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Engineering away our natural defenses: An analysis of shoreline hardening in the US


Abstract and Figures

Rapid coastal population growth and development are primary drivers of marine habitat degradation. Although shoreline hardening, a byproduct of development, can accelerate erosion and loss of beaches and tidal wetlands, it is a common practice globally. Here, we provided the first estimate of shoreline hardening along United States coasts and predicted where existing or future hardening may result in tidal wetland loss if coastal management changes are not made. Our analysis indicated that 22,842 km of continental U.S. shoreline, 14% of the total, has been hardened. We also considered how socioeconomic and physical factors relate to the pervasiveness of shoreline hardening and found that housing density, GDP, storms, and wave height were positively correlated with hardening. Over 50% of South Atlantic and Gulf Coast shorelines are fringed with tidal wetlands that could be threatened by hardening based on projected population growth, storm frequency, and a lack of shoreline hardening restrictions.
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© The Ecological Society of America
lthough coastal regions constitute less than 4% of the
Earth’s land area, coastal habitats (eg beaches and
tidal wetlands; Figure 1) rank among the most valuable
natural resources globally (MA 2005). Over one-third of
the human population lives within 100 km of a coastline
and development is increasing rapidly in most coastal
regions of the world (MA 2005). Coastal development is
vulnerable to damage and loss from erosion, including
ambient waves, flooding, storms, and sea-level rise (SLR)
(Peterson et al. 2008).
Historically, shoreline hardening has been a common
response to coastal erosion, storm risks, and SLR, particu-
larly in industrialized countries with large coastal popula-
tions, such as the US, the Netherlands, and Japan
(Dugan et al. 2011). Shoreline hardening or armoring is
defined as the construction or placement of vertical sea-
walls or bulkheads, sloped riprap (eg rocks) revetments,
groins, jetties, or breakwaters along a shoreline (Figure
1). Although humans have been armoring coastlines for
centuries, the extent and rate at which this occurs has
increased markedly since 1900, and the effects of armor-
ing on coastal ecosystem functions and services have only
recently begun to be evaluated (NRC 2007).
Hardening of shorelines by means of seawalls or bulk-
heads can steepen and shorten shallow intertidal habitat
over time (Dugan et al. 2008). The structures themselves
also provide less physically complex habitat as compared
with natural shorelines, so that hardened shorelines gen-
erally support fewer species (Figure 1; Seitz et al. 2006;
Gittman et al. in press). When constructed landward of
tidal wetlands, hard structures may also increase seaward
scour during storm events and can prevent upslope migra-
tion of tidal wetlands as sea level rises, leading to their
eventual loss (termed “coastal squeeze”; Doody 2004).
Despite its adverse ecological effects, efforts to under-
stand the underlying causes and rates of shoreline armor-
ing have been limited (Dugan et al. 2011). Filling these
data gaps could help determine where and how much
shoreline is at risk. More specifically, identifying shore-
lines with tidal wetlands that are threatened with harden-
ing will help coastal managers to adopt more stringent
regulations for shoreline armoring that could prevent
future wetland losses. The purpose of this study was to:
(1) catalog tidal shoreline hardening in all continental
US coastal counties; (2) determine which physical and
socioeconomic features of a coastal region may be more
commonly associated with armoring; (3) identify regions
of the US likely to experience continued hardening and
subsequent coastal habitat loss; and (4) identify alterna-
tive management strategies for coastal protection.
We selected the US for our analysis because of its exten-
sive coastline, high coastal population density (39% of
the population lives in coastal counties), and data avail-
ability, as well as its vulnerability to shoreline erosion,
flooding, and property damage (MA 2005; NOAA 2013).
Furthermore, in the US, there is growing interest in
developing a national coastal protection and adaptation
Engineering away our natural defenses: an
analysis of shoreline hardening in the US
Rachel K Gittman1*†, F Joel Fodrie1, Alyssa M Popowich2, Danielle A Keller1, John F Bruno3, Carolyn A
Currin4, Charles H Peterson1, and Michael F Piehler1
Rapid population growth and coastal development are primary drivers of marine habitat degradation. Although
shoreline hardening or armoring (the addition of concrete structures such as seawalls, jetties, and groins), a
byproduct of development, can accelerate erosion and loss of beaches and tidal wetlands, it is a common practice
globally. Here, we provide the first estimate of shoreline hardening along US Pacific, Atlantic, and Gulf of Mexico
coasts and predict where future armoring may result in tidal wetland loss if coastal management practices
remain unchanged. Our analysis indicates that 22 842 km of continental US shoreline – approximately 14% of
the total US coastline – has been armored. We also consider how socioeconomic and physical factors relate to the
pervasiveness of shoreline armoring and show that housing density, gross domestic product, storms, and wave
height are positively correlated with hardening. Over 50% of South Atlantic and Gulf of Mexico coasts are fringed
with tidal wetlands that could be threatened by future hardening, based on projected population growth, storm
frequency, and an absence of coastal development restrictions.
Front Ecol Environ 2015; 13(6): 301–307, doi:10.1890/150065
1Institute of Marine Sciences, University of North Carolina at
Chapel Hill, Morehead City, NC; current address: Marine Science
Center, Northeastern University, Nahant, MA *(r.gittman@; 2US Coast Guard, Portsmouth, VA; 3Department of
Biology, University of North Carolina at Chapel Hill, Chapel Hill,
NC; 4Center for Coastal Fisheries and Habitat Research, National
Oceanographic and Atmospheric Administration, Beaufort, NC
Shoreline hardening in the US RK Gittman et al.
302 © The Ecological Society of America
framework in response to changing climatic conditions
(Peterson et al. 2008; Arkema et al. 2013).
Estimation of shoreline hardening along the US
We used the National Oceanic and Atmospheric
Administration (NOAA) Office of Response and
Restoration’s Environmental Sensitivity Index (ESI) geo-
databases to calculate the linear kilometers of total shore-
line and kilometers of hardened shoreline for each coastal
county within the continental US (NOAA 2005). The
ESI identifies 15 shoreline types (eg Type 1: exposed
rocky shore or seawall) that are further subdivided into
more specific shoreline types (eg Type 1A: exposed rocky
shores; Type 1B: exposed, solid, man-made structures [sea-
walls]; Figure 1; WebTable 1). We grouped all man-made
structures (seawalls, bulkheads, riprap structures [revet-
ments, breakwaters, groins/jetties], and hybrid
seawall/bulkheads with riprap) to calculate the cumula-
tive lengths of hardened shoreline (Figure 1; WebTable 1).
