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Side weir is one of the most important structures in flood treatment projects that are designed provided the main channel bed is rigid. However, in the most practical channels, the main channel bed is movable and the changes in bed can produce wavelike patterns known as bed forms and another additional effect of side weir on bed forms is that it causes an aggradation of sediment particles in front of itself. These two products of using side weir in movable beds cause an additional bed resistance in comparison with the state that there isn’t any weir on sidewall of main channel and so the flow level with using side weir will rise subsequently and this means more diversion ratio. Thus, In order to study the effect of side weir hydraulic and geometric properties through Froude Numbers, diversion discharge ratios and flow depths on bed forms and its effect on design conditions, the present research work is carried out. A set of 9 experiments were conducted in a flume with dimensions of 0.85 m width, 0.40 m height and 10 m length on a mobile bed having median sediment particle size of 0.23 mm running with side weirs of crest lengths of 20, 40 and 60 cm . In addition a set of 3 experiments without using a weir were considered as bench mark experiments for comparison purposes. After running a determined flow in flume regarding with the bed profiles should reach a balance it was stopped after 1-3 hours. The bed topography at the end of each experimental run was recorded using automatic bed profiler in a length of 220 cm of main channel. This topography was recorded in a net of points that were distributed in a distance of 5 cm in length and 3 cm in width. The dimensions of bed forms were then determined using the well-known crest-through method. The results indicated that the effect of flow depth, discharge, and diversion ratio on bed form dimensions are significant and increasing these parameters cause an increasing influence on bed form dimensions. In this study 4 equations are suggested that both of them are for bench mark experiments and two others are for main experiment. The coefficients of these relations were determined by Solver Add In program in Excel with using 80 percent of data that were selected in chance and then they were verified with using remaining 20 percent of data. Verification of relations also illustrated that for both length and width of bed form in main experiments the maximum error of related equations is 50 percent and maximum error of relations for bench mark experiments is 30 percent. Analyzing these relations revealed an important influence of applying side weirs on lateral variation of bed form dimensions in the main channel so that it is indicated that bed form length and height near side weir will increase up to 70 and 2 percent of channel width, respectively, in comparison with the experiments that side weir is not used.

Keywords: Side weir, bed form dimensions, Experimental study, Froude Number effect, diversion ratio

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