Costal erosion and costal stability

Abstract

Coastal erosion is caused by many different processes like changes in prevailing wind direction, coastal currents, re-establishment of a new equilibrium profile, sea level rise, sea level fall, exceptional storms, hurricanes/cyclones, and tsunami events. These coastal factors are reviewed with special attention to effects due to changes in sea level. In the Indian Ocean, sea level seems to have remained virtually stable over the last 40-50 years. Coastal erosion in the Maldives was caused by a short lowering in sea level in the 1970s. In Bangladesh, repeated disastrous cyclone events cause severe coastal erosion, which hence has nothing to do with any proposed sea level rise. Places like Tuvalu, Kiribati and Vanuatu – all notorious for an inferred sea level rise – have tide gauges, which show no on-going sea level rise. Erosion is by no means a sign of sea level rise. Coastal erosion occurs in uplifting regions as well as in subsiding regions, or virtually stable areas. Coastal morphology provides excellent insights to the stability. The coastal characteristics tell us about sea level changes, changes in prevailing wind and currents, and the occurrence of storms, hurricanes and tsunamis. Erosion in one part of a coastline implies deposition in another. The formation of a shoreline (notch, terrace, rock cut platform, beach ridge, coastal bar, etc.) takes time. Therefore, most of the prominent fossil shorelines (uplifted above sea level as well as submerged below sea level) represent halts or oscillations in the general postglacial rise of sea level.

Full text

In: Coastal and Beach Erosion:
Processes, Adaptation Staregies and Environmental Impacts
Editor: Dianna Barens, Nova Science Publishers, 2015
p. 69-82
Chapter 4
COSTAL EROSION AND COSTAL STABILI T Y
Nils-Axel Mörner
∗
Paleogeophysics and Geodynamics, Stockholm, Sweden
ABSTRACT
Coastal erosion is caused by many different processes like changes in
prevailing wind direction, coastal currents, re-establishment of a new
equilibrium profile, sea level rise, sea level fall, exceptional storms,
hurricanes/cyclones, and tsunami events. These coastal factors are
reviewed with special attention to effects due to changes in sea level. In
the Indian Ocean, sea level seems to have remained virtually stable over
the last 40-50 years. Coastal erosion in the Maldives was caused by a
short lowering in sea level in the 1970s. In Bangladesh, repeated
disastrous cyclone events cause severe coastal erosion, which hence has
nothing to do with any proposed sea level rise. Places like Tuvalu,
Kiribati and Vanuatu – all notorious for an inferred sea level rise – have
tide gauges, which show no on-going sea level rise. Erosion is by no
means a sign of sea level rise. Coastal erosion occurs in uplifting regions
as well as in subsiding regions, or virtually stable areas. Coastal
morphology provides excellent insights to the stability. The coastal
characteristics tell us about sea level changes, changes in prevailing wind
and currents, and the occurrence of storms, hurricanes and tsunamis.
Erosion in one part of a coastline implies deposition in another. The
formation of a shoreline (notch, terrace, rock cut platform, beach ridge,
∗ Nils-Axel Mörner e-mail: morner@pog.nu.