We divided each state’s ESI shoreline by coastal county
and, for the Pacific and Atlantic coasts, by whether the
shoreline was “open” (ie directly exposed to the ocean) or
“sheltered” (ie located in a bay, sound, lagoon, or tidally
influenced river). We did not divide the Gulf of Mexico
coast into “open” or “sheltered” categories because much
of the Gulf coastline (eg the Louisiana coastline, the Big
Bend region of Florida) consists of reticulated wetlands
that cannot be easily classified in this way. We then calcu-
lated the length (in kilometers) of tidal shoreline (total
and armored), as well as the percentage of hardened shore
for each county. Additionally, we calculated the length of
tidal wetland shoreline (total and armored).
Regression tree analyses
To evaluate the relationship between potential drivers and
the percentage of hardened shoreline in each US coastal
county, we considered the following factors: 2010 housing
density (units per square kilometer), 2010 gross domestic
product (GDP, expressed in US dollars), coastal slope (%),
accretion/erosion rates (meters per year [m yr–1]), geomor-
phology, mean tidal range (m), mean wave height (m),
relative SLR (millimeters per year [mm yr–1]), storm fre-
quency between the years 1970–2010, relative county
shoreline position (north to south or west to east along
the coast), and years since a ban on shoreline hardening
was passed (sources included: the US Census Bureau, the
National Ocean Economics Program, the US Geological
Survey Coastal Vulnerability Index [USGS CVI] [for a
description of the variables, see Hammer-Klose and
Thiehler 2001], the Federal Emergency Management
Agency’s US Presidential Major Disaster and/or
Emergency Declarations [FEMA 2014], and federal and
state legislation and permitting procedures).
We ran separate regression trees for the Atlantic open
and sheltered coasts, the Pacific open and sheltered coasts,
and the Gulf of Mexico coast, to describe differences
among county-level shoreline hardening patterns, vis-à-
vis repeated partitioning of county armoring percentages
into increasingly homogeneous groups based on serial,
bimodal splits among potential drivers of hardening
(De’ath and Fabricius 2000). Because USGS CVI data
were not available for the Pacific sheltered coastal coun-
Figure 1. Types of natural and artificially hardened shorelines
found in the US: (a) rocky shore; (b) beach; (c) tidal marsh;
(d) mangrove; (e) seawall; (f) riprap revetment; (g) bulkhead;
and (h) breakwater. For images of other shoreline types found in
the US, refer to the NOAA ESI shoreline types image gallery
(a–h) R Gittman
RK Gittman et al. Shoreline hardening in the US
© The Ecological Society of America
ties, the regression tree for the Pacific sheltered coast does
not include these factors. We developed regression trees
using the analysis of variance (ANOVA) method of recur-
sive partitioning and we pruned over-fitted trees using k-
fold cross-validation. We ran all regression tree analyses
using R version 3.1.0 and rpart (Therneau et al. 2014).
The continental US was estimated to have 160 168 km of
tidal shoreline, of which 22 842 km (14%) was hardened
(WebTable 2). Brackish and salt marsh were the domi-
nant types of tidal wetland found along US coasts, mak-
ing up 48% of the total US shoreline (WebTable 3).
Approximately 1% (886 km) of existing tidal marsh
shoreline has been armored (eg a hard structure has been
built along and typically landward of the marsh)
(WebTable 3), although this does not account for marsh
already lost on the seaward side of man-made structures.
Shoreline hardening along US open coasts
Along the open coasts of the Atlantic and Pacific, 846 km
(9%) of the shoreline has been
hardened (Figures 2a and 3a).
Atlantic coastal counties that
experienced 17 or more storm
events between 1970 and 2010
(in Massachusetts, Maine, and
New Hampshire) had a higher
percentage of hardened shoreline
(µ = 30.7 ± 9.6%) than counties
that experienced fewer than 17
storms (µ = 8.3 ± 1.4%, R2=
0.25; Figure 2a). The percentage
of armoring along the Pacific
open coast was higher (µ = 24.1
± 2.8%) in counties where the
mean wave height was <1.3 m (ie
counties in southern California)
(Figure 3a). Counties with mean
wave heights 1.3 m and located
south of San Francisco County,
California, had more hardening
(µ = 11.0 ± 2.7%) than counties
north of San Mateo County,
California (µ = 2.4 ± 0.9%).
Mean wave height accounted for
70% of the variance in hardened
shoreline along the open Pacific
coast (R2= 0.80, full tree).
Shoreline hardening along US
sheltered coasts
Despite a considerable amount
of hardening along open coast-
Figure 2. The percentage of total tidal shoreline hardened by county and the regression
tree results for (a) the Atlantic open coast and (b) the Atlantic sheltered coast. “Housing
density” is the number of individual housing units per square kilometer (as defined by the
US Census Bureau), “Storms” are the total number of storms that resulted in a US
Presidential Major Disaster and/or Emergency Declaration from 1970 to 2010 (FEMA
2014), “Tide” is the mean tide range (m), “State” is the state in which the shoreline is
found, and “n” is the number of counties split into each node and used to calculate the
percentage of hardened shoreline. Error bars represent one standard error.
(a) Open coast
Hardened shoreline (%)
(b) Sheltered coast
Hardened shoreline (%)
Shoreline hardening in the US RK Gittman et al.
304 © The Ecological Society of America
lines, hardening was more prevalent on sheltered
Atlantic and Pacific coasts (14% or 14 607 km;
Figures 2b and 3b; WebTable 2). Atlantic coastal
counties with housing densities 658 units km–2
and counties in southern Florida with housing den-
sities 126 units km–2 had the highest percentages
of shoreline armoring (µ = 60.9 ± 4.8% and µ =
62.3 ± 14.5%, respectively; Figure 2b). When
housing densities were <126 units km–2 and in
counties outside South Florida, fewer storms and a
smaller mean tide range were associated with less
hardening. Housing density accounted for 41% of
the variation in hardened shoreline among
Atlantic sheltered coastal counties (R2= 0.72, full
tree). New Hampshire, Rhode Island, and
Connecticut had the highest percentage of hard-
ened marsh shoreline on the Atlantic coast (7%,
6%, and 4%, respectively; WebTable 3).
