Adaptive Uniform and Non-uniform Configuration of Boulders on Block Ramps for River Restoration

Block ramps are a cost-effective assembly in river restoration projects to sustain river morphology. In practical applications, block ramps are generally made of rock boulders with mean diameter between 0.3m and 1.5 m. This experimental study determines the various energy dissipation factors as a result of uniform and non-uniform configuration, which includes spacing and arrangement of the blocks (representing boulders), in the laboratory scale. The objective was to translate the dissipation of excess energy in the flow to minimize river bed aggradation or degradation, such as downstream of hydropower structures, spillways, bridges, etc. Semi-hemispherical blocks of 5 sizes representing boulders with a protruding part equal to half the diameter, are used to represent boulders as in mountain streams. In the present study, staggered pattern is adopted on 4 test slopes of the ramp and it was found that this arrangement is more effective in energy dissolution than the row or random arrangements. Analysis of data reveals that energy dissipation increases as the spacing decreases for various sets of uniform arrangements. The results are used to develop non-uniform configurations of the blocks with the intention to stabilize the flow and increase localized energy dissipation within the ramp. As it was observed that when low flow strikes with the boulders, the hopping distance of the flow jet is less and vice versa in respect of the discharge. Non-uniform arrangement was fabricated such that lesser spacing of the boulders were made for the upstream 1.0 m run of the ramp and greater spacing for the remaining downstream portion of the ramp. The results indicate that energy dissipation is greater in non-uniform configuration than uniform arrangement. Further evaluation of the different non-uniform arrangements, highlight that alternate spacing inherits the flow accordingly producing quantitative loss of energy. An empirical relation has been developed for both the uniform and non-uniform configurations for evaluating the relative energy dissipation (∆E_rB), in terms of length of ramp (L_R), ramp height (H), critical flow depth (h_c), boulder concentration (Γ), and coefficients (a₁, a₂ and a₃, which depend on the relative boulder configuration): ΔE_rB = L_Rx a₁ x exp[a₂⁄{a₃+Γ(h_c+H)}]

The relation is found to hold good within the ± 5% deviation from the line of perfect agreement, and may be conservatively used. The derivations made from the study should permit formulation of design guidelines and plots that can aid water resources engineers and planners in practical applications of boulder block ramps.