sr-012373-10a1~Ifl&;F'ATT ~C NICIIflZ,, EtiGI\EEI2S
LONG BEACH, GALfPORNtA PORTLAN O, OAEGON
November 6, 1972
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Mr. Perry Scott
City Manager,.
City of Santa Monica
1685 Main Street
Santa Monica, California 90401
Subject: Report Our L-1470
Dear Mr. Scott:
Enclosed are six {6) copies of our
Report to you of anticipated shoreline
effects of the proposed island shoreward-
of the Santa T+Inica breakwater,
ide are pleased to have had ths-
opportunity to be of service zo
.you, and look forward to any future
service we may rendere
Sincerely,
MOFFATT & NICHOL, ENGINEERS
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John G. T~loffatt, President
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enels. _ .° _.
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P. O. BOX 7707 250 WeST WAROLOW ROAD LONG EEACH CALIFORNIA 90607 (2:3) 426-9551, 7?4-5650
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ABSTRACT
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,_ In order to create the 35 acre island behind the Santa P4onica
Breakwater and maintain the beach south of Colorado Avenue during
construction approximately 2,500,000 cubic yards of sand will have.
to"be removed from the existing salient behind the breakcaater.
This will essentially .remove the salient and straighten the exist-
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ing alinement of the beach. Approximately 250,000 cubic yards of
littoral sand will be trapped each year in the. lee of the island
and will have to be bypassed to the beach south of Colorado Avenue
'_ in order to prevent serious erosion of this beach.
The most desirable sand bypass program is continuous sand
bypassing by a floating pipeline dredge. However, this program
j must be modified as it is not desirable to interfere ~~rith usage
of the beach during the sur2mer period nor is it desirable to
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.operate floating equipment during the winter months of storm i~raves
.and strong winds. Ideally the bypass operation should be aceom-
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plished each year between October and January. This will result
in a reasonably uniform alinement of the beach behind and to the
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:_ north of the island. This semi-continuous, or annual sand
~, bypassing will result in a stable beach to the south with fluctua-
tions in width comparable to present seasonal changes. This pro-
gram, by a City owned and operated dredge is the most economical-means
of bypassing littoral sand movement and, based on 1872 prices, would
cost about X200,000 per. year.
If the sand bypass program is to`be done by contracting with a
commercial dredging company, it should be done by a larger dredge on
a biennial bass. The annual cost to the City would be about $250,000
per year and there would be a corresponding larger fluctuation in
widths of adjacent beaches, although not to such an extent as to en-
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L, danger structures or severely interfere with recreatior_al use off'
~ the beaches.
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PRELIMINARY REPORT ON
' SHORELINE EFFECTS
OF
PROPOSED ISLAND SHORELTARD OF
SANTA MONICA BREAKS+IATER
INTRODUCTION
It is proposed to build an offshore island at Santa P4onica.
The area will be about 35 acres and the present 2000 foot offshore
breakwater will be incorporated into the seaward face of the island.
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` While the island perimeter will be curvilinear. it will, in general,
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~, extend about 800 feet shoreward of the breakwater (see Plate I).
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It is planned to dredge from 2.0 to 2.5 million yards of sand from
i_ the existing beach salient behind the breakwater which will be used
i to create the island and counteract shoreline effects. This will
`" straighten the alinement of the beach and create an initial channel
I some 700 to 800 feet in width and 20 feet or more deep betcreen the
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proposed island and the shore.
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j_ The purpose of this study is: to develop the best pattern
for excavation to create the island; to develop the natural rate of
littoral, or longshore, sand movement in this area; to develop the
probable effects of the island construc£ion on the upcoast area, the
area behind the island and the do~rrncoast area; to determine the
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E feasibility of bypassing sand by several alternative methods and
~~ to estimate t'rie probable quantity and costs..
`. HISTORY
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In its natural state, the entire shoreline of Santa P2oniea Bay
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consisted of a rather narrow beach backed by soft sedimentary bluffs.
