The California Coast and Living Shorelines - A Critical Look

Abstract

California and most other coastlines around the world are being impacted by both long-term sea-level rise and short-term extreme events. Because of California’s long and intensively developed coastline, it is an important area for evaluating responses to these challenges. The predominant historic approach to coastal erosion in California and globally has been the construction of hard coastal armoring such as seawalls and rock revetments. The concept of living shorelines – defined as using natural elements like plants, sand or rocks to stabilize the coastline – has been widely proposed as a soft or green response to coastal erosion and flooding. These approaches have very limited application in high-energy environments, however, such as California’s 1,100-mile-long outer coast and are not realistic solutions for protection from wave attack at high tides or long-term sea-level rise. Each of the state’s coastal communities need to identify their most vulnerable areas, develop adaptation plans, and plan eventual relocation strategies in response to an accelerating sea-level rise.

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The California Coast and Living
Shorelines - A Critical Look
Gary B Griggs *
Posted Date: 8 January 2024
doi: 10.20944/preprints202401.0619.v1
Keywords: living shorelines, coastal protection, shoreline erosion,
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Article
The California Coast and Living Shorelines - A
Critical Look
Gary B. Griggs
Department of Earth and Planetary Sciences, Institute of Marine Sciences, University of California Santa
Cruz, California
Abstract: California and most other coastlines around the world are being impacted by both long-term sea-
level rise and short-term extreme events. Because of California’s long and intensively developed coastline, it is
an important area for evaluating responses to these challenges. The predominant historic approach to coastal
erosion in California and globally has been the construction of hard coastal armoring such as seawalls and rock
revetments. The concept of living shorelines – defined as using natural elements like plants, sand or rocks to
stabilize the coastline – has been widely proposed as a soft or green response to coastal erosion and flooding.
These approaches have very limited application in high-energy environments, however, such as California’s
1,100-mile-long outer coast and are not realistic solutions for protection from wave attack at high tides or long-
term sea-level rise. Each of the state’s coastal communities need to identify their most vulnerable areas, develop
adaptation plans, and plan eventual relocation strategies in response to an accelerating sea-level rise.
Keywords: living shorelines; coastal protection; shoreline erosion
1. Introduction
For virtually the entire history of human civilization, roughly the past 8,000 years, global sea
level was relatively stable. Many of the earliest human settlements were situated on coasts for well-
known reasons including an abundant food supply, a moderate climate, and access to the ocean. Over
time these small settlements became villages, which evolved over time into towns and then cities.
These eventually grew into the megacities of today (defined as having over ten million people). At
present, fourteen of the world’s 17 largest cities are on coasts. However, the essentially stable sea
level of the past 8,000 years was altered beginning about 150 years ago by the industrial revolution
and the increasing use of fossil fuels, first coal, followed by oil and then natural gas. The combustion
of these fuels continued to increase the greenhouse gas content in the atmosphere, with the carbon
dioxide concentration now about 50% above pre-industrial levels (421 ppm). Approximately 80
percent of the world’s energy is now provided by fossil fuels.
The continuing warming of the planet has melted land ice and raised sea water temperature,
both contributing to higher global sea levels. Countless coastal towns and cities around the world
that were built at or very close to the ocean are now experiencing gradually rising seas as well as
short-term extreme events such as hurricanes and cyclones, high tides and storm waves, as well as
the occasional tsunami. The impacts of a continuing and accelerating rise in sea level, particularly on
low-lying shorelines such as the Atlantic and Gulf coasts of the United State and a number of Pacific
island states, are becoming increasingly clear. Short-term elevated water levels from extreme high
tides as well as storm waves, ENSO events on the Pacific coast, and hurricanes along the east and
Gulf coasts, have led to increasing coastal storm damage and shoreline erosion in recent decades.
While our projections of future sea-level rise are becoming more accurate (Sweet, et al. 2022) until at
least mid-century, short-term extreme events will be more damaging; although the trends are
becoming clearer, there is not yet complete agreement on whether the frequency and magnitude of
these extreme events is increasing.
Adaptation options for responding to elevated sea levels and shoreline recession, whether short
or long-term, are limited, and many coastal cities around the planet of all sizes are now struggling
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with future scenarios and risks to oceanfront development, whether public infrastructure or private
development.
