Criteria for the Depths of Dredged Navigational Channels (1983)

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4 EVALUATING THE ADEQUACY OF CRITERIA FOR DREDGED DEPTHS OF NAVIGATIONAL CHANNELS In the course of assessing the criteria used in the United States for determining the depth of dredged navigational channels serving major ports and harbors, the panel requested information about the criteria used in other countries from two international organizations--the Permanent International Association of Navigation Congresses (PIANC) and the International Association of Ports and Harbors {IAPH)--and from several maritime countries. Shippers were also consulted for criteria respecting underkeel clearance. The panel tested the adequacy of existing depths in the United States by taking as input to a mathematical model (developed to provide a check of channel design) the dimensions of nine representative channels of the East, Gulf, and West coasts. The results of the simulation are presented in the succeeding section, followed by the criteria of international organizations, foreign maritime countries, and shippers. Dredged Depths of Navigational Channels in the Uhited States Navigation The criteria used to design the depths of navigational channels in the United States are described in Chapter 2, "Regulatory and Institutional Considerations." For the major channels of the United States, the pertinence of the design ships used for the original 4-l

4-2 determination may have long since been superseded by the characteristics of the ships using the channels. The panel therefore tested the dimensions of nine major channels of the United States on three coasts (supplied by the U.S. Army Corps of Engineers) with passages of existing ships in a computer simulation. The computer model ~ described in Appendix D ) is a simple technique f or testing various channel designs. For each channel, the analysis was carried out using existing dimensions and using a hypothetical future depth of 60 ft. In addition, selected increments of depth for the Galveston Ship Channel were considered. The results for existing depths are set out in Table 9, and those for hypothetically deepened channels in Table 10. For each of the analyses performed, the model evaluated specific parameters resulting from the passage of the specified vesselts) through a specified section of the channel. These parameters serve as the basis for governing vessel operations in the channel. fit is essential to note that all calculations included the effects of a 1.5 kn current and a 30 mph wind, both acting perpendicular to the vesselLs). Speed limits were supplied by the U.S. Coast Guard. Speed limits are not specified for many channels, and the following estimates were made for the purpose of squat and other computations: Channel 1. Oakland Harbor 2. Columbia River 3. Chesapeake and Delaware Canal 4. Calcasi eu River Speed Limit Assumed 5 kn 6 kn 6 kn 6 kn From the panel's observations and brief ings during f held trips, the assumptions--including the design ships--are conservative. Even so, the underkeel clearances of the shi ps considered are well below the minions specified by the Corps ' criteria, squat t+3 ft. or 0.9 m) + rolling and pitching estimate + clearance (2 ft. or 0.6 m, soft bottom; 3 ft. or 0.9 m, rocky or hard bottom) . Although the rule of thumb is generous in squat allowance compared to the panel ' s results, the speeds assumed by the panel may be low. ~ Figures in Appendix D show how squat increases with speed for these ships in these channels.) As indicated in the sections addressing considerations of ships in channels, the maximum vertical excursion of a ship, and the part of the ship that experiences it, is sensitive to many factors. It is clear from the results of the simulation, for example, that the amount of squat changes for the same ship in different channels of the same depth . The tanker Lenino experiences squat of 2 . 4 f t ~ O . 7 m ~ in the Lower Columbia River Channel ~ 40 ft. or 12 . 1 m, deep ), but only 0 . 9 ft ~ O .3 m ~ in the inner Calcasieu River Channel ~ also 40 f t, or 1; . 1 m, deep), even assuming the same ship speed, current velocity, and wind. Changes in speed, traffic (passing or overtaking), and the physical environment will also affect the underkeel clearance actually available to these ships in passage through the selected channels.

