Three-dimensional flow dynamics around deflectors

Many river rehabilitation projects to enhance the aquatic habitat focus on the creation of pool and riffle habitat by the implementation of flow deflectors, with various degrees of successes and failures. A more comprehensive understanding of the complex three-dimensional flow dynamics that induces scour around instream structures is required for a more effective design. The objective of this study is to examine the three-dimensional mean and turbulent flow characteristics around paired flow deflectors for various types of deflector design in a laboratory flume. Three deflector angles (45 degrees, 90 degrees and 135 degrees) and two 44 deflector heights (with flow under and over the deflector height) were tested over a smooth (plexiglas) bed and a sand bed. Three-dimensional velocity measurements were taken with an acoustic Doppler velocimeter at several planform positions at two heights above the bed. Results show that the 90 degrees deflectors create the most important disturbance in the mean flow field, in turbulence intensity and bed shear stress. There is, however, a marked difference in the spatial distribution of the mean and turbulent parameters over a mobile bed and over a smooth, fixed bed. This stresses the importance of understanding the 14 feedback between bed topography and flow dynamics and limits the applicability of conclusions drawn from plane bed experiments to natural rivers. Copyright (c) 2005 John Wiley & Sons, Ltd.