Erosion: A human induced threat for the northwestern depositional coasts of Rhodos Island (SE Greece)

Abstract

Rhodes is the largest island of Dodecanese island complex in the SE Aegean Sea. Due to its geological structure it is characterized by a relatively high percentage of depositional coasts. The depositional northwest coasts of Rhodes Island have experienced strong erosion during the last few decades, due to a progressive decrease in sediment supply, resulting mainly from human activities along the coastline and in the hinterland. The extensive erosion has significant environmental and socioeconomic impacts, since considerable recreational infrastructure has been developed at the coastal zone. Coastal geomorphological observations were conducted and the coastal and sediment dynamics at certain locations of the northwest coasts were studied. To protect eroding coasts, the engineers suggest usually ‘hard works’, such as constructions parallel or normal to the shore, stabilization works, etc. We consider that the selection of the most suitable method is subject to environmental, technical, economical, political, social and aesthetic limitations and in contrast to the local defense measures (groins normal to the shore) that have been used unsuccessfully so far, we propose for the most of the places along the eroding northwest coastal area of Rhodes Island periodic beach nourishment.

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Erosion: A human induced threat for the NW coasts of
Rhodes Island (SE Greece)

Anagnostou, Ch.1, Sioulas, A.2, Hatiris, G.2, Karageorgis, A.1
& Chronis, G.1

(1) Institute of Oceanography, Hellenic Centre for Marine Research, 19013
Anavyssos, Greece, Tel: +30 22910 76369, Fax: +30 22910 76347
chanag@ath.hcmr.gr
(2) Hydrobiological Station of Rhodos, Hellenic Centre for Marine Research, Rhodos,
85100, Greece, Tel: +30 22410 27308, Faxl: +30 22410 78321, asioulas@hsr-ncmr.gr


Abstract

Rhodes is the largest island of Dodecanese island complex in the SE Aegean
Sea. Due to its geological structure it is characterized by a relatively high percentage of
depositional coasts. The depositional northwest coasts of Rhodes Island have
experienced strong erosion during the last few decades, due to a progressive decrease in
sediment supply, resulting mainly from human activities along the coastline and in the
hinterland. The extensive erosion has significant environmental and socioeconomic
impacts, since considerable recreational infrastructure has been developed at the coastal
zone. Coastal geomorphological observations were conducted and the coastal and
sediment dynamics at certain locations of the northwest coasts were studied. To protect
eroding coasts, the engineers suggest usually ‘hard works’, such as constructions
parallel or normal to the shore, stabilization works, etc. We consider that the selection
of the most suitable method is subject to environmental, technical, economical, political,
social and aesthetic limitations and in contrast to the local defense measures (groins
normal to the shore) that have been used unsuccessfully so far, we propose for the most
of the places along the eroding northwest coastal area of Rhodes Island periodic beach
nourishment.

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Introduction

In the present work, the natural phenomena that shape the northwest coast of
Rhodes Island and the factors that diminish sediment supply, resulting to a considerable
loss of beach material and finally erosion, are investigated. Rhodes Island, due to its
geological structure is characterized by abundance of depositional coasts. The
depositional northwest coasts of Rhodes Island have experienced strong erosion during
the last few decades, due to a progressive decrease in sediment feed, resulting mainly
from human activities along the coastline and in the hinterland. Coastal
geomorphological observations were conducted, the geological background of the area
is taken into account, and the coastal and sediment dynamics at certain beach locations
were defined, coupled with an assessment of the wind and wave climate. Recent
changes in coastline configuration were also analyzed and available historical wind data
were utilized to acquire an image of the wind climate in the last decades. The attempt
was to determine the drivers of erosion and to propose potential methods of coast
protection to involved local authorities.

The dynamics of the coastal area are mainly driven by sediment supply and
transport, the first depending on the geomorphology and the geological substratum of
the greater region, the local hydrology and the land coverage; the latter greatly relies on
the wind/wave regime, since the role of tides in coastal dynamics in Greece is not a
remarkable one, and seasonal variations or extremes of meteorological phenomena
determine erosion and accretion patterns. Various human activities influence coastal
dynamic leading to a progressive deficit in sediment supply or by obscuring the natural
transport processes.


The study area

The study area includes the N and NW parts of the coasts of Rhodes Island (Fig.
1). The geological structure of the island shows in the southern half of the island the
domination of alpine geological formations, and in the northern part the extension of
post-alpine sedimentary depositions (Fig. 2). This geological structure led to a
geomorphological configuration of the island and a differentiation of the coasts in rocky
coasts and depositional coasts. Plio-Pleistocene formations of marl, sandstones and
conglomerates build the geological substratum of the study area. These formations are
the main source of sediment of the area. The coasts are influenced by waves of W, SW
directions, which cause a sediment transport in a south-north direction. In the NW
coasts of Rhodes Island the extensive depositional coasts, consisting on gravel and
coarse sand dominate. These coasts are occupied by tourist facilities (sun chairs,
umbrellas, etc.). The area is also relative densely populated. The urban development of
the area as well as different human activities along the coasts and in the hinterland has
destructed the sediment supply – sediment distribution mechanisms. Removal of
sediments from the small rivers (the main sediment source of the study area), building
of small ports and other constructions along the coasts have led to sediment starvation
and have intensified coastal erosion.

