Experimental Study of Hydraulic Properties of Flow over Vertical Shaft Spillway with Bow-Tied Shape Inlet

A spillway is a structure used to provide the controlled release of water from a dam or levee downstream, typically into the riverbed of the dammed river itself. One of the types of spillway used in dams is vertical shaft spillway or the same as morning glory spillway. One of the methods to increase the flow rate and reduce the occurrence of vortex currents in the spillway is to modify the shape of the inlet of the spillways. In recent years, spillways with daisy, piano key and zigzag inlets have been proposed known as spiral spillways. A review of the results of previous research shows that the common goal in the study of vertical shaft spillways is to pass the flood flows with the highest possible discharge and the lowest water level and reducing the eff destructive effects of vortex currents. In previous researches, issues such as the effect of the inlet type of vertical shaft spillway and geometric parameters of the structure on the discharge coefficient and the study of vortex occurrence conditions and hydraulic flow of vertical shaft spillway. In this research a new shape of vertical shaft inlet in form of Bow-tied spillway was introduced and discharge coefficient of these spillways was investigated using a series of experiments.

The experiments of this research were performed in a metal cube tank with 1.5 m length, 1 m height and 1 m width with transparent Plexiglas side walls in the hydraulic laboratory of Department of Water Engineering at Zanjan University. Bow-tied spillways were examined with different angles of the bow-tie (60-150 degree) in both cases of with middle arc and without middle arc and the results were compared with a morning glory spillway. In this research, by preparing eight bow-tie spillways and a morning glory spillway with a total length of 60 cm and considering the hydraulic conditions (H/D), 72 experiments were run to investigate the effect of the geometric shape of the spillway on the discharge coefficient.

Results showed that in Bow-tied models without middle arc, increase of the bow-tie angle leads to reduce the flow interference and increase of flow discharge over the spillway. So that B-150 model (with 150 bow-tie angle) had higher discharge coefficient than the other models of bow-tied. This model had better hydraulic performance than morning glory spillway (10 percent). Also, economic advantage of this model was higher than morning glory spillway (31 percent). Results of the tip position of the bow-tie spillway show in tip angle of 120 degree, the tip position of the bow-tie has a significant effect on the discharge coefficient of the spillway, but with increasing the bow tip angle from 120 to 150, the tip position of the bow-tie spillway did not have much effect on the discharge coefficient. In Bow-tied models with middle arc, change of bow-tie angle had no significant effect on discharge coefficient and hydraulic performance of these models was lower than morning glory spillway (5.3-9.7). This could be due to the interference of fall currents and the low internal area of these models.

Comparison of the hydraulic performance of bow-tie models with morning glory spillway shows that the B-150 model has the highest hydraulic performance (10%) compared to other models. On the other hand, this spillway has a better economic advantage (31%) than the morning glory spillway. In the construction of reservoir dams, especially earthen dams, the major part of the executive cost is related to the implementation of concrete structures of emergency spillway. In this situation, by choosing Bow-tied spillway with optimal geometry, a suitable economic savings can be achieved along with improving the hydraulic conditions of the flow.