Is this doable or crazy? Most regulations require lowest floor to be 1' above the base flood elevation for a 100 year flood, and I have found a nice riverfront lot where I would like to build at an elevation 5 feet below base flood elevation - i.e. need to build up house 6 feet above ground.

Cost of compacted earth buildup w/slab on grade versus built on slits/pier and beam? Long term concerns and maintenance of each?

There could be an issue with "floodway" vs "flood plain". Floodway oes not allow building

Where is this? Each county/municipality different criteria.

Williamson county on North San Gabriel River, outside city limits. Houses on similar adjacent lots but most likely built before recent flood maps and regulation. Old houses that probably haven't been flooded, but real floods don't matter - only the base flood elevation and getting permitted to build. And it affects insurability, too.

Floodway or floodplain? You cant build in floodway. You can build in the floodplain, but I wouldnt rec it. 6+ ft of quality fill and the other factors (insurance etc) arent going to be cheap. Also,dont go by adjacent lots either, they arent necessarily in the same causeway. Next, you might want to go back and see when the last 100 year flood occured in the area and how the neighborhood fared. Texas has recently had several 100, 200 & 500 year floods, and thus while your only focusing on whats needed to build, you also need to focus on once its built if you want to live in a raft.

[This message has been edited by nnichols (edited 6/8/2010 5:26p).]

My wife is a civil engineer working in hydrology and hydraulics and has had experience preparing FEMA permits for developers wanting to fill to get lots out of the floodplain. She says that you will have to place fill to raise the building pad as opposed to using piers. The FEMA permit requires a surveyed elevation of the lowest adjacent grade (LAG) of the property next to the foundation. This LAG must be at or above the BFE (Base Flood Elevation). This website explains further (your situation would be a LOMR-F instead of LOMA)

http://www.fema.gov/plan/prevent/fhm/ot_lmreq.shtm

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[This message has been edited by Jay@AgsReward.com (edited 6/8/2010 8:35p).]

Thanks for the replies, I really appreciate it.

It is a floodplain, not floodway, as far as I know. It is adjacent to the river but the banks are very steep up to a certain elevation, beyond which it is mostly level with a very slight rise. I believe there was a 100+ plus flood in 2007 and water did not reach within 7 feet of what they say is the base flood elevation of that area, so it errs on the safe side for sure.


There is already a two story structure on the lot, about 16X16 which is house-like and current owners said required county approval. It was raised up pier and beam, but just needed a couple feet to be OK. It may not have FEMA insurance, they just did what they had to do to get the county permit?

Jay, I'd be interested in ballpark costs of compacted fill per foot of vertical height. I imagine your wife stays on the technical side of the permits and doesn't deal with financial costs to owner, but if she's ever heard anything mentioned I'd be interested to hear how pricey it gets.


Thanks again.

I'm a general contractor in Houston and have been directly involved in building houses in the flood plain. However, in Houston, they do not allow fill to be brought in to build up since there is no way to mitigate the offset water shed. A pier and beam foundation is required with a certain flood vent/square foot ratio. One particular house had at least 5-6' piers.

Another thing to consider may be to bring in 3' of fill and doing a 3' pier and beam. I wouldn't recommend doing anything less than 3' since that actually only gives you a 2' tall crawlspace and you will need to access this area occasionally for mainenance.

Also, a company I worked for previously built a home in the floodway and a friend of mine just built in the floodway on a lot in the City of Houston jurisdiction near Lake Houston so even that can be done. Not sure about lender/insurance specifications but I'm sure it's not cheap and a ton of red tape to work through.

Thanks for that info, it's pretty obvious the rules will vary by jurisdiction so I just need to go talk to the county.

I grew up in New Braunfels on a house on the edge of a canyon, and the house was on stilts. At least we called it stilts, I don't know the various types of pier and beam construction or if what we had wasn't technically pier and beam. There was a basement/garage, mostly above ground, about 16' square. You could drive under the house and park in it. But other than that, the house was on steel beams from around 8' tall in front to 20+' feet in back. So I have this nostalgic reason to want to build on piers/beams, but the cost and some maintenance issues (pipes freeze easily) might outweigh nostalgia.

I am a civil engineer in Austin and we do this type of work if you need a study, CLOMR/LOMR, elevation certificates, etc. My email is XXXXXXXXXXXXXXXXXX (I will take down the email in a couple of hours). I would be happy to help.

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Jay, I'd be interested in ballpark costs of compacted fill per foot of vertical height. I imagine your wife stays on the technical side of the permits and doesn't deal with financial costs to owner, but if she's ever heard anything mentioned I'd be interested to hear how pricey it gets.

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Sounds like the fill would be in a difficult area to get to. Aside from that, you are looking at cost in the $20 per CY range. Obviously there are a lot of factors that are unknown to me.


[This message has been edited by BBDP (edited 6/9/2010 4:19p).]

BBDP, I missed your email. Could you please send me one at @yahoo.com?

We wouldn't be building in the near term, but I'd still like your contact info.

I spoke to the county environmental office today, building in a flood plain (there is no floodway per FEMA, even though its on the river) is OK with the proper permits and following the 1' above base flood elevation rule. Depending on where we built on the lot, it would be from 1' to 8' lift to get 1' over the base flood elevation. He didn't care if it was fill, slab, or stilts as long as an engineer signed off on it and it was 1' above BFE. He didn't seem to recommend fill though, he actually thought an 8' slab would be better. Don't know about that.

Sorry Big12, I did not have a chance to get back to the board until now. looks like BBDP has you covered and you are right in that my wife is much better in the technical details then the practical details. She is an engineer after all.

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email sent.

I am a GC in Houston too, and I agree with Aggie SAM. What he said has been my experience. You will likely have to build it like a beach house if it is 6ft. Otherwise your dirt expense will be outrageous if they even allow you to bring it in.

Also forgot to mention that regardless you will want to:

1) Obtain a geotech (soil) report

2) Do a proof roll before you bring in any dirt which will tell you if any portions of the lot are unstable and get them fixed with fly ash or lime

3) If you build any type of structural pad, get a proctor test for the select fill you are using and make sure you do compaction tests for every 6'' lift

Your local floodplain administrator may require a hydraulic impact analysis on flood level upstream of the proposed development, which would need to be done by a registered professional engineer. In Houston, even piers constructed within the floodplain require a hydraulic impact analysis to insure that the placement of the piers do not cause the flood levels upstream of development to rise.