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Natural changes and human impacts on the sand budgets and beach widths of the Zuma and Santa Monica littoral cells, Southern California

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Abstract and Figures

The intensively developed southern California coastline from Malibu to the Palos Verdes Peninsula can be divided into two littoral cells, which have both undergone significant but markedly different changes over the past century. The Zuma Cell, extending from Pt. Mugu to Pt. Dume, trends nearly east-west and has relatively little sand input. Continuous littoral transport and a general lack of barriers have led to limited beach development. Modest beach cottages built on the sand in the Broad Beach area of Malibu decades ago have been replaced by very large homes in recent years, but a reduction of littoral sand from the west is now threatening these homes. It appears that much of the sand moving through the Zuma Cell and that was responsible for the beaches was initially provided by leakage around the head of Mugu Submarine Canyon from the upcoast Santa Barbara Littoral Cell. Headward erosion of the canyon has now cut off this sand supply leading to progressive narrowing of the downcoast beaches. The adjacent 24 km of the western portion of the Santa Monica Cell has the same general east-west trend as the Zuma Cell and is also characterized by limited sand input and very narrow beaches. At Pacific Palisades the trend of the shoreline changes almost 90 degrees. The next 32 km of Santa Monica Bay shoreline is oriented almost north-south, nearly parallel to the wave approach, reducing littoral drift rates and allowing wider beaches to develop. Combined with the addition of approximately 23 million m3 of sand added to the Santa Monica Bay beaches from coastal construction and dredging projects over a 60-year span, and a series of sand retention structures, this 32 km stretch of shoreline and the 18.6 million people in the adjacent greater Los Angeles area, have benefitted from wide and generally stable beaches.
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Shore & Beach Vol. 86, No. 1 Winter 2018 Page 1
The recognition of littoral cells or
beach compartments and their
importance in connecting the
sources, littoral transport, and sinks of
beach sand, initially along the southern
California coast, was one of the most
important contributions to coastal pro-
cess research in the last century (Inman
and Frautschy 1966). The delineation
of individual littoral cells, where rocky
headlands and submarine canyons
typically form the upcoast and downcoast
extent of these compartments, changed
our thinking and led to a new generation
of studies and papers that focused on
specic littoral cells, their sand sources
and sinks, and the quantication of sand
budgets and littoral dri rates (for sum-
mary see Patsch and Griggs 2006 a and
b and Orme et al. 2011).
e littoral cell concept was subse-
quently exported or applied to other
regions, but because of the unique nature
of the southern California coastline, in
particular the distribution of rocky head-
Natural changes and human impacts on the sand budgets
and beach widths of the Zuma and
Santa Monica littoral cells, Southern California
By
Gary B. Griggs*
Institute of Marine Sciences, University of California Santa Cruz, CA 95064
griggs@ucsc.edu
Kiki Patsch
Environmental Science and Resource Management Program
California State University Channel Islands
kiki.patsch@csuci.edu
e intensively developed southern California coastline from
Malibu to the Palos Verdes Peninsula can be divided into two lit-
toral cells, which have both undergone signicant but markedly
dierent changes over the past century. e Zuma Cell, extend-
ing from Pt. Mugu to Pt. Dume, trends nearly east-west and has
relatively little sand input. Continuous littoral transport and a
general lack of barriers have led to limited beach development.
Modest beach cottages built on the sand in the Broad Beach area
of Malibu decades ago have been replaced by very large homes
in recent years, but a reduction of littoral sand from the west is
now threatening these homes. It appears that much of the sand
moving through the Zuma Cell and that was responsible for the
beaches was initially provided by leakage around the head of
Mugu Submarine Canyon from the upcoast Santa Barbara Lit-
toral Cell. Headward erosion of the canyon has now cut o this
sand supply leading to progressive narrowing of the downcoast
beaches. e adjacent 24 km of the western portion of the Santa
Monica Cell has the same general east-west trend as the Zuma
Cell and is also characterized by limited sand input and very
narrow beaches. At Pacic Palisades the trend of the shoreline
changes almost 90 degrees. e next 32 km of Santa Monica
Bay shoreline is oriented almost north-south, nearly parallel
to the wave approach, reducing littoral dri rates and allowing
wider beaches to develop. Combined with the addition of ap-
proximately 23 million m3 of sand added to the Santa Monica
Bay beaches from coastal construction and dredging projects
over a 60-year span, and a series of sand retention structures,
this 32 km stretch of shoreline and the 18.6 million people in
the adjacent greater Los Angeles area, have benetted from wide
and generally stable beaches.
ABSTRACT
ADDITIONAL KEYWORDS: Lit-
toral cells; Southern California; sub-
marine canyons; beach sand budgets;
beach erosion.
Manuscript submitted: 27 April 2017,
revised and accepted: 26 September
2017.
lands, rivers and streams, and submarine
canyons, this region remains the typical
area for beach compartments and has
been extensively studied (Figure 1).
Southern California’s major littoral
cells range in length from about 24 km
to 230 km and each consists of: 1) beach
sand sources (such as rivers, creeks, erod-
ing coastal blus, or articial nourish-
ment) that provide sand to the shoreline;
2) beach sand sinks (such as submarine
canyons and coastal dunes) where sand
leaves or is removed from the shoreline;
and 3) littoral dri or longshore trans-
port that moves sand along the shoreline
within each cell, from sources to sinks.
Sediment within each cell includes the
sand on the exposed or dry beach, as well
as the sand on any adjacent dunes, or sand
that lies just oshore and moves on and
o the beach seasonally and alongshore
throughout the year.
Each of the major southern California
littoral cells (Figure 1; Santa Barbara,
Zuma, Santa Monica, San Pedro, La-
guna, Oceanside, Mission Bay, and Silver
Strand) has been altered to varying de-
grees by human intervention (Patsch and
Griggs 2006a and b). ese interventions
include: 1) dams and reservoirs or debris
basins in the watersheds that have im-
pounded large volumes of sand destined
for the beaches (Willis and Griggs 2003;
Slagel and Griggs 2008); coastal engineer-
ing structures such as groins, jetties and
breakwaters, which have impounded
large volumes of sand along the shoreline
(Griggs 1985; Everts Coastal 2002; Kins-
man and Griggs 2010); the construction
of many kilometers of seawalls and revet-
ments to protect coastal development and
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 2
Figure 1. Index map delineating littoral cells for the Southern California Bight.
Figure 2. The Zuma and Santa Monica Littoral Cells, neighboring cells,
predominant littoral drift directions, contributing drainage basins, and
submarine canyon sinks (from Zoulas and Orme 2007).
infrastructure, but which have reduced the
sand provided by formerly eroding clis
and blus to the beaches (Runyan and
Griggs 2003; Patsch and Griggs 2006b);
and the addition of millions of cubic me-
ters of sand, from the construction of new
harbors and marinas, dredging of existing
harbors and stream channels, and large
coastal construction projects (Flick 1993;
Wiegel 1994; Griggs and Kinsman 2016).
e wide sandy beaches of Los Angeles
County, stretching from Santa Monica to
Redondo Beach, are widely recognized
as one of the region’s greatest “natural
resources” and typically host more than
50 million visitors annually. is stretch
of shoreline, located adjacent to the Los
Angeles metropolitan area (population
of greater Los Angeles area is about 18.6
million), provides innumerable recre-
ational activities including swimming,
sunbathing, walking, jogging, volleyball,
surng, paddle boarding, and shing.
