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OCR for page 101
SEDIMENTATION IN HARBORS
J. W. Johnson
Introduction
The discussion to follow is concerned primarily with the type of
sedimentation that normally might be expected to create design,
operation, and maintenance problems in harbors, ports, and offshore
terminals. The most comprehensive and yet concise coverage of this
important sediment problem is that presented by Caldwell.i The types
of harbors discussed in his treatment are listed as follows, with some
actual harbors given as examples:
River-Channel Harbors
Baton Rouge, Louisiana
St. Louis, Missouri
Pittsburgh, Pennsylvania
Sacramento, California (old harbor)
In such harbors, the 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 problems with silts and clays. The solution to shoaling is
dredging, training walls and dikes, and use of locks or floodgates.
Fall-line harbors
Troy, New York
Washington, D.C.
Richmond, Virginia
101
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102
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
Washington, D.C., Channel Harbor
Houston, Texas
Sacramento, California (new harbor)
Shoaling is usually due to suspended silt and clay. Improvement is
the same as for off-channel river harbors--namely, dredging, training
dikes, and use of locks.
Shoreline harbors
Santa Barbara, California
Santa Monica, California
Camp Pendleton, Californi
The problem at such localities is the deposition of sand moved into
the harbor by littoral currents (discussed in the Succeeding section).
Maintenance of such harbors usually involves a "and-bypassing
operation, as also discussed in another section.
Sand Transport by Littoral Currents
General Considerations
The result of waves breaking at an angle to a shoreline is
generation of an alongshore or littoral current. It is this current,
combined with the agitating action of the breaking waves, that is the
primary factor causing the movement of sand along a coastline. This
movement takes place in two manners--in suspension, and by rolling in a
zigzag motion along the beach face. For a beach with an equilibrium
profile formed by waves of relatively large steepness, which is
characteristic of storm conditions, the sediment movement is mainly in
suspension. 2 In the came of an equilibrium beach profile formed by
wave" 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 an much as 80 percent of the material
moved by wave action Is moved in the area eboreward of the breaking
point.
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103
Rate of Drift
As yet, no general relationship between wave and sediment
characteristics is available for estimating the rate of littoral
transport that occurs along a given shoreline. A few early laboratory
experiments have assisted in defining the important variables. 3/4
Since these early studies, a considerable number of field and
laboratory investigations have been conducted. The fundamental
mechanics of littoral transport have been summarized recently by Komar
and Inmans and Komar. 6 Numerous measurements of rates of transport
along natural shorelines have been estimated from the amount of
material trapped by man-made shoreline structures. A summary of such
measured rates along U.S. coasts, as recently compiled by the Coastal
Engineering Research Center, is presented in Table 1. The reader is
referred to the Shore Protection Manual for the general procedure in
estimating rates of drift for a locality where the wave characteristics
are known.
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 1~. On many coastlines,
important reversals in the direction of littoral drift occur because of
the seasonal variation of the direction of wave attack. Usually,
however, the intensity of wave attack predominates in one direction,
with the resulting in a net or predominant direction of drift. For the
locations for which rates of transport are given in Table 1, the
predominant direction also is 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;
however, 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.
