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Criteria for the Depths of Dredged Navigational Channels (1983)

Chapter: Considerations Important to Determining Channel Depth

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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 28
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 29
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 30
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 31
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 32
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 33
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 34
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 35
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 36
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 37
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 38
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 39
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 40
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 41
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 42
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 43
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 44
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 45
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 46
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 47
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 48
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 49
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 50
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 51
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 52
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 53
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 54
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 55
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 56
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 57
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 58
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 59
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 60
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 61
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 62
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 63
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 64
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 65
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 66
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 67
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 68
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 69
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 70
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 71
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 72
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 73
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 74
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 75
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 76
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 77
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 78
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 79
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 80
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 81
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 82
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
×
Page 83
Suggested Citation:"Considerations Important to Determining Channel Depth." National Research Council. 1983. Criteria for the Depths of Dredged Navigational Channels. Washington, DC: The National Academies Press. doi: 10.17226/1707.
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Page 84

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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

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

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.

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.

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

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

- . 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. .

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

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.

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 SP£ED · 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.

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, .

3-12 Improved and validated models are needed for the prediction of ship motions, vertical and horizontal, in the environmental and operational situations found in harbor entrances. These models are needed in the development of channel design geometry (depth, cross-section, shape, and planfo`~), in the assessment of operating limits and traffic capacity, and to support the training of operators (simulators). An attempt to measure the continuously varying draft and trim of ships under way in a port approach channel was undertaken in the United Kingdom using specially adapted and calibrated cameras at selected observation points (National Ports Council, 1976~. Wang (1980), Wang et al. (1980), Wang and Noble (1982) report the results of an extensive program to validate design widths and depths for proposed improvement of the entrance channel of the Columbia River. Instrumentation developed to measure and record heave, pitch, roll, and vessel position was carried on 53 voyages of ships in the channel, and information was collected or calculated for each voyage about weather, wave and swell height and direction, and other factors. Tables 6 and 7 give the draft and loading conditions of the ships, and their maximum vertical penetration of the channel during each voyage. The statistical distribution of motion amplitudes f or a particular voyage was found to follow a Rayleigh distribution. The long-term statistical pattern of vertical vessel excursion was found to follow a log-normal distribution. The extreme-value analysis indicates that for any given transit, the vessel may experience a vertical excursion to as much as 23 ft (7 m), or that on the average, one of every ten transits will experience so great an excursion (Wang and Noble, 19821. As the project depth of the entrance channel varies from 48 ft (14.5 m) in the initial segment to 40 ft (12.1 m), Wang and Noble conclude that "a deeper channel will Improve the movement of the existing vessel fleet and will be required for the consistent movement of larger vessels." An interesting result of the program is that "tne magnitude of vertical motion ~ is] a function of position along the channel axis." Ship Speed The speed of the ship is a factor of considerable importance in both confined channels and harbor entrances. Model tests, verified By full-scale experience in the Panama Canal, indicate that for confined channels, the suction-moment rate coefficient increases with increasing speed: the coefficient remains fairly constant at low speed" but rises abruptly above certain speeds in shallow channels (water depth to ship draft ratio = 1.18~. Ship controllability could be expected to decline above these speech, but rudder effectiveness would be increased owing to greater propeller slip. Thus, for shallow underkeel clearance, a critical speed can be projected at which a ship

3-13 Table 5 Controlling depths, tidal range, draf ts and nether of ships in channels of four ports of the United States* Port Controlling Tidal Deptha Rangeb feet (meters) Ves sel Traf f icC Draf t Nether Baltimore 42.5 (12.9) 1.1 (0.3) 41 (12.4) 40 (12.1) 39 (11.8) 38 (11.5) 9 145 67 82 Philadelphia 40 (12.1) 6.7 (2.0) 41 (12.4) 40 (12.1) 39 (11.8) 38 (11.5) 37 (11.2) 36 (10.9) 35 (10.6) 230 112 223 244 197 177 Hampton Roads, 45 (13.6) 2.5 (0.8) Virginia 47 (14.2} 46 (13.9) 45 (13.6) 44 (13.3) 43 (13.0) 42 (12.7) 39 30 69 25 48 71 Galveston 42 (12.7} 1.4 (0.4) 43 (13.0) 40 (12.1) 39 (11.8) 38 (11.5) 37 (11.2) 36 (10.9) 162 453 413 301 308 anShoalent," or shallowest, point at mean low water. x~m~m, from National Ocean Survey (1982) Tide Tables 1982: High and Low Water Predictions (Washington, D.C.: National Oceanic and Atmos- pheric Administration). CFrom U.S. Army Corps of Engineers (1979) Waterborne Commerce of the United States Calendar Year 1979 (Ft. Belvoir, Va.: Water Resources Support Center). *SOURCE: "Real-Time Digitized Marine Navigation Data," National Ocean Survey, National Oceanic and Atmospheric Administration, October, 1982, p. 121.

3-14 will be most difficult to control. The abrupt change is not noticed in wider, deeper channels (Eda, 1971~. Yamaguchi et al. (1966, 1967; also cited in Eda (1971~) give the equation below for determining the 1 ~ ting Froude number from the required underkeel clearance for tankers: F - ~ 2pqtm - 1) )[q(1 ~ me) - n] - where FrL = limiting Froude n~mber based on ship length p = draft/ship length H/L ~ = 1/~1 + e), e determined from model tests to be 0.24 m = water depth/ship draft DW/H n = ship beam/canal width, B/W Contours of limiting speed are shown in Figure 8 for various channel cros"-sections . Wang (1980) notes that damping of heave and pitch decreases with increasing ship speed and that it is common practice for Asters to reduce speed in rough seas to control ship motions. He observed in 53 instrumented voyages in the entrance to the Columbia River that reduction of speeds below the average (12 kn, tankers; 14 in bulk carriers; 16 kn, containerships) directly followed increased ship motions in swells, tides, and waves. Ship speed is also a compensating factor pilots can use to reduce squat, particularly in passing, overtaking, or maneuvering turns. As one pilot states, "If a vessel will float, we will move it safely at a slow rate" (Knier~m, 19811. On the other hand, Bertsche and Atkins (1981) point out that while the "findings of several port-related studies have indicated that safety may be inversely dependent on ship's speed over a limited range...with reduced speed comes a reduction of maneuverability and an increase in crosstrack variability. Increased speed not only increases maneuverability, but also significantly reduces the required drift angle for adverse wind and current conditions" (p. 63~. Speed, therefore, cannot be reduced below certain minims for given ships in given channels. In some cases, tug assistance will be required. Few speed limits are stated for the major channels of the United States. Some are given as guidelines by the U.S. Coast Guard or local pilots association=. Other Factors The extensive studies reported by Wang (1980), Wang et al. (1980), and Wang and Noble (1982) indicate the dominance of swell as the environmental factor of concern in navigating the entrance channel of

3-15 Table 6 Sununary of vessel condition at transit of Columbia River entrance channel, full-sca~ e measurement program* _ 1 ~~= "~ I me Amp wow c~rv - n HOW NA=UD .. HOW WOW at O. - GON .. HOW Mat It6D~I AN OUVItOII WURS~ chevron ~~ - Q0~H ~~ TV - N 1~11 at ~ ''Y"Z . He- _ __ S/2077. 2 S/29~78 6~0S/~. 6/07/78 5 6/21~1J 6 6/21778 ~ 6~24lt. 8 ll/O~?J 9 11~04~78 O 1~09/1J 11 l~lOJ78 12 1~28~78 13 12~03~78 ;4 12704~?. IS 12~1S/78 16 12~17/78 17 12~20 30~?8 18 I/16~79 19 1/19~9 :0 1/21/?9 i 21 1/24~79 1 2 2 1/ 28~?9 1 23 ~ ~",~e 24 25 26 27 28 29 . 2/11/?9 2/22/?9 2/2t/?9 3/14/79 3/22779 3/23/19 _ CNZVIION CO1DRaDO CHZV~ CO:O - DO H2~ B RsrLvD' s A~SXA MaBU "U" ~ ma LB: I NIXA" ~U GOLDCN AIUW ~r~c~ ~W ~S='5 CA" 6~=E BElSNU ~ GOW2:N ~JUW ~5" MJUV BEI=U ~ C~N WAsHlt6;SON ~HI - . n~r OSL ~" OSL ~e s~ ~" = O_L CA~R B~ ~" BU." ~" OSL C1Wle:" O_L ~" OSL t~lat 01L ca=~r - e ~= OSL ~Zll OSL ~= OSL ~-= "L c~zla CO~ c~rse con~r~ camsza CO~S=& c~rza c~mrN" ra..rtR CONSASNER r.--"r~ a~s - x c&a~ C~ ^ '~our=, CO~ C~R "ar=s C~R CON~N" cw~rza CONSA7N" ca~s" or~ CaJU~ER OSL C~tZ. _ ~ST DI~OH 221 SH WS rs o" D' our rN OUT In rs ous S~ ous rs 1N o~r rx rN IN 2x rN rr' rN In rN o" __ ar~ ~-} 9-11 11-14 12-14 14 12-14 -14 -14 9-13 12-1S 12-14 14-16 10-19 12-14 10-13 6 1? 13 8 14 1? 17 16-21 ~6 17 16 1S 13 7-14 P=~" D~1~ DtSI" "a t~ JII~Uml DU~ D~S (FT, (FT] (FO (F'S, (LS) ~ _ 651. 3 62S. 0 96.0 34.0 3S, 000 6S1. 3 62S . 0 96. 0 34, .0 3S, 000 6S7.8 623. ~ 101. 1 33.0 36, 000 6S7.8 623. ~ 101. 1 33.0 36, 000 651.3 625.0 96.0 341,.0 3S,000 6S7.J 623.4 101.1 33.0 36,000 657. ~ 623.4 101. 1 33.0 36, 000 6S1. 3 62S.0 96. 0 34.0 3S, 000 6S1.3 62S.0 96.0 34.0 35,000 6S1.3 625.0 96.0 34.0 3S,000 6S1.3 625.0 96.0 34.0 35,000 68S.7 639.8 - .4 34.4 23,0= 651. 3 62S.0 96. 0 34.0 35, 000 651.3 62S.0 96.0 34.0 3S,000 523. S S03. 0 68. 0 32. 1 18, 000 S23. 5 S03. 0 66.0 32. ~ 1S, 000 68S. 7 639.8 98. ~ 34. ~ 23, 000 630. 3 606.0 71. 5 32.9 1S, 000 630. 3 606.0 ?1.S 32.9 18, 000 700. 3 656. 0 101. 7 34. 4 2 3, 000 616.8 S74. ~ 82. 7 35. 2 19. 0Q0 64S . 7 639.53 ~ .4 34.4 21. 000 718. 5 669.3 102. ~ 36.8 27, 000 697.2 ES6. 2 98.4 34.5 24, 000 616.8 574. ~ 82.7 3S. 2 19, 000 68S. 7 639.8 697. 2 (S6 . 2 98.. 98.4 34.5 34. ~ 23, 000 24, 000 6S1.3 62S.0 96. 0 34. 0 3S, 000 6S1.3 62S .0 96.0 34.0 3S, 000 "D ('S, ~ro (rr, 32.S 33.S 33.0 33.4 33.9 33.? 21. ~ 27.6 24. 5 23.; 27.8 2S.S 341.5 3S.5 3S.0 26.3 28.8 27.6 27.8 2J.3 28.0 32.8 34~. 2 33. S 27.0 30 . 0 28 . S 33.2 34.7 33.9 22.S 24.5 23.S 26.S 30.9 28.7 33.4 34.1 24.3 19.3 24.4 29.3 25.1 28.2 28.3 29.1 21.8 30.0 33.8 21.8 26.8 26.6 28.8 2S.9 22.8 31.5 2?. 2 28.9 26.8 30.9 25.? 33.7 28.S 30.6 32 -6 32.6 32.6 28.1 30.5 29.t 28.S 34.6 31.6 30.0 30.9 27.? 32.4 30.4 30.1 26.3 28.0 2?. 2 . 23.3 26.9 2S.1 LC*DS~C C0001 DSSP tLS) . (IS]_1_ {FS) 43,022 322.9 25. 44, 100 319.6 29.4 35,S44 311.8 2?.5 37,071 319.3 27.9 46,120 319.S 27.3 40,394 32S. 2 28.7 4l, 12S 329.1 30.6 43,840 318.8 29.2 36, 3aS 318.9 22.6 ·4,519 118.4 29.2 29, 2S6 325.5 23. 28,288 302.4 35.5 44, 240 319.3 29.1 26,733 317.7 23.2 19, 20S 248. ~ 20.9 19, 028 2S2.9 22.2 2?, ?78 307 . S 3S . g 22,296 299.3 23.2 23, 670 295. ~ 24.1 28, 926 308.8 37.6 29.7 22,483 268.8 29.0 29.6 1 29,176 302.3 34.6 3S, 101 320.0 37.6 29, 544 318.3 31.4 24,722 266.6 31.0 30, 280 306.0 35.9 30, 714 316.7 1 34.5 1 34,430 1 320.9 23.4 _ 31,S20 1 320.0 1 23.9 (IS] 322. 9 319.6 311.8 319.3 319. S 32S. 2 329.1 318.8 318.9 318.4 325.5 302. 319.3 317.7 248.7 2S2.9 307. S 299.3 295.d 308.8 268.8

