Optimal Design Criteria of Bendway Weirs from Numerical Simulations and Physical Model Studies

INTRODUCTION The objective of this study is to provide a synthesis of the pertinent design criteria for submerged bendway weirs. The technical paper is an extension of previous research conducted by the National Center for Computational Hydroscience and Engineering (NCCHE) at the University of Mississippi on the hydrodynamic effects on flow characteristics and effectiveness of submerged bendway weirs for improving navigation in navigable waterways. This article provides the following: Background information on bendway weirs in alluvial river systems; NCCHE numerical simulations of channel flow with submerged bendway weirs and prototype studies at Victoria Bend on the Mississippi River; NCCHE numerical simulations of submerged weirs and comparisons with physical model data; Analysis of design criteria developed from the numerical models, laboratory model tests and prototype model tests. Design parameters include weir height and length, orientation angle, weir spacing, weir side-slope, weir roughness; and Determination of first generation design criteria for bendway weirs. Bendway weirs are submerged linear rock structures, permeable or impermeable, usually positioned in series near the thalweg of river bends. Bendway weirs are different from spur dikes, jetties, or groins, and were developed for the improvement of navigable waterways. Bendway weirs typically reduce the magnitude of secondary flows in channel bends. optimize the benefits of bendway weirs, several structures are normally placed in series along the river thalweg near the concave bank. Resistance of the weir establishes a high-pressure zone in front of the weir near the center of the weir, while a low pressure develops behind the weir, near the tip. These zones redirect the high-energy flow away from the concave bank while creating a low-velocity recirculation zone, or eddy pool. Since bendway weirs are positioned normal to the downstream direction, the flow streamlines redirect toward the center of the channel minimizing bank erosion and assisting in navigation. Figure 2.1 illustrates the flow near a submerged weir. 5 A submerged weir also suppresses the flow near the center of the weir due to the high-pressure zone. Due to the suppression of flow, the flow accelerates over the weir and decelerates downstream. Further downstream of the weir, the influences of the weir decrease causing the flow to gradually recover downstream. A submerged weir also alters the secondary current by creating two counter direction zones. The pattern and strength of these secondary currents varies with flow conditions but the intensity of secondary current induced by the weir decreases and then …