2. The California Coast – Responses to long-Term Sea-Level Rise and Short-Term Extreme
Events
California has the longest shoreline in the lower 48, is the most populous state and also many
miles of developed coast, which makes it an appropriate place to evaluate the challenge of how to
respond or adapt to both long-term sea-level rise and short-term extreme events along the shoreline
and take a realistic look at the available options. In recent years, the concept of living shorelines has
been advanced in many areas as a response to coastal flooding and shoreline recession (O’Donnell,
2017; Mitchell and Bilkovic, 2018); NOAA Fisheries) Are living shorelines a realistic solution for the
coast of California?
While California lost population over the past several years, the state still is the most populous
in the nation, being home to about 39 million people (2023). Although the state’s 19 coastal counties
only account for 22 percent of the state’s land area, they are home to 68 percent of its population or
26.5 million people. They are not distributed evenly, however, with the three southernmost counties
alone (San Diego, Orange and Los Angeles) home to 41.8% of the state’s residents (16.3 million
people), and those counties around San Francisco Bay containing another 17.8% of California’s
population.
The state also has the longest coastline in the lower 48 states at 1,100 miles (1,760 km), much of
which has been intensively developed, with oceanfront property typically being the costliest in
California. Sea-level rise and coastal storms, however, don’t differentiate between homes or
properties of different values as the El Nino winters of 1982-83, 1997-98, and 2015-16 made
abundantly clear. The only difference may be in the armoring that individual coastal property owners
may have been able to afford and obtain approval for.
We learned about 25 years ago that the climate along the California coast and in the offshore
Pacific Ocean oscillates over periods of several decades between warmer (or positive) and cooler (or
negative) intervals, termed Pacific Decadal Oscillations (Mantua and Hare, 2002; Figure 1). The
warmer or positive periods are characterized by higher ocean surface water temperatures with
greater evaporation rates and subsequent rainfall, which can generate flooding and landslides in
coastal regions. These warmer periods also experience more frequent and severe El Niño events with
coastal storms and more energetic and damaging waves along the shoreline, typically approaching
from a more westerly or southwesterly direction which have been particularly damaging to south or
southwest facing coasts.
Figure 1. Pacific Decadal Oscillation Index from 1900-2023.
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The period from about the mid-1920s to the mid-1940s along the California coast was dominated
by warmer ocean surface conditions and a generally stormier and more damaging coastal climate
(Figure 1). About 1945, however, the PDO transitioned to a generally cooler and calmer period. This
coincided with the end of World War II when California’s population exploded. The state’s
population grew from 9.34 million in 1945 to 22.35 million in 1977, a 240% increase. In order to meet
the housing needs of all those new residents, the home construction industry boomed, particularly
in southern California. In addition to hundreds of new housing tracts, much of the coast was also
subdivided and homes were built on what appeared at the time to be stable and safe cliffs, bluffs,
even sand dunes and the beaches themselves. Those ocean view homes and properties became
increasingly valuable in subsequent years, but as their owners discovered, also more vulnerable to
coastal storm waves and high tides.
In the late-1970s, the PDO switched rather abruptly back to a warm and stormier cycle with more
frequent and powerful El Niño events (see Figures 1 and 2). Many of those homes that had been built
on the coast or along the shoreline were now threatened by elevated sea levels that accompany the
warmer water of an El Niño, as well as larger storm waves, typically arriving from the west and
southwest, rather than the more frequent arrival from the northwest. Damaging winters along the
shoreline arrived in 1978, 1982-83, 1997-98, and 2015-2016, which changed perceptions about the
wisdom and risks of living on the edge, although, somewhat surprisingly, haven’t yet diminished the
values of oceanfront homes.
Figure 2. Multivariate ENSO (El Nino-Southern Oscillation) Index 1950-2018.
