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Two prominent arctic coastal erosion mechanisms affect the coastal bluffs along the North Slope of Alaska. These include the niche erosion/block collapse mechanism and the bluff face thaw/slump mechanism. The niche erosion/block collapse erosion mechanism is dominant where there are few coarse sediments in the coastal bluffs, the elevation of the beach below the bluff is low, and there is frequent contact between the sea and the base of the bluff. In contrast, the bluff face thaw/slump mechanism is dominant where significant amounts of coarse sediment are present, the elevation of the beach is high, and contact between the sea and the bluff is infrequent. We show that a single geologic parameter, coarse sediment areal density, is predictive of the dominant erosion mechanism and is somewhat predictive of coastal erosion rates. The coarse sediment areal density is the dry mass (g) of coarse sediment (sand and gravel) per horizontal area (cm ² ) in the coastal bluff. It accounts for bluff height and the density of coarse material in the bluff. When the areal density exceeds 120 g cm ⁻² , the bluff face thaw/slump mechanism is dominant. When the areal density is below 80 g cm ⁻² , niche erosion/block collapse is dominant. Coarse sediment areal density also controls the coastal erosion rate to some extent. For the sites studied and using erosion rates for the 1980–2000 period, when the sediment areal density exceeds 120 g cm ⁻² , the average erosion rate is low or 0.34 ± 0.92 m/yr. For sediment areal density values less than 80 g cm ⁻² , the average erosion rate is higher or 2.1 ± 1.5 m/yr.
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Geologic Controls on Erosion
Mechanism on the Alaska Beaufort
Coast
Thomas M. Ravens
1
*
and Sasha Peterson
2
1
Department of Civil Engineering, University of Alaska Anchorage, Anchorage, AK, United States,
2
Department of Geological
Sciences, University of Alaska Anchorage, Anchorage, AK, United States
Two prominent arctic coastal erosion mechanisms affect the coastal bluffs along the North
Slope of Alaska. These include the niche erosion/block collapse mechanism and the bluff
face thaw/slump mechanism. The niche erosion/block collapse erosion mechanism is
dominant where there are few coarse sediments in the coastal bluffs, the elevation of the
beach below the bluff is low, and there is frequent contact between the sea and the base of
the bluff. In contrast, the bluff face thaw/slump mechanism is dominant where signicant
amounts of coarse sediment are present, the elevation of the beach is high, and contact
between the sea and the bluff is infrequent. We show that a single geologic parameter,
coarse sediment areal density, is predictive of the dominant erosion mechanism and is
somewhat predictive of coastal erosion rates. The coarse sediment areal density is the dry
mass (g) of coarse sediment (sand and gravel) per horizontal area (cm
2
) in the coastal bluff.
It accounts for bluff height and the density of coarse material in the bluff. When the areal
density exceeds 120 g cm
2
, the bluff face thaw/slump mechanism is dominant. When the
areal density is below 80 g cm
2
, niche erosion/block collapse is dominant. Coarse
sediment areal density also controls the coastal erosion rate to some extent. For the
sites studied and using erosion rates for the 19802000 period, when the sediment areal
density exceeds 120 g cm
2
, the average erosion rate is low or 0.34 ±0.92 m/yr. For
sediment areal density values less than 80 g cm
2
, the average erosion rate is higher or
2.1 ±1.5 m/yr.
Keywords: arctic, coastal erosion, mechanism, coarse sediment, areal density
INTRODUCTION
The Arctic is experiencing high and accelerating coastal erosion rates. For example, Mars and
Houseknecht (2007) used remote sensing techniques to study coastal erosion-derived land loss on a
60-km segment of the Beaufort Sea coast (between Drew Point and Cape Halkett, Alaska, Figure 1)
and found that the amount of land loss was signicantly greater in 19852005 (1.08 km
2
yr
1
) relative
to the loss in 19551985 (0.48 km
2
yr
1
). Jones et al. (2009) working in the same area determined that
the average rate of erosion increased from 6.8 m yr
1
(19551979), to 8.7 m yr
1
(19792002), and to
13.6 m yr
1
(20022007). Erosion rates are high in this location because of the high ice content of the
coastal bluffs and the absence of coarse material (sand and gravel). At other locations, erosion rates
are often lower but still accelerating. For example, on Barter Island, where coastal bluffs contain
signicant amounts of coarse material, bluff retreat rate averaged 1.8 myr
1
between 1955 and 2004
and 3.8 m yr
1
between 2004 and 2010 (Gibbs et al., 2010). Erosion rates are generally accelerating
Edited by:
Scott Raymond Dallimore,
Geological Survey of Canada, Canada
Reviewed by:
Henry Patton,
Arctic University of Norway, Norway
Jennifer Frederick,
Sandia National Laboratories,
United States
*Correspondence:
Thomas M. Ravens
tmravens@alaska.edu
ORCID
Thomas M. Ravens
orcid.org/0000-0002-4613-4632
Present address:
Sasha Peterson,
Department of Environmental Science
and Engineering, University of Texas
at El Paso, El Paso, TX, United States
Specialty section:
This article was submitted to
Cryospheric Sciences,
a section of the journal
Frontiers in Earth Science
Received: 12 April 2021
Accepted: 24 September 2021
Published: 15 October 2021
Citation:
Ravens TM and Peterson S (2021)
Geologic Controls on Erosion
Mechanism on the Alaska
Beaufort Coast.
