Investigation of the effect of outflow from a sharp bend, influenced by a pipe installed at the bend exit, on the scour pattern in the downstream straight reach under unsteady flow conditions

A major threat to the stability of pipes crossing rivers at width is scouring around such structures. Pipe failure and the consequent serious damage to and life-threatening cutoff of vital streams are possible outcomes of this phenomenon. Although numerous studies have investigated scour around pipes in straight channels and bends, changes in bed topography along the straight reach downstream of a sharp bend, which is affected by the complex flow pattern issuing from the bend, have received less attention. Therefore, this study examines the effect of pipe installation at the end of a sharp bend and its influence on the downstream bed topography along the straight reach. In addition, from both an operational and hydraulic perspective, placing the pipe outside the bend would be a preferable option for designers and engineers. The study investigated, among others, the pipe position relative to the bed level as well as the hydrographic parameters in terms of flow unsteadiness and the effect these could have on the pattern of scouring downstream of the 180-degree sharp bend. The experiments were conducted under three pipe-installation conditions relative to the initial bed level, namely level with the bed, above the bed, and buried, with the pipe positioned perpendicular to the flow direction and under four different ratios of hydrograph peak timing to flood duration in unsteady flow. To evaluate the effects of different pipe installation conditions as well as the flow hydrograph, parameters such as the extent of scour-hole development, the maximum scour depth, the maximum deposition height, and the bed topography changes in the straight reach downstream of the bend were measured and analyzed. The main focus of the research was toward identification of the scour hole development and the maximum scour depth variations and how this phenomenon interacted with the stated parameters. The study findings indicated that the most critical case of bed topography variations under the influence of pipe occurred when the pipe was in a level position with the bed, where altering the pipe level from bed level to buried levels decreased the maximum scour and sedimentation by respectively 37.1% and 38.0%. Furthermore, alteration of unsteady-flow related parameters had less effect toward scour reduction than that of pipe level position with respect to the initial bed level. The findings of this study can be used to identify the most suitable pipe installation condition in the straight reach downstream of sharp bends, reduce scour risk, predict bed changes, and improve the design and protection of pipelines in river engineering projects.