1) Where's the project located?
2) Sounds like you meant "detention pond" not "retention pond" ..? Presuming that's correct, your only recourse for lack of volume is find more volume. Exercise:
Print the post-development inflow hydrograph. Draw a line across it at your allowable discharge rate. Now draw another line from where it intersects the tail end of the inflow hydrograph back down to the origin of the hydrograph. Shade everything above the slanted line and below the hydrograph with a blue marker. If you do the calculus, the area under a hydrograph is a volume. The blue markered volume is approximately how much detention volume you need, to detain that hydrograph to that allowable discharge rate. If that volume isn't available in your pond, then you need a bigger pond. Raw science says that, there's no tricks you can use to bypass it.
Well, almost none. You could add a retention element to the detention pond to provide more peak discharge capture. Examples:
Your photo looks awfully flat, like Florida flat. I heard some rumblings that NFWMD (once it got going) was planning to allow folks to irrigate out of wet retention ponds, and if you can do a water balance calculation showing your irrigation draws the pond down to a certain level, then you might be able to justify some extra capture volume for your event storm.
Another idea, presuming your pond invert is above the seasonal high water table, is dig the pond out deeper and rig it to an infiltration system. Do the Darcy math to show that the retention volume is recaptured in a minimum time frame (usually 24 to 72 hours in the regs I've seen) and you can credit it as capture volume for your event storm.
Third idea is go underground with it. Stormtech chambers or an HDPE manifold or something, to catch your volume. You can make them cisterns too, again depending on a water balance calculation.
Someone mentioned parking lot storage - I've done that quite a bit in central and southern Florida. It helps that their maximum design storm is the 25 year, and they've all evacuated when it hits anyway.
As far as sediment ponds go, in any sediment pond sizing methodology that's based on Stokes Law (falling velocity of spheres) the volume turns out not to be the controlling factor for sediment trapping efficiency - the surface area is. That's unintuitive, but it has to do with length canceling out when you're calculating flow velocity across the pond. (if I recall correctly anyway, saw it in school)
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
