Greek case studies: Long term geomorphological changes in the coastal zone of the Thermaikos Gulf, Salonika Region, North Greece

Department of Physical & Environmental Geography, School of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

Abstract

During late Pleistocene (Würm Glacial) the area surrounding the city of Thessaloniki and the northern part of Thermaikos Gulf was a low-lying valley bounded by a hilly terrain and drained by the Axios River and its tributaries (rivers Gallikos, Aliakmon and Anthemoundas at present). The Holocene transgression caused the inundation of the lower parts of the valley and gradually led to the present shoreline configuration. Along the north and northwestern parts of the area the coastal zone has been impacted by depositional processes of the Gallikos – Axios – Aliakmon rivers forming a very extensive deltaic complex. Thessaloniki delta plain is the largest (~2 000 km²) deltaic area of Greece. In contrast, along the eastern part of the region beaches are backed by coastal terraces and with some exceptions are characterized by erosional trends. A number of both prehistoric and historic coastal settlements existed in the region and have been affected in many ways and various degrees by the prolongation or the retreat of the coastline. The city of Thessaloniki is built on a hilly terrain in the upper most part of Thermaikos gulf, around a smaller embayment where the harbor and the seafront of the city are. Generally, the morphology of the city’s surrounding area involves a hilly terrain at the north and two lowland areas at the west, where the big deltaic plain of Axios is formed and at the east, between the capes Mikro Emvolo and Megalo Emvolo where is the Anthemuntas basin.

1. Introduction

The present morphology of both Thessaloniki Bay and Thermaikos Gulf is the result of numerous natural processes having taken place during the past 10,000 years. The most important of these processes has been the rise of the sea level resulted from glacial melting after the last glacial maxima (Würm) and the continuous sediment deposition from the Gallikos, Axios and Aliakmon Rivers which formed a large deltaic complex on the north and western parts of the Gulf. As the sea level rose the low lying regions were inundated and new equilibrium was established in the area. In particular, along the northern part of the advancing shoreline numerous river deltas were formed. The base level migration resulted in changing erosion/deposition conditions both on terrestrial and marine environments. Humans have appeared in the region during the past 7000 years (Grammenos, 1991; French, 1967). As expected, human settlements been filled with sediments delivered by the main rivers whereas others have been destroyed by coastal erosion and the retreating shoreline. Natural processes have been shaping the landscape until the beginning of the 20th century. Major human impacts include a series of hydraulic works as dam constructions, drainage of the floodplains, sand quarrying, canalization, land reclamation, and construction of levees and weirs. As a result, the evolution of the present landscape is not related to the sequence of natural processes but to the intensity of human impacts (Konstantinidis, 1989; Psilovikos & Psilovikos, 1997).

2. Study area

The study area includes Thessaloniki Bay and the northern part of Thermaikos Gulf (Fig. 1). The morphology of the northern and eastern parts of the study area is characterized by rolling hills composed by Tertiary and Quaternary sediments. Erosional trends exist there with only exception the coastal zone of Anthemoundas River.

Figure 1. Location of the study area together with the type of coasts found in the North Thermaikos Gulf and the Thessaloniki Bay.

In contrast to the eroding coasts on the north and eastern parts the western part of the study area is dominated by the prograding deltas of Gallikos –Axios – Aliakmon Rivers (Lykousis et al., 1981; Poulos et al., 1994). The present location of their channels and deltas is the result of numerous human interventions that have been taking place since the 1930’s (Albanakis et al., 1993). Most important the mouth of Axios River has been displaced from its natural course outside Thessaloniki Bay across cape Megalo Emvolo. The displacement of the river mouth was important in that it decelerated the siltation of Thessaloniki port (Evmorphopoulos, 1961). The low relief prograding coasts on the western part give place to the eroding coastal terraces on the eastern part even though deposition along the coastline occurs on isolated parts. Finally, deposition caused by humans along numerous parts of the coast of Thessaloniki Bay has resulted in manmade coasts.

