ENERGY DISSIPATION ON BLOCK RAMPS

Abstract

1 lydraulic imbalance and instability of the flow in steep channels due to natural and anthropogcnic causes has often been resulting in the degradation of riparian river conditions. Practicing engineers and planners have been confronted with the task of finding a feasible and cost-effective solution for restoring such rivers and streams, especially in the downstream reaches of mountainous water courses where structures as small dams, diversion weirs, hydropower units, etc, have been constructed. Energy dissipaters are generally adopted whenever the velocity of flow leaving a hydraulic structure exceeds the erosion velocity of the downstream channel system. Armoring of bed controls bed degradation and scour and the increase in hydraulic roughness of the bed control structure may dissipate a minor amount of hydraulic energy. Various stream restoration structures have been established upon as a solution to this manifested problem. Block ramps have recently been indicated to be viable stream restoration structures with respect to its energy dissipation characteristics and efficient composition. Block ramps can be defined as grade-control structures applied to maintain the morphological continuity of the stream especially in mountainous reaches of rivers and are created by superimposing one or more rock layers on the original river bed. These structures are generally adopted where there is an appreciable fall in the river grade and are made of natural boulders whose mean diameter varies between 0.3 m and 1.5 in. The deployment of block ramps induces a considerable dissipation of energy with the creation of high turbulence IF zones within the macroroughness boulder elements. Few anomalies were noted during the review of studies conducted on block ramps. It has been pointed out by Pagliara and Chiavacinni (2004) that the energy dissipation on block ramps generally increases with the decline of the bed slope, and the differences tend to diminish with decreasing discharges. 1-lowever the same authors (2006a) indicated that for the same discharge values, the dissipation is directly proportional to the height of the ramp and, for the same slope, to its length. Further when the same authors (2006b) adopted macroroughness boulders on block ramps in their study, they found that the slope seems not to influence the increase in energy dissipation. The authors also stated that the effect of Reynolds number is negligible, because Re was greater than l01 in their test conditions, as also stipulated by other investigators (Lawrence 1997 Ferro 1999). Further the authors concluded that as the boulder concentration increases, the dissipation increases, with a maximum of 10 - 12 % for a v concentration of 30 -35 % for boulders in row and random arrangements. it has been suggested that the Reynolds number is an important parameter of highly turbulent flows (Nezu and Nakagawa, 1993; Jimcnez, 2004). The effect of boulder spacing and distribution on the energy dissipation process for block ramps with boulders has not been examined in detail so far. Also the ranges of boulder concentration to achieve optimum energy dissipation needs to he ascertained for other boulder arrangements as the staggered configuration, and variant boulder sizes. The correlation of flow resistance with the energy dissipation on block ramps has not been directly examined, though several studies in close relation to block ramp have been done. Canovaro and Solari (2007) investigated flow resistance developed by macroroughness pebbles positioned on a granular layer according to a regularly spaced stripe pattern on steep bed slopes. Pagliara and Chiavacinni (2006c) proposed a combined friction factor for the boulder block ramps and have concluded that boulders on the block ramp increases the flow resistance; the increase in flow resistance depend on the ramp slope, on the boulder concentration and disposition, and on the boulder roughness. These facets are further examined and addressed in the present experimental study. The objective of the study was to comprehend the energy dissipation characteristics on boulder block ramps with focus on the staggered configuration of boulders. and find out the boulder spacing and distribution which can generate efficient energy dissipation, and to ascertain its practical application. b.xperiments were conducted to investigate the manifold parameters characterizing energy dissipation on block ramps for different configurations such as smooth condition, ramp with base material and ramp with protruding boulders over the base material. The experiments were carried out at the I-Iydrauhcs Laboratory of the I)epartmcnt of' Civil Engineering, Indian institute of l'echnology Roorkee, India, and were performed on a concrete flume having a rectangular ramp of width (W) 0.30 m, horizontal length 4.0 in with side walls of height 0.45 in. 'Ibree ramp slopes (S) in the order of IV:511, lV:7H and 1V:911 were investigated. The flow discharge used in the experiments varied from 0.007 to 0.039 m3/s. The ramp was tested in the smooth condition for the three slopes followed by the block ramp with base material using angular stone aggregate with size distribution for d10 = 16 mm and d84 = 25 mm. To test the effect of angularity of the base material on the energy dissipation, round river bed aggregate with d 0 20 mm were also used. Then boulders using semi-hemispherical blocks of various sizes (0.042 in < DB 5 0.10 m) were used to examine the effects of the primary variables as boulder concentration, size, spacing and distribution along with the flow parameters on the energy dissipation process for boulder concentration (F) in the range of 8 to vi 32 %. The staggered configuration formed the main criteria of the experimental study within which two boulder distributions were tested: (i) uniform distribution with a regular S/D13 + longitudinal spacing of the boulders, and (ii) non—uniform distribution with a differential SID13 longitudinal spacing of the boulders, on the block ramp. Observations were recorded and analysis were carried out to delve at the energy dissipation characteristics for the different configurations of block ramps. Experimental data observed for block ramps in various configurations were analyzed to examine the effects of the various parameters related to the relative energy dissipation for the case of smooth ramps, block ramps with base material and block ramps with boulders under various permutations and combinations of test conditions. The smooth ramp condition was analyzed to check its consistency in dissipating energy and serve as a benchmark for comparing the magnitude of energy dissipation for other block ramp conditions. It was found that the