Dry retention ponds permanently hold back, or retain, the first half inch of runoff from a site. They use the underlying soil as a natural sand filter to remove pollutants from stormwater runoff. As the water infiltrates, or soaks into, the soil forming the pond bed, pollutants are trapped. The filtered water then flows underground into the Floridan Aquifer. Dry ponds are the simplest type of ponds and have the fewest problems.
Although dry ponds are supposed to be too dry for wetland plants such as cattails to survive, there are several situations when their presence is not a violation of your SWFWMD permit.
How To Recognize: Dry retention ponds are designed to be dry, except for 72 hours following a rain event, or a series of rain events if they occur frequently. They are sodded or grassed, with a concrete overflow structure that has a metal grated top and a rectangular weir cut in the side of the concrete structure.
How They Work: A portion of the site’s stormwater percolates through the top soil. The pollutants settle out and are trapped in the pond’s bottom. Exposure to sun and oxygen helps break down the greases and oils.
Why They Fail: Accumulated sediments with silts, oils and greases eventually seal off the porous bottom sands resulting in little or no percolation through the filtering sands. Untreated water may discharge through the overflow structure if this occurs.
1. On a monthly or quarterly basis, and following a storm event, the entity responsible for maintenance should make an inspection of the pond and its outfall structure to ensure that the system is operating properly. If standing water persists longer than 72 hours after a normal summer rain event, or if wetland vegetation such as cattails grow in the pond, the stormwater facility maybe in need of repair. Repairs may be as simple as scarifying or raking the pond bottom, or may consist of removing the bottom sediment (approximately the top foot of soil) and replacing the soil with clean sand. For more information contact your local SWFWMD service office.
2. Mow frequently enough to prevent thatch build up. Pick up grass clippings after cutting. Limit fertilizer use around the pond, and do not fertilize grass in the pond area.
3. Resod any areas (sides or bottom) where grass or sod has been removed or eroded.
4. Keep the outfall structure clear of debris and vegetation.
* The information on this page below the gray line is quoted from SWFWMD's pamphlet, "How to Operate & Maintain Your Stormwater Management System", VISKH0003 (8/03). Use this link to download a PDF of the complete pamphlet from SWFWMD's website: https://www.swfwmd.state.fl.us/media/755 .