3-16 Table 6, continued* ~ 1 1 ~" I)YAGE .~. Da" ~ 30 10/16/79 C - :VltON at~ZONA 31 10~17/79 C—:V1ON ARIZONA 32 10/28/79 a~SXA YOU 33 11/14/79 - ~ ~ 34 1~1?~79 - 35 11721/79 36 11726/?9 =~= 37 11/28/79 Ecu MASCOT 38 12703/79 .. 39 12/16/79 MESON "SI~I 40 12/18/79 ~H ~~H 41 1720/80 42 1/24/80 = 43 2/04/80 - 44 2/06/80 - = 45 2710/80 0~ H= i 46 2/14/80 47 3/04/80 1~ ~: 48 3/10/80 LION '5 "" B= 49 3718/80 =~ ~ 50 3/22/80 }I MEK:llaNS S1 3/26/80 COLD8N AJtl~l 52 4/01/80 ~S" t~U 53 4/03/80 ' TYPE 01L ~= OIL CMRIZR CONTAIN C~ER BU" alul,X ~—D.~= C~IUNI:R CA~ZA CON~N" CUUUZR 8ULIC BU1JC r.--~ ort c~ 0= ~"t" C - S==R aUIRIER CO~N" CW\ZER AUSO CU~ AUSO C~F~ BWK C~:CR a CONTA2NI!:R aU~R COttSAlt~ C' - ~R B~ ~= B CO - A~ CADUZ~ CONTaINCR ~TF~ ~ ca~a "aNSIS 5~=D D1~:C=ON I { 1~5 . ) Il1 11-14 0= 11-14 ~ 16 IN —14 Ot~ —14 OUS 13 IN 17 IN —14 OUT -14 ~il 11-14 OUS 11-14 It' 14 U1 17 rN 16-21 OU! 16-21 ~ —14 OU! —14 IN 13 IN 16 - 21 ~ _~4 00~ —14 1N lt IN 16 ~ -14 623.4 101.1 623.4 101.1 606.O tI.S S74.1 a2. 7 65? .8 623.4 657.8 623.4 6S1.3 625.0 651. 3 625.0 700.3 6S6.0 616.8 574.1 566.7 557.7 566.7 557. ? 657.8 623.4 657.8 623.4 630.3 606.0 718.5 669.i 6S7.8 623.4 657.8 623.4 616. ~ 574. 685.7 639.S 33.0 36, 000 33.0 36,000 32.9 1S,000 3S .2 19 .000 101. ~ 101.1 96.0 96.0 101.7 82.7 90.6 90.6 101.1 101.1 71.5 102 .4 101.1 101.1 82.7 98.4 33.0 36,000 33.0 36,000 34.0 35,~ 34.0 3S,OOO 34.4 23,000 3S.2 19,000 29.0 21,00O 29.0 23, 000 33. 0 36, 000 33.0 36, 000 32.9 18,000 36.8 27, 000 33.0 36,000 33. 0 36, 000 3S.2 19, 000 34.4 23, 000 P=NCS,JU. DD~SalS j LOADSlIG COIIDS=alB _ ~ ~ DRAFT' ~ ~DISPIJU:E - NS D=ICN l ~A ~IP BEWUml DRAFS DWT Iwo "S MEAN (~) "G | VCG (Pr) {F~) (FT) (FT) (~5) (FT) (FT) (FT, (FT), (FT) S1.3 62S.0 96.0 34.0 3S,000 32.8 33.5 33.2 43,300 1 320.4 ~l 29.2 51.3 625.0 96.0 34.0 3S, 000 21.9 23.4 22.7 28,000 1 321.5 1 24.0 8S.7 639.8 98.4 ".4 23,000 30.9 31.6 31.3 31,108 329.3 1 36.1 S7.8 623.4 101.1 33.0 36,000 25.7 29.3 27.S 40,774 324.0 28.8 57.8 623.4 101.1 33.0 36,000 27.0 31.2 29.1 43,286 322.6 30.0 30.3 606.O n.s 32.9 1S,000 19.0 29.0 24.0 20,600 294.4 24.a 16.8 S74.l 82.7 3S.2 19,000 31.2 34.2 32.7 25,775 275.8 31.9 34,2S2 318.6 32.5 39,070 318.7 26.8 44, 100 320.7 29. 3 29, 200 319. 7 24.2 29, 049 310.9 36.6 23,960 217.6 30.8 26,905 296.3 31.6 25,686 300.8 31.6 43, 685 327 ~ 27. 44, 351 330.3 1 31.2 22, 010 308., ~ 24. 8 33, 637 318.4 37 .9 46,094 ~ ~ ~ 48,260 322.4 21.0 23, 960 298.0 31. 7 33,175 335.7 37.2 40, 261 311.7 25 .8 25.7 29.3 27.0 31.2 19.0 29.0 31.2 34.2 21.8 2S.S 25.3 28.2 33.4 33.9 21.5 25.5 28.3 30.6 29.5 32. 22.S 24.3 22.3 21.8 27.3 31.5 28.2 31.3 25.5 2S.S 30.4 32.6 28.5 33.0 30.7 33.7 28.8 32.8 31.7 27.S 29.1 24.0 32.7 23.7 26.8 33.7 23.5 29.5 30.8 23.4 22.3 29.3 29.8 25.5 31.5 30.8 32.2 30.8 33.5 32.6 6S7.8 623.4 101.1 33.0 36,000 35.0 30.0 27.5 _ *SOURCE: S. Wang, C. Butcher, M. Kimble, and G. Cox (1980), "Columbia River Entrance Channe] Deep-Draft Vessel Motion Study, Tetra Tech Report No. TC-3925, Final Report to U.S. Army Corps of Engineers, Pasadena, Calif., September 1980.