!- The- exception is the Marina-del Rey-Ballona Creek area, w'riich at
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times was the mouth of the Los Angeles River. There have been
numerous man-made structures and fills created along this beach since
the early 1900's and it is very difficult to develop a clear-cut
picture of littoral processes throughout the bay.
It has been developed in previous Corps of Engineers studies
that the littoral drift is predominantly from north to 's outki,although
there is limited upcoast movement in the summer that must be taken
into account. They, established a net. rate of movement of 270,000
cubic yards per year to the south-. They also developed that primary
sources of littoral beach sand are from upcoast, mostly from upcoast
beaches and to a lesser extent from the numerous small local drainage
features. For the purposes of this study, only the area from the
Light House Cafe southward to the Ocean Park. Pier will be given
detailed consideration.
ANALYSIS OF LSTTORAL SAND P~IOVENLETdT
The rate of littoral sand movement is a function of wave
energy and wave direction when it reaches the beach. In the Santa
Nonica area the long term condition of the beach is related to the
winter and spring waves from the North Pacific Ocean areas as compared
to summer and fall .waves from ocean areas to the south extending
as far as Peru and Australia. Ldhile day to day wave action is very
important, the severity and frequency of local or off-shore storms
also make major contributions to the total wave energy-sand movement
relationship. On the sand supply side, the rainfall rates and
severity of storms such as those in 1936, 1938, 1843 and 19og
contribute to local sand supply as idell as influencing the rate of
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sand movement from the upcoast, or updrift beaches. In view of
' these variables, it is remarkable how consistent the average annual
rate of littoral sand movement is along the shoreline df southern
California. Frequent field measurements have been made for ever
35 year s. at Santa Barbara, 30 years at Channel Island, and 25 years
in north Orange County, and it has been found that the annual rate
of sand movement does not vary more than about + 15 percent.
The most reliable method of measuring littoral sand movement
is actual field measurements at an obstruction to littoral drift.
In order to determine the total amount of drift, measurements are
only reliable so long as the structure is a total obstacle to
longshore sand movement. During this period either accretion on the
updrift side or erosion on the downdrift side may be a measure of
movement. The exception is an area like Channel Island and Santa
Barbara where a measured amount of impounded sand is removed
periodically and placed in the littoral stream sufficiently-far down-
coast to not return to the trap. and confuse the measurement.
~ At Santa Monica a relative short period of time i~ras available
for this type of measurement. i~lhile there were a fe~~r open pile
piers in the Santa P~Tonica area, such as the Santa Monica pier built
in 1907, they appeared to have little effect or_ the shoreline and
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the surveyed mean high tide line in 1921 sho~,red a generally straight
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': alinement with a narrow beach through the City of Santa Monica.
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In a situation such as that at Santa Monica, with an offshore
breakwater parallel to the shore and open at both ends, the sand
moving along the shoreline is influenced by 3 different factors:
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(1) The-sand moving downcoast is deposited in the shelter
of the breakwater causing large scale accretion; (2) at the
southerly end of the breakwater a limited amount of sand is
moved upcoast sufficiently to be sheltered from the pre-
dominantly dotancoast wave action; and (3)-the lotr profile of
of the breakwater and the voids in the cap rock permit a
sufficient amount of *rrave energy to pass through the structure
and move sand along the beach in the lee of the breakvrater.
(a) Downeoast-Sand Movement:. This is the predominant
cause of shoreline accretion and development of the large
salient behind the existing Santa Monica breakwater. Between
the start of construction of the breakt•.rater in ?riarch 1933
and the survey of 1946, over 3,500,000 cubic yards of sand
were impounded behind the breakvrater. S+?ost of this
resulted from the interference with dot~rncoast littoral
drift. This beach sand is moved by wave induced littoral.
currents. The breakwater influences this sand movement in
tyro trays: first it eliminates, or greatly reduces, the
amount of wave energy and thus long shore sand moving
capability along the shoreline behind the breakvrater.