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Fig. 1: Location of Rhodos Island in the SE Aagean Sea




Fig. 2: Geological structure of Rhodos Island

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Methods

In the present work observations are made along the N and NW coasts of
Rhodes aiming at a better understanding of coastal dynamic processes. These
observations are combined with a study of the local wind-wave regime. All acquired
information as the basis for the proposal of the most suitable measures for the protection
of the coasts.

Results

Observations


as well have recorded some technical measures applied against the erosion threat (Fig.
3). The sand-gravel composition of the coasts is also documented and places with
sediment accumulations indicate the S to the N dynamics of sediment transport.

Field observations carried out, documented the extensive erosion of the coasts,
Wind – wave regime

Analysis of the available historical wind data from the meteorological station of
the airport of Rhodes, for the years 1955 – 1986, as well as high resolution hindcast
wind and wave results of the HCMR POSEIDON system (Soukisian et al., 2000) show
that winds of SW, W and NW directions dominate, which affect the study area (Fig. 4).

Sediment transport

The coastline configuration and the main angle of incident waves are the force
for the sediment transport along the coasts. The sediment transport follows the S to N
direction. At the NE tip of the island seasonal coastline changes are observed
(Anagnostou et al. [in press]). During the spring - summer period, at the western part of
the tip, the dominant waves move sediments in a clockwise direction towards the north,
and the sediments are deposited on the northeastern side of the tip forming a sand spit
towards the east. During winter months, the eastern part of the tip is dominated by
easterly and southeasterly winds and the wave action forces sediments to move
counterclockwise back to the west. This seasonal variation has been identified over the
years and is expressed through seasonal changes in the coastline configuration.


Discussion

When it comes to protecting coasts under erosion, the available coastal
engineering solutions include mainly coastal defense measures such as constructions
parallel or normal to the shore, stabilization works (such as sea walls) (Fig. 5)
(Anagnostou et al., 2002). However, the selection of the most suitable method is a more
complicated procedure, because it incorporates environmental, technical, economical,
political, social and aesthetic limitations and should be seen as part of an integrated
management approach (Antoniou et al., 2009).

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Fig. 3: Examples showing the coastal erosion and the measures against erosion.





Fig. 4: The wind dynamic regime in the study area, showing the dominated winds from
SW, W and NW direction.

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In contrast to the local defense measures (groins normal to the shore) that have
been used largely unsuccessfully so far, the optimal solution proposed for the most of
the places along the eroding northwest coastal area of Rhodes Island is that of periodic
beach nourishment. However, as this remediation remains one more human
intervention, several environmental impacts should be anticipated and deeper
understanding of the beach and coastal dynamics should be achieved. Also, it must be
only considered as part of an integrated coastal management strategy, which would
assist to a sustainable use of a priceless resource of Rhodes Island.




Fig. 5: Coastal defense measures with constructions normal to the shore. This is a
engineering solution to the coastal erosion problems.


Conclusions

In the present work an attempt was made to combine geological –
geomorphological information and wind/wave data, in order to define which physical
processes drive coastal erosion and according to this natural dynamics to propose
feasible protection measures. Following the “adaptation principle” and in contrast to the
local defense measures (groins normal to the shore) that have unsuccessfully been taken
so far, the optimal solution proposed for the most of the places along the eroding coastal
zone of the study area is that of periodic beach nourishment.

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References

Anagnostou, Ch., Drakopoulou, P. & Soukissian, T. (2002), “The morphodynamics of
the west coast of Messiniakos Gulf and the role of the Agios Andreas marina”
Proceedings of the 2nd Pan-Hellenic Conference on Management and
Development of coastal Zones (Athens Greece), pp 249 – 252 (In Greek).

Anagnostou, Ch., Antoniou, P. & Hatiris, G. (in press), “Erosion of a depositional coast
in NE Rhodos island (SE Greece) and assessment of the best available measures
for coast protection” Journal of Coastal Research, SI 64.

Antoniou, P., Kyriakidou, H. & Anagnostou, Ch. (2009), “Cement filled geotextile
groynes as a mean of beach protection against erosion: a critique of application in
Greece”. Journal of Coastal research, Special Issue No 56, pp. 463 -466.

Soukissian, T., Chronis, G. & Nittis, K. (2000), “POSEIDON: Operational Marine
Monitoring System for Greek Seas”. Sea Technology, 40(7), 32-37