While considered a natural resource,
very little sand enters this cell today to
naturally maintain the beaches; much of
the sand was added articially. e sand
budget for the Santa Monica littoral cell
is one that has been marked by extensive
human inuence. In contrast, narrow and
discontinuous beaches generally charac-
terize the Zuma littoral cell, immediately
to the west, due to a signicant reduction
in natural sand supply.
Net sand movement along the shoreline
in California trends in the down-coast
direction under the inuence of winter
waves approaching from the northwest to
west direction. It is the long-term balance
of sand entering and leaving a littoral cell,
as well as the shoreline orientation and the
presence or absence of littoral barriers,
which govern the width of beaches within
a particular cell. Where natural sand sup-
plies are reduced due to the construction
of dams or debris basins in coastal wa-
tersheds, or from sea cli armoring, sand
mining, or restriction of littoral transport
through the construction of large coastal
engineering structures, beaches may tem-
porarily or permanently narrow (Runyan
and Griggs 2003; Willis and Griggs 2003).
Conversely, where sand has been arti-
cially added to the beach through dredging
of harbors, marinas, or river channels,
from shoreline construction projects,
or through direct nourishment, beaches
can be temporarily widened (Flick 1993;
Wiegel 1994; Griggs and Kinsman 2016).
THE ZUMA AND SANTA MONICA
LITTORAL CELLS
e combined Zuma and Santa Mon-
ica littoral cells extend approximately
85 km from Point Mugu on the west to
Palos Verdes Peninsula on the southeast
(Figure 2). An important factor aect-
ing beach width in these two cells is the
coastline orientation. From Point Mugu
to Pacic Palisades, the shoreline trends
nearly east-west, such that littoral dri
driven by waves from the west moves
sand rapidly along this stretch of coast
and beaches are generally narrow to non-
existent. At Pacic Palisades, the shore-
line changes orientation to approximately
north-south, nearly parallel to the wave
approach, reducing littoral dri rates and
allowing wider beaches to develop.
e Zuma Littoral Cell: Sand supply
e beaches of the 27 km long Zuma
Cell receive sediment primarily from
seacli erosion, from small streams drain-
ing about 195 km2 of the western Santa
Monica Mountains between Point Mugu
and Point Dume, and, historically, from
sand moving across the head of Mugu
Submarine Canyon from the upcoast
Santa Barbara Littoral Cell. (Figure 2).
e rocks types, geologic structure, and
steep slopes in the watersheds favor ero-
sion and mass movements.
e Pacic Coast Highway was com-
pleted between Malibu and Oxnard in
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 3
Figure 4. Coastline from Point Dume
looking west to Lechuza Point and
beyond to Point Mugu showing the
smooth, gently curved shoreline
that extends to Lechuza Point.
(Photo: Bruce Perry, California State
University Long Beach, 2005.)
Figure 3. The Great Sand Dune near La Jolla Valley climbing a steep hillside. This dune
is now isolated from the beach and is gradually being vegetated. (Photo: Kenneth and
Gabrielle Adelman, California Coastal Records Project, www.californiacoastline.org.)
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 4
Figure 5. Middle portion of Broad Beach in June 2005 showing the relict sea cli below Pacic Coast Highway
adjacent to Broad Beach Road. Note the narrow width of the dry beach at this time. (Photo: Bruce Perry, California
State University Long Beach, 2005.)
1929 and involved cutting the roadbed
into the active sea clis with the removal
of about 2.3 million m3 of rock debris
from the cli base between Point Mugu
and Santa Monica Canyon, which was
used as base rock for the highway and
also for coastal protection. is was the
largest articial source of sediment to
the beaches of the Malibu coastline, and
calculations indicate about 138,000 m3
of sand sized material was added to the
beaches of the Zuma cell. is was 80
years ago however, so it would have been
broken down, dispersed, or transported
downcoast decades ago, and should no
longer provide any littoral material.
Twenty-four groins, 24,000 m3 of rip-
rap, and several long seawalls provided
additional roadway protection for the
original Pacic Coast Highway. e rip-
rap and seawalls reduced the erosion of
the sea clis and, therefore, reduced the
amount of sand and gravel that would
have been supplied to the beaches in
subsequent years (Orme 2005; Zoulas
and Orme 2007). The amount of po-
tential beach sand generated along any
stretch of coast from cli or blu retreat
is dependent upon the type of rock, how
much littoral sized sand it contains, the
rate of cli erosion, and the height and
alongshore extent of each rock type. e
cli forming materials in this area, how-
ever, are primarily shale so do not provide
large amounts of beach-forming material
even under natural conditions. Using rea-
sonable estimates for these parameters,
as well as the reduction from shoreline
armoring (Knur 2000), the clis along the
Zuma Cell yield only about 3,900 m3/year
of sand under present conditions.
Progressing eastward from Point
Mugu, sand and cobbles are brought
to the shoreline through a number of
ephemeral or intermittent streams. With
the exception of the lower reaches of
Big Sycamore Creek, channel slopes are
mostly steep and sediment reaches the
coast in short duration peak discharge
events following significant rainfall.
Compared to the size of the watersheds
and sand volumes discharged by the riv-
ers of the upcoast Santa Barbara Littoral
Cell (Patsch and Griggs 2006a, b), all of
the streams draining the Malibu coast
are small, and their sand discharges are
quite low, totaling about 26,000 m3/year
(Knur 2000; Knur and Kim 1999). For
comparison, the Santa Clara River alone,
in the adjacent Santa Barbara Cell, deliv-
ers about 720,000 m3/year of sand to the
coast on average at present, or about 28
times more sand than all of the streams
draining into the Zuma Cell combined.
Knur (2000) calculated that prior
to human intervention, approximately
39,000 m3 of sand was transported along
the shoreline annually at Point Dume,
the end of the Zuma littoral cell, from a
combination of cli retreat and stream
Table 1.
Changes in sand supply to the Zuma
Littoral Cell (in cubic meters/year)
Sources Natural Present day
Stream runo 34,000 26,000
Cli erosion 5,000 4,000
Totals 39,000 30,000
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 5
Figure 6. A) Middle portion of Broad Beach in 1972 showing a wide beach and the relict sea cli below Pacic Coast
Highway adjacent to Broad Beach Road. (Photo: Kenneth and Gabrielle Adelman, California Coastal Records Project,
www.californiacoastline.org.) B) Middle portion of Broad Beach in 2005 (same area pictured in Figure 6A) with small
beach cottages replaced by large homes and beach width signicantly reduced. (Photo: Kenneth and Gabrielle
Adelman, California Coastal Records Project, www.californiacoastline.org.)