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4-5 Overdredged Depths Many local conditions affect the accuracy of dredging processes, and the range of various kinds of dredging equipment is not precisely known. Nevertheless, the evidence cited in this report ~ "Dredging" in Chapter 3 ) suggests that the "plus 2 ft" rule of thumb (actually O ft to 8 ft. or 0 m to 2.4 m; but generally 2 ft. or 0.6 m) allowed by the Corps to achieve the design prism is within the order of dredging accuracy but may need to be adi usted in particular channel ~:er~ti one: or for certain types of work. For advance maintenance overdredging, the procedures outlined by Trawle ~1981b~ and described in Chapter 2, "Regulatory and Institutional Considerations," offer appropriate guidance for shoaling analysis and progressive improvement in estimation through frequent surveys. Frequent surveys may also yield needed information to adjust the "plus 2 f t " pay-overdepth specif ication . ~ ~ . Criteria of International Organizations, Other Maritime Countries, Shippers Criteria of International Organizations PIANC Criteria In 1974, the Permanent International Association of Navigation Congresses ( PIANC ~ sponsored an International Commission for the Reception of Large Ships ~ ICORELS ~ . Working Group IV was charged with the optimal layout and dimensions f or large ships in shallow waterways. Group IV published a final report (ICORELS, 1980) designed to establish criteria to regulate the problem of navigation of large ships in shallow seas and sea straits (e.g., North Sea, Baltic Sea, Straits of Dover, Straits of Malacca). In this report, PIANC gives advice about technical aspects of the possible works to be undertaken, such as the dimensions of dredged channels, navigational aids, wreck removal, and evaluation of safety. Approximately 20 countries participated as members of ICORELS. The report of Working Group IV includes a summery of studies and developments pertinent to various aspects of channel design, navigation in sea straits, and dredging for construction and maintenance, together with conclusions and recommendations in each area. A set of recommendations is given for determining the depth of channels: Recommendations The conclusion Lsection 2.1.1.31 drawn by ICORELS is that it is not possible to state a general rule for minimum underkeel clearances and port approaches and maneuvering areas, because of the influence of local conditions, currents, and swell. The Commission does note that general criteria for gross 1lnderkeel clearances ~ section 2 .2 .2 .8 ~ can be given for drawing up preliminary plans: -

4 - 6 Open sea area-en exposed to strong and long stern or quarter swells where speed may be high, the gross underkeel clearance should be about 20 percent of the maximum draft of the large ships to be received. Waitina area-When exposed to strong or long swells, the gross underkeel clearance should be about 15 percent of the draft. Channel-Sections exposed to long swells, the gross ~nderkeel clearance should be about 15 percent of the draft. Maneuvering and berthing areas-Protected gross underkeel clearance to be about 7 percent of the draft. ~ Figure 37] shows the def inition of underkeel clearances used by the Commission in their recommendations and are described as follows Lsection 2.2.2.41: THE GROSS UNDERKEEL CLEARANCE is by definition the margin between the keel of a vessel and the nominal channel bed level, considering the water ref erence level during its pas sage and the maximum draught of the vessel, measured at rest in calm water. THE NET UNDERKEEL CLEARANCE is by definition the minim margin remaining between the keel of the vessel and the nominal channel bed level, the vessel moving at planned speed under the influence of the most severe wind and wave conditions it was designed to withstand (operational limit conditions). The net underkeel clearance, which should be at least O.5 m (1.7 ft), has to be assessed as a safety margin against striking the bottom. Other factors are also involved--types and sizes of ships, commodities transported, environmental consequences, and density of traffic. Summary The recommendations of the Commission are for gross , under~eel clearances in restricted channels of approximately 15 percent, a factor that allows for the admissible draft of the ship plus its vertical motions due to swells, squat, due to speed, and net underkeel clearance. Further tolerances are added to this nominal channel bed level to allow for sounding accuracy, sedimentation deposits between two dredging campaigns, and tolerances for dredging, to produce the final channel dredged level. lAPH Criteria The International Association of Ports and Harbors (IAPH) also assembled a Committee on Large Ships (COLS), now the Committee on Port Safety, Environment, and Construction. In the section "Depth of Entrance Channel," COLS (1981) cross-references

