Floodplain questions 1

[wds1]

We want to extend a buiding addition into a "Zone A" 100 yr floodplain. I'm hearing conflicting information from FEMA and others, and would like to get it clarified. Zone A is determined by approximate methods and does Not have a Base Flood Elev(BFE) or a defined Floodway. We know we have to do an engineering study.
1.) Will this be a Flood Insurance Study(F.I.S.) or Hydrology & Hydraulic study(H&H)?
2.) Will they define a new Floodway or Will they do a Pre and Post const. H&H and limit the rise in elevation to 0' or 1.0'?
3.) If we fill in the flood fringe do we have to excavate the same volume so we do Not reduce the storage volume?
4.) If they define a Floodway, can we have the engineer shift it slightly to the other side of the stream(Floodways are Not symmetric about streams) to miss our building?

 

[RWF7437]

1) Probably an H&H study
2) They will probably expect you to define the floodway
3)No, you don't have compensate for lost storage in the floodway fringe
4)Yes, you can define the floodway in any one of 5 acceptable ways.

Your engineer should know these things. Find one with good experience in this area of practice and good relations with your local FEMA reps.

good luck

 

[queque]

The FEMA staff have engineers to provide help.
If you can get a Project number assigned, you can talk to the FEMA staff member assigned to it and expedite the process of understanding what is needed.

 

[cimmeron]

1. A FIS is generally for a watershed or significant part of a watershead and you would not want to pay for such a revision. When the flood plain does not have a flood plain elevation specified, the general proctice is to determine the ground elevation at the edge of the flood plain and use it as your flood plain elevation.

2. No, floodways are part of the FIS study and are not changed for new developments. Also a small building addition is too small to affect 99% of the FIS studies. If no floodways are shown, don't worry about it.
3.Normally FEMA and local jurisdictions require the flood plain storage volume to remain unchanged with new construction and will require an excavation from the floodplain equal to the volume of the building within the flood plain.
4. Floodways are normally only revised when the FIS study is revised as it is a major project.

You will probably also need a LOMR ( letter of map revision ) from FEMA. Discuss this with your professional enineeer.

 

[RWF7437]

Methinks Cimmaron's answer No. 3 is incorrect. Obstructions in the floodway (not the floodplain), such as fill, are not allowed unless one can demostrate a zero rise in water surface. The floodway is defined by assuming the flood plain IS obstructed up to the point where the water surface rises by one foot, in most cases. See the HEC-RAS User's manual for a complete explanation.

good luck

 

[francesca]

Where we have to design a subdivision on the banks a creek that did not have its 100-year elevations determined by FEMA, we develop a HEC-RAS model to calculate a 100-year flood elevation based on USGS Regression Equation or SCS Curve Number method flows.

We consulted the guidelines for undertaking Flood Insurance Rate Maps when coming up with this methodology.

HEC-RAS is free software, available for download off the US Corps of Engineers website. Their Hydraulic Reference Manual is essential reading if this is your first model.

 

[RCEJD]

We are currently performing a study for a large tract development in Zone A for a major channle and have submitted a CLOMR for FEMA review by Michael Baker and Associates. From my experience and the literature available thru FEMA, I see alot of mis-information and misunderstandings on this thread.

If there is an FIS, this is the basic information and you will (as noted correctly) need to evaluate whether you are in the FEMA floodway and what the Base ELevations are. FIS report can be purchased on line, and you can obtain the electronic models thru FEMA, otherwise you will need to go to Michael Baker (They are helpful once you get to them). You will need to obtian the effective model, and then create a duplicate effective model. You will also need to then perform a FLoodway analysis to determine encroachments based on the surcharges.

Some of the FEMA language is very specific and the Fed Regs are quoted line and verse by them. Essential that you get an engineer EXPERIENCED with FEMA, not just running HECRAS. There are several publications on the FEMA website that explain the process and how they manage there floodplains and Zone A's. It is essential to obtain this information ...

Good luck.