A
B
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 6
Figure 7. A) Mean beach width and time-series beach width for all six beaches
in the Zuma Littoral Cell (1928-2002). B) The Pacic Decadal Oscillation (PDO)
index for November through March from 1925-2002, with 10-year moving
average from Mantua (2007), (from Zoulas and Orme 2007).
A
B
input (Table 1). is littoral sand volume
has been reduced to about 30,000 m3/year
due to human activities, including cli
armoring and watershed impoundments
(Knur 2000). is is a historic reduction
in the already low volume of sand sup-
plied naturally to the shoreline of about
25% for the entire, but small, Zuma Cell.
e Zuma Littoral Cell: Sand losses
Most sand is lost from California’s lit-
toral cells in one two ways: 1) it is blown
o the beach and inland by onshore winds
to form dunes; or 2) it is carried oshore
at the end of a littoral cell as it is inter-
cepted by the head of a submarine canyon
and is then transported downslope and
into deep water. e major littoral cells
of southern California all terminate in
submarine canyons (Figure 1). Mugu
and Hueneme Canyons mark the end of
the Santa Barbara Cell. Dume Canyon
marks the end of the Zuma Cell; Redondo
Submarine Canyon terminates the Santa
Monica Cell, and Newport Submarine
Canyon lies at the end of the San Pedro
Cell.
The Dume Submarine Canyon is
located just oshore of Point Dume and
is generally recognized as the end of the
Zuma Littoral Cell (Figure 2). e mouth
the canyon reaches to within about 250
m of the shoreline and descends from a
depth of approximately 30 m to a depth
of over 600 m (Shepard and Dill 1966;
Knur and Kim 1999). Point Dume and
Dume Submarine Canyon act as partial
barriers to littoral dri. ere has not
been complete agreement, however, on
the importance of Dume Submarine
Canyon on the sand budget for the Santa
Monica Cell, located just downdri; how
much sand is lost into Dume Submarine
Canyon and how much makes its way
around Point Dume and continues into
the Santa Monica Cell is not agreed upon.
Inman (1986) reported that during
moderate wave conditions, 90% of the
material traveling as littoral dri east-
ward towards Point Dume is transported
around the promontory, bypassing the
canyon head. Orme (1991), however,
concluded the opposite, that only 10%
of littoral sediments bypass the point and
canyon mouth. Knur and Kim (1999)
attempted to resolve this discrepancy
by performing their own analysis and
determined that 70% of the littoral dri
enters Dume Submarine Canyon, and is
eectively removed from the littoral cell
budget. More recently, based on oshore
work, Normark et al. (2009) report: “Be-
cause the shelf is so narrow at Dume point
(<3 km), Dume Canyon also continued to
transfer sediment to the (Santa Barbara)
basin although at lower rates and volumes
relative to Hueneme and Mugu Canyons.
Small dunes have developed within
the Zuma Cell, but they are minor in
volume and are not believed to constitute
a signicant sand loss from the littoral
budget. Onshore winds did build small
dunes downcoast of La Jolla Valley, Zuma
Beach, and at Point Dume. Only the
dunes at Zuma Beach and Point Dume
are still in contact with their source
beaches, however. e Great Sand Dune
climbing the cli at La Jolla Valley was cut
o from the beach by the construction
of Pacic Coast Highway around 1926
and is considered a fossil dune (Figure
3; Knur 2000).
e Santa Monica Littoral Cell:
Natural sand supply
The most important historic event
impacting the natural sand supply to the
56 km long Santa Monica littoral cell was
the change in the course of Los Angeles
River in 1825. Prior to that time, the river
discharged through Ballona Creek and
provided a substantial amount of sand
to this cell. In 1825, however, unusually
heavy ooding caused the river to change
its course across the low relief Los Angeles
coastal plain and discharge into San Pedro
Bay, approximately 43 km southeast of its
original outlet and in a completely dier-
ent littoral cell (Knur and Kim 1999; Wie-
gel 1994). us, for almost two centuries,
the Santa Monica littoral cell has lacked a
large natural source of sand.
Malibu Creek is the largest stream
draining the Santa Monica Mountains
between Point Dume and Ballona Creek.
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 7
Figure 8. A) Mean beach width at Broad Beach from 1928 to 2002, and (B)
Pacic Decadal Oscillation (PDO) Index from 1925 to 2005. Values from PDO
warm phases are highlighted in orange and cool phases are highlighted in
blue.
A
B
Under natural conditions it discharged
about 41,000 m3/year of sand to the
shoreline (Knur 2000). Twenty-three
dams have been built within the Malibu
Creek watershed over the last 135 years,
the largest being Rindge Dam, which re-
duced that creeks sand discharge to about
14,000 m3/year (Knur and Kim 1999;
Willis and Griggs 2003). Using stream
discharge information, Willis and Griggs
(2003) determined that Ballona Creek,
a small ephemeral stream draining into
Santa Monica Bay, delivers only about
1,800 m3/year of sand to the shoreline at
present. Sand input from streams drain-
ing into the Santa Monica Cell, therefore,
only totals about 15,800 m3/year, assum-
ing that the great majority of sand in the
Zuma Cell is transported oshore into
Dume Canyon (Figure 2)
e lack of a large sand contribution
from streams increases the potential role
of blu erosion in contributing sand to
the beaches of the Santa Monica Cell.
Using alongshore cli length, cli or blu
height, terrace deposit thickness, grain
size of cli or blu and terrace sediments,
erosion rate, and littoral-cut-o diameter
(the smallest grain size of sediment pres-
ent on the beach face; Limber et al. 2007),
blu erosion was determined to contrib-
ute only a moderate amount of sand to
this littoral cell (Patsch and Griggs 2006
a and b). Overall, erosion of coastal blus
between Point Dume and the downcoast
end of the Santa Monica Cell at the Palos
Verdes Peninsula, naturally contributed
an average of ~85,000 m3/year of beach-
sand-sized material (coarser than 3Ø
or .125mm). Armoring has reduced
the historic, or natural, volume slightly
(~1,200 m3/year). Although blu erosion
is not contributing large volumes of sand
to the littoral budget, because the overall
natural sand volumes in the cell are so
low, this source still constitutes 60% of the
“natural” sand supplied to this cell. Most
of this sand (~80,000 m3/year), however,
is derived from the stretch of coast from
Malaga cove to Palos Verdes Peninsula
where it travels north and is lost into the
Redondo Submarine Canyon, providing
little to no sand to beaches in the north-
ern and central stretches of this cell.
