ENERGY DISSIPATION ON BLOCK RAMPS WITH PROTRUDING BOULDERS

Abstract

Block ramps (in the European and Australian literature) and Rock chutes (in the American literature) represent, nowadays, a valid alternative as grade control structure. 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 than in 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). They mainly allow for fish passages (Ferro et al. 2004) and, owing their surface geometrical complexity, they indeed enhance the development of self-aerated flows (Rice et al. 1998), improving the water quality of poorly aerated rivers (Essery et al. 1978; Moog and Jirka 1999). Various studies have been conducted to study the hydraulics of block ramps. Flow resistance has been investigated in terms of Manning coefficient and Darcy-Weisbach friction factor for riprap in mountain streams with steep slopes. Anderson et al (1970); Janet (1984); Bray (1979) and Bathurst (1985); Abt et al. (1987, 1988); Julian and Ramon (1990); Ugarte and Madrid (1994); Rice et al.(1998);Brian et al (2002); proposed equations to estimate flow resistance in terms of the friction factor. Aberle and Smart (2003) studied the influence of roughness structure on flow resistance on steep slopes. Pagliara and Chiavaccini (2006) studied the flow resistance of rock chutes with protruding boulders. Cascon et al.(1991),Stephenson (1991),Peyras et al.(1992),Christodoulou (1993) and Chamani and Rajartnam (1999a,b) investigated the energy dissipation on stepped channel which hydraulic behavior, in case of skimming flow similar to the block ramp. The hydraulics of stepped channel was investigated by Essery and Homer (1978), while Chanson (1994) gave the equations for the energy dissipation in condition of nappe and skimming flow. 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, 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 iv 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, the experimental results 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 to its failure. Boulders in random, row, and are (reinforced or not) disposition were investigated. Pagliara and Lotti, (2007) presented the first relationship of specific discharge for local failure of base block ramps. Two aspects viz, design aspect and application aspect were studied during the present study. In design aspect, the stability of loose base material of different size was investigated on different channel slopes and on the basis of experimental results the existing relationships for critical discharge estimation were checked. A new relationship was proposed which correctly predicts the critical discharge estimation at less percentage error. With regard to application aspect, first; energy dissipation phenomenon was studied over loose and grouted base material and after that over grouted base material reinforced with protruding boulders in staggered arrangement at different spacing.