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OCR for page 13
HARBOR/PORT ENTRANCE DESIGN
Eugene H. Harlow
_ _ .
The design of entrances to ocean harbors, like most design problems
in engineering, is an exercise in achieving a compromise among
conflicting aims. In the open ocean, a vessel has virtually unlimited
space to maneuver. Collision with land is not a hazard, but the ship
may be buffeted by waves and swells, shrouded in rain" and fog, blown
off course by wind, caught in transverse currents or endangered by ice
floes or icebergs. Approaching a harbor, all these factors may still
be present, but to gain shelter from the hazards of the sea, the vessel
now has to follow an accurate route, avoiding collision or grounding on
the shores, or on the very breakwaters that provide the shelter.
No two harbors are alike. Each approach pits the skill of the ship
captain or pilot against the natural forces that prevail as the vessel
moves closer to the obstacles it must at all costs avoid. The contrast
between the safety soon to be reached in the harbor and the hazard to
the ship's hull in traversing the entrance can hardly be more
chilling. The presence of large rocks or irregular masses forming a
breakwater -- ideally suited to ripping a jagged gash in a ship-- form a
narrow slot through which the vessel must pass in order to reach quiet
water and a place to unload its cargo or its passengers. The slot must
be narrow in order to exclude wave energy, but it must be wide enough
to allow ~safe" entry.
"Safe" is a subjective term that depends on judgment. The harbor
designer, the port agency, the ship captain, and the pilot may have
differing views about the safety of a harbor entrance--views affected
by the weather, the alternative harbors that may be available, and the
time constraints under which the vessel operates.
To design a harbor entrance, assuming the harbor is not a natural
one that needs no man-made props, one must of course consider the types
of vessels that will enter or leave it. For example, in the days of
sailing vessels, a harbor entrance could not be lined up with a
strongly prevalent wind direction, else ships could seldom enter or
leave it. Today, the channel dimensions must be large enough to pass
the largest "hip expected to call at the port, despite the possibility
that these dimensions may require a wide opening between breakwaters,
admitting more wave energy than desirable or needed for smaller
vessels, and despite the sedimentation that may occur at an accelerated
rate in the deeper channels needed for the larger ships.
· -
13
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14
l
_
VEER
ANGLE
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DREDGED
CHANNEL
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Figure 1. Change of course necessary
to steer into current at harbor entrance.
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Figure 2. Placement of buoys between those
marking changes in direction reduces pilot error
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15
Navigation in restricted waters remain' more an art than a
science. As the training of pilots becomes mare sophisticated,
simulation devices are frequently built to aid in the training
process. Yet so far, man's visual perception of movement is the
ability upon which we must ultimately rely in steering a vessel and in
regulating its propeller speed to accomplish a passage between the
obstacles represented by a harbor entrance.
Perception is almost entirely the controlling element in
negotiating curves. Even on a straight course, the harbor designer
must be aware of the pilot's predicament in countering the varying
-effects of environmental forces on his vessel.
When entering a harbor under adverse conditions of current and
wind, as illustrated in Figure 1, there must be a change in direction
or rudder angle to steer a vessel on a straight course.
The ship must be steered at some angle into the current and wind to
compensate for a varying lateral force, if it is to remain on a
straight course that will maintain adequate clearance between the ends
of the harbor breakwaters. The current is likely to be variable, and
may be stronger near the ends of the breakwaters than in the sea. The
wind direction is not steady, but veers through an angle at irregular
intervals. The rudder angle must be increased for stronger current or
wind, and decreased for weaker current, or for more favorable direction
of either one of them. Steering is easier if the desired path is a
straight line and if two or more range markers on shore can be lined up
visually along this path. Many recent tracking tests have shown that
pilot error in following a channel is reduced considerably if
intermediate buoys are placed between those that mark changes in
direction, as in Figure 2. This is simply because one's perception of
a straight path depends on a reference line that is marked by two or
more fixed objects. Rudder angle can then be adjusted to maintain
alignment with these fixed objects. On the other hand, with only one
object in view, a pilot tends only to steer toward it, relatively
unaware of possible side drift. The course is then parabolic rather
than straight, the curvature a function of the relative strength of
current (or wind) to the ship's forward speed.
Channel design must allow for this kind of deviation, unless a
sufficient number and arrangement of channel buoys and range markers
are provided to give pilots at least two of them ahead as a reference
line at all times.
Because of the steering angle of the vessel as it approaches the
harbor entrance, the vessel sweeps across a greater width of channel
than its own beam width. The width that is swept can be as much as
twice the beam, depending on the ratio of speed to the lateral wind and
current.
