Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings (1987)

The information developed during the symposium and the committee’s deliberations led to the following responses to the charges to the committee:

Issue 1. Is knowledge of sediment…adequate to support the design and installation of navigational facilities for minimal maintenance dredging?

Knowledge of sediment types, transport processes, and circulation patterns available today can be effectively applied to the design, installation, and operation of new facilities that will have minimum sedimentation rates or minimum maintenance dredging costs. The minimum sedimentation rates obtained, will depend on the location of the facilities in an estuary, their configuration and orientation, the amount and characteristics of sediment supplied to the estuary, the hydrology of the watershed, and tides, winds, and estuary configuration. Techniques for minimizing sedimentation rates include the installation of harbor enclosures to exclude local waters having high concentrations of suspended sediment, configuring and orienting facilities in open waters to cause minimum rates of aggregation of suspended particles and minimum sheltering of adjacent navigable waters, and designing bed cuts for channels and wharf approaches to maintain scouring shear stresses during the strength of tidal flows. These features can often be included in the design of new facilities at little added cost. Selecting a site for a new facility that has low concentrations of suspended sediment or currents that are sufficient to erode material that deposits during slack periods is especially important for minimizing sedimentation rates.

Application of present knowledge to modify existing facilities in order to reduce sedimentation rates or maintenance costs is restricted by the cost of modifying installed works or relocating facilities. Modifying dredged cuts and constructing enclosures have successfully reduced sedimentation rates in existing facilities. Significant lowering of maintenance dredging costs can be achieved in some instances now by selecting appropriate equipment, advanced maintenance, and by relocating the shoaling region. There are emerging structural and mechanical means for reducing sedimentation rates in existing

facilities in the field testing stage, including jet arrays, tethered hydrofoils, and moveable curtains.

Sedimentation control measures must be designed for each individual facility according to the local conditions. Effective design requires field studies and possibly model studies. Capabilities for the application of existing knowledge to such design are available now in federal laboratories and private engineering firms. Both are supported by knowledge contributed by university research.

Issue 2. What is the current state of technology and practice in management techniques and mitigating systems?

The current state of technology in management techniques and mitigating systems ranges from techniques that have been demonstrated in practice to experimental techniques and conceptual approaches. Demonstrated techniques for minimizing sedimentation rates include basic structural approaches to promoting the passsage of suspended sediments through facilities that are located in open water, and to limiting the entrance of sediment-laden water into harbor enclosures. Experimental techniques being tested at field scale in existing harbors include jet arrays, moveable curtains, and tethered water foils.

Dredging plant is available to perform efficient operations in essentially all situations. Costs of maintenance can be minimized by selecting the most appropriate equipment for a project, and often by doing advanced maintenance dredging to increase the interval between dredgings. Virtually all dredging contracts are awarded to the lowest bidder, even though selection of optimum equipment will reduce costs. Government contracting procedures do not make it easy to select contractors that operate the most modern equipment, which is efficient, produces little turbidity, is fully instrumented and partly automated. Selecting the optimum equipment for the job can also ameliorate some environmental concerns about dredging, along with scheduling operations so that they do not occur during environmentally sensitive times.

Altering the physical characteristics of an estuary to cause shoaling to occur where it is most economical to remove and where it will interfere least with navigation is a viable means for reducing maintenance costs in some estuaries.

Equipment is available to perform hydrographic surveys rapidly and economically. Frequent and accurate hydrographic surveys provide an excellent means for determining the locations and rates of channel shoaling and for indicating remedial measures.

These presently available techniques and management approaches could be more widely applied.

Issue 3. What actions are needed to improve the state of knowledge, technology, and practice in management of sedimentation?

Some maintenance dredging in navigation facilities in an estuary will always be required. Land disposal sites have limited capacities

and increasing costs. Present restrictions on open-water disposal significantly elevate maintenance costs. The development of long-term plans for sediment disposal, based on actual environmental effects and economic considerations, is urgently needed in most estuaries in the United States.

Knowledge of basic sediment transport processes is vital to the successful design of sedimentation control works and management strategies. Further research on estuarial sediment transport processes is needed to improve the accuracy of predictive models used for design and to identify new means for controlling sedimentation rates. In particular, research is needed on the response of muddy beds to wave action, estimation of aggregate settling velocities under various physical conditions, the interactions of highly concentrated suspensions with waves and currents, and the transport behavior—including deposition and erosion—of mixtures of cohesive and cohesionless sediment particles. In addition, knowledge about the chemistry and physics of grain size distribution, mineral behavior and flocculation characteristics in cyclic currents in three-dimensional salt- and freshwater is needed.

Continuing development of instruments to reliably and continuously measure flow fields and concentrations of suspensions is needed for field studies of sediment transport.

Sediment supplies and freshwater discharge to an estuary are episodic events that vary from year to year. Circulation of sediment in an estuary is typically distributed over spring and summer. Records of inflows and bed configurations over a number of years are needed for siting of new facilities and for designing sedimentation control works and dredging programs. Also needed are data acquisition programs that include more frequent suspended solids and discharge measurements during episodic high flow events than are available now, and regular hydrographic surveys of estuaries. Hydrographic surveys of navigation works that suffer undesirable sedimentation rates should be made at intervals of a year or less.

Additional field testing of newly developed methods for reducing sedimentation rates is needed in order to evaluate large-scale and long-term effects. Testing is also needed to evaluate the environmental impacts of the methods, such as movement of benthic larvae and effects on dissolved oxygen and turbidity.

The findings of this study lead to the following recommendations:

1. The U.S. Army Corps of Engineers, together with the Environmental Protection Agency and interested agencies, should immediately begin the development of long-range plans for management of dredged material for all coastal and estuarine navigation facilities in the United States. These plans should address all methods of disposal, including beneficial uses, and should include consideration of actual

environmental effects, as determined by the Corps’ Dredged Material Research Project, and economics of waterway maintenance. These plans should be implemented as soon as possible.

2. Consideration of sedimentation and sediment management should be included in the selection of sites for new facilities and in their planning, design, construction, and operation. Successful application of knowledge of sediment transport to the completion of facilities that have minimum sediment management costs will require field studies to provide the necessary data. It is especially important that consideration of sedimentation be included in the selection of the site.

3. Available knowledge concerning modifications to reduce sedimentation rates and maintenance costs can usefully be applied to improve operations of existing ports. Evaluation of sedimentation processes and feasibility of works or management options for reducing sedimentation rates and maintenance costs in facilities that suffer sedimentation problems should proceed now. The evaluation should include operational, environmental, and economic benefits of sedimentation management measures, and such measures should be installed where the benefits are sufficient.

4. A research program, on the scale of the Dredged Material Research Project, should be established to develop additional knowledge of estuarial sediment transport processes, to develop and test new sedimentation control methods, and to develop design capabilities including more accurate numerical models of sediment transport. This program should include participation of universities, government laboratories, and military and private port operators. It should be funded for successive five-year periods with reviews at the end of each period. The cost of such a research and development effort would be a minute fraction of the nation’s dredging bill and would be repaid by environmental and economic savings in perpetuity.