HYDRAULICS OF PIANO KEY WEIRS

Abstract

Piano key weirs (PKW) are basically installed on the low head dams to augment their discharging capacity and are often preferred over other traditional weirs for the dam rehabilitation purposes. The long-established dams require renovation/up gradation due to the sedimentation which reduces the storage capacity and changes in the hydrology. Therefore, high efficient weirs/ spillways must be deployed to increase the discharging capacity of low head dams. PKWs are best suitable for this kind of rehabilitation purpose. Construction of hydraulic structures disturbs the flow and sediment continuity which results into upstream siltation and downstream scouring. Mostly PKWs are constructed in countries where sedimentation is not a big issue but in countries like India where sedimentation is a serious concern and requires a good expanse of research on the upstream siltation, flow field, movement of sediment and scour. Literature review on PKW reveals that scarce investigations are found related to sediment movement, flow field, upstream siltation and scour downstream of a PKW. This study basically deals with the hydraulics of PKWs. The prime focus of this investigation is to deal with the discharge and flow characteristics, mechanics of sediment movement and sour downstream of weir to understand the hydraulics of PKWs. Both experimental and numerical approaches were adopted to accomplish the research objectives of the study. Experimental tests were conducted to check the accuracy of the available equations of coefficient of discharge of PKWs and dimensional analysis were performed to formulate empirical equations for the discharging capacity with and without upstream siltation considering free flow and submerged flow conditions. The flow field in the vicinity of weir and shear stress along the inlet key were investigated experimentally and numerically using Computational Fluid Dynamics simulations. The mechanics, kinematics and dynamics of the sediment particle tracking was performed using a high speed camera during its movement over the key and bed shear stress required to pass the sediment over the weir was also determined. Scour downstream of a PKW with and without solid aprons were conducted experimentally. All the experiments were conducted in the Hydraulics Laboratory of IIT Roorkee in a 15 m long flume and CFD simulations were performed using Ansys 18.0 adopting k-ε turbulent model and volume of fluid method. Initially, investigation was carried out on a triangular weir with an upstream ramp (TW-UR) model which behaves as an inlet key of PKW. Later, three PKW models having different cycle (One cycle, two cycles and three cycles) were considered in the study. Three solid aprons of length 0.05, 0.15 and 0.20 m were used for studying the scour downstream of weir.