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Flood Loading per ASCE

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I am looking for a little piece of mind on my interpretation of Chapter 5 of ASCE 7-02 for Flood Loading.  A courageous towing company owner wants to place his PEMB in a flood zone where the flood depth is almost 13ft above the finished floor.  I told him that the building didn't have a chance of withstanding flood waters, even if they put in flood gates as specficied in the Corps publications - 1 square inch per square foot of floor, on each side of the building, or something like that...
I used section Hydrodynamic Loads which calculates an effective flood depth for flow rates up to 10 ft/sec.  This equals the flood depth plus about a 2 ft surcharge depth for V=10 ft/sec.  For this case, the effective flood depth is about 15 ft.
The end of section states that surfaces wetted by the tailwater can be designed for the flood depth only, 13 ft in this case.
Is it correct to assume that for a wall that is subject to V=10ft/sec flow rate, and that is also wetted by the tailwater, the forces would be determined by 15ft of water on one side and 13ft of water on the other?  Simplifying, the wall would essentially be subjected to 2ft surcharge x 62.4 lbs/cf = 124.8 psf from ground level up to 13 ft + the gradient to 0 psf at 15 ft.
I am second-guessing myself because it sounds like an excessive amount of force for what seems like a low-flow flood.  But I don't have any flood design experience to draw on to give me a better feeling about these numbers.  Any thoughts?
Jim Wilson, PE
Stroudsburg, PA

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