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An impervious surface is a surface that does not allow passage of water, whereas a pervious surface will allow water to pass through without barrier. Disconnection of stormwater allows runoff from connected impervious surfaces the opportunity to infiltrate into a pervious area; such as a lawn or garden or capture the water for use later on such an area. It has been shown that when a watershed becomes more urbanized and therefore has more impervious cover the biota, or living organisims, in the stream are negatively impacted. It has also been shown that effective disconnection can have a positive benefit. Basically if we can alter the impervious cycle it is possible to decrease some of the impacts.
Stormwater is often directly connected from a rooftop, driveway or road to the stormwater collection system, picking up whatever pollutants might be there: oil from the car; mercury deposited from the air on the roof; bird droppings, dog droppings; fertilizer and pesticide from the lawn; and raccoon droppings in the storm sewer itself. Without an opportunity to infiltrate and with numerous pipes discharging into the stream at relatively the same time the peak volume is increased. More runoff is also generated during even smaller storms so the frequency of flooding events increases. With less water infiltrating there is less water to recharge the ground water and provide for baseflow in the summer months. Some studies have shown that lack of flow in the dry months has a greater negative impact on aquatic organisms than high flows.
There are various means of disconnection of stormwater. A few examples:
- Rain Gardens
- Roof Gardens
- Low Impact Development
- Infiltration Basins
- Rain Barrels
There are many other ways to help disconnect stormwater runoff.