Modern developments are designed to self-manage all the rain that falls within them; they are a small “watershed.” The stormwater management system of a neighborhood directs stormwater off streets and buildings via inlets, pipes and swales; the water is then directed into a detention or retention area for flood control and to improve the quality of the runoff. Most of the water is detained long enough for it to seep into the ground and replenish the aquifer or allow natural processes to improve the water’s quality before releasing it downstream.
Wet Detention Ponds are neighborhood ponds that are common to Southwest Florida developments. They are the low spot in the neighborhood watershed. They are designed to receive storm runoff and detain it long enough to allow natural processes to improve the water’s quality before releasing it downstream. Excess water is released from the pond at one exit structure called the “control structure," so called because it controls the elevation and the rate at which water leaves the pond.
Dry Retention Areas capture runoff from surrounding areas and release it only into the ground by percolation. They are designed to never discharge water to other surface waterbodies. They only have inlet structures, and no outlet structures. The bottom elevation is higher than the ground water level so that after the runoff percolates into the ground the area is dry. These are 100 percent efficient at stopping any pollutant from proceeding downstream. It is important to NOT FERTILIZE these areas to avoid polluting the water that percolates into the ground (fertilizer is a very serious pollutant in ground water aquifers).
Dry Detention (aka, Pre-treatment Areas) are the same as Dry Retention Areas except that instead of retaining all the runoff that enters into them, they are grassed areas designed to capture the dirtiest “first flush” of runoff from paved areas and only allow water from larger rain events to move on to Wet Detention Ponds or wetland preserve areas.
Control Structures that are located in neighborhood stormwater systems are the only pathway for excess stormwater to leave the neighborhood and flow downstream to join natural waterways. There are weirs that control the elevation and the rate at which stormwater leaves the detention area. These structures are designed to prevent flooding within the neighborhood while not overburdening development downstream. They are a part of the entire neighborhood stormwater management system which was engineered and permitted when the development was initially built. No alterations can be made to them without approval by a permitting agency. Their openings may not be blocked, partially blocked, re-sized or relocated on the structure. It is essential to keep them clear of trash and vegetation so that water passes through them unobstructed.
Skimmers (aka, trash guards) are mounted on the face of control structures to keep floating debris away so it doesn’t block the outlet holes, which are only a few inches in diameter. These outlet holes are the only location for flood water to escape the detention area without water levels rising to an emergency flood height. There should be about 1 foot of clearance UNDER them so water rises up behind them in order to reach the holes in the control structure.
Swales are gently sloped, elongated low areas that collect stormwater and guide it to drop inlets, swale inlets or a pond. They also function to store water, usually temporarily, until it has a chance to flow downstream. Sometimes they are designed to capture water and release it only into the ground by percolation (in which case they function as a Dry Retention Area). It is important to NOT FERTILIZE swales to avoid polluting the runoff and ground water with dissolved fertilizer; in general there is plenty of fertilizer that leaches to them from adjacent areas. For this reason thatch (thick vegetation) builds up more rapidly in swales, filling them in and reducing their water storage capacity. Fertilizing swales only accelerates the frequency for reshaping them to their designed width and depth, restoring their capacity to prevent flooding.
Drop Inlets allow water and debris from the ground level to enter, or “drop” into a storm sewer pipe through a grate on top. The metal grate is usually square or rectangular, but sometimes round. They can be located just about any place; a street gutter, parking lot, swale or other grassed area. They should be at the low point of the area that drains to them. The metal grate can become dislodged and can fall into the inlet. If they are broken they should be replaced with a grate of the same type. They should be maintained regularly to keep them clear of landscape material; this will prevent flooding and prevent the pipes from becoming clogged.
Curb Inlets are inlets with an opening along a street curb. They collect stormwater and debris from streets and send it to storm sewer pipes; these sewer pipes then lead to a pond. Consequently, when pollutants such as grass clippings, pet waste and fertilizer are washed, or blown into these inlets they will ultimately degrade the pond.
Pipes are underground conduits for stormwater. They can be round, oval, or rectangular and made of cement, metal, plastic or more rarely, PVC. Their ends may be protected with grates to keep debris out, and a headwall or concrete apron to protect them from being crushed. It is necessary to keep thatch and debris from building up at the pipe ends so that water flow is not compromised. Even small sinkholes over them indicate that the pipe has a significant hole in it and needs replacement.
Culverts are pipes which are open on both ends. Generally they are less than 100 feet long and most commonly are found under driveways and in-line with swales. They can be round, oval, or rectangular and made of cement, metal, plastic or more rarely, PVC. Their ends may be protected with grates to keep debris out, and a headwall or concrete apron to protect them from being crushed. It is necessary to keep thatch and debris from building up at the ends so that water flow is not compromised. Even small sinkholes over them indicate that the pipe has a significant hole in it and needs replacement.
Manholes provide access to pipes from the surface. They have a solid lid labeled “stormwater.” Lids labeled, “sanitary” belong to a separate piping system for sending sewage to a treatment plant. In the rare event that a sinkhole develops near or next to them, this typically indicates that a pipe joint is compromised and needs rebuilding. The problem should be addressed as soon as possible.
Updated February 22, 2017