Santa Monica Littoral Cell: Sand losses
Santa Monica Submarine Canyon lies
between Dume and Redondo Canyons
(Figure 2) but the canyon head lies about
11 km offshore near the shelf break;
thus, it is not a sink for modern littoral
sand. Redondo Submarine Canyon is
located just oshore of King Harbor in
the southern portion of Santa Monica Bay
(Figure 2). With its head located within
185 m of the shoreline near the end of
the King Harbor breakwater, sand is es-
sentially funneled into the canyon head,
eectively removing it from the littoral
system. Historically, about 100,000 m3/
year of sand was captured by the canyon
head and funneled oshore (Moatt &
Nichol 2009). Today, due to the extensive
compartmentalization of the shoreline in
Santa Monica Bay with multiple retention
structures, however, it is believed that
little sand is actually lost into the canyon
from the north.
CHANGES IN THE NATURAL
SAND SUPPLY
e Zuma Littoral Cell:
Historic changes in beach width
Broad Beach is an approximately
1.9 km-long stretch of sandy shoreline
contiguous with Zuma County Beach to
the east, which extends another 4.5 km
to Point Dume, where a signicant por-
tion of the littoral dri is believed to exit
the cell and move oshore into Dume
Submarine Canyon (Figure 4). is 6.4
km-long beach is the most extensive
stretch of sandy shoreline within the
Zuma Littoral Cell and owes its existence
to the presence of Point Dume, a volcanic
headland that forms a major littoral bar-
rier. is headland has impounded nearly
2 million cubic meters of sand, allowing
a beach 75 m to 110 m wide to form for
6.5 km upcoast. is continuous beach
has historically been narrowest at the
west end near Lechuza Point (10 m to
20 m wide), 15 m to 30 m along much
of Broad Beach, and then reached its
greatest width (50 m to 75 m on average)
along Zuma Beach (Figure 4). However, a
signicant portion (as much as 75 m) of
the landward portion of Zuma Beach has
been covered by the access road along the
back beach, as well as beach parking lots,
and the Pacic Coast Highway.
Similarly, Broad Beach received its
name because it originally was very wide,
but this has changed over the past 15 or
so years. A careful look at the shoreline
indicates that under pre-development
conditions the beach extended from the
present shoreline inland to the blu below
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 8
Figure 9. A) West end of Broad Beach at Lechuza Point in 1972 showing
the wide beach that existed at that time and the low level of development.
B) Same view in 2006 illustrating extensive beach and blu development
and virtually no beach. (Photos: Kenneth and Gabrielle Adelman, California
Coastal Records Project, www.californiacoastline.org.)
A
B
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 9
Figure 10. Eastern end of Broad Beach in 2013, with temporary rock revetment and lack of dry beach at high tide.
(Photo: Kenneth and Gabrielle Adelman, California Coastal Records Project, www.californiacoastline.org.)
the Pacic Coast Highway, or the inland
side of present-day Broad Beach Road
(Figure 5). e blu begins below the
Pacic Coast Highway at Trancas Creek
where it is very low, and then gradually
increases in height proceeding west to
a maximum of 15-25 m approaching
Lechuza Point. is steep blu is the old
abandoned sea cli, now separated from
the shoreline by Broad Beach Road as
well as the homes and other development
along Broad Beach.
While nearly all of the homes con-
structed west of Lechuza Point towards
Point Mugu are well above beach level or
on the marine terrace at the base of the
mountains, along most of Broad Beach
the homes have been built on the sand.
Although the average summer beach
fronting the homes along Broad Beach
(measured from the dune line seaward),
historically varied in width from 10 m to
40 m, there is an additional 75 m to 100
m of beach sand extending from the outer
edge of the dunes, beneath the homes and
Broad Beach Road to the base of the old
sea cli below the Pacic Coast Highway
(Figures 4 and 5).
The exposed or usable portion of
Broad Beach became signicantly nar-
rower when the original lots on the sand
were subdivided, and home construction
took place. Historic aerial photographs
indicate that initial development took
place in the 1930s and 1940s, with most
of the major construction occurring be-
tween the 1947 and 1959 photographs.
In addition, as older beach houses were
remodeled or replaced with newer and
larger homes, seaward encroachment
further reduced the width of the original
beach and dune buer zone (Figures 6a
and 6b).
Hapke et al. (2006) calculated long
and short-term changes in beach widths
along the stretch of shoreline between
Point Mugu and Point Dume as part of a
statewide U.S. Geological Survey assess-
ment of shoreline change for California.
e shoreline was dened as the wet-dry
line on the beach, and the position of this
shoreline was determined from a combi-
nation of early maps (from the mid- to
late-1800s), historic aerial photographs
(late 1920s to 1970s), and LiDAR from
1998-2002.
e long-term change (from the mid-
1800s to ~2002) in shoreline position for
the beaches extending from Point Mugu
to Trancas Creek (the dividing line be-
tween Broad Beach on the west and Zuma
Beach on the east) shows a moderate
erosional trend of about 20-30 cm/year
on average, indicating that the beaches
have gradually narrowed over the past
century or so. Between Trancas Creek
and Point Dume, however, the long-term
trend is accretional, and indicates that
Zuma Beach widened overall during this
period at about 20-30 cm/year
e short-term (1950s-1970s to 1998-
2002) trend indicates that most of the
beaches between Point Mugu and Point
Dume show a signicant erosional trend
over the past 30 to 50 years. e shoreline
retreat or beach erosion rates are typically
in the range of about 60-120 cm/year,
signicantly higher than the long-term
average. e short-term beach erosion
rates increase at Trancas Beach (which
includes the area from Lechuza Point to
Trancas Creek, or the entire Broad Beach
area) and range from about 60 cm to over
200 cm/year. ese rates are based simply
on the changing position of the wet-dry
line over the 30-50 year period ending
in 2002. e short-term beach erosion
rates at Trancas Beach or Broad Beach
are designated as the highest of any area
in the entire Zuma and Santa Monica
cell region.
Zoulas and Orme (2007) carried out
a more detailed study of beach changes
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 10
Figure 11 (above). Sandbags were placed temporary along nearly the entire
length of Broad Beach in an eort to halt additional shoreline retreat until a
permanent solution could be approved (2008).
Figure 12 (left). The sandbags lasted for less than a year and were replaced
by a rock revetment (2012).
throughout the entire Zuma Littoral
Cell, although it did not initially include
the Broad Beach area. ey measured
changes in beach widths from eight target
beaches from historic aerial photographs
available between 1928 and 2002. Beach
transects were measured at 20 m inter-
vals along the shoreline on each historic
photograph to document the distance
between the wet-dry line and a backshore
feature, typically the vegetation line or
dune edge. eir data on beach widths
show considerable seasonal and annual
changes within and between beaches,
but also signicant trends over longer
time-scales.
Beach behavior throughout the Zuma
Cell during the period analyzed (1928-
2002) is characterized by short-term
episodes of erosion related to winter
storms and recurrent El Niño events, and
longer-term changes that appear to have
a cyclical pattern over decades (Figure 7).
e longer-term patterns of beach width
change correlate reasonably well with the
phases of the Pacic Decadal Oscillation
(PDO), which appear to correspond to
period of greater or lesser storminess
related to sea surface temperatures across
the northeast Pacic Ocean.
