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Review
Coastal Adaptation to Climate Change and Sea-Level Rise
Gary Griggs 1, * and Borja G. Reguero 2
Citation: Griggs, G.; Reguero, B.G.
Coastal Adaptation to Climate
Change and Sea-Level Rise. Water
2021,13, 2151. https://doi.org/
10.3390/w13162151
Academic Editor: Giorgio Anfuso
Received: 25 June 2021
Accepted: 31 July 2021
Published: 5 August 2021
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1Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA
2Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA;
breguero@ucsc.edu
*Correspondence: griggs@ucsc.edu
Abstract:
The Earth’s climate is changing; ice sheets and glaciers are melting and coastal hazards
and sea level are rising in response. With a total population of over 300 million people situated on
coasts, including 20 of the planet’s 33 megacities (over 10 million people), low-lying coastal areas
represent one of the most vulnerable areas to the impacts of climate change. Many of the largest
cities along the Atlantic coast of the U.S. are already experiencing frequent high tide flooding, and
these events will increase in frequency, depth, duration and extent as sea levels continue to rise at
an accelerating
rate throughout the 21st century and beyond. Cities in southeast Asia and islands in
the Indo-Pacific and Caribbean are also suffering the effects of extreme weather events combined
with other factors that increase coastal risk. While short-term extreme events such as hurricanes, El
Niños and severe storms come and go and will be more damaging in the short term, sea-level rise is
a long-term permanent change of state. However, the effects of sea-level rise are compounded with
other hazards, such as increased wave action or a loss of ecosystems. As sea-level rise could lead
to the displacement of hundreds of millions of people, this may be one of the greatest challenges
that human civilization has ever faced, with associated inundation of major cities, loss of coastal
infrastructure, increased saltwater intrusion and damage to coastal aquifers among many other
global impacts, as well as geopolitical and legal implications. While there are several short-term
responses or adaptation options, we need to begin to think longer term for both public infrastructure
and private development. This article provides an overview of the status on adaptation to climate
change in coastal zones.
Keywords: sea-level rise; climate change; shoreline erosion; adaptation; managed retreat
1. Introduction
The climate has been changing for as long as we have had the Earth and the Sun. The
amount of solar radiation we receive from the Sun has varied over tens of thousands of
years due in large part to the Milankovitch cycles, which control the distance between the
Earth and the Sun. Sea level is intimately tied to ocean warming and therefore climate
change. As temperatures rise during warm or interglacial periods, seawater expands,
and the ice covering Antarctica, Greenland and the mountain glaciers of the planet melts,
increasing sea levels globally. During cooler (glacial) periods, sea level is lowered as
seawater cools and takes up less volume, and more precipitation falls as snow freezes to
ice and allows ice sheets and glaciers to expand.
However, global warming from greenhouse gas concentration during the last century
has led to higher sea levels, globally driven by the melting of ice sheets and the thermal
expansion of the ocean. Sea levels will continue to rise in the future, critically threatening
low-lying coastal zones [
1
]. The potential for high-end sea-level rise may remain despite
the ambition of the Paris Agreement to limit global temperature increase well below 2
◦
C
above pre-industrial levels, given the inertia in ocean processes.
Coastal zones are particularly vulnerable to the impacts of sea-level rise. However, sea-
level rise is not the only way climate change affects coastlines. Climate changes also affect
Water 2021,13, 2151. https://doi.org/10.3390/w13162151 https://www.mdpi.com/journal/water
Water 2021,13, 2151 2 of 26
processes and dynamics in coastal zones through interannual and long-term changes in
winds, storm surges or wave action [
2
–
6
]. Changes in wind patterns, wave power, extreme
waves and sea levels [
7
–
12
] all drive important effects on coastlines. However, these
changes vary between regions and coastlines, from seasonal to interannual and long-term
temporal scales, triggering different impacts locally, such as flooding and erosion [
3
,
13
,
14
].
Expected rises in water temperatures and ocean acidification will also impact coastal
ecosystems, with important implications for the services they provide, such as fisheries,
coastal protection or carbon sequestration [15–22].
Increasing coastal hazards combined with development and demographic concen-
tration in coastal areas makes the need for adaptation urgent. However, the status of
implementation is still limited along many coastlines, challenged by important technical,
economic, financial and social factors [
23
]. Coastal communities require targeted responses,
plans and informed action in order to address the present and future effects and costs from
sea-level rise and climate change.
This paper is intended to serve as a comprehensive introduction or overview to this
Special Issue of WATER on Coastal Adaptation to Climate Change and Sea-Level Rise. Here,
we provide a summary of the current issues and challenges involved in this important and
timely subject. The article describes existing knowledge on coastal hazards, their effects
in coastal areas and adaptation responses. The article also describes the main challenges
and current advances. We rely on recent research and examples from many regions. The
article aims to provide an overview on coastal adaptation that can provide the reader with
a useful document for future reference.
2. Sea-Level Rise
2.1. Historic Changes in Sea Level
Geological evidence, primarily from sediments and fossils collected from the continen-
tal shelf, provides clear global confirmation that the rapid rise of the sea level following the
end of the last ice age approximately 20,000 years ago slowed to nearly a halt approximately
7–8000 years ago [
24
]. From that time until around the mid-1800s, sea level rose at less than
1 mm/year. With the onset of the industrial era and the increasing combustion of fossil
fuels (coal, oil and natural gas), the greenhouse gas content of the atmosphere gradually
increased. Since the onset of the industrial revolution the content of carbon dioxide in the
atmosphere has increased from natural levels varying from approximately 175–275 ppm to
419 ppm today, which is an increase of approximately 50 percent. Greater greenhouse gas
concentrations have amplified the Earth’s natural greenhouse effect, leading to a gradually
warming planet. Over the past 100 years, the Earth’s climate has warmed by approximately
1
◦
C (1.8
◦
F; Figure 1). As temperatures rose, ice sheets and continental glaciers melted at
an increasing rate and seawater warmed and expanded. Global sea levels rose in response,
raising sea levels at a more rapid rate than over the previous 7000–8000 years [25].
Today, hundreds of tidal gauges around the coastlines of the world are recording sea
water levels, but the first measurements date from the mid-1800s. Tide gauges track the
local or relative sea level, which is the elevation of the local sea level relative to land motion,
including uplift and subsidence. Globally averaging historic records documented sea-level
rise values ranging from ~1.2 to ~1.7 mm/year (4.7 to 6.8 in century) over much of the
20st century [
26
,
27
]. These tide gauges are not evenly distributed, however, with most in
the northern hemisphere (U.S. and Europe). While tide gauges provide relative or local
sea-level rise rates, a recent evaluation of 32 tide gauge records from all U.S. coastlines
revealed that, with the exception of the U.S. northeast coast and Alaska, every coastal
location in the continental U.S. has experienced an upturn in relative sea-level rise rate since
2013–2014, despite wide differences in the magnitude and trending direction of relative
sea-level rise acceleration [28].
Water 2021,13, 2151 3 of 26
Water 2021, 13, x FOR PEER REVIEW 3 of 28
since 2013–2014, despite wide differences in the magnitude and trending direction of rel-
ative sea-level rise acceleration [28].