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70
coastal bar, etc.) takes time. Therefore, most of the prominent fossil
shorelines (uplifted above sea level as well as submerged below sea level)
represent halts or oscillations in the general postglacial rise of sea level.
Keywords: Erosion, Coastal erosion, Changes in sea level, Coastal stability,
Coastal morphology, Water level, Wind, Currents, Shoreline formation,
Sea level oscillations and halts, Crustal movements, Groundwater level,
Long-term events, Short-term events, Tide gauge records
INTRODUCTION
Erosion is a general geological term for removal of sedimentare deposits
and bedrock material. It operated in the mega-scale successively wearing
down whole mountains (in the famour Bod Dylan wording: ”How long can a
mountain exist, before it is washed to the sea”) as well as in more normal rates
where the effects can be observed over days or years.
Coastal erosion denotes the removal of sediments or rocks building up a
coastal segment by wave actions driven by currents, tides, and winds. This
paper will be limited to coastal erosion in association with sea level changes
and local dynamic changes; organized in the form of thematic examples and a
few concluding case studies.
Erosion in one point will always be balanced by deposition in another
point, even if a sediment would become graded up in fractions set in different
sedimentary environments. It is in this context we discuss the establishment of
equilibrium beach and off-shore profiles (e.g. Bruun, 1962; Pilkey et al., 1993;
Nummedal et al., 1993; Cooper et al., 2000).
SEA LEVEL CHANGES
The science of sea level changes is old. It has evolved rapidly over the last
half-a-century (e.g. Fairbridge, 1961; Mörner, 2013a). After the last glaciation
maximum at around 20,000 BP, sea level rose by about 120 m. This rise was
punctuated (i.e. oscillatory) rather than smooth as evidenced in uplifted areas
(e.g. Mörner, 1971) as in submarine areas (e.g. Rufin-Soler et al., 2014).
Figure 1 shows how an oscillating eustatic sea level signal gets
increasingly deformed by uplift and subsidence, respectively.

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71
Figure 1. Illustration of the sussessive deformation of a primary eustatic sea level
signal (red) by increasing rates of uplift and subsidense (Mörner, 1971, 2013a).
1. Raised Beaches
In uplifted areas like Fennoscandia, Canada, Tierra del Fuego, the
postglacial sea level rise is generally marked by a maximum upper
transgression limit. The corresponding rise up over former land also implied a
corresponding rise of the groundwater level. Therefore, this upper limit is
more conspicuous being formed both by exceptional beach ridges, shore cuts
or cliffs, and hanging dead valleys marking the level of dewatering of raised
groundwater levels. I have personally investigated numerous such sites in
Sweden (Mörner, 1969) and also at sites in Tierra del Fuego (Mörner, 1991).
2. Beach Morphology
The beach has a wide range of morphological configurations; protected
lagoonal shores, gently dipping sand beaches, shingly shores with beach
ridges, rock-cut platforms, erosional notches and erosionsl cliffs (in sediments
or bedrock).

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72
In a sea level investigation, it is often imperative to investigate all
different varieties of coastal settings; where only the combined analyses
provide a reliable establishment of the mean sealevel of a fossil shore (e.g.
Mörner et al., 2004; Mörner, 2007a).
3. Coastal Erosion
Coastal erosion is by no means a sign of sea level rise. It occurs today in
uplifted areas as well as in subsiding areas. Usually it is local environmental
conditions that generate erosion in response to changes in prevailig wind,
changes in coastal currents, changes in run-off, and changes in sea level.
4. Storm Events
All costal segments sooner or later have to face storms. Usually, they
come more or lesss regularily. Sometime the stormes are of exceptional
vigour, and we may speak of a ”century storm” or even a ”millennium storm”.
A storm usually affects ony parts or segments of a coast; all depending on
exposure and shore morphology. Therefore, very severe destruction in one part
of the coast may coinside with only wague effects in another part of the same
coast.
In 2014, southern Sweden suffered the storm ”Sven”. In parts of SW
Sweden, it affected the shore in such a strong way that it had not happened in a
century, several centuries or maybe even a millennium (Figure 2).
It must be stressed, however, that the erosionsl/depositional effects of
storm ”Sven” were local and varied quite significanly along the coast. In the
village of Torekov, the sandy beach and the dune field inside was extensively
eroded in such a way that we have to talk not only about a ”century storm”,
but probably about a ”millennium storm”.
Along the coasts of SW Sweden, the effects of storm ”Sven” range from
very extensive to hardly notisable, and this is quite typical for local storm
effects.