Front. Earth Sci. 9:693824.
doi: 10.3389/feart.2021.693824
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938241
BRIEF RESEARCH REPORT
published: 15 October 2021
doi: 10.3389/feart.2021.693824
because of 1) greater spatial extent of open water, which allows
for the generation of larger waves, 2) greater open water
period, and 3) increased rate of coastal permafrost thaw
(Barnhart et al., 2014a;Barnhart et al., 2014b;Frederick et al.,
2016). Erosion threatens coastal infrastructure throughout
the Arctic including governmental assets and community
infrastructure. The US Army Corps of Engineers (2009) has
designated 26 Alaska communities (including Barrow) Priority
Action Communitiesdue to the threat of erosion.
A number of arctic coastal erosion mechanisms affecting
high coastal bluffs in the Arctic have been identied including
niche erosion/block collapse (prevalent in the Drew Point area
(Ravens et al., 2012;Barnhart et al., 2014a)) and bluff face
thaw/slump (also referred to as translational-shear ice-thaw,
Gibbs et al., 2013,andthermaldenudation,Barankaya et al.,
2021). The erosion mechanisms affecting Arctic coastal bluffs
differ from the erosion of non-Arctic bluffs (e.g., Carter and
Guy 1988) because of the role played by thermal processes in
FIGURE 1 | Map of the north coast of Alaska showing color-coded shoreline change rates for the period circa-1940s (1947 and 1949) to circa-2000s (19972012,
Gibbs and Richmond, 2015).
FIGURE 2 | Conceptual model of the niche erosion/block collapse erosion mechanism (from Ravens et al., 2012).
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938242
Ravens and Peterson Geologic Controls on Arctic Erosion
the Arctic. With the niche erosion/block collapse erosion
mechanism, typically a small beach is present before the
bluff (Figures 2,3). During a storm surge event, waters rise
allowing contact between sea and the base of the bluff. Waves
and currents thermally and mechanically carve a niche at the
base of the bluff (Kobayashi 1985). Niche growth undermines
the bluffs leading to block collapse due to an overturning
failure (Hoque and Pollard 2016). The lower failure plane
intersects with a shore parallel ice wedge (Figure 4). The
upper failure plane is at interface of the ice wedge and the
soil. The failure is governed by the tensile strength of the frozen
soil, as well as the niche depth, the ice wedge location, and the
depth of the ice wedge. Niche erosion/block collapse is the
predominant erosion mechanism in settings where the coastal
bluffs have high ice content (70%, Ping et al., 2011), and
where the bluffs lack signicant amounts of coarse material
(sand and gravel). The lack of coarse material leads to a low
elevation beach at the base of the bluff and frequent contact
between the sea and the coastal bluffs (Ravens et al., 2011;
Ravens and Peterson 2018).
Bluff face thaw/slump is the predominant erosion mechanism
in settings where signicant amounts of coarse sediments are
common (e.g., at Barter Island, Ravens et al., 2011;Ravens and
Peterson 2018). With signicant amounts of coarse sediments in
the coastal bluffs, the elevation of the beach before the bluff is
relatively high (12 m above mean sea level) and contact between
the sea and the base of the bluffand niche erosionis
infrequent. For example, data provided by the USGS (Ann
Gibbs, personal communication) indicates that only a single
signicant niche erosion/block collapse event occurred in the
19552010 time period at Barter Island which has signicant
amounts of coarse sediments (Figure 5). The bluff face warms
due to the combined effect of a number of heat transfer processes
including solar (shortwave) radiation, longwave radiation
emission from the Earths surface, absorption of downward
longwave radiation from the atmosphere, sensible heat ux,
and latent heat ux (Westermann et al., 2009;Ravens and
Ulmgren, 2020). When the bluff face is warmed sufciently, it
thaws and material slumps to the beach face (Figures 5,6).
Relatively small storms (e.g., the 1-year return period storm)
are sufcient to remove the sediment that accumulates on the
beach (Ravens et al., 2011).
Ravens et al. (2011) dened a parameter, the coarse sediment
areal density, and they hypothesized that this parameter
FIGURE 3 | Photos of (A) an erosional niche from Elson Lagoon Alaska and (B) a fallen block by Drew Point, Alaska (image courtesy of Christopher Arp of the Alaska
Science Center, U.S. Geological Survey).
FIGURE 4 | Sketch of the bluff cross-section assumed by Hoque and
Pollard (2016) in their analysis of overturning failure.
FIGURE 5 | Photo showing material that has slumped onto the beach
face following bluff face thaw at Barter Island (2011 image courtesy of Li
Erikson, U.S. Geological Survey). The bluff height is about 10 m and the
sediment areal density is about 600 g/cm
2
, based on USGS data.
Interestingly, the photo was taken soon after the 2008 niche erosion/block
collapse event and the niche is still in evidence.