4 -7 e ~1 1 ~ 1 1 1 t wAT iE;? ~ £FEF!c NC£ LEVY L ~ . . ~ . Adm~ssibt. drought t St`,, sized - I _ vertical sh'¢ mowamcnts du. to s - MU, squat due to speed Net ~~,t`~.t cleo= Gross ~^dericeel C l" rG He ~ Sounding cecu~cy Sed~n`ent deposit ~. t no c, dog · n9 co m" ·9 as . ~~~~~~~~/~~ L h~m^C I coopt D" level Toleronce tor ded9~9 | ~hC^n~l O. - 9.d level } Figure 37 Underkeel clearances as defined by PIANC Permanent International Association of Navigation Congresse *SOURCE: COLS (Committee on Large Ships) Environmental Protection of Ports and Harbors (Tokyo: Association of Ports and Harbors) (1981), Guidelines for Safety and International .

4-8 the ICORELS recommendations: S.3.1.2.1 The depths of entrance channels are determined by allowing a minimum margin between the lowest point on the keel of the vessel and the channel bed (underkeel clearance) in addition to the draught of the design vessel. There are two types of underkeel clearance, one gross and the other net: Gross underkeel clearance is the margin remaining under the keel when the ship is motionless in quiet waters and net underkeel clearance is the margin which continues to exist under the keel when a vessel moves at a scheduled speed and when it undergoes the expected maximum inf luence of swell and wind. The net underkeel clearance must be at least equal to O .5 m ( 1. 7 ft ~ for a sandy seabed and 1. 0 m ~ 3 .5 f t] when it is rocky. 5.3.1.2.2 The minimum value of the gross underkeel clearance taken, consistent with compliance with the lowest value of the net underkeel clearance, depends upon the following factors: , , _ _ _ - Factors related to channel bed; *the level of the channel bed below chart data, *the allowance made for the degree of accuracy _ in hydrographic surveys (chart data) and dredging tolerances, *the incidence and degree of channel silting between maintenance dredgings and dredging tolerances, *the latest maintenance soundings and *the eventual percentage of suspended silt. Factors related to tide; *tidal variations (maximum and minimum), *tables of predicted tide levels, *details of any tidal surges, wind and atmospheric pressure ef fects on water level and *the accuracy of predicted tidal heights and the predicted times of high and low water (for particular tides ~ . - Factors related to the ship; *the actual maximum draught of the design ship, *the increase in ef f ective draught due to the rolling, pitching and heaving of the ship under wave action within the channe 1, *the estimated squat and change of trim f or the design ship calculated f or each critical depth area based on the maximum Fermi ssible operating ship speed and the most constricted channel section within the critical depth area, The normal loaded condition of the design ship and *the draught and trim changes attributed to any change in water density. -

4-9 5.3.1.2.3 It is not possible to establish accurate rules concerning the minimum depth of port channels because of the ma jor importance of local conditions . . In the initial planning stages, the following generalizations may be valuable: - sections exposed to strong and long swell - gross underkeel clearance to be about 15 percent of the maximum draught, - sections less exposed to swell--gross underkeel clearance to be about 10 percent of the maximum draught. 5 .3 .1.2 .4 Detailed recommendations for the depth of channels are given in ICORELS Report of PIANC ~ Working Group No . IV) .... In order to define the nominal level of dredging, it would be advisable to allow for the accuracy of the soundings, for siltation between maintenance dredging and f or dredging tolerances . Criteria Used in Other Maritime Countries Canada' s design criteria for channel depth primarily address the need for precise and reliable measurement of the environmental risks associated with the location and operation of marine terminals, particularly those for large on tankers. They can be found in the "TERMPOL" Code of Recommended Standards (Canadian Coast Guard, 1977 ~ . The Ministry of Transportation, through the Canadian Coast Guard, established the code TERMPOL ~ ire coordination with other departments . The TE1W?OL code is a set of recommended standards used in Canada f or the prevention of pollution in me rine term nal systems . The code outlines acceptable ship terminal standards, def ines the required ship terminal system analysis and assessment criteria, and sets out operating practices and procedures for ship terminals. Although published by the Canadian Coast Guard, the TERMPOL code is a coordination and correlation or the separate requirements of the Canadian Department of Fisheries and the Environment, Public Works, And -m, and Reg~ona-l and Economic Expansion. Each participating department is individually responsible for all contributions made and decisions taken within its area of responsibility. ~ Provisions of the code are not themselves mandatory, but the assessment criteria of the code are used by the Ships Safety Branch of the Canadian Coast Guard to determine the technical needs, if any, for making regulations or implementing special precautionary measures to licit navigation within the ship terminal system under review. Canadian Criteria for Underkeel Clearance

