You can calculate the flooding rate of any size hole:
Q = 20 x d x square-root of h
Q = flooding rate in gpm
d = diameter of hole in inches
h = depth of hole underwater in feet
so with a 3/4" drain hole and a depth of 2 feet, we do the math: 20 (3/4) x 1.41= 21.15GPM
GPH would be 1269 (21.15 x 60) so a 1500GPH might keep up with it if they actually pump that amount..
Here’s a good link too...
http://www.whsyc.org/Flooding/Flooding.html