Effect of Clearwater Scour on the Flow Field at a Single Bendway Weir: Two-Dimensional Numerical Modeling Supported by Flume Data

This study used two-dimensional (2D or depth-average) numerical modeling to examine the effects of clearwater scour on the flow field at a single bendway weir (BW) set at angles of 30°, 60°, and 90° pointing into the approach flow. The flow field was characterized in terms of depth-average velocity (location and magnitude) through the contraction formed around the BW and was supported by data and observations obtained from large and small flumes. In addition, a three-dimensional (3D) model was used to assess when a 2D model is of design use and when it is not. The 2D modeling showed that, when the ratio of flow depth (about four BW lengths upstream) to BW height ($Y:H$) equaled 2.0, the differences (pre- and post-scour) in depth-average velocity magnitude and location of flow around the BW were fairly negligible for all orientations of the BW. However, when $Y:H$ equaled 1.25, the effects of the scour on the flow field were more prominent. Similar results were obtained for all three orientations of the BW. The value of $V_{\max }$ obtained with the 2D model was closer to that measured in the large flume when $Y:H$ decreased from 2.0 to 1.25. This was due to the shallower flow’s greater effect in directing the flow around the BW than occurred for the deeper flow, in which flow had a larger vertical component of velocity when passing over the top of the BW. The practical implications of using a 2D model versus a 3D model were briefly assessed, and 2D models were found to be suitable for designing BWs to manage thalweg position but inadequate for estimating near-bank velocities.