Schemes to restore fish habitat in rivers often involve installing instream structures such as current deflectors to create and maintain riffle-pool sequences. However, there is a lack of field studies on the impact of these structures on flow dynamics and bed topography. The objective of this research is to characterize flow dynamics and sediment transport around paired deflectors used to enhance fish habitat in the Nicolet River, Quebec. Bed and bank topography surveys were taken with a total station, and velocity and bed shear stress estimates were obtained from an acoustic doppler velocimeter. Bedload sediment transport was assessed by two methods: tracer rocks (painted "particles" and passive integrated transponder tags) and sediment traps. Results show marked differences in bedload sediment transport patterns between the left bank and the right bank downstream of the deflectors. This is surprising considering that paired deflectors should produce a relatively symmetrical disruption to the flow field on each side. More high-flow dynamics data during overtopping conditions are required to understand the complex interactions between these instream structures and bedload transport. Key words: stream restoration, pool, bedload transport, radio frequency identification (RFID), passive integrated transponder (PIT) tags, fieldwork, deflectors, fish habitat.
Few studies have examined sediment transport patterns around in-stream structures used to enhance fish habitat despite the importance of this variable in the successful design of stream restoration schemes. The objective of this study is to examine interactions between the (excavated) pool morphology, flow and sediment transport in a restored reach of the Nicolet River (Quebec, Canada). Bedload transport was investigated using passive integrated transponder (PIT) tagged particles that were followed from positions upstream of a pair of current deflectors which were designed to maintain the excavated pool downstream. Three-dimensional numerical simulations of the flow field at various flow stages (with emerged or submerged deflectors) were used to relate near-bed velocity and bed shear stress to transport patterns and to assess the impact of varying the pool location and geometry on the flow field and water surface profiles. Results show that from 2005 to 2008, of the 117 pit-tagged particles that fell in the pool, only 27 are known to have exited. None of the 30 largest rocks entering the pool escaped. Bed shear stress values simulated at high and peak flow (slightly above bankfull level) are not sufficient to move the largest rocks in the pool exit zone. Simulations also reveal a complex water surface topography when flow is above the height of deflectors, with negative water surface slope in the pool zone. When modifying the pool geometry so that the deepest zones are close to the apex of the in-stream structures instead of in the centre of the channel, both water surface slope and near-bed velocity patterns are greatly modified. Understanding the interactions between excavated pools, bedload and 3D velocity patterns around in-stream structures is essential for long-term success of fish habitat restoration projects, and using 3D models to test various designs of artificial pools is a promising approach. Figure 2. (a) Location map of the Nicolet watershed; (b) field site location within the Nicolet watershed (black triangle); the grey zone represents the Appalachian part of the watershed, whereas the white area is located in the St. Lawrence Lowlands; (c) photograph showing the upstream and downstream paired deflectors at the field site; (d) bed topography showing the downstream excavated pool and the upstream pressure transducer (indicated by a black star).This figure is available in colour online at wileyonlinelibrary.com/journal/rra
Despite the widespread use of in-stream structures in stream restoration projects to enhance the quality of physical habitat, it is only recently that the hydraulics of these structures have been studied in detail, typically using simplistic geometries in laboratory experiments. The objective of this study is to examine hydraulics around complex flow deflectors using a combination of laboratory, field and three-dimensional (3D) Computational Fluid Dynamics (CFD) approaches. In the laboratory, Acoustic Doppler Velocimeter (ADV) measurements revealed that flow overtopping the structure modifies the scour zone and bed shear stress pattern compared to a greater structure height. In the field, a 3D CFD model, which was calibrated at low flow using ADV and Particle Image Velocimetry measurements, was used to investigate both low and high flow (overtopping) conditions. A complex 3D pattern in the recirculation zones downstream of the deflectors is observed in the overtopping simulations, highlighting the limits of habitat structure studies based on depth-averaged (2D) models. The comparison with laboratory data is complicated by the fact that a dug pool was used in the field, which does not correspond to the position of the pools that developed near deflectors over a mobile bed in the laboratory. As natural rivers exhibit a wide range of discharges with various overtopping ratios, it is essential to pursue work using 3D CFD to test how different deflector height, length and angle designs affect the position and dimension of scour zones and the long-term viability of fish-habitat restoration projects.
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