HYDRAULIC PERFORMANCE OF BOTTOM RACK UNDER SUPERCRITICAL APPROACH FLOW

Abstract

Trench weir is a standard hydraulic diversion structure used as intakes in mountainous terrains, to divert water flow for various purposes like power generation, irrigation, water supply schemes and for retaining sedimentation flowing along with the stream flow. Attainment of the desired dimensions of the bottom rack, trench, intake well, flushing of sediments, protection works upstream and downstream of the trench etc. are dealt with via hydraulic designing of the trench weir. Conventional types of raised-crest weirs employed for diverting water from the boulder bedded streams for catering the need of generation of hydropower, irrigation and water supply schemes etc. marred the very purpose of the trench weir as the afflux brings about significant change in the flow behaviour of the stream. In such a scenario the trench weir, simply built across the stream below its bed level have been most widely adopted. Such type of a weir provided a confident & unequivocal advantage over the conventional design by not affecting the general bed level of the stream. l-lowever, due to being below the level of river bed, the bed sedimentation of size less than the clear spacing of the rack bars enters into the trench and gets collected in it. Such cardinal disadvantage makes post-monsoon clearance of the trench obligatory. Due to scope of enormous work over trench weir, various studies have been conducted exhaustively by investigators to study the hydraulic performance of trench weir. In this report we discussed discharge characteristics of a trench weir under supercritical approach flow. Many researchers worked extensively on trench weir like Kuntzmann and Bouvard (1954), Noseda (1956), Mostkow (1957), Lauterjung and Schmidt (1989), Brunella et al. (2003), Ghosh and Ahmad (2006) and Kumar et al. (2010) etc. Experiments were carried out under free flow condition with rack spacing 5cm, 10cm and 15cm with rack slope 0.0036, 0.05 and 0.1035. Three bed slopes were used 1/190, 1/110 and 1/55. Coefficient of discharge decrease with increase in many factors like rack slope, spacing of rack, approach bed slope etc. individually by keeping other parameters constant. Equation for coefficient of discharge is obtained by dimensional analysis.