HYDRAULIC CHARACTERISTICS OF FLOW OVER RAMP WITH ROUNDED BOULDERS

Abstract

Block ramps represent a valid and low environmental impact alternative as fishway passages, and also by improving the re-aeration in rivers with low dissolved oxygen content, due to the turbulence enhanced by the three dimensional macroroughness conditions. They have been identified as an effective energy dissipater downstream of hydraulic structures like trench weirs, overflow dams, spillways, etc. due to the large roughness. They are characterized by relatively large bed slopes along which the flow is subjected to greater energy loss compared to traditional chutes (Pagliara and Peruginelli 2000; Pagliara and Dazzini 2002). The concept of artificially introducing the macro roughness has been extended to these rock chutes to increase energy dissipation (Ghare et al. 2010). Pagliara and Peruginelli (2000) studied the energy dissipation relationship on block ramps with regard to skimming flow condition, Pagliara and Chiavaccini (2006) with regard to various roughness conditions and Pagliara et al. (2008) with regard to submerged flow conditions. Pagliara and Lotti (2009) studied the effect of permeable bed on energy dissipation. Ahmad et al (2009) studied the effect of staggered arrangement of boulders on energy dissipation. Ghare et al. (2010) presented a mathematical model which correlates the representative bed material size of block ramp with the step height of stepped chute using multiple regression analysis so that, findings of stepped chutes can be used for the design of block ramp system. Stability is another important aspect in the design of block ramps. In fact, to ensure ramp functionality the boulders should remain stable during design flood events. The stability of block ramp has been investigated by different researchers. Ibash (1936) developed a relationship relating the minimum velocity necessary to move stones of a known size and specific gravity. Anderson et al. (1970) developed a design procedure for riprap-lined drainage channels by testing round stone on relatively flat slopes. Whittler and Abt (1990) investigated the effect of stone gradation on chute performance. Abt et al. (1987) and Abt and Johnson (1991) tested angular and rounded stone in their study. Maynord (1988) developed a riprap sizing method for stable open channel flows. Frizell and Ruff (1995) examined riprap for embankment overtopping protection. Whittaker and Jaggi (1986) and Robinson et al. (1997) investigated the stability of rock chutes. Pagliara and Chiavaccini (2004) studied the effect of boulders for increasing the ramp stability. Pagliara and Chiavaccini (2007) analyzed the stability of the rock ramps in terms of shear stresses or densimetric Froude number for both the base and the reinforced ramp and to evaluate the bed evolution of the rock chute up III to its failure. Boulders in random, row, and arc (reinforced or not) disposition were investigated. Pagliara and Lotti, (2007). They also presented a first relationship of specific discharge for local failure of base block ramps. This thesis aims to study the effect. of gradation and slope of ramp on energy dissipation. Experiment was performed on a rectangular ramp of width 0.83 m, for three set of length and slope (6.27m and 1V:9.87H, 4.20m and 1V:6H and 2.10and 1V:3H). A 20 cm broad crested weir was provided at upstream of the ramp. The effect of nine different gradations of rounded boulders on energy dissipation was observed on three slopes. On each slope, water surface profile was observed. Effect of friction factor on energy dissipation was also observed. At the three different slopes critical discharge required for failure of the ramp was also observed. From the present study it was observed that water surface profiles increase with increase of discharge. It was also observed that at smaller slopes (1V:9.87 and 1 V:6H) flow depth was uniform and uniform flow condition existed, but for larger slope (lV:3H) flow depth was not constant. It was also observed that the effect of energy dissipation per unit length increases with increase of slopes. It was also found that the large size of boulders were more efficient in energy dissipation as compared to small sized of boulders. It was also observed that critical discharge required for failure of ramp decreases with increase in slope and it was increases with increase of non- dimentional boulders sizes. A new relation has proposed for predicting of critical discharge . required for ramp - failure taking in to consideration of effect of uniformity coefficient.