California had the highest percentage of sheltered
armored shoreline on the Pacific coast (28%) and
possessed four of the five counties with the greatest
amounts of hardening (Figure 3b; WebTable 3).
California was also characterized by the highest
percentage of hardened marsh shoreline (2%) on
the Pacific Coast; in San
Francisco County, for in-
stance, 71% of marsh shore-
line had been armored (Web-
Table 3). Counties with the
highest GDP ($8.95 ×
1010) along the Pacific shel-
tered coast also had the high-
est percentages of hardened
sheltered shorelines (µ = 56.7
± 10.4%; Figure 3b). When
GDP was < $8.95 × 1010, and
housing density was 52
units km–2, counties with
more frequent storms also
had higher percentages of
armoring (µ = 43.9 ± 6.0%)
than counties with the same
housing densities but with
fewer than 12 storms (Figure
3b). GDP and housing den-
sity accounted for 53% of the
variance in hardened shore-
line percentages along the
sheltered Pacific coast (R2=
0.69, full tree).
Shoreline hardening
along the US Gulf of
Mexico coast
The Gulf of Mexico coast
had the same percentage of
Figure 3. The percentage of total tidal shoreline hardened by county and the regression tree results for
(a) the Pacific open coast and (b) the Pacific sheltered coast. “Wave height” is the mean wave height
(m); “GDP” is the 2010 US gross domestic product (US$, as defined by the US Bureau of Economic
Analysis); and “Housing density”, “Storms”, “State”, “n”, and error bars are defined as in Figure 2.
(a) Open coast
Hardened shoreline (%)
(b) Sheltered coast
Hardened shoreline (%)
Wave height
RK Gittman et al. Shoreline hardening in the US
hardened shoreline (16%) as the entire Pacific coast (this
percentage drops to 9% when all of the highly reticulated
marsh shoreline of Louisiana is included) (WebTable 2;
Figure 4). Counties with higher housing densities (91 units
km–2, eg Orleans Parish, Louisiana; Harris County, Texas)
had higher percentages of armoring (µ = 41.7 ± 4.7%) than
counties with lower housing densities (Figure 4). When
GDP was < $3.09 × 109, Gulf counties in Texas had more
hardening (µ = 16.3 ± 2.5%) than Gulf counties outside of
Texas (µ = 3.7 ± 1.1%; Figure 4). Housing density alone par-
titioned 46% of the variation in the percentage of hardened
shoreline along the Gulf coast (R2= 0.56, full tree). Texas
had more than five times as much armored marsh shoreline
(6%) than other Gulf states (0.4–1.4%) (WebTable 3).
Our analysis indicates that 14% of the contiguous US shore-
line is hardened and that 64% of armoring has occurred
along Atlantic and Pacific sheltered shorelines, such as estu-
aries, lagoons, and tidally influenced rivers (WebTable 2;
Figures 2b and 3b). Thus, shoreline hardening is likely a sub-
stantial yet largely understudied means by which humans
modify and degrade coastal ecosystems in the US.
Potential drivers of shoreline hardening
Understanding the potential drivers of shoreline harden-
ing could help identify where and how much shoreline and
© The Ecological Society of America
associated habitats are at risk of being lost to this process in
the near future. Our analyses revealed that housing density
and GDP, respectively, were the best predictors of armoring
on US Atlantic and Pacific sheltered coasts, as well as
along the Gulf of Mexico coast (Figures 2–4). Globally,
man-made shoreline structures are associated with densely
populated coastlines (eg around the Mediterranean), and
have been used to protect both commercial and residential
development and infrastructure for generations (Dugan et
al. 2011). Most major US coastal metropolitan areas are
located on sheltered coasts and tend to be heavily armored
(eg New York, Los Angeles, New Orleans), regardless of
other physical shoreline characteristics or processes.
Coastal metropolitan areas also support development in
neighboring coastal counties (eg for the purposes of
industry and tourism); this likely contributes to the
spread of shoreline hardening, despite relatively low
housing densities in these adjacent areas (Figures 2b, 3b,
and 4; NOAA 2013). Specifically, these regions have a
history of coastal modification that includes dredging of
canals, to support shipping traffic from major ports (eg
New York, Corpus Christi, Miami) and for flood control
(eg South Florida canal systems), that probably con-
tributed to hardening (US Census Bureau 2010).
Outside of metropolitan areas, the addition of man-
made structures is more closely related to the vulnerability
of coastal developments to damage from physical processes
(eg storms, erosion). Along the Pacific open coast, the
rocky shorelines of northern California, Oregon, and
Figure 4. The percentage of total tidal shoreline hardened by county and the regression tree results for the Gulf of Mexico coast.
“Housing density”, “GDP”, “State”, “n”, and error bars are defined as in Figures 2 and 3.
Hardened shoreline (%)
Housing density
Shoreline hardening in the US RK Gittman et al.
Washington are associated with wave heights greater than
1.3 m, and are therefore not suitable for most types of
development; this may be the reason for the lesser degree
of armoring in these regions (Figure 3a). The regression
tree selected a single variable that reduced the most vari-
ance in shoreline hardening; however, wave height was
also strongly positively correlated with both county loca-
tion (R2= 0.82) and the number of years since a ban on
shoreline hardening was implemented (R2= 0.67).
Greater storm frequency was the most important predic-
tor of armoring on the Atlantic open coast (Figure 2a). Hard
structures are often built along coastlines in response to
damage and erosion from major storm events (eg seawall
construction in Galveston, Texas, following a major hurri-
cane; Hansen 2007); areas prone to major storms would
therefore be expected to have more hardened shoreline.
However, there is evidence that natural beach dune and
marsh shorelines experience less erosion and damage than
hardened shorelines during single storm events (Thieler
and Young 1991; Gittman et al. 2014). Given the uncer-
tainty associated with the performance of bulkheads and
seawalls during these storm events, the use of hard struc-
tures in response to storms should be evaluated further.
