Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
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
OCR for page 2
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
OCR for page 3
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.
OCR for page 4
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
OCR for page 5
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.
OCR for page 6
Representative terms from entire chapter:
international organizations
