Food for Thought: Stormwater

Friday, January 8, 2010

Please see this information on stormwater from Mary Gutierrez, Environmental Planner with the West Florida Regional Planning Council:

Before urbanization, water recharge happened when precipitation fell on pervious surfaces (including grassland and woods) and infiltration occurred. When cities developed and the amount of pervious surfaces decreased, leading to less ground water recharge and a huge increase in surface runoff.

Impermeable surfaces tend to become fully saturated very quickly and thereafter all of the precipitation becomes runoff, though some of that runoff may be absorbed by adjacent permeable areas and may not enter any drainage network. Once these impermeable surfaces have been wetted the percentage of runoff does not vary greatly. With the increase in impervious surfaces (roads, rooftops, parking lots and other hard surfaces that do not allow stormwater to soak into the ground), the rate of stormwater runoff increases. This means more water reaches the waterway faster and less water infiltrates into the ground. In streams, more erosion of stream banks and scouring of channels occurs because of runoff. This degrades habitat for plants and animals that depend on clear water. Sediment in the water clogs the gills of fish and blocks light needed for plants. The sediment also settles to fill in channels of streams, lakes, and reservoirs.

Rainwater will carry chemicals, nutrients, sediments and other substances into local streams (either directly or through storm sewers) if the water is not absorbed by soil and vegetation. The increased runoff can also carry along debris such as litter, cigarette butts, motor oil poured down the storm sewer, air pollutants that settle from car exhaust, and fertilizers, and pesticides from lawn care. The reduced amount of infiltrating water can lower ground water levels, which in turn can stress downstream environments which depend on steadier flows of water. New sources of groundwater can also develop in urban areas, although they are not from the most desirable places (septic tanks, percolation basins, industrial waste injection wells, agricultural and residential irrigation).

Once an area is cleared of vegetation, graded and compacted, and an impervious surface or partially pervious surface is constructed or installed, the area generally will not return to a naturally vegetated state. New impervious surfaces change natural drainage patterns and impact the environment by affecting the way that stormwater and, in some cases, tidal water moves over the landscape and through the soil. New impervious surfaces can affect the quantity, velocity, and quality of stormwater resulting in impacts to nearby land and water bodies.

Permeable surfaces react differently. As the storm progresses the upper layers of the soil become wetter and wetter and when the rainfall exceeds the rate at which it can soak into the ground the rainfall is turned into runoff. When the rainfall intensity drops below the soakage rate the runoff ceases even though rainfall may still continue. Therefore the percentage runoff varies throughout the duration of the storm.

One method of reducing stormwater runoff is to minimize the amount of impervious surfaces such as concrete sidewalks, roads, and asphalt driveways. These surfaces do not allow runoff to seep into the ground; they are not pervious. Use pervious surfaces instead. A paving surface that allows water to soak in may seem impossible, but there are many materials that provide the durability of concrete while allowing rainwater to filter down into the ground. If you are planning a new patio, walkway or driveway, there are several attractive alternatives to concrete such as wood decking, bricks, interlocking pavers, or flat stones. If used properly these can create a permeable paving surface that is not as harmful to the environment.

Stormwater flows into the stormwater system through storm drains, which are frequently located along the curbs of parking lots and roadways. The grate and holding tank that prevents larger objects from flowing into the storm sewer system are called a catch basin. Once below ground, the stormwater flows through pipes that lead to an outfall where the stormwater enters a stream, river or lake.

In some areas, the outfall may lead to a stormwater management basin. These basins control the flow of stormwater and can also improve water quality, depending on how they are designed.

In some urban areas, the stormwater and sanitary sewer systems may be combined (not here). In this situation, both stormwater and sewage from households and businesses travel together in the same pipes. Both stormwater and sewage are treated at sewage treatment plants except during heavy rains. During these occasions, both the stormwater and untreated sewage exceed the capacity of the treatment plant and this overflow is directed into local waterways untreated.


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