smooth ramp yielded a maximum relative energy dissipation of 56% on the steepest slope tested lV:51-I. Block ramps with base material were investigated in the next case, and it was found that this ramp condition produced a maximum relative energy dissipation of 82%, on the slope lV:51-1. The angularity of the base material was analyzed using angular and round stone aggregates, and no significant variation was being found in their energy dissipation characteristics. The slope showed a dominant role and it was observed that maximum energy dissipation was realized in the steepest slope tested. It was also found that the relative energy dissipation AEr computed using the Pagliara and Chiavacinni (2006a) equation overestimated the values by more than 5% when compared with the observed values for the range 0.07 <h/H 0.26. For the block ramps with boulders, which formed the main component of the study, the variation of relative energy dissipation with respect to the primary energy dissipation variables were exhaustively analyzed for each tested slope under various boulder concentration and configuration in the flow parameter range of 0.05 to 0.26. The Reynolds number was found to range from 2.55 x to 10.68 x 10 with a distinct correlation for each tested slope with respect to the flow parameter in the tested conditions for boulder block ramps. Initially the flow resistance associated with the macroroughness boulders on the block ramp were analysed in terms of the resistance function (8//)1/2 using the observed dataset and that computed using the relation of various investigators. The present study showed lower values of the resistance function (8/1)1/2 in comparison with the other authors and was found to range from 2.4 to 4.1. A comparative assay of boulders in rows and staggered configuration showed that the staggered vii configuration generated higher relative energy dissipation when tested with boulders of the same size at the spacing under. The two roughness parameters E and F (functions of arrangement and roughness of boulders) which were postulated by Pagliara and Chiavacinni (2006h) were re-examined and reinstated to be 0.23 and 11.6 respectively, for boulders bigger than 0.042 in diameter for rows configuration and rounded (smooth) condition. Within the staggered configuration, a subjective analysis was conducted for the boulder distribution in the form of uniform and non-uniform configurations. The differential and integral effects of the boulder concentration, spacing, distribution and size were investigated for each individual slope as the relative energy dissipation due to boulders only i.e. AErBIAEr, showed an apparently unvaried effect with the slope. On the lV:511 sloped-boulder block ramp, the relative energy dissipation observed was between 73 - 92 % for the range of boulder concentration 8 to 32 %. On this slope, closer spacing with S/D13 < 1 .5 and certain non-uniform configurations exhibited higher dissipation of energy than the other configurations. On the 1V:7H sloped-boulder block ramp, the relative energy dissipation observed was between 74 - 83 % for the range of boulder concentration 14 to 29 /o. At this slope intermingling trends of the relative energy dissipation were observed. On the I V:91 I sloped-boulder block ramp, the relative energy dissipation observed was in the range of 67 to 77 % and AErB was found to decrease considerably as the flow parameter increases. for the range of boulder concentration 19 to 29 %. At this slope, a particular nonuniform configuration (NU-4) exhibited higher dissipation of energy than the other tested configurations in the case of boulders bigger than 0.065 in diameter. It was found that the behaviour of the uniform and non-uniform boulder distributions had varied asymptotes and dependent on the boulder size and concentration at each tested slope. From the sequence of analyses, a threshold boulder concentration beyond which there is negligible or diminishing effect on the relative energy dissipation could be earmarked. This threshold boulder concentration was found to be in the range 22 - 25 % for the tested boulder configurations and sizes for all the three slopes tested. Certain deductions were also made as the ratio of channel width to boulder size which characterized the energy dissipation trend along with the boulder configuration for particular slopes. Specific examinations were done to examine the variance of the major energy dissipation parameters and it was generally found 'that along with the flow parameter, the boulder concentration, spacing and size were primary parameters in describing the energy dissipation trend. A hypothetical analysis was also done to examine the direct analogy and possible relationship of flow resistance with the energy viii dissipation on boulder block ramps and it was found that the resistance function, in terms of (811) 112 varied inversely with the relative energy dissipation function, asserting that higher flow resistance described by the friction factor f showed increase in the energy dissipation on boulder block ramps. This concurred with the findings by Ferro (1999) and Pagliara and Chiavacinni (2006c). The multitude of the energy dissipation parameters were further analyzed using statistical techniques to select an optimal datasct which best reciprocated the relative energy dissipation function on boulder block ramps in staggered configuration. The selected optimal dataset were tested for consistency with regard to the representation of the whole dataset observed in the present study. Relations are proposed for computing the relative energy dissipation for block ramps with boulders in staggered and non-uniform arrangements using 3/411) of the optimal dataset with boulder concentration 10 to 30 % and the same is validated using the remaining 114th of the dataset. The equation can be used satisfctorily within + 5% error margins in the similitude range of the tested conditions. Conclusively the relations proposed for the estimation of energy dissipation on boulder block ramps with boulders in staggered configuration were validated along with existing relations, and thereby projected as the main outcome of the study. The derivations made from the analysis of the observed data were used to formulate design guidelines and plots that can aid in practical applications of boulder block ramps. The design methodologies have been formulated by also taking into consideration the philosophy of relevant studies and findings reported for block ramps structures. The element of block ramp application in the field was demonstrated using a case study highlighting the riparian river 'V conditions and the effectiveness of the design approach in applying boulder block ramps for actual site conditions.