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938243
Ravens and Peterson Geologic Controls on Arctic Erosion
determined whether the bluffs at a given coastal site were
controlled by niche erosion/block collapse or by bluff face
thaw/slumping. The sediment areal density is the dry mass of
coarse sediment (sand and gravel) contained in a column of bluff
sediment/soil per unit horizontal area (g cm-2). If there was a
virtual column in the bluff extending from mean sea level to the
bluff top, the coarse sediment areal density would be the dry mass
of coarse sediment (sand and gravel) per unit horizontal area in
the column. In this paper, we test this hypothesis by examining
the extent to which coarse sediment areal density can predict
coastal erosion mechanism. We also examine the relationship
between coarse sediment areal density and coastal erosion rate.
FIGURE 6 | Conceptual depiction of the bluff face thaw/slump erosion mechanism, which includes 1) the thawing of the bluff face, followed by 2) the slumping and
deposition on the beach face, followed by 3) the offshore transport due to storm surge and waves.
TABLE 1 | Photographic and geologic data used in the analysis.
Photo ID Photo location Erosion
mechanism
Barrier
island
present
Ping
et al.
(2021)
site
Average
sediment
density
Bluff
height
Coarse
material
(sand)
content
Coarse
sediment
areal
density
Erosion
rate
Latitude Longitude ——(g cm
3
) (cm) (%) (g cm
2
) (m/year)
IMG_9510 70.899 153.367 Niche/block N BSC17 0.69 50 21.9 7.5 3.47
IMG_8113 70.1629 145.845 Niche/block N BSC39 0.64 250 54.1 86.0 0.35
IMG_0238 71.02287 154.623 Niche/block N BSC15 0.37 40 48.4 7.1 2.14
IMG_9428 70.78902 152.271 Niche/block N BSC20 0.61 250 30.8 46.8 2.7
IMG_8136 70.04606 145.447 Niche/block Y BSC40 0.57 280 47.3 76.1 0.12
IMG_0065 71.33132 156.566 Niche/block Y BSC01 1.03 40 53.2 22.0 0.31
IMG_0087 71.29122 156.438 Niche/block Y BSC02 0.52 230 32.6 39.3 1.56
IMG_0124 71.21429 156.047 Niche/block Y BSC03 0.38 140 34.6 18.6 4.57
IMG_0184 71.12589 155.548 Niche/block Y BSC04 0.62 160 54.3 53.9 2.25
IMG_8366 70.03766 142.72 Bluff face
thaw
N BSC46 0.60 300 88.4 158.9 0.54
IMG_8210 69.99457 144.546 Bluff face
thaw
N BSC42 0.66 200 62.7 87.7 0.26
IMG_8385 69.98949 142.556 Bluff face
thaw
N BSC47 0.70 320 74.4 238.0 0.96
IMG_8470 69.65694 141.039 Bluff face
thaw
N BSC50 0.54 350 48.7 91.5 3.88
IMG_8772 70.00185 144.828 Bluff face
thaw
N BSC41b 0.81 400 11.6 82.7 0.36
IMG_9327 70.55583 151.709 Bluff face
thaw
N BSC24 1.53 320 90.1 441.7 0.24
IMG_7869 70.4919 149.226 Bluff face
thaw
Y BSC31 1.12 200 90.3 202.1 -1.55
IMG_7924 70.40772 148.778 Bluff face
thaw
Y BSC32 0.66 260 71.3 122.6 1.24
IMG_8225 70.03146 144.319 Bluff face
thaw
Y BSC42 0.66 200 62.7 87.7 0.26
IMG_8241 70.08234 144.002 Bluff face
thaw
Y BSC43 1.32 170 83.0 186.9 -0.24
IMG_7571 70.33116 148.08 Bluff face
thaw
Y BSC34 1.03 300 62.5 193.2 0.33
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938244
Ravens and Peterson Geologic Controls on Arctic Erosion
METHODOLOGY
Coastal locations with both sediment data and aerial photo data
from the north coast of Alaska between Utqiagvik (formerly
Barrow) and the Canadian border were sought. Data on sediment
grain size distribution (percent sand, silt, and clay) as a function
of depth into the bluffs, sediment bulk density, and bluff height
were obtained from 22 coastal sites according to Ping et al. (2011).
Note, Ping et al. (2011) did not report on the presence of gravel so
we concluded that it was negligible in their samples. However, the
USGS, working at their Barter Island site, found signicant gravel
(Gibbs et al., 2010). The samples were collected from undisturbed
areas between ice wedges after removal of slumped material. We
examined oblique aerial photos from Gibbs and Richmond
(2009) at locations proximal to the sites with sediment data to
determine if the coastal erosion mechanism was niche erosion/
block collapse or bluff face thaw/slump (Table 1). On average, the
distance between location with sediment data and photos was
about 6 km. For each photo, sand and gravel content data from
one proximal core or bluff sample was used to determine the
sediment areal density (Figure 7). Locations experiencing niche
erosion/block collapse were readily determined based on the
characteristic erosional blocks (Figure 8). Locations dominated
by bluff face thaw/slump were evident based on the presence of a
high elevation beach before the coastal bluff and the presence of
material (e.g., vegetation) that was slumping on the bluff face
(Figure 9).Thecoarsesedimentarealdensity(gcm-2)was
calculated as the product of the coarse sediment (sand and
gravel) content (%), sediment bulk density (g cm-3) and the bluff
height (cm), using data from Ping et al., 2011. The ice content of the
bluffs was implicitly included in the sediment bulk density.
