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3 CONSIDERATIONS IMPORTANT TO DETERMINING CHANNEL DEPTH Channel Types Dredged navigational channels may be open or confined (Figure 3 ~ . Open channels characterize entrances from the open sea to the port or harbor ; conf ined channels may be canals, dredged estuaries or rivers, or excavated extensions of waterway systems. Definition of Water Level Depths of navigational channels must be specified relative to the water surface and channel bottom (special problems of defining the bottom in silty channels are addressed in a separate section). The water surface is defined by reference to a datum, mean low water (MLW) or mean lower low water (MEOW) on the Pacific Coast. These are averages of low (or lower low) water" for 19 years. The actual water surface in the channel is often cliff icult to specify. The main tides produced by the relative motions and positions of the earth, moon, and sun are not experienced instantaneously. The tide is a gravity water wave with a very long wavelength and, owing to coastal bathymetric and hydrodynamic effects (such as losses to friction and ref lections f rom the seabottom and boundaries), may have significantly different elevations at different locations in a bay or estuary at the acme time. The mean surface itself is an irregularly warped plane that is sensitive to changes in the shoreline and the depths of channels 3-1

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3-2 Figure 3 Types of dredged navigational channels 1 ~ Open Confined . '///// 7/ ~~ r ~~\ LDEPTH - ~TH CHANNEL WIDTH

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3-3 (Johnson, 1929~. Few tide gauges are located in the open waters of major coastal channels. Dramatic (and sometimes rapid) vertical changes may occur witn the Impoundment of river outflow owing to constricted harbor entrances, or with strong prevailing winds in one direction. Wind stress at the water surface may induce a surface current in the direction of the wind that forces more water to the leeward side and lowers the windward side (with a resulting return flow below the surface). This wind setup, or letdown, depending on the point of . measurement, can be considerable in confined or estuarine channels; in open channels Located in coastal areas, the phenomenon is called storm surge. If the storm surge is caused by hurricane winds, the lowering of atmospheric pressure will cause water levels to rise well above those induced by wind stress alone. A few channels and harbors are subs eat to seiches, or standing wares of relatively long period, induced by an intermittent or periodic series of changes in local atmospheric pressure and winds, or by oscillations communicated through a harbor entrance from the open sea (resulting, for example, from wave groups, offshore earthquakes or landslides, or changes in atmospheric pressure and winds). These standing waves may achieve large amplitude if the causative force or forces are periodic, and their period is close to that of the resonance frequency of the waterway system, harbor, or bay. The measurement and prediction of these changes in water level present formidable challenges to designing channel depths and to evaluating the effect of changes. These are carefully addressed in the Engineer Manual. As the manual states, "The determination of these effects may be accomplished by means of difficult computations, hydraulic models, or electric analogs....For investigations of problems within a waterway, where it is not contemplated that significant modifications in the geometry of the waterway will be made, it is necessary that tidal data be available at a sufficient nether of stations along its course to define the tidal establishment throughout. " With the exception of some sites, tide, current, and channel depth information is provided by an annual prediction of daily tides and currents, published by the National Ocean Survey (NOS), tide-gauge readings, nautical charts, and the surveys conducted by the U.S. Army Corps of Engineers. At the request of ship pilots or the U.S. Coast Guard, NOS or the Corps will sound the channel bottom or check shoaling areas, but as indicated in Chapter 2, surveys are typically conducted annually or semiannually, and nautical charts are typically updated annually (National Ocean Survey, 1982~. Information is rarely available in real time on water levels at points of interest in the channel, and the difference between predicted and actual high or low tide may be as great as 1 ft.

