Mechanisms of interaction between dry sand flow and multiple rigid barries : flume and finite-element modelling

Abstract

Multiple barriers are commonly installed along predicted flow paths to intercept large volumes of geophysical flows. Current practice ignores the impact and overflow dynamics between multiple barriers and leads to an overly conservative design by using prescriptive approaches. There is certainly a need to develop a scientific framework and recommendations by investigating the flow dynamics during interaction with multiple barriers. In this study, a series of physical tests were carried out using a 5-m long flume to understand the mechanisms of interaction between two rigid barriers in series, using dry sand flow. The physical test data was used to calibrate a three-dimensional finite-element model and to back-analyse the flume test results. A numerical parametric study was also carried out to investigate the effects of upstream barrier height and barrier spacing on downstream impact characteristics. This study reveals that there are two key interaction mechanisms that should be considered in the design of multiple-barrier systems: (i) runup at the upstream barrier, in which flow momentum changes direction from towards the barrier to parallel to it, which reduces overflow momentum at the downstream barrier; and (ii) downstream flow-thinning at landing, that reduces the lateral pressure acting on the downstream barrier. Based on the physical model tests and numerical analyses, a new, quantitative scientific framework and formulations for multiple-barrier systems are developed. This framework accounts for (i) runup upon barrier impact; (ii) the trajectory of overflow; and (iii) landing velocity attenuation which facilitates the optimization of multiple-barrier systems. These interaction mechanisms have profound effects on downstream impact characteristics, when the upstream barrier height is taller than twice the maximum flow depth. Thus, the downstream barrier height and impact pressure can be reduced up to 17% and 35%, respectively. Proper design of upstream barrier height and spacing between barriers can avoid unnecessarily large impact pressures on the downstream barriers. These imply that structural capacity for the downstream barriers can be safely reduced. It is acknowledged that small-scale flume tests of dry sand flow entail scaling discrepancies with the absence of pore fluid. If high-quality data from large scaled tests becomes available, it would be worthwhile to utilise the data to enrich the proposed multiple-barrier framework for enhancement of engineering applications in future.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology