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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
14 l _ VEER ANGLE 1~// :/'~l~D f C UR RENT l ,,, IV ', '~'i" ~ ~ _-- ~` BREAKWAT! it' ~ ?~J DREDGED CHANNEL , . Figure 1. Change of course necessary to steer into current at harbor entrance. / 1 / ~ ) N /S~L 77~V / A~: ~ / DREDGED at/ AR" It's ~ "T0 - L pC~ ~' I, \ _~l 5~0AL \ \ Rlv~R \ FLOW Hi,' Figure 2. Placement of buoys between those marking changes in direction reduces pilot error
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
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
17 it, ~ \ ~ ~ if, ~ ~ 3~ ___ ;, 1 ~ ~ ~ figure 3. Harbor entrance with breakwater parallel to coast. "e ~ - - on _, ~ was __ . e__ a_ ~_; _~ _ fir ~ Hi. t ~ ! _~. Figure 4. Test harbor for evaluation of tanker maneuvering. Figure 5. Entrance to harbor of Manfredonia,
18 ~ I: ~ ~ ~3~,. it. _ : ~:~ I ~~ e - ~-_~ _ ... _, _ ~ y a;; ~ L , ~ ~ _' .~ _ _ Go_ Figure 6. Harbor of Ashod, Israel. E:_ _I lo_ _- Ad: _ ,~ "i~ Figure 7. Dual entrances to power-plant island. .~ ~3 GIG_ ~ I. 1 · _ :~_ - _ _ _ ~ :" :'" -_ -a: _. Go; _ _ ~9 Figure 8. Simple entrance to pleasure marina.
19 ,^~ . \\ : :\ \ .~,*~: .: · it., . ~ . \. ~, F Figure 9. Sea berths protected by rocky islands. '1 _ ~ _ ~_ Figure 11. Burmah-Shipment Channel for small ships, behind berths of Figure 10. ~ .. Figure 10. Sea berth for tankers, Grand Bahama Island. ~ my' ~ .. .. -_ ~ ) : ,, , Am, ~N Figure 12. Simple, straight, but unforgiving entrance to port. ~ '. _""; ' ~ -. Figure 13. Port of Bilbao, Soain--diffi- cult entrance to an ample harbor. Figure 14. Entrance to East Rota Harbor, Spain-wide and easy,
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
21 in oL~- Figure 15. Port of Bandar Abbas, Iran, with breakwaters designed to min- imize current velocities. ........ ~ _ ~ ~ ~iC~i3 0'- ~+ -air '/ ~3 ~ are Figure 17. Port of Los Angeles, California i_ ~ C I , ~A- ~ -:' ~ .j it_ _ ~ . . i; ~.~ ~. ~6 . . , , A, A,,-- '2~ .' ~ALAS . S ~ '' . ~ ~ _ ' 'an .~ ~ ~ ~ ~ . ~V ~~ · 41- a ~ ~ 'id ~;~ Figure 16. Straight breakwater at entrance to Escombreros, Spain, demands curving approach. _i' ~_ Figure 18. Proposed oil terminal for port of Los Angeles. s~uT J~ ~ <,>~bL^MO \ as, or Taco ~: J \ - Tams Call_-, ~ -AT ~.t music INKED / Figure 19. Entrance and turning basin, Port Aransas, Texas.
22 , 7,, ., ~,. ,8: I. r -._ lo.. .., a... Figure 20. Entrance to port of Charleston, South Carolina. ... - .. ~.: .... ... . ~ : A. .~. ., I: , .-. ~-~ ... .~ ~ ~ A:. ·- ~ A; -: ~ .. ,,i - ~ ~ ~~ .;, .:.. ~ s~ ;0 ~ > · ~ ~ arm ! or.. .A.6.' ~ ~ 8 :, ;., Ott ~ -. ~ ~ ~ ~ > '' ~ ~j~ f ~j~i~_ _~-;2~*'~< , .2,..~ ,''~'~.'"~:~'- 'I'' I. I'".. ,:.~'~"~''~'' .~"' Figure 21. Riviere-au- Renaud, Quebec. I: ~ 1 '`= ~ ", mat VALOEZ ~ 1/~ ~ Figure 23. Curved entrance and narrows leading to port of Valdez, Alaska. Figure 24, Constricted entrance to Suez Canal. 7~_. j. AL 1 C~de Od TO 2 500~tWUO V R- 3 __ f-~_P-d4c~r~, 5 ~ ~_, . ram s__ 7 A__ _d I_ Bit J For __ - d 9 it_ - _ to _cr Cal 11 _#b_t~ 12 D_ - ~ - 13 Po_t and Sew #A 14 ~ 15 a_ Woe To_ lot 10 bead ~ To 17 r~ 18 ~ Few to Airport "d ~1 20 Fee _n at bl_d A_-~ Al _ C_ Figure 22. Proposed industria island and port--curved entrap ces would. be challenging.
~- Figure 25. Long, straight entrance to Misu- rata Iron and Steel Port, Sirte, Libya. 1~ ~ >14~ :~.: 1\ ( t^~~ \ tt~,:_._~ \ LOCATOR OF MELVILLE ~e ~"o ~6 Figure 27. Long entrance to Melville Island, Quebec. W^^ ' ~/,, f~Et~lY ~ ~ ~ ¢~ ~< ~;~#-~.~'`~- ~ i' .\ i,, lk ~ ~. ,\ \ _, ~J ': ~ Figure 26. Another long, straight entrance, Freeport, Tex. s or;.. .~~ .~ Fiji .. a. lo.' ~ -t ' *~12 1 ~ ·',' ~ e o t.' ¢.1 , ~ : i' ' ;11 .~ .~ ~ ## Figure 28. Open entrance to port of Sines, Portugal. -~1.\ ! ~ 7 ~ And' ~ _ We0 ~ :,J,I\~: \ ,: 0e 000 0 "0 · ~ ~ _ tic ~ Prey . ~ Figure 2 9 . Pincers breakwaters of Kahului Harbor, Hawaii ~
24 ~,Z5 I - . ~_ · ~ ~ Amp,- .._~ '.~ _ ._ _. ram ~ ,. . . .. I At, · a, 1 1 -_ of at _ '`~ _ItSwit, cow - et - - red the _-, pedaet of the T~ coot ~ ~. 1979. P_ Cam, · tow' me, "erased eb"_ (_ _ t- 111 - a~ Ha. u319). .; Figure 30. Tidal-inlet entrance to Port O'Connor, Texas. .. ... r EVE ~ ~ ,4. . · I .~..~ _ , ,, , , ., !. .~ ..'tJ~- ~ l^ · `` ~^ ~ >I ~l~ll~b~l^; ~ .' 1 ~ ^~l · ~ ~^ · '~4 I-' ~ t' ~ ^, - \, ~ ~ , ~ 5 ,~ .. - ~ _~,~ I:_, ~ *,- ,? ~ ..' ,, i.<' _,. t ' ~ ! Figure 32. Marina harbor design- ed for minimal harm to environment. : . . ~_. - . of.. . ~ . ~: .~ ~ _. ~_~~ \\ ~ ''' .. - loam ~-~ I a; . _ I_ I'd: :: ' ~ - ~' fit - L LA - U' :Pigure 31. Harbor designed to reduce wave penetration for barge anchorage. Ha. ~ . J Figure 33. Tug assistance required to maneuver vessel through harbor entrance.
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.