In 1993, two satellites were placed in orbit (Topex and Poseidon), followed by Jason-
1, -2 and -3, with the objective of measuring global or absolute sea level accurately and
precisely from space using lasers. The average sea-level rise rate from these satellite meas-
urements over their 27 years of operation is now 3.4 mm/year (13.4 in./century), but this
rate is accelerating [29,30]. More recently, independent data from European satellites [31]
has been used in order to increase both the time period covered (1991–2019) as well as the
geographic distribution of data (from 66 degrees to 82 degrees latitude). Satellite-based
observations now allow us to measure that the average acceleration of sea-level rise,
which has been 0.1 mm/year2 between 1991 and 2019. The average rate of rise of 3.4
mm/year over the past 27 years has now increased to about 4.8 mm/year, or approximately
18.9 in./century (Figure 2), based on observations of the past 10 years
(https://www.aviso.altimetry.fr/en/data/products/ocean-indicators-products/mean-sea-
level.html) (accessed on 1 July 2021)
Figure 1. Annual global surface temperature 1850–2019. Source: (NASA Earth Observatory/Robert
Simmon). The line plot below shows yearly temperature anomalies from 1880 to 2019 as recorded
by NASA, NOAA, the Berkeley Earth research group, the Met Office Hadley Centre (United King-
dom), and the Cowtan and Way analysis. NASA’s temperature analyses incorporate surface tem-
perature measurements from more than 20,000 weather stations, ship- and buoy-based observa-
tions of sea surface temperatures, and temperature measurements from Antarctic research sta-
tions. Credits: NASA’s Earth Observatory, obtained from: https://earthobserva-
tory.nasa.gov/world-of-change/global-temperatures, accessed 1 August 2021.
Figure 1.
Annual global surface temperature 1850–2019. Source: (NASA Earth Observatory/Robert
Simmon). The line plot below shows yearly temperature anomalies from 1880 to 2019 as recorded by
NASA, NOAA, the Berkeley Earth research group, the Met Office Hadley Centre (United Kingdom),
and the Cowtan and Way analysis. NASA’s temperature analyses incorporate surface temperature
measurements from more than 20,000 weather stations, ship- and buoy-based observations of sea
surface temperatures, and temperature measurements from Antarctic research stations. Credits:
NASA’s Earth Observatory, obtained from: https://earthobservatory.nasa.gov/world-of-change/
global-temperatures, accessed on 1 August 2021.
In 1993, two satellites were placed in orbit (Topex and Poseidon), followed by Jason-1,
-2 and -3, with the objective of measuring global or absolute sea level accurately and pre-
cisely from space using lasers. The average sea-level rise rate from these satellite measure-
ments over their 27 years of operation is now 3.4 mm/year (13.4 in./century), but this rate is
accelerating [
29
,
30
]. More recently, independent data from European satellites [
31
] has been
used in order to increase both the time period covered (1991–2019) as well as the geographic
distribution of data (from 66 degrees to 82 degrees latitude). Satellite-based observations
now allow us to measure that the average acceleration of sea-level rise, which has been
0.1 mm/year
2
between 1991 and 2019. The average rate of rise of 3.4 mm/year over the
past 27 years has now increased to about 4.8 mm/year, or approximately 18.9 in./century
(Figure 2), based on observations of the past 10 years (https://www.aviso.altimetry.fr/en/
data/products/ocean-indicators-products/mean-sea-level.html) (accessed on 1 July 2021).
2.2. Future Sea Levels
Tide gauges and satellite-based observations provide a good understanding of past
and present sea level. However, the challenge for coastal regions around the planet is
projecting sea-level rise and its impacts into the future. This is an important objective of
the Intergovernmental Panel on Climate Change (IPCC), but individual geographic entities
(local to national governments) are simultaneously involved in developing future sea-level
rise projections for their own regions [
32
]. Future climate projections are developed through
global climate models, which include uncertainties and assumptions of future greenhouse
gas emissions (i.e., Representative Concentration Pathways) and model the inputs or
factors that will affect global climate, including ice melt and consequently sea-level rise [
25
].
Today, the predictions or projections for the next few decades are in general agreement
but estimates for the end-of-century vary between models and depend on Representative
Concentration Pathways (RCPs), with increasingly wider uncertainties and ranges by 2100.
The latest estimates indicate that values for the end-of-century (2100) range from a low
of ~50 cm (~20 inches) to as high as ~310 cm (~10 feet), as a function of greenhouse gas
emission scenarios and various probabilities or uncertainties, especially concerning the
extent of Greenland and Antarctica ice melt [33] (Figure 3).
Water 2021,13, 2151 4 of 26
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Figure 2. Sea-level rise from satellite altimetry 1993–2020. Source: based on [30], obtained from:
https://www.aviso.altimetry.fr/en/data/products/ocean-indicators-products/mean-sea-level.html,
accessed 1 August 2021. The reference global mean sea level (GMSL) is based on data from the
TOPEX/Poseidon, Jason-1, Jason-2 and Jason-3 missions from January 1993 to present, after re-
moving the annual and semi-annual signals and applying a 6-month filter. By applying the post-
glacial rebound correction (−0.3 mm/yr), the rise in mean sea level has thus been estimated as 3.4
mm/year (straight line on the figure).
2.2. Future Sea Levels
Tide gauges and satellite-based observations provide a good understanding of past
and present sea level. However, the challenge for coastal regions around the planet is pro-
jecting sea-level rise and its impacts into the future. This is an important objective of the
Intergovernmental Panel on Climate Change (IPCC), but individual geographic entities
(local to national governments) are simultaneously involved in developing future sea-
level rise projections for their own regions [32]. Future climate projections are developed
through global climate models, which include uncertainties and assumptions of future
greenhouse gas emissions (i.e., Representative Concentration Pathways) and model the
inputs or factors that will affect global climate, including ice melt and consequently sea-
level rise [25]. Today, the predictions or projections for the next few decades are in general
agreement but estimates for the end-of-century vary between models and depend on Rep-
resentative Concentration Pathways (RCPs), with increasingly wider uncertainties and
ranges by 2100. The latest estimates indicate that values for the end-of-century (2100)
range from a low of ~50 cm (~20 inches) to as high as ~310 cm (~10 feet), as a function of
greenhouse gas emission scenarios and various probabilities or uncertainties, especially
concerning the extent of Greenland and Antarctica ice melt [33] (Figure 3).
Understandably, while projections of future sea levels typically only extend out to
2100 due to increasing uncertainties, sea-level rise will not stop then, but will likely con-
tinue for decades and even centuries into the future. Even in the absence of further green-
house emissions, the sea-level rise inertia will continue, and sea levels will increase in the
future. There is approximately 66 m (~216 feet) of potential sea-level rise contained in the
ice sheets and glaciers of Antarctica, Greenland and the mountain glaciers of the planet
(http://www.antarcticglaciers.org/glaciers-and-climate/estimating-glacier-contribution-
to-sea-level-rise/, accessed on 1 August 2021). No one believes that these will all melt this
century, but this is the total potential that exists if it were all to melt.
Figure 2.
Sea-level rise from satellite altimetry 1993–2020. Source: based on [
30
], obtained from: https://www.aviso.
altimetry.fr/en/data/products/ocean-indicators-products/mean-sea-level.html, accessed on 1 August 2021. The reference
global mean sea level (GMSL) is based on data from the TOPEX/Poseidon, Jason-1, Jason-2 and Jason-3 missions from
January 1993 to present, after removing the annual and semi-annual signals and applying a 6-month filter. By applying the
postglacial rebound correction (
−
0.3 mm/year), the rise in mean sea level has thus been estimated as 3.4 mm/year (straight
line on the figure).
Understandably, while projections of future sea levels typically only extend out to
2100 due to increasing uncertainties, sea-level rise will not stop then, but will likely con-
tinue for decades and even centuries into the future. Even in the absence of further
greenhouse emissions, the sea-level rise inertia will continue, and sea levels will increase
in the future. There is approximately 66 m (~216 feet) of potential sea-level rise con-
tained in the ice sheets and glaciers of Antarctica, Greenland and the mountain glaciers
of the planet (http://www.antarcticglaciers.org/glaciers-and-climate/estimating-glacier-
contribution-to-sea-level-rise/, accessed on 1 August 2021). No one believes that these will
all melt this century, but this is the total potential that exists if it were all to melt.