The determination of the predominant direction of littoral
transport has long been a study of interest to the geologist. In many
instances, it is necessary to know both the direction of littoral
transport at any one time and the predominant direction of littoral
transport over a normal climatic cycle. The predominant direction is
the more difficult to determine, and may involve locating the position
of natural and unnatural littoral barriers and those areas called nodal
zones in which the net littoral transport changes direction. In these
zones, the net littoral drift is zero, or in other words, the downdrift
components of littoral drift are equal to the updrift components. An
excellent example in this respect is the coast of New Jersey where
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104
TABLE 1. LON(:SHORE TRANSPORT RATES FROM U.S, COASTS*
PREI)(]MINANT LONGSHORE (a)
LOCATION DIRECTION OF TRANSPORT DATE OF
TRANSPORT Ccu.yd, /yr, ) RECORD
Atlantic Coast
Suffolk Coun~cy, N.Y, W 200,,000 1946-55
Sandy Rook, N. J. N 493 000 1885-1933
Sandy Hook, N. J. N 436 000 1933-51
Asbury Park, N.~. N 200,000 1922-25
Shark River, N. J. N 300,000 1947-53
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. (I) S 400,000 1935-46
Cold Spring Inlet, N. J. S 200,000 --------
Ocean City, Md . S 150 ~ 000 1934-36
Atlantic Beach, N. C. E 29,500 1850~1908
Hillsboro Inlet, Fla. S 75,000 1850-1908
Palm Beach, Fla. S 150~000 1925-30
to
225,000
(:ulf of Mexico
Pinellas County, Fla' S 50,000 1922-50
Perdido Pass, Ala. W 200,000 1934-53
Pacific Coast
Santa Barbara, Calif. E280,000 1932-51
Oxnard Plain Shore, Calif. S1,000,000 1938-48
Port Huene~e, Calif. S500,000 _-_______
Santa Monica, Calif. S270,000 1936-40
E1 Segundo, Calif . S162,000 1936-40
Redondo Beach, Calif . S30,000 -----a
Anaheim Bay, Calif . E150 000 1937-48
Camp Pendleton, Calif. S100 000 1950-52
Great Lakes
Milwaukee County, Wis . S 8,000 1894-1912
Racine County, Wis. S 40,000 1912-49
Kenosha, Wis. S 15,000 1872-1909
Ill. State Line to Waukegan S 90,000 --------
Waukegan to Evans ton, Ill. S 57,000 --------
South of Evanston, Ill. S 40,000 ----a
Hawaii
Waikiki Beach (b)
10,000
aTransport rates are esti - ated net transport rates, Qn In some cases, these
approximate the gross transport rates, Qg.
bMethod of measurement is by accretion except f or Abeecon Inlet, and Ocean City,
New Jersey, and Anaheim Bay, California, by erosion and Waikiki Beach, Hawaii, by
suspended load samples.
*SOURCE: U.S. Army Corps of Engineers, Shore Protection Manual, Vol. I
(Washington , D. C .: Government Printing Off ice , 1973), p . 9 (Table 4-6) .
D.ect_ ~ ht - -
ret - ~ - .~' ._
Figure 1 . Components of wave ve lo
city when waves break at angle to
.~hc~rml; no
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105
extensive study of sand movement by the Corps of Engineers and others
has established that a nodal point occurs at Manasquan, New Jersey,
with the predominant drift being northward north of this point and
southward south of this point.
Although the methods used in determining the direction of littoral
transport may differ from place to place, determination of the
instantaneous and predominant directions of littoral transport and the
location of littoral barriers and nodal zones ordinarily is
accomplished by consideration of such factors as (a) shore patterns in
the vicinity of headlands, (b) the configuration of the banks and beds
of inlets and streams, (c) accretion of erosion effects of man-made
structures, (d) statistical analysis of wave energy, (e)
characteristics of beach and bed materials, and (f) current
measurements.
Another item of~importance with respect to currents is the
confusion that often occurs between the strength of the littoral
currents and the strength of the large-scale ocean currents. These
latter currents, as measured by the drift of bottles and floating
debris, usually are relatively weak as a sand transporting agent
compared to the wave-induced littoral current. At localities where
these two currents are opposed, the wave-induced littoral current
usually is the stronger of the two, and therefore determines the
predominant direction of littoral drift.
Sediment Transportation, Deposition, and Erosion at Man-Made Littoral
Barriers
There are three basic types of man-made coastal structures that
function as littoral barriers: a dredged channel, a jetty or groin,
and an offshore or detached breakwater. 7 The littoral processes in
the vicinity of such works are summarized briefly here.