Given the strong relationships between both housing
density and GDP and shoreline hardening, other socio-
economic factors we did not include – such as commercial
and recreational shoreline uses, coastal land ownership
(public versus private), and coastal property values – may
provide additional insights into the patterns of observed
shoreline hardening in the US. Further refinement of the
relationship between socioeconomic factors and armoring
will allow policy makers and coastal-resource managers to
develop targeted strategies (eg tax breaks for waterfront
property owners that choose not to build a bulkhead or
seawall) for reducing shoreline hardening in areas with
vulnerable coastal habitats. These approaches may
become increasingly fine-tuned as we learn more about
how construction/repair costs or the cascading effects of
neighboring shoreline hardening (Scyphers et al. 2015)
drive decisions regarding shoreline maintenance.
Predictions for future hardening and habitat loss
Although European countries have been adding man-
made structures to their shorelines for centuries, in the
US, most of this construction likely occurred after 1900
(Dugan et al. 2011), making the rate of shoreline harden-
ing in the US about 200 km yr–1. If this rate remains con-
stant and coastal populations continue to increase
according to NOAA projections (NOAA 2013), the per-
centage of hardened shoreline will double by 2100, result-
ing in nearly one-third of the contiguous US coastline
being armored. This projected rate is probably conserva-
tive but assumes that no additional restrictions are placed
on shoreline hardening (only eight states have imple-
mented total or partial bans on construction of hard
structures along the shore).
Some of the largest increases in population density are
predicted to occur along the South Atlantic and Gulf
coasts, where most of the US’s remaining tidal salt
marshes (> 50%) and mangrove forests (100%) are cur-
rently found (WebTable 3; Kennish 2001). As much as
50% of US salt marsh has been lost over the past century,
largely as a result of human activities (Kennish 2001).
Although only a small percentage (1%) of existing tidal
wetlands is currently hardened, this percentage does not
account for the wetlands likely lost in the past to harden-
ing (WebTable 3). On the Atlantic coast, 60% of the
land between 0–1 m above current sea level is expected
to be developed and hardened, thereby resulting in a
large-scale coastal squeeze on tidal wetlands (Titus et al.
2009). Given the prevalence and ecological conse-
quences of shoreline hardening, steps should be taken to
reduce the rate of armoring and to implement alternative
stabilization strategies (eg submerged sills, marsh plant-
ing; see Gittman et al. in press).
Our assessment demonstrates that much of the US shore-
line is vulnerable to future habitat loss if actions are not
taken to revise current coastal management strategies.
Our estimate – that 14% of US tidal shoreline is hard-
ened – is conservative, as all types of tidal shoreline were
included in our analysis. When shorelines that are not
likely to be armored (eg naturally rocky shores) are
excluded, the majority of the natural shoreline most vul-
nerable to future hardening (eg beaches, wetlands) is
found along the Gulf of Mexico and South Atlantic
coasts. Given the projected increases in population for
these two regions, these coasts will likely experience the
highest rates of future hardening and the associated loss
of marsh and other vegetated intertidal habitat to coastal
squeeze (Doody 2004). This, in turn, will result in the loss
of critical coastal ecosystem services such as provision of
nursery habitat for commercially and recreationally valu-
able fish and crustaceans, filtration of nutrients and pol-
lutants from terrestrial runoff, carbon burial, and erosion
protection (Peterson et al. 2008).
Further analyses of the socioeconomic drivers of shore-
line hardening are needed to determine how to prevent
or reduce further destruction of sensitive habitats such as
tidal wetlands. Although our analysis provides baseline
estimates of armoring, continued updates will be required
to calculate region-specific rates. Coastal managers could
use these rates to assess the cumulative impacts on coastal
habitats and to assess the risk of future habitat loss. Policy
makers should also use these assessments to develop
informed legislation and regulations, including a revision
of the US Army Corps of Engineers’ policy on nationwide
permits to account for the future loss of habitat as a result
of coastal squeeze, which will likely extend well beyond
the construction footprint of a hard structure. Finally, we
recommend that new management guidelines be devel-
306 © The Ecological Society of America
RK Gittman et al. Shoreline hardening in the US
oped to incorporate green infrastructure and planning for
shoreline migration (eg rolling easements, bulkhead
removal) with SLR. Without substantial changes to
coastal management policies and development practices,
the US coastlines will likely lose their natural defenses.
We thank J Grabowski, T Rodriguez, and S Scyphers for
their guidance and advice. This research was funded by a
NOAA National Estuarine Research Reserve System
graduate fellowship to RKG, a US National Science
Foundation grant to FJF (NSF OCE-1155628), and the
University of North Carolina at Chapel Hill. The scien-
tific results and conclusions, as well as any views or opin-
ions expressed herein, are those of the author(s) and do
not necessarily reflect the views of NOAA or the US
Department of Commerce.
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... These social decisions (e.g., large-scale policies or individual level choices) can have long-lasting consequences for both the environment and society, especially as coastal development increases. Decisions that modify and change the biophysical nature of the environment (e.g., waterfront residents' decision to use artificial structures for storm protection and shoreline stabilization) impact its ecological functionality 7 . At the same time, these alterations may change the degree of connectivity that individual humans have to their environments, which might extend to broader societies' ecological knowledge 8,9 . ...
... (2015)'s county-level estimates of NOAA's Environmental Sensitivity Index (ESI) for the percent of armored (hardened) shorelines (see ref.7 for more details). ...
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Coastal ecosystems nearby human societies collectively shape complex social-ecological systems (SESs). These ecosystems support high levels of ecological biodiversity while providing resources and services to humans. However, shoreline armoring, land transformation, and urban homogenization across urbanized coastal areas may degrade natural ecosystems and alter how humans and nature are connected. We hypothesize that these alterations extend to residents’ knowledge of SESs. We explore evidence of such cognitive outcomes in graphical mental models of more than 1350 coastal residents across eight states in the Northeast United States. Our results revealed that, in more urbanized areas, residents’ mental models underrepresented complex interdependence between humans and natural components, indicating limited systems thinking. Additionally, urbanization and shoreline armoring were associated with homogenization of mental models. We refer to these results as Urbanized Knowledge Syndrome (UKS). Importantly, respondents with more symptoms of UKS were less likely to self-report adoption of pro-environmental behaviors. These results indicate a potential societal-level erosion of ecological knowledge associated with urbanization in the same way more urbanized areas are associated with diminishing ecological function. Thus, diagnosing and treating UKS is an essential component of urban sustainability.
