Sediment Transport Upstream of Orifices

Abstract

Sediment transport upstream of orifices has not been studied outside the realm of the restoration of reservoir storage capacity. This experimental study used a large basin with a circular orifice outlet and a movable sand bed leveled with the invert of the orifice. Three sizes of non-cohesive, uniform sand were used with d50 values of 0.29mm, 0.72mm, and 0.89mm. Bed profiles at equilibrium scour conditions were measured for three different head levels for each sediment size. The maximum scour depth, length, and scour width were related to the particle size and the available head. The longitudinal and lateral extents of the scour hole were defined by the stable angle of the sand. The primary mechanism of sediment removal from a scour hole was vertically-oriented vortices that entrained sediment from the base of the scour hole, lifting it up and out through the orifice. Equations were developed to predict the length, width, and depth of the scour hole as well as to define its longitudinal and transverse shapes. The flow field upstream of the orifice was measured with an ADV after the equilibrium scour conditions had been reached. Similar measurements were taken over a fixed bed located at the orifice invert. It was found that the decay of the centerline velocity resembled an unbounded orifice within the scoured area before transitioning to a decay rate similar to that of an orifice over a fixed bed approximately at the extent of the scour area. The velocity profiles in the horizontal plane behaved similarly for both the fixed and movable beds, and their growth rate was calculated. The profiles in the vertical direction showed exceptional similarity at each x/D location. The vertical velocity at various locations clearly showed the flow diving into and rising out of the scour hole. The experimental data were compared to predictions using a numerical model. The numerical model did not provide accurate predictions of the flow field above the fixed bed or the movable bed geometries