3-17 u so . - u . - o ~ U ~ -3 ~ ~ ~ 3 ~ ~ ~ ~ ~ ~ ~ 4 ~ Q ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~5 _ _ 1 ~ . U' ~ _ _ , - _ o US ~ . _ E. 2 - Z ~ ~ ~ _ ~ ~ S Z ~ _ ;~ ~ z ~ _ :u - ~ ~ ~ ~ ~o ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ t I ~ I l l I I l I l l I l l I l I l l l 10 ~1 ~ ~ ~ ~ \0 ~ \0 `0 ~ `0 ~ ~ ~ ~ ~ ~ ~ \0 'O 4~ q~ O 4o { I I ~ 1 — 1 1 1 4e ~ _ ~ ° al ~ ~ ~ ~ O ~ ~ ~ ~ ~ ~ ~ ~ ~ z ~ ~ ~ ~ s ~ ~ 31 ~ ~ ~ ~ ~ 1 . ~ ~ ~ ~ t~ ;~ ~ ~ 1 - ~ ~ _ ~ ~ ~ ~ ~ ~ .-. ~ ~ ~ ~ ~ — ~ ~— ~ '0 0 _ ~ ~ ~ ~ ~ . . · . . · · · . . . · · · · · · · · · · · · · · · . . . ~ ~ ;~ ~ _ ~ _ ~ .t ~ ~ O ~ 1 - 1 - 0 ~ — ~ ~ 1— ~ ~ — O O ~ ~ ~ \0 ~ ~ ~~ ~ ~ t~ O ~ ~~~ 3~ ~ ~ ~- ·— ~4 ~ t~ C- ~\ O O ~ ;d O Cl ~ ~ ~ _ _ _ ~ _ _ ~ ~4 ~ r~ _ _ ~ ~ ~ _ _ t" `" ~ — §O !_ #_ ;l O ~ ~ — ~ ~ ~ ~ ~ 01 ~ ~ ~ — — ~ O ~ ~ _ _ _ . _ . . . . . . . . · · — tn t~ ~ 0 0 ~P ~ ~o 0 0 ~t O tn ~ ~ — \0 ~ ~ ~ ~ ~ ~ ~ ~ `,0 ~ _ _4t O O ~ _ _t _ 1 - ~ ~ _ ~ ~ ~i 0 _ ~4 0 _ ~ ~ ~t _ _ O _ _ —t — _ ~ ~ ~ ~ t~ ~ ~— ~ _ ~ ~ _ ~ ~ ~ ~ ~ t~ ~, ~ ~ ~ O ~ ~— _ ~ ~ ~ r*4 ~n cr ~ ~ ~ ~ ~4 ~ ~ ~ ~ ~ ~ t~ ~ ~ ~ ' - ~ ' - ~ ''~ _ ~ ~ \0 ~ ~ _ O ~ ~ ~ — . . . . · . · . . . . . . · · . · . . · · . · . . . r. ~ ~ _ ~ ~ ~ ~ O ,n ~ ~ ~ ~ .~ ~ ~ c~ ~ ~ ~o D :~ ~ ~ ~ , ~ \0 0 ~ ~ ~ ~ ~ ~ ~ ~ O ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ O ,. . . . . . . . . . · · . · · . · . · · . · · . . . · .. . ~ ~ r- ~ r~ ~ ~ ~ ~ ~ ~n ~ ~ ~4 ~4 ~ ~0 cn _ _ ~ r~ ~ ~ r~ _ ~4 _ ^ ~ iE o ~ ~ _ ~ 0 _ ~ ~ 0 ~o ~ ~ 0 ~ ~ ~ _ ~ ~ ~ ~ _ ~n ~ _ ~ 0 0 ~ ~n 0 ~ `0 ~ ~o ~ ~ In ~ ~ ~ _ ~ ~ ~ ~ ~o ~ ~ __ _ ~___ c~_~- _ _ _ __ \0 ~ ~ ~ ~ _ ~ ~ `0 ~ _ ~ `0 ~ O O O ~ r" O ~ ~ ;o ~ ~ O .··.···.···· .···· · · · · · · · · . · . 0 ~ ~o ~ ~ r~ ~ ~ _ ~ ~ ~ ~ ~ ~o _ ~ ~ r. ~ ~4 ~ ~ ~ ~ ~ ~— ~r _~-- 5~ ~ ~ o ~ ` ;~go~ ~Y `0 ~ _ ~ O ~ ~ O ~ O ~ ~ ~ _ ~ ~ ~ ~ r. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~___:~__~r~0 ~<~0 _ ~ 0 — — O O O O O —~ , _ ~— ~ ~ O ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ O ~ ~ · ~ ~ o. _ In . . · . . . · · . · · . . . . . . . . . · . . . . . . . . ~— ~ r~ ~ ~P ~ ~ ~ ~ Y1 ~ _ r~ ~ ~ ~ ~4 ~ ~4 ~ ~ ~ ~ _ _ _ _ ~ ~ ~ _ _ ~ ~ ~ ~_ ~ ~~ _ _ ~ _ ~ f_ _ _ P_ ~ ~ ~ ~ ~ ~ ~ _ 0 0 — _ 0 _ ~ _ 0 _ 0 ~ · ~ 0 _ qr ~ ~ ~ _ _ .04 _ ;~ _ _ q~ d~ ~ ~ :o 4o \~ ~ ~ ql _ 0 ~ ~ ~ O ~ ~ ~ ~ ~ ~ ~ ~ O _ _ ~ ~ _ ~ ~ ~ r~ ~ , - ~ ~ ~ ~ ~ ~ ,n _ ~ ~ ~ ~o ~ ~ I_ _ ~ ~ ~ ~ ~ ~ u~ ~ ~ ~ ~ ~ ,-, ~ ~ ~ ~ ~o ~ ~ ~ ~ ~n ~ ~ O O O O O O O O O O O O _ _ ~ 0 0 _ O O O O 0 ~ O 0 0 j a, ~ ~ ~ ~ ~ ~ ~ ~ o ~ r. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ;~ E. ~ ~ _ _ ~ — — O ~ ~ O — O — O ~ ~ O ~ O ~ -4 0 ~ — ~ ~ - C} P — 0- ~: ~ a: ~ :a ~ ~ ~ ~ ~ ~ s ¢ s a: ~ ca ~ — — ;a — ~ ~ ~ :a :a za ~ ~ ca :d s - ~ ~ - - ~ - ~ ~ - - - ~ ~ x ~ ~ ~ o: 3: ~ s ~: ¢ ¢ a: ~ ~ ~ a: o: ct ~ ~ ~ ~ ;~: ~ ] :~] ~, ~ ~e a: ~ ~ ~ ~ ~ x s z ~ ~ ¢ o: ~ z ~ z ~ — ~ z ~ — ~ z ~ z ~ z ~ z ~ z ~ _ ~ _ ~ ~ u U ~ ~ ;, ~C ~ e ~ _ ~ ~ ~ ~ ~ ~ ;a - ~ ~ ~ ~ ~ - ~ ~ w - ~ - :~2 —~ - ~ - ;~ ~ e t~ CJ X y U ~ y C) ~ ~ ~ ~ ~ U ~ ~ 'J ~ ~ ~ ~ ~ ~ ~ - ~ S ~ ~ ~ ~ ,e S- ~ ~ ~ s ~ ~ '~ ~ ~ ~ ~ — ~ ~ ~ ~ ~ ~ ~ 'Z ¢ ~ ~ ~ ~ ~ ~ ~ ~ ~ ¢ Z ~ Z ~ 2 ~ ~ ~ Z S — - Z S Z ~ ~ _ __~_~~-__~a:__-— ~O~O~ ~O~__ O O ~ ~ O ~ :D O O O O ~ ~ O O O O ~ ~ ~ U ~ ~ ~ ~ '~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ -' O O ~ "z z .8 'd ~ ~ - ~ ~ ~ ~ 2 Z _ _ _ _ 8 8 , _ _ ~ ~ Z Z 3 ~ ~ S ~ ~ ~ e ~ ~ e ~ 0 `3 ~ ° - - ' ° ° _ _ ~ _ ~ 2 ~ ~ Q ~ ° °~ ~ ~4 ~ e e _ _ c ~ ~ z ~ Q Q u Q u - s ~ 8 c~ ~ _ = _ t; ~ c' ~ c, <; = s e ~ ~ ~ m ,_ D ~J D ~J ~ S~ ~t D ~ ~ ~ 5t <D ~ ~ ~D (~~ @~ C 3~ O ~ :~~ ~~ ¢~~ (~~ ~~~ ~~ ~ {_ t~ ~ ~ ~ ~ ~ 1~ ~ ~ _ ~ _ ~ ~ ~ ~ ~ ~ ~ O — ·~ ~ ~ ~ `0 _ ~ _ _ _ _ _ _ _ _ _ _ ~ C~ ~4 ~ ~ ~4 ~ C C _ U C : O _ _ ~ _ O _ _ _ ~ 5 _ _ _ _ S O i _ > . -

Q EN E. Ci US o ~ §~ ~ 613 ~ ~ ~ ~ ~~ 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3 ~ ~ of Z ~ ~ ~ ~ ~ . ~ ~ I, ~ ~ C': ~ at. ~ ~ ~ ~ lo ~ ~ z ~ - ° o , o o O ~ D D ~ I ~ 1 ~ 3 ~ ~ ~~ ~ ~: ~ ~ ~ ~ 0 ~ O ~ ~ 0 ~ Or ED 10 01 ~ to (D O rat ad (D J a . . . . . . . . ~ . . . . . . . . . I ~ ~ ~ ~ ~ D D ~ ~ ~ ~ ~ ~ ~ t ~ E. ~ ~ -4 cat u, ~ l D: ~ << I^ ME ;,~E" {9 x ad ~ a l w §o xI x ~ '~ l ~ A: §~ !c ~ x ~ at; 2= ~ X ~ ~ _ P . ~ Z tD ~ ¢: ~ · ~ ~ ~ I ~ ~r a, ~c ~ a, ~ ~ ~ ~ , ~ ~o ~o ~ ~ ~ ~ ~ ~ ~D ~0 r~ I ~D ~ \0 ~ qr ~o ~ ~r ~o ~ I tD ~r ~? ~D Z Z Z Z Z Z Z Z Z P Z Z Z Z ~Z Z ° ~ ~ O O ~ ~ O ~ O ~ ~ ~ O ~ O ~ ~ ~ O ~ ~ D: ~ ~ a: .: ~ ~ ~1 ~ C) -4 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ W ~ U ~ ~ — ~ C' ~ ~ ~ ~ 8" ~ ~' ~ ~ ~ C) ~ ~ ~ ~ ~ ~ ~ ~ 3` ~ ~ E" ~ ~ Y E" ~ E" o o ~ ~ ~ 3 8 ~ ~ ~ ' 3 o o 8 ~ 8 ~ ~ ~ ~ ~ ~ ~ 8 Z N ~0 ~ ~ ~ 8 8 ~ ~ ~ ~ ~ ~ ~ 3 S S ~ , ~ ~ ~ tn ~ ~ tn tn ~ ~ ~ Oz cn ~ ~ ~ ~ ~ ~ ~ ~ W ~ ~ ~ Z S ~ ~ ~ S S ~ ~ ~ ~ ~ ~ ~ Q ~ 52 ~ ~ : ~ ~ ~ Q a~l ~ ~ Q ~ S c.' ~ Q ~ ~ ~ o ~ s ~ ~ ~ ~ o ~ ~ ~ ~ ~ 8 ~ Q CQ ~ O C) V O ,1 ~ L~ Q I ~ o O · U O - Z ~ '= Q 0 0 a oo ~ ~; _ tn X V O V E~ ~ . - · ~ _ ~ ~ ' Q ·,] U) · O ' O V ~Q . ~ 1 3 ~ · U) ·a o U) ~Q · - o U] o V

3-19 Figure 8 Contours of limiting speed for ships in confined channels* oW/H 2 0 1 ~ 1 ~61 P I\ Otis DO b0110~ 1 - 6 x~15 ~ -A 10 - ,4;- ACCEPTABLE f O lSO A, / O t' S \~0.100 ~ Mono .// W/B D: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: 87.

