Underdrain ponds can either have a dry bottom or have a permanent pool of water for aesthetic purposes.  They use a man-made sand filter to remove pollutants from stormwater. The process is similar to that of sand and DE filters used for swimming pools.  As the water flows through the underdrain, pollutants are trapped by the special sand filter media.  The filtered water is collected by an underground perforated pipe inside the underdrain, and then flows out through the control structure.

Underdrain ponds are the most complex type of ponds and have the most problems.  They often have one or more plumbing clean-outs visible. The upper photo shows a typical underdrain that is covered by coarse gravel ballast and has a white plastic clean-out.  The lower photo shows an underdrain pond that has a permanent pool of water, and the underdrain is covered with grass rather than ballast.

How To Recognize:  These ponds may either be dry or wet, but rely on a filtration system made of perforated pipe laid in a bed of filter media, such as sand, to remove pollutants.  “Cleanouts” or vertical pipes with caps screwed on top, are connected to the buried pipes and extend up to the pond surface or bank.  It is essential that the cap is always kept secured on the cleanout so that untreated water is not discharged through the cleanup. 

How They Work:  A portion of the site’s stormwater percolates through the filter media into the perforated pipe and out through the control structure.  Pollutants settle out or are trapped in the filter media.  In addition, exposure to sun and oxygen helps break down the greases and oils.

Why They Fail:  The filter bed may become clogged with accumulated sediment, oils and greases, resulting in little or no percolation through the filtering sands.   Untreated water may discharge through the overflow structure if this occurs.

Maintenance Suggestions:

In general, if approximately 36 hours after a rain event you notice that water discharges over the top of the concrete control structure, rather than through the perforated pipe, it may be a signal that the pond is not functioning properly. 

1. On a monthly or quarterly basis, and following a storm event, inspect the pond and its outfall structure to ensure that the system is operating properly.  Repairs may be as simple as scarifying or raking the filter sand, forcing water through the cleanout to cleanse the perforated pipe, or as involved as replacing the filter media.  Check construction plans or contact SWFWMD for more information if any questions arise. 

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 where grass has been removed or eroded.  Do not sod over the filter media. Place stone or gravel over the filter media for stabilization, if necessary. 

4. Keep the outfall structure clear of debris and vegetation.