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Finding a Balance: Stream Restoration Objectives in a Vertically and Laterally Constrained Urban River

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The American Heritage Dictionary defines restoration as “a return of something to a former, original, normal, or unimpaired condition.” The National Research Council expands this definition to define aquatic restoration as “the reestablishment of pre-disturbance aquatic functions and related physical, chemical, and biologic characteristics” (NRC, 1992). This is the general perception by many of the expectations of stream restoration. Ideally, full restoration to pre-development conditions or functions is the ultimate goal of any project, but is often not realistic, given existing site constraints. An article in the 2007 Freshwater Biology journal states, “Restoration of urban stream channels is highly constrained, thus it is unlikely that an urban stream will ever be restored to its pre-urbanization state” (Bernhardt/Palmer, 2007). Therefore, it is critical to the success of any stream restoration project that project-specific achievable goals be identified within limits set by schedule, budget, and other issues and factors. Identifying achievable goals and setting realistic expectations for stream restoration in an urban environment is particularly important, given the physical restraints that are typically associated with urban development. The Longview Branch restoration project provides an example of the types of issues and constraints that dictate the degree of restoration that can be reasonably achieved on an urban stream.

Longview Branch is located in Raleigh, North Carolina, and is a tributary of the 303(d)-listed Crabtree Creek, which flows into the Neuse River. The 0.5-square-mile watershed draining to Longview Branch is comprised predominately of single family residential and institutional development, with a 32-percent impervious cover, and has been nearly built-out since about 1980. Development impacts are reflected in the severely eroded, incised, and sediment-burdened channel and the downstream impairment listing for biological integrity. The City of Raleigh Stormwater Management Division conceived the project while seeking to improve water quality throughout the Crabtree Creek Watershed, one of the city's most impacted watersheds. With funding assistance from the North Carolina Clean Water Management Trust Fund, the city was able to support the design of Longview Branch improvements that address the geomorphic impacts resulting from urbanization.

An evaluation of Longview Branch and its contributing drainage area identified common symptoms of urban development, such as high, flashy storm flows; decreased sediment loads from increased imperviousness and a piped drainage network; toxic pollutants, such as oils and greases, pesticides, and fertilizers; increased water temperatures; and horizontal constraints that do not allow lateral channel migration (i.e., meandering) without exacerbating flooding and endangering infrastructure. In the case of Longview Branch, each of these constraints, as well as other factors limit the ability to utilize natural channel design procedures, such as restoring sinuosity, reconnecting the floodplain, and reducing peak flow shear stress and velocity, thus preventing the ability to restore habitat, ecological diversity, and water quality to pre-development conditions.

The inability to restore Longview Branch to its pre-development condition should not discount the improvements that can be made. Setting achievable goals and managing expectations can improve the chances of having a successful project that can still provide positive impacts towards improving habitat, ecological diversity, and water quality. The goals for the Longview Branch Restoration Project were developed based on direction from the city of Raleigh, feedback from local stakeholders, and an understanding of the limitations. They include:

Reduce stream bank erosion

Address existing structural impacts due to erosion and reduce the likelihood for future structural impacts

Improve habitat and ecological diversity

Improve water quality

Improve aesthetics

Improve public education and awareness

For the Longview Branch design, the project goals to reduce bank erosion, address existing and future structural impacts, and improve ecological diversity and water quality were addressed by utilizing natural channel design techniques to control the migration of the stream and attempts to simulate natural conditions by connecting the channel to a floodplain bench; controlling velocities and erosion with in-stream vane structures; providing habitat with riffle pool sequences, deep pools, log vanes, and stream bank vegetation; and planting of a dense native riparian buffer. The use of rock gabions, reinforced earthen slopes, and retaining walls were required in areas where physical constraints prevented the use of natural channel techniques. Improving aesthetics and encouraging public education and awareness is not a direct component of stream restoration, but helps to preserve some of the restoration aspects incorporated into the project and improve water quality. The design includes incorporating signage to educate citizens on the project, facilitating public meetings to garner support, and communications with the local high school's environmental classes. Permanent restoration easements will prevent development within and mowing of the riparian buffer.

Longview Branch is a common example of an urban stream incapable of being restored to pre-development conditions. Understanding limitations and identifying other project constraints allowed achievable goals to be developed and expectations managed. This enabled a design tailored specific to Longview Branch to be developed, which provided a positive impact on the watershed and helped to address downstream impacted waters by reducing stream bank erosion and improving habitat, ecological diversity, water quality, aesthetics, and public education and awareness.
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Keywords: Urban stream restoration; ecological diversity; natural channel design; water quality

Document Type: Research Article

Publication date: 2010-01-01

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