3-20 the Colombia River. The environmental factor ts) of greatest concern in other channels may be wind setup and setdown or storm surges, waves, currents, or ice. Intensive study of the physical environment and of ship interactions with environmental factors and the navigational channel is essential for design and maintenance. Practices have evolved in the channels of the United States to accommodate the increased traffic of larger ships--for example, loading and lightening at sea, agreements among pilots against passing or overtaking in certain areas or between certain sizes of ships, and the use of high water for underkeel clearance. Many vessels--notably tankers, but other vessels as well--sail at considerably less than full load to maintain adequate underkeel clearance. A report to the U.S. Coast Guard acknowledges that (among other reasons) a systematic plan for aids to navigation must compensate for the navigational deficiencies of channels and indicates that in congested or particularly difficult areas "the Coast Guard ~ ght limit the transit speed for user vessels or limit operations to one-way traffic" (Eclectech Associates, Inc., 1982~. Thus, while aids to navigation and local practices may adjust for the deficiencies of navigational channels, the accumulation of such compensations eventually contradicts the basis of the original channel design, which was to provide for existing and projected ship traffic or to reduce delays (and the propulsion requirements of ships, although it is doubtful if this can be demonstrated) at an economical price. Compensating factors also reach a licit. In an analysis of casualties in the Houston-Galveston area, the U.S. Coast Guard found that over the period 1969-1977, cargo tonnage increased 100 percent, but transits only 15 percent, owing to larger and more heavily laden ships. Over this period, the number of ship casualties rose steadily, then accelerated with 22 accidents in a 6 month period. The report suggests that the Houston-Galveston system has reached saturation and that safety can be expected to continue to decline (Thompson et al., 1981) . Bertsche and Atkins (1981), Kray (1973), Waugh (1971), and Eden (1971) note that studies of ship characteristics in navigational channels undertaken for one purpose often yield insights for another (the value of training pilots on simulators, the implications for ship design of channel Dimension validations, placement of navigational aids to reduce crosstrack variances in maneuvering bends and turns, the need for additional ship controllability at low speeds), and all note that as yet there are few mechanisms for communication of these insights among the community of interests. "It would appear," Eden concludes, "that there has been little or no effort made to consider the design of vessels, canals, and controllability facilities as a system...an interdisciplinary system approach could indicate that transportation costs could be reduced without increase in risk, or even that both hazard and costs could be reduced to the end that projects could be constructed for less money and in less time."

3-21 Sediment Transport and Deposition Two mechanisms of sediment movement may affect navigational channels in harbors and estuaries, denoted here as interior and exterior Processes. The interior Process is the transport of material delivered to a harbor from freshwater inland sources. Exterior processes refer to the transport of sediment into a harbor from tne ocean, i.e., the sediment carried by longshore processes, or suspended sediment carried into the harbor by tides or storms. Johnson (1981) discusses several modes of deposition at harbor entrances and summarizes the problem of transport by littoral currents. Interior and exterior processes are associated with different types of material as well as transport mechanisms; thus, they may represent different dredging problems. Exterior Processes In most cases, the sediment carried by longshore transport processes is composed of sand with a median diameter varying from about 0.1 mm to 0.4 mm. This material has usually been transported along the beaches outside the harbor by waves that refract as they propagate from deeper water toward the coast, breaking at an angle to the nearshore contours. This creates a component of momentum flux parallel to the beach, which (once material is put into suspension by wave-breaking) can move the suspended material along the beach. In addition, sand is moved onshore and offshore by wave activity. Komar and Inman (1970) give a brief summary: [The] movement takes place in two manners--in rolling in a zigzag motion along the beach with an equilibrium profile relatively large steepness, which is suspension, and Dy beach face. For a formed by waves of _ _ _, , ~ characteristic of storm conditions, the sediment movement is mainly in suspension. In the case or an equilibrium beach profile formed by waves of low steepness, which is typical of calm summer conditions, the transport appears to be the result of rolling or skipping along the beach face. It is believed that as much as 80 percent of the material moved by wave action is moved in the area shoreward of the breaking point. Sand moved along the coast can become the material that must be dredged from harbor entrances and shipping channels. By its very nature, the material usually forms relatively firm deposits (or shoals), with a well-defined boundary between the sea and the bottom. Numerous measurements of rates of transport along natural shorelines have been estimated--for example, from the amount of material trapped by man-made shoreline structures. A summary of such measured rates along U.S. coasts, compiled by the Coastal Engineering Research Center, is presented in Table 8 . The Shore Protection Manual

3-22 Table 8 Longshore transport rates for U.S. coasts. Predominant Longshorea Locate or. Direction of Transport Date of Transport (cu. yd./yr. ) Record Atlantic Coast Suf f olk County, NY W 200, 000 1946-55 Sandy Hook, N.J. ~ 493, 000 1885-1933 Sandy Hook, N.J. ~ 436, 000 1933-51 Augury Park, N.~. N 200, 000 1922-25 Shark River, N.J. N 300, 000 1947-S3 Manasquan, N.J. N 360, 000 1930-31 Barneget Inlet, N.J. S 250, 000 1939-41 Absecon Inlet, N.J.b S 400,000 1935-46 Ocean City, N.J.b S 400, 000 1935-4 Cold Spring Inlet, N.J. S 200, 000 -a Ocean City, Md. S 150, 000 1934-36 Atlantic Beach, N.C. E 29,500 1850-1908 Hillsboro Inlet, Ella. S 75, 000 1850-1908 Palm Beact;, Ela. S 150, 000 1925-30 to 225, 000 Gul f of Mexico Pinellas County, Fla. S 50, 000 1922-50 Perdido Pass, Ala. W 200, 000 1934-53 Pac~f ic Coast Santa Barbara, Calif . E 280, 000 1932-51 Oxnard Plain Shore, Calif . S 1, 000, 000 1938-48 Port Huer.eme, Calif. S 500, 000 ------- Santa Monica, Calif . S 270, 000 1936-40 E1 Segundo, Calif . S 162, 000 1936-40 Redondc Beach, Calif . S 30, 000 ~~~~~~~ Anal~.e~m Bay, Calm . E 150, 000 1937-48 Camp Pendleton, Calif . S 100, 000 1950-52 Great Lakes M, 1~-aukee County, Wis. S 8, 000 1894-1912 Racine County, Wis. S 40, 000 1512-49 Kenosh.a, Wis. S 15, 000 1872-1909 Ill. State Line to Waulcogan 5 90, 000 --------- Waukegan to Evanston, Ill. S 57, 000 -a South. of Evanstor., Ill. S 40, 000 --------- Hawai i Waikiki Beach.b . - 10, 000 --------- aTransport rates are estimated net transport rates. approximate the gross transport rates. In ';ome cases, these tMethod of measurement is by accretion except f or Abeecon Inlet, and Ocean City, New Jersey, and Anaheim Bay, California, which is by erosion, and Waikiki Beach, Hawaii, which is by suspended load samples. ~SOURCE: I). S. Army Corps of Engineers ( 1977 ), Shore Protection Manual t Washi ngton , D . C .: Government Printing Of f ice ) .

3-23 ( U. S. Army Corps of Engineers , 19 77 ~ gives procedures for estimating rates of drif t f or particular localities, but as Komar and Inman note, "no general relationship between wave and sea; ment characteristics is available for estimating the rate of littoral transport that occurs along a given shoreline." Predominant Direction of Littoral Transport The direction of littoral transport at a particular time is dictated by the direction of the alongshore component of the wave velocity at the breaking point (Figure 9~. On many coastlines, important reversals in the direction of littoral drift occur because of the seasonal or long-term variation in the direction of the wave attack. Usually, however, tne intensity of wave attack predominates in one direction, resulting in a net or predominant direction of drift. For the locations for which rates of transport are given in Table 8, the predominant direction is also given. Undoubtedly, the drift occurs in one direction along the various coastlines at certain times of the year, and in the opposite direction during the remainder of the year; nevertheless, a net drift occurs in the direction and at the rate indicated. For example, along the south Atlantic coast of the United States, the littoral drift is northward during the summer season, when light winds from the south and southeast prevail, but during the fall and winter, strong northeasterly storms, accompanied by relatively high seas, drive the sand southward. These winter storms are more severe than the summer storms, with the result that the predominant drift is southward along the south Atlantic coast ~ Johnson, 1981 ) . Sand Waves A factor that may affect channel entrances and navigational channels is the generation of sand waves. Sand waves of considerable height and length can occur at locations where water velocities are high: examples cited in the literature include Brazil, Tampa Bay, Florida; and San Francisco, California (Gibson, 1951; Johnson, 19 7 3 ~ . Sand waves observed at several locations along the coast of northern Brazil, for example, are generated by the high velocities resulting from large tidal ranges and large inland tidal basins. At the easterly approach to the Golden Gate in San Francisco, the velocity of the tidal current increases with the constriction of the seaway, and sand waves occur in depths between 48 f t and 51 f t , having average lengths from crest to crest of 240 ft and heights varying from 6 ft to 17 ft. These waves do not now appear to present a problem, because the channel depths where sand waves occur are less than the 50 ft depths maintained in the balance of the dredged channel across the San Francisco Bar. Nevertheless, if ships are transiting navigational channels with little underkeel clearance, the accuracy of nautical charts may present problems, as it will not be clear whether the soundings Known represent the crest, bottom, or face of sand waves. Because sand waves develop with relatively high currents, they will constantly be moving, and the soundings will be unreliable unless frequent surveys are made.

3-24 Figure 9 Direction of littoral current and littoral drift determined by direction of alongshore component of wave velocity at breaking point* breaker line H~ l - - ~ Wave velocity at irk breaking point _4 ~ 4 ~ ~ ~ ~ ~ ~ ~ _ 4 ~ ~ ~ \ _ ~ wave crests\ \ ~ normal component \ \ direction of littoral current and littoral drift - \ % *SOURCE: J. W. Johnson (1981), "Sedimentation in Harbors," Problems and Oppor- tunities in the Design of Entrances to Ports and Harbors (Washington D.C.: National Academy Press) ~ p. 104.