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73
Figure 2. At the storm ”Sven” in 2014, a new beach ridge was formed. This ridge of
fresh shingle was thrown in over an old land surface of weathered beach shingle. This
ridge reached an elevation higher than previous beach material in, at least, a century;
i.e. it was a ”century storm”.
4. Hurricanes and Cyclones
Hurricanes or cyclones (depending upon nomenclature) hit the coasts with
very strong forces. Therefore the effects are often disastrous; for example
hurricanes Katrina and Sandy striking the coasts of the US in 2005 and 2012.
The Bangladesh region is regularly cursed by big cyclons; often with
tremendeous erosionsl effects. The Sidar Cyclone hit the coasts in 2007. It
caused extensive erosion and redeposition of material. I visited the region in
2009, and could see remnents of a dead forest sticking out of a sandy shore at
low-tide (Mörner, 2010).
Many visitors had taken the exposed trees stumps on the beach as
evidence of a recent sea level rise in accordance with the IPCC scenarios of
global warming and rising sea level (IPCC, 2007). The truth is completely
different, however (as demonstrated in Mörner, 2010).

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74
A closer examination of the trees left standing on the shore, indicates that
several of them have horozontal root systems hanging some 60-80 cm above
the shore (Figure 3).
This indicates that the trees are mangroove trees that must have been
standing in the delta deposits with the mud flat just above their horizontal
roots. This gives evidence of an extensive erosion, removing about 80 cm of
the shore deposits; but no sea level rise. We know that this heavy erosion was
caused by the cyclone Sidar in 2007.
It also implies that there is no records of any sea level rise, on the
contrary; a stbility (Mörner, 2010).
5. Tsunamis
Tsunamis are generated by different forces as illustrated in Figure 4. Their
coastal damage is often directly disastrous.
From: Mörner (2010).
Figure 3. A closer examination of the tree stumps sticking out os the shore deposits at
low tide indicates that the trees have horizontal roots, which once radiated in the mud
deposits. This indicates that about 80 cm of sediments have been eroded away (by the

Costal Erosion and Costal Stability
75
Sidar Cyclone in 2007), and that the old mud flat had the same elevation as that of
today (excluding any recent sea level rise).
From: Morner, 2013b.
Figure 4. Five main factors generating tsunami waves.
Figure 5. The shore of the Yokosuka coast in Japan before (black line) and after
(photographed land/sea configuration at the March 11, 2011, tsunami are recorded by
Tanaka et al. (2012) and redrawn by Mörner (2013b). The coastal remodelling is,
indeed, very extensive.
The tsunami events of Lisbon 1755, Messina 1908, Indonesia 2004 and
Japan 2011 are all notorious for their terrible effects. They were all driven by
high-magnitude submarine earthquakes.
The erosional effects stike the subaereal and subaquatic coastal
environments and may remodel the coastal morphology quite substantially
(e.g. Tanaka et al., 20012; and collective volumes like Mörner, 2011, and
Kontar et al., 2014).

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Figure 5 illustrates the thorough remodelling of a coastal segment in
association with the 2011 tsunami event in Japan (Tanata et al., 2012).
SOME CASE STUDIES
Coastal erosion occurs all over the globe and can be extensively discussed
with examples from almost any coastal segment. In this case, I will confine the
discussion to a few sites because they offer fundamental aspects for the
understanding of coastal erosion and its real or imagined relation to sea level
changes; at stability, at regression (fall) and at transgression (rise). The main
examples will be taken from the Maldives (Mörner, 2007a).
1. Qatar
A costal segment at the Al Khor–Al Dakhira area in Qatar (Mörner,
2014a) offers a good example of erosion, longshore drift and the formation of
coastal barrier systems at periods of stable sea level conditiona at minor sea
level high-stands (Figure 6).
Figure 6 gives a Google image of the shore segment in question. It
documents 3 separate Holocene coastal barrier systems. They all represent
minor sea level high-stands; number 1 being the presently active beach system,
number 2 being undated and at about the same sea level hight, and number 3
representing the Holocene maximum at +0.3 m and dated at about 5000 cal.yrs
BP (Mörner, 2014a). Each of the three coastal barrier systems represents a
period of time with stable sea level conditions allowing the barrier systems to
be built out in a more or less regular fashion. This may be especially significan
for the present beach indicating a formation under stable sea level conditions
(Mörner, 2014b) contradicting model scenarios of a present sea level rise (e.g.
IPCC, 2007).
2. The Maldives
The Maldives is a nation of some 1200 islands arranged in 20 major atolls
(in the Google definition of the word ”atoll”, we can read: ”the islands of the
Maldives are grouped in ring-shaped atolls, each enclosing a relatively
shallow lagoon with a flat sandy bottom”).

