Deflector Designs for Fish Habitat Restoration

Abstract

Paired current deflectors are structures that are installed on each bank of a river to locally reduce the width of the channel, thereby creating flow acceleration and promoting scouring. These instream habitat structures have been used extensively in restoration projects to create pool habitat for fish, but there are many discrepancies in deflector design recommendations in terms of orientation, height, and length. Our objectives were to (1) examine how the angle, height, and length of paired deflectors affect scour hole dimensions and potential for bank erosion; and (2) test the applicability to paired deflectors of existing equations for scour hole depth and volume. Three deflector angles (45 degrees, 90 degrees, and 135 degrees), two deflector heights (with flow under and over the deflector height), and two lengths (reducing the width by 25% and 50%) were investigated using uniform sand in a laboratory flume. Results showed a 26-30% smaller scour depth resulting from 45 degrees deflectors than from 90 degrees deflectors and a 5-10% smaller scour depth for 135 degrees deflectors compared to 90 degrees deflectors. The volume of scour and the potential for bank erosion were greater when flow was under the height of the deflectors rather than overtopping and when the length of deflector was increased. When flow was under the deflector height, 135 degrees deflectors had the highest amount of bank erosion; whereas during overtopping flow conditions, 90 degrees deflectors had the greatest bank erosion potential. Values predicted by the model of Kuhnle and others were closest to observed scour depth and volume measurements. The assumption that upstream-oriented deflectors always generate the largest scour should be revised.

Experimental Investigations of Scour Pools around Porous Obstructions

“…Thompson [8] conducted flume experiments on the effect of scour pools at low and high deflectors, and concluded that high deflectors have been proven much more useful to create scour pool habitat. Some previous research reported that the orientation of deflectors is associated with the volume of scour pools [3,7,9]. Maximum scour was observed at spur dikes oriented perpendicular to the flow [3].…”

“…There are a lot of instream habitat structures such as deflectors, rock vanes, and spur dikes. A usual design of instream habitat structure is the use of flow deflectors, which are recognized as one of the best tools for habitat restoration techniques [3,4]. Deflectors provide various flow conditions by constricting and diverting the water, thereby enhancing ecosystem diversity.…”

“…The scour pools can offer favorable places for fishes because they can hold and feed during low flow conditions, and take refuge during high flow conditions [6]. In order to successfully plan, design, and construct restoration projects that provide a variety of flow conditions and favorable habitats for fishes, two parameters should be considered: (i) maximizing scour pool habitat and (ii) minimizing potential for bank scour [3,7].…”

Hydraulics of stream deflectors used in fish-habitat restoration schemes

“…Velocities were also higher than for the first set of experiments, in order to examine the combined effect of increasing flow stage and velocity. Bed shear stress was calculated from the turbulent kinetic energy (TKE) approach, which has proved more accurate in a complex flow field than other methods based on mean velocities (Biron et al [11]). The 3D CFD model Phoenics was used to run simulations of the flow field at the Nicolet River for low and high flow conditions.…”

“…The impact of increasing both water stage and velocities around deflectors is not clear because many laboratory experiments have used constant flow intensity conditions for various overtopping ratios (e.g. Kuhnle et al [10]; Biron et al [11,12]). …”