It appears that the detention basin was designed to manage storm water. I wouldn't say "the water level could never really raise above this 2 foot high drain hole on the side of the Drain box (culvert)" because it does when the rate of inflow exceeds the outflow during designed storm events. I wouldn't change the design of the pond without a new design sealed by an engineer.

Van Duong
Vduong@saintcharlescounty.org