4-10 The "TERMPOL8. representatives of the the assessment and can Coordinating Committee participating __ ~ (T.C.C.), composed of government departments, performs require terminal planners to submit data for environmental impact assessment. Port authorities seeking to improve their waterways may be called upon by the TERMPOL Assessment Committee to provide the following surveys: o o o o o o o a Ori gination and Destination Survey; Transit Time and Delay Survey; Marine Traf f ic Volume Survey; Fishing Vessel Operation Survey; Approach Characteristics and Navigability Surrey; Special Underkeel Clearance Survey; Site Plans/Technical Data; Environmental Stud' es. Thus, the Canadians use a systems approach in which the specif ication of underkee 1 clearance is integrated with studies of marine traf f ic and required navigability . For the purposes of this report, the study of direct interest is the Special Underkeel Clearance Surrey. The requirements are as follows: Nominally, the design ship's minimum underkeel clearance should be 15 percent of her maximum permissible draft. A proposal for a minimum underkeel clearance in approach channels of less than 15 percent of the design ship's deepest draft will be considered but should be supported by explicit details or calculations for each of the following factors: o Minimum chart datum measurements supplemented with tidal heights over a specified period; o Accuracy of predicted tidal heights and the predicted tomes of high water and low water; 0 Details of any tidal surges and wind setup; o Allowances for the degree of accuracy in the hydrographic survey ~ chart datum) and for that of dredging processes; o Incidence and degree of channel silting between maintenance dredgings and the identif ication of all critical depth areas; 0 Increase in effective draft due to the rolling, pitching, and heaving of the ship under wave action within the ship channel; o Estimated squat for the design ship calculated for each ~rit~ ; Moth - treat based ~a - anal maximum' per=rse~peret~ns ship speed and the most constricted channel section within the critical depth area; o Nominal trim and changes of trim experienced by the design ship; o Draft and trim changes attributed to any changes in water density; 0 Climatological and related depth anomalies; and 0 Nature of the bottom (rock, sand, mud, etc. ~ .

4-11 Underkeel Clearance and Depth Criteria in Japan The Japanese Ministry of Transport has established its criteria for marine facilities through j oint action of its Bureau of Ports and Harbours and its Port and Harbour Research Institute. These are summarized in a booklet entitled "Technical Standards for Port and Harbour Facilities in Japan" published in English in 1980. The Japanese version, published in March 1979, represented the first compilation of all advanced Japanese port and harbor engineering techniques. The English version differs from the Japanese version only in that it excludes the official procedures for compliance with the standards. The Japanese relate the depth of channels to their basic specif ication for depth of harbor basins . The design criteria for basin depth, in turn, are ~ Bureau of Ports and Harbours and Port and Harbour Research Institute, L980 ): DEPTH OF BASIN 1 ) The depth of basin shall be 1.1 times full load draft of the ship below the datum level, in considering the extent of the oscillatory motion of the ship due to the natural conditions such as waves and tidal currents. However, this provision shall not apply to a basin for outf it of ships and a basin used for special anchorage or mooring of ships. In the case of a basin f or ferryboats, the draf t difference between stern and bow during car-to handling, should be considered to determine the depth of the basin. Furthermore, where the sea level of a basin may be below the datum level, because the seasonal changes of mean sea level are larger than the tidal level change due to astronomical tide, or where the basin may be attacked by high waves and swells, these influences should be considered. 2) The depth of a basin can be determined in reference to the values given in tTable ll] when the full draft of the ship is not known. The depth of waterways is related to that of basins as f allows: DEPTH OF WATERWAY 1 ) The depth of waterway shall be an appropriate value of no less than the full load draft of the ship in consideration of the extent of oscillatory motion of the ship due to the natural condition such as waves, winds, and tidal currents and the trim. In this case, "a proper depth" means a depth obtained by an allowance added to the depths specified in [DEPTH OF BASINI. The allowance varies with such conditions as roll and pitching, trim and squat of the ship and the conditions of seabed materials. This provision may not apply to at special waterway where the draft Of the ships mires thii-S

