MOVEMENT OF SEDIMENT OVER A PIANO KEY WEIR

Abstract

Piano Key Weir (PKW) is an upgradation of labyrinth weir and it was introduced in 2003. It is capable of enhancing the discharge capacity of conventional weirs by several times and till date around 30 PKWs are already constructed. It was observed that very limited research has been carried out on the flow field in vicinity of PKW and bed load transport over it. Any obstruction such as a weir placed across a stream reduces the upstream flow velocity and disturbs the flow and sediment continuity, and finally leads to deposition in the upstream side and scouring in the downstream side. Most PKWs are constructed in countries where sedimentation is not a big issue but in countries like India, where sedimentation is a matter of concern, a good amount of research is desired on the flow pattern near PKW and capability of PKW to pass sediment. Literatures suggest that the flow velocity is the governing parameter for the sediment movement and the sloping inlet key can be useful in sediment passage than the conventional weirs, but no detailed study was carried out earlier on the movement of sediment over PKW. This dissertation is focused on the experimental study and Computational Fluid Dynamics (CFD) analysis on the flow field in vicinity of an inlet key of PKW, and experimental study on the mechanics, kinematics and dynamics of sediment through its tracking using high-speed camera during its movement over the inlet key, and determination of the bed shear stress in the upstream of PKW required to pass sediment over the weir. Experiments were conducted in the Hydraulics Laboratory of IIT Roorkee in a 15 m flume and the CFD simulation was performed in Spatial Sciences Laboratory using the standard k-ε turbulent model and Volume of Fluid method. Initially, the study was carried out on a Triangular Weir with an Upstream Ramp (TW-UR) model which represents an inlet key of PKW and having the same slope, height and length as a key of PKW. Later a single unit/cycle PKW was studied. It was observed that the flow field in vicinity of PKW was very active and the velocity components along all three directions (especially the lateral and velocity components) were increased as the flow approached towards the inlet key. The CFD simulation estimated the resultant velocity with a considerable error range. The velocity increment along the flow direction was higher for PKW than that for TW-UR. It was also found that to pass sediment of some definite size, the lower discharge was required in case of PKW than that of TW-UR. There was a large variation in particle velocity along its path of movement; it decelerated while approaching the inlet key and accelerated beyond the first few centimetres over the key. In few cases, the saltation regime of sediment movement was also iii observed. About 16% to 35% additional shear stress on the upstream bed was required to pass sediment over PKW and the percentage went on reducing beyond the particle size 2.0 mm. The angle of orientation of the particle velocity vector was 0 to 50.9 degree to the horizontal axis. Increasing velocity and shear stress along the flow direction over the inlet key due to flow contraction in the vertical plane are helpful in enhancing the particle acceleration. While analysing the discharge characteristics of TW-UR it was found graphically and using statistical parameters that the equation of coefficient of discharge for TW-UR proposed by Di Stefano et al. (2016) is more accurate than the equation proposed by Azimi et al. (2013). For both TW-UR and PKW, CFD simulation estimated higher discharging capacity than the observed results. It can be concluded that PKW has very active flow pattern in its vicinity and can pass sediment more easily than TW-UR, and some additional bed shear stress is required to overcome the tangential component of weight that opposes the sediment movement. The conducted study was on a single-cycle PKW type A model and further research on other PKW types with different length magnification ratios and inlet key slopes need to be carried out for better understanding on the sediment transport capability of PKW and establishing an improved design guideline to maximize both discharge and sediment passage over PKW.