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5 CONCLUSIONS AND RECOMMENDATIONS Conclusions General 1. The uniqueness of the physical environment and characteristics of each port must be appreciated. One factor or some combination of factors such as swell, wave climate, tides, or currents may dominate in a particular location. Thus, criteria of channel depth are site-specific. Overdredging depth considerations are also site-specif ic . 2. General criteria have been developed to provide guidelines and procedures: those of the United States can be improved. Institutional Factors 3. The characteristics of design ships which have been used to determine the dimensions of channels in the United States no longer correlate with the characteristics of ships that use them. 5-1

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5-2 4. Criteria for the dredged depths of navigational channels are strongly inf luenced by the uncertainties of sediment deposition and the irregular time constraints imposed by financial and institutional needs and procedures. 5. The tome lapses of 15 years to 20 years between initial studies of navigational projects and the initiation of work do not allow for changes in ship technology. 6. Despite skills of pilots and improved aids to navigation, most major navigational channels have experienced such increases in traffic and changes in ship characteristics that the channels no longer offer the underkeel clearances for which they were designed, or those recommended by international organizations. _ Underkeel clearances in the United States are as little as 2.5 percent of ship draft; international criteria specify a minimum of 10 percent to 15 percent of ship draft. Design of Depth 7. The ratio of water depth to ship draft is important in navigation: course stability is enhanced with decreasing ratio of water depth to ship draft--e.g., 1.2 or 1.1--but turning performance is much reduced. 8. Depth cannot be considered in isolation f rom other dimensions and features of the channel, of ships, and of ships in the channel. 9. The "nautical depth" concept is important to defining tne usable depth of "silty " channels . 10 . Physical models and full-scale trials, particularly for passing, overtaking, and turning, assist understanding of ship behavior, and the results also assist in mathematical modeling and in simulation. 11. There seems to be little exchange of inf oblation or correlation between ship owners, pilots, and ship channel designers in the design of channels. Surveys and Overdredged Depths 12. In the United States, the practice is to rely on historical data rather than channel surveys to determine dredging frequency (except for emergencies). European practice is to survey certain channels more frequently, e.g., weekly. 13. Dredging accuracy appears to be specified overdredging depths. of the same order as

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5-3 Recommendations I. For adequate channel design, intensive site studies must be made, and the design criteria based on the results of the studies. 2. It is suggested that the recommendations of ICORELS* (Working Group IV of PIANC**) be substituted for the several criteria now used for channel depth and underkeel clearance in the United States. 3. It is recommended that immediate steps be taken to reduce the amount of time between identification of physical ~ and other) constraints against major dredging projects and the time that work can begin. 4 . Frequent surveys should be made in known, high-shoaling areas . The nautical depth concept should be adopted for silty channels of the United States. 6. Better information about operational practices and the hydrodynamic behavior of ships needs to be collected and incorporated in channel design. *International Commission for the Reception of Large Ships **Permanent International Association of Navigation Congresses

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