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4-13 waterway is always smaller than the full draft, such as an approaching waterway to dock of shipyard or a waterway exclusively for partially loaded ships. WATERWAY MAT NTENANCE 1) The depth and width of a waterway shall be maintained properly for the smooth use of harbour and the safe navigation of ships. When a waterway is planned on a river mouth or a beach where a big amount of littoral drift Is expected, the degree of maintenance dredging requ, red in the future should be forecasted by estimating the rate of sediment transport by a flood or the rate of littoral drift by waves and tidal current. Presumably, this latter calculation is used for an initial schedule of maintenance dredging or for determining additional depth f or advance maintenance dredging. Channel Depth Criteria, Port of Zeebrugge, Belgium The Belgian approach to cnanne1 Repin criteria IS outlined in a 1~8U bulletin of the Permanent International Association of Navigation Congresses. Waters of the Belgian coast experience dense me rine traffic. As a result, traffic has been channeled into compulsory routes. One of the main routes through the North Sea arrives at the f rontier of France and Belgium approximately 16 miles of f the coast, then runs east/northeast to the "Scheur, " which is the access channel to the River Scheldt estuary and the Port of Zeebrugge. The channel continues to its terminus at Antwerp. In 19 7 0 , the Be lgian government decided to extend the port of Zeebrugge, using a sea lock to connect the inner port with the sea (Simoen, 19801. The seaward extension was to accommodate vessels UD to 125,000 DWT and, specifically, to accommodate roll-on, roll-off (RO-RO) cargo vessels and containerships. The port will also have an LNG terminal. Since the projected traffic involved large vessels with draf ts of 5 0 f t or more, the Adraini stration of Waterways, Ministry of Public Works, initiated a study of needed channel dimensions . The study plan included a preliminary phase to def ine the design criteria and .to chaise the chancel repute., bS~;£~ d~led~; s`~di~ Are then made for the design of the chosen channel route which included the determination of the channel cross-section profile, various studies of sedimentation, the removal of wrecks and mines, and the study of required nautical equipment. - The dredged channel depth criteria were derived in this second phase as follows: Channel Depth Methodology The provisional channel depth was first determined on the basis of a combination of design draft and tidal "windows" for the design vessels. The necessary keel clearance ,