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3-4 Ship Behavior and Channel Depth Ships moving in navigational channels are subject to forces that no counterpart in the open ocean, such as bank and bottom suction. They also experience significant changes in the effects of hydrodynamic phenomena associated with forward motion and maneuvers, forces of the physical environment, and the forces generated between ships in passing and overtaking. The effects of channel depth can be considerable, particularly when underkeel clearance is small, and may be significantly amplified or dampened by interactive forces. A vessel's six degrees of freedom in motion are illustrated in Figure 4. The interactive effects experienced by a ship vary with the type of channel: if confined, the channel's sides and bottom will increase the longitudinal flow of water under and around the ship, improving dynamic course stability and consequently reducing maneuverability (Eda, 19711. If the channel is in open water, the restricted flow of water under the ship increases water resistance, but less linkage, or squat, is experienced in such a channel than in a confined channel of equal depth (Kray, 1973~. Frequently critical for ship controllability is the harbor entrance. Ships may experience extraordinary vertical (and horizontal) excursions owing to the characteristics of the ship--its speed and trim, for example, or length and natural frequency in heave and pitch--and its interactions with waves and swells, currents, winds, and salinity and temperature gradients. Squat A ship in motion depresses water levels around it, and its draft increases. The difference in a ship's dynamic draft and static draft, or squat, depends primarily on speed and water depth, becoming more pronounced at higher speeds and shallower depths. The panel's preliminary calculations of the squat of typical ships in nine major channels in the United States are set out in a section of Chapter 4 ("Evaluating the Adequacy of Criteria..."~. Squat increases in passing or overtaking. This case is not so well understood as that of a ship in motion in a confined channel, but some research and modeling has been undertaken in recent years (Eda, 1973; Eda et al., 1979; Dand, 1980~. Combining the curves and tables developed by Dand (1980) from model tests and data for shallow and confined water given by Sjostrum (1965), Kimon (1982) developed a set of guidelines {Table 4) for the squat of the slower vessel in passing situations. {The complex hydrodynamic forces generated in overtaking are essentially the same as those for passing, with the critical difference that the duration of the encounter is longer in overtaking.) Other factors may influence the amount of squat: Kiman t1982) indicates that squat can double in executing turns, reversing the propeller, and experiencing sudden changes in water depth.

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3-5 Figure 4 Ship's six degrees of freedom in motion SURGE X- ~ , ROLL ' B\, HE HEAVE Z ~ YAW ED E _ _ I_ _ _ PITCH\~ SWAY 1 X, Y. Z = System of coordinates, related to ship's principal axe. x, y, z = Linear displacements A, , ~ = Angular displacements

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3-6 Table 4 Multipliers for additional squat, ships passing* Own Ship Speed Passlog Ship speed O.S . "hers: Seperetio. of "" Center ~ - tarter ID. "~- esSne Ship HultSolSer - ova 9"thtD - t ,6 1 en ~ cn ~116~- stem ~ _ Bomb lithe ~ = - O 6 Passing Ship Speed . I - tamp S.per.~ of "Ip C - ~~r "~ - 2 - roar Snle's " _~2~e Snte lt~tIolIsr - ~ ,868~e "1' ~UltlP\~.S - stem llo~ "pt5~ It \^iO \_~ ]~56 ~ "ptht="t 1.10 1.20 1.50 1.0 I.0 I.0 . ~f~ O.7 Passing Ship Speed .0 3.4 . 3.6 I.0 1.0 1.0 _ Latere1 Seperation of ShS,. C - tar ~ - [__ ~ Iean ~ l~,tt~hr~t I.~~ I~= I~ ~ _ ~! "p~t~ft 1.10 1.20 1.50

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- . 3 - 7 Owst Whip Speed Passing Ship Speck - 0.8 _ . ,.~' Thee MuitIeSter - ~ ~ as"e shin HuttIotIer - stem C ~ 1.10 1. 0 I. - I.10 1.10 1.50 _ At 2.0 2 ~ _~.. i_ 2.0 t.0 2.2 3.! 2., 2.. _ I 2.0 2.0 l _2.0 l,.' l.l 2.0 I ~ ~ ~ I., I., I., I., ~. ~ 1.1 1.0 I.0 I.0 I.0 I.0 1.0 _ - I - tere1 Seperet~ o! ships Center Sareer Shio I.1 I., _ :~ ?.2 ,.6S 4.0 . OWSI Ship Speed gal sing Ship Speed ~~~ tesetes PIP ~ . ~ I ?~2ne ~e shalt ~:~ax - ~ - ~i[~i~i= ~ "Ipe Center Laresr ShI-s " ~2~1 1~pth~t 10 1.20 1.50 I., 3.65 4.0 I.7 1., 2.. 1.' I.1 2.! 1.. 1., 1.. ~ _ I.1 I-. _ . _ I.0 1.0 1~0 _ ~~u3, ~t~~~t t-~6 t.20 1.50 I:!! 2., .5 L~ 1.0 I.. 2.2 ~ - 2.. 2.1~:~ 1.2 I.2 _ S.O . *SOURCE: P. M. Kimon (1982), "Under3ceeJ Clearance in Ports, " Paper presented at SHIP-TRANS-PORT Symposium, September 8-10, 1982, Rotterdam. .

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3-8 Trim The difference in a ship's draft over its length, or trim, also changes with varying conditions. Changes in squat are generally accompanied by changes in trim. Theories and - that the change in trim (from bow down to bow critical speed, V/(gL)1/2 = 1, where ~ is the experiments have shown up) occurs at the ship's speed, g is the gravitational acceleration, and L is the ship length. At the critical speed, the vessel's bow will be at its lowest point relative to the channel bottom. Ratio of Channel Depth to Ship Draft: Confined Channels Curves for acceptable ratios of channel depth to Ship draft ~ and channel width to ship beam), based on digital simulations' observed performance of pilot-controlled ships in small and large model tests, and full-scale ship trials in deep and shallow water, are shown in Figure 5 (Eda, 1971~. Contours A, B. and C represent differences among pilots steering straight channel segments in the degree of their sensitivity to small changes in heading angle and in their response with changes in rudder angle, A being least and C being most sensitive and responsive. (Actually, the pilots of contours A, B. and C are simple mathematical models suggested and validated by observation of the complex human activity of ship handling.) While any channel configuration within the acceptable contour offers equal ship controllability by the selected criteria (maximum rudder angle, directional stability, and underkeel clearance), one or another of the three factors dominates the limits of acceptable ship control for various channel cross-sections, as indicated in Figure 6. The experiments and analysis were directed to determine the relationship between channel dimensions and acceptable ship size, with a particular view to the proposed interoceanic canal across the Central American Isthmus, and thus assume calm water and tanker traffic. The results indicate that canal width is f ar more important than depth in ship controllability. "When canal width is increased at any water depth," the report concludes, "there is a significant improvement in ship controllability" (Eda, 1971~. Shallow channel depths need not affect ship control, but only "if sufficient bottom clearance exists for sinKaae at a alven Steen (emunasls alien). Figure 7 dramatically illustrates the underlined statement with the turning trajectories of various tankers in deep and shallow water. Figures 7a and 7b are computer simulations; Figure 7c shows the results of marine trials. In all sets of curves, the limits of acceptable channel depth to ship draft ratios are reached at channel depth = 1.1 x ship draft. Pilots indicate a preference for channel depths 20 percent deeper than ship draft (Knierim, 19817. As an approximate check, Table 5 indicates the controlling depth (~shoalest,~ or shallowest, point) in

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3-9 Figure 5 Composite 1 ~ iting contours of relationships between canal dimensions (confined channel, calm water, tanker-form ships)* offs 2.S~ 20L .u _ to 1 TANKER f - M (Cee0.83) AT Fr aO.OS UNACCEPTABLE t.ACCEPTA~E ., lo,. I; W'8 Day H = water depth/ship draft W/B = channel width/ship beam *SOURCE: Haruzo Eda (1971), "Directional Stability and Control of Ships in Restricted Channels," Trans. SNARE, 79: 88. Figure 6 Ranges of influence of directional stability, turning moments, and bottom clearance on ship controllability for various channel cross- sections (dimensionless ratios)* <2.0 J - - ~ 1,5 o ~ 1.0 o`~eC1C\~* /~,~0 {U N ACCEPTA BLE} ~ ~ } RANGE ~ ~ WE A . / Morton \.: / CLEARANCE Au`.,. .: .C, ,.l',rtI.C,A,L Anglo ~ o THOR -~107H CHANNEL VESSEL I *SOURCE: Edwin W. Eden, Jr. (1971), "Vessel Controllability in Restricted Waters," J. Waterways, Harbors and Coastal Eng., Proc. ASCE, _ (WW31: 486.

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3-10 . . Figure 7 Turning trajectories for various ships with changing water depth* Bt0 000 lYT ~ER FULL-L - D C - D I ~ I "~F AHEAD SPEED S 3S SAC ItUDOER ~ 2 ~ in_ ~ ~6 ~ In_ [~1 Ad. a) Effect of water depth on toning p~om~r~ r 280.000 ~11 TANK~. ESSO ~" FULL-L"D CONDI TION - hALF AHEAD SPEED 35-DEG RUDDER 4 244 400.000 GYP = - E. I ~ "E, AND S - LL~ HATER - Lf-~"O SPED FULL-L"D 35 OEC ~UDKR SHALLD~ WATER D~/~=1-2 l fZ~ 1 1 2 3 4 5 b) 7wni~ ~apmo, ~ /~=1.2 ~ \ ~g ~ C ) Eff.Ct ot wat. depth on tuning perfom~ (Esso 0~ ~bl r~s h d~p ard *~Ibw w ter) *SOURCE: Haruzo Eda, Robert Falls, and David A. Waiden (1979), "Ship Maneuvering Safety Studies, " Trans. SNAME, 87: 231.

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3-11 the channels of four major ports of the United States, the tidal range, drafts of the larger ships using the channels, and their number. The implication of the table is that the tidal range is being used to assure minimum underkeel clearance over the shoaling points. Assuming maximum high water, clearance of the shallowest points of the channels of Baltimore and Hampton Roads is below the acceptable minimum for calm water given by Eda (1971) and Eda et al. (1979), and at Galveston and Philadelphia it is near the minimum. Moreover, the predicted values may differ from those observed (predicted and observed high-water levels differed by more than a foot 79 times in Baltimore in 1981~. Several uncertainties and compensating or aggravating factors affect the interpretation of Table 5 ~ these are discussed in succeeding sections), and while caution must therefore be exercised in drawing conclusions from the table, it does indicate clearly that a significant number of ships using the channels of these ports have less than rule-of-thumb underkeel clearance. The information presented agrees with the reports of ship pilots to the panel during its site visits that underkeel clearances are less than 2.5 percent for many ships in the navigational channels of major domestic ports and harbors. Ships in Harbor Entrance Channels As noted by Gray (1973), vessels in an entrance channel are subjected to many external forces, such as crosswinds, turbulent waters, breaking waves, tides, and currents. Some of these physical phenomena are strongly interactive; for example, extreme standing waves may be caused by the opposing oscillatory forces of waves and tides in the face of a strong opposing current. Wang (1980) notes that for swells occurring in the predominant direction at the entrance channel of the Columbia River, an ebb tide tends to steepen the wave front and may cause it to break, whereas a flood tide tends to lengthen the swell and reduce the likelihood of breaking. The range of motion experienced by a ship in this transition between the open ocean and the sheltered waters of a fort or harbor is a function of the outward _ , current from river flow; Of wave and swell heights, directions, and celerity; of the ship's length and natural frequency in heave and pitch, and the ship's speed and trim; and of salinity and temperature gradients (Waugh, 1971), and it may be considerable. Entrance channels must therefore be deeper "to allow a safe vessel's entrance and provide for some reasonable reserve of depth...to compensate for harmful effects of elements acting on vessel, and for the storage of sediments" (Kray, 1981, p. 100), but relatively few data have been collected for the variables of interest. Participants in an interdisciplinary meeting on the design of entrances to ports and harbors gave the highest priority for research to the prediction of ship motions (Marine Board, 1981~. Specifically, .

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=~ - ~ - ~ e a ~~ I. mat'& A. ' 5~ a - _e ~ ~ ~ `~ ~ . : _~ 3,_. it_ __ 1 a_ Figure 29 Split-hull seagoing hopper dredge (inset shows split hull open to discharge dredged material ~ ; ~ r , Hi.' .'r `.~: Ale' ''''Wit __ by_ 4_ _ - ._ ,. _ . ~ ' ' ;~ A. ~q _. _ %;1_ .' . _ :i_9~ _ . '^''' ~_. _ ~ _ _ _ Il~_-

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3- 5 3 ~,,~ Am_

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3-54 Am- " ~_; . ~;a' ~ -. ~ 71 Bezel '1 .. ~ ~ ~1 *01 In.. _ At. . ~ tot, it`' t,`- f ~ I . '(. ~ ~ Ail' hi? { ~ '^- . ~~- r,4~ _~] "~ _~r~- : ~1 r -I - rl u - a' h -

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3-55 Environmental Conditions landlocked channels present fewer problems - than open channels for controlling the dredg'ng process and for pre- and post-dredging surveys. Exposed waters allow less control of both operations. Tides, especially those of some magnitude, affect dredging and surveying, particularly if the gauge is far from the site . ~ See Chapter 3 under "Def inition of Water Level. " ~ Where the location of the tide-gauge station is known to be unrepresentative of the dredging site, signif icant "no-pay" overdepths may be dredged to ensure that the required depths will be measured in the postdredging survey. Currents have a marked ef f ect on hopper dredge accuracy, influencing not only the movement of the ship, but the attitude of the ship in relation to the channel. New Work versus Maintenance The accuracy of dredging is little af f ected by the change f rom new work to maintenance . However, there may be substantial variations in the no-pay yardage, depending on the type of material, so de slopes, previous dredging, consi stency of the material in the cut, underlying material, and other variables. Examples of Accuracy of Dredging Operations . Cutterhead Dredge A typical section of maintenance dredging by a Butterhead dredge is shown in Figure 32. A channel such as the one shown is dredged in two passes. How the maintenance dredging is accomplished depends principally on how the original channel was dredged, the type or soil encountered within design depth (silt or sand ), and the virgin material encountered j ust below grade. If the channel was previously well overdredged, then mai ntenance dredging will be easy. This is shown clearly in the cross-section, particularly on the right side (250 ft to 500 ft) of the channel. Here the maximum swing speed of the dredge is the limiting factor for production, owing to the low bank, so if no hard ~ virgin ~ material is encountered, the total pay quantity can be removed with dispatch. The channel is dredged too widely on the right side because of the previous overdredging. Almost all maintenance material must be removed to achieve grade in the corners (toes) of the cut, as can be seen on the left side, where higher banks were encountered and where the virgin slope is closer to the required prism. New Work in Protected Waters Figure 33 shows a typical section for new work in protected waters performed by a Butterhead dredge. Positioning was very accurate; stakes could be set out because there was no traffic, and the water was calm and shallow. The soil consisted mainly of soft to medium clays, which is ideal for cutting slopes, as can be seen in the cross-section. However, the channel is overdug on one side, owing to the inaccuracy of width indication and spud position. No electronic positioning system was used on this dredge, but the cross-section indicates very accurate dredging to the required depth, which can be attributed to good tidal information and ~ in this case ~ small tidal dif f erences.

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3 -57 .m ED .N

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3-59 New Work in Unprotected Waters The cross-section pictured in Figure 34 is for a hydraulic dredging project 10 miles (16 km) offshore. Use of an electronic positioning system was essential to th, s project, but accuracy can vary between 7 ft and 50 ft ~ 2.1 m and 15.2 m) . This explains why the channel is overexcavated on both sides. Depth control is very difficult in open waters, owing to the movement of the dredge hull and consequent problems controlling the position of the draghead. Maintenance Work In Sand and Silty Sand The irregular bottom made by a hopper dredge in progress as Illustrated an Figure 35. For width accuracy, the dredger relies on electronic positioning and thus experiences the problems noted for cutter dredges. Moreover, this type of dredge is free-sailing, which necessitates maneuvering the ship while dredging. Assuming that experienced personnel are operating the dredge, the important f actors in control of the ship are currents (especially crosscurrents ), winds , and swells , as these factors influence position. Another factor important to accurate work with a hopper dredge is frequent surveying, as a hopper dredge gradually brings a large area to the required depth. Maintenance Work in Silt and Sof t Clav If the material is soft, _ overdredging is likely, as is shown in the cross-section of Figure 36. Soft clay was encountered in the corners, which is much more difficult to remove with a hopper dredge than with other types of dredges. Implications The overdredged depths actually left by dredging are likely to be greater than those allowed by the pay overdepth specif fed . Estimates compiled by Lacasse (1981) of overdredging to achieve design prism indicate it may represent 10 percent to 15 percent of total volumes dredged. As implied by the examples, the pay overdepth specified is an incentive to dredging accuracy. Although each case will be different, accurate pre- and post-dredging surveys may yield multiple benefits--better depth information for navigation, for example, and increasingly precise pay-overdepth specifications.

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