Abstract: Problem of Misplaced Sediment

Maynard M. Nichols, Richard W. Faas

Disposal of dredge material in U.S. East Coast estuarines is a major depositional process with important sedimentologic consequences. Massive disposal has altered the long-term quasiequilibrium of sea-level rise and sedimentary infilling and has broken down the dynamic balance between sediment supply, bottom geometry, and tidal flow. A small change in a single variable produces large effects. As greater channel depths are reached, dredge material is replaced by finer sediment that attains equilibrium depths at a higher bed level than coarser sediments. With sediment misplaced on lateral shoals or behind shore dikes, tidal flow is confined to a single channel and the effects of river flooding extend farther seaward. At the same time, lateral mixing over the shoals is eliminated, the tidal prism is reduced, and stratification of channel water is enhanced. Consequently, transport in the estuaries has changed from an escape mode under natural conditions to a trapping mode produced by dredging and disposal.

Longtime disposal in the James Estuary has built up shoals to a near-surface level where they are subject to wave erosion and redistribution by currents. Most consequential is the return of misplaced sediment to the main channel down slopes steepened by years of disposal. In brief, the estuary has lost its capacity to absorb more dredge material. Added to this, the problems of disposing of polluted sediment, limited disposal sites, and ecologic side effects make ocean disposal seem an attractive alternative.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC