Sediment Transport and Backfilling of Trenches in Oscillatory Flow

By means of detailed mathematical modeling the processes responsible for the morphological evolution of small-scale excavations exposed to wave-induced flow are investigated. The problem is relevant when assessing rates of sedimentation of trenches dredged for the purpose of laying out pipelines or cables in the marine environment. A flow and sediment transport model formulated in curvilinear coordinates is applied. The unsteady Reynolds-averaged Navier-Stokes equations along with a standard k -ε turbulence model and the equation of continuity constitute the flow model. The action of waves is simulated by an oscillating pressure gradient imposed on the horizontal momentum equation. The sediment transport is composed of bed and suspended load taking into account the effect of gravity on the former. The convection-diffusion concept is applied to calculate the distribution of suspended sediment concentration. Intraperiod flow and sediment transport processes are resolved, and the effect of these on the backfilling is quantified by a parameter defined by the ratio between the amplitude of oscillation of the flow and the width of the trench. The role of this parameter is twofold. It indicates the extent of flow separation developing over the lee-side slope of the trench. The energetic turbulence associated with the presence of a separation zone impedes sedimentation; it indicates the relative-to-the-trench-width distance covered by advected suspended sediment. For small values of the parameter, sediment will only be exchanged over relatively small distances as opposed to large values where sediment from the undredged regions will encounter the deepest sections of the trench thus enhancing sedimentation. Consequently, with respect to backfilling, the parameter in question governs two counteracting processes. For large values, the separation bubble is the prevailing mechanism for the distribution of the period-averaged transport of sediment and, in turn, for the evolution, of the trench morphology. The flow model is validated against measurements from a U-tube experiment.