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Calculating Base Flood Elevations

Calculating Base Flood Elevations

Calculating Base Flood Elevations

I am working on a 20-acre site that lies within a FEMA designated zone "A".  Only 63-cfs enters the site and is conveyed safely through a small portion of the site back to its natural channel.  My problem is that the whole site lies within the flood zone but only 2 of the 6 buildings are really impacted by water.  How am I supposed to calculate the BFE's for the remaining 4 buildings?  Even if I base it off of local flow of 2 or 3cfs I would have to raise the minimum finished floor elevations 18" above that local flowline and then they don't comply with ADA.

Any guidance would be very appreciated.

Justin Stoker

RE: Calculating Base Flood Elevations

1) Many times, in FEMA flood zone "A", you are given the 100 year BFE or there is a nearby transsect with a BFE.
2) You also may have a local regulatory constraint regarding a flow line, pond TOB or roadway crown and the minimum FF elevation.
3) 18" is no biggie with a 1:12 ramp with rails, it is only 18 feet long (under the magic 20' for another landing).  If you lack the length, consider switchbacks.  You could always place the ramp parallel to the 5' van isle or parallel to the sidewalk if you have the room.  Trust me, railings are cheaper than fill on a 20 acre site.  
4) Consider placing the HC spaces adjacent to local high points in your grading plan.  Remember that the HC space may be at 2% max slope and the rest of the lot may be steeper if necessary.
5) Think out of the box. (like a 3%-4% slope leading to the HC parking at a 2% slope, just call out the slope break for the contractor) 20 acres is like a 1320' x 660' and 18" does not seem like alot here. At 1%, you can do 18" in 150 feet, at 2% in 75 feet.  What is the problem?
6) Remember, keep your s/w flush of 6" above the edge of pavement.
7) Ask a more experianced person in your firm or pump a regulator for some specific hints to solving your problem.

Clifford H Laubstein
FL Certified PE #58662

RE: Calculating Base Flood Elevations

Thank you for the help, perhaps I can elaborate a little more into the problem.  This 20-acre development is the start of a great deal of development in the area, so BFE's have not yet been established.  In fact, it is my responsiblity to establish them.

Local regulations state that the finished floor must be 6" above the closest flowline highpoint and 18" above the BFE.  

I have spoke with others in the firm, local officials, and officials at a regional flood control district and all disagree with the manner of determining BFE's.  FEMA's National Flood Insurance Program (NFIP) design manual is extremely vague.  It is my hope to find some kind of clarifying advice about how BFE's are to be determined, specifically for buildings not directly impacted by flood water.

Again thank you for your advice.


RE: Calculating Base Flood Elevations

1) The BFE exists for all lands within the 100 year flood zone.  Due to a lack of development, this elevation may not yet have been calculated, but it exists for all riverine, tidal and "closed basin" systems.

2) You job is to determine this elevation.  In a riverine system, this elevation continually falls as the stream flows downhill.  In a closed basin, this is a fixed static elevation.

3) To determine this elevation, you use a FEMA approved model, and you model the WHOLE drainage basin for the 100 year event.  You will need 100 year rainfall data  (available on the internet), USDA soil maps (available at your local soil conservation service office), contour maps and current aerial maps.  You also should get selected survey data of critical points and typical X-sections.

4) Your model output will be the BFE's.  I would place more sections in areas of greater interest.  I would also submit the model output for FEMA approval.

5) Once the BFE's are determined, this is how I would apply them.  I would determine the 1' countours of the BFE across my 20 acre site.  If part of the site is above the 100 year flood zone, then I would uses the adjacent 100 year flood zone elevation as a BFE.  I would interpolate a BFE for each building pad, using the maximum BFE that crosses the pad of immediately adjacent sidewalk.  (I would consider future building expansions unless you want to step the finished floor)   

6) If a building is not directly affected, because it is outside the 100 year flood zone, no problem. Just do not make the finished floor lower than 18" above the nearest adjacent BFE.

7) I almost forgot, if your jurisdiction does not require attenuation with development, then you should model your basin with maximum development based upon current/projected zoning.  If attenuation is required, just the small exempt lots should be modeled as developed.  This means that you will need property appraiser data also.  The major roads should be modeled as build out.
Best of luck....

Clifford H Laubstein
FL Certified PE #58662

RE: Calculating Base Flood Elevations

Please excuse me, maybe I did not directly answer your questions, please let me clairify my remarks.

1) If your whole site is in the 100 year flood zone, then all six buildings are in the 100 year flood zone.  If the original flood study is not available, then I would simply overlay the contours on the fema map and see where the  100 year flood limits are.  At each building, the furtherst upstream building point perpandicular to your natural channel flow path, I would determine the 100 year base flood by interpolation.  I would conservatively interpolate to minimize flood damage.  Please remember to mitigate all fill placed within the 100 year flood plain.

2) If you mean desire to place two buildings in the 25 year flood plain or flow channel, forget it.  The 25 year flow channel should not be disturbed, if you disturb it then you must remodel the system.   (I do not really understand how your whole site can be in the flood zone but only 2 building really affected by the water, unless the 2 building are in some lower flood zone, like the 25 year.  Unless, the whole site is in the 500 year flood zone and 2 buildings are in the 100 year flood zone.)

3) Please see my previous remarks(11/27) about grading, 18" is no big deal.  Remember, ADA pedestrian paths may be under 5% maximum slope without a ramp and under 2% maximum slope is considered flat.  DO NOT FIGHT YOUR SITE.  Accept it and do your best within its limitations.  Remember, if you cheat, then you may pay for it, but if you do it right, then it is a just development cost paid by the owner.

4) Consider using catch basins to make grading easier.  Consider alternatives for lowering the local flow line, like putting all the building on the same side of the roadway and collecting the water on the other side or using a 20' wide grassed median to collect the runoff, etc.   

5) If the 100 year flood zones have been determined, then the 100 year Base Flood Elevations are determined.  They may be given implicitly in a graphic format by the FEMA flood map  (See comment #1).  If you have some non-FEMA flood map. then you need to determine the flood zone via a FEMA acceptable methodology.  (See previous 12/11 comments, #3-#4)

6) You only need an acceptable method to determine the BFE.  There are many acceptable methods but no single method may satisfy everyone.  You only need to satisfy the stormwater reviewer and follow accepted practice.  See comment #5.

7) I assume that you are working under a PE.  You may need to conference with him.  Also, your problem may not be unique.  Speak to the stormwater and ADA reviewers.  Ask to see simmilar projects.  Ask a contractor how he would solve your problem.

8) An old boss once told me - "Just do it, do not play with it."  Start from the easy and work to the hard.

Best of Luck......

Clifford H Laubstein
FL Certified PE #58662

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