In just this century, raising the sea level just 1 m will create substantial issues for
developed shorelines around the planet. A recent global assessment determined that
approximately 110 million people live below the present high tide today, and 250 million
occupy land below current annual flood levels [
34
]. For the first few meters of sea-level
rise, more than 3 million more people are at risk with each vertical 2.5 cm (one inch) of rise.
One billion people today, approximately 13% of the entire global population, live less than
10 m (33 feet) above today’s high tide.
Water 2021,13, 2151 5 of 26
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In just this century, raising the sea level just 1 m will create substantial issues for
developed shorelines around the planet. A recent global assessment determined that ap-
proximately 110 million people live below the present high tide today, and 250 million
occupy land below current annual flood levels [34]. For the first few meters of sea-level
rise, more than 3 million more people are at risk with each vertical 2.5 cm (one inch) of
rise. One billion people today, approximately 13% of the entire global population, live less
than 10 m (33 feet) above today’s high tide.
Figure 3. Possible future sea levels for different greenhouse gas emission pathways. Observed sea level from tide gauges
(dark gray) and satellites (light gray) from 1800–2015 show historical trajectory. The scenarios differ based on potential
future rates of greenhouse gas emissions and differences in the plausible rates of glacier and ice sheet loss. Source: NOAA
Climate.gov graph, adapted from Figure 8 in [35].
3. Effects of Climate Change and Sea-Level Rise in Coastal Areas
3.1. How Future Sea-Level Rise Will Affect Coastal Areas
Looking to the future, the potential loss of public infrastructure and private develop-
ment due to sea-level rise will have enormous economic impacts on coastal nations glob-
ally [1]. Different coastal environments face unique hazards, however, as a result of their
geology and topography, regional climatic settings and development patterns. Coasts dis-
play a variety of landforms (e.g., estuaries, beaches, dunes, low bluffs, high cliffs and steep
mountains) and also differing development patterns (low to high density). Lower-lying
shoreline areas are more vulnerable to flooding from wave action, hurricanes and large
storm waves acting simultaneously with very high tides and atop the higher sea levels of
the future. Higher-elevation areas, such as bluffs, cliffs and coastal mountains, are more
vulnerable to coastal erosion from wave attack during high tides or elevated sea levels.
Nonetheless, higher sea levels in the future will mean: (1) more frequent and higher ele-
vation flooding of low relief shoreline areas [36,37], followed by permanent inundation
and loss of beaches and coastal wetlands [14,38]; and (2) waves reaching and impacting
the base of coastal cliffs, bluffs and dunes more often, leading to increased erosion rates.
The economic impacts of coastal hazards will also vary with the degree and type of
development and whether it is public or private. Passive erosion, or the gradual loss of
beaches from continuing sea-level rise where the back beach has been fixed by a seawall,
rock revetment or some other structure, will be a major challenge along highly developed
and armored coasts [24,38,39]. Along the intensively developed ~325 km (233-mile) coast-
line of southern California, for example, where millions of people use the beaches, 38 per-
cent of the entire shoreline has now been armored (Figure 4), and with rising sea levels,
the issue of passive erosion and beach loss will become more pressing [40].
Figure 3.
Possible future sea levels for different greenhouse gas emission pathways. Observed sea level from tide gauges
(dark gray) and satellites (light gray) from 1800–2015 show historical trajectory. The scenarios differ based on potential
future rates of greenhouse gas emissions and differences in the plausible rates of glacier and ice sheet loss. Source: NOAA
Climate.gov graph, adapted from Figure 8in [35].
3. Effects of Climate Change and Sea-Level Rise in Coastal Areas
3.1. How Future Sea-Level Rise Will Affect Coastal Areas
Looking to the future, the potential loss of public infrastructure and private devel-
opment due to sea-level rise will have enormous economic impacts on coastal nations
globally [
1
]. Different coastal environments face unique hazards, however, as a result
of their geology and topography, regional climatic settings and development patterns.
Coasts display a variety of landforms (e.g., estuaries, beaches, dunes, low bluffs, high
cliffs and steep mountains) and also differing development patterns (low to high density).
Lower-lying shoreline areas are more vulnerable to flooding from wave action, hurricanes
and large storm waves acting simultaneously with very high tides and atop the higher sea
levels of the future. Higher-elevation areas, such as bluffs, cliffs and coastal mountains,
are more vulnerable to coastal erosion from wave attack during high tides or elevated sea
levels. Nonetheless, higher sea levels in the future will mean: (1) more frequent and higher
elevation flooding of low relief shoreline areas [
36
,
37
], followed by permanent inundation
and loss of beaches and coastal wetlands [
14
,
38
]; and (2) waves reaching and impacting the
base of coastal cliffs, bluffs and dunes more often, leading to increased erosion rates.
The economic impacts of coastal hazards will also vary with the degree and type
of development and whether it is public or private. Passive erosion, or the gradual
loss of beaches from continuing sea-level rise where the back beach has been fixed by
a seawall
, rock revetment or some other structure, will be a major challenge along highly
developed and armored coasts [
24
,
38
,
39
]. Along the intensively developed ~325 km
(233-mile
) coastline of southern California, for example, where millions of people use the
beaches, 38 percent of the entire shoreline has now been armored (Figure 4), and with rising
sea levels, the issue of passive erosion and beach loss will become more pressing [40].
3.2. Nuisance Flooding
To date, much of the research on the impacts of sea-level rise has focused on the
occurrence and damage of sea level extremes, such as from tropical cyclones or other
storms [
41
–
43
], as sea-level rise contributes to more flooding by increasing the probability
of extreme floods [
36
,
44
]. However, nuisance flooding (also known as sunny day floods)
has increased on U.S. east coasts in recent decades due to sea-level rise [45,46].
Water 2021,13, 2151 6 of 26
Water 2021, 13, x FOR PEER REVIEW 6 of 28
Figure 4. Rip-rap revetment armoring a section of the Malibu, California (USA) shoreline. Courtesy: California Coastal
Records Project.
3.2. Nuisance Flooding
To date, much of the research on the impacts of sea-level rise has focused on the oc-
currence and damage of sea level extremes, such as from tropical cyclones or other storms
[41–43], as sea-level rise contributes to more flooding by increasing the probability of ex-
treme floods [36,44]. However, nuisance flooding (also known as sunny day floods) has
increased on U.S. east coasts in recent decades due to sea-level rise [45,46].
Coastal nuisance flooding is considered to be minor flooding from the sea that causes
problems such as flooded roads and overloaded stormwater systems, which can be major
inconveniences for people and provide a habitat for bacteria and mosquitoes. Based on
over 70 years of observations from the U.S., a study found that the total number of nui-
sance flooding events caused by tidal changes have increased at an exponential rate since
1950, adding 27% more nuisance flooding occurrences in 2019 [47]. Estuaries show the
largest changes because of tide changes associated with anthropogenic alterations, such
as the dredging of channels, land reclamation, changes in river flows and other develop-
ments.
Frequent high-tide flooding also affects local economic activity. For example, a study
in Annapolis, Maryland, found that frequent high-tide floods have reduced visits to the
historic downtown by 1.7%, but with 8 and 30 cm (3 and 12 inches) of additional sea-level
rise, high-tide floods would reduce visits by 3.6% and 24%, respectively [48]. The impacts
of high-tide flooding should also be better characterized and understood in order to help
guide efficient local responses and include them in urban planning.
4. Short-Term Coastal Hazards versus Long-Term Sea-Level Rise
Climate change will also influence coastal hazards besides sea-level rise. While con-
siderable research and planning effort today is focused on increasing the accuracy of fu-
ture sea-level rise projections, in the short- or near-term (until perhaps the mid-century),
it will likely be the extreme events that will be more damaging to coastal development
Figure 4.
Rip-rap revetment armoring a section of the Malibu, California (USA) shoreline. Courtesy: California Coastal
Records Project.