RESULTS AND DISCUSSION
The locations of the 19 coastal sites subject to analysis, as well as
the erosion mechanisms attributed to those sites based on the
analysis of the aerial photos, are shown in Figure 10.Itis
noteworthy that the majority of the sites experiencing niche
erosion/block collapse are on the western side of the study
FIGURE 7 | Plot showing sand content (%) as a function of normalized
bluff position (depth/bluff. height) at the various sites for which sediment data
was available. The plot also identies the erosion mechanism inferred based
on aerial photo analysis. Note, in some instances, only a single bluff
sample was analyzed and these data are plotted as dots. Note, the low sand
content of one core (BSC41b in Table 1), identied as a site of bluff face thaw/
slump erosion, appears to be an outlier. However, the coarse sediment areal
density of this site (82.7 g/cm
2
,Table 1) is similar to that calculated for other
bluff face thaw/slump sites.
FIGURE 8 | Example photo of coastal bluffs where niche erosion/block
collapse was the predominant mechanism (image courtesy of Ann Gibbs, U.S.
Geological Survey).
FIGURE 9 | Example photo of coastal bluffs where bluff face thaw/
slumping was the predominant erosion mechanism (image courtesy of Ann
Gibbs, U.S. Geological Survey).
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938245
Ravens and Peterson Geologic Controls on Arctic Erosion
domain, whereas the sites experiencing bluff face thaw/slump are
mainly on the eastern side. Note also that there was relatively little
variation of erosion mechanism with position according to our
analysis. The frequency of occurrence of the niche erosion/block
collapse mechanism and the bluff face thaw/slump mechanism
relative to the course sediment areal density (g cm
2
,Figure 11)
shows that with sediment areal density greater than 120 g cm
2
,
the dominant erosion mechanism was bluff face thaw/slumping.
With sediment areal density less than 80 g cm
2
, the dominant
erosion mechanism was niche erosion/block collapse. One might
wonder whether the erosion mechanism at specic sites, inferred
based on the 2006 areal photos, might vary over time. It is
noteworthy that, for example, Elson Lagoon, Drew Point, and
FIGURE 10 | Map of the north coast of Alaska showing the locations of the coastal sites studied as well as the erosion mechanism attributed to those sites. Base
map imagery courtesy of Esri.
FIGURE 11 | A histogram showing the frequency of occurrence of the
niche erosion/block collapse erosion mechanism and bluff face thaw/slump
mechanism as a function of coarse sediment areal density.
FIGURE 12 | Dependence of coastal erosion rates for the
19802000 time period on coarse sediment areal density, for sites
experiencing niche erosion/block collapse and bluff face thaw/slump. Note,
the gure provides data on coastal sites that are protected by barrier
islands as well as ones without protection as indicated in the legend. Trend
lines are provided for sites with niche erosion/block collapse (orange line, R
2
0.37) as well as considering all sites (black line, R
2
0.25). For bluff face thaw/
slump sites, the correlation was negligible (R
2
0.09) and no trend line is
provided.
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938246
Ravens and Peterson Geologic Controls on Arctic Erosion
Barter Island have been subject to numerous research papers over
the past few decades, and there has been no mention of a change
in erosion mechanism although there are some caveats. First,
Barter Island has eroded mainly due to bluff face thaw/slump (as
expected due to its high sediment areal density), but it was subject
to a signicant niche erosion/block collapse event during a large
2008 storm (Gibbs et al., 2010;Ravens et al., 2011). Also, Gibbs
et al. (2019) point out the seasonality of erosion mechanism. In
early to mid-summer, there tends to be more bluff face thaw/
slumping because of the high levels of solar (short wave)
radiation. In the second half of the summer, after the thaw of
sea ice, storm surges and wave action bring aggressive mechanical
forces to the coast removing previously thawed and deposited
material, and potentially causing niche erosion if the beach
elevation is sufciently low.
Erosion rates for the 19802000 period (from Ping et al., 2011)
are plotted relative to coarse sediment areal density (Figure 12).
For sediment areal density values greater than 120 g cm
2
(coincident with the bluff face thaw/slump mechanism), erosion
rates ranged from 1.24 m/yr to 1.55 m/yr (i.e., an accretion of
1.55 m/yr) with an average erosion rate of 0.34 ±0.92 m/yr,
Table 2). For sediment areal density values less than 80 g cm
2
(coincident with the niche erosion/block collapse mechanism),
erosion rates ranged from 4.57 to 0.12 m/yr with an average of
2.1 ±1.5 m/yr. Thus, the presence of elevated coarse sediment areal
density appears to control (or reduce) the coastal erosion rate.
Analysis was also performed to determine whether the
presence of barrier island protection translated to reduced
erosion rates for the two ranges of sediment areal density and
the associated erosion mechanisms. For locations with coarse
sediment areal density above 120 g cm
2
(i.e., bluff face thaw/
slump sites), the average erosion rate was reduced from 0.34 ±
0.92 m/yr (considering all sites) to 0.06 ±1.17 m/yr, when only
sites protected by barrier islands were considered (Table 2). For
locations with coarse sediment areal density less than 80 g
cm
2
(i.e., the niche erosion/block collapse sites), the average
erosion rate was reduced from 2.1 ±1.5 m/yr (considering all
sites) to 1.8 ±1.8 m/yr, when only sites protected by barrier
islands were considered (Table 1). Thus, barrier island
protection appeared to provide a small reduction in erosion
rate for all levels of coarse sediment areal density (and for both
erosion mechanisms) though the reduction was less than the
standard deviation. When all of the data (Figure 11)was
subject to linear regression, the erosion rate (ER, m/yr,
19802000 period) was found to be somewhat correlated
with coarse sediment areal density (ρ
areal
,gcm
2
) with an
R
2
of 0.20: ER −0.0068 ρaerial +1.88 .This indicates that the
erosion rate is negatively correlated with sediment areal density.