For most of the beaches studied within
the Zuma Cell, erosional cycles coincide
with the warm phases of the PDO, one
extending from 1925 to 1947, and an-
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 11
Figure 13. The Channel Islands Harbor (middle of photo), Silver Strand
Beach and Port Hueneme (bottom of photo). Some of the sand dredged from
Channel Islands Harbor is placed on the Silver Strand, but most if piped
under the entrance to Port Hueneme and nourished Hueneme Beach. (Photo:
Bruce Perry, California State University Long Beach, 2005.)
other from 1978 to about 1998 (Figure 7).
ese are periods characterized overall by
more frequent and more intense El Niño
events, greater wave energy, and storms
generally approaching the coastline from
the west or southwest. ese conditions
are responsible for much of the beach
and coastline erosion and also coastal
storm damage along the California coast
between 1978 and 1998 (Storlazzi and
Griggs 1998; Storlazzi and Griggs 2000).
Beach widening, or accretion, on the
other hand, generally coincides with the
cool phase of the PDO (La Niña dominat-
ed with fewer strong coastal storms, less
wave energy and as a result, less shoreline
erosion), which extended from 1947 to
1978 and from about 1998 to 2013. ere
are some lag eects, however, so that the
beaches don’t respond immediately to
changing climatic conditions.
Broad Beach was recently studied
using 18 sets of aerial photos spanning
the 1928 to 2002 period and using the
techniques described in Zoulas and
Orme (2007; Griggs & Associates 2011).
Eighty-nine transects were established
at 20 m spacing along the entire mile
of Broad Beach from Lechuza Point to
Trancas Creek. Measurements were made
from the front edge of the dunes to the
wet-dry line, to accurately document
changes in beach width over this 74-year
period. Photographs taken under eroded
or narrowed winter conditions were
omitted. Where the time of day of the
photograph was known, corrections were
made for tidal stage. ese measures were
all taken to ensure that the photographs
were as comparable as possible and any
trends in beach width over time could be
recognized.
e overall historic trends in the aver-
age width of Broad Beach over time are
similar to those previously determined
for most of the other beaches in the Zuma
Cell (Zoulas and Orme 2007; Figure
8). e average beach width gradually
increases from 1947 (the beginning of
a cooler and calmer PDO phase) to a
maximum of about 40 m in 1971. e
average width for all of Broad Beach then
gradually narrows through the warm
PDO cycle that began in about 1978 and
remained about the same width, between
18 m to 20 m, between 1990 and 2002
(Griggs & Associates 2011).
e long-term conditions along Broad
Beach began to change around 2000,
however, as the beach adjacent to Lechuza
Point began to narrow, and by 2002, had
disappeared altogether (Figures 9a and
9b). Two years later (2004), there was
no dry beach exposed for the rst 200 m
downcoast from Lechuza Point. While
there are some year-to-year uctuations,
by 2006, the first dry beach is 400 m
downcoast to the east. By August 2008,
the month when beach width should
normally be the greatest, there was no
dry beach for fully half the length of
Broad Beach, and proceeding towards
Trancas Creek, the beach was continuing
to narrow. As of September 2013, there
was no dry beach fronting the entire 1.9
km of Broad Beach (Figure 10; California
Coastal Records Project).
Most of the homes at the narrow west
end of Broad Beach were either already
protected by seawalls or supported on
concrete caissons, but for the remainder
of the beach level homes, in response to
the continuing loss of sand and progres-
sive narrowing of the beach, large sand
bags or rip-rap was installed following the
2007-2008 winter to protect the homes
(Figure 11). An additional concern was
that most homes have a septic tank and
leach eld for wastewater disposal buried
beneath the beach in front of their homes.
Waves from the winter of 2008 damaged
or destroyed the sand bags, however,
which lead to the placement of a “tempo-
rary” rock revetment by the next winter
that extended virtually the entire 1.9 km
length of Broad Beach (Figure 12).
e narrowing and loss of Broad Beach
For well over a century, and perhaps
much longer, a very wide (100 m or more)
beach existed along what became known
as Trancas or Broad Beach. e beach was
wide enough that dunes formed, and was
presumably above mean high tide for a
suciently long time period to have been
considered private rather than state land.
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 12
Figure 14. Bathymetry at the head of Mugu Canyon showing the inner canyon head extending virtually to the
shoreline where it now intercepts virtually all of the littoral drift. (Image: Digital Globe 2009.)
The backshore was subsequently sub-
divided, and small beach cottages were
constructed, initially in the 1930s and
1940s. Many of these have been demol-
ished in recent years, lots combined and
replaced by much larger structures that
became homes for a number of people
from Hollywood, which also led to con-
tinuing publicity about the progressive
erosion of Broad Beach.
Aer well over a century of a relatively
permanent, stable, and broad beach, why
did this stretch of shoreline change so
quickly and the beach narrow or erode to
the point where a 1.9 km-long rock revet-
ment was required to save the homes and
protect their beach level septic systems
from being exposed? e revetment is
considered a “temporary” solution by
the California Coastal Commission, the
coastal agency with authority for essen-
tially all coastal land use decisions in the
state, and the Broad Beach homeowners
are now in the midst of a $19 million plan
to nourish the beach with up to 230,000
m3 of sand from an inland source.
e two obvious sources of sand for
the Zuma Littoral Cell, summarized
earlier, are the small streams draining the
Santa Monica Mountains as well as cli
retreat between Point Mugu and Point
Dume. However, the contributions from
these sources were quite small under
natural conditions (~39,000 m3/year),
and they have been reduced to about
30,000 m3/year due to human activities,
including cli armoring and watershed
impoundments (Knur 2000). Nonethe-
less, the presence of Point Dume trapped
enough sand over the years to allow a very
wide beach to form.
e observation that the narrowing of
Broad Beach began at the Lechuza Point
(west) end of the beach and progressed
eastward for the entire length of the
beach, such that there is virtually no dry
beach exposed now at high tide, even in
the summer months, suggests that the
explanation lies in the reduction of littoral
sand from the west or upcoast. is raises
the question of whether the Hueneme and
Mugu Submarine Canyons were complete
barriers to littoral transport in the past;
or, did some signicant volume of sand
move across the canyon heads historically
to continue downcoast, around Point
Mugu and continue eastward to nourish
the Zuma Littoral Cell?
Just 3 km east of Point Mugu, which is
considered the end of the Santa Barbara
Littoral Cell, the Great Sand Dune (Fig-
ure 3) provides evidence of a signicant
upcoast sand supply in the recent past; yet
there is just one small, very steep, inter-
mittent creek as a potential source of sand
between Point Mugu and the Great Sand
Dune. It is extremely unlikely that this
small steep stream was able to provide the
volume of ned-grained sand necessary to
build this large climbing dune, which is
gradually being vegetated and stabilized as
Shore & Beach Vol. 86, No. 1 Winter 2018 Page 13
the Pacic Coast Highway now separates
the dune from its shoreline sand supply.
About 760,000 m3 of sand is dredged
annually, on average, from the entrance
channel of Channel Islands Harbor,
17.5 km upcoast or west of Point Mugu.