Coastal nuisance flooding is considered to be minor flooding from the sea that causes
problems such as flooded roads and overloaded stormwater systems, which can be major
inconveniences for people and provide a habitat for bacteria and mosquitoes. Based on
over 70 years of observations from the U.S., a study found that the total number of nuisance
flooding events caused by tidal changes have increased at an exponential rate since 1950,
adding 27% more nuisance flooding occurrences in 2019 [
47
]. Estuaries show the largest
changes because of tide changes associated with anthropogenic alterations, such as the
dredging of channels, land reclamation, changes in river flows and other developments.
Frequent high-tide flooding also affects local economic activity. For example, a study
in Annapolis, Maryland, found that frequent high-tide floods have reduced visits to the
historic downtown by 1.7%, but with 8 and 30 cm (3 and 12 inches) of additional sea-level
rise, high-tide floods would reduce visits by 3.6% and 24%, respectively [
48
]. The impacts
of high-tide flooding should also be better characterized and understood in order to help
guide efficient local responses and include them in urban planning.
4. Short-Term Coastal Hazards versus Long-Term Sea-Level Rise
Climate change will also influence coastal hazards besides sea-level rise. While consid-
erable research and planning effort today is focused on increasing the accuracy of future
sea-level rise projections, in the short- or near-term (until perhaps the mid-century), it
will likely be the extreme events that will be more damaging to coastal development and
infrastructure [
40
]. These events include cyclones, typhoons and hurricanes, large storm
waves arriving simultaneously with very high tides or elevated water levels and tsunamis.
4.1. Interannual Changes
The large El Niño of 1982–83 raised sea levels along the California (USA) coast, as
recorded at tide gauges, to the highest values ever registered, ranging from 29 cm (11.4 in.)
above predicted tidal heights at San Diego, California, 32.3 cm (12.7 in.) at Los Angeles and
Water 2021,13, 2151 7 of 26
53.9 cm (21.2 in.) at San Francisco. These extreme El Niño-related tides were the highest
water levels recorded during the prior 77 years in San Diego, 59 years in Los Angeles
and 128 years in San Francisco. Using average rates of global sea-level rise from satellite
altimetry (3.4 mm/year or 13.4 inches/century), these elevated 1983 El Niño water levels
were equivalent to 85, 95 and 158 years of sea-level rise at recent rates, respectively, at
these three locations. In addition to elevated water levels, seven major storms brought
large waves during periods of high tides. These combined to produce USD 265 million
in coastal damage to oceanfront property (in 2020 USD). Damage was not restricted to
just broken windows and flooding of low-lying areas—33 oceanfront homes were totally
destroyed and dozens of businesses, parks, roads and other public infrastructure were
heavily damaged (Figure 5).
Water 2021, 13, x FOR PEER REVIEW 7 of 28
and infrastructure [40]. These events include cyclones, typhoons and hurricanes, large
storm waves arriving simultaneously with very high tides or elevated water levels and
tsunamis.
4.1. Interannual Changes
The large El Niño of 1982-83 raised sea levels along the California (United States of
America, USA) coast, as recorded at tide gauges, to the highest values ever registered,
ranging from 29 cm (11.4 in.) above predicted tidal heights at San Diego, California, 32.3
cm (12.7 in.) at Los Angeles and 53.9 cm (21.2 in.) at San Francisco. These extreme El Niño-
related tides were the highest water levels recorded during the prior 77 years in San Diego,
59 years in Los Angeles and 128 years in San Francisco. Using average rates of global sea-
level rise from satellite altimetry (3.4 mm/yr. or 13.4 inches/century), these elevated 1983
El Niño water levels were equivalent to 85, 95 and 158 years of sea-level rise at recent
rates, respectively, at these three locations. In addition to elevated water levels, seven ma-
jor storms brought large waves during periods of high tides. These combined to produce
USD 265 million in coastal damage to oceanfront property (in 2020 USD). Damage was
not restricted to just broken windows and flooding of low-lying areas—33 oceanfront
homes were totally destroyed and dozens of businesses, parks, roads and other public
infrastructure were heavily damaged (Figure 5).
Figure 5. Storm waves arriving simultaneously with high tides broke through the front of these
homes built along the northern Monterey Bay, California, shoreline (Photo: Gary Griggs).
Yet again, in mid-January 1988, very large waves struck the southern California coast
suddenly and left USD 62 million in losses [40]. A decade later, during the El Niño winter
of 1997-98, intense rainstorms hit southern California, washing out roads and railroad
tracks, overflowing flood control channels and battering the coast, leading to 17 fatalities
and over half a billion dollars (109) in damage [49]. Recent El Niños along the US West
Coast and Pacific have also set new records and caused widespread flooding and erosion
[3,4,50].
Figure 5.
Storm waves arriving simultaneously with high tides broke through the front of these
homes built along the northern Monterey Bay, California, shoreline (Photo: Gary Griggs).
Yet again, in mid-January 1988, very large waves struck the southern California coast
suddenly and left USD 62 million in losses [
40
]. A decade later, during the El Niño winter of
1997–98, intense rainstorms hit southern California, washing out roads and railroad tracks,
overflowing flood control channels and battering the coast, leading to 17 fatalities and over
half a billion dollars (10
9
) in damage [
49
]. Recent El Niños along the US West Coast and
Pacific have also set new records and caused widespread flooding and erosion [3,4,50].
4.2. Hurricanes, Cyclones and Typhoons
While other coastal areas around the planet do not experience the impacts of El Niños,
they have their own extreme events to contend with. On 8 November 2013, Typhoon
Haiyan, the strongest tropical cyclone ever to make landfall based on wind velocities, cut
a devastating
swath across the central Philippines in the tropical western Pacific. The storm
strength was equivalent to a Category 5 hurricane (the highest level of intensity) with
sustained wind speeds at landfall of 312 km/h. (195 mph), the highest ever recorded, and
gusts of up to 376 km/h. (235 mph). When wind speed reaches approximately
192 km/h
.
(120 mph), it is no longer possible for a human being to stand up. Thirteen percent of
the nation’s entire population, nearly 13 million people, was affected. There were at least
6300 fatalities and 28,700 injuries. As a result of the lightweight construction materials
Water 2021,13, 2151 8 of 26
commonly used in the Philippines and the extreme wind velocities, over 281,000 houses
were reported as destroyed, with 1.9 million people displaced (Figure 6).
Water 2021, 13, x FOR PEER REVIEW 8 of 28
4.2. Hurricanes, Cyclones and Typhoons
While other coastal areas around the planet do not experience the impacts of El Ni-
ños, they have their own extreme events to contend with. On 8 November 2013, Typhoon
Haiyan, the strongest tropical cyclone ever to make landfall based on wind velocities, cut
a devastating swath across the central Philippines in the tropical western Pacific. The
storm strength was equivalent to a Category 5 hurricane (the highest level of intensity)
with sustained wind speeds at landfall of 312 km/h. (195 mph), the highest ever recorded,
and gusts of up to 376 km/h. (235 mph). When wind speed reaches approximately 192
km/h. (120 mph), it is no longer possible for a human being to stand up. Thirteen percent
of the nation’s entire population, nearly 13 million people, was affected. There were at
least 6300 fatalities and 28,700 injuries. As a result of the lightweight construction materi-
als commonly used in the Philippines and the extreme wind velocities, over 281,000
houses were reported as destroyed, with 1.9 million people displaced (Figure 6).
Figure 6. Damage from Typhoon Haiyan in the Philippines, 2013 (Photo: Trocare via Wikimedia).
A year earlier, Superstorm Sandy produced a record storm surge in New York City.
The water level at the southern tip of Manhattan (New York, NY, USA) topped 4.2 m (13.9
feet), exceeding the 3.1 m (10.2 feet) record set by Hurricane Donna fifty-two years earlier,
driven by the size and angle of superstorm Sandy along the U.S. East Coast. For perspec-
tive, the average long-term sea-level rise rate for the NOAA tide gauge at the southern
end of Manhattan Island has been 2.87 mm/yr. since 1856. Seventy-two lives were lost and
losses reached USD 50 billion from damage to homes and other buildings, roads, board-
walks and mass transit facilities in low-lying coastal areas of both New York and New
Jersey (USA) from storm surges and large waves.