Dredged channels. Harbors are often connected with deep water
offshore by means of a dredged channel through the littoral zone
(Figure 2~. Such a channel creates greater than normal depths with the
result that littoral material accumulates therein. Sediment of small
enough size to be moved in the deeper depths seaward from the end of
the dredged channel would not, of course, be affected. Measurements
indicate that most of the longshore transport of material occurs in the
vicinity of the breakers where the available wave energy is converted
suddenly from an oscillatory motion into the form of turbulence. For
that portion of the wave that moves over a dredged channel, however,
breaking does not occur, because of the increased depth, and the wave
energy passes the normal point of breaking to be spread by refraction
and dissipated further inshore. The degree of turbulence, therefore,
is insufficient to transport material across the channel and the
material accumulates approximately as indicated in Figure 2. To
maintain the channel in a navigable condition, this accumulation of
littoral material must be dredged periodically. If this material is
removed and redeposited on the downcoast side of the channel, normal
littoral transport will occur in that region, and the shoreline will
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106
remain in an equilibrium position. If, however, the channel deposits
are placed elsewhere, then the supply of material to the dowocoast
beach is reduced and erosion and retreat of the shoreline probably will
result (Figure 2~. In a harbor such as shown in Figure 2, the action
of the waves is to restore the natural littoral transport of material
and thus reduce the area of the entrance to a size compatible with the
tidal prism. Tbe equilibrium size of entrance to be expected might be
estimated by the relationships between entrance area and tidal prism as
given by O'Brien. B
Harbors created by shore-connected breakwaters.
The effect of a
structure that extends seaward from the shore and across the littoral
zone is to act as a dam and trap the littoral drift. The impounding
capacity is dependent on the height of the structure, the bottom slope,
and the equilibrium alinement of the shore in that region. Tbe
equilibrium alinement is one which is normal to the resultant littoral
forces. Thus, in Figure 3, if the original shoreline was stable with
respect to the material balance and a breakwater is constructed as
shown, accretion will first occur in the form of a fillet on the
upcoast side with an alinement tending toward equilibrium. This will
create a deficiency in material supplied to the downcoast shoreline, in
which erosion probably will occur with the shoreline also tending
toward equilibrium. As the upcoast fillet approaches equilibrium,
littoral material will move along the outer face of the breakwater and
be deposited in the relatively calm water in the lee of the structure.
Thus, the turbulent character of the wave action upcoast from the
breakwater tip is sufficient to transport littoral material at capacity.
As the waves reach the tip, however, and are refracted and diffracted
into the lee of the structure, the turbulence is insufficient to
transport the material and deposition occurs. The deposit continues to
grow toward the downcoast shoreline, and when it reaches the shoreline
the material balance will be re-established on each side of the
barrier. The alinement of the harbor deposit depends primarily on the
predominant wave direction. A typical example of such a harbor deposit
is that at Santa Barbara, California.
A variation of a harbor formed by a shore-connected breakwater is
the case where two breakwaters must be provided to ensure protection
from storm waves that may approach the entrance from various
directions. Pronounced reversals in the direction of littoral drift
usually occur in such instances.
Detached breakwater. This type of structure intercepts the waves
and creates a protected area of relatively calm water. Tbe original
theory of suab a breakwater location was that the littoral material
would move along the coast uninterrupted by the presence of the
structure, and consequently, no maintenance problems from sediment
deposition would be created. This assumption, however, is in error.
The result of the refraction and diffraction of the waves behind the
structure is to reduce the energy available for littoral transport in
the lee of the structure as compared with the energy available on both
the upcoast and downcoast shorelines (Figure 4~. The result of this
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107
~ TRIPOLI ARE ~ ~
, 77~71 ~ [','''
I Predict ~ -~d~" ~~~onote;ce
I`ttom' currmt ~ cret~ I
~1
L.llorol It .~81't1 0? t" I
_ .ovc' .' of "copac'?~- ~ 'Rc r - ~I ~col.~q d - ' ~ ocean ~ clown
-~"e ' - ~s °~ ~0.'.9 ! "come d - r's zesty tne,.'ore
_
Figure 2. Schematic representation of transporta-
tion, deposition, and scour of littoral sediments at
channel dredged through littoral zone.
~~ ~_~s d ~ret_.
Fir, 'a
I \~\ \ \ \ \
Re.uct~ d ~ron..or' - owlet,
at cow Scow at ,.~roct~ _
*~ c_' ec - Ace d mot
Figure 3. Schematic representation of transpor-
tation, deposition, and scour of littoral sediments
at shore-connected breakwater.
