FEMA No-Rise Analysis

[oregonpe]

I'm trying to perform a no-rise analysis in a shallow floodway (1 to 2 feet deep) for a mobile home development. The park is existing (grandfathered in before 1979 when BFE's were determined), fully within the floodway and they want to replace single wide homes with double wides.

The local jurisdiction will allow it if they can get a no-rise certification. I've obtained the original HEC-2 output reports which were done in 1979, based on USGS data. They are based on 40-foot contours and show no existing obstructions even though the mobile home park was there at the time as well as several farm buildings, etc. I guess the HEC-2 modelers just assumed that stuff was minor insignificant clutter with negligible effect.

The FEMA guidelines require recreating the original model, and reestablish the original BFE's. Fine, no problem. Then revise the duplicate model to reflect site-specific existing conditions by adding new cross-sections (two or more) in the area of the proposed development, without the proposed changes in place. Then create another model with the proposed new improvements and compare the resulting BFE's.

Given that this river is fairly large 1500 to 2000 feet wide with 12fps average velocities what is the likelihood of success? I assume that any new structures will be elevated on piers and maybe parallel sheer walls so in effect the new development will probably have less obstruction than the duplicate model which has whole homes below the BFE with no flow-through or under allowed?

Has anybody done this before?

 

[msquared48]

Have you checked the appropriate USGS quadrangle map to see if the mobiles are shown?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[francesca]

I wouldn't consider mobile homes to be an obstruction to 12 fps flow. More like tinder.

Don't assume the new structures will be elevated on piers unless they will be. And if anyone has any sense, they will be. This sounds like a horrible idea.

 

[msquared48]

Doesn't sound like 12 fps at the mobiles at 1-2 feet deep. That's the average. It's probably a lot more in the main channel and a lot less at the mobiles...depending.

With double-wide mobiles there will be a minimum of four pier lines - a lot of area to consider.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[gbam]

Okay I have to ask how can WSEs be established with 40-foot contours provided that the error is +/- 20-feet? Oh well, have you contacted FEMA directly to dicuss the scenario?

 

[oregonpe]

12 fps is the average velocity, not necessarily where the mobiles are. Any new structure will have to be built on foundations capable of withstanding the flows through them. The long dimension of the mobiles is parallel to the flow so its possible (and fairly common hereabouts)to design and build an elevated structure. FEMA has a whole publication on designing elevated residential structures (FEMA 54).

Not my idea of a great home site but people pay good money to be near the river and its not my place to judge I guess.

I haven't contacted FEMA directly yet. Just wondering if anybody had experience with this type of analysis.

 

[beej67]

Maybe I'm not envisioning this correctly. If the river is huge (1500+ ft wide) and the flooding at the trailer park is only 1 to 2 feet deep, how is the trailer park in the floodway?



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

http://www.campbellcivil.com

 

[oregonpe]

Good question beej67. Just a quark of the original model. If I remember right from my fishing days, the entire river width is fairly shallow here due to a riffle. In many places the river is only a few feet deep with many rapids and riffles.