3-25 Interior Processes The material delivered to a harbor from inland sources presents a more serious problem with regard to the required dredged depth of shipping channels because, in general, the material is of a type that does not allow the boundary between the bottom and the fluid to be well defined. Indeed, the density in shoals created from material delivered from inland sources may vary from 1050 g/1 and less to 1300 g/1; this corresponds to 20 percent to 30 percent of dry solids :~~^ . ^~~: ~ ~_~ _; ~ as silt, but it consists of clay and minerals. Particle sizes are less than 100 microns and may consist of material less than 1 micron in diameter. The material in suspension, which may comprise the shoals, could be colloidal. (The special problems of silty channels for dredging and navigation are taken up in a separate section.) Very little is understood about the transport of fine material--even for noncohesive sed~ments--in unsteady flows. Thus, in harbors and estuaries with tidal flows, the mechanics of sediment transport are not firmly known, and considering that the transported sediment may behave as a cohesive material, the development and movement of shoals and shoallike material does not appear to be easily predicted. The process of wave-induced transport of noncohesive material is understood in a general way but is not easily or reliably quantified. This understanding permits description of certain aspects of the process of sedimentation and shoaling owing to the fine sediments delivered by freshwater inland sources to harbors (discussed in some detail by Ippen (1966~. The flow within the harbor where seal ment is being delivered from inland sources is complex and relates to aspects of freshwater flow into a saline body. An important aspect of the flow is the currents induced by the tide, together with the lag between currents and tides from friction effects. The estuary may be well mixed or stratified; the degree of stratification strongly influences the shoaling characteristics of the harbor or estuary tHer,-mann, 1981~. A simplified case can be described where a well-defined density stratification occurs. In this case, the more dense seawater travels up into an estuary or harbor and freshwater travels seaward over the top of the intruding saline wedge. Since flow velocities are present within the wedge due to tides and because continuity conditions must be satisfied, at the furthest upstream penetration of the wedge (within the wedge, near the toe) the direction of velocity must reverse. In this region of flow reversal, fine material transported ( liens Cobb ) . Generallv this material is classified toward tne ocean entrance of the estuary by the freshwater flow may be deposited by sedimentation processes. Thus, the region of maximum intrusion of the saline wedge originating from the ocean entrance into an estuary may be a region of excessive shoaling. In the case of an estuary which is well mixed by turbulence, such large density differences are not observed, nor will the extremes of velocities characteristic of a highly stratified system be seen; however, similar features are evidenced.

3-26 A simplif fed description of the velocity and salinity distributions in a fully stratified estuary and a well-m xed estuary is presented in Figure 10. The upper part of the figure shows the velocity distribution in a stratified estuary, demonstrating the general seaward flow of fresh water near the surface, with velocities being upstream near the bottom within the saline wedge and seaward near the top of the wedge to satisfy continuity conditions. Similar characteristics of a well-mixed~estuary are presented in the lower part of Figure 10. Fine material transported in estuaries and harbors is generally in suspension. Ippen (1966) gives an excellent description of the shoaling process in a turbulent flow: Turbulence near the bottom boundary is increasingly damped by the presence of silt which settles downward continuously as long as only moderate currents exist. Diffusion is hindered locally and with increasing consistency the material will gradually be able to resist the shear exerted, and damp the turbulence, for longer and longer periods of the tidal cycle. A shoal will grow. Occasionally, with application of exceptionally large shear due to variations in tides or flow, the shoal may be failing again. It can be eroded from the top downward as the turbulence and shear forces overcome the resistance to deformation inherent at that level of consistency of the mixture. On the other hand, large portions of the shoal may fail along shear planes where local shear and consistency combine to produce an optimum condition for rupture. Large masses of mud may then be displaced in bulk and slide to new positions or may be broken down and dispersed to be redeposited elsewhere. The establishment of shoals in estuaries and harbors and the breakdown of these -shoals may take place over a long period during which an averaging process takes place in terms of saline intrusion or fresh-saline mixtures in a well-mixed estuary. Changes in the saline intrusion brought about by dredging can have significant effects in terms of the location of shoals as well as the amount of shoaling after dredging. For example , channel deepening can allow the salt wedge to travel deep-' into an estuary, creating shoaling in regions where shoaling has never before occurred. In addition, theme tidal-induced bottom currents transport material within a harbor. If dredged materials are disposed of within the harbor, it is quite possible they will migrate to the position from which they were removed. An excellent example of some of the problems that arise witn dredging i" presented in Figure 11, which shows the shoaling characteristics of Savannah Harbor. Two curves are shown in this figure, one for a 39,000 ft (11,818 m) section of the estuary in the lower harbor, and the other for a 27,000 ft (8,181 m) reach of the estuary in the upper harbor. The average annual shoaling rates in million cubic yards (cy) of material are plotted as a function of

3-27 Figure 10 Schematic representation of Sal ~ ity intrusions in estuaries* Forerun" eaten" ~~ "knit, Veloett, Allots - ~ do~atrea. - bulb. dletrib tarpon m _i h,o (if) Us ~.e. ~ =0 _~ ~ _ . _ . ~ If E Of _ _ __ ~ t_ ~ ~ _ ~Q, _ Plo ~ co. __~ _ h~o7~7'~~ _ _Q,- ~ s 3~ r^= _ _ _ ~ _ ~ ; l ~z ~ ~ ~z ~yheo ~ ~h Well-~ed ·~" ~eum "knit, iteloett, do~ ·tre_ ~ acrib d! ·trt b. A E~1 ~ ~ ~ I ~ - 3 ~ , e~ ~ ~ ~ _ r~ `i `0 3 1 1 (~) ~ ~)h neo== ,,,,,,,__ I ~ ~ I l~h+~h Sts (lo) Ste (l`) Pree~~ ap~t - |h ~74` ~ U. 1 "S 1 1 1 St. (lo) St. (l`) )6 ~ ~* (Y ~ ^~) _ —(Ya) ~ *SOURCE: A. T. Ippen, ed. (1966), Estuary and Coastline Hydrodynamics (New York: McGraw-Hill) - (~)

3-28 Figure 11 Shoaling characteristics of Savannah Harbor* 26'.22' cb~el ( 1923 - 1925) 30' - 26' - 22' channel ( 193 1 - 1932) 30' channel ( 1939 - 194 1) 36'-34' channel ( 19S3_1gS4) Joe ~un1 shoot rates ~ ~i''io. cubic yards 0 1 2 3 ~ 5 ._ . ILLau66tostau1~6r 1 \ ~ (3g.~ ft) ~ an, . 1 ~ 1 1~ / I Statim 107 to station ~ 134 1 1 ~ ~ I (Stations numbered from mouth of Savannah River upward) *SOURCE : A . ~ . Ippen , ed . ~ 1966 ), Estuary and Coastline Hydrodynamics (New York: McGraw-Hill).

3-29 changes in channel configuration. From 1923 to 1925--when nearly 3 miles (4.8 km) of channel were dredged to depths of 22 ft to 26 f t ~ 7 m to 8 m)--the amount of shoaling in the lower reach of the estuary was nearly 10 tomes that in the upper reach. However, as the channel that was dredged was both deepened and lengthened, the situation changed. In the early 1950s, a 7-1/2 mile (12 km) section of the channel was deepened to 34 ft to 36 ft (10 m to 11 m), resulting in nearly 40 times more shoaling at the upstream station than at the downstream station. Thus, the material brought into the estuary from inland source" was deposited farther and farther upstream each tome the channel was dredged. The vertical distribution of velocity at several sections in Savannah Harbor can be seen in Figure 12. Station 193 is located near the harbor entrance, and Station 130 is near the upstream licit of salt-water intrusion. The agreement with the simplified description of Figure 10 is evident. Considering the complex hydrodynamic aspects of the flows in the interior of a harbor or estuary due to the hydrography of the fresh- water inflows and the nature of tidal-induced currents, and the importance of the details of these flows on the effects of changes of depth (by dredging, for examples on shoaling, it appears that numerical modeling can, at best, reveal only very approximate ideas about dredging criteria. This review suggests that more or less a case-by-case investigation may be necessary to indicate the problems likely to accompany overdredging. On the other hand, it is possible that the overdredging is not nearly so important as the initial dredging decision in modifying shoaling characteristics in an estuary. Some coastal inlets are believed to have reached a state of equilibrium; some have not and will continue to be unstable. O'Brien (1969) found that the equilibrium minimum flow area of a tidal inlet, with or without jetties, is controlled by the tidal prism. A reduction of the tidal area by sedimentation will reduce the flow area. Figure 13 shows a typical tidal inlet which has approximately ideal proportions: a crescent-shaped bar seaward having a center of curvature near the throat section, a swash channel at each end of the bar, and a controlling depth over the bar much smaller than at the throat section. The flood currents are shown in Figure 13a and ebb currents in Figure lab. Tidal currents through the inlet must sweep the littoral sediment drift out of the inlet, moving the sediment into the bay, or Seaward to the offshore bar. Along the Pacific coast, the ebb currents predominate, owing to diurnal inequality, and move the sediment seaward. Along the Gulf of Mexico, the opposite is true, and the littoral drift tends to accumulate inside the bay. This accounts for the instability of many Gulf of Mexico inlets prior to stabilization by jetties and dredging.