4-14 was then calculated in accordance with the recommendations of PIANC, in comparison with other harbors, and by using the wave and tide records taken at Zeebrugge. Provisional channel depths were then selected ranging between 13 m and 16 e 7 m (43 ft and 55 ft) for a variety of design ships (LNG tankers, VLCCs, containerships). More detailed studies were used to determine the cross-section prof ile of the waterways . Basic data were gathered in these studies f or the determination of the cross-section: o The velocity and pattern of the current in different parts of the access channel, as well as variation during the tidal cycle inf luence on channel width ); o The wave heights induced by dif f Brent wind f orces and tide-level records ~ inf luence on channel depth ); o Regularly taken soundings of the channel bottom. These echo soundings provided information about the variation of the channel bottom ~ inf luence on clearance and channel depth ~ . Kee 1 Clearance Criteria The channel depth needed in the access route to Zeebrugge was determined as a function of: o The draft of design vessels; o The keel clearance to be observed in the access to Zeebrugge; o The water leve 1 at the time of entering or leaving the harbor , which is a function of the tidal windows for each type of ship. Keel clearances for good and bad weather were calculated separately. The criterion for good weather conditions is normal maneuverability. The criterion for bad weather is "the chance of the ships ' touching bottom should be acceptably small. " Fi gure 38 shows the method used to determine keel clearance. Starting from a reference water level, vertical ship motion due to squat and waves is added to ship draft. This incremental value of keel clearance is described as a net keel clearance (about 4 ft for LNG carriers). Further allowances are added to the clearance to allow for the accuracy of soundings and tide measurements. Still another allowance is made for divergence from average tome levels, and an allowance is made for sedimentation between two sounding campaigns, plus a tolerance for dredging work. Finally, during bad weather conditions, it is recognized that the ship may experience increased draft owing to wave-induced motion. Additional allowances are specified to prevent the ship from hitting the bottom. Given the wave characteristics, it is possible to calculate the probability of a ship's vertical movements. Each of these individual allowances is determined by calculation or measurement. Summery The keel clearance criteria used by the Belgium Port Authorities are experimental and analytic, rather than regulatory, and are determined f or both good and bad weather.

4-15 Figure 38 Access route to Zeebrugge, Belgium: prescription of the underkeel clearance ~ ~ - . SO ~ t ! i Reference Water Level Ship's draft Vertical ship movement (squat and heave) Draft keel clearance Accuracy of the soundings and tide measurements Divergency from average tide level _ Sedimentation between two surveys and inaccuracies of dredging work \ ~ 1

4-16 Calculations are made of all key factors affecting vertical ship movement (design draft, squat, waves, maneuverability requirements, accuracy of soundings, and tide measurements) on an analytical or statistical basis, or both. Additional allowance is made for maintenance dredging to provide for sedimentation between two sounding campaigns. The result for the channel leading to Zeebrugge is the specification of depth, 13.4 m (44 ft), and gross keel clearance for normal and bad conditions of 1. 4 m and 2 .7 5 m, respectively ~ 5 f t and 9 ft). Inasmuch as the design draft of the vessel was 11 m (36 ft), the s underkeel clearance amounts to 12 . 7 percent of ship draft for normal conditions and to 22 percent for bad weather conditions. One notes that the Belgian design criteria for underkeel clearance approximate those of other countries, although the allowance for bad weather conditions is more conservative. "Nautical Depth" Concept, Europoort and Rotterdam, Netherlands The . "nautical depth " concept ( de scribed in a preceding section ~ evolved from study of the behavior of vessels in Europoort and Rotterdam harbors. The present operational practice in the Europoort/Rotterdam area requires that supertankers approaching Europoort from the North Sea to the Maas-Center Buoy (Surogeul) have a minimum underkeel clearance of 20 percent of their draft, and from the Maas-Center Buoy to the pierheads an underkeel clearance of 15 percent of draft. The approach channel is maintained at 23.5 m (78 ft) depth. From the buoy to the pierheads, the channel depth is 22 .5 m (74 ft.) . me Caland and Beer channels allow vessels to proceed at an underkeel clearance of 10 percent of draft. Supertankers with a maximum draft of 20 .7 m ( 68 ft ~ are required to proceed very slowly with a mi nim fin underkeel clearance of 10 percent ~ 2. 1 m, or 7 f t) . The nautical depth is defined as the depth to silty layers of specific gravity 1.2. The specific gravity of bottom sediments is monitored weekly by survey vessels, and decisions are made each Friday about maintenance dredging for the following week. Density charts are prepared showing the variations throughout the channels, as shown in Figure 39. Underkeel Clearance Criteria, Port of Hamburg, West Germany The approach channel to the Port of Hamburg f rom the North Sea is 6 0 miles long and 13 . 5 m ~ 44 ft ) deep at ELM. The tidal range is 3~ m tS . ft). The port is now conducting tests for squat in various conditions; in the meantime, allowances for vessels have been determined from experience, taking into account greater squat at the higher speeds in the approach channel, insufficient depth, and narrow curves. For arriving vessels , the depth of the River Elbe (13.5 m, or 44 ft~ and tidal range ~3 m, or 10 ft) , as well as tidal uncertainty, are considered, with the following results:

