The filling in of river channels, harbors, and estuaries by river-borne sediments is a relentless process, and one that brings with it a high cost to society. Keeping channels and harbors navigable for commerce and defense can require constant dredging, or some means of otherwise diverting sediment or keeping it moving away from channels and harbors. The price tag for this activity in the United States is about a half billion dollars a year—and it is increasing.

In response to a request by the U.S. Navy to assess what is known about sedimentation processes—transport, flocculation, settling, shoaling, and erosion—and ways to lessen the financial burden of maintaining navigable waters, the Marine Board of the National Research Council initiated this study. Reducing the sediments associated with navigational facilities, especially access channels and berths around piers, is an issue of serious national importance. Yet the principal effects of sedimentation are manageable, even though the choice of techniques may be complicated by existing facilities, competing design objectives, and environmental considerations.

In 1985 the Marine Board agreed to a request of the U.S. Naval Facilities Engineering Command to appraise sedimentation control technology and practices to reduce maintenance dredging of navigational facilities in estuaries. A committee representing a broad spectrum of expertise was appointed and directed to assess the state of knowledge of sedimentation processes and its application to the problem of maintaining adequate water depths in ports. (See Appendix A for members’ expertise.) The committee was asked to consider three issues:

1. The state of knowledge of sediment types, transport, circulation patterns, and rates of deposition in estuaries and tidal inlets to support the design and installation of navigational facilities for minimal maintenance dredging.

2. The state of technology and practice in sediment management techniques and mitigation systems.

3. Actions needed to improve the state of knowledge, technology, and practice in the management of sedimentation.

During its initial discussions, the committee recognized that assessment of management techniques and mitigating systems had to include review of maintenance dredging management and techniques, because in existing harbors improved maintenance dredging techniques may be the only practical means for reducing costs and maximizing readiness. Furthermore, even if facilities are modified to achieve minimum maintenance dredging, periodic dredging may be required and should be accomplished using the most effective technology. The three issues were viewed from overall sediment management and technical perspectives, recognizing that a huge investment may be required in order for existing and new facilities to accommodate deeper draft vessels in the future.

The committee convened a symposium that included leading practicing engineers and academicians to obtain pertinent information. (Symposium participants are listed in Appendix B.) Part I of this report presents a synopsis of the papers presented at the symposium, it also contains the committee’s conclusions and recommendations, which are based on the information presented at the symposium and on the experience of the committee members. While the committee’s conclusions and recommendations are not the product of a more comprehensive technical assessment than that presented herein, the committee believes its advice has a high level of validity. Part II is a record of the symposium proceedings. Although the committee recognizes that the papers in these proceedings may contain some duplication of material, they nevertheless offer a wealth of information for federal, state, and port decision makers.

The committee gratefully acknowledges the generous contribution of time and information provided by the government liaison representatives and their agencies, and the many individuals who participated in the data gathering process. Special thanks are extended to all symposium, participants, both speakers who presented papers and participants who contributed to a stimulating and productive exchange of information. Our appreciation is also extended to the U.S. Army Corps of Engineers and the U.S. Naval Facilities Engineering Command for identifying this project as an issue of national importance to be addressed by the Marine Board.

Every year in the United States about 497 million yd³ of material are dredged by the U.S. Army Corps of Engineers and private operators to create and maintain navigable waterways and harbors. Without such activity, the natural processes of sediment deposition and shoaling would make many waterways and port facilities impassable by most large commercial and defense vessels. The process of keeping these waters passable is an expensive one—approximately $500 million dollars annually (in 1986 dollars). In addition to the financial cost of dredging, the disposal of dredged material has become more difficult with the passage of laws to protect the marine environment and the consequent dwindling supply of suitable places to put it. Thus, consideration of alternatives to dredging has become an issue with serious implications for this nation’s maritime trade and naval installations.

This report addresses the state of knowledge about sediment sources and transport processes; dredging procedures, equipment, and dredged material disposal; and considerations for modifying existing facilities and designing new ones to reduce maintenance dredging. The committee members concluded that despite aspects of sediment transport common to many locations, each harbor, channel, and turning basin is unique, and design or modification to reduce deposition rates be the subject of site-specific study. Three options can be investigated—

• stopping the sediment before it reaches the site,

• keeping the material in suspension as it pases through the site

• diverting the sediment flow away from the site.

In its assessment of sedimentation processes the committee concluded that present knowledge can be applied to many existing facilities to reduce sedimentation rates, but that modifying or relocating facilities can be costly. They recommended that sedimentation and sediment management should be considered in the process of selecting sites for new facilities and in planning, design, construction, and operation. Successful application of current knowledge will require field studies. More and better hydrographic data are important for advanced planning.

The committee noted that increasingly restrictive environmental regulations and dwindling disposal sites are putting greater pressures on dredging activities. They therefore recommend that the Corps of Engineers, together with the Environmental Protection Agency and interested agencies, develop long-range plans for managing dredged material disposal for all coastal and estuarine navigation facilities in the United States. Consideration of environmental effects—as well as beneficial uses of dredged materials, such as beach nourishment—are important aspects of such a planning process.

Finally, the committee recommends additional research in the areas of estuarine sediment transport processes, development of new sedimentation control methods, and development of design capabilities—such as numerical models—of sediment transport. The cost of a research and development effort, the committee noted, would be a minute fraction of the nation’s dredging bill and would be an investment repaid in cost savings and environmental protection.