Tropical cyclone-induced coastal flooding will worsen under climate change from
the combined effects of sea-level rise and changes in storm activity. For the U.S., the com-
pound effects of SLR and tropical cyclone climatology changes will turn the historical 100-
year flood levels into annual events in the New England and mid-Atlantic regions and 1–
30-year events in the southeast Atlantic and Gulf of Mexico regions in the late 21st century
[51]. Even in regions where the effect of strengthened storms could be compensated with
the displacement of storm tracks, as in New York, the effects of higher mean sea levels
Figure 6. Damage from Typhoon Haiyan in the Philippines, 2013 (Photo: Trocare via Wikimedia).
A year earlier, Superstorm Sandy produced a record storm surge in New York City.
The water level at the southern tip of Manhattan (New York, NY, USA) topped 4.2 m
(13.9 feet), exceeding the 3.1 m (10.2 feet) record set by Hurricane Donna fifty-two years
earlier, driven by the size and angle of superstorm Sandy along the U.S. East Coast. For
perspective, the average long-term sea-level rise rate for the NOAA tide gauge at the
southern end of Manhattan Island has been 2.87 mm/year. since 1856. Seventy-two lives
were lost and losses reached USD 50 billion from damage to homes and other buildings,
roads, boardwalks and mass transit facilities in low-lying coastal areas of both New York
and New Jersey (USA) from storm surges and large waves.
Tropical cyclone-induced coastal flooding will worsen under climate change from the
combined effects of sea-level rise and changes in storm activity. For the U.S., the compound
effects of SLR and tropical cyclone climatology changes will turn the historical 100-year
flood levels into annual events in the New England and mid-Atlantic regions and 1–30-year
events in the southeast Atlantic and Gulf of Mexico regions in the late 21st century [
51
].
Even in regions where the effect of strengthened storms could be compensated with the
displacement of storm tracks, as in New York, the effects of higher mean sea levels will
drive significant increases in flood levels [
52
], with important consequences for coastal risk
and adaptation needs.
Apart from geographic coastline variability and sea-level rise, whether climate change
will drive a future increase in the frequency and magnitude of climatic patterns, such
as El Niños or hurricanes events, remains uncertain. There seems to be an emerging
consensus, however, that warmer surface ocean water will raise evaporation rates and
increase the frequency and magnitude of hurricanes, cyclones and typhoons, potentially
delivering more damage when they make landfall. NOAA has suggested that an increase in
Category
4 and 5 hurricanes
is likely (https://www.gfdl.noaa.gov/global-warming-and-
hurricanes/, accessed on 1 August 2021), with hurricane wind speeds increasing by up to
10%, and with 10–15% more precipitation in a 2 ◦C scenario.
Recent storms, such as Hurricane Harvey (2017), which dropped over 152 cm
(60 inches
) in
some locations near Houston, Texas (USA), Florence (2018), with over 89 cm (
35 inches
),
Water 2021,13, 2151 9 of 26
and Imelda (2019; 112 cm/44 inches), in addition to the impacts of wind, storm surges
and wave action, as demonstrated with Hurricanes Irma and Maria in the Caribbean in
2017, demonstrate the devastating effects that can be triggered by more frequent, intense
or wetter hurricanes. The connection between climate change and hurricane frequency is
less straightforward. It is likely that the number of storms will remain the same or even
decrease, with the primary increase being in the most extreme storms. Areas affected
by hurricanes are also shifting poleward, likely with expanding tropics due to higher
global average temperatures. The changing patterns of tropical storms (a shift northward
in the Atlantic) could also put more property and human lives at risk, but much more
research is required to better characterize and predict how such patterns will change in
the future. Independently, historic data show how the cost of hurricanes is also increasing
globally, associated with a conjunction of climate, more intense coastal development and
other factors [53].
4.3. Wave Action
Changes in wave action can be one of the most important drivers of coastal change.
Wave energy has increased and is globally associated with global warming [
7
]. Increases in
wave heights are more significant for higher percentiles, especially in high latitudes [
11
].
Future projections also point to increases in wave heights along many coastlines, including
the extremes, although with strong spatial variability [
10
,
12
,
54
]. Local changes in wave
action, as well as the strong influence of interannual patterns, such as El Niño or the
North Atlantic Oscillation [
55
–
57
], necessitate including local projections and effects when
predicting and forecasting wave-driven impacts in coastal areas. Furthermore, recent
research has also indicated that wave forces could drive important impacts, especially for
low-lying areas, sandy shorelines and islands, or coastal infrastructure [
58
,
59
]. Changes in
wave action will accelerate coastal erosion (Figures 7–9), require upgrading and increased
costs of coastal defenses [
60
,
61
], port activity and operations [
62
] and alter sediment
transport and beaches [
14
,
63
,
64
]. Local changes in wave action, as well as the strong
influence of interannual patterns, such as El Niño or the North Atlantic Oscillation [
55
,
56
],
need local projections for predicting these local impacts. These impacts are occurring
globally in developed regions such as the U.S., but also represent important challenges in
developing countries and island nations (Figures 7and 8).
Water 2021, 13, x FOR PEER REVIEW 10 of 28
Figure 7. Beach erosion in Saint Louis, Senegal, threatens houses and the livelihood of many. Source: B. G. Reguero.
Figure 8. Saint Louis, the old colonial capital of Senegal, faces a flooding threat that has already seen entire villages lost to
the Atlantic. Source: The Guardian—https://www.theguardian.com/environment/2020/jan/28/how-the-venice-of-africa-is-
losing-its-battle-against-the-rising-ocean, accessed on 1 August 2021.
Figure 7.
Beach erosion in Saint Louis, Senegal, threatens houses and the livelihood of many. Source:
B. G. Reguero.
Water 2021,13, 2151 10 of 26
Water 2021, 13, x FOR PEER REVIEW 10 of 28
Figure 7. Beach erosion in Saint Louis, Senegal, threatens houses and the livelihood of many. Source: B. G. Reguero.
Figure 8. Saint Louis, the old colonial capital of Senegal, faces a flooding threat that has already seen entire villages lost to
the Atlantic. Source: The Guardian—https://www.theguardian.com/environment/2020/jan/28/how-the-venice-of-africa-is-
losing-its-battle-against-the-rising-ocean, accessed on 1 August 2021.
Figure 8.
Saint Louis, the old colonial capital of Senegal, faces a flooding threat that has already seen entire villages lost to
the Atlantic. Source: The Guardian—https://www.theguardian.com/environment/2020/jan/28/how-the-venice-of-africa-
is-losing-its-battle-against-the-rising-ocean, accessed on 1 August 2021.
Water 2021, 13, x FOR PEER REVIEW 11 of 28
Figure 9. Collapse of road from beach erosion and wave action (left) and wave overtopping during high tides (right) in
the Seychelles. Source: B.G. Reguero.
4.4. Other Effects of Climate Change in Coastal Areas
In addition to flooding and erosion, sea-level rise will also produce other effects in
coastal areas. The six main concerns for low-lying coasts from sea-level rise include (IPCC
2014): (i) permanent submergence of land by mean sea levels or mean high tides; (ii) more
frequent or intense extreme flooding; (iii) enhanced erosion; (iv) loss and change of eco-
systems such as wetlands; (v) salinization of soils, ground and surface water; and (vi) im-
peded drainage.