3-30 Figure 12 Normal distribution of f low ~ in vertical ~ in Savannah Harbor* o: 20 ~0 - - ~ 60 0. ~0 1 1 1 1 ~~ - tide river Q ~ S670 cfe ~ .~ -20 ~ Station 19 100 ~ O _ . _ Station IS: lo _~~ Station 130 40 60 80 1 1 1 30 40 SO Percent totem not doubt - 60 70 t~ *SOURCE : A. T. Ippen, ed. (1966), Estuary and Coastline Hydrodynamics (New York: McGraw-Hill)

3-31 Figure 13 Tidal inlet, showing (a) flood currents; (b) ebb currents* BAY ,:: ~ ~\ (a) OCEAN _ _ _ ,-~.'`~-:, Nm ~~~' Contours __- (b) OCEAN _ — Figure 14 Relationship between min~mum flow cross-section of entrance channel (ft2) and t~dal prism (ft3) for diurnal range of tide* ~ loll - z z IGlt cr o 9 IU. _ V G CL 8 107 t2 _ i 1 1 1 ~ I i i 1 1 i 1 1 1 1 ''1 1 1 Tt | DELAWAR ~ / _ _ _~ /GOLDE N GAT] '? ~ COLUMBIA RIVER X~ ,tD 1 1 - - ~PEND!ETON | ~ I Ix 53-T 9ASI~NI | ! I I I I I I l I I ! I I | 32 103 104 lo5 1 o6 107 a- M IN IMUM fLOW AREA ( Ft21 SOURCE: M. P. O'Brien (1969), "Equilibrium Flow Areas of Tidal Inlets on Sandy Coasts," J. Waterways, Harbors and Coastal Eng., Proc. ASCE , 9 5 (WW1 ): 4 3- 52 .

3-32 The relationship between entrance area and tidal prism (based on available data) is given by A = 4.69~10~~4 p0.85 where A = area (in ft ), the minimum flow cross-section of the entrance channel (throat) measured below mean sea level, and 3 P = the tidal prism (in ft ~ corresponding to the diurnal range of tide. Figure 14 shows the relationship between A and P for several inlets. Harbor Entrances Sedimentation by Type of Harbor, and Mitigation Strategies Sediments tend to collect at different types of harbor entrances for different reasons. These harbors (Figure 15) have been classified (Caldwell, l9S0; Komar and Inman, 1970) as: River-Channel Harbors Fresh river waters keep the clays moving; consequently, the principal sedimentation problem becomes one of sand. Dredging, training walls, and diversion of the river are the usual corrective measures in such harbors. Off-River Harbors These harbors have little difficulty with sand - and gravel but do often have probl-~ with silts and clays. The solution to shoaling is dredging, training walls and dikes, and the use of locks or floodgates. Fall-Line Harbors Sedimentation problems generally result from . . sand and gravel deposits. Solutions usually consist of dredging, training walls, or the creation of an off-channel harbor. Off-Channel Harbors in Tidal Estuaries Shoaling is usually due to suspended wilt and clay. Improvement is the same as for off-channel river harbors--namely, dredging, training dikes, and use of locks. Shoreline Harbors The problem at such localities is the deposition of sand moved into the harbor by littoral currents (discussed in the preceding section). Maintenance of such harbors usually involves a sand-bypass~ng operation. Thus, mitigation strategies may encompass efforts to control sedimentation as well as dredging. Minimizing Sedimentation Sedimentation can be minimized in several ways. these were studied in the MOO project in the Netherlands (van Oostrum, 19791. The balance of particle transport with current velocity, with variations in deposition density, and with possible silt traps was researched, as well as recirculation from offshore dumping. Studies of the orientation of harbor approaches can lead to designs for partial self-cleaning of the dredged channel. River, tidal, or littoral currents can sometimes be directed to help scour the navigational channel, carrying sediments into deeper water outside the dredged area.

3-33 Figure 15 Types of harbors IS Dew J9~ed~ed Isis ~ Burning \ / /~asi~z tic - ~Jarlorma Td~ \ Estuary ' Off-~anneZ En\ }lessor ~ ,',' —'of c ~ ~ i ~ s ~ ~ ' , ' ' if ~ c ~ ~ ~ i ~ s ,,' Ol~/Y LIONS ~S~/ _ _ _ _ _ _ ~ '\ aim ~,~a,'ged wit), \ Chd~nnc! ~ ' \ \ j ~ ~ 'I O~cd~cdCh~nnel al ~ fl//e6 :LANL~6LrM60@ ~~ KLd~cd ~~ ~ ,~F~L-L'reN,reo~ ~ ~ ) -RIVER HARBOR ' I \ 0~W - - / OFF-CHANHE' ANO / TIML &TUARYRARB~S . OCCUR A: _ ''~~~red~ed basin 1 Ll ~1 ` ,'7 Shorel in e // / \: / I' / TIC It \~e£tics SORREL/ HARBOR

3-34 Sedimentation can be minimized by increasing the channel velocity in a number of ways, such as o o o o o o o o channel; Groins, to reduce the width of f low; Training walls to direct flow; Closing of side channels; Straightening channels; Increasing the tidal prism; Increasing flushing by reservoirs, releasing at ebb tide; Reducing flocculation by diversion of freshwater flow; Avoiding river sediments passing through the navigable and o Creating turbulence artifically. very well in some busy channels. Ships ' propellers do this Model tests are helpful in observing the comparative velocities of water in different areas of a harbor entrance. Extensive model studies are routinely conducted at many laboratories in the world, including the Waterways Experiment Station of the U.S. Army Corps of Engineers. Fixed bypassing plants for moving sediments from one side of the channel to the other have been employed, working steadily or intermittently. These may be land-based or floating. Since major sediment movements generally occur during major periods of wave action, or storms, the rate of bypassing becomes variable. Because of the standby plant costs, the procedure may evolve into periodic dredging, as in normal channel maintenance with floating mobile equipment. Examples of Mitigation Strategies and Effects One example (Figure 16) is the sand transfer arrangement at the entrance to Santa Barbara Harbor, California, which was constructed in 1930. Here the dominant direction of littoral drift is from west to east. Five years after the west shore-connected breakwater was built, sand had filled up the area to the west. The sand then moved along and around the seaward end and settled in the protected harbor in the form of a bulb. The rate of drift was about 270,000 cy (205,200 m3) per year. Severe erosion prevailed downcoast until the first bypassing arrangement in the country was initiated in the late 1930s. Another bypassing operation designed to handle all the littoral drift moving to an inlet was put into service at Ventura, California (Figure 17~. The plan involved an offshore rubble breakwater, 2S00 ft (758 m) long and parallel to the shore, to minimize wave action at the channel entrance and create a trap for the easterly moving littoral drift. The trapped material is then transferred to the shores downdrift of the Port Hueneme east jetty. The purpose was both to minimize maintenance of the entrance channel to the Ventura County Harbor and to stabilize the shores downdrift of Port Huene~e, whose harbor was acquired by the Navy in 1942. From 1938 when the west jetty was completed, littoral drift was diverted into the Hue neme Submarine Canyon, thus removing from 800,000 cy to 1,600,000 cy (608,000 m3 to 1,216,000 m3) of drift and practically eliminating all maintenance in the 35 ft ( 11 m) channel. However, erosion began southeast of Port Hueneme at a rate about equal to the average annual drif t .

Santa Barbara, \ \4 California \V\~/ _ 7 ~ ~\~—~—~ · wench \ ) Dredge and discharge line · . ALLY 1930~_ j/ '_~ ~932 ~ Present --am ~ -—-—..t A! SOO 1000 Figure 16 Figure 17 Breakwater SANTA BARBARA BYPASSING SYSTEM \ `. \ ~ \ \_ County| \ (~-1 \ )201 1 ~1 \ Borrow area for emergency biassing operation 19531954 0 2000 4000 l I I .1 1 Seal in Get \~ fort lluename V'!~ ~ ~ ~C,' "i':~ Wee Cd VENTURA COUNTY HARBOR, CALIF.

3-36 In 1953-1954 an emergency bypassing operation was conducted moving about 2 million cy (1.5 million m3) from the impounding trap to the shores east of the harbor, and from there by conventional floating dredge to points below Hueneme. Decisions and results are site-specific. Ediz Hook, Washington, is a low, bare sand and gravel spit extending easterly from the mainland some 3-1/2 miles (5.6 km) into the Strait of Juan de Fuca (Figure 18~. The protective arm of Ediz Hook acts as a natural breakwater and protects the city of Port Angeles and its harbor from the direct wave attack of swells from the west. The development of the present spit began when the sea had worn away the cliffs to the west and carried detritus from the Elwha River east and southeast. A dam was built on the Elwha River, however, and erosion of the cliffs was reduced by works constructed to protect the water-supply pipeline to Port Angeles (O'Brien and Johnson, 1980~. The more westerly portions of the suit then began to migrate southward by a process of Preaching and overtopping. This movement into the deeper water of Port Angeles Harbor thinned the spit progressively so that now it is in danger of becoming a mere shoal. In 1974, extensive _ . . . . . . - studie. were started to determine the details of a nourishment program at 5 year intervals of feeding, based on unsupported theories about the nature and behavior of the coarse materials involved here in overfilling to anticipate erosion. About 100,000 cy (76,000 m3) of selected gravel were planned each cycle. Some harbor entrances, because they are subjected to both tidal and storm wave attack from the open ocean and because the shoreline is composed of sandy soil, are so amorphous that they defy economy Cal methods for providing safe navigation. In Chatham Harbor {Cape Cod) for example, flood and ebb deltas shift with almost every change in tide. Attempts at stabilization have so far not been economic, because the harbor is mostly for fishing and pleasure boating. However, as an example of the problems of harbor entrances in the United States, this is one of the oldest: the f irst recorded shipwreck in the New World was in this vicinity in 1626. Perhaps a classic case of a migrating harbor entrance, fed by littoral drift predominantly from one direction, is that of Fire Island, New York, which serves a series of small ports and pleasure boat centers in Great South Bay. It is a tidal estuary with a large tidal prism and a mean tide range of 4.1 ft (1.2 m). Maximum is from -6.0 ft to +9.4 ft (-1.8 m to +2.8 m). Figure 19 shows historical shoreline changes (Survey Report, N.D.~. Note the radical alteration in the shape of the elongating tip between 1939, when the jetty was built (1939-1941), and 1955, when sand had fully impounded behind the jetty and was spilling over into the navigable inlet. - Figure 20 shows dredging and shoaling between 1955 and 1962, as sand continued to spill around the jetty. A major problem of erosion to the west had threatened to destroy the newly built Ocean Parkway. Sand was imported from Great South Bay to make up the deficit. The

3-37 Figure 18 - ' - i . EDIZ HOOK - ~ PORT ~ PORT ANGELES HAR90R ~C._. CC-_ ~ =;' ,.._ ~ Cy ~ .. · CC ~ __ ~ - " - C" ste,' - LECEND {1 ~ r- r ~ REY£Tbl£NT ,777~ PREACH FEE D see's: ''.2000' 1 N

