STABILIZATION OF RECONFIGURED CHANNELS BY IN-STREAM STRUCTURES: IS IT AN EFFECTIVE METHODOLOGY?

North Carolina has spent $10s of millions on stream restoration projects during the past decade. Many of these projects involve the installation of in-stream structures and a reconfiguration of channel width, depth, and planform morphology. During this investigation, cross-channel survey data collected annually as part of the North Carolina Ecosystems Enhancement Program were examined for 221 cross sections from 25 sites (26 stream-reaches). Approximately 45 % of the examined cross sections exhibited at least a 20 % change in channel capacity following construction. Average at-a-cross section change in channel capacity for sites older than 3 years exceeded 25 %, and in most cases exceeded 30 %. Younger sites tended to be more stable, but net change at some locations ranged up to 60 % indicating that rapid adjustments could occur. Major channel adjustments were often associated with rather frequent floods, including the first significant overbank event. Multivariate statistical analysis, combined with field observations, revealed that the magnitude of channel change was controlled by a combination of the stream's sediment transporting ability, sediment supply, and bank cohesion.

In most cases, in-stream structures (e.g., cross vanes, J-hooks, and rock vanes) were used to stabilize reconfigured channels following construction. A rapid assessment of in-stream structures at 26 locations, including a number of those for which monitoring data were analyzed, showed that structures at reconfigured sites exhibited a high rate of impairment where adjustments in channel capacity exceeded 20 %. Presumably, channel adjustments between structure emplacements and along their margins where they were attached to the banks led to their failure. Thus, it is questionable whether in-stream structures can stabilize reconfigured channels as generally intended along dynamic rivers characterized by non-cohesive banks, high stream powers, and large sediment supplies. Natural channel recovery may be a more appropriate approach where space is available and the potential damage to infrastructure is limited.