Salinization is caused by rising sea levels that drive seawater intrusion into coastal
aquifers, surface waters and soils. Salinization also increases with land-based drought
events, decreasing river discharges in combination with water extraction and SLR. Sea-
water intrusion is already contributing to the conversion of land or freshwater ponds to
brackish or saline aquaculture in low-lying coastal areas of southeast Asia, such as the
Mekong Delta [65]. SLR will also affect agriculture mainly through land submergence, soil
and fresh groundwater resource salinization and land loss from permanent inundation
and erosion, with consequences on production, livelihood diversification and food secu-
rity, especially in coastal agriculture-dependent countries, such as Bangladesh [66]. Sali-
nization is already a major problem for traditional agriculture in deltas and low-lying is-
land nations [67,68].
SLR may also affect tourism and recreation through impacts on landscapes (e.g.,
beaches), cultural features and critical transportation infrastructures, such as harbors and
airports [69]. Coastal areas’ future tourism and recreation attractiveness will, however,
also depend on changes in air temperature, seasonality and sea surface temperature. Alt-
hough ocean warming and acidification will be more influential in global fisheries and
aquaculture, sea-level rise may produce indirect effects through adverse effects in habitats
or facilities.
5. Responses to the Inevitable and Accelerating Rise in Sea Level and Coastal Hazards
With the thousands of kilometers of developed beaches, dunes, barrier islands, bluff
and cliff tops around the planet, virtually every coastal nation has all types of develop-
ments, whether private or public, whether new or old, and land threatened by erosion
and/or flooding in the decades ahead. What can or should be done with the communities
and cities, homes and hotels, streets and parking lots, airports and power plants,
wastewater treatment plants, pump stations and transmission lines or other infrastructure
built on the beach or at the edge of a cliff or bluff? This challenge has affected and will
Figure 9.
Collapse of road from beach erosion and wave action (
left
) and wave overtopping during high tides (
right
) in the
Seychelles. Source: B.G. Reguero.
4.4. Other Effects of Climate Change in Coastal Areas
In addition to flooding and erosion, sea-level rise will also produce other effects
in coastal areas. The six main concerns for low-lying coasts from sea-level rise include
(IPCC 2014): (i) permanent submergence of land by mean sea levels or mean high tides;
Water 2021,13, 2151 11 of 26
(ii) more
frequent or intense extreme flooding; (iii) enhanced erosion; (iv) loss and change
of ecosystems such as wetlands; (v) salinization of soils, ground and surface water; and
(vi) impeded drainage.
Salinization is caused by rising sea levels that drive seawater intrusion into coastal
aquifers, surface waters and soils. Salinization also increases with land-based drought
events, decreasing river discharges in combination with water extraction and SLR. Seawater
intrusion is already contributing to the conversion of land or freshwater ponds to brackish
or saline aquaculture in low-lying coastal areas of southeast Asia, such as the Mekong
Delta [
65
]. SLR will also affect agriculture mainly through land submergence, soil and fresh
groundwater resource salinization and land loss from permanent inundation and erosion,
with consequences on production, livelihood diversification and food security, especially
in coastal agriculture-dependent countries, such as Bangladesh [
66
]. Salinization is already
a major problem for traditional agriculture in deltas and low-lying island nations [67,68].
SLR may also affect tourism and recreation through impacts on landscapes
(e.g., beaches
),
cultural features and critical transportation infrastructures, such as harbors and airports [
69
].
Coastal areas’ future tourism and recreation attractiveness will, however, also depend on
changes in air temperature, seasonality and sea surface temperature. Although ocean
warming and acidification will be more influential in global fisheries and aquaculture,
sea-level rise may produce indirect effects through adverse effects in habitats or facilities.
5. Responses to the Inevitable and Accelerating Rise in Sea Level and Coastal Hazards
With the thousands of kilometers of developed beaches, dunes, barrier islands, bluff
and cliff tops around the planet, virtually every coastal nation has all types of developments,
whether private or public, whether new or old, and land threatened by erosion and/or
flooding in the decades ahead. What can or should be done with the communities and
cities, homes and hotels, streets and parking lots, airports and power plants, wastewater
treatment plants, pump stations and transmission lines or other infrastructure built on the
beach or at the edge of a cliff or bluff? This challenge has affected and will increasingly
affect nearly every coastal community on Earth and will only become more acute and costly
over time [70].
There are a limited number of options, however, and all come with some costs,
benefits and impacts. Depending on the location, some of these may require successfully
navigating and negotiating through a complex, expensive and time-consuming permitting
and environmental review process. Future losses will be high. The threat from future
sea-level rise to coastal cities and low-lying areas around the world, combined with the
storms, erosion and inundation, and the rapid degradation of natural coastal systems, will
be one of the major societal and infrastructure challenges of this century [
18
,
22
,
43
,
71
–
73
].
Threatening levels of sea-level rise, however, are a longer-term issue, at least for now, but
require mainstreaming adequate planning and rethinking of how coastal communities plan
for new development in coastal areas as well as existing development, manage ecosystems
and other coastal resources and prepare action to mitigate the impact of existing hazards,
such as El Niño, hurricanes or tsunamis.
Throughout the 20th century, developed coastlines around the world have been
responding to the hazards of shoreline flooding and coastal erosion or shoreline retreat in
several ways:
•Do nothing (or wait and see);
•Beach nourishment or adding sand to beaches;
•
Preventive actions in order to maintain the shoreline (i.e., hold the line) through either
soft or hard solutions that may include armoring or hardening the shoreline;
•Managed or unmanaged retreat or realignment;
•Regulatory and restriction options on new development.
Each of these options has its positives and negatives and different geographic ar-
eas, political entities, communities, cities, states or nations have either intentionally or
Water 2021,13, 2151 12 of 26
unintentionally made decisions to use one or several approaches as short to intermediate
term responses.
5.1. Do Nothing (or Wait and See)
This strategy may have the lowest cost upfront, but also the greatest risk of potential
consequences. Whereas it is difficult to predict when any particular structure built on the
back beach or on the edge of an eroding bluff will be inundated, damaged or collapse,
doing nothing almost guarantees that the day will come when it’s too late and damage or
complete loss will result. This approach incurs no costs until a major event finally does
occur, which usually cannot be predicted very far in advance, and then the losses may
be high or catastrophic as last-minute protection might not be permissible, possible or
effective. Depending on the setback of a particular structure from the shoreline or bluff
edge, its elevation relative to sea level and wave runup (also known as freeboard), age or
condition, past erosion or flooding problems, maintenance level and the future actions from
local sea-level rise and storm impacts (which may exceed their original design conditions),
this approach may work for a limited period of time.
5.2. Beach Nourishment
Approximately 80% to 90% of the sandy beaches along the U.S. Atlantic and Gulf
coasts are experiencing erosion, with rates averaging 0.6 m per year (Heinz Center, 2000).
While many factors contribute to shoreline recession, sea-level rise is the underlying factor
accounting for the nearly ubiquitous coastal retreat [
74
]. This land loss has enormous
economic impacts because some of the most expensive real estate in the United States is
beachfront property.
One approach used for temporarily forestalling shoreline retreat or beach erosion
is to artificially widen a beach with sand from some outside source, usually from the
offshore continental shelf. Beach nourishment is usually carried out as very large-scale
projects, where thousands of meters of shoreline are at least temporarily widened with
thousands or millions of cubic meters of sand. Restoring beaches through beach nourish-
ment can greatly increase their attractiveness to tourists [
75
]. Beach nourishment has been
employed for decades along the low relief, typically barrier island-backed sandy shorelines
of the Atlantic and Gulf coasts of the U.S. Over 1.35 billion m
3
of sand has nourished
the beaches of 475 U.S. communities since 1923, at a 2020 real cost of USD 10.8 billion
(https://beachnourishment.wcu.edu/, accessed on 1 August 2021)
. While several states
have long-term beach management plans, the great majority of the funding for placing
sand on beaches comes from the federal government through the Army Corps of Engineers.
