RIASSUNTO
ABSTRACT
Sidecast dredging has been employed annually in the New England region for the past four years. Operation of the dredge is restricted to inlet channels and ocean sand shoals. Physical and chemical analysis of sediment samples from eighteen navigation projects confirm that the dredged material consists primarily of clean fine to coarse sands. Some channels, however, are characterized by eelgrass patches with subsequent higher percentages of fine grain sediment. Preliminary studies by the New England Division Army Corps of Engineers have concentrated on evaluation of dredging effects on . turbidity, dissolved oxygen and mobilization of trace metal constituents. Four projects have been selected for comparison of pre- and postdredging impacts on benthic communities.
INTRODUCTION
Sidecast dredging has been used extensively by the U. S. Army Corps Wilmington District since 1964 and at Barnegat Inlet, New Jersey, since 1967. The need and economic benefits which led to the development of this dredging method are described by Long.1 The function of this dredge type is to maintain as frequently as necessary shallow inlets where rapid shoaling has reduced authorized depths or otherwise impeded safe navigation.
The prospects for local application of sidecast dredging in New England waters were first introduced in 1971. Eleven Federal small boat navigation projects were originally selected for experimental operation. These included from Massachusetts: Newburyport Harbor, Chatham Stage Harbor, Andrews River, Menemsha Creek and Cuttyhunk Harbor; Hampton Harbor, New Hampshire; Pig Island Gut, Wells Harbor and Saco and Scarboro Rivers, Maine; and Little Narragansett Bay, Rhode Island-Connecticut. In the past two years sidecast dredging was proposed for an additional six projects (Figure 1).
The development of a maintenance program utilizing this dredging method evolved around using the following criteria and assumptions:
Areas of operation are limited to outer ocean bars, shoals and coastal inlets.
The shoaling rate and effects of shoaling in these channel environs are such that frequent dredging by conventional means is not practical or economically feasible.
The dredged material is composed primarily of sand containing little or no harmful pollutants.
Deposition of the dredged sediments in areas adjacent to the channel will have no long-term adverse effects upon water quality or marine biota.
The proposed employment of sidecast dredging in New England was assailed vigorously by state and Federal environmental agencies. The objections encountered were not entirely without merit as many of the project sites chosen are characterized by relatively strong tidal currents and have adjoining shellfish beds, beaches and productive wetlands.
It was the concensus of the regulatory agencies that turbidities and siltation from sidecast operations would seriously disturb a much greater area than either hydraulic or bucket methods. Of particular concern was the threat that dredging operations would create conditions which might inhibit migratory activities of anadromous fish species or cause disposed material to drift into areas supporting valuable shellfish resources. Further, it was thought that the currents might transport the suspended sediments back into the channels or inner harbors creating secondary shoal formations.