Hydrodynamic models of boulder berm deposition

Abstract

The hydrodynamic environment associated with boulder berm deposition at flow expansions during high-magnitude flood is considered with respect to the plan geometry of surveyed berms. Hydraulic computer simulation of a prototype channel expansion and Froude-scaled physical hydraulic models are used to demonstrate the processes of berm formation. All the geometric features observed in the Field and temporal scales of berm-formation can be reproduced in the laboratory and compared in principle with hydraulic energy diffusers used in engineering practice. The experiments indicate that berms form most readily at low Froude numbers ( F < 1.5) owing to the deposition of individual bedload clasts and the shearing of transverse bars in the shear flow zone at the edge of the flow separation zone. The resultant accretionary berms minimize energy losses and ensure that high flows are transmitted efficiently through channel transitions. Berm formation may occur with either steady or unsteady supercritical flows ( F < 2.0) through a variety of channel expansions where initial flow already maintains a minimal backwater effect. It would appear that formation of berms is not associated with initial flow into “dry” expansions where no backwater depth is maintained. In all cases stable berm formation occurs extremely rapidly and the depositional process is not sensibly affected by steady or unsteady sediment feed rates.