Got stormwater runoff?
By Melanie Brethauer on 03/16/2009
If you drive a car, have a pet, fertilize your lawn, grow a garden, live in a house, have a gravel or paved driveway—then you are affecting stormwater runoff. When rainfall or snowmelt flows over the ground and drains into natural or constructed drainage ways, we call it “stormwater runoff” or simply “stormwater.” In some cases, this runoff drains directly into streams, rivers, lakes or oceans. In other cases—particularly in urbanized areas—runoff drains through constructed drainage systems that consist of inlets and underground pipes, commonly referred to as “storm sewers” or “storm drains.”
Stormwater entering the storm-drain system usually does not receive treatment. Increasing urbanization can increase the amount of stormwater entering the streams, which also can increase the transport of pollutants into the rivers and streams. The amount of stormwater can be controlled by “detention” ponds. The pollutants can be mitigated by “retention” ponds.
There are three basic types of stormwater ponds:
1) Retention = a water-quality device, where water is stored to remove pollutants, pathogens or nutrients (the permanent pond depth is typically 3 feet or more)
2) Detention = a water-quantity device, where water is detained to reduce discharge rates and lessen the impact on a downstream storm-water system
3) Erosion Control = a temporary device to protect downstream properties from silt
To protect our streams from pollution, we need to be concerned about nutrients, sediment, pathogens and temperature changes. Nutrients are compounds that stimulate plant growth, mainly nitrogen and phosphorous (lawn fertilizer and pet waste are major contributors of this type of stream pollution). Drinking-water contamination can cause health problems. Excess nutrients running off the land and reaching surface waters can cause massive algae blooms. When the algae decays, it creates odors and consumes the dissolved oxygen in the water, which results in fish kills. While this type of pollution is typically not a big concern for our mountain streams, it is a problem for our lakes and an even larger problem for the areas downstream of us, such as the piedmont and coastal zones of North Carolina and parts of South Carolina.
Sediment is the silt, sand, dirt and gravel that’s eroded by stormwater runoff and flows into the streams and lakes. Sedimentation can alter stream flow, erode channels, deposit silt in new locations and suffocate fish and plant life. On the construction or agricultural sites, erosion-control devices are designed to retain sediment.
Pathogens are organisms—bacteria, viruses or protozoan—that cause illnesses, such as typhoid and dysentery. Urbanization can increase the temperature of cold-water stream environments by transferring solar radiation captured by pavement to receiving water bodies through stormwater runoff. Due to the sensitivity our native trout have to water temperature, such increases are of particular concern in Western North Carolina.
Nutrients, pathogens and temperature changes can be mitigated by Best Management Practices. The BMP selected varies depending on location (soil types, annual temperatures and rainfall, for example).
In WNC, we use rain gardens, bio-swales, wet and dry ponds, porous pavement, rooftop runoff management (green roofs/cisterns) and rainwater harvesting (rain barrels) as common BMPs. Clustering development and reducing the overall impervious footprint also reduces stormwater runoff and increases infiltration into the natural soil. The basic goal of any BMP is to try and restore the natural stormwater patterns that have been disrupted by development. The exact combination of BMPs used is site specific. The North Carolina Arboretum has many examples of these. Whether designing a large site or adding to an individual lot, the trick is to be creative and use multiple devices.
Some BMPs can reduce the construction cost for a building site. Allowing streets to sheet flow into bio-swales can cost less than curbs/gutters with inlets and concrete pipe, for example. The individual property owner may consider adding anything from rain barrels to rain gardens to help improve our stormwater.
For additional design information, the N.C. Division of Water Quality publishes a BMP manual with specifications (https://h2o.enr.state.nc.us/su/bmp_forms.htm). The NCSU Stormwater Engineering Group has a Web site with examples of the latest research for innovative treatment practices for developments or individual homeowners (https://www.bae.ncsu.edu/stormwater/). The Land-of-Sky-Regional Council has “Stormwater Fact Sheets” that include a wealth of information (https://www.landofsky.org/planning/p_water.html). And Green Streets Program (Portland, Ore.) has an urban stormwater program (https://www.portlandonline.com/BES/index.cfm?c=44407).
Melanie Brethauer, PE, CFM, is a partner at the Asheville-based WNC Professional Engineers & Surveyors, where she heads the Civil Engineering division. She is a member of the LEED for Neighborhood Development Corresponding Committee, the Congress for New Urbanism, the U.S. Green Building Council and the WNC Green Building Council. She brings more than 24 years of design and permitting experience, having worked as an engineer in 10 states, plus the United Arab Emirates. She can be reached at email@example.com or at (828) 277-5074, ext. 103.