Flow Dynamics and Bedload Sediment Transport Around Paired Deflectors for Fish Habitat Enhancement

Schemes to restore fish habitat in rivers often involve installing instream structures such as current deflectors to create and maintain riffle-pool sequences. The presence of deep pools is a critical component of a high-quality physical habitat for fish as they provide shelter and cool temperatures during the summer months. Natural rivers exhibit many complexities that need to be taken into account when implementing instream structures. To improve the success rate of restoration schemes, many of which currently fail or require costly maintenance, we need to develop tools such as numerical modeling to test scenarios prior to installing such structures.;The study reach is in the Nicolet River, Quebec, where several deflectors and excavated pools were implemented between 1995 and 1999 to improve fish habitat. Calibrating and validating a 3-D CFD model requires accurate measurements of hydraulic parameters in the natural river. Acoustic Doppler Velocimetry and Particle Image Velocimetry data were used to calibrate the model during low-flow conditions. A three-dimensional numerical model was then used to simulate both low and high flow (overtopping) conditions. A complex 3-D pattern in the recirculation zones downstream of the deflectors is observed in the overtopping simulations, highlighting the limitations of habitat structure studies based on depth-averaged (2-D) models. Bed load sediment transport was assessed in the field using two methods: tracer rocks (painted particles and passive integrated transponder tags) and sediment traps. These data allow the 3-D model of the study reach to be validated at high-flow (overtopping) conditions. The results show that the simulation can qualitatively predict erosion and deposition zones when installing deflectors for all flow conditions. It is less appropriate to generate quantitative predictions of the threshold of motion of individual particles. However, predicted bed load transport compared well with field measurements obtained from traps when bed shear stress was estimated with the quadratic law. These results suggest that 3-D simulation might be used to estimate the transport rate and pattern of sediment transport. It is hoped that this model will help maximizing deflector efficiency and success rate in future installations.;The objective of this research is to examine flow dynamics and sediment transport around complex flow deflectors using a combination of field measurements and computational fluid dynamics (CFD).