Dredging Coastal Ports An Assessment of the Issues (1985) / Chapter Skim
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8. Assessment of Technical Considerations and Needs to be Met in Dredging U.S. Ports
8. Assessment of Technical Considerations and Needs to be Met in Dredging U.S. Ports
Pages 95-116
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From page 95... ...
ENGINEERING DESIGN OBJECTIVES FOR DREDGED NAVIGATIONAL FACILITIES There are two important design objectives for navigational £acilities-accommodating the maneuvering requirements of vessels, and reducing as much as possible the future maintenance dredging required. Some general considerations of the vessel in the waterway and sedimentation are briefly described in succeeding subsections.
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From page 96... ...
. Bank Effects While water flow past the sides of the hull is symmetrical if the vessel is on the channel's centerline and aligned with it, moving off the centerline will decrease the flow area between the vessel and the near bank, causing the flow rate on that side to accelerate, with corresponding loss of pressure.
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From page 97... ...
Modelling of navigational channels usually assumes uniform side slopes and unvarying bottoms, but general and local conditions usually favor rapid shoaling on one or another side of a bend or turn, or formation of a spit that encroaches on the channel at breakwaters or jetties, with the result of narrowing the width or reducing the depth of channels in locations where width and depth are most critical to
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From page 98... ...
Successful shiphandling by a pilot in navigational channels demands smooth, skillful integration of several very important elements: directing vessel movements; assessing other traffic movements in meeting and overtaking, as well as crossing traffic; evaluating waves and surges created by the ship; assuring that the helmsman clearly understands and executes rudder commands and steering directions without error; analyzing radar information; knowing the magnitude and effects of currents, wind, the hydrodynamic interaction of ship and channel; and anticipating possible changes in high-shoaling areas. Harbor piloting in ports of the United States is typically of foreign vessels, of unknown maneuvering characteristics, designed and equipped primarily for the deep ocean.
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From page 99... ...
Criteria for Dimensions of Dredged Navigational Facilities General guidelines for the dimensions of dredged navigational facilities have been developed taking into account the over-all vessel maneuvering requirements described in preceding subsections. In the United States, the guidelines are developed by the U.S.
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From page 100... ...
Many of the projects, however, represent minimal improvements for existing vessel traffic: the design basis assumes, for example, that design vessels will not be fully loaded, or width calculations are minimal, assuming tug escort. In general, many proposed improvements are for relatively modest sizes of vessels (which may or may not be appropriate)
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From page 101... ...
To gain an understanding of the operators' views of navigational facilities in U.S. ports, the technical panel sent a questionnaire to the pilots organizations (Appendix C)
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From page 102... ...
DESIGN OF NEW CONSTRUCTION DREDGING PROJECTS FOR MINIMAL MAINTENANCE DREDGING As pointed out in Chapter 9, thorough understanding of local tidal hydraulics and circulation is necessary to design dredged navigational facilities for minimal shoaling (see also Marine Board, 1983~. Site-specific hydrographic surveys, measurements of currents, and an understanding of existing patterns of sedimentation in the port are all necessary; in addition, a physical model can be a helpful tool in assessing interactions of the facility with currents and (possibly)
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From page 103... ...
shows the annual average maintenance dredging costs for each port. As the total approaches a half-billion dollars a year, reducing the sedimentation associated with navigational facilities, and achieving the lowest-cost maintenance dredging program are important dredging needs.
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From page 104... ...
As a result of these and other in-channel forces, the channel ages and changes shape, with corresponding shifts in areas and rates of sedimentation. Thus, determining an effective maintenance dredging program in a particular port depends on a great deal of historical and current local knowledge, and frequent hydrographic surveys.
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From page 105... ...
indicates that every developed maritime nation funds the new construction and maintenance dredging of its major navigational facilities, and that every nation is concerned to keep down the costs. The report recommends that (where appropriate)
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From page 106... ...
Contractor Corps of Engineers Total Federal contracts $331 $86 $417 Private contracts 110 0 110 $441 $86 $527 SOURCES: Federal contract dollars from U.S. Army Corps of Engineers.
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From page 107... ...
Having applied average price per cubic yard to work performed under federal contracts to estimate work in the private market, a note of caution needs to be added about making dollars-per-yard comparisons between the contractor fleet and the Corps fleet. Comparisons using annual averages or totals are virtually meaningless owing to differences in types of projects, measurement of yards dredged, and equipment utilization rates.
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From page 108... ...
2 Hopper Dredges Dustpan Dredges Sidecaster Dredges Special Purpose Dredge Total 4 3 l 13 One cutter dredge is scheduled for retirement during fiscal year 1985. The total does not include a number of small two-man cutter dredges which have very low utilization.
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From page 109... ...
Position Indicator System This type of indicator provides the operator with immediate visual indication of the position of the suction pipe, depth of the suction head and the angle of the lower part of the suction pipe in both the horizontal and vertical planes.
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From page 110... ...
Automatic Draghead with Winch Control System The automatic draghead winch controller was developed to regulate the movements of the suction pipe and draghead throughout the dredging cycle. It is programmed to swing the pipe outboard, to lower the pipe, and, in conjunction with the swell compensator, maintain the correct pressure of the draghead on the bottom.
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From page 111... ...
Individual automatic systems that have already been developed are vacuum-relief valve, bypass valve, automatic draghead winch controller, automatic light mixture overboard, and draghead visor controller (Van Zutphen, 1983~. For example, the automatic suction pipe controller moves the suction pipe.
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From page 112... ...
, but obsolete dredges. In 5 years, the dredging industry invested more than $250 million in efficient, productive hopper dredges.
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From page 113... ...
None of the existing authorizations for dredged navigational channels is as recent as the advent of large dry-bulk carriers or the latest-generation containership. The lack of timely improvements places the burden on local pilots, the ports, and U.S.
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From page 114... ...
The dimensions of dredged navigational facilities in the U.S. appear minimal for the vessels now using them, and emergency anchorages are small or lacking.
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From page 115... ...
(1983) , "A Nuclear Silt Density Meter," Dredging Engineering Short Course Notes, Texas A&M University, College Station, Texas.
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From page 116... ...
, Engineer Manual: Hydraulic Design of Deep-Draft Navigation Projects, EM 1110-2-1613 (Washington, D.C.: U.S. Army Corps of Engineers)
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