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Flood Run-off Facts
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Natural vs. Urban Run-off
Natural Run-off
Surface run-off : 0.3%
Urban Run-off
Surface run-off : 30%
Interesting water runoff facts
* There is NO absorption of water into the ground when it is covered with paving.
Think of what happens when open space, pastures, woodlands, dirt, or gravel driveways and parking areas are paved over?all that water that would have been absorbed is now turned into sheets of water running across the pavement looking for somewhere to go. It gains speed and volume until it creates a gully, then a small stream, then a swollen creek.
* Did you know that a moderate storm that dumps 1" of rain produces about 600 gallons of runoff from a roof surface of 1,000 square feet?
Imagine the number of rooftops in your community and you can quickly see how much runoff is headed toward the storm drains, picking up pollutants common in urban areas along the way, and headed toward our lakes, streams and rivers.
* Did you know that because of impervious surfaces such as pavement and rooftops, a typical city block generates 9 times more runoff than a woodland area of the same size?
* The increased volumes and flow rates of runoff produced by urban watersheds have a number of harmful effects including flooding and stream erosion.
The water flow velocity of runoff over a smooth, impermeable surface such as a road or parking lot is about ten times faster than over a vegetated surface.
Low Impact Development
Designs To Reduce Run-off
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