Second, the waves at the ends of the structure ere
changed in height and direction as they. round the north
end of. the breakwater by the process knot~m as grave
diffraction. The projection of these tt~;o factors from
the breakwater to the shore varies from day to day,
depending on the direction of the graves and the result
is a transition curve forming the sal;ents of the general
shape shown on Plates 1 and 3. Additional accretion is
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caused when the salient .advances a sufficient
. distance seataard to act as a groin, so that at
Santa D'fonica accretion has occurred along the
beach for some three miles northward of the break-
vrater. This effect does not change the rate of
accretion but instead slo~r~s the seaward advance
of the salient. This is shown on Plate 1 by the
rapid growth of the salient bettueen 1933 and 1946
- as compared to later years.
(b) Upcoast Sand Movement. While the net ahnual
sand movement at Santa Monica is dotvncoast, there
is an appreciable amount of upcoast movement through
the summer and fall caused by i~raves generated by
storms off the coast of Mexico or in the South
Pacific Ocean. .This sand moves into the shelter
' behind the south end of the breakwater. It is
.further influenced by variable wave directions and
vrave diffraction around the south end of the
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j; structure. This impounded sand tends to form the
~- south flank of the salient from which the Corps cf
~' Engineers estimate of 270,000 cubic yards per year
!~ represents an average rate of accretion, more than
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average rate of littoral .drift.
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~_, (c) Wave Energy Through the Breakwater. While the
Santa Monica Breakwater reduces the amount of wave
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energy acting on the shoreline behind the structure,
it is not a total barrier. The orieina7 hrFakUratar
was of capstone with 35 to 40~ voids from elevation
+10 to -10 feet P-?LLLd when completed in 1935. This
elevation was reduced to +7 feet by 1957 and has
further deteriorated since that date. Hence, an
increasing amount of wave. energy is going through and
over the structure. It is not within the scope of
this study. to quantify this residual energy but,
particularly during the higher tides, it has a capa-
bility to move sand. Tiro effects can be noted: it
.tends to further blend the ti~rinter and summer sand
deposits on-the flanks of the salient making identifi-
cation of source more difficult, and in recent years
is assisting the predominant do.aneoast waves to move
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,_, sand beyond the salient and help substain the down-
~_ coast beaches.
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Tn the early stages of development of the large sand salient
behind the breakwater, this eras probably a negligible factor
due to dispersion of this energy betT~reen the breakwater and the
i~ beach. The Corps of Engineers findings that between 1935 and
r, 1946 the average. annual rate of accretion was in the order of
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270,000 cubic yards vaas undoubtedly correct.
Between completion of the breakwater, in 1935, and 1959,
some 1,800,000 cubic yards of sand was dredged from the salient
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and placed on the beach dotirncoast of the Santa P~Tonica Pier. This,
in combination vrith placement of the Hyperion Beach fill, has
~- prevented serious erosion of the downcoast beach. Examination of
the historical mean high tide lines shows on Plate 1, shows a
relatively stable beach alinement since 1946 indicating that
!~ either a considerable portion of the littoral sand has been
,_ carried through the area to the down-coast beaches or that this
- sand- or a portion of it- is impounded along the entire 3 mile
sand fillet to the north and is not easily identified. Ordinarily
examination of the doivncoast beach would show compensating erosion
in the same manner as occurred between 1035 and. 1939 but the
~_, enlargement of the Pacific Ocean Park Pier and the placement of
the Hyperion beach fill plus the. three dredging-sand episodes
prevents a reliable accretion vs erosion analysis. At present,
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it is probable that upti~rard of 100;000 cubic yards per year is
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being carried past the existing sand salient and, along with-the
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Hyperion beach fill, ;s reasonably sustaining the downcoast beaches.
L., Construction of the proposed island. and the removal of most
r of the existing beach salient will essentially recreate conditions
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as they were in 1935• In addition, the breakvrater-island structure
~ will completely prevent waves passing .through or over the structure
and the inshore side of the island will be-some 800 feet shore,va'rd
~_. of the 1935 structure thus insuring a complete obstacle to littoral
~' currents. we must assume that the rate of impoundment will be
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the same as that experienced bet~nreen 1935 and 1846 and that
scheduled sand-bypassing will be required to sustain the downeoast
beaches.
