Sanjay Pahuja and Ole S. Madsen

MIT Parsons Lab
Room 48-319
Cambridge, MA; 02139

Shoaling of waves creates an increase of near-bottom wave orbital velocity and wave-induced bottom shear stress with decreasing depth in shoreward direction. This results in a cross-shelf gradient of near-bottom sediment concentration. Addition of shore-normal oscillatory (tidal) current to this scenario results in a water column having different histories immediately before it traverses a point in the offshore and the onshore directions. A water column moving offshore comes from a region of higher near-bottom sediment concentration as compared to the same column when it moves shorewards. The finite response time of suspended sediment concentration in the water column thus leads to a net offshore sediment transport.

The problem is formulated for Lagrangian analysis of an oscillating water column. A depth-averaged but temporally varying eddy diffusivity is assumed. For a simplified case of linear near-bottom concentration variation, the partial differential equation governing the physical system is solved analytically using a perturbation approach. By considering the difference in suspended sediment concentrations when a water column passes a fixed point in up-gradient and down-gradient directions, the amount of sediment transported during an oscillation period can be calculated. The results are verified by numerical computation.

The proposed mechanism is seen to cause a net offshore sediment transport proportional to the gradient of the near-bottom reference concentration. The transport rate is higher for fine sediments. This diffusion-type mechanism may contribute significantly to offshore movement of fine sediments across the shelf.