Greybar said:60 mph ~= 5000 ft/min ~= 100 5ft squares/round
(Hey, that was easy!)
Assume 10x10 standard dungeon tunnel as installed by union kuo-tao contractors. Look for the label of Kuo-Tao Stonemason Local 403 for quality!
So that's a 10x10x5 block of water for every round of PC drownage.
That should be 10x10x500, assuming you're working in feet. Volume flow rate equals area * velocity; area is 10x10, velocity is 100 squares/round is 500 feet/round.
So we need to challenge twice their Con to make it worth it! So let's say 30 rounds worth of water. That's 30x10x10x5 = 15000 cubic feet of water. So a pond 30 long by 50 wide by 10 deep would be sufficient. No problem, piece of cake.
That would make this bit 30 rounds duration x 10x10 area x 500 feet of water/round = 1,500,000 cubic feet of water, or a pond about 100 by 150 by 100 feet deep. Sounds much better to me, since that little dinky cistern you first postulated would drain out in about 1/3 a second at this flow rate.