Asignicant amount of the variance in the measured erosion
rate could not be explained using the coarse sediment areal density
alone. Various explanations for the unexplained variance exist.
First, we had to work with a signicant distance (order 1 km)
between the location of the erosion measurement and the borehole
from which the sediment areal density was derived. Given spatial
non-uniformity in the coastal stratigraphy, it is reasonable to
suggest that the sediment areal density at the location of the
erosion measurement differed from the density at the borehole.
Second, there are many environmental variables that affect erosion
but were not included in the regression including: nearshore water
surface elevation, nearshore wave condition, and nearshore water
and air temperature. Third, the way in which environmental
variables affect arctic coastal erosion can be quite complex as
indicated by process-based approaches to determine erosion rate
(Ravens et al., 2012;Barnhart et al., 2014a).
The analysis presented above focuses on the predictability of
Arctic coastal erosion mechanism based on sediment areal
density. However, once this relationship has been established,
it is noteworthy that sediment character can be inferred to some
extent based on the erosion mechanism. For example, in locations
where niche erosion/block collapse is dominant, we can infer that
the coarse sediment in the eroding bluffs in limited. Such insights
could be used in sediment transport and other studies.
CONCLUSION
The research presented here suggests that a single geologic
parameter, the coarse sediment areal density, controls the
dominant arctic coastal erosion mechanism of coastal bluffs
on the North Slope (i.e., north coast) of Alaska. The coarse
sediment areal density is the dry mass (g) of coarse sediment
(sand) per horizontal area (cm2) in the coastal bluff. When the
coarse sediment areal density exceeds 120 g cm
2
, the bluff face
thaw/slump erosion mechanism is dominant. When the coarse
sediment areal density is below 80 g cm
2
, the niche erosion/block
collapse erosion mechanism is dominant. The coarse sediment
areal density also has some inuence on coastal erosion rates.
Considering the 22 sites addressed in this study, the sediment
areal density was found to have a controlling effect on erosion
rate. Using erosion rates for the 19802000 period, when the
sediment areal density exceeds 120 g cm
2
, the average erosion
rate was of 0.34 ±0.92 m/yr. For sediment areal density values less
than 80 g cm
2
, the average erosion rate was as high as 2.1 ±
1.5 m/yr. Linear regression between coarse sediment areal density
and erosion rate found that 20% of the variance in erosion rate
was explainable by coarse sediment areal density.
DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in
the article/Supplementary Material, further inquiries can be
directed to the corresponding author.
TABLE 2 | Average erosion rates (for 19802000 period) for different ranges of
coarse sediment areal density.
Range of sediment
areal density (g
cm
2
)
Average erosion rate
(m/yr) considering all
locations
Average erosion rate
(m/yr) considering sites
with barrier island
protection
>100 g cm
2
0.22±0.92 0.06±1.17
<80 g cm
2
2.1±1.5 1.8±1.8
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938247
Ravens and Peterson Geologic Controls on Arctic Erosion
AUTHOR CONTRIBUTIONS
TR conceived the overall manuscript and did the majority of the
writing. SP did the analysis of the aerial photos and
sediment data.
FUNDING
Funding for this research came from the National Science
Foundation (Award # 7416374) and from the Bureau of Ocean
Energy Management (NSL# AK-17-01).
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Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 6938248
Ravens and Peterson Geologic Controls on Arctic Erosion
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Article
Full-text available
Coastal erosion in the Arctic has numerous internal and external environmental drivers. Internal drivers include sediment composition, permafrost properties and exposure which contribute to its spatial variability, while changing hydrometeorological conditions act as external drivers and determine the temporal evolution of shoreline retreat. To reveal the relative role of these factors, we investigated patterns of coastal dynamics in an enclosed bay in the southwestern Kara Sea, Russia, namely the Gulf of Kruzenstern, which is protected from open-sea waves by the Sharapovy Koshki Islands. Using multitemporal satellite imagery, we calculated decadal-scale retreat rates for erosional segments of the coastal plain from 1964 to 2019. In the field, we studied and described Quaternary sediments and massive ground-ice beds outcropping in the coastal bluffs. Using data from regional hydrometeorological stations and climate reanalysis (ERA), we estimated changes in the air thawing index, sea ice-free period duration, wind-wave energy and total hydrometeorological stress for the Gulf of Kruzenstern, and compared it to Kharasavey and Marre-Sale open-sea segments north and south of the gulf to understand how the hydrometeorological forcing changes in an enclosed bay. The calculated average shoreline retreat rates along the Gulf in 1964–2010 were 0.5 ± 0.2 m yr−1; the highest erosion of up to 1.7 ± 0.2 m yr−1 was typical for segments containing outcrops of massive ground-ice beds and facing to the northwest. These retreat rates, driven by intensive thermal denudation, are comparable to long-term rates measured along open-sea sites known from literature. As a result of recent air temperature and sea ice-free period increases, average erosion rates rose to 0.9 ± 0.7 m yr−1 in 2010–2019, with extremes of up to 2.4 ± 0.7 m yr−1. The increased mean decadal-scale erosion rates were also associated with higher spatial variability in erosion patterns. Analysis of the air thawing index, wave energy potential and their total effect showed that inside the Gulf of Kruzenstern, 85% of coastal erosion is attributable to thermal denudation associated with the air thawing index, if we suppose that at open-sea locations, the input of wave energy and air thawing index is equal. Our findings highlight the importance of permafrost degradation and thermal denudation on increases in ice-rich permafrost bluff erosion in the Arctic.
