com