Whether New Jersey, New York or Florida (USA), while literally billions of federal
dollars have been spent moving sand from offshore to the shoreline for both recreational
and shoreline protection benefits, the lifespan of the sand added artificially to these beaches
in many cases has been relatively short, and in some instances has been less than a year.
For some perspective on lifespans of individual nourishment projects, Florida (USA) has
15 beaches that have each been nourished 15 or more times, and Palm Beach has been nour-
ished 51 different times (https://beachnourishment.wcu.edu/ accessed on
1 June 2021
).
It is clear that beach nourishment should not be seen as a permanent or even long-term
solution to beach or bluff erosion, but simply buys a little more time at great
public expense.
However, beach restoration can have important benefits too. The resurgence of
Miami Beach, Florida, was largely attributed to saving the Art Deco architecture in South
Beach, which was a significant achievement, but beach restoration was paramount to the
recovery [
76
]. Beach nourishment in the late 1970s and early 1980s rejuvenated Miami
Beach, which brought back the visitors and hence the economy [
76
,
77
]. The massive beach
nourishment project, which was the largest such project of its kind undertaken in the
world at that time, cost USD 51 million. White coral sand was pumped from deposits
a few
kilometers offshore at the cost of only a few dollars per cubic meter. Miami Beach
was widened by approximately 60 m along the 16 km of barrier shoreline. This beach
Water 2021,13, 2151 13 of 26
nourishment project is often cited as the most successful beach restoration in the USA
because of its longevity and positive economic impact. Most recently, Miami is investing
USD 16 million in fresh sand to push back against erosion, whilealso maintaining adune belt.
The economic benefits of these projects are clear but face challenges from the costly maintenance,
the duration and the rising threats from sea levels and storm action (Figure 10).
Water 2021, 13, x FOR PEER REVIEW 14 of 28
Figure 10. The U.S. Army Corps of Engineers dumps new sand from Central Florida (USA) along
the Miami Beach shoreline. Source: U.S. Army Corps of Engineers.
In the USA, six states account for over 83% of the total volume of sand placed on
beaches: California, Florida, New Jersey, North Carolina, New York and Louisiana [78].
The largest recipients have been Florida, New York and New Jersey, who have received
approximately 500 million m3 of sand since the 1930s, much of this funded by the federal
government. Adding in the remaining Atlantic and Gulf Coast states, the amount of sand
dredged from offshore and dumped on the beaches totals approximately 1.3 million m3
(ASBPA). This is a difficult volume of sand to visualize but is enough sand to build a beach
50 m wide, 3 m deep and 8667 km long, or a beach extending all the way down the Atlantic
seaboard from Maine around the southern tip of Florida and west across the entire state
of Texas and beyond. Much of this sand has been moved around at federal expense and
in recent years, with federal budgets being stressed, these projects have been more diffi-
cult to fund.
Overall, beach nourishment remains controversial. Shorefront cities and develop-
ment have come to rely upon continuing projects funded by the federal government to
restore and maintain beach values. While maintaining the beach is essential for recreation,
the storm protection of property and beach-dependent economic activities, re-nourish-
ment projects are a costly intervention, paid by taxpayer money and maintaining high real
estate interests for those who live by the beach, which can be considered a form of subsidy
to the wealthy [79].
Traditional approaches to beach nourishment that merely add sand have also had
important limitations in effectiveness. Beach replenishment success and environmental
impacts may arise when one or more of the following factors is lacking: (1) a realistic as-
sessment of potential borrow area sand volume; (2) compatibility of added sand to the
beach being nourished, (3) construction costs; (4) the vulnerable geomorphic elements of
Figure 10.
The U.S. Army Corps of Engineers dumps new sand from Central Florida (USA) along
the Miami Beach shoreline. Source: U.S. Army Corps of Engineers.
In the USA, six states account for over 83% of the total volume of sand placed on
beaches: California, Florida, New Jersey, North Carolina, New York and Louisiana [
78
].
The largest recipients have been Florida, New York and New Jersey, who have received
approximately 500 million m
3
of sand since the 1930s, much of this funded by the federal
government. Adding in the remaining Atlantic and Gulf Coast states, the amount of
sand dredged from offshore and dumped on the beaches totals approximately 1.3 million
m
3
(ASBPA). This is a difficult volume of sand to visualize but is enough sand to build
a beach
50 m wide, 3 m deep and 8667 km long, or
a beach
extending all the way down the
Atlantic seaboard from Maine around the southern tip of Florida and west across the entire
state of Texas and beyond. Much of this sand has been moved around at federal expense
and in recent years, with federal budgets being stressed, these projects have been more
difficult to fund.
Overall, beach nourishment remains controversial. Shorefront cities and development
have come to rely upon continuing projects funded by the federal government to restore
and maintain beach values. While maintaining the beach is essential for recreation, the
storm protection of property and beach-dependent economic activities, re-nourishment
projects are a costly intervention, paid by taxpayer money and maintaining high real estate
Water 2021,13, 2151 14 of 26
interests for those who live by the beach, which can be considered a form of subsidy to
the wealthy [79].
Traditional approaches to beach nourishment that merely add sand have also had
important limitations in effectiveness. Beach replenishment success and environmental
impacts may arise when one or more of the following factors is lacking: (1) a realistic
assessment of potential borrow area sand volume; (2) compatibility of added sand to the
beach being nourished, (3) construction costs; (4) the vulnerable geomorphic elements of
the coastal zone; and (5) environmental impacts. Additionally, pre- and post-replenishment
monitoring studies have frequently been inadequate in answering the questions of envi-
ronmental impacts [
80
,
81
]. The ecological consequences of beach replenishment can also
be significant [
82
]. In order to be effective, beach nourishment needs to be combined with
sediment management techniques; ideally sand retention efforts [
83
,
84
], whether groins
or some other mechanism to hold the sand in place so that it survives for a longer period
of time and avoids frequent and costly sand feeding cycles. This requires understanding
and characterizing the historic causes of erosion, either episodic or chronic, the long-term
littoral drift rates and directions, and the natural processes [63,85].
With sea-level rise and increased wave action, beach nourishment will also need
to be combined with other options, including adequate setback zones that can naturally
nourish the system. Although the Army Corps of Engineers, as well as many local beach
communities and coastal organizations, continue to put forward beach replenishment as
a “soft”
solution to coastal erosion or beach loss, other lines of evidence indicate that beach
replenishment (alone) may not be
a sustainable
strategy in the long term to mitigate climate
change [
86
]. For example, projections in California indicate that beach replenishment will
only marginally delay the long-term inevitable loss of southern California beaches due to
sea-level rise [38] (Figure 11).
Water 2021, 13, x FOR PEER REVIEW 15 of 28
the coastal zone; and (5) environmental impacts. Additionally, pre- and post-replenish-
ment monitoring studies have frequently been inadequate in answering the questions of
environmental impacts [80,81]. The ecological consequences of beach replenishment can
also be significant [82]. In order to be effective, beach nourishment needs to be combined
with sediment management techniques; ideally sand retention efforts [83,84], whether
groins or some other mechanism to hold the sand in place so that it survives for a longer
period of time and avoids frequent and costly sand feeding cycles. This requires under-
standing and characterizing the historic causes of erosion, either episodic or chronic, the
long-term littoral drift rates and directions, and the natural processes [63,85].
With sea-level rise and increased wave action, beach nourishment will also need to
be combined with other options, including adequate setback zones that can naturally
nourish the system. Although the Army Corps of Engineers, as well as many local beach
communities and coastal organizations, continue to put forward beach replenishment as
a “soft” solution to coastal erosion or beach loss, other lines of evidence indicate that beach
replenishment (alone) may not be a sustainable strategy in the long term to mitigate cli-
mate change [86]. For example, projections in California indicate that beach replenishment
will only marginally delay the long-term inevitable loss of southern California beaches
due to sea-level rise [38] (Figure 11).