Technical Report
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Permafrost-dominated coastlines in the Arctic are rapidly disappearing. Arctic coastal erosion rates in the United States have doubled since the middle of the twentieth century and appear to be accelerating. Positive erosion trends have been observed for highly-variable geomorphic conditions across the entire Arctic, suggesting a major (human-timescale) shift in coastal landscape evolution. Unfortunately, irreversible coastal land loss in this region poses a threat to native, industrial, scientific, and military communities. The Arctic coastline is vast, spanning more than 100,000 km across eight nations, ten percent of which is overseen by the United States. Much of area is inaccessible by all-season roads. People and infrastructure, therefore, are commonly located near the coast. The impact of the Arctic coastal erosion problem is widespread. Homes are being lost. Residents are being dispersed and their villages relocated. Shoreline fuel storage and delivery systems are at greater risk. The U.S. Department of Energy (DOE) and Sandia National Laboratories (SNL) operate research facilities along some of the most rapidly eroding sections of coast in the world. The U.S. Department of Defense (DOD) is struggling to fortify coastal radar sites, operated to ensure national sovereignty in the air, against the erosion problem. Rapid alterations to the Arctic coastline are facilitated by oceanographic and geomorphic perturbations associated with climate change. Sea ice extent is declining, sea level is rising, sea water temperature is increasing, and permafrost state is changing. The polar orientation of the Arctic exacerbates the magnitude and rate of the environmental forcings that facilitate coastal land area loss. The fundamental mechanics of these processes are understood; their non-linear combination poses an extreme hazard. Tools to accurately predict Arctic coastal erosion do not exist. To obtain an accurate predictive model, a coupling of the influences of evolving wave dynamics, thermodynamics, and sediment dynamics must be developed. The objective of this document is to present the state-of-the-science and outline the key steps for creation of a framework that will allow for improved prediction of Arctic coastal erosion rates. This is the first step towards the quantification of coastal hazards that will allow for sustainable planning and development of Arctic infrastructure. Chapter 1: Problem Statement and Infrastructure Concerns Chapter 2: Review of Arctic Coastal Erosion Studies: Observations and Trends Chapter 3: Changes in Arctic Sea Ice and Oceanographic Conditions Chapter 4: Arctic Coastal Geomorphology Chapter 5: Review of Existing Models Chapter 6: Putting It All Together to Create a Predictive Tool Chapter 1 provides a snapshot of the magnitude of Arctic coastal erosion as well the social and economic costs associated with the hazard. The consistency in erosion trends is indicative of a major disruption to oceanographic/geomorphic equilibrium. Billions of dollars are being spent to relocate or fortify infrastructure. Chapter 2 synthesizes decades of observation-based studies aimed at quantifying long-term rates of coastal erosion across the Arctic. These kinds of studies typically rely upon ground survey, aerial imagery, or remotely-sensed data. The collective efforts of researchers leads to a fairly consistent conclusion: erosion rates in the Alaskan Arctic are among the highest in the world and they are accelerating. Chapter 3 discusses how Arctic Ocean conditions are changing. Sea ice is melting earlier and forming later. Perennial ice is being replaced by thinner first-year ice. By some accounts, the Arctic Ocean may experience ice-free summers by 2018. As the duration of open water conditions in the Arctic increases, more powerful ocean waves are expected to form. These changes will facilitate the delivery of heat to the permafrost-laden coastlines of the Arctic. Chapter 4 describes the characteristics of permafrost in the Arctic and discusses those traits relative to the geomorphic nature of the coastline. The character of Arctic coastal permafrost varies widely. Some of the permafrost coastlines are lithified, but many consist of unconsolidated sediment with grain sizes ranging from fine to coarse. It is not uncommon for the volumetric ice content of the permafrost to be greater than 50 percent. The sediment type and degree to which the permafrost is ice-bonded affects its thermal and mechanical (i.e., sediment strength) properties. Chapter 5 reviews the models that have been (or could be) used to model oceanographic and geomorphic conditions in the Arctic. Wave, sea-ice, near-shore circulation, permafrost thermal, and permafrost erosion models are discussed. Ocean wave modeling is a well-established discipline, but understanding of how waves form and propagate in the vicinity of sea-ice is an area of active research with efforts split among wave-ice, weather and storm ocean-ice, and earth systems models. Near-shore circulation modeling is also a well-established method that is critical to finely resolve the sea water-temperature, -salinity, -velocity, and -level in the vicinity of a permafrost bluff. Permafrost thermal model complexity often depends upon the scale of interest. Physically-based thermal models, typically employed at the field scale, are highly parameterized. Earth system thermal models require simplifying assumptions about the physics, but can be applied on regional scales. Existing permafrost bluff erosion models are typically calibrated to operate within a narrow range of geomorphic conditions. A common theme that emerges from the literature is that water setup (i.e., depth and duration) and temperature are first-order controls on the rate of erosion. Chapter 6 provides an outline for a new modeling strategy that could be used to predict coastal erosion rates in the Arctic. The inputs and outputs of each major model type (i.e., the sea-ice-wave model, ocean circulation model, permafrost thermal model, and the permafrost erosion model) are presented. Ten advancements associated with the proposed effort are identified. These improvements will introduce more physical processes into each model component and result in a degree inter-model coupling greater than previous efforts. Despite the fact that the Arctic coastline comprises one-third of the global coastline length, much of our current understanding of coastal landscape evolution is applicable to coasts that are fundamentally different than the Arctic. This whitepaper demonstrates that Arctic coastal erosion is driven by complex oceanographic and geomorphic feedbacks. The social and economic costs associated with these destructive processes are large. Although surrogate-type models have shown promise, care should be taken such that the hydrologic, thermal, and mechanical processes associated with the Arctic system are properly coupled. With this complete approach, models based upon ground truth and physical parameters will facilitate the process-based understanding needed to inform Arctic stakeholders.