... In most regions of the world, the economic and social benefits in coastal zones tend to increase when population grows together with industrial and recreational activities Dugan et al., 2011;Gittman et al., 2015). This attracts stakeholders with different perspectives and interests. ...
... In order to solve a predefined coastal erosion or flooding problem with a solution that is adapted to the specific socioecological context, a variety of scenarios must be considered in a multiphase process before the design and construction phases are undertaken (USACE, 2006). In most cases, decision-making has traditionally been limited to engineers, experts and scientists Sauvéet al., 2020), and have led to a high rate of shoreline artificiality worldwide, the majority consisting of hard coastal defense structures (Koike, 1996;Valloni et al., 2003;EEA, 2006;Gittman et al., 2015;Cooper et al., 2020;Sauveé t al., 2020). In the past decade, a trend reversal has been observed with the implementation of soft techniques like beach nourishment or vegetation Sauvéet al., (2022), ecological approaches (Morris et al., 2018;Morris et al., 2019) such as Engineering With Nature in the U.S.A. (Bridges et al., 2018) or Building With Nature in The Netherlands (de Vriend and Van Koningsveld, 2012;van Slobbe et al., 2013), and the use of ecological or socio-economic enhancements in the design of hard structures (Evans et al., 2017;Schoonees et al., 2019;Vuik et al., 2019). ...
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Coastal socio-ecological systems are complex adaptive systems with nonlinear changing properties and multi-scale dynamics. They are influenced by unpredictable coastal hazards accentuated by the effects of climate change, and they can quickly be altered if critical thresholds are crossed. Additional pressures come from coastal activities and development, both of which attracting stakeholders with different perspectives and interests. While coastal defence measures (CDMs) have been implemented to mitigate coastal hazards for centuries, a lack of knowledge and tools available to make informed decision has led to coastal managers favouring the choice of seawalls or rock armours with little consideration for socio-ecological systems features, and stakeholders’ priorities. Though it is not currently widely applied in coastal zone management, multicriteria decision analysis (MCDA) is a tool that can be useful to facilitate decision making. PROMETHEE, an outranking method, was chosen to support the multicriteria decision analysis for the evaluation of CDMs in the context of four study sites characterized by distinct environmental features. The aim was to determine the relevance and benefits of a MCDA by integrating coastal zone stakeholders in a participatory decision-making process in order to select CDMs that are better adapted to the whole socio-ecological system. First, in a series of five workshops, stakeholders were asked to identify and weigh criteria that were relevant to their local conditions. Second and third, CDMs were evaluated in relation to each criterion within the local context, then, hierarchized. Initial results show that vegetation came first in three of the four sites, while rock armour ranked first in the fourth site. A post-evaluation of the participatory process indicated that the weighting phase is an effective way to integrate local knowledge into the decision-making process, but the identification of criteria could be streamlined by the presentation of a predefined list from which participants could make a selection. This would ensure criteria that are standardized, and in a format that is compatible with the MCDA. Coupled with a participatory process MCDA proved to be a flexible methodology that can synthetize multiple aspects of the problem, and contribute in a meaningful way to the coastal engineering and management decision-making process.
... Meanwhile, sediment inputs from shoreline erosion in Chesapeake Bay are decreasing as the shoreline is gradually hardened by human development (Gittman et al., 2015;Halka et al., 2006;Hardaway and Byrne, 1999;Isdell, 2014;Patrick et al., 2014;. For sheltered coasts within the Chesapeake Bay and its tidal tributaries, approximately 25-50% of previously natural shorelines have been hardened, depending on location (Gittman et al., 2015;. ...
... Meanwhile, sediment inputs from shoreline erosion in Chesapeake Bay are decreasing as the shoreline is gradually hardened by human development (Gittman et al., 2015;Halka et al., 2006;Hardaway and Byrne, 1999;Isdell, 2014;Patrick et al., 2014;. For sheltered coasts within the Chesapeake Bay and its tidal tributaries, approximately 25-50% of previously natural shorelines have been hardened, depending on location (Gittman et al., 2015;. Regional shoreline erosion adds slightly more sediment to the Bay than the two largest rivers combined (Table S1), and in the mainstem mid-to lower-=Bay shoreline erosion is the largest single source of inorganic solids (Cerco et al., 2013). ...
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Water clarity is a key indicator of the ecosystem health in the Chesapeake Bay. Estuarine water clarity fluctuates due to external inputs from the watershed as well as processes occurring within the estuary itself, such as sediment resuspension and organic matter production. Therefore, water clarity requires study at multiple spatial and temporal scales and with multiple metrics. One local-scale process potentially influencing water clarity is shellfish aquaculture. One part of this dissertation examined how water quality and hydrodynamics varied among oyster farms as well as inside versus outside the extent of caged areas located in southern Chesapeake Bay. Current speed and water quality were measured within and adjacent to four oyster farms during two seasons. Results revealed minor effects of oyster farms on water quality, likely due to high background variability, relatively high flushing rates, relatively low oyster density, and small farm footprints. Minimal impacts overall suggest that low-density oyster farms located in adequately flushed areas are unlikely to negatively impact local water quality. At a larger spatial scale, another potential influence on water clarity is shoreline erosion. The second part of this dissertation examined the impact of shoreline erosion on water clarity via a numerical modeling study. Experiments were conducted to simulate realistic shoreline conditions representative of the early 2000s, increased shoreline erosion, and highly armored shorelines. Together, reduced shoreline erosion and the corresponding low seabed resuspension resulted in decreased concentrations of inorganic particles in surface waters, improving water clarity overall. However, clearer waters relaxed light limitation on phytoplankton, which often increased organic matter production, sometimes yielding opposite effects on water clarity according to different metrics. Clarity improved in mid-Bay central channel waters in terms of light attenuation depth, but simultaneously degraded in terms of Secchi depth because the resulting increase in organic matter decreased the water’s transparency. A final water clarity process considered was the long-term trend in water clarity from satellite remote sensing. The third part of this dissertation examined how remote sensing reflectance changed over time in Chesapeake Bay from 2002 to 2020 using the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on satellite Aqua. MODIS-Aqua remote sensing reflectance trends were evaluated from 2002 to 2020 at multiple wavelengths and spatial resolutions for surface waters of the Chesapeake Bay. Trends showed long-term decreasing reflectance in the upper estuary yet increasing reflectance in the lower estuary in the green wavelengths. Band ratios involving red-to-green and red-to-blue have decreased, suggesting improved water clarity, while green-to-blue ratios have increased over time, suggesting increasing contribution of phytoplankton to water cloudiness. Reflectance change over time relates well to observed decreases in total suspended solids and light attenuation, yet inconclusive trends in chlorophyll-a, suggesting a long-term change in particle properties such as size and composition that affect light scattering behavior.