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EFFECT OF PROPOSED ISLAAiD ON LITTORAL DRIFT
The construction of the island by excavation of some
2,500,000 cubic yards of sand from the existing beach salient
will result in conditions as sho~~rn on Plate 2. The construc-
Lion of either the island or the excavation of the beach area
j will cause major changes in the rates of accretion and/or
erosion of sand to the beaches behind and adjacent to the
project. These effects would become evident at about the 25
percent of completion point of either the breakwater rehabili-
tation, construction of the island or excavation of the beach
salient.
EFFECT GJITHOUT SAND BYPASSING:
.Sand bypassing must be an intergral part of this project
but because the beach salient and the adjacent beaches have
not been subjected to noticable changes since the last bypass
dredging in 1958-59 this approach is discussed.
..Construction of the island and rehabilitation of the
breakwater as shown on plate 2 grill constitute a complete
barrier to waves impinging on the structure. Wave action
around the ends .vill penetrate to a limited extent behind
j the island due to the wave diffraction effect but as the
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water gag will be only some 700 feet wide as compared to an
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~-- island-breaksrrater length of 2000 feet wave diffraction around
( each end of the island will form separate beach salients
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instead of the smooth combined salient presently existing.
Most of the southerly impoundment caused by upcoast wave
action can be intercepted by a land filled base
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` for the bridge to the island at Colorado Avenue, as shown on
Plate 2, and thus remain in circulation.
Excavation of the beach salient as shovrn on Plate 2 brill
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i_ delay the formation of the nevr salient behind the north 1/3 of
~ the island until this dredged area is brought up to normal
~ beach slope. Then a tombola ti~rill form-similar to that shovrn
on plate 3, connecting the island to the shore. This tiaould
require some 5 to 7 years.
{_ -. In the mean time, there would have been no supply of
~-- littoral sand to the beach south of the pier. and some 1,000.,000
1 to 2,000,000 cubic yards of sand would have moved on dotivncoast.
This represents a loss of 20 to 35 acres of shoreline,
including public beach, parking lots and private property.
I_ betvreeri the beach and the promenade.
(- EFF,FC^ ~dITH SAND BYPASSING
E' j~Tith a properly planned sand bypass program, during and
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~ after construction, there will be no adverse effects on the
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hew alinement of beach behind the island or on the adjacent
~ beaches..
~ The most desirable approach would be to establish a
`-" continuous sand bypassing program but this is neither econo-
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mical or desirable. The beach is very heavily used during
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the summer season and between early April through September
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~_ should not be cluttered T.aith pipelines and heavy equipment.
~- Also, it is not desirable to be placing sand on the south
beach during periods of summer upcoast drift as the newly
deposited sand would tend to move nori;h~rard into the lee of
the island where it would ultimately have to be rehandled.
Therefor the program should be to bypass the total year of
sand accretion from behind the island to the southerly beach
between October and February;. before the worst of the tivinter
winds and storm waves would endanger the equipment.
During Construction, a 22 to 26 month period, some
. 2,500,000 cubic yards of sand will be dredged from the shore
salient behind the island as shown on Plate 2. Approximately
2,000,000 cubic yards will be needed to create the nets island.
The remainder will be needed to maintain a stable beach to
the south during the construction period. A coordinated
schedule will be needed in which, preferably during each
fa.11 of the construction year 250,000 + cubic yards of sand
will be placed south of Colorado Street to replace the natural
littoral drift to the south.
the"landside base!'for the bridge, extending from Colorado
Avenue to the island should, in conjunction tivzth sealing of
the offshore breakwater and dredging of the salient to fill
the island area, be a first start construction item. The
island is of adequate size to allotir filling the center area
from -24 to -10 ft. MLLtd
Optimum dredging conditions vrill be obtained if the
breaks%rater rehabilitation is done from south to north and.
dredging also be started near the south boundry to obtain
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maximum protection from wave action. The norther]_y 800 feet
of the borrow area is exposed to wave action from winter
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~__ .waves and this area should be scheduled for dredging; during
the late summer and fall season. There Lvill be very little
disruption of recreational use of the beach during the 2 year
construction season as most of the dredge spoil will be placed
on the island. The two sessions of sand placement on the
beach south of Colorado Avenue can be scheduled. for late fall
or early winter vahen recreational.use of the beach is minimal.