3-39 'at/// 11 "] ~ <"~"':~- an, It I_ . \ / / ~ I ~ , ~ In C) hi: _; J ~ ° _ ' ' lo. L/~y foal ~ ~ 'at 'an IL Z o ~ Z J ~ 1t 5h a · ~ : - r v 2 ON Z 0~ _ —~ ~ 2 ~ \~a ~ \. \ ~ d .. , - ., ~ EN ~ c O~ _ ~ , ~ §, . Li 8 i 11; IN !~ so a,. Co a. ? or o w Z a: ~ _ C' ~ J @: Q

3-40 dike within the Fire Island entrance channel had been built in 1959 to train tidal flows through the shoaling area. Various plans for fixed sand-bypassing plants were then considered. An extensive model study was carried out at the Waterways Experiment Station of the U.S. Army Corps of Engineers in Vicksburg, Mississippi, in which 14 different basic plans, along with many alternatives, were modeled using both fixed- and movable-bed models. Twelve agencies and subcontractors made contributions. Figure 21 shows the recommended plan (3A), which required dredging a littoral trap (1.2 million cy, or 912,000 m3) to E1. -18, and a rehandling basin (2 million cy, or 1.5 million m3) to E1. -30, as well as land reclamation to the west (3,370,000 cy, or 561,200 m3) and a feeder beach to supply it (2 million cy, or 1.5 million m3) initially, with biennial nourishment of 1.2 million cy (900,000 m31. The estimated volume of littoral drift was 600,000 cy (456,000 m3) per year (U.S. Army Corps of Engineers, 1970~. The design studies continued until April 1971, alternative methods of dredging were studied in 1972, and the first dredging contract in 1973 and 1974 placed 1 million cy (760,000 m3) along 9000 ft (2727.2 m) of feeder beach. A second contract placed 930,000 cy (706,800 m3) in 1975 but ran short of funds. A third contract, begun in June 1976 and completed May 1977, placed 2,272,000 cy (1,726,700 m3} directly on the beach to the west, and the rehandling basin was eliminated, as was the revested sand dike. The 1977 report on construction states that "the configuration of the inlet shoal experiences such significant changes over short periods of time that dredging operations are often tomes hampered and delayed." This statement came 22 years after shoaling and erosion nad reached a critical stage, endangering both navigation and highway transportation. From these examples, the criteria for dredged depths at harbor entrances appear to be dominated more by the uncertainties of deposition and the irregular time constraints imposed by financial and institutional arrangements than by the maneuverability of ships. Special Considerations of Silty Channels In many ship channels, the interface between the sediment and overlying water is clearly defined and the sediment-water interface is easily seen on depth sounders. In many other channels and coastal inlets with active circulation systems carrying suspended sediment, siltation of the channel occurs through the deposition of silt or the presence of fluid mud (static suspension). Research work on cohesive sediment suspensions in coastal areas (Parker and Kirby, 1977) has revealed the behavioral link between sediment suspended in the water column and dense cohesive sediment suspensions on the seabed (Figure 2210 This work shows that high-energy events (tidal currents or storms) erode cohesive sediments and transport them into navigational channels. When first eroded, the sediments are mixed throughout the water column an a homogeneous mobile suspension. AS energy levels decline, the mobile suspensions begin to differentiate by settling, forming marked steps that continue

- lo go - N _~_ - N ISO.OOO ~o 96 - . · 'I . - Cot ., · - - 5L'00 I MOVABLE-BED LIMITS 3-41 O a a N OAK BEACH i, GEL - 20 /LIrrOR~L I / FLOP 1 ~ EL - J~ I . . . . . . Elf CECIL it:. /< / / DEMOCRAT POINT / i_ _ f IRE ISLAND A TL AN TIC OCEAN NOTE: EL£V^1tONS ARE 1 - Bitt REfERREO TO ALA. ELEMENTS OF PLAN 3A SC^etS He FEET PROtOT'PE MODEL Zooo o zooo Solo Figure 2 1

3-42 Figure 2 2 Relationships between mobile and static suspensions and settled mud* COHESIVE SEDIMENT DYNAMICS Mobile SusDensions .,`~ it. Settled Mud Static Suspensions - 2~7,a~ Figure 23 Structures developed in mobile suspensions during spring to neap cycle* SUSPENSION STRUCTURES Mobile Suspensions / ~~- \ / Suspensions of 1 \ ~. / O lOk~m~s ~ \ .0 m. V _ ~ ~ Stratified 20 =1 43~ ~ ~ Q Din - - tote ,0- ~ :: go_ .- . . ! a, l~ ~ O 1 201~'m~3 `. l~ \ Differentiated ~ / o- ~ . \ O k' m~3 40 / Static Suspensions / *SOURCE: W. R. Parker and R. Kirby (1977), "Fine Sediment Studies Relevant to Dredging Practice and Control," BHRA Fluid Engineering/Texas A&M University, 2nd Int. Symp. on Dredging Tech., Vol B.2, pp. 15-26.

3-43 to settle through the water column until dense layers several meters thick of specific gravity equal to about 1.15 are observed flowing along the bed. Eventually, these mobile layers stagnate to form dense static suspensions (mud flow). It is patent, therefore, that where static suspensions occur, deposition has been rapid. Mobile suspensions commonly show multiple stratif ication, and although layering within the static suspensions is also common, there is as yet no unequivocal explanation of the cause. Following stagnation, the static suspensions continue to settle, and within a few hours they consolidate to a stage at which they can be detected by normal hydrographic survey echo sounders. Once they become detectable, these suspensions--"fluid mud" or "silty bottom"--present a survey problem, since no evidence is presented on the echo sounder record that allows confident selection of the layer to be regarded as the seabed. Such static suspensions appear very suddenly after storms, commonly have two or more layers, and may reach 2 m or 3 m (7 ft or 10 ft) in thickness--a significant height above the channel bottom. Field studies in the United Kingdom and the Netherlands indicate that cohesive fine sediments occur in estuaries in three forms (Figure 23~: 1. Mobile suspensions, which are common in estuaries with high tidal exchange and significant turbidity. These may also occur during major storms, such as hurricanes in the Gulf of Mexico or along the eastern seaboard, or may be caused (on a smaller, local scale) by engineering operations. 2. Static suspensions, which may be referred to as "fluid mud,' or "fluff." These suspensions develop during neap tides and may be dispersed by spring tides. If not dispersed, they will settle at the bottom of the channel and eventually consolidate. 3. Settled mud, formed by consolidation of static suspensions. Implications The consequences of arbitrarily choosing one layer or another on the echo sounder record in the absence of supporting information were considered by Parker and Kirby (19771. Among these are the evident hazard to navigation of uncertain information about channel depths and the location of the bottom, and the expense of dredging "false" bottoms that may actually be navigable, as described in a succeeding section, "Nautical Depth Concept." Mobile and static suspensions also create problems for dredging operations. The mobile phase may pass undetected by the usual survey techniques (particularly where surveys are infrequent). In the static phase, suspended sediments may be detected by an echo sounder, but the time-dependent properties of the suspensions control the actual readings. These readings, in turn, affect the evaluation of navigable depths, the determination of the maintenance dredging required, and the measurement of new depths

3-44 following dredging. They may also influence the timing of maintenance dredging operations. Measurement Technology The suspensions described here have long been recognized by hydrographic surveyors. In the early days of lead-line techniques, a special "mud lead" was devised for static suspensions. Echo sounders have largely replaced lead lines; however, the acoustic detection of dense suspensions is difficult to achieve. In the simplest cane, particles settle from a sharp-surf ace concentration zone onto the bottom material, which may be loosely defined as the channel "bed" (Figure 24~. The simplest case is not the usual case. Echo sounders respond to both the density and the acoustic velocity gradients of the medium, and theme acoustic properties cannot readily be converted into information about the altitude within the suspension at which its mechanical properties may significantly affect ship handling. As pointed out by Kirby et al. (1980}, information was needed about the behavior of vessels sailing through such areas (see the section "Nautical Depth Concept") and in situ measurements. Two radioactive probes were tested for measurement of specific gravity in situ: the transmission type and the back~catter type (Figure 25~. The transmission probe is an H-shaped instrument with a radioactive source in one leg and a detector in the other. The source radiates directly to the detector. The transmission probe is able to average the specific gravity with a resolution of 3 cm in depth (which is the height of the detection crystal). Since the instrument must sink into the silt by its own weight, the H-shape is a disadvantage. The sturdier back~catter probe carries source and detector in a single pencil-shaped tube. The source radiates in all directions; the detector, placed above the source and screened from it Dy a shield, receive" only part of this radiation. The back~catter probe registers densities with a resolution of 15 cm (i.e., the distance between the source and the detector). Because of its shape, this probe penetrates the bottom more readily than the transmission probe. The probes and associated equipment were developed by the Atomic Energy Research Establishment ( United Kingdom) and tested for f ield use by the hydrographic survey department of the Rijkewaterstaat. The method, now in regular use in Europoort, is essentially a modern mud lead and shares with the older technology the disadvantage of providing only point-by-point information. A towed, continuously measuring probe is under development in the Netherlands. Present practice is to superimpose the point-by-point measurements of specific gravity on the continuous bottom profile of the echo sounder. Although far from ideal or convenient, the combination of these two techniques offers considerable advantages in establishing "true" channel depths, and in pre- and post-dredging surveys in silty channels, as fluid muds Rewater slowly and may be subject to considerable tide-created movement.

3-45 Figure 24 Diagrammatic illustration of the settling of a static suspension showing bed and density structure development* Suspension ~ Suspension difficult to increasingly \ detect with I detectable with ~\ ed~osou~der1ed-~osour~de~- tn~ I \ WAILER 1 vat ply) (a Ps) 1 - CO ~:D: Sus~\ PS<PB WATER P~<<PEl) 1 I Face density gradient r deve aping ~ I^, A a/~: BED V—~ To O (minutes) 60 TIME Chum. d Heft at To) 180 \and ate straw/ *SOURCE: Based on data from A. J. Mehta and E. Partheniades (1973), "Depositional Behavior of Cohesive Sediments," Technical Report 16, Coastal and Oceanographic Engineering Laboratory, University of Florida, p. 274.