The typical historic response by coastal homeowners, as well as local, state and federal
government agencies in California, when threated with coastal erosion or flooding has been to protect
private property and public infrastructure with some sort of coastal armoring, typically concrete
seawalls or rock revetments. While there were a few large coastal armoring projects built early in the
last century - the massive O’Shaughnessy seawall constructed in 1928 along San Francisco’s Ocean
Beach, for example - for the most part, armoring on a large scale didn’t begin in earnest until the
1970s. Wave attack on shoreline development began to present serious threats to public infrastructure
and those homes built on cliffs, bluffs, dunes and beaches following the calm period that persisted
along the California coast from the mid-1940s until about 1978. Damages to coastal properties and
homes led to a significant increase in the number of permit applications to the California Coastal
Commission for some type of protection, followed by the construction of an increasing number and
the extent of coastal protection structures.
In 1971, only 27.1 miles of the entire state’s coast had been armored (2.5%), with 17 miles of that
being in the four most densely developed southern California counties (Ventura, Los Angeles,
Orange and San Diego). By 1998, after two major El Niño winters, this had increased 400%; 110.3
miles of the state’s shoreline, or 10.3% had been armored. A decade later, as more homes were
threatened, armor coverage continued to expand so that by 2018, 148.7 miles had been protected, or
13.8% of the entire 1,100 miles of state coastline. For the four most-populated and developed southern
California counties, these numbers reached 88.1 miles or 37.8% - over a third of that entire 233 miles
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of shoreline had been protected with some type of structure by 2018 (Table 1; Figure 3; Griggs and
Patsch, 2019).
Table 1. Progressive increase in the length and percentage of armoring along the shoreline of
California’s coastal counties in 1971, 1998 and 2018 (from Griggs and Patsch, 2019).
Figure 3. Continuous seawalls along the back beach in Encinitas, northern San Diego County where
34% of the entire county shoreline has been armored (Kenneth and Gabrielle Adelman, California
Coastal Records Project, Californiacoastline.org).
The California Coastal Act of 1976 established a Coastal Commission of twelve politically
appointed individuals to make decisions on coastal land use and permit issues and their consistency
with the Coastal Act. The legislation also included policy language regarding coastal erosion and
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protection (California Coastal Commission, 1975). Over time, however, with each new permit
request, concerns have been increasingly raised about the impacts of armoring on the coast. The
potential impacts of hard structures (seawalls and rock revetments or rip rap) on the shoreline are
now well understood and include visual effects, impoundment or placement losses from placing a
structure on the beach, reduction of beach access, loss of sand supply from previously eroding bluffs
or cliffs, and passive erosion (Griggs, 2005). With nearly 14% of the entire coast of California now
armored including 37.8% of the shoreline of the southern California coast (Griggs and Patsch, 2019),
these have become very real concerns to the Coastal Commission.
3. Living Shorelines
Creating living, organic or green shorelines has been put forward in recent years as a potentially
less-impactful response to coastal erosion or recession than hardening the coastline with concrete
seawalls or rock revetments. From a global perspective, living shorelines would include creating,
protecting or preserving natural habitats and their ecosystems, coral reefs, mangroves, seagrass and
wetlands, for example. This concept seems to have originated from tropical and subtropical
environments where both coral reefs and mangroves are important in stabilizing shorelines and
buffering coasts from direct wave attack. Living shoreline, like the word resilient, seems to provide a
very appealing answer to the issues of shoreline recession and/or flooding that many coastal
communities and cities in California and around the world are facing today.
An important starting point in any discussion of this potential solution or approach is to be clear
on just what is meant by this term as different definitions or interpretations seem to be used. A
sampling of several of these definitions include the following:
• National Oceanic and Atmospheric Administration (NOAA) – A living shoreline is a shoreline
management practice that provides erosion control benefits; protects, restores, or enhances
natural shoreline habitat; and maintains coastal processes through the strategic placement of
plants, stone, sand fill, and other structural organic materials...
• NOAA Fisheries - A living shoreline is a protected, stabilized coastal edge made of natural
materials such as plants, sand, or rock. Unlike a concrete seawall or other hard structure, which
impede the growth of plants and animals, living shorelines grow over time. Natural
infrastructure solutions like living shorelines provide wildlife habitat, as well as natural
resilience to communities near the waterfront. Living shorelines are sometimes referred to as
nature-based, green, or soft shorelines.
