INTRODUCTION
Maintenance dredging of navigation and port facilities in estuaries is a continuing and unrelenting burden. Most operators of ports in estuaries suffer interference with the use of facilities during dredging operations, in addition to ongoing costs of dredging. This burden has been exacerbated during the last two decades by increasing environmental restrictions on dredged material disposal. Longer hauls to a very limited number of disposal sites, expensive onshore disposal, and long delays in processing permit applications add to the problem of maintaining navigable water depths.
Navigable waterways in estuaries are vital to the economy and defense of the United States. As a nation we face increasing competition in world markets from other industrialized nations, and to successfully compete requires production of goods and their transportion to offshore markets at competitive costs. Economies of scale resulting from the use of large, deep-draft ships can contribute significantly to this nation’s competitive edge. In addition, the United States Navy is constructing weapons systems requiring navigable waters as deep as those needed for the largest commercial vessels. Continuous access and egress of Navy units to and from port facilities are essential to national defense. Further development of deep-draft navigation facilities and provision of technology for their continuing maintenance are essential to our future prosperity.
It is apparent from these considerations that means for reducing the rate of sedimentation in navigation facilities, for reducing the costs of dredging, and for long-term disposal of dredged material are sorely needed. Reducing sedimentation rates in navigation facilities is an attractive prospect because it diminishes the need for dredging. Most navigation and port facilities in the United States were constructed long before the fine sediment transport and deposition processes in estuaries became known, and many are designed in ways that exacerbate sedimentation. Costs to modify existing facilities can be very high; therefore, to be useful, modification of these facilities aimed at reducing sedimentation must cost less than the resulting decrease in maintenance dredging costs. The design of new facilities, however, provides opportunities to apply current knowledge to minimize sedimentation rates. This will become increasingly important as the
federal contribution to dredging costs decreases and local costs increase as the result of enactment of P.L. 99–662 (effective April 1, 1987). Under the new cost-sharing formula, local contribution to dredging of most ports may be as high as 60 percent of the total cost (depending on depth dredged). These cost were previously borne by the federal government.
Continued use of existing facilities, and the limited efficacy of measures to reduce sedimentation in new facilities in some locations, will prolong the need for maintenance dredging indefinitely. Reduction of the costs of maintenance dredging, even without reducing the rates of sedimentation, has been accomplished through improvements in dredging technology and by modifying the waterway. This approach warrants further consideration.
Sediment production is a geologic process that will continue forever, on a human time scale. Thus, the need to establish means for long-term disposal is clearly evident. Successful development of means for continuing disposal of dredged material depends on the use of procedures that cause minimum real environmental stresses together with regulation that is based on knowledge of the environment and actual stresses, and recognizes the value of operational navigation facilities. Each of these issues must be addressed to achieve optimum maintenance operations.
Knowledge about reduction of maintenance dredging, dredging costs, and dredging impacts is dispersed among port and waterway engineers and academic researchers. Information has not been organized to assess the prospects for feasible reductions in maintenance requirements and maintenance costs with available knowledge, or to identify research and development needed to advance the design of facilities and the management of maintenance dredging. The material presented in this report represents the assembled contributions of leading practicing engineers and academic researchers that were presented in the symposium, “Sedimentation Control to Reduce Maintenance Dredging in Estuaries,” held in Washington, D.C. on July 8–10, 1986. Session topics were sediment sources and transport processes, existing facility modifications, optimizing dredging procedures, dredged material disposal considerations, and new facility design considerations. Summaries of these sessions and the conclusions derived therefrom constitute the body of this report. All of the papers presented at the syposium are included to document the proceedings and to provide detailed references.
