EXPERIMENTAL STUDY ON SPATIAL FLOW CHARACTERISTICS FOR A PIANO-KEY WEIR MODEL

Abstract

In recent years, The Water Resource projects are reassessed for safety, provision for an increased estimate of the probable riaximum flood (PMF) has to be made in many cases. It is therefore necessary to provide more flood storage and/or larger capacity of spillways to pass the PMF safely. An innovative and effective way of increasing the spillway capacity is to use a Labyrinth weir. The concept of the Labyrinth weir is to vary the plan shape of the crest to increase the effective crest length. This increases the discharge per unit width of the spillway for a given operating head. A Labyrinth weir has advantages compared to a straight overflow weir and the standard crest. Labyrinth weir can be used to increase outlet capacity for a given spillway crest elevation and length or to increase storage by raising the crest while maintaining spillway capacity. A new concept of a Labyrinth weir has been proposed with a new shape like black and white Piano keys when viewed in plan. Keeping in view the relatively reported better performance of Piano-Key Weirs in comparison to linear and Labyrinth weirs, following objectives are considered for the present study. 1. To have an in-sight of the hydraulics involved with the flow over the Piano-Key Weir 2. To propose a geometry of Piano-Key for the given case study so as to provide required in-stream storage. 3. To perform experimental study on a Piano-Key Weir model at 3 different discharges so as to determine the spatial flow characteristics, like; a. The head-discharge relationship. b. The water surface profile as it flows in the up-stream side as well as over the weir. c. Velocity distribution of all the three components of the velocity at different points at up-stream of the weir and over the weir, d. Velocity vector diagram in up-stream of the weir as the flowing water approaches the PK-Weir and over the weir as it spills across the crest e. The change in discharge along the flow in an inlet cell. Hence to study the emerging concept of Piano-Key weir and in-stream storage, a case study of Brahinani River in Orissa was undertaken. A Piano-Key weir was proposed near Kharagprasad in Brahmani River to create the in- stream storage. This storage volume of water is created for the industrial and drinking requirement in the upstream of Kharagprasad. The requirement of 2.1 cumec of water was to be satisfied for 90 days in low flow condition with the help of created storage. The Experimental program includes the details of the three stages of the experiment. In the first stage of experiment which was planned with a Piano-key weir model, the water surface profile was to be traced in order to know the variation of stage (in direction of flow) with change in discharge. In the second stage of experiment, the measurement of velocities was carried out at different nodes of a predefined grid (3D-mesh) in the space up-stream to the weir, so as to draw the velocity profile along the direction of flow as well as to show the variation of z component of velocity along the direction of flow. In the third stage of experiment, the measurement of velocities was carried out at different nodes of a predefined grid in the space over the weir, so as to draw the velocity profile along the direction of flow as well as to show the velocity vector diagram as the water flows over the weir crest of the Piano-Key Weir. With the help of these velocity profiles, the discharge across various sections (across the inlet cell) were found out and after that the variation in discharge along the direction of flow in the inlet cell of the Piano-Key weir was found out. With the velocity profile the kinetic energy correction factors were computed in order to know the total head at different sections in inlet cell. Using these, the slope of energy line was found out. And finally the Tractive stress exerted by the flowing water on the Piano-Key Weir surface was computed. This study gives an insight of the hydraulics involved with the flow over the Piano-Key weir and is also very helpful in determining the sediment flushing capacity of Piano-Key weir.