DISCHARGE CHARACTERISTICS OF BOTTOM RACKS WITH T-SHAPED BARS

Abstract

Trench weir is most common type of weir adopted for diversion of water from the boulder streams for the hydropower generation, water supply, irrigation etc. Bottom racks in trench weir are a standard hydraulic structure used as intakes in mountainous regions, to retain sediment and other solid matter larger than rack spacing. The present study is aimed at to study experimentally the effect of T-shaped bars on the discharge characteristics of trench weir under free and submerged flow conditions. It also involves comparison of discharge characteristics of T-shaped bars and flat bars. At present, trench weir is most common type of weir constructed across the boulder streams. Trench type of weirs have been widely used for small hydro schemes in India and other countries like U.S.A., Sweden, Norway, New Zealand, China, Bhutan, etc. Subramanya (1994, 1990) defined the different types of bottom racks and the different type of flow conditions over the racks depending upon the characteristics of approach flow. A first hydraulic description of bottom intakes was given by Orth et al. (1954) investigating flows on a 20% sloping channel with different transverse rack geometries, including the simple T, the semi-circular shape, the fully circular shape and the ovoid profile. The ovoid bar profile required the minimum structural length, whereas the T-shaped bar the poorest discharge performance. The bottom slope of the rack has small effect on rack clogging. Kuntzmann and Bouvard (1954) presented a first computational approach for free-surface profile over bottom racks by assuming constant specific energy head. Mostkow (1957) derived expression for the water surface profile for flow over the racks. Subramanya and Shukla (1988) classified flows over bottom racks into five categories, depending on the upstream and downstream flow conditions. 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, Ahmad and Mittal (2003) observed that the optimum length of bottom rack is obtained when diverted discharge is equal to incoming discharge in the channel. Other investigator like Ranga Raju et al (1977), Naseer et al (1980) and Ramumurthy et al (1986) have studied the flow characteristics of' bottom racks of transverse bars, perforated plate and slots in the bed of channel. Analysis of experimental data collected in the presented study revealed that the coefficient of discharge decreases with increase in the specific energy of the approach flow to the rack and decreases with increase in other dimensions of the rack. Moreover, the coefficient of discharge also decreases with the increase in the spacing between bars and slope of the rack. A relationship is proposed for prediction of coefficient of discharge for T-shaped bars. However, the use of the proposed relation is limited to particular value of slope of racks, clear spacing between the rack bars and dimensions of the bars used in the rack.