MODELING INDICATORS OF CHANNEL CONDITION FOR LOCAL WATERSHED PLANNING TO TARGET STREAM RESTORATION AND MANAGEMENT EFFORTS
Abstract:The North Carolina Ecosystem Enhancement Program (NCEEP) is a non-regulatory program established by the North Carolina General Assembly in 1996, charged with the implementation of watershed restoration and preservation projects to satisfy Clean Water Act Section 401/404 mitigation requirements for stream and wetland impacts across the state. To increase the ecological effectiveness of wetland, stream, and riparian restoration projects, NCEEP has sought to integrate mitigation projects into comprehensive local watershed assessment and planning.
Working with NCEEP staff and teams of local stakeholders, Tetra Tech is conducting technical assessments and providing local watershed planning support in several targeted 14-digit hydrologic units in the Upper Cape Fear River Basin. In the initial phases of these projects, several key stressors having the potential to degrade watershed functions are identified, including, but not limited to: stream erosion/instability, eutrophication in water supply reservoirs, terrestrial habitat loss, wetland loss, and flooding potential. Subwatersheds within each 14-digit hydrologic unit where existing degradation of watershed functions is documented are targeted for restoration efforts, while those with the least disturbed conditions are targeted for preservation. In addition, those subwatersheds found to have the greatest risk for future degradation are targeted for the development of management and protection measures to prevent functional losses.
The physical impact of stormwater runoff from urban and agricultural land uses on channel morphology is often identified as one of the primary stressors affecting watershed functions. As a result, stream stability is one of the indicators analyzed as part of the detailed watershed assessments. Hydrologic and hydraulic models (HEC-HMS / HEC-RAS) are used to evaluate channel velocity and specific stream power associated with bankfull flows for existing and potential future conditions. The outputs of these models are compared to published thresholds, as well as in-situ visual assessments of channel condition, to characterize channel degradation and the potential for restoration. The modeling framework allows for proposed future urbanization scenarios to be assessed, in addition to the existing state of development. As each of the individual methods (specific stream power, critical channel velocity, and in-situ visual assessments) has shortcomings, the synthesis of the results of each method provides a stronger foundation on which watershed functions can be evaluated. Taken together, these methods have been shown to be a valuable assessment tool allowing for targeting field investigation efforts where existing degradation is expected. These tools have also allowed for prediction of morphological conditions associated with various potential future land use scenarios, and for determining the potential outcomes of restoration and management efforts to address adverse impacts. Through the evaluation of various management and restoration scenarios, this modeling framework is being used to target financial resources to stream restoration and stormwater BMP applications in high-priority subwatersheds, with the goal of providing the greatest long-term functional benefits on a watershed scale.
Document Type: Research Article
Publication date: January 1, 2004
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