• Virginia Institute of Marine Sciences (VIMS) - Living shorelines are nature-based approaches for
shoreline protection. These stabilization techniques not only protect shorelines and
infrastructure, but they also conserve, create or restore natural shoreline habitats and ecosystem
services.
• National Geographic - A living shoreline is a way of managing coastal areas to protect, restore, or
enhance the habitat. This is done through the placement of plants, stone, sand, and other
materials.
We now also have the power of ChatGP that can do searches of vast amounts of on-line
information and produce summaries or definitions literally within seconds. The following is just one
of several that were instantly generated, each slightly different –
• Living shorelines are coastal management approaches designed to protect and stabilize
shorelines using natural and nature-based elements, in contrast to traditional “hard” or
engineered structures such as seawalls and bulkheads. These approaches aim to create resilient
and sustainable shoreline protection while promoting the health of the environment and natural
ecosystems. Living shorelines work with the natural processes of erosion and sediment
movement to reduce wave energy and erosion while maintaining or enhancing the ecological
function of the shoreline.”
Commonalities in these definitions include the objectives of erosion control and protecting,
restoring or enhancing habitat. In addition, most of these definitions also include the use of plants,
sand or rock. Sand and rocks seem to significantly expand the definition beyond what many might
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perceive as a living or green shoreline. Do these definitions mean that rip rap or a rock revetment is
considered a living shoreline (Figure 4)? Does sand and rock actually “grow over time”?
Figure 4. About half of the three miles of West Cliff Drive in the city of Santa Cruz has been armored
with rip rap. Does this rock constitute a living shoreline? (Kenneth and Gabrielle Adelman, California
Coastal Records Project, Californiacoastline.org).
Where protected and preserved, coral reefs and mangroves have provided important natural or
living shoreline protection, although there are also many locations where reefs and mangroves have
been either removed or destroyed, often for aquaculture, shoreline access, marinas and harbors and
a variety of tourist-serving facilities. There have now been a number of landmark studies on the
importance of these naturally occurring living shorelines that have also calculated their economic
benefits, which has led them to be considered or recommended for coastlines well outside of the
tropical or subtropical ranges of coral reefs and mangrove forests (Beck, et.al. 2016; Storlazzi, et.al.
2021; Reguero, et.al. 2021).
What does not always seem to be appreciated or understood, however, is that these prime
examples of the importance and effectiveness of living shorelines are their geographic restrictions
and the energy conditions that they can withstand. Mangroves, for example, only grow in sheltered
tropical and subtropical coastal areas, which in general are between latitudes of 25 degrees north and
south of the equator, although these can vary somewhat depending upon local ocean climate and
water temperatures (Smithsonian). The closest mangroves to California, for example are distributed
from the southern tip to the center of the Gulf of California, mostly in small bays, estuaries and
isolated pockets.
Reef building corals have similar temperature constraints and grow optimally in water
temperatures between 73° and 84° Fahrenheit (23°-29° Celsius) and cannot tolerate temperatures
below 64° F (18° C), which generally means between 30° north and south of the equator (NOAA-
NOS). For reference, the southernmost part of California is at 32.5 degrees north, but because of the
cool offshore California current, historical ocean water temperatures off the southern end of the state
(San Diego) get as cold as 57° F (14°C) in the winter months. The closest recognized coral reef to the
south is at the tip of the Baja California peninsula, about 800 miles south of the border with California.
Mangroves and reef building corals do not grow along the California coast because water
temperatures are too low.
Another consideration for the feasibility of utilizing living (or green) shorelines along
California’s outer coast is the typical energy conditions. The 1,100-mile California coast is frequently
exposed to very high wave energy, which does vary seasonally, as well as from north to south, and
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also as a function of the orientation of the coastline and any offshore barriers, such as the Channel
Islands.
The issue or question of appropriate energy conditions suitable for living shorelines is described
in a NOAA report (NOAA, 2015) and illustrated in a NOAA graphic (Figure 5), which depicts a range
of shoreline protection approaches ranging from green to gray. This is a useful and simplified way to
frame the issue of where living shorelines may be appropriate to consider or encourage and where
they would be inappropriate.
Figure 5. Conditions suitable for Green vs. Gray shoreline solutions (from NOAA, 2015).