; _=
~ K~x,,..~.
' ~ ~ O' ·~.d - - In, ht - .
Ltt~~' tren'.~'-~ _ ~ / tram. ~ 01~, ~ Add. t~.e _
Moo of lo Ivy ~ d ~~ / Item sac-~ _
·~c''~'-~_ B,-~- I / ,,
_ \ _ _ _
Figure 4. Schematic representation of transporta-
tion, deposition, and scour of littoral sediments at
detached breakwater.
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108
reduction in available energy is that littoral material accumulates in
the protected area. If this accumulation of material is not removed
periodically by dredging, the accretion eventually may extend
completely out to the breakwater in the form of tombola.
On the upcoast side of a detached breakwater, the accretion
advances beyond the region directly affected by the structure itself,
and corresponding erosion occurs on the downcoast side (Figure 47. A
typical example of a harbor of this type is that at Santa Monica,
California.
Sand Bypassing
A coastal inlet may be considered, for the purpose of this section,
as any relatively narrow waterway connecting the sea or large lake with
interior waters. Such inlets, either in their natural state or
improved to meet navigation requirements, tend to interrupt the normal
littoral transport along the shore. In the case of natural inlets that
have a well-clef ined bar formation on the seaward side of the inlet by
way of the outer bar, but intermittent, rather than regular, supply
reaching the downdrift shore, the result is that the shore downdrift
from the inlet is normally unstable for a considerable distance. If
the strength of tidal flow through the inlet into the interior body of
water is appreciable, part of the available littoral drift is
permanently stored in the interior body of water in the form of an
inner bar, reducing the supply available to nourish downdrift shores.
In the case of migrating inlets, the outer bar normally migrates with
the inlet, but the inner bar does not; the inner bar increases in
length as the inlet migrates, thus increasing the volume of material
inside the inlet.
When the natural depth of an inlet is increased by dredging, either
through the outer or inner bars or the channel, additional storage area
is created to trap the available littoral drift, thereby reducing the
quantity that would naturally pass the inlet. If the material dredged
(either for opening or for channel maintenance) is deposited beyond the
limits of the littoral zone, as in the case of disposal in deep water
at sea, the supply to the downdrift shore may be virtually eliminated,
with consequent erosion at a rate equivalent to the reduction in supply.
The normal method of inlet improvement teas been to provide jetties
flanking the inlet channel. Jetties may bave any or all of the
following functions: to block the entry of littoral drift into the
channel; to serve as training walls to increase the velocity of tidal
currents and thereby flush sediments from the channel; to serve as
breakwaters to reduce wave action in the channel; and to prevent
further inlet migration. In cases where there is no predominant
direction of littoral transports, jetties also serve to stabilize the
adjoining coastal shores. In the more common cases where littoral
drift in one direction predominates, jetties cause accretion of the
updrift shore and erosion of the downdrift ebore.
Stability of the shore downdrift from inlets, with or without
jetties, may be improved by artificial nourishment to make up the
OCR for page 109
Figure 5. Area of littoral drift affected by
waves from several directions, Bahia Blanca,
Argentina.
Figure 6. Sedimentation patterns in Figure 7.
Bahia Blanca.
Harbors in tidal estuary.
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110
deficiency in supply. When such nourishment is accomplished by using
the available littoral supply from updrift sources, the process Is
called sand bypassing. A number of mechanical methods of sand
bypassing have been employed; however, this is still a relatively
recent engineering development, and additional methods will no doubt be
developed as experience is gained.
Several teaboiques bave been (and are now) employed for
mechanically bypassing littoral materials at inlets. Sometimes a
combination of techniques has proved to be the most practicable and
economical. The basic methods which have been used are:
.
Land-based dredging plants,
Floating dredges,
Mobile land-based vehicles.
For details on these methods, the reader is referred to the Coastal
Engineering Research Center's Shore Protection Manual, which describes
the use of these methods at specific localities.
