Stormwater is water that originates during rain, melting snow, or runoff water that enters the stormwater system. Stormwater that does not soak into the ground becomes "surface runoff," which either flows into surface waterways or is channeled into storm sewers. When the volume of stormwater is high, soil can be moved by the water and swept away; this is called "soil erosion".

Healthy soil should be able to absorb and hold water in pore space. When the pore space is reduced, water infiltration is reduced or restricted. The organic matter content of the soil is the key to absorbing and retaining water; healthy soil with high organic matter content reduces the amount of runoff.

Soil erosion is the single greatest water pollutant and comes with a very high price: death of fish, smothered fish eggs, destroyed aquatic food webs, and increased water-filtration and hydro-electric-power-generation costs.

Stormwater pollution is a serious problem. Because impervious surfaces (parking lots, roads, buildings, compacted-soil) prevent rain to infiltrate into the ground, more runoff is generated in urban areas than in the undeveloped areas. This additional runoff can erode watercourses (streams and rivers) and cause flooding when the stormwater collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during a storm, little water actually infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather. The soil is then carried away.

Water pollution can be caused by daily human activities such as gardening, driving, and building, resulting in the deposit of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. The quantity of human activity results in significant pollutants in these receiving waters.

Flood control is necessary to prevent excessive pollution. Stormwater runoff has been a problem ever since the beginning of urbanization. Modern drainage systems which collect runoff from impervious surfaces (eg. roofs and roads) ensure that water is efficiently conveyed to waterways through pipe networks, meaning that even small storm events result in increased flows in our waterways.

In a traditional stormwater design the water is collected and stored in a pond allowing pollutants and impurities to settle. The treated water is then discharged downstream. The problem with this traditional approach is that the water is not really "treated" and the pipes may be undersized, which may be causing street flooding.

For an ecologically friendly approach to site development and stormwater management:
-Preserve open space and minimize land development.
-Protect Natural systems and process drainage-ways, vegetation, soils, sensitive areas
-Re-examine the use and design of the infrastructure (lots, streets, curb & gutter, sidewalks), and customize the design to each side.
-Incorporate Natural Site Elements (wetlands, stream corridors mature forests) as Design Elements.
-Decentralize and Micromanage Stormwater at its source.

This means that everybody can:
-Use fertilizer and pesticides sparingly and keep them off of paved surfaces. Sweep it back onto the lawn.
-Don't fertilize before a rainstorm.
-Don't bag grass. Leave it on the lawn to reduce the need to fertilize.
-Mow at the proper height. (Fescue thrives at 3 inches. Mowing any closer will create favorable conditions for weeds.)
-Wash your car in the grass. The wash water will be treated by the soil, and the grass will benefit from the water.
-Use native trees and shrubs to landscape
- they help to filter pollutants, are easier to take care of and don't need fertilizer. (see Xeriscaping)
-Minimize water use through landscape design and xeriscaping
-Preserve open space and minimize land disturbance and reduce impervious surfaces.
-Protect and incorporate natural systems as design elements.
-Use retaining walls to preserve trees and soil
-Use a rain gardens design
-Pave your driveway with "pervious material" or "Permeable Paving Alternatives" such as gravel or bricks that are set farther apart to let the water through. This is a way that has been traditionally used in Europe. These surfaces reduce site runoff by increasing infiltration into the soil. There are a number of permeable paving surfaces available from paver block systems to geoweb reinforced grass surfaces. These systems can be designed to infiltrate virtually any design storm including the 100-year storm or they can be used strategically with impervious surfaces to capture the high frequency lower intensity storms.
-Use rain barrels. Rain barrels are designed to hold rainwater collected from residential rooftops. Water is retained in the barrel and can be used for yard watering of vegetation. The barrels are designed with overflow options to allow water to infiltrate beneath the barrel or be redirected to such features as a rain garden. Barrels would be especially effective in areas of cities with combined sewers.

If you have a septic tank:
-Have your septic tank pumped every three to five years to extend the life of your system.
-Don't put chemicals into your septic system. Household chemicals kill the good bacteria that help your septic system work.
-Maintain and repair your septic or waste treatment system to avoid malfunctions.
-Divert roof drains and runoff away from the septic system area to keep extra water out.

Rain Gardens (see also Xeriscaping)
-Create a shallow area planted with native vegetation that absorbs and infiltrates runoff from impervious surfaces and may discharge to groundwater, a storm drain, or surface outlet.
-Depending on site conditions, a subgrade tile system may be recommended to enhance infiltration.
-Rain gardens reduce the volume of stormwater runoff pollutant loads delivered to surface water.
-Rain gardens can be used in individual residential, commercial, or institutional settings to mitigate impervious surface runoff.

Vegetated Roof Covers, also called Green Roofs, reduce the amount of impervious surface by capturing and evapo-transpiring rainwater. The roof is multilayered and typically consists of a drainage layer covered by a manufactured soil matrix protected by growing vegetation. Green roofs may extend the life of roofs, reduce energy costs, significantly reduce stormwater runoff, and ultimately reduce the size of typical stormwater controls within the community. Roofs can be designed to accommodate specific low or high intensity storms by varying the media depths.

When building:
-Immediately after grading a site, seeding is the next thing to do, especially if rain is expected.
-Use erosion control devices to prevent downstream contamination.

Following are some ways urban planning tackles the stromwater problem. It goes a little beyond what the average home owner can do, but it might give you ideas to how you can use this at your own home and gives you some understanding what these ponds are you see popping up around your area.

The following is taken from: Stormwater Management and Post-Construction Best Management Practices

Bioswales are an alternative to standard below-ground stormwater sewers. Bioswales intercept or receive impervious surface runoff and blend infiltration and slow conveyance of stormwater. The soil matrix of the swale can be amended to enhance infiltration and percolation. These systems can be engineered to absorb the high frequency low intensity storms but can convey the large storm events while providing vegetative filtering. Bioswales can discharge to groundwater, storm sewer intakes, or directly to surface water.

Concave Medians are essentially constructed similarly to rain gardens and biocells except that they are placed in a median strip between two lanes of traffic or in parking lots. Depending on the setting, they may be confined to narrow cross-sections, which may restrict their capacity to low intensity storms. Raised medians can be retrofitted by excavation and curb cuts to allow water to enter.

Detention Ponds temporarily store runoff and control the rate of release to reduce downstream flooding. In the past, detention ponds have not provided significant water quality benefits due to the short duration of storage and the lack of control of low intensity storms. The ponds will trap some sediment, but are not designed to capture the first flush of contaminants. Two-stage outlets can enhance the removal of these contaminants.

Retention Ponds are designed primarily to manage stormwater discharge to prevent flooding. They can enhance sediment trap efficiency and can reduce some nutrient loading while controlling rate of release to control downstream flooding. They also provide an aesthetic amenity that doubles as wildlife habitat and a recreation source, and which also enhances property value. Two-stage outlets can enhance the removal of contaminants by increasing retention time of low intensity storms.

Constructed Wetlands provide water storage benefits similar to retention ponds. The filtering, biological, and chemical removal mechanisms provided by wetlands can also improve the quality of stormwater discharges. Wetlands are aesthetically pleasant and provide wildlife habitat and recreational outlets. The soils underlying the wetland should be relatively impermeable in order to maintain a permanent water level. Remnant native wetlands should never be used to store or treat stormwater. Thermal and contaminant loads will degrade vegetative communities thus reducing biological diversity.

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