عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Construction of weirs and diversion dams in river bends has been inevitable in some cases around the world, and many examples are available to demonstrate the use of conventional types of weirs in the channel bends. The lack of design criteria for the construction of weirs on channel bends is evident. However, the operational experiences of these weirs in river bends indicate the non-uniformity of water surface levels, sediment distribution, and flow rates across the channel bend, hence, the poor performance of intake structures on both sides of the river. The main hypothesis for providing uniform distribution of flow across the bend is the change of the weir-crest profile from horizontal to sloping crest. Review of previous studies indicates a few researches on the performance of sloping-crested weirs in channel bends. The aim of the present study was to test the performance of horizontal and sloping Crump weirs in a channel bend, in order to achieve the most uniform distribution of the unit flow rates across the river bend. In this study, the theory of the design of a weir crest in a channel bend is presented. Flow equation was derived from mathematical and dimensional analysis. The discharge coefficient, Cd, is related to the relative curvature of the bend, the position of the weir along the bend, the geometry of the weir and the upstream head water. Flow characteristics were evaluated in a laboratory flume consisting of a 90 degree bend with a relative radius of curvature 3, with and without the standard Crump weirs. The experiments were carried out using 3 different heights of weir, 4 different weir crest slopes (3 sloping crests, and 1 horizontal crest), and at 7 different positions of the channel (2 cross sections in straight reach and 5 cross sections in curvature reach of the channel). All experiments were repeated under 3 different flow conditions. A total of 252 tests (63 with horizontal weirs and 189 experiments with sloping crested weirs) were carried out. Flow discharge, 2D velocity and water surface profiles at the control sections of weirs were measured.
The results indicated that horizontal crested weirs are appropriate for straight reaches upstream and downstream of the channel bend, and are still sufficient at the entrance and exit of the bend. The control section is parallel to the weir crest. It is located at a distance of (2-4.7)h upstream the weir at upstream and bend’s entrance sections (h is the upstream head). For the horizontal weirs at the downstream of the bend, the control section is located at (1-3.45)h, and on 2B-Downstream of the bend is at (2.1-3.5)h. In the channel bend, sloping crest weirs provide better convergence of the unit flow rates across the river. The slope of the weir crest is recommended to be in the range of 85?