Study of blockage effects on scouring pattern downstream of box culverts

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Culverts are locations in water courses where a constriction in the water occurs and therefore where blockage of some type is likely to occur. Blockage of culverts commonly occurs during flood events. When a culvert is partially blocked at the inlet, the flow structure in the barrel and at the outlet will change and consequently may result in the failure of the culvert. Despite this, the impacts of blockage on culvert hydraulics and downstream waterways have not received consideration in the literature. The purpose of this study is to mitigate this deficiency by reporting on an investigation into scouring at the outlet of partially blocked culverts. The aims of this study can be categorised as: • to investigate the effects of blockage on scour geometry and to develop predictive equations; • to study the flow structure at the outlet of partially blocked culverts and compare it with a non-blocked culvert to explain the previous findings; • to conduct the experiments in unsteady flow conditions which are more similar to flood events to validate the general findings To achieve these aims experimental tests were conducted under steady flow to investigate a relationship between the maximum scour extents, blockage ratio of the culvert and the flow characteristics. Both non-blocked and partially blocked conditions were considered. Consideration of the results obtained showed that the scoured area and maximum scour depth increased with partially blocked culverts conditions compared with equivalent non-blocked culverts. Because complete protection against scour is too expensive, generally, the maximum scour depth of the scour hole has to be predicted to minimize the risk of failure. Therefore, empirical equations were developed to predict the maximum scour depth, length and width at the outlet of blocked culverts. In the next stage and to have a better understanding of the turbulent flow conditions at the outlet of a partially blocked culvert an experimental program was designed to measure the longitudinal, lateral and vertical velocity components through the culvert barrel centreline and at the outlet. Tests were conducted in both non-blocked and partially blocked condition. Based on that, the turbulent flow characteristics such as turbulence intensity, turbulent kinetic energy and Reynolds shear stress were obtained for each condition. Results showed that the turbulent flow characteristics in the partially blocked condition are significantly different in the barrel and at the outlet compared to a non-blocked condition of the same discharge. As the final step of this study, a test program was conducted under unsteady conditions in partially blocked and non-blocked culverts. The results were compared with previous results of steady tests and showed an agreement with previous steady flow tests.
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