Article
Full-text available
The Arctic climate is changing, inducing accelerating retreat of ice-rich permafrost coastal bluffs. Along Alaska's Beaufort Sea coast, erosion rates have increased roughly threefold from 6.8 to 19 m yr −1 since 1955 while the sea ice-free season has increased roughly twofold from 45 to 100 days since 1979. We develop a numerical model of bluff retreat to assess the relative roles of the length of sea ice-free season, sea level, water temperature, nearshore wavefield, and permafrost temperature in controlling erosion rates in this setting. The model captures the processes of erosion observed in short-term monitoring experiments along the Beaufort Sea coast, including evolution of melt notches, topple of ice wedge-bounded blocks, and degradation of these blocks. Model results agree with time-lapse imagery of bluff evolution and time series of ocean-based instrumentation. Erosion is highly episodic with 40% of erosion is accomplished during less than 5% of the sea ice-free season. Among the formulations of the submarine erosion rate we assessed, we advocate those that employ both water temperature and nearshore wavefield. As high water levels are a prerequisite for erosion, any future changes that increase the frequency with which water levels exceed the base of the bluffs will increase rates of coastal erosion. The certain increases in sea level and potential changes in storminess will both contribute to this effect. As water temperature also influences erosion rates, any further expansion of the sea ice-free season into the midsummer period of greatest insolation is likely to result in an additional increase in coastal retreat rates.
Article
Full-text available
A predictive coastal erosion/shoreline change model has been developed for the North Slope (Alaska) coast by Drew Point. This coastal area has been experiencing rapid and accelerating erosion in the past few decades (to about 20 m/yr in the recent past). The coast has 3-m high permafrost bluffs with high ice content and fine-grained soils. The bluffs are typically fronted by a small (~ 5 m wide) beach. During a storm surge, the warming Beaufort Sea is able to contact the base of the bluff and erode a niche which will grow and eventually undermine the bluff, leading to block collapse. The fallen block is eroded by waves and currents. The erosion model explicitly accounts for and integrates a number of processes including: (1) storm surge generation due to wind and atmospheric forcing, (2) erosional niche growth due to wave-induced turbulent heat transfer and sediment transport, and (3) thermal and mechanical erosion of the fallen block. Historic and projected nearshore water temperature and sea-ice conditions were calculated using a fully-coupled ocean-ice model. The coastal erosion model was calibrated and validated with historic shoreline change data for two long time periods (1979-2002 and 2002-2007) and two recent annual cycles 2007 and 2008 which were quite different. The model indicates that the shoreline erosion rate will continue to increase exponentially in this area, reaching an erosion rate of about 30 m/yr by 2045.
Article
Full-text available
Analysis of a 60 km segment of the Alaskan Beaufort Sea coast using a time-series of aerial photography revealed that mean annual erosion rates increased from 6.8 m a-1 (1955 to 1979), to 8.7 m a-1 (1979 to 2002), to 13.6 m a-1 (2002 to 2007). We also observed that spatial patterns of erosion have become more uniform across shoreline types with different degrees of ice-richness. Further, during the remainder of the 2007 ice-free season 25 m of erosion occurred locally, in the absence of a westerly storm event. Concurrent arctic changes potentially responsible for this shift in the rate and pattern of land loss include declining sea ice extent, increasing summertime sea surface temperature, rising sea-level, and increases in storm power and corresponding wave action. Taken together, these factors may be leading to a new regime of ocean-land interactions that are repositioning and reshaping the Arctic coastline.
