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

Abstract

A 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.Note that quoted discharge for acoustic Doppler velocimeter (ADV) surveys is based on the downstream gauge recording (at 92% of contributing watershed), whereas discharge for acoustic Doppler profiler (ADP) surveys is from ADP data collected on-site (at 90% of contributing watershed) based on an average of repeated transects. Where available, the discharge from the downstream gauge is also provided (in brackets) for comparison with M9 discharge measurement at the site. Water level data are from the on-site water level logger. 616 E. C. JAMIESON et al.

Design of fascines for riverbank protection in alpine rivers: Insight from flume experiments

“…Overall, however, they have indirect effects through modification of the local hydraulics, damping turbulence at the fascine toe and displacing scouring towards the centre of the channel, thus promoting gravel deposition against the fascine and bank toe. Used alone or in association with indirect protection (such as stream barbs) redirecting flow coming from upstream [Jamieson et al, 2013], anti-scouring twigs offer promising perspectives against scouring even in erodible beds. New experiments are needed to optimise this technique and to determine up to which level of vertical scouring they remain an option and above which level of scouring threat, heavier options such as riprap remain necessary.…”

Influence of Meander Confinement on Hydro-Morphodynamics of a Cohesive Meandering Channel

“…In coherence with the Page and Nanson conceptual model for development of a concave-bank bench, the encroachment of the railway embankment on the M4 sampling reach in the study area confined the lateral erosion and caused the meander to migrate downstream, which provided room for fine-grained accretion on the outer meander bends. On the other hand, a lower ratio of the bedload to the suspended load in this semi-alluvial cohesive-bed river may have limited downstream development of the point bar [43,47]. This may have created an open space for separation zone and flow expansion.…”

Continuous prediction of clay‐bed stream erosion in response to climate model output for a small urban watershed

“…This study is focused solely on that open water year (from April 1 to October 31) neglecting snowmelt. A study on a similar nearby watershed (17 km from Watts Creek) Sawmill Creek (Jamieson, Ruta, Rennie, & Townsend, ) found that extreme summer rainfall events may exceed the freshet peak flow. Therefore, the open water year study period may be expected to capture most of the total annual flow volume and the largest erosion‐producing events.…”