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Detention Basin Retrofits Have Environmental Benefit

Kyle Leonard

In many communities, the urban environment and human intervention have impacted our natural streams and rivers. Impervious surfaces such as roads, parking lots and buildings take away areas where rainfall would normally be slowed down, spread out and soaked into the ground. Further exacerbating the problem, streams in urban areas are often straightened or channelized to make more room for development.

Stream banks can be stabilized using a number of bioengineered treatments.

Stream banks can be stabilized using a number of bioengineered treatments.

In addition, floodplain encroachment (fill) and other human intervention adversely impacts natural vegetation, habitat on the banks and within the riparian corridor. That means there is more runoff, carrying more pollutants, flowing directly to the streams at a faster rate – resulting in increased flooding, stream bank erosion, sediment load, water quality problems and habitat degradation.

Streams attempt to adjust to these flow changes and increased energy through an evolution process. Incision occurs first, when the bed of the channel erodes downward. The sides of the bank erode once the stream invert reaches a stable elevation. It is important to note that not all erosion is bad – it is a way for a channel to become stable. However, when there is not enough room (as is often the case in urban areas), the adjustments can threaten roads, bridges, buildings and other infrastructure.

When stream problems are identified in urban environments, there are several types of solutions. On one end of the spectrum are hard engineered or artificial solutions. These types of solutions can resolve a problem and be integrated into a project. However, they are usually a shorter term solution with little ecological value, are less attractive and weaken as they age. On the other end are bioengineered solutions. The goal in these types of retrofitting projects is to create a long-term stable stream corridor that strengthens over time and provides ecological value.

Stream banks can be stabilized using a number of bioengineered treatments, depending on the severity of the erosion and the physical constraints of the stream. Native vegetation, rock toe protection, soil lifts, geocells and/or strategic placement of logs within the stream bank are all bank treatments that can be effective depending on site specific conditions and the goals of the project.

  • Native vegetation is a key component to almost any bank treatment. The toe of the bank, where shear stresses and velocities are highest, may be stabilized with stone. A natural fabric and native vegetation can be used for the upper banks. Native vegetation is important to use as the deep root structure will help keep the banks in place by creating erosion resistance. Riparian vegetation buffers are now seen as a way of transitioning from the built to the natural.
  • Soil lifts bank stabilization treatments consist of a thicker natural fabric used to create steps filled with cobble, gravel, soil or a mix. The lifts are staked in place and seeded with native vegetation as well as native plant species.
  • Another bank stabilization technique is the use of geocells. They are often used to create an immobile bank where steep slopes are necessary to protect adjacent property and structures. The cells are stacked in steps and each cell is filled with soil and the entire bank is seeded and planted.
  • Large woody debris/log bank stabilization is one of the most involved types of solutions. Logs are driven or trenched into the stream bank and are often anchored to existing trees or ballasted depending on drag force or uplift concerns. Due diligence must be done to consider and design for risks associated with installing large woody debris in streams.

Another key attribute of stream restoration is to dissipate energy. When the flow is slowed down, it alleviates erosion as well as creates habitats. This can be achieved by using tree root wads so that water hits the roots and creates scour pools. Not only does this method dissipate energy but it creates good habitat zones. Log steps are another method wherein a log is placed across the stream and anchored. When the water flows over the log, a plunge pool is created on the downstream side creating riffles and pools. When developing a riffle and pool reach, rock is often placed in the stream. These riffles slow down the water, creating pools. Similar to riffles and pools, step pools can be created with larger rock when stream bed slopes are greater. Other examples of urban stream restoration may include raising the channel elevation to reconnect it to the floodplain and including in-line infiltration underneath the stream or lowering the floodplain in cases where flood storage is necessary.

If our streams are degraded, they offer a mirror into our management of the land, air and water.
Important to remember is that it’s not just the streams, but the watersheds that need help too. In other words, if our streams are degraded, they offer a mirror into our management of the land, air and water. Since the Clean Water Act of 1972, human intervention through the implementation of stormwater management Best Management Practices (BMPs), has attempted to address problems caused by urbanization of watersheds. One of the most common types of BMPs has been stormwater detention basins.

Traditional stormwater detention basins are areas that detain storm runoff from large storms for a short period of time before allowing the runoff to flow out of the basin. Over the last 30 to 40 years, the primary purpose of most traditional detention basins in urban developments has been to reduce peak flows during significant storm events. However, these basins do little to filter pollutants before they enter streams. Other common problems include sediment and debris collection and clogging of low flow orifices, stagnant water and mosquito issues, low aesthetic appeal and community use benefits, and general mowing and maintenance costs. Traditional detention basins are assets that can be transformed or retrofitted effectively to address these problems and realize other benefits.

Rock is placed in the stream to create riffles, which slows the water, creating pools that are essential for habitat.

Rock is placed in the stream to create riffles, which slows the water, creating pools that are essential for habitat.

There are many ways to retrofit existing detention basins, all the way from simply naturalizing the plant palette using native species to adding enhanced filtration techniques. Goals that coincide with detention basin retrofits are often to add features within its existing footprint that will enhance water quality while maintaining its performance to provide flood protection. Ancillary benefits that result can be: reduced peak flow discharges and volumes, improved visual appearance and reduced maintenance costs. For example, by using native plantings instead of simply turf grass, mowing is minimized or even eliminated. The mature vegetation can improve wildlife habitat and provide ecological benefits. Allowing the vegetation to grow and flower will enable birds and native pollinators to utilize this “urban refuge.” Examples of retrofit solutions may include removing existing concrete pilot channels, performing minor excavation and grading, and installing influent sediment forebays, enhanced filtration, infiltration trenches and native plantings.

All of these methods help to create a more natural and environmentally friendly way to manage stormwater and protect our natural assets. This process of naturalization and habitat restoration requires the support of the local residents. Residents need to be educated about the benefits and techniques used in stream restoration and maintenance to ensure its acceptance and success. A stormwater management program must also incorporate a good public relations plan. Visual presentations or field trips to existing projects can be powerful persuasions to skeptical citizens. After the initial installation of the restoration or retrofit methods, volunteers can aid in planting of the native vegetation with guidance and support from experienced staff.

The result of a successful detention basin retrofit is to slow down stormwater runoff and provide the time and space for the water to infiltrate into the ground, while providing necessary flood protection. Native vegetation can allow a site to serve as a place for environmental education while providing the local community with increased ecosystem diversity. A successful retrofit will not only alleviate flooding problems but create natural habitat and enhance the area for uses beneficial to the surrounding neighborhood.

To learn more about retrofits to stormwater detention areas, contact HR Green’s Josiah Holst at jholst@hrgreen.com

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