What is clear but not always acknowledged when living shorelines are proposed or considered
in any alternatives analysis for shoreline protection is the distinction spelled out in this graphic
between Living Shorelines (Green – Softer Techniques) and Coastal Structures (Gray – Harder
Techniques). The living or green shoreline consideration states “suitable only for low wave energy
environments”. The increasingly gray or harder techniques are the options proposed for those
locations exposed to high wave forces. The entire 1,100 miles of California’s exposed coastline is, at
least seasonally, if not year around, a high wave energy environment.
There are certainly low-energy estuarine and lagoonal environments along the state’s coastline,
including Humboldt Bay, San Francisco Bay, Elkhorn Slough, Morro Bay, Agua Hedionda, Batiquitos
Lagoon and San Diego Bay, to name a few of the larger ones. These are locations where living
shorelines have or can be restored and protected as defense measures against sea-level rise, for at
least the near-term future. Many of our Southern California estuaries and bays, such as San Diego
Bay, Newport Bay, Ballona Wetlands and others, as well as San Francisco Bay on the central coast,
are surrounded or backed by development or infrastructure, however, so there is a limit to how much
future sea level can be accommodated (Figure 6).
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Figure 6. West side of San Francisco Bay showing Foster City and end of San Mateo Bridge with little
room for shoreline migration (Google Earth image).
Another factor in the consideration of the appropriateness of a living shoreline is the shoreline
topography or landforms. As a result of the state’s active tectonic setting, the great majority (72
percent, or about 790 miles) of the California coast consists of eroding bluffs or sea cliffs (Griggs,
Patsch and Savoy, 2005). Of this 790 miles, about 650 miles (~59%) is relatively low-relief bluffs
typically eroded into uplifted marine terraces (Figure 7); the other 140 miles (~13%) consists of high-
relief cliffs and coastal mountains (Figure 8). The remaining 310 miles or 28 percent of the coastline
is of low relief and relatively flat. These include the wide beaches, sand dunes, as well as the bays,
estuaries, lagoons and wetlands, which form many of the state’s coastal recreational environments
and parkland (Figure 9).
Figure 7. Eroding coastal bluff fronting the lowest marine terrace at Solana Beach, northern San Diego
County(Kenneth and Gabrielle Adelman, California Coastal Records Project, Californiacoastline.org).
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Figure 8. Steep mountainous coast in the Big Sur area of Central California with Mud Creek landslide
(May 2017- Jon Warrick, U.S. Geological Survey).
Figure 9. Beach front development backed by wetlands at Imperial Beach (Kenneth and Gabrielle
Adelman, California Coastal Records Project, Californiacoastline.org).
4. Coastal Dunes and Living Shorelines
If rock revetments are considered as living shorelines, which seems somewhat questionable as
they are certainly not alive, there are already many miles of these structures along the state’s coast.
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In terms of actual living vegetation, however, the main efforts along California’s outer coast have
been focused on planting to stabilize existing coastal dunes (Judge, et al. 2017). There are a number
of large well-known dune fields along the state’s outer coast, but they have some specific
requirements in order to form (Griggs, Patsch & Savoy, 2005). These include 1) a large supply of fine-
grained sand; 2) a dominant onshore wind direction; 3) a wide flat beach so that sand can dry out
between tides and can be moved by the wind; and 4) low relief topography inland of the beach where
sand can be transported and accumulate.
Field studies have shown that the roots of dune vegetation tend to bind the sand together which
helps to keep sand grains from blowing or washing away. The exposed portions of vegetation also
act to reduce the wind velocity near the ground surface, thereby diminishing the potential for the
wind to transport or move the sand. These are the reasons why efforts to restore degraded sandy
shorelines and stabilize coastal dunes have often turned to planting dune vegetation or the
construction of dune fences as solutions.
