OPENING AND MAINTAINING TIDAL LAGOONS & ESTUARIES

Abstract

Sedimentation in tidal lagoons and estuaries has become an acute problem, particularly where there has been over-dredging to accomodate today's deep draft vessels, 0(15m). The rise in mean sea level, which created natural estuaries, has not kept pace with the demand for greater draft ships. The limited number of navigable lagoons confronting a growing demand for more inland waterways has encouraged reconstruction dredging of some closed or partially filled lagoons. Man's efforts to deepen existing or relict estuarine systems have disturbed the steady state equilibrium of the systems. Sedimentation acts continuously to restore this equilibrium by either of two,processes. One is floculation of fine-grained fluvial born sediments, a process accelerated when an estuary is deepened thereby allowing greater salt wedge intrusion. The other is the interception of the longshore transport of coarse grained beach sediments by the lagoon inlet. The larger tidal prism of an enlarged lagoon draws a greater percentage of the longshore transport into the lagoon from the adjacent surf zone. In the absence of wave suspension within the lagoon, very .little of this sediment is carried back out of the lagoon on ebbing tides. These sedimentation processes confront the coastal engineer with several distinct problems. A particular set of counter-measures are needed against accumulations of cohesive fines from the rivers, another set against accretion of cohesionless coarse grained sediments from the beaches. Still other methods are needed to reopen a closed lagoon. Dredging has been the most widely practiced solution to all of these problems for the past 150 years. It is a solution we may not be able to afford indefinitely. In addition to intrinsic high rates of energy consumption and equipment breakdown, there are ever-growing costs associated with dredge spoils disposal. Ninety percent of the material annually dredged is contaminated by heavy metals, concentrated in the floes from either natural erosion of country rock or industrial sources (Malloy, 1980).