Effect of Piers Geometric on the Hydraulic Properties of Piano Key weirs

Background and objectives: Piano Key weirs (PKW) used in spillway of dams and regulating structures in irrigation and drainage network because of increasing the discharge efficiency. In the present study, the effect of geometric shape of piers, where are placed under outlet keys, have been evaluated on hydraulic efficiency. These structures increase the water level and water storage volume in the reservoirs of dam. In the previous study physical model has been used to extract discharge coefficient of Piano key weirs without any research on pier shape. Also some dimensionless relationships have been derived for discharge coefficient.
Materials and Methods: To achieve these purposes two weirs built with the ratio of height to width of one cycle, P/Wu, 1.33 (Model 1) and 0.50 (Model 2). As a comparison to the hydraulic efficiency the different piers were constructed including of continues, Triangular, half cylindrical and without piers. However, eight models were made and the hydraulics experiments with various discharges were performed. Both of the free and submergence flow on the weirs could be considered with the variation of discharge. The Minimum and Maximum discharge in the all of experiments were 1 L/S and 130 L/S, respectively. The Weber Number as surface tension was controlled in the minimum of discharge. All of tests were performed at a flume with 0.8m width and 12 m length in physically and hydraulically models laboratory of Shahid Chamran University (SCU).
Results: Generally, the results of this study showed that hydraulic discharge efficiency in model 1 with P/Wu=1.33 is more than model 2 with P/Wu=0.5. So this condition is independent of piers shape. However, discharge of weir with triangular pier was more than the other states in the same heads of both models. In addition, analysis of experiments result in both models showed that head-discharge curve of weirs is linear trend which it does specify this structure have an appropriate regulating of upstream water level. The general discharge equation of weirs was used for hydraulic efficiency analysis. In this stage results showed that the hydraulic efficiency of triangular pier is more than other piers. In addition, the half cylindrical piers is placed at second rank and there are no significantly different between continues and without piers. The streamlines with smoothly trends become curved at the half cylindrical pier and friction head losses decreased. Hence, the maximum of measured discharge coefficients in models 1 and 2 are more than continues and without any piers, 15 and 17 percents, respectively.
Conclusion: Using the piers with various forms has a significant effect on the efficiency of the discharge Piano Key weirs. So that the maximum discharge coefficients are 0.61, 0.54, 0.48 and 0.46 for triangular, half cylindrical, continues and without piers at model 1, respectively. However, these coefficients obtained in the constant Ht/P (Ht is upstream head water and P is weir height) which was about 0.1.