FLOW CHARACTERISTICS. OVER BOTTOM RACKS

Abstract

Bottom racks are standard hydraulic structure used as intakes in mountainous region to divert water from mainstream for different purposes. This structure is consisted essentially of a transverse trench in channel bed covered with a metal racks to prevent the transport of unwanted solid material through the opening of the racks. Bottom racks are generally made of thin perforated metal sheet or of bars placed either parallel or across the flow. Bottom racks placed over a rectangular or trapezoidal shaped trench built across the stream below its bed level is known as trench type or drop type weir. At present, trench weir is most common type of weir constructed across the boulder streams. There are many advantages in adopting trench weir as an intake structure likes it does not effect the general bed level of the stream. Trench type of weirs have been widely used for small hydel schemes in India and other countries like U.S.A., Sweden, Norway, New Zealand, Bhutan, China, etc. Mostkow (1957) derived expression for the water surface profile for flow over the racks by making assumptions that specific energy over the racks remains constant along the length of the bars. Subramanya (1994, 1990) defined the different types of bottom racks and the different types of flow conditions over the racks depending upon the characteristics of approach flow. They also developed relationships for coefficient of discharge, CD for different types of flow of longitudinal bottom racks of circular bars. Brunella et al. (2003) proposed equation for velocity distribution, water surface profiles over the rack and coefficient of discharge for bottom racks of circular bars. From the design consideration, the optimum length of bottom rack is obtained when diverted discharge is equal to the incoming discharge in the channel (Ahmad and Mittal 2003). Other investigators like Ranga Raju et al (1977), Naseer et at (1980), Ramumurthy et al (1986) have studied the flow characteristics of bottom racks of transverse bars, perforated plate and slots in the bed of channel.