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EXECUTIVE SUMMARY At the request of the U.S. Army Corps of Engineers, the Panel on Criteria for Dredged Depths of Navigational Channels of the Marine Board appraised the criteria used in the United States to design the depths of dredged navigational channels and to specify overdredged depths. Rules of Thumb The criteria used by the U.S. Army Corps of Engineers are empirical--rules of thumb. For design depth, or underkeel clearance, the rule is to select the design ship, add its draft + squat* (3 ft) + rolling and pitching allowance (estimate) + clearance (2 ft for soft channel bottoms; 3 ft for rocky or hard bottoms). The Corps' criteria recommend model tests and site evaluations. Nevertheless, the references cited for assistance in determining squat and other characteristics of the design ship (aside from consultation with ship owners and local pilots) are at least 20 years old, some more than 30. Selection of the design ship is critical. While the regulations *Sinkage of a ship in motion E-1
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E -2 indicate that the choice of design ships) should be made for safety of the largest expected ships, for economy, and for accommodation of projected trends, completing the process required for federal navigational projects takes 15 years to 25 years. So long a process does not allow for changes in ship design or technology. The criteria for overdredged depths (for ensuring that specified dimensions are achieved and for advance maintenance dredging to reduce dredging intervals) have evolved from experience in the district offices of the Corps. Recently promulgated criteria update these historical rules of thump or formulas and suggest more frequent surveys of channel dimensions. In most districts, surveys are no more frequent than semiannual. Navigation Data necessary for an adequate criterion of underkeel clearance are sparse, but model tests, full-scale trials, and computer simulations have increased understanding of ship behavior in open and confined channels and in the entrance channel to a port or harbor from the open ocean. Generally, vertical excursions may be large in the unprotected waters of a harbor entrance; squat is much increased by decreasing water depth and in maneuvers such as turning, passing, and overtaking. Squat in narrow channels is greater than that in open or wider channels. While channel width has been shown to be more important than depth in providing for ship controllability, the improvement depends on sufficient underkeel clearance. Although small depths of water under the keel tend to dampen ship movements, tne possible effects of resonance among the complex variables preclude a general rule. As ship speed influences squat and maneuverability, the speed assumed for channel design is the minimum at which effective steering is possible. The results of experiments and models, and the several relationships they suggest, offer useful means for evaluating the adequacy of existing channels. The panel used them in a simple computer analysis of nine major dredged channels* of the three coasts of the United States. The panel's choices of design Snips and ship speeds for each channel were based on moderate assumptions, but the underkeel clearances in these channels were shown to be inadequate. Ships of greater draft are using the channels, and any increase in speed over those assumed will result in greater squat, further reducing underkeel clearances, and increasing the probability of grounding or striking bottom. The panel emphasizes tnat these results _ *Calcasieu River (inner and outer channels), Lower Columbia River, Delaware River, Mobile Harbor, Norfolk Harbor, Oakland Harbor, San Francisco Bay, Thimble Shoal, and Galveston Channel
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E-3 are more indicative than definitive. A thorough, site-specific understanding of a channel and its environment is necessary for complete assessment of a channel's adequacy. Sedimentation a Data for control of sedimentation and shoaling by dredging are also site-specific. Essentially, two mechanisms of sediment movement may affect navigational channels in harbors and estuaries: interior and exterior. Interior mechanisms deliver sediment to a harbor from freshwater inland sources; exterior processes carry coastal sediment into a channel or harbor. Dredging may significantly alter existing relationships. While there are probably an optimum overdredged depth and an optimum dredging interval for the average conditions of channels, they may not be constant, owing to the flux of the variables. A major storm can undo all calculations. Examples studied by the panel indicate that the criteria for the dredged depths of harbor entrance channels (and perhaps for other channels as well) are dominated by the uncertainties of deposition and the irregular time Interior mechanisms deliver sediment to a harbor from constraints imposed by financial and institutional arrangements--not by the maneuvering requirements of ships. "Nautical Depth" Concept The results of research in the Netherlands preceding and following construction of the very deep draft channels and harbors of Europoort/Rotterd~m could be applied to silty channels in the United States. Ascertaining the channel bottom is difficult when fine suspended sediments are in motion or settling. Dutch experience indicates that while the maneuvering characteristics of a ship change, it can navigate through suspended sediments to specific gravity of 1.2 ("fluid Rudy. This "nautical depth" concept depends on frequent surveys for navigational safety. Density meters are used in weekly surveys in the Netherlands to determine the areas to be dredged ~ where specific gravity exceeds 1.2 above design depth>, in connection with a conventional bottom-contouring echo sounder. Overdredged Depths Although the rules of thigh f or overdredged depths to achieve the specified dimensions of a channel have evolved and become f ixed, they are within the order of accuracy of various dredging processes. For some locations, they may need to be adjusted, and more accurate pre- and post-dredging surveys may need to be conducted.
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E-4 Criteria of International organizations, Other Maritime Countries, Shippers Criteria for channel depths developed by international organizations and those used in other maritime countries have been established in concert with the evolving body of data, tests, and experience. Those of the Permanent International Association of Navigation Congresses (PIANC) have found application in the ports and harbors of many maritime nations and are included by reference in the standards of the International Association of Ports and Harbors (IAPH). PIANC criteria are for gross underkeel clearances in confined channels of approximately 15 percent of ship draft (the figure found to allow for vertical excursions), plus additional depths to allow for sounding accuracy, for sedimentation between two dredging intervals, and for dredging inaccuracies. This last figure varies from country to country: those making an allowance for dredging inaccuracies specify amounts close to the allowances used in the United States, but some countries allow none. A statistical method has been developed by shipping interests that combines the values and uncertainties of all the factors known to be most important to the required underkeel clearance of ships to produce an acceptably small probability of grounding. Owing to the scarcity of data, the uncertainties for some factors are large, but these can be updated as information becomes available. This statistical method has been used to specify the design depths of at least one port (Zeebrugge, Belgium, under construction). Conclusions and Recommendations In light of its reviews and appraisals, the panel concludes that the physical environment and characteristics of each port are unique and that criteria for channel depth, as well as the specification of overdredged depths, must be site-specific. Nevertheless, general criteria provide a useful first approximation for design and practice, and basic standards of adequacy. The general criteria are old and need to be updated, but their inadequacy is determined more by institutional constraints than by their content. The very long tine between initial studies and completion of the approval process in the United States for major navigational projects f ar exceeds the half-life of the world fleet. Channel improvements of the future will almost certainly be for the ships of the past. Immediate steps need to be taken to reduce the time between identification of physical and other constraints against major dredging projects and the tome work can begin. The panel recognizes that depth cannot be considered in isolation from other dimensions and features of channels, of ships, and of ships in the channel but notes that underkeel clearances in the channels of 1
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E-S the United States are less than those of other countries, and much less than those recommended by international organizations. They may be as little as 2.5 percent of ship draft. Moreover, practices in the United States tend to rely on historical data to determine dredging frequency, rather than frequent channel surveys. The panel suggests that o The recommendations of PIANC be substituted for the general criteria now used in the United States for channel depths (and other dimensions)--these recommendations reflect the results of recent research and represent international consensus; o Specific design criteria for particular channels be based on the results of comprehensive and detailed site studies; o o Frequent surveys be made in known, high-shoaling areas; The nautical depth concept be adopted for silty channels of the United States; and o Better information about operational practices and the hydrodynamic behavior of ships be collected and incorporated in channel design.
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