... piers, pontoons, artificial reefs) and offshore activities (e.g. wind turbines, oil platforms) (Baine, 2001;Bugnot et al., 2020;Firth et al., 2016;Gittman et al., 2015). These artificial structures often support fewer species than natural hard substrate habitats and may facilitate bioinvasion, leading to changes in the composition of assemblages and biotic homogenization (Bulleri and Chapman, 2010;Chapman, 2003;Firth et al., 2016;Todd et al., 2019). ...
Marine artificial structures such as pilings are replacing natural habitats, and modifying surrounding areas, often resulting in local decreases in species diversity and facilitation of bioinvasion. Most research on the impacts of artificial structures in marine ecosystems has primarily focused on rocky bottom habitats and biodiversity, overlooking the effects of these structures on the functioning of nearby sedimentary habitats. Here we compared, for the first time, benthic metabolism (O2 fluxes) and sediment-water nutrient (inorganic nitrogen, phosphate, and dissolved organic nitrogen) fluxes in shallow water sediments adjacent to pilings and natural reefs. We also measured sediment properties (grain size, total organic carbon, total nitrogen, C:N ratio and chlorophyll-a content). We found that sediments near pilings were generally finer with greater C:N ratios than those near reefs, while differences in other sediment properties between types of habitats were dependent on the site. We found significant differences in the oxygen consumption, primary productivity, and net ecosystem metabolism in sediments around pilings compared to sediments near natural reefs, but these patterns differed by site. Net nutrient fluxes were similar in sediments near pilings and reefs at both sites. This study showed that although pilings can be associated with changes in the functioning of sedimentary habitats, patterns and the direction of change seem to vary depending on local conditions.
... Assessing tidal wetland vulnerability to loss is increasingly important due to recent escalations in relative sea level rise and coastal development and, consequently, the need for their strategic protection and management. Many coastlines have impediments that limit the landward migration of tidal wetlands (Gittman et al., 2015;Thorne et al., 2018;Titus et al., 2009), and therefore, their survival partly depends on their ability to maintain an elevation above sea level. Wetland elevations increase through the accumulation of sediment and organic material (Reed, 1995), and as sea level is predicted to increase up to 0.6 to 2.0 m by 2100 (Parris et al., 2012;Najjar et al., 2000), rates of material accumulation will have to escalate to limit coastal wetland loss particularly in areas with barriers to inland migration. ...
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Tidal wetlands in the Mid-Atlantic, USA, are experiencing high rates of relative sea level rise, and it is unclear whether they will be resilient in the face of future flooding increases. In a previous study, we found 80% of our study areas in tidal freshwater and salt marshes in the Delaware Estuary and Barnegat Bay had elevation change rates lower than the 19-year increase in mean sea level. Here, we examine relationships between marsh elevation dynamics and abiotic and biotic parameters in order to assess their utility as indicators of vulnerability to relative sea level rise. We further apply a range of marsh vulnerability indicators including elevation change rates to evaluate their ability to corroborate marsh habitat change over the last 30 years. Of the field measurements, soil bulk density and belowground plant biomass were among the strongest predictors of elevation change and accretion dynamics across all marsh types and settings. Both tidal freshwater and salt marshes tended to have higher rates of elevation increase and surface accretion in areas where soil bulk density and live belowground biomass were higher. Nine of the ten marshes experienced a net loss of area from the 1970s to 2015 ranging from 0.05 to 14%. Although tidal freshwater marshes were low in elevation and experienced variable elevation change rates, marsh area loss was low. Conversely, salt marshes closest to the coast and perched high in the tidal frame with a higher degree of human modification tended to experience the greatest marsh loss, which incorporated anthropogenic impacts and edge erosion. Thus, our regional assessment points to the need for a comprehensive understanding of factors that influence marsh resilience including human modifications and geomorphic settings.
... A number of assessments have been conducted to determine and quantify the extent of global coastal sprawl (Table 1). These include an assessment of the global extent of artificial residential canals in estuaries (Waltham and Connolly, 2011), mapping urban and industrial seascapes of the Great Barrier Reef, Australia (Waltham and Sheaves, 2015), determining the status, loss and trends of coastal habitats in Europe (Airoldi and Beck 2007), an assessment of shoreline hardening in North America (Gittman et al., 2015), and an assessment identifying and determining the distribution of artificial structures along the coast of Chile, South America (Aguilera, 2018). A recent survey of the global extent of marine structures concluded that 32 000 km 2 of engineering structures exist and estimated this will increase by 7400 km 2 by 2028 (Bugnot et al., 2020). ...
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Coastal ecosystems are increasingly being transformed from natural to artificial owing to increasing population growth, development pressures and impacts from climate change. With the threat from sea-level rise the increase in armouring of coastlines and other coastal defence structures is a particular concern. This study used Google Earth to define and map the extent of artificial structures along the coastline and within estuaries of South Africa. Infrastructure along the coastline was mostly concentrated around major cities with 85.71 km of armouring mapped along the entire coast and 51.74 km mapped within estuaries. Jetties were the dominant infrastructure type found within estuaries and armouring was the most dominant type along the coastline. Although large sections of the coast were found to be underdeveloped, hotspots around cities show that much of these areas are affected by infrastructure. In these areas in particular, haphazard and ad hoc development can have cumulative environmental impacts. As no extensive record of structures along the coastline of South Africa has been compiled, this study provides the first baseline inventory of the extent of infrastructure within the coastal environment of South Africa. This baseline can therefore be used to record and measure changes in infrastructure development of the coastal environment and guide future coastal development practises.