Recent research using the very large flume at Oregon State University (230-feet long), however,
produced results that were somewhat contrary to conventional thinking. The researchers compared
19 hours of wave impacts on a dune that had been vegetated for six months with those on a bare
dune. As wave heights in the flume were increased to 3.3 feet (one meter), both vegetated and bare
dunes formed near-vertical scarps, although the scarp on the vegetated dune formed several hours
earlier, moved farther inland, and was twice as high as the scarp on the bare dune. From
observations and measurements during the experiments, wave energy periodically deposited sand
on the dune and then eroded it again. Without any vegetation, larger waves could deposit sand on
top of the bare dune. Where plants were present, however, waves couldn’t runup as much, so the
water percolated into the sand. This pore water made the sand more prone to erosion when
subsequent waves washed up the dune. On the unvegetated dune, the wave runup infiltrated over a
larger area, so had less of an effect (Wilke, 2023).
While planting may help to stabilize dunes and reduce sand transport during moderate weather,
during severe wave conditions (hurricanes along the Atlantic or Gulf Coast, or when large waves
occur at high tides along the California coast), dunes, whether vegetated or not, can erode quickly.
Vegetated dunes along the central and southern Monterey Bay shoreline have been repeatedly
eroded back during large El Niño events, threatening coastal homes and historically leading to the
construction of coastal armoring (Figures 10 and 11).
Figure 10. Erosion of a vegetated dune at the Pajaro Dunes development along the central Monterey
Bay shoreline during the severe El Nino of January 1983.
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Figure 11. Rip rap revetment placed against eroded dune face at Pajaro Dunes (Photo April 2006).
Even along the low relief, usually less energetic Atlantic coast of the U.S., vegetated dunes can
be eroded quickly during periods of large waves, whether tropical storms, hurricanes or nor’easters
(Figure 12). Dunes are wind-blown piles of unconsolidated sand and are highly susceptible to
recession when impacted by large waves, and as sea levels continue to rise, waves will reach coastal
dunes more frequently, accelerating the erosion. There is a limit to what dunes, whether vegetated
or not, can withstand and how much sea level rise they can resist.
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Figure 12. Erosion of vegetated dunes at Sea Isle along the New Jersey coastline during Tropical Storm
Ophelia September 24, 2023 (Courtesy of Sea Isle News-Donald Wittowski).
5. Discussion and Conclusions
The reality is that California’s coastal communities face a challenge of increasing magnitude —
and we have no clear agreed-upon or simple solutions. There is some vegetation that can thrive and
stabilize or trap sediment in the state’s low-energy coastal environments such as estuaries, lagoons,
marshes and sloughs. With much of the shoreline of these marginal marine environments backed by
development or infrastructure along the central and south coast of California, there is only so much
sea-level rise they can tolerate before being permanently inundated.
For the 1,100 miles of the state’s exposed, high-energy outer coast, 72 percent consisting of
coastal bluffs or high cliffs, a living shoreline is not a realistic solution for protection from wave attack
or for adapting to long-term sea-level rise. Some of the state’s shoreline communities and
neighborhoods are already experiencing coastal flooding during king tides and periods of large
waves (Figures 13 and 14), and many intensively developed coastal bluff and cliff areas are suffering
or threatened by continuing retreat or recession.
Figure 13. Flooding of East Cliff Drive in Santa Cruz during high tide and large waves in early January
2023 (Photo: Michael Beck).
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Figure 14. Flooding of Beach Drive in Rio Del Mar along the northern Monterey Bay shoreline during
the 1983 El Niño.
Every coastal community, city and county in California needs to determine its most vulnerable
areas and assets and develop response and adaptation plans with thresholds for relocation or
beginning the process of moving back from the shoreline. Thresholds could include when sea level
reaches a specific elevation, or when a street, neighborhood or home is flooded or damaged a certain
number of times during a given period, or when the bluff edge gets to within X feet of a home or
some public infrastructure. These will need to be community-specific decisions, and it’s not too early
to start these discussions with the rate of sea level rise accelerating. Even if we don’t know exactly
how high the sea level will be at a specific point in the future, the Pacific Ocean is rising and we are
in the way. And for the high-energy outer coast of California, there are no living shoreline solutions
that are going to hold back the Pacific Ocean.
Funding: N/A
Institutional Review Board Statement: N/A
Informed Consent Statement: N/A
Data Availability Statement: N/A
Conflicts of Interest: None
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Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 January 2024 doi:10.20944/preprints202401.0619.v1