Examples
The action of these forces, and their interaction with harbor and
port design, can be seen in a particularly challenging area on the
coast of Argentina (Figure 5~. The large estuary of Bahia Blanca has
no streams of any importance feeding into it: the sources of sediments
are the Rio Colorado, the Rio Negro, or both. Notice that whatever the
direction from which waves come--south, southeast, west, even to some
extent, northeast--littoral drift will occur along the coast, moving
material into the entrance of Bahia Blanca. The sink where all this
material arrives is shown in Figure 6. The material is principally
sand from the large rivers, and from minor beach erosion and small
streams up coast. There are no structures at the entrance: the only
developments are the buoys.
Some harbors within the tidal estuary are dredged back into the mud
flats pictured in Figure 7. The problem in this area of tidal flats is
almost entirely one of cohesive sediments, or wash flow. With each
range in tide (about 15 to 20 feet), the sediments are washed back and
forth. About 10 feet of sediments are deposited each year, and must be
removed by dredging. Dredging is accomplished by the dredge shown in
Figure 8, a museum piece, and barged to the middle of the stream where
they are dumped, most likely to return with the next tidal range. The
pier in the harbor pictured in Figure 9 projects into the tidal
stream. Notice the steeply banked channels. The high particle
velocity in these channels creates turbulence in flow through the
pier: the particles collide and settle faster than they would
otherwise.
The grain elevators shown in Figure 10 store the principal export,
grain from the pampas. The principal import is oil for the surrounding
area.
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111
~b ~
: ~
3 -
l
-
- ~
l i ~ - -
- ~
- -
''skiddy
~ id
~ 3
:
. lo;. ~
Cal ~.. .. ;
~ ~ ,
_-
Figure 10. Pier facilities for grain
exports.
. .
~-
Figure 8. Dredge used to remove sedi- Figure 9. Pier projecting
meets in channels and harbors. stream.
into tidal
wit . _ _ · _ 1 ~
.
.
9
Figure 11. Pier built parallel to tidal
stream.
am_
_e
Hi_ ~
Figure 12. Mud flats.
·~) ,\
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112
Downstream from this pier is another (Figure 11) that offers a
useful contrast. Notice that the pier is parallel to the flow of the
tidal stream, and almost sediment-free. The depth is always 23 to 30
feet along the pier face.
A small-craft harbor for tugs and boats is also illustrated in
Figure 11, to the left. Some sedimentation can be seen at the
entrance, a dead-water area. The principal tidal currents keep the
main channels open, but the accumulations of fine materials eventually
become cohesive sediments at the entrance to the tug harbor, which is
somewhat restricted. The flow of sediments from the mud flats (Figure
12), owing to the high tide range, cuts extremely steep, sloped
channels, and the fine sediments are continuously entrained and
redeposited.
References
2.
1. Caldwell, J. M., "Sedimentation in Harbors, n Chap. 16, Applied
Sedimentation, P. D. Trask, ed. (New York: John Wiley and Sons,
Ince ~ 1950) ~ ppe 290 - 291.
Saville, Te ~ Jr., n Model Study of Sand Transport Along an
Infinitely Long, Straight Beach,. Transactions of the American
Geophysical Union, 31 (Aug. 1950~: 555.
3. Krumbein, W. C., "Currents and Sand Movement in a Model Beach,"
Beach Erosion Board, Techn . Memo. No. 7, U.S. Army, Washington ,
D. C., 1944 .
Saville, op. c it .
Komar, P. D. and D. I. Inman. "Lc~nashore Sand qYransnort on
4.
5.
7.
Clt.
. and D. I. Inman, "Longshore Sand Transport on
Beaches," Journal of Geophysical Research, 75 (Oct. 1970~: 5914.
6. Komar, P. D., "The Mechanics of Sand Transport on Beaches, Journal
of Geophysical Research, _ (Jan. 1971~: 713.
Johnson, J. W., The Littoral Drift Problem at Shoreline Harbors, n
Transactions of the American Society of Civil Engineers, 124
(1959~: 525.
O'Brien, M. P., "Equilibrium Flow Areas and Tidal Inlets on Sandy
Coasts," Journal of the Waterways and Harbors Div., American
Society of Civil Engineers, Feb. 1969, p. 43.
