No impact to FEMA modeled streams

[TxPE]

What are the FEMA rules regarding flood plain elevation increases from development. Where can you get copies of these rules and is an impact to a stream ever considered to be insignificant? For example the development will increase the CWSEL by 0.01 feet. Any thoughts out there? Thanks.

 

[bltseattle]

Contact your local FEMA office for the details. I was a designer on a project in a FEMA floodplain & the process for coordinating a change in flood levels was quite tedious. For that project, HEC-RAS modeling was used to show the project impacts. The original hydraulic model was obtained/replicated to generate a base flood level that essentially matched FEMAs flood elevations, then it was modified to reflect the project. The issue is that if the flood plain limits change, it can affect upstream flood insurance maps, and revising/reissuing those maps is a major effort.

Your project will go faster and easier if you can find a way to prevent the flood level from increasing. For the project I worked on we put in some big pipes near a floodway restriction to ensure there would not be upstream impacts.

Again, talk to your local FEMA folks to get details appropriate for your system, and please let us all know what you find out!

Good luck
BLT

 

[staffgage]

I believe rules depend on the region. My understanding was in CA that the floodplain was set artifically 1 foot above the existing, and subsequent development was permitted to impact the floodplain incrementally until that limit was reached. If you are adjacent and discharge to the floodplain of a stream in Western WA, and you have to provide water quality control, then a pond might be oversized to provide additional floodplain storage as mitigation for development.

 

[RWF7437]

You can find much information, including digital flood maps at FEMA's website.

http://www.fema.gov

Most of it is downloadable including all of the regulations (CFR).

Good luck

 

[johnhan76]

TXPE,

How did you go about obtaining the original HEC model. I have called FEMA to get info regarding pipe inverts, # of pipes, top of road elevations etc that they used in their study but I didnt get very far with them. Thanks for the help.

 

[hhengineer]

Johnhan76,

Good luck trying to get anything directly from FEMA. You have to go through one of the contractors who maintain the data. See the FEMA website for the contractor for your region of the country. As you might have guessed this is not a free service. You have to pay up front and they might not find anything anyway. In NY the DEC maintains a pretty good collection of FIS and the corresponding HEC-2 models (paper format). You can get a copy at no cost. I suspect some other states might have something similar. If that fails, you can always try the original study contractor (listed in the FIS) but that's a real shot in the dark.

TxPE
As for FEMA rules it spells it out pretty good in the FIS, try reading that first. That may not be the final word however. The State and Local governments may impose stricter requirements than FEMA. In NY I have had more than a couple of projects that NYSDEC has mandated NO increase in the 100 year WS Elev.

 

[RWF7437]

Also try your "local" Corps of Engineers office. This may be the district office or the Division Office. I've had good luck in Oregon getting HEC-2 models through the Portland District.

Also, be aware that when you import HEC-2 data into HEC-RAS you will lose all the bridge and culvert data which you then have to input manually.

Good luck

 

[Bayou]

FEMA rules state that the effective BFE cannot be impacted more 1.0 feet. Therefore a development can fill within the floodplain fringe (the area within the floodplain, but outside the floodway) without imposing on FEMA regulations.
However, development cannot occur within the floodway.
The requirements from FEMA is that and change in the floodplain map (be it from channel improvements, new roadway crossing, or fill) be properly documented with them. This documentation, either through a LOMR or PMR, is very tedious and time consuming.

Overruling FEMA regulations are any local regulations that are more strict. The local state or county regulatory agency typically has a 0.00' impact requirements.

As far as getting information out of FEMA, if it is a HEC model, then contact the FEMA Library. The address is on the FEMA website. There is an initial payment of $120 required.

Johnhan76, if you are in Texas, PBS&J are the review contractors for Region VI. Be sure and call the reviewers often to explain any details of your project. Do not assume they will make an engineering assumption. Be overly detailed in your form explanations as well.