Motor Vehicles and Stormwater Runoff
Roads and highways play a vital role in shaping the economic prosperity of a community, as well as the quality of life for its citizens. Unfortunately, the motor vehicles that regularly utilize them are not only polluting the air, but a community’s stormwater runoff as well. The most common pollutants can include: oils and grease; heavy metals from car exhaust, worn tires and engine parts, and rust; road salts; and sediments from construction vehicles
Across the country, landscape architects are introducing ecologically-safe design elements that address stormwater runoff problems and improve water quality. Several of these strategies, which fall under the heading of “green” infrastructure, are detailed below.
Bioswales and wetland mitigation
Many landscape architects are implementing bioswales in projects. Placed within medians or parallel to a road, a bioswale captures stormwater and slowly treats it before it infiltrates the ground. Bioswales typically consist of a sloped canal featuring specially engineered soils and plants, as well as rock barriers. These materials help reduce the amount of harmful chemicals—such as phosphorous, nitrogen, and lead—often found in stormwater runoff.
In addition to mitigating the contaminants found in runoff, bioswales can help curb the peak runoff rateby adding a detention or retention pond between the runoff area and the receiving body. These ponds temporarily store stormwater runoff while containing suspended solids and other contaminants. This helps reduce the erosion impact of stormwater events.
Bioswales allow for the contaminants present in a large volume of water to be treated in a shorter time than with other stormwater management methods. Because they are often used for the drainage of large impervious areas, bioswales are typically featured in highway projects.
Also, many transportation projects are also tackling stormwater issues through wetland mitigation. Wetlands have many widely known functions, including stormwater retention and water-quality improvement. Wetland mitigation typically refers to the act of replacing the function and value of an affected wetland by creating, restoring, or enhancing a wetland in another suitable location. The process includes the consideration of groundwater, soils, plantings, and other features.
Complete streets practices
Many studies have shown that water quality becomes degraded when total impervious surface exceeds 10 percent (and in some cases, at even lower levels). This indicates that a close relationship does exist between urbanization and water quality.
Increased urbanization has become an issue for countless communities across the U.S. More hardscapes and impervious surfaces results in more rain that runs off roads and highways in excessive amounts. This stormwater often contains pollutants, meaning it can have a harmful effect upon the public and economic health of a community.
Complete Streets practices have proven to be effective in reducing pollutants in stormwater. Complete Streets projects feature design elements that promote the design and construction of safe and efficient transportation network designed for all users, whether they bike, walk, or use mass transit.
Many Complete Streets projects feature rain gardens, which consist of specially chosen plantings that can lower nitrogen and phosphorus levels in stormwater. These gardens—which are generally located on a sidewalk—can also include: specially structured tree planting soils; grates and drains that irrigate the plantings; and engineered composite soils, which store and treat stormwater runoff.
Also, many Complete Streets projects are utilizing porous pavement systems, which often feature a permeable pavement surface with a stone reservoir located underneath. Porous pavement is created without “fine” materials and incorporates void spaces, which allows for infiltration. When stormwater passes through the pavement, it’s filtered of any automobile oil, grease and sediment frequently found in parking lots. The reservoir then holds the runoff before being infiltrated into the soil.
Lastly, Complete Street projects are featuring the planting of distinct species of trees and shrubs. This vegetation can help reduce thermal pollution and urban heat island effects. This occurs when stormwater runoff gets hotter as it washes across impervious surfaces and then enters a natural water source. The heated runoff can negatively impact fish and other wildlife that need cold water to live and breed.
The benefits of green infrastructure
One of the key differences between the green infrastructure solutions detailed here and traditional “grey” infrastructure: The latter requires increased operations and maintenance over time as equipment and materials deteriorate, while the former will increase in resilience and function as vegetation matures and adapts to local resource cycles.
Plus, because it must be designed and built to fit specific conditions at individual sites, green solutions are generally more context-specific. Therefore, green design elements are better designed and implemented to address local concerns and values. This, coupled with grey infrastructure’s additional need for operations and maintenance, means green infrastructure solutions can be significantly less expensive over time.
Finally, green infrastructure can be better at improving the quality reliability in municipal drinking water supplies, as well as increase the predictability of water quality. It can also allow for better flood control and groundwater recharge, as well as a reduction in stormwater entering municipal sewage treatment plants.
The negative effects of motor vehicle emissions on air quality have been well documented. Lesser known is how harmful automobiles can be to water quality. Thankfully, landscape architects are developing solutions that are not only cost effective and environmentally safe, but quite effective in ensuring the quality of water in communities across the country.
Thomas R. Tavella, FASLA, is director of design for Fuss & O’Neill’s Landscape Architecture Studio and president-elect of the American Society of Landscape Architects. Manchester, Conn.-based Fuss & O’Neill is ranked among the top 200 environmental firms and top 500 design firms in the United States. Tavella can be reached at TTavella@fando.com.
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