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4-18 Maximum draf t of ves se 1 s up to 3 0 0 m ~ 9 90 f t ~ in length: 13 .72 m (45 ft ) arri ving at Hamburg Pilot Stat' on 2 hours before high water Maximum draft of vessels more than 300 m (990 ft) in length: 12.5 m (41 ft), arriving at Hamburg Pi lot Station 2 hours before high water. The largest ship allowed in Hamburg has a draft of 14 m (48 ft) and must be brought in at high water. Conta~nerships must have an underkeel clearance of at least 1 m (3 ft). These are, of course, operational rather than design criteria. The Port of Hamburg does not pay for overdredged depths . Underkeel Clearance Criteria in the United Kingdom Ports and harbors - in the United Kingdom general ly apply one standard for underkeel clearance to sheltered waters and another to open waters, but other criteria are occasionally used. The British Ports Association offered the following summary to the panel: Traffic Underkeel Clearance Port A Almost exclusively VLCCs Port B Port C Port D Port E Port F Port G VLCCs, bulk carriers, general cargo ships VLCCs, containerships, general cargo ships and passenger ships VLCCs, bulk carriers, general cargo ship s VLCCs, bulk carriers, general cargo ships ROW RO f erries and general cargo ships to 4.8 m, or 16 ft. draft Similar to Port F ~ 1096 in excess of draf t f or well -sheltered channe 1 0 .9 m ~ 3 f t ~ f or sheltered waters; 2.3 m {8 ft) for ships to 150,000 DWT in open waters; 10% in excess of draft for ships 150 ,000 DWT to 250,000 DWT in open waters 1 m ~ 3 ft ~ sheltered waters; 1.6 m (5 ft) open waters 1 .2 m ~ 4 f t ~ in relatively sheltered channels For VLCCs in excess of 12 m (39 ft.) draft only: 0.9 m (3 ft) on the flood tide, 1.5 m (5 ft) on the ebb tide O.3 m (1 ft) over muddy bottoms {but often less); 1 m (3 ft) over sandy bar 0.6 m to 0.8 m (2 ft to 3 ft) in sheltered harbor

4 -19 me Association points out that "open" and "sheltered"' may be variously interpreted from port to port, that depths change more rapidly in some ports than in others, and that traffic conditions, channe 1 width, and the nature of the cargo may imply a need f or dif f Brent standards . The National Ports Council and the Department of Transport have sponsored studies to improve understanding of ship behavior and channel characteristics ( e . g., National Ports Council, 1976 ) . In the Port of Southampton, the design channel depth is 50 ft (15.2 m). The maximum draft allowed is 48.5 ft (14.7 m) at high tide. Minimum underkeel clearance required by the port is 4 .2 ft ( 1.3 m) . The overdredged depth is 0.3 m ( 1 ft) . The South Wales Ports specify overdredging depths of O .7 m ( 2 f t ); in areas of high shoaling, 1 m ~ 3 .3 f t ~ . The ports operator, the British Transport Docks Board, owns and operates the dredges . Shippers' Criteria Ship operators must appraise ports to decide their suitability f or ships of particular dimensions. Among their concerns is channel depth. Crane ( 1981 ~ gives a brief summary of the methods shippers use to make such appraisals, including extens' on of experience, hydraulic model studies, and simulations of varying sophistication. For bottom-clearance appraisals, Crane points out, there are several procedures, the simplest being experiences of other ships and the reports of pilots. Another simple technique is essentially similar to the criteria used for designing channel depths: addition of allowances for squat, trim, and other factors to the ship's static draft (Figure 40). This method will produce overly conservative results for a particular ship, Crane notes, because it assumes the coincidence of maximum vertical excursions. "Therefore, statistical addition of allowances for each factor should be substituted ~ Figure 411 . " This statistical method has been elaborated by Kimon ( 1982 ~ for comparison against a standard for underkeel clearance, namely a very small probability of grounding. The generalized method is a statistical combination of all the factors known to be most important in dete,.~ining the depth of water required by ships of certain drafts (or required underkeel clearance, or both). The statistical combination produces an acceptably small probability of grounding in cocoon with a semiemp~r~cal coefficient, whose value is derived from known ports (similar to that in question) with many ship-entry years ' experience. The method can be applied by a ship's master to a particular port using preprinted worksheets, sample arithmetic calcu- lations, and a series of graphs developed by Simon. The uncertainty associated with some factors is as great as a factor of 10, and judgment is inevitably decisive in certain cases, but the characterization of various f actors ~ such as ship response to waves) can be updated as data become available.