This construction dredging of 2,500,000 cubic yards of
sand in 2 years crith the complexities of building up a
35 acre island requires the use of a large commercial dredge
(24 inch diameter or larger) operated by experienced
contractor personnel.
SAND BYPASSING PROCEDURE
Construction of the island will create a complete barrier
to the longshore movement of beach sand by wave action. This
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will cause accretion of sand behind the north portion of the
', island and denial of replacement sand to the beach south of
Colorado Avenue vahere the beach is not protected from normal
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wave action with the resultant longshore movement of sand.
Based on previous experience this interference will amount
~ to about 250,000 cubic yards pe'r year. Within one year
~' after completion of the island construction a system must
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be established for transfer of this sand. Even in nature the
width of a sand beach will vary seasonally or there may be a
temporary loss of beach after a severe storm so a limited amount
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of erosion can be tolerated 4vithout undue interference with
recreational use of the beach. However, 2 years would
represent a loss of 500,000 cubic yards of sand or 10 to 12
acres of beach which is about as much as can be tolerated.
The associated accretion of 500,000 cubic yards of sand .would
also begin to seriously narrow the protected crater area as
shown on plate 3 and cause other inconveniences to the beach
users in that area. For these reasons the sand should be
bypassed at no greater than 2 year intervals.
There are several established methods of sand bypassing,
varying from continuous bypassing to a complete mobilization
and demobolization of equipment for a short highly productive
effort every two years, Any of these tivill result in a
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`' stable beach alinement in both the accreting and eroding
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zones with little more variation than is presently observed
fror!m seasonal effects. Obviously the more frequent the
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~,, dredging the more the beaches will vary in width in the same
`- manner- as they no?-~r do from natural causes. However, this
` stability must be measured against cost, clutter of the: beach
with eauipment that might be hazardous to bathers, and
noise: and visual pollution during summer periods of-heavy
beach usage.
ItSETH0DS OF SAND BYPASSING
Examples of sand bypass systems successfully in opera-
bons are as follocas:
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Santa Barbara: Between 1956 and 1970 this harbor was
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maintained by a small City-owned continuously operated
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pipeline dredge, Approximately 300,000 cubic yards
per year c~rere bypassed at an annual cost of around X0.40
a yard. It is not certain that this included all of the
overhead and plant rental costs. In the future this
project ~~rill be done by commercial dredge bypassing
300,000 cubic yards in about four months of dredging.
A contract was ac~rarded in September 1972 to bypass 225,000
cubic yards at a contractor cost of y1.21 per cubic yard.
Channel Island Harbor: The rate of littoral sand movement
-here is in the order of 1,200,000 cubic yards per year.
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The sand trap can accumulate a 2-year. supply so recent
eontractorr costs for biennial bypassing of 2
500
000 cubic
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~- yards in a 6-month dredging season is in the order of
$0.50 per cubic yard.
Ventura varina. The rate of littoral drift here is in
'~, the order of 400,000 cubic yards per year. The first
t-- contract in 1972 for biennial bypassing was for 825,000
~ cubic yards ata0.92 per cubic yard.
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~ Oceanside: P~laintenance here is on a biennial basis at
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about 1400,000 cubic yards per dredging episode at a cost
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f_, of X0.80 per cubic yard.
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T%ixed Sand Bypass Plants: (land based) Two such plants
~ operate off the ends of jetties at North and South Lake
Ldorth Florida but neither are over 50t~ effective as their
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lack of mobility prevents interception of all the littoral
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stream.
