Laboratory Investigation on Discharge Coefficient of Trapezoidal Piano Key Side Weirs

 The flow passing through a side weir, one of the varieties of water diversion structures, is a variable flow with decreasing flow rate. Labyrinth weir is the basis for piano key weirs. They are often constructed with vertical walls and are much more efficient than the linear weirs. Nevertheless, the flow, especially the bottom flow, enters this type of weirs and passes through two vertical walls of the side crests. Then it becomes squeezed and therefore the upstream and downstream crests come up with inappropriate hydraulic behavior. Also, the most outstanding disadvantage of this type of weirs is the large foundation area needed to be constructed on the concrete dams. The piano key weirs are a modern type of the nonlinear weirs which have been developed by Hydro Coop Institute of France and the Hydraulic and Environmental laboratory of the Biskara University of Algeria. In general, these weirs comprise of 4 different types, the differences of which lie in the presence or the absence of slopes created for them. Type A is sloped both upstream and downstream, Type B is sloped upstream, Type C is sloped downstream and Type D lacks any slope. The present study conducted show the effective geometrical parameters on the hydraulic performance and discharge coefficient (CM) of the trapezoidal piano key side weirs (TPKSW). The type of flow and its variations in a side weir can be considered as the C_M of the side weir, using simplifications and assumptions suggested by De-Marchi in 1934 to obtain suitable equivalents for side weirs.  

All tests have been conducted in a closed-loop rectangular Plexiglas flume in Soil Conservation and Watershed Management Research Institute (SCWMRI), Tehran, Iran. The study canal  was10m long, 0.6m wide and 0.6m high. All tests have been carried out on the 0.6m wide canal. To prevent flow turbulence upstream of the canal, tranquilizing racks were used at the upstream. A calibrated triangular weir was also applied to measure the flow at upstream. Also, a calibrated rectangular sharp-crested weir was used downstream. The water surface profiles were measured longitudinally. For this purpose, a digital depth profiler with 0.1 mm precision was used. The profiler accuracy was valid for almost stable water surface but it could be decreased in highly turbulent flows. An electromagnetic velocity meter with 0.001 m/s precision was used to determine velocity components to obtain parallel (𝑉𝑥) and perpendicular (𝑉𝑌)to the side weir. The profiler and the velocity meter could move on a rail in both X and Y directions. Flow rates at the main and the collection canal were measured by a calibrated 90° V-notched and a rectangular weir, respectively. Figure 6 shows a general view of the laboratory. In this research, 16 models of Type-A trapezoidal weirs have been studied in two cases of 1 and 2. The weirs had 3mm thickness made of Plexiglas. The tests were carried out preventing the effects of viscosity and surface tension over the weir and considering the height of more than 3 cm.  

In this research, for investigating the effects of a number of inlet cycles, the weirs were tested at two different directions of the side weirs located in the main canal. The results showed that the weirs with 15cm and 20cm had the highest discharge coefficient CM in dimensionless ratios of 0.2> H/P> 0.4 and H/P>0.5 respectively. Having reviewed previous studies, it could be concluded that the trapezoidal piano key side weir was capable of releasing a flow 1.2 times more than that of the linear trapezoidal labyrinth weir with 12 degrees angle and 1.87 times more than the one with 6 degrees angle, and 1.5 times more than that of the triangular labyrinth weir. 

The results of the present study showed that these types of weirs could have a better lateral performance than other weirs. In addition to this research, other researchers can conduct detailed studies on sedimentation and sediment control in this type of weir.   Keywords:Discharge, Flow, Water Diversion Structure