4-20 WATER REfERENCE LEVEL GROWS ~ UND£RKEEL CLEARANCE t ADMISSABLE DRAFT e I`JERttCAL MOTION )(SWELL AND SQUAT) ~ _ _ _ _ _ _ _ _ _ _ _ NET UND£RKEtL CLEARANCE SOUNDING ACCURACY SEDI MENTATION BETWEEN DREDGINGS TOLERANCE FOR DREDGING Figure 4 0 Conventional net underkeel clearance calculation, def initions from PIANC TIDAL UNCERTAINTY PREDICTED TIDE 1 CHA.R:IED DEPTFt 1 CHART I'ATU M ~ _ _ _ _ _ _ STATIC DRAFT UNCERTAINTY UNCERTAl~JS=~7 CHARTED DEPTH UNCERTAINTY NOMINAL SEABED ~ SHIP rWATER LINE STATIC DRAFT 4_ _ _,~ _ . _ __ - ~ W, tVE RESPONSE UNDERCUT SlLTAtlON Figure 41 S-atis=~c~l ~d¢~keel clearance caTc~;~ti~*< Permanent International Association of Navigation Congresses *SOURCE: C. Lincoln Crane, Jr. (1981), "Concerns of Ship Owners, " Problems and Opportunities in the Design ~ _ = ~ — (Washington, D. C.: National Academy Press ), pp . 4 5-52 .

4-21 This statistical technique could prove useful in the validation of channel design. While Kimon's analysis and results are specific to tankers, the generalized method could be applied to other ships (or the design ship). It is interesting to note that a similar method was used to design the channels of the Port of Zeebrugge, with a some lar aim: a very small statistical probability of grounding. Discussion The criteria recommended by PIANC were developed by ref erence to recent research and approved by voluntary consensus of many nations ~ including representation f ram the United States ) and the IAPH. Particularly for advance maintenance dredging, the criteria assume more f requent bottom surveys than is normal practice in the United _ . . . . _ . . . states, wnlcn is no more often than annual or semiannual. Taking the largest vessel in 1980 (by draft) that transited each of the nine channels selected by the panel, underkeel clearance was calculated by the lower value of the U. S . rule of thumb and of PIANC criteria in Table 12 . There channels were designed long ago ~ 2 0 years or more ), and vessels have been built that are much larger than the design ship or ships . The margins of saf ety that were assumed in the original design are no longer offered to the vessels using these channels . Actual use of the U. S. rule of thumb in the design of channel depths today would prove cumbersome, as it relies on several estimates f or ship behavior . Substitution of the PIANC criteria seems a reasonable step. The statistical technique also offers attractive features--for example, incorporation of a sub jective judgment of safety (an "acceptably small probability of grounding" )--that could prove useful in builds ng consensus among ship operators, pilots, and local Coast Guard off icials about the otherwise vexing question, How safe is safe enough?

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