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There have been attempts at use of draglines and
shovels with truck haul but unit costs a-re high and
visual, noise and traffic problems were e:ctremely
troublesome. The surest and most successful bypass
system to date is the establishment of a large sheltered
or semi-sheltered impounding area from which the periodic
removal and placement downcoast of sand by a floating
.pipeline dredge can be accomplished.
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RECOP+i1+IENDED SAiJD BYPASS SYSTEtii
It is recommended that the original dredging of 2,500,OQ0
cubic yards be done by a large commercial dredge. This s~rork
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i, should be scheduled so that near the completion of the project
~: sufficient sand be placed on the beach downcoast of Colorado
~- Avenue to replace any losses incurred during the 2-year
construction period.
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A permanent sand bypassing program should be initiated
vrithin one year after completion of construction dredging. At
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the present stage of knowledge the following tyro methods are
recommended.
(1) Continuous Sand Bypassing. The required annual
bypassing of 250,000 cubic yards averages out at only
700 cubic yards per day. However, this downcoast sand
-. movement occurs during about 9 to 10 months of the the
year with reversals or no drift during the summer period.
For 9 months at 21 working days per month the average
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rate of bypassing would be 1,330 cubic yards per day.
Theoretically this could be handled by an ~ or 10 inch
diameter pipeline hydraulic dredge. However, a safety
', factor must be provided for in oase of breakdowns,
strikes or very severe storms which may increase the
rate of littoral drift. Also a pumping capability for
a distance of 3,000 feet is required which taxes the
efficiency of small dredges.
A 1~4-inch diameter pipelin2 hydraulic dredge pumping
10 percent solids vrould have a capacity of about 250
cubic yards of sand per hour or 2500 cubic yards per 10
hour shift. Thus about 4 months, October to January
inclusive, would be required for annual bypassing. ^lcvo
vrays are available to accomplish this annual bypass
operation:
(a) By contract, and based on 1972 costs, the
costs to the city would be about X1.25 per cubic
yard or X315,000 per year.
(b) If the City i?ere to purchase a dredge and
operate it themselves, and could supply adequately
skilled personnel from their public vaorks depart-
ment for 'the 4 months of dredging, the unit costs
could be reduced to about $0.75 per cubic yard.
This approach would require an initial city
expenditure of $400,000 to X500,000 for a 14-inch
dredge, 3.000 feet of pipeline, a vrork boat and
'- other miscellaneous equipment. Taking into account
time for mobilization and demobilization the dredge
would not be needed for about 7 months of the year.
The dredge could be stored on site but, considering
the character of development planned :for the island,
' it 4~rould probably be most desirable to store the
', dredge in a corrli-nercial yard in Los Angeles-Long Beach
Harbor.
(2) Biennial Sand Bypassing: A biennial program would
require the bypassing of 500,000 cubic yards of sand
~' during the same 4 months fall period as before and would
require operating about 20 hours per day. Triis quantity
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is large enough so that, with shore based electrical
power, the larger dredges could be economically used and.
- complete the bypass effort in a shorter time,
Based on 1972 prices, the cost to the City would be
about $1.00 pe'r cubic yard if done by a contractor, It
v.ould be quite. difficult for the city public works
department to provide sufficient man power for a 3 shift
~- operation and the main savings in cost by use of a city
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`J owned. dredge would be the elimination of the contractor's
profit, _
It is recommended that the city consider acquiring
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,__ and operating their oti~rn hydraulic pipeline dredge on an
annual basis of 4 months operating time. An additional
6 to 8 weeks will be required for mobilization and
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demobilization, This will result in a more stable beach
alinement both behind the breaYwater-island and to the
south of Colorado Avenue and in fact changes in beach
width would not greatly exceed present seasonal changes.
Annual costs, based on 1972 prices would be in the order
of X200,000 per. year, including depreciation, maintenance,
supervision and administration.
The alternative would be to establish a biennial
program by contract dredging v`rhich would average about
X250,000 per year and would result in a greater fluctua-
tion in ti~ridth of the beach, particularly the eroding
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