Base Flood Elevation 3

[JAS34]

Hello Engineers,

I am currently looking at ASCE 7-05 Flood Loading.

I have found the FIRM for my area and it gives a Base Flood Elevation (BFE) of 19 feet for the exact area where my structure is located.

(1) Is this stating a base flood of 19 feet above sea level or that the base flood reaches a height of 19 feet above the ground at this particular location? The map legend does not specifically say.

(2) If it is height above sea level, is there a resource/reference used to find the ground level’s height above sea level at a particular location?


Thanks in advance,

JAS34

 

[SteelPE]

I had to deal with this same problem a few months ago. I wanted to go on a rampage soon after.

The base flood elevation is base upon the North American Vertical Datum of 1988(NAVD 88). This should information should be located somewhere on your FIRM map (or similar reference information).

You will need to have your site engineer give reference the ground elevations in the same manner (NAVD 88). From what I can tell no NAVD 88 topo..... no dice. From there you should be able to apply ASCE7-05.

On a side not, you may also need ASCE24 as well. This gives you all the information ASCE7 doesn't with regards to designing for flooding. If your case is severe enough (coastal flooding subjected to high velocity wave action) it is a must have.

 

[cvg]

flood elevations are referenced to a vertical datum which should be indicated in the notes to users. the base flood elevations are not given in depth of water. recommend if you have difficulties with this to call your floodplain administrator and ask for clarification.

 

[charliealphabravo]

in my experience the ground elevations will usually come from the surveyor. the base flood elevation (BFE) will come from the flood map. the design flood elevation (DFE) will come from the building official if they want to use a higher elevation than the BFE.

if you fill out a flood elevation certificate as part of the construction documents you will also need to specify the lowest and highest adjacent grade and the height of the lowest horizontal structural member supporting a floor (for V Zone). i would typically put these elevations on the drawings as part of my design criteria.

 

[cvg]

be careful.
not all flood insurance maps use NAVD88. Many are based on NGVD29 still and others are based on more obscure vertical datum's such as mean sea level. you have to confirm which datum is being referenced. Sure, you can get ground elevations from a surveyor, but make darn sure he is referencing the correct datum in his survey. He should verify the elevation of the nearest elevation reference mark which is shown on the Flood Insurance Study. around here a vertical datum bust can amount to a difference of nearly 3 feet.