so, do you mean "flood gates" or "stop log / closure" or "flap gates" or "slide gates"? usually the interior drainage pumps get turned on when the basin gets full, and not tied to the level on the river side of the levee. Peak flow into the interior basin may not occur when the river is at its peak, but you will need to determine if the peaks are coincident.

Your site design criteria are generally set by the city or county permitting agency and probably at least 100-year if higher. doesnt matter if "gates" are closed, you still need to protect your site from flooding.

Pump size is dictated by the size of your detention basin. since you have 21 acre-feet of water, you will either need a large basin or a very large pump.

the following USACE publications are available online and might help
EM 1110-2-1413 HYDROLOGIC ANALYSIS OF INTERIOR AREAS
EM 1110-2-1913 DESIGN AND CONSTRUCTION OF LEVEES
EM 1110-2-3102 GENERAL PRINCIPLES OF PUMPING STATION DESIGN AND LAYOUT

Thanks CVG. Not flap gates. Gates do slide up and down. Cities operational manual specifies closing the gates when gauge reaches certain level on river and also turn the pumps on since water will not be escaping. I am looking at all scenarios. Existing conditions had 3 pumps totaling 17,000 gpm for this outfall. I will look at those publications.

Design your pumps to their efficiency curves. Multiple pumps can keep individual pumps operating in their respective best efficiency range and allow more flexibility in varying flows, plus provide backup.