About 5%-10% of the sand is placed on
the adjacent Silver Strand Beach. In order
to eliminate the need to dredge this sand
again from the adjacent Port Hueneme
channel, just 1.6 km downcoast to the
east (Figure 13), however, the sand is
pumped through a pipeline that passes
under the entrance to the port, bypasses
the head of the oshore Hueneme Subma-
rine Canyon and is then discharged onto
Hueneme Beach. is very large volume
of sand is then transported as littoral
dri about 12 km downcoast to the head
of Mugu Submarine Canyon (Figure 2).
Even a modest percentage of the
760,000 m3 of sand being transported on
average along Hueneme Beach annually
could have had a signicant impact on
the downcoast Zuma Littoral Cell. e
concept of “leaky” littoral cells has been
around for some time, and there are good
examples where sinks (typically submarine
canyons) don’t form complete traps and
some littoral sand exits one cell and enters
the next. In the case of Mugu Canyon,
Moatt and Nichol (2009) from an inves-
tigation of submarine canyons along the
southern California coast reported that:
Between 1938 and 1995, the portion
of the net longshore sand transport rate of
1,065,000 yds3/yr (810,000 m3) that was
captured in Mugu Canyon progressively
increased from 0.88 to 1.0 as determined
by measurements of the position of the
canyon head relative to the shoreline in
the canyon lee (Moatt and Nichol 1995).
In the 1960s, a seawall (actually a rock
revetment) was constructed at the back
of the adjacent beach… In response to a
retreating shoreline, the rim of the canyon
intercepted the structure and progressively
more of the net transport rate was cap-
tured. e result was a measurable level
of shore retreat up to 15 miles (24 km)
downcoast. During a storm in 1995, the
seawall was undermined and failed. Sub-
sequently a large portion of what remained
of the structure and many of the buildings
behind it were removed. Aerwards the
beach reestablished its equilibrium width
providing for the recovery of some of
the alongshore transport that previously
passed the canyon.
Using 810,000 m3 as the average long-
term annual littoral transport rate along
Hueneme Beach approaching the canyon
head, a capture rate of 88% would amount
to 713,000 m3 that would have entered
the canyon head, with the remaining
approximately 97,000 m3 bypassing the
canyon (Figure 14) and moving to the
east into the Zuma Cell. is volume of
bypassed sand would have provided a
signicant augmentation to the modest
amount of sand added from eroding
blus and small streams along the Santa
Monica Mountains between Point Mugu
and Point Dume (~39,000 m3/year) and
provides an explanation for the presence
of the Great Sand Dune, for beaches along
this stretch of shoreline, and importantly,
the nearly two million cubic meters of
sand that accumulated along Broad and
Zuma beaches. e headward growth of
Mugu Canyon, however, approached the
shoreline to the point where the revet-
ment failed in 1995, and then began to
capture virtually all the littoral dri that
had exited this leaky cell for well over a
century. With very little littoral sand now
entering the western portion of the Zuma
Cell from either the adjacent Santa Bar-
bara Cell, or the small streams and blu
retreat, Broad Beach has progressively
narrowed and disappeared beginning
at the west end. is beach erosion has
gradually migrated eastward leaving the
homes along Broad Beach unprotected
and exposed to additional shoreline ero-
sion, without some protection system.
In addition to the signicant reduction
of sand to the sediment budget in the
Zuma Littoral Cell as a result of the head-
ward growth of Mugu submarine canyon,
the following issues are compounding the
erosion of Broad Beach:
1) e initial development of Broad
Beach involved construction of homes
and other improvements that encroached
60 m to 75 m onto the original beach and
dunes, leaving only a narrow fronting
beach with little seasonal buer. With
sea level rise and the associated process
of shoreline retreat, passive erosion of the
active beach is occurring, which is caught
between high tides and wave run-up and
the shoreline protection structures. e
beach and dunes can no longer retreat
towards the old sea cli and have con-
tinued to narrow.
2) e Santa Monica tide gage shows
sea level rising there at 1.52 mm/year (6
inches/century) from 1933 to 2016, which
would lead to gradual shoreline retreat.
e Malibu Coast fault runs just inland
from the Malibu coast but the elevated
marine terraces suggest that coastline is
being uplied rather than subsiding. It is
not clear what the local relative sea level
rise rate is at Broad Beach specically and
whether it is contributing to the increased
rate of erosion seen on this beach.
e Santa Monica Littoral Cell:
Articial nourishment
and beach accretion
Since 1926, beach nourishment has
been the main source of sand for the Santa
Monica Littoral Cell, far overshadowing
the amount of sand supplied naturally by
streams and blu erosion. Leidersdorf et
al. (1994) estimated that ~23 million m3
of sand were placed on the beaches of the
Santa Monica littoral cell over a 60-year
span, which averages out to ~383,000
m3/year. This is reasonably consistent
with the estimate made by Flick (1993)
of 23 million m3 of sand over a 50-year
span (or ~460,000 m3/year). However,
since 1970, beach nourishment has sig-
nicantly decreased. Leidersdorf et al.
(1994) determined that only 1.3 million
m3 (~53,000 m3/year) was added to the
beaches between 1970 and 1994. ey
attributed the decline in nourishment to:
1) a decrease in large coastal construction
projects; 2) more stringent regulations
and standards for the size and quality of
acceptable sand used for nourishment;
and 3) the relative stability of earlier ll
as a result of retention structures.
Some of the largest and earliest op-
portunistic beach nourishment projects
came along with the construction and
expansion of the Hyperion Sewage Treat-
ment Facility inland from Dockweiler
Beach. In 1938, 1.37 million m3 of sand
were excavated from the sand dunes at
the future site of the Hyperion Facility
and placed on the beach in anticipation
of construction (Wiegel 1994). From 1946
to 1948, during construction of the plant,
an additional ~13 million m3 of sand
were excavated and disposed of along
an eleven mile stretch of beach from the
Santa Monica Pier to El Segundo Beach,
widening the beaches by an average of
200 m (Leidersdorf et al. 1994; Wiegel
1994). One of the most recent beach
nourishment projects associated with
Hyperion occurred in 1989 when about
835,000 additional cubic meters of sand
were transported by conveyor belt from
Shore & Beach Vol. 86, No. 1 Winter 2018
Page 14
Hyperion, across Pacic Coast Highway,
and deposited on Dockweiler beach
(Flick 1993).
e construction of Marina del Rey
between 1960 and 1963, which was
dredged from the wetlands of Ballona
Lagoon, where the Los Angeles River
formerly flowed, added an additional
760,000 m3 of sand to the beaches of the
area (Flick 1993; Wiegel 1994). It is one
of the largest created recreational boat-
ing and residential marinas in the world,
providing moorings for 6,000 boats, and
facilities for thousands more.
