Masters Thesis


The State of Stream Restoration in the North Carolina Piedmont:
An Assessment of Instream Structure Durability and Stream Bank Erosion

submitted to Ecological Engineering

Kim Hamlin


ABSTRACT

We evaluated 583 instream structures in 19 stream restorations that were between 4 and 12 years old, focusing on damage to structure components and stream banks. The structures included rock cross vanes, single arm vanes, j-hook vanes, rootwad revetments, and boulder revetments. We also calculated Bank Erosion Hazard Indices (BEHIs) for the restored stream sections and sections of six reference streams. A threshold value for both structure condition and BEHI score was specified. This threshold value indicated the point where structure and bank condition may be questioned. Rock cross vanes and rootwad revetments were the least durable structures, reaching the threshold value in 4 and 6 years, respectively. Single arm vanes and J-hook vanes did not pass the threshold value until 10 and 11, respectively. Boulder revetments approached but never reached the threshold value and were the most durable structures. The majority of BEHI scores in restored and reference sections were above the threshold value. No relationships were found between structure condition, structure density, or number of structures per stream and BEHI score. This suggests erosion is the result of large scale disturbances within the watershed to which both restorations and references are reacting similarly. Limiting the number of instream structures will reduce costs associated with stream restoration. Instream structure design should be critically examined to improve or remove structures that are not contributing to restoration success. Collecting pre-restoration erosion data and reference stream bank data annually will also be useful in determining success by providing tangible evidence of improvement during the monitoring period.