... It occurs over a full range of time scales, including short-term events such as waves, tides, and storms, and long-term changes due to sea-level rise (National Research Council, 2007). Shoreline hardening (e.g., bulkhead, revetments) has been the industry standard for controlling shoreline erosion problems (Gittman et al., 2015). Several studies have shown that the construction of erosion control structures (i.e., armoring or shoreline hardening) results in the decrease, and permanent loss in some cases, of living resources along impacted shorelines (e.g., Bilkovic and Roggero, 2008;Myszewski and Alber, 2016;Tavares et al., 2020). ...
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The Shoreline Management Model (SMM) is a novel geospatial approach used to assess conditions along a shoreline, and recommend best management practices for defended and undefended shorelines. The SMM models available spatial data in order to identify areas where the use of living shorelines would be suitable to address shoreline erosion. The model was developed to support and inform decision-making by shoreline managers responsible for management of shoreline resources, shorefront property owners, and tidal habitat restoration actions. Recommended erosion control strategies are based on scientific knowledge of how shorelines respond to natural conditions and anthropogenic measures used to stabilize shorelines. The SMM uses input variables representing current conditions and recommends a strategy that falls into one of three general categories: living shorelines, traditional approaches, and special considerations. Areas of special consideration are areas where the model may not be able to provide an appropriate recommendation due to ecological, geological, or highly developed conditions. These areas are given recommendations that include the instruction to seek expert advice. Data required to run the model include presence of tidal marsh, beach, submerged aquatic vegetation (SAV), riparian land cover, bank height, nearshore bathymetry, fetch, and shoreline erosion control structures. The model has been calibrated and validated along Virginia's Chesapeake Bay shoreline, USA. The model results are largely consistent with field recommendations (i.e., shoreline management recommendations made by scientists based on on-site observations during shoreline evaluation visits). The SMM performed with an overall accuracy of 82.5%. The SMM is exportable; the model code can be adapted to other systems. This geospatial model provides a robust screening tool for local and state governments, coastal and environmental planners and engineers, as well as property owners, when considering best management practices, including living shorelines as an alternative for erosion control.
... Coastal defence measures (CDMs) have been implemented to mitigate these hazards since the establishment of human societies along shorelines [1]. However, coastal erosion and flooding have been exacerbated in recent decades by the effects of climate change, sea-level rise [2][3][4][5], and anthropogenic activities [6][7][8], which have led to an increase in shoreline armouring worldwide [9][10][11][12]. ...
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The decision-making process of the coastal defence measures (CDMs) is complex and filled with uncertainties due to site-specific interactions between hydrodynamic and geomorphological conditions, which have repercussions on the ecological and social aspects of coastal communities. Scientific knowledge of the effects of CDMs contributes to the reduction in inherent uncer-tainties and facilitates the decision-making and design processes. The goal of this article is to present an algorithm designed to evaluate and hierarchize CDMs in relation to different coastal environments. Drawn from 411 published scientific case studies, a total of 1709 authors’ observation statements regarding the effects of CDMs on the study sites’ environmental features (type of coast, type of substrate, tidal range, and wave climate) were entered in a database, categorized, and weighted according to a qualitative scale. The algorithm processes the information by establishing a correspondence between user-selected environment features and those stocked in the database, and it evaluates user-selected CDMs in relation to the specified coastal characteris-tics by identifying, collating, and rating the effects as observed in similar contexts. The result is a tool able to process, structure, and concretize scientific knowledge regarding CDMs and their effects on coastal systems. It is complementary to existing tools currently used in the decision-making and design processes of the CDMs. The results present the hierarchization of CDMs according to a multilevel aggregated structure, which can be used in different ways by coastal managers, decision-makers, and engineers. The algorithm, based on standardized coastal characteristics, can be applied to any shoreline worldwide.
Living Shorelines (LS) refer to the combined use of man-made and natural materials to build a resilient and ecologically vibrant shore. LS are an emerging alternative to hardened shorelines (HS), which employ engineered structures to reinforce eroding shorelines. LS better protect coastlines against erosion and flooding, which are of increasing concern due to climate change and rising sea levels. New Jersey (NJ) is a leader in LS policy, but lack of knowledge regarding these structures hinders further LS implementation. Progress has been made to reduce regulatory hurdles for LS projects. However, decision-making power rests with many private property owners (PO) who default to familiar approaches, like HS. Therefore, we advise the NJ state legislature to encourage LS development by appropriating funds to the NJ Department of Environmental Protection or other relevant agencies to conduct an awareness campaign in key coastal communities. Additionally, PO can be incentivized to convert from HS to LS by restructuring the existing NJ Shoreline Protection Fund. This proactive intervention will provide environmental benefits, in addition to protecting the coastline of NJ.
Oceans play critical roles in the lives, economies, cultures, and nutrition of people globally, yet face increasing pressures from human activities that put those benefits at risk. To anticipate the future of the world's ocean, we review the many human activities that impose pressures on marine species and ecosystems, evaluating their impacts on marine life, the degree of scientific uncertainty in those assessments, and the expected trajectory over the next few decades. We highlight that fundamental research should prioritize areas of high potential impact and greater uncertainty about ecosystem vulnerability, such as emerging fisheries, organic chemical pollution, seabed mining, and the interactions of cumulative pressures, and deprioritize research on areas that demonstrate little impact or are well understood, such as plastic pollution and ship strikes to marine fauna. There remains hope for a productive and sustainable future ocean, but the window of opportunity for action is closing. Expected final online publication date for the Annual Review of Environment and Resources, Volume 47 is October 2022. Please see for revised estimates.
