Experimental Study of Downstream Scour of Piano Keys Weirs

Experimental Study of Downstream Scour of Piano Keys Weirs
Abstract
Background and objectives: The Piano Key Weir (PKW) is a rectangular-labyrinth-type weir that used to maximize the allowable weir length that can fit in a given spillway channel width, while reducing the required structural footprint. Piano key spillways is a new form of long crest spillways that have a relatively simple structure and high economic efficiency structures. Due to the advantages of this spillways, we need to study and investigate the downstream scouring of this structures. In this research, the downstream scour of piano key spillway has been studied. For this purpose, two different physical models of piano key spillways in different hydraulic conditions have been used.
Material and methods: The experiments were carried out in a flume with a 10 meters length and 50 cm width and height. Two type of piano key weir with 5 and 9 keys were used in this experiments. In order to compare the scour hole area a linear spillway with a height of 20cm and a width of 50 cm were used in comparison with linear spillways, the corresponding hydraulic conditions was also studied. The scour hole dimensions were measured with a point gauge with ±1 millimeter reading accuracy which has located on top of the channel that can moving along to the channel. The effect of the particle number on the maximum scour depth and location of maximum scour hole was investigated.
Results: The results show that in piano key weirs, with increasing effective crest length, water depth decreases over spillway crest and so on decreases the amount of scour depth by decreasing the flow velocity at downstream of spillway. Bed profile shape at eroded area is completely three dimensional. Increasing rate of Scour hole depth is related to number the of piano keys. By increasing the number of keys the maximum depth, length and location of the scour hole as well increased. The results show that even at the low level downstream water depth, scouring of this spillway is less than the sharp crested linear spillways. The profile of the scour hole in the piano key spillway is almost similar to the linear spillway but the only difference is in the height of the dune at the downstream of scour hole. The height of the dune generated at higher downstream water depth of piano key spillway is even lower than sharp crested linear spillway and also, with increasing the particle Froude number, the maximum scour depth parameter and length of occurrence was increased.
Conclusion: In piano key spillways reducing downstream water depth will led increasing scour depth and far it away from downstream apron. Also by increasing the particle Froude number, the maximum scour depth, the location of the maximum scour depth and the maximum length of the scour hole was increased. The rate of increase in the scour depth is also dependent on the number of overflow keys, and in a fixed Froude number and downstream water depth, increasing the number of keys, the maximum depth and location of maximum scour depth also increased.

Keywords: Experimental Study, Froude Number, Piano Key Spillway, Scour Hole, Water Depth