Chapter
An Arctic coastal erosion process or mechanism is distinct from a non-Arctic erosion process due to the importance of thermal processes in addition to mechanical ones. The Arctic contains permanently frozen soil (permafrost) as well as soil and sediments that freeze seasonally. Thawing of the coastal permafrost and seasonally frozen soils/sediments is a critical and distinctive feature of Arctic coastal erosion. Thus, the Arctic coastal erosion modeler must include both thermal and mechanical processes in their models, either implicitly or explicitly. Arctic coastal erosion modeling features the identification of particularly Arctic coastal configurations and the development of process-based and predictive Arctic coastal erosion models for those specific configurations. In this chapter, recent advances in Artic coastal erosion modeling are presented, with a particular focus on the work done in Arctic Alaska. In addition, suggestions for next steps are offered. Much of the Arctic coastal erosion modeling has focused on cross-shore processes and sediment transport. In the future, Arctic modelers will need to include longshore transport processes and account for their contribution to erosion and shoreline change. © 2018 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
Article
Block failure is considered to be an important component of coastal retreat in permafrost regions. A comprehensive model is developed to study the effects of thermoerosional niche and ice wedge morphology on the stability of permafrost dominated coastal cliff against block failure. The model is formulated by coupling slope stability analysis with a time dependent progression of thermoerosional niches and the morphology of the nearby ice wedges. Model computations are initially performed for failure conditions for a given cliff height, frozen soil strength, ice content, water pressure in the active layer, thermoerosional niche depth and ice wedge morphology. Under these conditions block failures are found to be predominantly overturning failures and are governed by the tensile strength of frozen soil, thermoerosional niche depth and ice wedge location and depth. The effects of ice wedges are then examined by analyzing failure conditions for ice wedges of different locations and depths. For a given cliff height, strength and thermoerosional niche, block failure may occur at a range of different combinations of ice wedge locations and depths. Two stability nomograms are developed through repeated model calculations for range of cliff heights and frozen soil tensile strength. These nomograms can be used to determine the critical combinations of thermoerosional niche depth, ice wedge distance and ice wedge depth that lead to block collapse of a cliff of known height and soil strength. Some analytical expressions are also derived to determine potential block failure criteria along Arctic coasts.
Article
Sea ice limits the interaction of the land and ocean water in the Arctic winter and influences this interaction in the summer by governing the fetch. In many parts of the Arctic, the open-water season is increasing in duration and summertime sea-ice extents are decreasing. Sea ice pro-vides a first-order control on the physical vulnerability of Arctic coasts to erosion, inundation, and damage to settle-ments and infrastructures by ocean water. We ask how the changing sea-ice cover has influenced coastal erosion over the satellite record. First, we present a pan-Arctic analysis of satellite-based sea-ice concentration specifically along the Arctic coasts. The median length of the 2012 open-water sea-son, in comparison to 1979, expanded by between 1.5 and 3-fold by Arctic Sea sector, which allows for open water dur-ing the stormy Arctic fall. Second, we present a case study of Drew Point, Alaska, a site on the Beaufort Sea, characterized by ice-rich permafrost and rapid coastal-erosion rates, where both the duration of the open-water season and distance to the sea-ice edge, particularly towards the northwest, have in-creased. At Drew Point, winds from the northwest result in increased water levels at the coast and control the process of submarine notch incision, the rate-limiting step of coastal re-treat. When open-water conditions exist, the distance to the sea ice edge exerts control on the water level and wave field through its control on fetch. We find that the extreme values of water-level setup have increased consistently with increas-ing fetch.
Article
Carbon, nitrogen, and material fluxes were quantified at 48 sampling locations along the 1957 km coastline of the Beaufort Sea, Alaska. Landform characteristics, soil stratigraphy, cryogenic features, and ice contents were determined for each site. Erosion rates for the sites were quantified using satellite images and aerial photos, and the rates averaged across the coastline increased from 0.6 m yr-1 during circa 1950-1980 to 1.2 m yr -1 during circa 1980-2000. Soils were highly cryoturbated, and organic carbon (OC) stores ranged from 13 to 162 kg OC m-2 in banks above sea level and averaged 63 kg OC m-2 over the entire coastline. Long-term (1950-2000) annual lateral fluxes due to erosion were estimated at -153 Gg OC, -7762 Mg total nitrogen, -2106 Tg solids, and -2762 Tg water. Total land area loss along the Alaska Beaufort Sea coastline was estimated at 203 ha yr-1. We found coastal erosion rates, bank heights, soil properties, and material stores and fluxes to be extremely variable among sampling sites. In comparing two classification systems used to classifying coastline types from an oceanographic, coastal morphology perspective and geomorphic units from a terrestrial, soils perspective, we found both systems were effective at differentiating significant differences among classes for most material stores, but the coastline classification did not find significant differences in erosion rates because it lacked differentiation of soil texture.
Article
A new quantitative coastal land gained-and-lost method uses image analysis of topographic maps and Landsat thematic mapper short-wave infrared data to document accelerated coastal land loss and thermokarst lake expansion and drainage. The data span 1955-2005 along the Beaufort Sea coast north of Teshekpuk Lake in the National Petroleum Reserve in Alaska. Some areas have undergone as much as 0.9 km of coastal erosion in the past 50 yr. Land loss attributed to coastal erosion more than doubled, from 0.48 km2 yr-1 during 1955-1985 to 1.08 km2 yr-1 during 1985-2005. Coastal erosion has breached thermokarst lakes, causing initial draining of the lakes followed by marine flooding. Although inland thermokarst lakes show some uniform expansion, lakes breached by coastal erosion display lake expansion several orders of magnitude greater than inland lakes.