• An Evaluation of the Effectiveness of Existing North Carolina Department of Transportation Wetland Mitigation Sites

    NCDOT Research Project Number: 1999-10

 Executive Summary

  • In this study, 49 NCDOT wetland compensatory mitigation sites and 11 reference sites were evaluated on-site in 1999. Seventeen of the larger sites consisted of more than one type of mitigation (restoration, creation and/or preservation). In total, 71 mitigation parcels (approx. 3,000 acres) were evaluated to assess the likelihood that mitigation sites would achieve some level of structure and functioning similar to natural, self-sustaining wetland ecosystems and to provide recommendations for improvements. Ecological success was related to whether or not natural geomorphology had been successfully restored. Sites from which fill was removed were generally successful. Sites in which water impediment structures were constructed showed mixed results for vegetation survival, presumably because it was difficult to determine how wet to make a site. Wetland creations were generally unsuccessful because most all involved excavating soil to reach the underlying saturated zone, thus inhibiting growth of vegetation on sub-soils. Predictions of success were difficult due to the immaturity of sites, but it appeared that many created wetlands would not likely resemble historic, natural ecosystems. Of the 71 compensatory mitigations examined, 26 were judged to be ecologically successful, 19 were preservation sites (automatically judged to be successful), 9 were judged to be unsuccessful, 10 lacked sufficient data (mostly hydrologic data) for judging success, 4 sites were too young to predict the outcome for vegetation survival, and 3 were undergoing construction at the time of our site visit. Alteration of and failure to restore natural geomorphology in compensatory mitigation sites was the major factor associated with the lack of mitigation success, regardless of whether success was defined by permit success criteria or by ecological success. More use of information from reference sites could improve outcomes. Compensatory wetland mitigation involving restoration and creation appears to have gravitated toward relatively narrow sets of conditions for hydrology and vegetation, with little room for flexibility. In contrast, no standards are being used for soil condition. Current success criteria and standards should undergo critical examination to see if they are consistent with no-net-loss wetland policies, and if alternative measures should to be taken.The Phase 2 component of this research examines five of the compensatory mitigation sites to provide a more in-depth analysis. The objective of the two reports is to help NCDOT and wetland regulatory agencies develop a framework to improve NCDOT's compensatory mitigation, to enhance communication between NCDOT and regulatory agencies, and to benefit wetland restoration overall. We encountered problems with various definitions (restoration, preservation, enhancement, etc.) that are not compatible with current scientific understanding of ecosystem functioning. This has led to avoiding the potential for improving the condition of severely altered wetlands because they meet the jurisdictional definition in spite of a highly degraded condition. Elsewhere, socioeconomic limitations may prevent complete restoration. In such cases, partial restorations may be better than none at all. For example, preservation through purchase or conservation easements of headwater streams and their buffers in a partially degraded condition would provide opportunities for improving water quality. Undue reliance on criteria for hydrology over criteria for soil, in extreme cases, has led to soil excavation that reduced survivorship of planted seedlings and lowered recruitment capacity. In general, reference sites have been little utilized to design restorations and to gauge success. Depending on initial conditions, the restoration of wetland structure and function may take many decades to achieve maturity. Presently, all monitoring stops once permit conditions have been met. Institutional memory then rests almost entirely with personnel in the NCDOT organization. To encourage long-term research, regulatory agencies must be willing to provide mitigation credit for establishing reference sites and to conduct long-term research in comparing them with a variety of restoration practices. To avoid unintentional shifting of distribution among one set of hydrogeomorphic classes to others, it will be necessary to track restoration at drainage basin scales according to hydrogeomorphic wetland classes. Many of these suggestions will require acceptance by regulatory agencies and implementation by all parties. Regulatory agencies would have to be willing to accept success criteria based on data from reference wetlands.​
Mark M. Brinson; Richard D. Rheinhardt
David Schiller
G. Dennis Pipkin
NC State University

 Report Period

  • April 1999 - June 2001


  • Complete


  • Environment and Hydraulics

 Sub Category

  • Wetlands Mitigation

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