Optimal selection of sediment disposal sites in open coast locations

Disposal of dredged sediment to the marine environment can result in modifications of suspended sediment loads and sedimentation rates that extend well beyond the immediate site of disposal. Dredging practices such as cuttersuction and trailer-suction dredging tend to liquefy fine sediment particles, and when dumped they can form turbid plumes near the seabed that will migrate with prevailing currents. When deposited into dispersive offshore environments they can be selectively mobilised, due to resuspension of fines by the combined actions of currents and waves. This process is enhanced by the high water content, requiring lower shear stresses for mobilisation. Over time, cycles of resuspension and settlement can result in the shunting of fine sediments over considerable distances in energetic ocean settings, with undesirable environmental, social and economic consequences. Disposal sites should desirably be non-dispersive, to retain dumped sediments, or if dispersive, the rate of flux and direction of migration should not be problematic. A procedure for the selection of optimal disposal sites within coastal waters using numerical modelling is illustrated in this paper for two coastal settings with different environments. Long-run (years) hindcasts of current velocities and waves are applied to calculate spatial and temporal trends in shear-stress at the seabed over the candidate region, to indicate the likely stability of given particle sizes. A three-dimensional sediment transport model (SSFATE), which accounts for resuspension, is then applied to simulate disposal into alternative sites to estimate the rates of sediment flux, transport routes and potential exposures for surrounding locations. The integrated procedure allows managers of a disposal programme to select potential disposal areas to minimise the return of sediments into ports or harm to sensitive habitats.