LIMITATIONS OF THE USE OF IN-STREAM STRUCTURES IN STREAM RESTORATION PROJECTS IN NORTH CAROLINA

A database of stream restoration projects is being developed for the State of North Carolina. Of the more than 400 currently recorded projects, nearly all utilize in-stream structures to (1) limit channel incision, (2) reduce bank erosion and sediment inputs, or (3) improve habitat. The widespread use of in-stream structures indicates that an evaluation of their integrity can provide insights into the physical performance of restoration designs. To date, we have evaluated in-stream structures at approximately 40 sites across the state, most of which were completed within the past 8 years using the methods of Natural Channel Design. The data are intended to provide guidelines for the improved design of future restoration projects.

Field surveys show that at about a third of examined sites, more than 70% of in-stream structures have sustained significant damage so that structures no longer perform their intended function; numerous others exhibit less severe damage. In some cases, structures were installed, damaged, and repaired, only to be damaged again. Damages vary, but a common problem was the erosion around the landward side of J-hooks and cross-vanes during overbank flows, accelerating bank erosion locally and stranding the rock structures as ineffective mid-channel islands. Approximately 30 % of the damaged structures were affected by either burial or bar development, which led to channel migration and flanking of the structure along newly eroded banks.

Channels with high failure rates are associated with easily erodible bed and bank materials (in comparison to stream power) and/or streams with high rates of bedload transport. A preliminary review of design documents shows that analyses of the past or current geomorphic conditions were rarely undertaken, and analyses of sediment transport continuity through the restored reach were virtually non-existent. The large number of sites exhibiting damaged structures within a few years of construction strongly suggests that more rigorous geomorphic and hydrological analyses are required to improve the performance of restoration projects in the State even though this will initially require more time, money, and trained personnel.