Controls on Post-Construction Channel Adjustments along Restored River Reaches: Implications to the Development of Site Screening Tools to Improve Project Success

Many restoration projects in North Carolina, like the nation as a whole, involve the reconfiguration of channel width, depth, slope, and/or planimetric configuration to reduce ecosystem degradation and/or to improve upon channel stability. As part of a statewide evaluation of restoration projects, annual cross sectional survey data, collected by the North Carolina Ecosystem Enhancement Program, were examined for 221 cross sections from 26 reconfigured stream-reaches. Post-project changes in channel capacity were highly variable from site to site, but tended to be large for most locations. More than 60 % of the sites underwent, on average, at least a 20 % change in channel capacity while net changes at sites older than 4 years were often much larger. Major channel adjustments were often associated with frequent floods, including the first significant overbank event. An analysis of site geomorphology and design documents revealed that large post-construction adjustments were associated with (1) highly dynamic stream channels characterized by high sediment transport rates, large sediment supply, and easily eroded bank materials, (2) a poor understanding of long-term geomorphic processes, process responses and adjustments to land use change, and (3) difficulties in adequately designing channels that match the current hydrologic and sedimentologic regime. With regards to the latter, meander design was found to be particularly important. In light of these findings, it is unlikely that stable channels can be effectively designed and implemented along highly dynamic channels characterized by high stream power, large sediment supply, and non-cohesive banks. Thus, we present an initial screening method that can be used to assess the likelihood of project success early in the design process, and argue that where space permits an enhanced natural channel adjustment approach is likely to be more effective than commonly used ‘rigid design' restoration methods.