The stability and dispersion of dredged material at an offshore disposal site was studied using integrated multibeam–geophysical surveys, sampling, in situ monitoring, and modelling techniques. The disposed material at the disposal site undergoes significant reworking by strong tidal currents and superimposed waves: 84% of the dumped material, mainly fine-grained sediments, was transported away from the disposal site and only 16%, mainly coarser material, remained near the disposal site centre to form an irregularly shaped spoil mound with ∼1 km radius and approximately 14 m height at the disposal centre. A large slump, 1.4 by 1.4 km in size, developed south of the spoil mound because of the failure of the dumped material. Tidal currents up to 60 cm s−1 were measured and can cause sediment mobilization over more than 50% of a tidal cycle. The total-load transport rate calculated using measured currents can reach 0.16 kg m−1 s−1. Superimposition of 1-year storm waves could enhance this by one to two orders of magnitude. Hydrodynamic and sediment transport modeling indicated that the maximum total transport rate for uniform fine sand at the disposal site can reach 0.024 kg m−1 s−1 under tide-only conditions, in the eastward direction during flooding and the westward direction during ebbing. The net transport averaged over a tidal cycle is relatively small (0.00027 to 0.0027 kg m−1 s−1) and to the southeast. The significant enhancement of sediment transport by storm waves clearly indicates that wave effects should be included in assessing the mobility of dredged material at ocean disposal sites. The majority of the disposed sediment was effectively dispersed by the wave-enhanced tidal transport; therefore the disposal site is deemed suitable for continued disposal of dredged material. Future disposals should be spread to a broader area to limit disposal mound build up.
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Vol. 2009 • No. 254