Once in the harbor, both currents and winds will be reduced, and
the vessel will require less steering compensation. On the other hand,
decreasing velocity will cause less steering response, so that the ship
may tend to move in the direction of its axis, rather than to follow
the desired path.
The strong current vectors across the channel may tend to sweep
sand and silt into the channel (in the shaded area of Figure 1, for
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16
example). Heavy sedimentation may occur both in the harbor and
offshore from the entrance when river flow is strong.
Tidal movements are critical in the transport of sediments, as well
as of suspended pollutants.
Long-period waves sometimes create surge and harbor oscillations
that not only may be damaging to ship mooring or cargo handling, but
may complicate entrance conditions.
Wind set-up, causing surface water to flow in the wind's direction,
is frequently the dominant factor causing water movement in both the
vertical and horizontal direction.
The variety of design considerations and the consequences of
particular design decisions can be seen in existing and proposed
examples of ports and harbors. Figure 3, for example, shows a
synthetic harbor with a protecting breakwater parallel to the coast.
To enter it, a ship must turn outside the roundhead and move along a
lee shore before reaching quiet water. The secondary and tertiary
breakwaters increase the protection offered smaller vessels.
Figure 4 shows a test harbor used to evaluate how tankers respond
to the challenges of maneuvering in current and around bends. Get to
the L-head pier, and you win a silver dollar'
The entrance to Manfredonia, illustrated in Figure 5, is a long
hockey-stick pier with breakwater. It is apparent in the photograph
that entering is easier than turning here. Figure 6 illustrates the
harbor of Ashod, Israel, at the east end of the Mediterranean. This
harbor has exactly the same shape an an ancient Roman harbor whose
remnants were discovered underwater a few miles away after this one was
designed. It is interesting that the harbor at As hod is a good one for
sailing vessels.
The synthetic island drawn in Figure 7 would have harbored twin
nuclear power plants. Dual entrances are indicated for support
vessels. One would nearly always provide entering shelter, and the
exit, of course, would always be straight ahead. Figure 8 shows a
simple entrance; in this case, to a marina in Ithaca, New York.
Figure 9 indicates how protection for big ships can be achieved
behind rocky islands. The design would have been for sea berths,
rather than harbors, in this instance. Aristotle Onassis tried to get
permission for a terminal in this area--the Isles of Shoals, New
Hampshire--to serve a refinery.
In Figure 10, the sea berth built by 8urmah Oil behind Grand Bahama
Island can be seen. It proved a deep, rough site for smaller vessels.
The Burmah-Shipment Channel, illustrated in Figure 11, leads to a
harbor for small ships that was dredged from the coral behind the
berth. The turn required by this snaky entrance is difficult, at
best. A rather different approach is shown in Figure 12. This
entrance would have been simple and straight, but unforgiving. Plans
to develop a terminal and refinery at this location in Machiasport,
Maine, were finally abandoned.
Three stages in the growth of a river port can be seen at Bilbao,
in Spain (Figure 13~. The latest requires a huge double-arm
breakwater. Berths for the large ships are just behind it. Entering
is somewhat like threading a needle, but once inside, there is ample
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it,
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figure 3. Harbor entrance with breakwater
parallel to coast.
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Figure 4. Test harbor for evaluation of
tanker maneuvering.
Figure 5. Entrance to harbor of Manfredonia,
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Figure 6. Harbor of Ashod, Israel.
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Figure 7. Dual entrances to power-plant island.
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Figure 8. Simple entrance to pleasure marina.
OCR for page 19
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Figure 9. Sea berths protected by rocky
islands.
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Figure 11. Burmah-Shipment Channel for
small ships, behind berths of Figure 10.
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Figure 10. Sea berth for tankers, Grand
Bahama Island.
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Figure 12. Simple, straight,
but unforgiving entrance to port.
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Figure 13. Port of Bilbao, Soain--diffi-
cult entrance to an ample harbor.
Figure 14. Entrance to East Rota Harbor,
Spain-wide and easy,
OCR for page 20
20
room to maneuver, provided the tugs are available. For contrast,
Figure 14 e bows the wide, easy entrance to East Rota Harbor in Spain
and its hook-shaped breakwater.
Figure 15 pictures the port of Bandar Abbes, Iran. A major
consideration in the design of its entrance was the current velocities
at the openings of the proposed breakwaters. Studies conducted at the
Delft laboratories in Holland helped indicate bow changes in the
configurations of the breakwaters and channels could create better
conditions. Another example of the consequences of breakwater design
is illustrated in Figure 16. The long, straight breakwater at
Escombreros, Spain, with berths on the inside, demands a curving
approach by loaded tankers, and tug assistance to move the vessel
laterally. Figure 17 shows the Port of Los Angeles, California,
behind the big San Pedro breakwater, and its several channels. A
proposed oil terminal for Los Angeles, sketched in Figure 18, would
make it necessary to turn a tanker 90 degrees immediately after
threading the needle at the harbor breakwater.