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During the past century, human modification of environmental systems has greatly accelerated tidal salt marsh deterioration and shoreline retreat in many coastal regions worldwide. As a result, more than 50% of the original tidal salt marsh habitat in the U.S. has been lost. Numerous human activities have contributed directly or indirectly to wetland loss and alteration at local, regional, and global scales. Human impacts at the local scale include those that directly modify or destroy salt marsh habitat such as dredging, spoil dumping, grid ditching, canal cutting, leveeing, and salt hay farming. Indirect impacts, which can be even more significant, typically are those that interfere with normal tidal flooding of the marsh surface, alter wetlands drainage, and reduce mineral sediment inputs and marsh vertical accretion rates. These impacts usually develop over a greater period of time. At the regional scale, subsidence caused by subsurface withdrawal of groundwater, oil, and gas has submerged and eliminated hundreds of square kilometers of salt marsh habitat in the Chesapeake Bay, San Francisco Bay, and Gulf of Mexico. At the global scale, atmospheric warming due to increased burden of anthropogenic greenhouse gases and tropospheric sulfate aerosols appears to be strongly coupled to glacial melting, thermal expansion of ocean waters, and eustatic sea-level rise. Changes in coastal water levels ascribable to eustatic sea-level rise pose a long-term threat to the stability and viability of these critically important coastal systems.
Coastal ecosystems provide numerous services, such as nutrient cycling, climate change amelioration, and habitat provision for commercially valuable organisms. Ecosystem functions and processes are modified by human activities locally and globally, with degradation of coastal ecosystems by development and climate change occurring at unprecedented rates. The demand for coastal defense strategies against storms and sea-level rise has increased with human population growth and development along coastlines worldwide, even while that population growth has reduced natural buffering of shorelines. Shoreline hardening, a common coastal defense strategy that includes the use of seawalls and bulkheads (vertical walls constructed of concrete, wood, vinyl, or steel), is resulting in a “coastal squeeze” on estuarine habitats. In contrast to hardening, living shorelines, which range from vegetation plantings to a combination of hard structures and plantings, can be deployed to restore or enhance multiple ecosystem services normally delivered by naturally vegetated shores. Although hundreds of living shoreline projects have been implemented in the United States alone, few studies have evaluated their effectiveness in sustaining or enhancing ecosystem services relative to naturally vegetated shorelines and hardened shorelines. We quantified the effectiveness of (1) sills with landward marsh (a type of living shoreline that combines marsh plantings with an offshore low-profile breakwater), (2) natural salt marsh shorelines (control marshes), and (3) unvegetated bulkheaded shores in providing habitat for fish and crustaceans (nekton). Sills supported higher abundances and species diversity of fishes than unvegetated habitat adjacent to bulkheads, and even control marshes. Sills also supported higher cover of filter-feeding bivalves (a food resource and refuge habitat for nekton) than bulkheads or control marshes. These ecosystem-service enhancements were detected on shores with sills three or more years after construction, but not before. Sills provide added structure and may provide better refuges from predation and greater opportunity to use available food resources for nekton than unvegetated bulkheaded shores or control marshes. Our study shows that unlike shoreline hardening, living shorelines can enhance some ecosystem services provided by marshes, such as provision of nursery habitat.
Pre- and post-storm aerial videotape surveys were made along 51 km of the barrier island coast of South Carolina from Garden City to Folly Beach. Before Hugo the shoreline from the beach landward to, and including the first row of development, was classified as dune field (45%), bulldozed dune ridge (25%), revetment (14%), bulkhead (12%), vegetated washover terrace (3%), and beach only (1%). After the storm, 80% of the shoreline was classified as washover sheet, and 5% as washover fan. The only areas that were not overwashed were sections of very high dune field (13%) and large bulldozed dune ridge (2%). Our most important observations can be summarized as follows. 1) Provided the dune field was not submerged, the minimum width of dune field required to survive Hurricane Hugo, was 30 m. 2) Two types of dunes survived the storm: those high enough to prevent being overwashed and wide enough to prevent being completely eroded. 3) All bulkheads and revetments were overtopped, and wave activity was carried inland to the first and succeeding rows of development. 4) Fifty percent of all buildings completely destroyed or removed from their foundations were fronted by a combination of dry beaches less than 3 m wide and dune fields less than 15 m wide. -from Authors
Rapidly growing populations and expanding development are intensifying pressures on coastal ecosystems. Sea-level rise and other predicted effects of climate change are expected to exert even greater pressures on coastal ecosystems, exacerbating erosion, degrading habitat, and accelerating shoreline retreat. Historically, society’s responses to threats from erosion and shoreline retreat have relied on armoring and other engineered coastal defenses. Despite widespread use on all types of shorelines, information about the ecological impacts of shoreline armoring is quite limited. Here we summarize existing knowledge on the effects of armoring structures on the biodiversity, productivity, structure, and function of coastal ecosystems.
Acting on the perception that they perform better for longer, most property owners in the United States choose hard engineered structures, such as bulkheads or riprap revetments, to protect estuarine shorelines from erosion. Less intrusive alternatives, specifically marsh plantings with and without sills, have the potential to better sustain marsh habitat and support its ecosystem services, yet their shoreline protection capabilities during storms have not been evaluated. In this study, the performances of alternative shoreline protection approaches during Hurricane Irene (Category 1 storm) were compared by 1) classifying resultant damage to shorelines with different types of shoreline protection in three NC coastal regions after Irene; and 2) quantifying shoreline erosion at marshes with and without sills in one NC region by using repeated measurements of marsh surface elevation and marsh vegetation stem density before and after Irene. In the central Outer Banks, NC, where the strongest sustained winds blew across the longest fetch; Irene damaged 76% of bulkheads surveyed, while no damage to other shoreline protection options was detected. Across marsh sites within 25 km of its landfall, Hurricane Irene had no effect on marsh surface elevations behind sills or along marsh shorelines without sills. Although Irene temporarily reduced marsh vegetation density at sites with and without sills, vegetation recovered to pre-hurricane levels within a year. Storm responses suggest that marshes with and without sills are more durable and may protect shorelines from erosion better than the bulkheads in a Category 1 storm. This study is the first to provide data on the shoreline protection capabilities of marshes with and without sills relative to bulkheads during a substantial storm event, and to articulate a research framework to assist in the development of comprehensive policies for climate change adaptation and sustainable management of estuarine shorelines and resources in U.S. and globally.