Perhaps a basic way to look at it would be (required fire flow x required time)+ losses from infiltration & evaporation. This volume would be the bare minimum, of course - you wouldn't want to drain the pond anywhere near the bottom and pull in all sorts of nasty stuff into your fire system.
Whether a standard, guideline, or simply rule of thumb, for fire department drafting operations I've heard that 2 feet of water needs to remain above your outlet (to prevent whirlpool effects that may introduce air into the line) and that the fire engine (if a manual dry situation) cannot draw greater than 30 feet of head.
Thanks, guys for the info. I asked FM and this was his response:
The typical method is as follows:
1. determine the occupancy of the building to find the needed sprinkler density,
let's say it's 0.3 gpm per sq. ft. over the most remote 3,000 sq. ft.
2. multiply the 0.3 x 3,000 to get 900 gpm
3. add the desired hose stream demand, let's say 500 gpm. 900 + 500 = 1,400 gpm
4. NFPA and FM Global standards define a desired length of time for adequate
water to be available for fire fighting. This time depends upon the occupancy, let's say
in our case it's 2 hours. 2 hours = 120 minutes. Multiply 120 minutes x 1,400 gpm
and we get 168,000 gallons.
5. We usually add a percentage for error and other reasons. In this example, we may look
for roughly 200,000 gallons of fire water availability.
Alternatively, if a sprinkler system is already hydraulically designed, we can use the
sprinkler system demand figure (gpm) and skip steps 1. and 2.
So, no, the square footage of a building is not typically a determining factor in sizing
fire water supplies.
