Experimental investigation of the effect of sloping-broad crested weir at bilateral water intakes in 90⁰ sharp bend

Most rivers are naturally meandering and are constantly eroding on the outer bank and sedimentation on the inner bank. As a result, cognition of the flow pattern in river bends is critical. Today, one of the constructions employed in river bends is the diversion dam. When the position of the bend changes, the diversion dam alters and its position shifts. Moreover, the adjacent agricultural lands' water requirements necessitate the intake construction on both sides. Hence, it is significant to study broad-crested weirs as one of the structures for maintaining uniformity of flow in river bends and during dewatering. In general, the presence of in-stream structures such as sediment control structures or out-of-river structures such as intakes at the bend of rivers require further investigations on sedimentation problems and flow patterns. One way to maintain a uniform flow in river bends is to create a secondary flow by broad crested weirs. Accordingly, the primary purpose of this study is to investigate the flow pattern and evaluate the uniformity of the distribution of unit flow discharge distribution at the intakes located on both sides of the broad crested weir in a 90-degree bend using a laboratory model.

The experiments were carried out in a rectangular flume trough 0.92 width, 0.6 depth, and 90-degrees bend. Indeed, the radius of relative curvature of the bend is 2.05, which falls into the category of sharp bends. The weir model is of the broad crested weir type, and the intakes were placed on both sides of the weir in the direction of flow and with an angle of zero. The weir was installed at an angle of 60 degrees from the bend inlet, and velocity and depth were measured in various transverse and longitudinal sections. All experiments were run at three flow rates of 37, 50, and 60 liters per second.

In general, the results show that bends and secondary flows affected the water level and caused a difference in water depth in the inner and outer banks in the case of without weir. This difference is indeed due to the effects of the bend. In this event, the side slope of the water surface deviates from the horizontal position and falls towards the inner bank of the bend, and rises towards the outer bank of the bend. One of the motives for the problems associated with sedimentation in river bend dewatering is related to this issue and the flow pattern in river bends. With the installation of a horizontal crest weir in the bend, the uniformity of the distribution of unit flow discharge has increased.However, in the case of the inclined sloped weir, the difference in unit flow discharge of the inner and outer bank is the largest compared to the other two cases. Therefore, the performance of the sloped crest weir was not acceptable in the sharp bend and upstream control section. The results of the discharge distribution in the intakes on both sides of the bend confirms the effectiveness of the hypothesis that the slope of the weir crest directs flow to the catchments on both sides of the bend. With the weir installation, more water flows into the intakes than without a weir.

 Regardless of the effect of the weir slope, the distribution of flow in the intakes on either side of the weir is more uniform than without the weir due to weir construction in the bend. The sloped crest weir has contributed to the distribution of intakes discharge. The highest uniformity of dewatering on either side of the weir was at an inlet in maximum flow discharge. The discharge ratio of the outer bank intake to the inner bank in the horizontal crest weir and sloped crest weir was 1.04 and 1.06, respectively. A sloped crest weir crown has the most significant influence on the uniformity water flow on both sides of the weir in all cases of the upstream inlet flows. An average, the discharge ratio of intakes in the outer bank to the inner bank in sloped crest increased by 11%, compared to the case of horizontal crest weir.