CONCLUSIONS
e combined Zuma and Santa Mon-
ica littoral cells encompass about 85 km
of coastline from Point Mugu on the
west to the Palos Verdes Peninsula on the
southeast and have undergone signicant
changes over the past century. e coast
of the upcoast or Zuma Cell to the west
trends nearly east-west such that waves
from the west drive littoral dri rapidly
along this stretch of shoreline. Combined
with the limited natural sand supply from
the adjacent Santa Monica Mountains,
beach development in the Zuma Cell
has historically been very limited. e
exception has been the trapping of ap-
proximately 2 million m3 of sand by Point
Dume, which led to the development of
the combined Zuma and Broad beaches.
Historically, leakage of sand from the up-
coast Santa Barbara Littoral Cell, across
the head of Mugu Submarine Canyon,
provided a signicant additional source
of beach sand to the Zuma Cell. About
1995, however, the headward growth of
Mugu Canyon led to the interception of
essentially all of the littoral transport.
An erosion wave has slowly migrated
through the Zuma Cell and has now led
to the essentially complete loss of Broad
Beach and threats to the 109 homes and
their sewage disposal systems built on
the back beach. A “temporary” rock
revetment was constructed in 2009 and
plans have been developed for nourishing
the beach with imported sand from an
inland source.
While the adjacent 24 km coastline
of Santa Monica Cell has the same
general east-west trend as the Zuma
Cell and is also characterized by very
narrow beaches, the orientation of the
shoreline changes almost 90 degrees at
Pacic Palisades. e 32 km shoreline
of Santa Monica Bay is oriented nearly
north-south, approximately parallel to
the wave approach, reducing littoral
dri rates and allowing wider beaches
to develop. Combined with the addition
of approximately 23 million m3 of sand
added to the Santa Monica Bay beaches
from coastal construction and dredging
projects over a 60-year span, and a series
of sand retention structures, this 32 km
stretch of shoreline and the nearly 20
million people in the adjacent great Los
Angeles area, have benetted from wide
and generally stable beaches.
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... While there are sections of the southern California coast where the beaches are very wide and stable (the low-lying, originally dune-backed shoreline of Santa Monica Bay is a good example; Figures 6 and 7), there are other areas where, for a variety of reasons, beaches are narrow or virtually nonexistent, including Isla Vista in western Santa Barbara County ( Figures 6 and 8), the Rincon coast of Ventura County (Figure 6), Broad Beach in western Los Angeles County ( Figures 6 and 9), and extensive reaches of northern San Diego County. There are multiple reasons for these differences along the southern California coastline, which include shoreline orientation and lack of littoral drift barriers, rates of littoral drift, human impacts on sand input, and shoreline processes, to name some of the most significant (Flick 1993;Wiegel 1994;Everts and Eldon 2000;Everts Coastal 2002;Patsch and Griggs 2006;Orme et al. 2011;Griggs and Kinsman 2016;Griggs and Patsch 2018). ...
... There was no Coastal Commission, no California Environmental Quality Act (CEQA) or environmental review process at that time. The groins did work effectively for a number of years along a stretch of shoreline that has only a very modest littoral drift rate due to the general lack of upcoast sand supply, a supply which has now virtually disappeared (Griggs and Patsch 2018). Over the subsequent 90 years, however, the concrete deteriorated, along with parts of the steel sheet-piles, leaving a jagged and dangerous set of metal projections extending across the shoreline ( Figure 16), which led to safety concerns, lawsuits, and proposals for removal and the construction of new groins. ...
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Beaches form a significant component of the economy, history, and culture of southern California. Yet both the construction of dams and debris basins in coastal watersheds and the armoring of eroding coastal cliffs and bluffs have reduced sand supply. Ultimately, most of this beach sand is permanently lost to the submarine canyons that intercept littoral drift moving along this intensively used shoreline. Each decade the volume of lost sand is enough to build a beach 100 feet wide, 10 feet deep and 20 miles long, or a continuous beach extending from Newport Bay to San Clemente. Sea-level rise will negatively impact the beaches of southern California further, specifically those with back beach barriers such as seawalls, revetments, homes, businesses, highways, or railroads. Over 75% of the beaches in southern California are retained by structures, whether natural or artificial, and groin fields built decades ago have been important for local beach growth and stabilization efforts. While groins have been generally discouraged in recent decades in California, and there are important engineering and environmental considerations involved prior to any groin construction, the potential benefits are quite large for the intensively used beaches and growing population of southern California, particularly in light of predicted sea-level rise and public beach loss. All things considered, in many areas groins or groin fields may well meet the objectives of the California Coastal Act, which governs coastal land-use decisions. There are a number of shoreline areas in southern California where sand is in short supply, beaches are narrow, beach usage is high, and where sand retention structures could be used to widen or stabilize local beaches before sand is funneled offshore by submarine canyons intercepting littoral drift. Stabilizing and widening the beaches would add valuable recreational area, support beach ecology, provide a buffer for back beach infrastructure or development, and slow the impacts of a rising sea level.
... About nine miles downcoast to the southeast, after being pumped under the entrance to Port Hueneme and deposited on downdrift beaches, this sand enters the head of Mugu Submarine Canyon and is transported to the deep-sea floor of Santa Monica Basin Patsch 2018, Patsch andGriggs 2006a). ...
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California is a major shipping point for exports and imports across the Pacific Basin, has large commercial and recreational fisheries, and an abundance of marine recreational boaters. Each of these industries or activities requires either a port or harbor. California has 26 individual coastal ports and harbors, ranging from the huge sprawling container ports of Los Angeles and Long Beach to small fishing ports like Noyo Harbor and Bodega Bay. Almost all of California’s ports and harbors were constructed without any knowledge or consideration of littoral drift directions and rates and potential future dredging issues. Rather, they were built where a need existed, where there was a history of boat anchorage, or where there was a natural feature (e.g. bay, estuary, or lagoon) that could be the basis of an improved port or harbor. California’s littoral drift rates and directions are now well known and understood, however, and have led to the need to perform annual dredging at many of these harbors as a result of their locations (e.g. Santa Cruz, Oceanside, Santa Barbara, Ventura, and Channel Islands harbors) while other harbors require little or no annual dredging (e.g. Half Moon Bay, Moss Landing, Monterey, Redondo-King and Alamitos Bay). California’s coastal harbors can be divided into three general groups based on their long-term annual dredging volumes, which range from three harbors that have never been dredged to the Channel Islands Harbor where nearly a million cubic yards is removed on average annually. There are coastal harbors where dredging rates have remained nearly constant over time, those where rates have gradually increased, and others where rates have decreased in recent years. While the causal factors for these changes are evident in a few cases, for most there are likely a combination of reasons including changes in sand supply by updrift rivers and streams related to dam construction as well as rainfall intensity and duration; lag times between when pulses of sand added to the shoreline from large discharge events actually reach downdrift harbors; variations in wave climate over time; shoreline topography and nearshore bathymetry that determine how much sand can be trapped upcoast of littoral barriers, such as jetties and breakwaters, before it enters a harbor; and timing of dredging. While there is virtually nothing that can be done to any of these harbors to significantly reduce annual dredging rates and costs, short of modifying either breakwater or jetty length and/or configuration to increase the volume of sand trapped upcoast, thereby altering dredging timing, they are clearly major economic engines, but come with associated costs.