DISCUSSION
KRAY: My question is in reference to the wharf or pier which
is free from sediments in Figure 11. What is the washout rate ahead of
that pier? The sediments consist primarily of the cohesive soils, and
~ presume that washout and movement are very considerable along that
face.
JOHNSON: You mean the scour? As far as I know, there are no
problems. The penetration of the pile is such that the lower portion
of the pile is never exposed.
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113
KRAY: It doesn't extend too deep?
JOHNSON: No.
KRAY: Do you know by any chance what the foundation i" of
that particular pier?
JOHNSON: It is piles.
KRAY: Steel sheet piles?
JOHNSON: I am not sure of that.
KRAY: So there is a solid wall, at any rate?
JOHNSON: Right. The solid wall is preferable. It is a
smooth wall and you don't get the high turbulence which Is conducive to
coalescence and deposition of the material.
RIEDEL: I would like to add a cautionary note to your
suggestion that you won't have deposition along the base of the pier
which is parallel to the main channel. I use as illustration what I
called down in Vicksburg a couple of days ago a parking lot problem,
sometimes referred to as a marine transportation problem, and it is. A
ship is turning around in an anchorage, experiences some failures and
runs into the ship which is tied up at the base of a pier very similar
to the pier you spoke of, and the ensuing fire closes the port.
I would suggest that we have to be careful about the best
solution for ease of maintenance; for example, docking along the base
of a stream. Sometimes we must counsel ourselves on the safety
problems as well as the advantages for maintenance of various
solutions. ~ don't think we want to be moved too far in any one
direction without a rationalization of all elements.
JOHNSON: Your point is well taken. In the particular case
you refer to, there isn't any parking area in that main stream, and if
you dredge a parking area from the mud flats, it will eventually fill
up; nonetheless, T agree with your point.
BERTSCHE: Would knowledge of the whole hydraulic water flow
of that area at the design stage help you in solving some of the
sedimentation problem" that occur? You pointed out the one flat that
was essentially draining into the docking basin at the side. That is
pretty obvious, perhaps it could be predicted by look ing at the chart,
but in more subtle cases , would a f ull , tbree-dimensional hydraulic
model--either mathematical or full scale--aid in looking at the
sedimentation, or is tbat part of the problem with the design process?
JOHNSON: Frankly, I don't think enough data exist for a
remote area like that to build a model. The model can only be as good
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114
as the prototype data. A new pier and expansion of this port have been
proposed: tbe pier would be parallel to the stream along the mud bank.
BERTSCHE: Let me pursue this point. Would the sedimentation
include a sand drop at the entrance? You were talking about transport
down the coast. I would assume that as soon as it hits the breakwaters
and jetties, it creates a problem.
JOHNSON: There are no breakwaters or jetties at that
particular port, but at others, material does accumulate against the
jetties. In my opinion, the rate in difficult to estimate unless there
is a record of experience at nearby harbors. Santa Barbara, for
instance, has a long period of record: in that vicinity, 250,000 to
280,000 cubic yards per year seems a reasonable expectation.
I think Bob Dean has worked at the Channel Islands Harbor
further down the coast. What was the annual rate you estimated, and
that dredging records show for that area?
DEAN: About a million cubic yards.
between ports.
JOHNSON: So, there is guise a difference in a short distance
SAVILLE: The case of Channel Islands Harbor is interesting.
The original design was based on Santa Barbara, and then upgraded to
about 700,000 to a million cubic yards per year. Predicting
sedimentation rates from past experience is a good practice, but you
need present experience, too.
JOHNSON: That i. correct. Between Santa Barbara and the
Channel Islands Harbor, for example, is the Santa Clara River, which
can get out of hand about every 25 to 30 years, suddenly dumping a huge
amount of material just up the coast from the Channel Islands Harbor.
HERBICH: You mentioned some equations in the Shore Protection
Manual that allow one to make estimates of Sediment transport. Other
,
equations have become available since that manual was published. What,
in your opinion, is the accuracy of any estimate of sediment
transport? Is it plus or minus 50 percent?
JOHNSON: It can be as much as 200 percent, and that is the
basis of my concern.
Representative terms from entire chapter:
littoral drift
