Initiation of motion and scour burial of objects underwater

Abstract

Laboratory experiments were conducted to examine mobility and burial of objects, including onset of motion on a rigid bottom, scour burial into a sand bed, and the interplay between scour and motion onset. Experiments covered a range of object diameter-to-bottom roughness ratios (D/k) of relevance to mobility of unexploded ordnance (UXO). Results indicate that the critical mobility parameter (ΘUcrit) varies as ΘUcrit ≈ 1.6(D/k)−0.72 for cylinders under currents and waves. Modifications of this relation are also presented for accelerating flows. Scour experiments produced formulae for the equilibrium burial depth (Beq) for UXO as a function of the sediment Shields parameter (Θsed) of the form Beq/D = a2 Θsedb2. For current-induced live-bed scour, relatively large cylinders (D >8cm) result in a2 ≈1.3 and b2 ≈0.36. Smaller cylinders (D <3cm) and a tapered shape bury more easily (a2 ≈15 and b2 ≈1.1). An alternate, physics based relation for Beq incorporating additional length scales is also presented. For objects on mobile sand under accelerating flow, behavior was observed to be a contest between scour burial and the current speed reaching critical mobility. It is shown that by considering pit depth at the moment of mobility to be the effective bottom roughness, a general explanation of the onset of motion can be obtained.