Port Aransas, Texas (near Corpus Christi), has a long, straight,
dredged entrance between twin jetties that leads to the large turning
basin (2200 ft) indicated in Figure 19. A similar design can be
observed in Charleston, South Carolina (Figure 20~. Notice how much
shorter the twin-jetty entrance is than that of Port Aransas. This
entrance is subject to shoaling from littoral drift through the inner,
permeable portions of the jetties.
Figure 21 depicts Riviere-au-Renaud, Quebec--a narrow slot through
a rubble mound, with the wharf just inside. The proposed industrial
island and port illustrated in Figure 22 would have vessels entering
from the left and exiting on the right. The curving approaches would
require considerable skill to navigate. Another curved opening is Port
Valdez, Alaska (Figure 23~. Notice that the excellent natural
protection of this landlocked bay is gained through the Valdez Narrows,
and that they are narrowest at exactly the point where the separation
of ship traffic ends. The entrance to the Suez Canal is also quite
constricted, as can be seen in Figure 24. A recent planning study
indicates there is little room for expansion.
Two long, straight entrances are illustrated in Figures 25 and 26,
the Mi~urata Iron and Steel Port in the Gulf of Sirte, Libya, and the
approach to Freeport, Texas, which meets the Intracoastal Waterway at a
turn in the channel. A long course through ice-bearing waters leads to
Melville Island, Quebec (shown in Figure 27~.
A very open entrance (Figure 28) is that of Sines, Portugal,
located behind a huge, rubble-mound breakwater. The design and master
plan are being restudied because of severe damage to this structure. A
design that also might be restudied is that of Kahului Harbor, Hawaii.
The pincer--shaped breakwaters, drawn in Figure 29, have been
repeatedly damaged at the roundheads. The displaced armor units can
create dangerous obstructions at the channel edges.
Natural forces, as I pointed out, are always an important
consideration in the design of entrances to ports and harbors. The
entrance to Port O'Connor, Texas--Pass Cavallo--i" actually a large
tidal inlet (Figure 30~. Acajutla, Salvador, was recently the subject
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in
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Figure 15. Port of Bandar Abbas,
Iran, with breakwaters designed to min-
imize current velocities.
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Figure 17. Port of Los
Angeles, California
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Figure 16. Straight breakwater at entrance
to Escombreros, Spain, demands curving approach.
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Figure 18. Proposed oil terminal for port
of Los Angeles.
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Figure 19. Entrance and turning basin, Port
Aransas, Texas.
OCR for page 22
22
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Figure 20. Entrance to port
of Charleston, South Carolina.
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Figure 21. Riviere-au-
Renaud, Quebec.
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Figure 23. Curved entrance and narrows leading to
port of Valdez, Alaska.
Figure 24, Constricted entrance to Suez Canal.
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Figure 22. Proposed industria
island and port--curved entrap
ces would. be challenging.
OCR for page 23
~-
Figure 25. Long, straight entrance to Misu-
rata Iron and Steel Port, Sirte, Libya.
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LOCATOR OF MELVILLE ~e
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Figure 27. Long entrance to
Melville Island, Quebec.
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Figure 26. Another long,
straight entrance, Freeport, Tex.
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or;.. .~~
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Figure 28. Open entrance to
port of Sines, Portugal.
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Figure 2 9 . Pincers breakwaters
of Kahului Harbor, Hawaii ~
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24
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ram ~
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-_ of at _
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_-, pedaet of the
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Figure 30. Tidal-inlet entrance
to Port O'Connor, Texas.
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Figure 32. Marina harbor design-
ed for minimal harm to environment.
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:Pigure 31. Harbor designed to reduce wave
penetration for barge anchorage.
Ha. ~ .
J
Figure 33. Tug assistance required to maneuver
vessel through harbor entrance.
OCR for page 25
25
of exhaustive studier of harbor oscillation, a special kind of water
movement characterized by large lateral and small vertical amplitude.
Figure 31 illustrates a harbor designed after studying the wave
conditions of a particular area. Outer and inner breakwaters provide
quiet water for barges. In Puget Sound, on the other hand, the
principal concerns in the design of a marina at Point Roberts (Figure
32) were the effects on the environment in this residential area, and
protection against shoaling from littoral drift.
The last illustration, Figure 33, although not strictly of a harbor
entrance, indicates the kind of difficult maneuver, with tug
assistance, that many require. One hopes the vessel is going full
astern at this point in its approach.
OCR for page 26
Representative terms from entire chapter:
tug assistance
