Controlling stormwater a simple concept, yet hard to do
Published: Monday, May 23, 2005 at 4:30 a.m.
Last Modified: Monday, May 23, 2005 at 12:00 a.m.
The management of stormwater runoff is at once a simple concept and a complex problem. Stormwater runoff is a major contributing factor to flood frequency and intensity, stream side erosion and, in our area, degraded trout habitat.
What is stormwater runoff? It is the rain and melting snow that flows off impervious surfaces, such as streets, parking lots and rooftops, and also off lawns and farm fields before it soaks, infiltrates, into the land. Much of this water makes its way directly into streams and other bodies of water, carrying with it several well known and some less known consequences for water quality and down-stream properties.
Most people have seen the most commonly thought of consequence of stormwater runoff every time they see a muddied stream after a heavy rain. The soil in the stream that makes it muddy was brought there by stormwater runoff.
Most people have also seen another major consequence of stormwater runoff, but may not have attributed it to this. That consequence is an increased volume of water being carried by streams and, in large rain events, spilled over the banks into back yards, fields, roads and homes.
Another consequence of stormwater runoff even less noticeable by those not living in the stream is something you can't directly see. Take off your shoes and splash through a stream of stormwater runoff running down a street the next time it rains after a sunny day. The runoff water absorbs heat from paved roads, parking lots and, to a lesser degree, roof tops, heating it by as much as 40 degrees before it runs into the streams. This consequence is known as thermal pollution and is a major threat to cold water trout residing in several Henderson County streams.
In general, these three consequences are measured by a simple equation. It is the amount of impervious surface times rainfall and the benefits from stormwater Best Management Practices equal the consequence of stormwater runoff.
For instance, one mile of an average width curbed road can produce nearly 66,000 gallons of stormwater runoff from 1 inch of rain, if there is no stormwater BMP in place. An average 3,000 square-foot home with gutters can produce more than 250 gallons of stormwater runoff if no BMP is in place. A large parking lot can produce even more. If this water isn't stored in a BMP, such as a stormwater pond, wetland, rain garden or other permeable land, it will typically go directly to a stream, increasing the downstream consequences by these amounts.
So, the more impervious surfaces put into a watershed, the more thermal pollution, flooding frequency and intensity, and stream bank erosion due to the increased volume of water in the stream channel there will be unless adequate stormwater management BMPs are implemented.
There are ways to increase impervious surfaces without increasing stormwater runoff. Management activities designed towards replicating natural hydrology on developed sites and which consider the watershed-wide impacts of stormwater runoff have the greatest potential for success.
To learn more about stormwater management, visit the N.C. Department of Water Qualities stormwater Web site at h2o.enr.state.nc.us/su/ stormwater.html.
Shaun Moore is the Mills River Watershed Protection project coordinator for the Henderson County Soil and Water Conservation District.
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