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In the period 1999 to 2005, we had a separate international sea level
research project devoted to the Maldives (Mörner et al., 2004; Mörner, 2007a,
2007b, Mörner et al., 2008; Mörner and Dawson, 2011; Rufin-Soler et al.,
2014; Mörner 2014b).
Figure 6. Google image of the Al Dahkira region in Qatar (from Mörner, 2014a),
recording a Last Interglacial shore cliff and three Holocene coastal barrier systems
representing stable sea level conditions at minor eustatic high-stands. The 3rd system
represents the local Holocene sea level maximum at +0.3 m and dated at 5000 cal.yrs
BP (red dot). The 1st system refers to the present beach system and indicates a
formation under stable sea level conditions.

Nils-Axel Mörner
78
Already at our first expedition in 2000, I noted that the present active
beach was located shortly below a subrecent beach, now left for overgowing
and weathering. The abondonned beach was located about 20 cm higher than
the present beach; indicating a fall in sea level which was likely to have
occurrec in the middle to end of the previous century.
The local fishermen could inform us that the change was likely to have
occurred in the 1970s (Mörner et al., 2004; Mörner 2007a).
Like in any other island regions or coastal areas, there are several sites of
active erosion, and this was sometimes claimed to give evidence of a sea level
rise. In the real world – i.e. nature itself – erosion is by no means an a priori
indication of sea level rise as demonstrated in Figure 7 (from Mörner, 2007b).
Consequently, Figure 7 gives evidence of erosion initiated by a sea level
fall (regression); not by a rise. This is, of course, of fundamental importance
for a realistic interpretation of our field observations.
It gave rise to the Figure 8 cartoon of erosion with respect to a sea level
rise (b) and a sea level fall (c), repectively. What we documented in the
Maldives followed the case (c) model.

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79
Figure 7. Above: Heavy activ erosion. Below: The lateral deposition of the eroded
material occurs outward-downward; i.e. to a lower sea level than before. The fossil
shore of the pre-1970 sea level is now located inlands and in the stage of becoming
owergrown.
Figure 8. Island wind- and lee-side morphology (a) and erosion/deposition in
association with a sea level rise (b) and a sea level fall (c), repectively. In the exposed
wind-side of (a) are marked the low-tide level (LT), mean-tide leve (MT), high-tide
level (HT) and washing limit (WL). When sea level rises in (b) minor erosion must
take place as the shore profile is pushed inlands. On the depositional lee-side, the
eroded material is redeposited upwards (arrow) in over previous land surfaces. When
sea level falls in (c) quite extensive erosion takes place because the old equilibrium
profile has to be reorganized. On the lee-side, the eroded material is redeposited
downwards and seawards, and the old beach surface is left for weathering and
overgrowth; i. e. just as observet in Figure 7.
3. Tuvalu, Vanuatu and Kiribati
The South Pacific islands of Tuvalu, Vanuatu and Kiribati are frequently
mentioned as islands in the process of becoming flooded and islands of heavy
erosion. In all of those islands, the local tide gauge stations indicate stable sea
level conditions over the last 20-40 years (Mörner, 2007b, 2014b).
A present erosion in any of those islands does not provide conclusive
evidence of a sea level rise; just a case on active erosion. The Figures 7 and 8
records provide key data for the evaluation of active erosion with respect to
sea level changes.

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The conclusion is straight forwards: erosion is not an evidence of sea level
rise; it might, but it might also refer to coastal stability (Figure 6) or a sea level
fall (Figure 7). The tide gauge information provides superior information in
these cases.
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Coastal and Beach Erosion: Processes, Adaptation Strategies and
Environmental Impacts
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