3-46 Figure 25 Radioactive density probes: ,L Crystal- Shield i//////, _ Radiation Source (a) (b) (a) backscatter, (b) transmission ~$ Crystal 1 Radiation Source

3-47 Nautical Depth Concept The "nautical depth" concept evolved from studies of the behavior of vessels in Europoort and Rotterdam harbors as part of an effort to minimize the maintenance dredging costs. The normal design depth of ship channels in sheltered waters includes a 10 percent underkeel clearance to provide safe navigation and maneuverability (Figure 261. The behavior of vessels sailing close to the surface and in the upper layers of dense suspensions was investigated, and extensive model tests were conducted in the Netherlands Ship Model Basin with a two—laver svst~m to stubs the sailing and man - Veins shared - victims — _ ;7 ~ of supertankers in a channel with a soft bed. Prototype field tests were made with the 240,000 DWT suDertanker LeDton sailing in the Eurooport channels. Extensive investigations were also made in the Chao Phraya River, Bangkok, and along the coast of Suriname where ships sail in mud with a negative underkeel clearance. On the basis of these investigations and a search of the literature, it was found that fluids of specific gravity to 1.2 had only a slight influence on maneuverability (van Nostrum, 1979; van Oostrum et al., 1981; Kirby et al., 1980~. Thus, channel depth can be increased if the upper layers of static suspensions are included in the underkeel clearance. Based on the results of the investigations and studies, the concept of nautical depth was developed (Nederlof, 1980~: "a density within the suspension above whose altitude vessels can safely sail" {Figure 27), and the density was defined as 1.2 g/cm3, or having specific gravity of 1.2. Use of the nautical depth concept to define channel depths depends critically on frequent (weekly) and accurate density measurements. Dredging Among the considerations important to judging criteria for the depths of dredged navigational channels is the accuracy of the dredging process. As indicated in a preceding section, the two types of overdepths specified in channel design and (more usually) maintenance are intended to (1) achieve and preserve the design depth (an allowance for inaccuracies of dredging and surveys) and (2) provide advance maintenance dredging, i.e., to reduce dredging frequency. Neither decision is simple; neither can be characterized by a general formula. By tradition and contract specification, the Corps pays only for the material removed within the overdepth specified. Because dredging is a highly competitive activity worldwide, technological advances that enable greater accuracy to be achieved are regarded as proprietary industrial information. Thus, the leading edge of technology in dredging is hidden within the industry, with a lag time of several years between the development and use of innovations and their description in the literature.

3-48 Figure 26 Required minimum water depth over a firm sandy bed: draft of vessel + 10~* Figure 27 Comparison of density profile and tanker cross-section to illustrate concept of nautical depth* too 1~ ~ ~- cross section I( ~ '~T ; 0 t Kale Requi~d tanie, Nautical t:)epth ._ ;\ . - ~ ~ .~ underfeed clearance V '_ ~ NN>~N ~~,\~_ we let 5L`~aCe solid sesecd _ em cm~3 OQ2 suspended mud ~ ~ ~ desirer static suspension causes _ _ ~ neCat~ve underkee' cleatancc _ ~ ~0 unconsolidated ~120 static suspension _ _ _ _ ~ _ _ A_ consod'deled stat 'c ~,pSJJ8~Si~ *SOURCE: L. Nederlof (1980), "Sailing through Water Rich in Floating Silt: A Vessel Behaves Differently, but Remains Manageable," Rotterdam Europoort Delta, p. 20.

3-49 For advance maintenance dredging, several countervailing consequences must be weighed: a deeper cut may reduce the velocity of water in the channel and increase the rate of sedimentation {as evidenced in a preceding section for Savannah Harbor ~ . Unit dredging costs, on the other hand, are likely to be less for larger than for smaller contracts. Some of the considerations most important to decisions about overdredged depths and (especially) dredging cycles are beyond the scope of this study. Among the most important are those pertaining to the disposal of dredged materials--the nature and distance of disposal sites from the channel to be dredged, the requirements to be met in the handling of dredged materials, and the tome needed to secure permits and access to new disposal sites. Accuracy of Dredging Processes Accuracy depends on the type of equipment used, the sediments encountered, control of the dredge's position, whether the work to be performed is new or maintenance dredging, and, if the latter, the previous dredging work. It can be influenced strongly by such local conditions as tides and currents and by the accuracy of pre- and post-dredging surveys. Type of Equipment Dredging plant is diverse: types used in the United States for dredging major navigational channels can be categorized as fixed relative to the channel bottom or independent. Fixed Equipment Fixed equipment includes cutter suction dredges (Figure 28) with a spud pole (pilelike leg) firmly implanted in the channel bottom and the dredge itself rotating about this axis. The distance from the Butterhead to the spud pole is fixed mechanically, as is the depth of the cutter beneath the surf ace . In theory, the only variable is the angular motion of the dredge itself as it pivots about the fixed spud pole. This motion is usually controlled by cables attached to anchors. It is imperative that the dredge operator know the exact location of the spud pole and the relative position of the butterhead to the spud pole as well as the transverse position of the Butterhead to the channel centerline at all times. The position of the spud pole can be assumed to be known within an accuracy of less than S f t . Spud placement has little effect on the dredging operation, except in terms of the horizontal cut. Therefore, the position of the spud pole in normal maintenance dredging is not of prime concern. Electronic positioning equipment or gyrocompasses are used to measure the deflection of the dredge from the channel heading. With the use of these two control methods, the accuracy of the channel width can be controlled within 5 ft to 10 ft. Other fixed-spud plants are grab dredges and dipper dredge`;. They dif f er f rom the cutter suction dredge in that their hulls do not move relative to the spud pole. The bucket used for the excavation is moved relative to the hull. The movement of the excavating bucket can be controlled more precisely than that of the butterhead.

- ~ Id. - ~ e,; rF4 ~ ~~ '_ ran-.- _' -' --at b _ ~3 -

3-51 Independent Equipment Hopper dredges (Figure 29) are the most common among independent equipment. As they are not fixed relative to the channel bottom, they have no fixed point of reference for determining the accuracy of operations. The head attached to the suction pipe (Figure 30) acts very much like the household vacuum cleaner, with the exception that the hopper dredge drags its suction head ("draghead") while the Butterhead is pushed forward in dredging operations. Unlike the cutter suction dredge, the hopper dredge removes a very thin layer of the bottom material as it dredges. It must therefore traverse the area to be dredged many times before the channel is substantially deepened. Electronic positioning and track plotters are used to indicate those areas that have been traversed. Continued surveys are required to assist the operator in controlling the dredging operation. Occasional passes outside the dredging area or in areas that have already been dredged entail very little damage or lost effort because only 2 or 3 vertical inches of material are removed. The accuracy of hopper dredges is much more dependent on the physical shape and dimension of the area to be dredged than that of the fixed dredges, which can more precisely control their location. The accuracy that can be achieved also depends on the type of soil encountered. Another example of independent dredges is the dustpan dredge Figure 31) , which propels itself by cables attached to anchors, using propulsion devices to aid steering. This dredge, like a cutter suction dredge, excavates a substantial depth of material at one time . It therefore moves much more s lowly, and because of the anchoring system, has much better control of its horizontal position. It relies very heavily on electronic positioning. The vertical control of the dustpan is such that accuracy within 1 ft ~ O .3 m) can be achieved and horizontal control within 3 ft ~ 0.9 m) . Type of Bottom The accuracy of the dredging process is affected ~ . by a change in the material encountered. Cohesive or hard soils lead to the development of trenches. Different materials may dictate the dredging method. Certain hard materials such as sandstone or limestone cannot be dredged with hopper dredges or dustpan dredges, since these types of dredges generally require the material to be loose and free-flowing. Such hard materials can be dredged with a butterhead dredge. To Achieve Design Prism Again, the accuracy of the equipment remains unchanged, but the quantity of no-pay material dredged to ensure achieving the design prism can vary signif icantly. Dredging narrow shoals adjacent to the slope with a hopper may result in dredging 200 to 300 percent of the pay quantity. Rock often must cut 3 ft to 4 ft (0.9 m to 1.2 m) below grade to preclude strikes during a bar survey. be

=~ - ~ - ~ 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~_-

3- 5 3 ~,,~ Am_

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 · -

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.

3-56 lo .o lo _ 1 ~ . .. it\ ~ \ I . ~ v Q 3 · - ~ V ~ ·rl U] a) . - - o .,, In 1 - .~ . - ~q . . j ~ \9 In ., o ~ 1 1 1 1 J ~ r 1 1 0 J / 1 I o 1 o / _. lo lo 1 G MU Digs /~z 0 ~ 0 G O G (A it') ~ , In C:

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-I r--~ _ . ~ ~ _. ' 1 ' . , . _. ' ~ .l . i. t ~ . . t-i t ~ _ ~ .1-1 ~ .. ret I r ~_~ . ~ - a,) — r _. _ ~ _ - ~_ F_, _ _,j.~_ _ _ _~. _ _ .. i_i _ _ - :.~. F. — ~ .~. . I . . _ ~ t_ t_ ~;: ~ (t . ~ .~., 1. ! - , ·t_. l_t- -t-F i ~ : +.,, ._, Rj~ ~ ~ t- . ,~! -~-H-~ .—.., _ ~ _=, , , . i_F I,~ ,'t r . ~ . ~t `. -r _ _ _ _,. _ h. . r - _ _ _. r.t 1- _ . . L ~ ~l . . _ t~ -i ,— T. ~ T : ~ . t - } - , ~ . ~ . - 1 1- —I I - ~ t;I.—T- .... h.~_~_ .. I, [, . ~.! _i I' - - . I _l ~ r - _~_ ~ t..l ,- l'' ~ t' 1__ -

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.

3 -60 . . · - ~ .... ·,1 US - : : . . .: .'>~~ . 1 . . . ,~= , _ . ~ ~ ~ ~ -!: . . . .. . ..... ~ . . ~ .` . , _ . . , , .... , - ~ ~ o o .. , ~ ~ c\~ ~~ — - ~ -- -A 1 ~ - ~ --- t- 1 -- 11 a-- - ., ._ ~ ~ .. . . . _ . _. ~ 'I _;~ . . . ~ . ~ ~ ~ ~ , . .. i _ ~ _ ; , . , - I i ... ,. ... , . - _. I -no; o i I

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