... In an ideal situation, there is little to no transport between cells. However, more typically, there may be some transport of sand around promontories and bypassing canyon heads (George et al., 2015;Griggs and Patsch, 2018). Sand within a littoral cell is predominantly sourced from rivers and streams and to a lesser extent eroding cliffs, bluffs, and dunes. ...
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... Several federal, state, and local government agencies, as well as NGOs and academic institutions, are debating how best to adapt California's coast to these dire threats as well as exploring the most effective ways to engage the general public in understanding these threats and acting to bring about needed change. Many of the coastal management options put forward, such as hardening the shoreline with seawalls or revetments as well as expanding beach replenishment, are detrimental to fronting beaches (Griggs, 2005;Griggs & Patsch, 2018;Runyan & Griggs, 2003). Communities will need to rely on local management to see the value in the sandy beach environment and make the choice to become part of the solution in preserving this valuable resource. ...
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Coastal environments are being affected on a global scale by excessive urbanization, industrial growth and tourism, which are causing erosion, loss of biodiversity and ecosystem alteration, constituting potential risks that entail negative environmental and socioeconomic consequences. In this sense, the Uruguayan coast, characterized by extensive sandy beaches separated by rocky promontories, is not excluded from to this global problem. Focusing on geomorphological, sedimentological and malacological aspects, we tackle the anthropic impact on littoral dynamics with the purpose of creating a database to evaluate different aspects related to the beach resource management. In this work, seven beaches from Maldonado Department were compared, one on which is on an estuarial environment while the others are on the Atlantic margin, and they all present different degrees of anthropization and urbanization. Each coastal zone (foreshore, backshore, dune and modified dune) was parameterized taking into account the area, width, slope and cross section through a geomorphological analysis, making use of satellite images, digital elevation models, and processing and storage of data in a geographic information system. The grain size analysis allowed the characterization of each site and coastal zone, and by statistics it was possible to establish a relationship between the increase of the standard deviation and the anthropization in the coastal strip. The faunal composition identified in the beaches and different index calculations showed the sites with the greatest diversity and dominance. The anthropization in the coastal strip is associated with incipient urban development, which modifies the dune sector generating a reduction of the coastal zones, exposing more vulnerable areas to littoral erosion. This results presented in this paper may contribute to a better management of the coastal environment and to identify critical zones.
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Seacliff erosion contributes to the natural sediment supply that provides California's beaches with sand. When an armoring structure (i.e. rip-rap, seawall) is built in front of a seacliff to hinder erosion and thus protect cliff-top development, the natural supply of sand from seacliff erosion is cut off. Thus, it is important to inventory the extent of eroding seacliffs along the coast of California and the degree to which they are armored to determine the human impact on natural sediment supply to the coast. The great majority (72% or 1275 km) of the coast of California consists of actively eroding seacliffs. More specifically, 13% of the coastline is resistant, high-relief, steep cliffs or mountains that contribute a minor amount of sand to the littoral budget, and 59% of the coastline is low relief (less than 100 m) wave-cut cliffs or terraces that, through erosion, produce a greater percent of sand-size material to the littoral budget than the high-relief, cliffed coastline. Using digital video footage of the California coast from 1998, it was determined that approximately 165 km (or 14%) of the state's more developed 1160 km coastline from Marin County to the US/Mexico border are presently armored; 93 km (56%) of this armoring protects back beach development, harbors, low bluffs, and dunes while the remaining 72 km (44%) of the armoring protects seacliffs. To assess the direct impact of coastal armoring on the contribution of littoral sediment from seacliff erosion, two littoral cells were chosen for detailed investigation. The Oceanside and Santa Barbara cells were selected to provide a littoral cell-specific sand budget analysis, including the pre-development budget and the extent of human impact on the budget. Overall, seacliff erosion plays an insignificant role as a source of sand for the Santa Barbara littoral cell in particular. The total amount of sand supplied to the beaches by seacliff erosion, whether under natural or actual conditions, is less than 1.0% of the total littoral budget for this cell. In the Oceanside cell, seacliff erosion contributes 12% of the sand to the overall littoral budget; thus, seacliff erosion is a significant contributor to this cell and future armoring proposals need to fully evaluate impacts on sand production.
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The use of coastal sediment budgets has garnered wide acceptance since its inception nearly 40 years ago. Since then, many researchers have used sediment budgets to quantify littoral transport rates and understand coastal processes on diverse coastlines including the high-energy Pacific coast of North America, the Black Sea, the Nile Delta and beyond. Here, we suggest further improvement on an already successful conceptual tool by questioning the broad definition of sand set forth by the classic Wentworth grain size scale (63-2000 microns) that is often used in quantifying coastal sediment budget inputs from sources such as coastal-draining rivers and eroding sea cliffs. A smaller range of sediment sizes is found on many beaches in California. This range is defined by a minimum grain size threshold, termed the littoral cutoff diameter. Sediment contributed to the littoral system that is smaller than this threshold, even if defined as sand by the Wentworth scale, may not remain on the beach in any significant quantity. The littoral cutoff diameter ranges from 88 to 180 microns on the California beaches studied herein, and results from a variety of locations show that yearly littoral sediment flux from coastal-draining rivers and eroding sea cliffs can be overestimated by 16-300% percent if the littoral cutoff diameter is not considered. The presence of the littoral cutoff diameter suggests that quantifying sediment inputs within the context of preexisting littoral sediments is of first-order importance when constructing sediment budgets in California and in other analogous coastal environments.
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This paper describes multidecadal-scale beach changes in the 27-km long Zuma littoral cell, southern California, over 75 years (1928-2002) and suggests explanations based on ocean-climate forcing and other factors. Over this period, beaches within the cell, between Point Mugu and Point Dume, have experienced little human interference compared with other beaches in the region. The methods involve selection of eight target beaches and measurement of changes in beach width from vertical aerial photographs obtained at irregular intervals between 1928 and 2002, supported by archival studies, repeat field surveys and statistical analysis. The photogrammetric data show considerable seasonal and annual changes in width within and between beaches, but also significant trends at longer time-scales. In temporal terms, beach behavior throughout the cell is characterized by short-term episodes of erosion related to seasonal storms and recurrent El Niño events, and longer-term changes that appear to operate in a cyclic manner over decades. The latter we correlate with the Pacific Decadal Oscillation (PDO), as reflected in greater or lesser storminess related to sea surface temperatures across the northeast Pacific Ocean. For most beaches, net erosion coincides in varying degrees with PDO warm phases from 1925 to 1946, and again from 1977 to 2002, whereas net accretion coincides with the PDO cool phase from the 1947 to 1976. The beach-sediment regime is complicated by lag effects, sediment delivered by local streams as storms move onshore, and by reduced seacliff erosion following coastal highway construction. These findings have important implications for coastal management at decadal time-scales.
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