Figure 11. A number of beaches on the northern San Diego County coastline, California, have been
nourished twice with a total of 2.7 million m3 of sand at a total cost of USD 46 million. Beach surveys
showed most of the sand had moved downcoast with littoral drift with a year or two (Courtesy:
SANDAG Regional Beach Sand Project).
5.3. Armoring or Hardening the Shoreline
Whether rock revetments, seawalls, levees or floodwalls, or any of a variety of other
engineered or non-engineered structures, hardening or armoring the shoreline has been
the most common historical approach to coastal erosion, shoreline retreat or flooding.
These solutions aim to protect the shore by defending against elevated water levels or
wave impacts. There is a long global history of coastal armoring, but in most cases, these
structures were not built with their potential impacts to the surrounding environment and
shoreline in mind. The potential effects of hardening the shoreline include visual impacts;
loss of public beach due to placement of the structure on the beach; loss of the sand pre-
viously provided by the eroding cliff, bluff or dune being armored; passive erosion or the
Figure 11.
A number of beaches on the northern San Diego County coastline, California, have been
nourished twice with a total of 2.7 million m
3
of sand at a total cost of USD 46 million. Beach surveys
showed most of the sand had moved downcoast with littoral drift with a year or two (Courtesy:
SANDAG Regional Beach Sand Project).
5.3. Armoring or Hardening the Shoreline
Whether rock revetments, seawalls, levees or floodwalls, or any of a variety of other
engineered or non-engineered structures, hardening or armoring the shoreline has been
Water 2021,13, 2151 15 of 26
the most common historical approach to coastal erosion, shoreline retreat or flooding.
These solutions aim to protect the shore by defending against elevated water levels or
wave impacts. There is a long global history of coastal armoring, but in most cases, these
structures were not built with their potential impacts to the surrounding environment
and shoreline in mind. The potential effects of hardening the shoreline include visual
impacts; loss of public beach due to placement of the structure on the beach; loss of
the sand previously provided by the eroding cliff, bluff or dune being armored; passive
erosion or the gradual loss of the beach fronting the armor with a continuing rise in sea
level [39] (Figure 12).
Water 2021, 13, x FOR PEER REVIEW 16 of 28
gradual loss of the beach fronting the armor with a continuing rise in sea level [39] (Figure
12).
Figure 12. Passive erosion (loss of beach) in front of a rock revetment in central Monterey Bay, Cal-
ifornia, while a beach continues to exist on either side where there is no armor. Courtesy: California
Coastal Records Project.
It is important to understand that coastal armoring (including seawalls and revet-
ments) protects what is behind the armor, at the cost of the fronting beach. Combating
erosion with a hard structure parallel to the shoreline is a choice to not protect the beach
at that location. It is only a matter of time before beaches in front of hard armoring struc-
tures will be flooded or disappear with a rising ocean [39] Figure 12). Structures also pro-
duce shoreline encroachment and coastal habitat squeeze [24]. Coastal development and
coastal armoring present physical barriers for the natural inland migration of coastal hab-
itats, and changes in hydrological connectivity reduce sediment inputs and the potential
for vertical accretion. Encroachment is also a term used to describe the advancement of
structures, roads, buildings and other developments into natural areas, including the
shoreline and the buffers around these areas. The term encroachment encompasses the
placement of fill, the removal of vegetation or an alteration of the natural topography.
This causes impacts to the functions and values of natural areas, such as water quality,
loss of habitat (both aquatic and terrestrial), loss of flood attenuation potential or modifi-
cations in ecological processes.
However, there are many locations where coastal structures have historically been
deemed necessary to protect critical infrastructure and other important assets. They will
no doubt be used in the future to protect high value coastal infrastructure (large interna-
tional airports, for example) that will be prioritized for protection for as long as possible.
For many other oceanfront development locations, however, hard armoring structures
will eventually become increasingly impractical, costly, unaffordable or unacceptable. The
effectiveness of existing armoring will vary depending upon the age, engineering, foun-
dation depth, height and lateral extent of the individual seawall or revetment, as well as
exposure to wave energy and elevated water levels. Overtopping of current armoring
structures during moderate to extreme events may demonstrate the need for existing sea-
walls and rock revetments to be engineered to stand taller. As proposed seawalls become
Figure 12.
Passive erosion (loss of beach) in front of a rock revetment in central Monterey Bay,
California, while a beach continues to exist on either side where there is no armor. Courtesy:
California Coastal Records Project.
It is important to understand that coastal armoring (including seawalls and revet-
ments) protects what is behind the armor, at the cost of the fronting beach. Combating
erosion with a hard structure parallel to the shoreline is a choice to not protect the beach at
that location. It is only a matter of time before beaches in front of hard armoring structures
will be flooded or disappear with a rising ocean [
39
] Figure 12). Structures also produce
shoreline encroachment and coastal habitat squeeze [
24
]. Coastal development and coastal
armoring present physical barriers for the natural inland migration of coastal habitats, and
changes in hydrological connectivity reduce sediment inputs and the potential for vertical
accretion. Encroachment is also a term used to describe the advancement of structures,
roads, buildings and other developments into natural areas, including the shoreline and
the buffers around these areas. The term encroachment encompasses the placement of fill,
the removal of vegetation or an alteration of the natural topography. This causes impacts to
the functions and values of natural areas, such as water quality, loss of habitat (both aquatic
and terrestrial), loss of flood attenuation potential or modifications in
ecological processes.
However, there are many locations where coastal structures have historically been
deemed necessary to protect critical infrastructure and other important assets. They will no
doubt be used in the future to protect high value coastal infrastructure (large international
airports, for example) that will be prioritized for protection for as long as possible. For many
other oceanfront development locations, however, hard armoring structures will eventually
become increasingly impractical, costly, unaffordable or unacceptable. The effectiveness
Water 2021,13, 2151 16 of 26
of existing armoring will vary depending upon the age, engineering, foundation depth,
height and lateral extent of the individual seawall or revetment, as well as exposure to
wave energy and elevated water levels. Overtopping of current armoring structures during
moderate to extreme events may demonstrate the need for existing seawalls and rock
revetments to be engineered to stand taller. As proposed seawalls become larger and
stronger, this will inevitably bring new concerns and conflict in the regulatory process
concerning increased environmental impacts.
In the USA, some coastal states have essentially banned any new hard structures
altogether, while others have made it more and more difficult to obtain a permit unless
the primary structure (a house for example) is under imminent threat. The era of routine
armoring of any eroding stretch of coastline in the United States is ending, as the negative
impacts of protective structures have been increasingly documented, recognized and
understood and the inevitability of future sea-level rise has become more obvious [
40
,
84
].
While armor can provide short- or intermediate-term protection for private property
and public infrastructure, with a changing climate and a rising sea, there are no future
guarantees that today’s armor will survive far into the future. A review of research and
experience also demonstrate that there exist a range of financial, policy, planning and
management tools, often used for different purposes, that can be readily implemented or
modified to address coastal squeeze and enable inland habitat migration. Awareness of
approaches/solutions can assist in accommodating the migration of habitats as a necessary
component of coastal management in an era of increasing rates of sea-level rise.
5.4. Soft Protection Approaches and Working with Natural Processes Rather Than against Them
The rising costs of coastal armoring and its intrinsic challenges in a changing climate
has driven an increasing interest in other soft approaches. These approaches include
a growing
number of ‘engineering’ and ’building with nature’ initiatives that leverage nat-
ural ecosystems for their coastal protection value [
87
–
90
]. Examples of “soft” approaches
are increasing and include managed sediment relocation and beach and dune restora-
tion with plants and landscape reshaping, as well as other living shorelines alternatives
(
e.g., oyster reefs
). For example, the Spanjaards Duin in the Netherlands is one of the
first examples of constructing artificial dunes in order to create natural dune habitats as
a compensation measure for port development. This project