Monitoring stream barb performance in a semi‐alluvial meandering channel: flow field dynamics and morphology

Abstract

ABSTRACTA series of seven stream barbs were installed in two consecutive channel bends of Sawmill Creek (Ottawa, Canada), a semi‐alluvial stream with bed and banks composed of consolidated clay. Stream barbs (also known as submerged groynes) are low‐profile linear rock structures that project out from the bank (in an upstream direction) to redirect flow and prevent erosion of the bank. As well as providing bank protection, barbs promote vegetated stream banks, create scour hole resting pools for fish habitat and can increase aquatic species diversity. Flow conditions (discharge and water levels), water velocity distribution using acoustic Doppler velocimeters and an acoustic Doppler current profiler, and bathymetry using a Total Station have been measured over a period of 5 years, both before (2 years) and after (2 years) the construction of the barbs, providing valuable data for understanding stream barb performance in a semi‐alluvial channel. Pre‐barb velocity measurements indicate that the flow field in the first bend is dominated by strong secondary circulation and high cross‐stream bed stresses that comprise a substantial proportion of the total near‐bed Reynolds stress. Post‐barb results indicate that velocity magnitude along the outer bank was reduced. Scour occurred in the channel centre near barb tips, but in general, little to no bathymetry changes were measured between seasonal surveys, suggesting that the barbs had limited impact on bed topography and/or that semi‐alluvial channel bends are resistant to change. Recommendations for future design and implementation of stream barbs are also included. Copyright © 2013 John Wiley & Sons, Ltd.