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OCR for page 23
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
OCR for page 24
3-2
Figure 3 Types of dredged navigational channels
1 ~
Open
Confined
.
'///// 7/
~~ r
~~\
LDEPTH
- ~TH
CHANNEL
WIDTH
OCR for page 25
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.
OCR for page 26
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.
OCR for page 27
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
OCR for page 28
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
OCR for page 29
- .
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.
.
OCR for page 30
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
OCR for page 31
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.
OCR for page 32
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 TANKŁ~. 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 SPŁED
· 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.
OCR for page 33
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,
.
OCR for page 74
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e
a
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mat'& A. '
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it_
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a_
Figure 29 Split-hull seagoing hopper dredge (inset shows split hull open
to discharge dredged material ~
; ~ r , Hi.' .'r `.~:
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OCR for page 75
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OCR for page 76
3-54
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OCR for page 77
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.
OCR for page 78
3-56
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OCR for page 79
3 -57
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OCR for page 81
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.
OCR for page 82
3 -60
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OCR for page 83
3-61
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OCR for page 84
Representative terms from entire chapter:
hopper dredge
