Criteria for the Depths of Dredged Navigational Channels (1983) / Chapter Skim
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Considerations Important to Determining Channel Depth
Considerations Important to Determining Channel Depth
Pages 23-84
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From page 23... ...
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)
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From page 24... ...
3-2 Figure 3 Types of dredged navigational channels 1 ~ Open Confined .
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From page 25... ...
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~.
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From page 26... ...
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~.
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From page 27... ...
x, y, z = Linear displacements A, ¢, ~ = Angular displacements
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From page 28... ...
"~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 \_~ ]
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From page 29... ...
.2 ,.6S 4.0 . OWSI Ship Speed gal sing Ship Speed ~~~ tesetes PIP ~ · .
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From page 30... ...
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.
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From page 31... ...
SNARE, 79: 88. Figure 6 Ranges of influence of directional stability, turning moments, and bottom clearance on ship controllability for various channel crosssections (dimensionless ratios)
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From page 32... ...
* SOURCE: Haruzo Eda, Robert Falls, and David A
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From page 33... ...
, 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.
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From page 34... ...
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~.
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From page 35... ...
x~m~m, from National Ocean Survey (1982) Tide Tables 1982: High and Low Water Predictions (Washington, D.C.: National Oceanic and Atmospheric Administration)
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From page 36... ...
, 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 .
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3-15 Table 6 Sununary of vessel condition at transit of Columbia River entrance channel, full-sca~ e measurement program*
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3-16 Table 6, continued*
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From page 39... ...
3-17 u so .
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From page 41... ...
SOURCE: Haruzo Eda {1971) , "Directional Stability and Control of Ships in Restricted Channels," Trans.
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From page 42... ...
, 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)
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From page 43... ...
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.
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From page 44... ...
3-22 Table 8 Longshore transport rates for U.S. coasts.
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From page 45... ...
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.
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From page 46... ...
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 \ % *
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From page 47... ...
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.)
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From page 48... ...
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.
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From page 49... ...
3-27 Figure 10 Schematic representation of Sal ~ ity intrusions in estuaries* Forerun" eaten" ~~ "knit, Veloett, Allots - ~ do~atrea.
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From page 50... ...
3-28 Figure 11 Shoaling characteristics of Savannah Harbor*
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From page 51... ...
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.
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From page 52... ...
3-30 Figure 12 Normal distribution of f low ~ in vertical ~ in Savannah Harbor*
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From page 53... ...
3-31 Figure 13 Tidal inlet, showing (a) flood currents; (b)
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From page 54... ...
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.
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From page 55... ...
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)
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From page 56... ...
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.
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From page 57... ...
Santa Barbara, \ \4 California \V\~/ _ 7 ~ ~\~—~—~ · wench \ ) Dredge and discharge line · .
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From page 58... ...
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.
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From page 59... ...
3-37 Figure 18 ' - i .
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From page 61... ...
3-39 'at/// 11 "]
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From page 62... ...
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.
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From page 63... ...
lo go N _~_ N ISO.OOO ~o 96 .
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From page 64... ...
\ O k' m~3 40 / Static Suspensions / *
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From page 65... ...
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.
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From page 66... ...
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.
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From page 67... ...
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)
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From page 68... ...
3-46 Figure 25 Radioactive density probes: ,L CrystalShield i//////, _ Radiation Source (a)
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From page 69... ...
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.
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From page 70... ...
- ~ ~ .~ 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~ *
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From page 71... ...
Fixed Equipment Fixed equipment includes cutter suction dredges (Figure 28) with a spud pole (pilelike leg)
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From page 73... ...
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.
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From page 74... ...
: _~ 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_ _ ._ ,.
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From page 77... ...
Examples of Accuracy of Dredging Operations . Cutterhead Dredge A typical section of maintenance dredging by a Butterhead dredge is shown in Figure 32.
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From page 78... ...
3-56 lo .o lo _ 1 ~ .
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From page 81... ...
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 .
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