3. Morphological changes in the coastal zone

3.1. The deltaic complex of the Aliakmon, Loudias, Axios, & Galikos Rivers

By the end of the last glacial age the study area comprised the lower part of a low lying river valley drained by the Axios River and its tributaries Gallikos, Aliakmon and Anthemoundas rivers together with other minor streams. The river mouth was located in the center of the present-day Thermaikos Gulf and below the 100m-depth contour (Lykousis and Chronis, 1989). The rise of the sea level flooded the river valleys and the coastline begun retreating to the north. Sea inundation reached beyond the present coastline about 30 km to the northwest totally drowning the area occupied today by the Thessaloniki – Giannitsa plain. The main port of the Macedonian era, Pella is found today 25 km inland. The higher base level shifted the accumulation area of river discharge more inland. Continuous sediment deposition took place in deltaic, lagoonal and marine environments (Fig. 2) (Fouache et al., in press). Today, the river deltas are characterized by relatively narrow prodelta slopes whereas their deltaic platforms extend much across the study area (Lykousis and Chronis, 1989).

Figure 2. Holocene reconstruction of the growth of the deltaic plain of Thessaloniki (Fouache et al., in press).

According to Fouache et al. (in press), the main stages in sedimentary and environmental conditions of the Thessaloniki plain are the following:

• The first stage corresponds to the maximum extension of the sea intrusion. This stage is dated around 4000 B.C., which corresponds to the peak of the Holocene transgression. * From 4000 B.C. and 3000 B.C., we have the maximum extension of the shoreline in the actual alluvial plain.
• During the second stage dated approximately from 3000 B.C. to 2700 B.C., we have a shallow marine to lagoonal environmental conditions all around the bay
• The third stage shows a lacustrine occupation around 1600 B.C.
• The fourth stage around 400 A.D. represents the initial formation of the present shoreline. In this stage we have archaeological evidences of Roman constructions (Bridge) very close to the coastal zone.
• The last stage is till the beginning of the 20th century. At early 30’s we have the human interference with public works in the channels of the rivers and the coastal zone.

3.2. Erosional processes and shoreline migration

On the other hand along the eastern sides of northern Thermaikos Gulf and Thessaloniki Bay, sediment deposition was localized and was taking place only near the Anthemoundas river mouth. But as the sea was advancing and filling the Bay the remaining beaches were subjected to the erosional action of waves. The case when beach formations and sediments are not cohesive enough to resist to wave action is considered identical for the formation of steep beaches and coastal terraces. The terrace front is subjected to continuous wave action, which creates a scour at its base and hence leads to the collapse of the upper part. In this way the shoreline is gradually retreating backwards and the eroded sediments are deposited along the beach (Fig. 3).

Figure 3. The coastal terraces formation due to the erosional action of waves on the eastern coast of the Thermaikos Gulf.

3.3. The sea front of the Salonica city

Stratigraphical data from boreholes drilled along the coastal area of the city of Thessaloniki (Vouvalidis et al., 2005) showed that there was a surface layer of human debris up to 8 m thick above marine sediments. It is remarkable that the marine sediments are covered immediately by human debris of various ages (hellenistic, - roman – ottoman - recent). This suggests that the shoreline was initially inshore and shifted offshore due to debris deposition. The human impact was focused primary on the seafront of the old city of Thessaloniki (from the harbour to White Tower). The city of Thessaloniki has evolved as a harbour for more than 23 centuries, ancient and recent human debris lies directly on top of marine sediments. The coastline along the city’s seafront, in the absence of significant sedimentation from river input, should have been inshore its present position as transgression proceeded. But borehole data suggested that human activity over the centuries not only kept the seafront stable but both the city and the harbour extended seawards in order to provide more space to the inhabitants. The Thessaloniki case provides a good example of how humans were protected from increasing sea level and can be used as a reference for future reaction of major cities to sea level rise.

4. Concluding Remarks

This article is an attempt to be described the long term geomorphological changes on the coastal zone of the Thermaikos Gulf. The case study of the Thermaikos Gulf is a very typical example, of the wide range geomorphological changes could take place in the coastal zone. The human occupation has been adapted to the shoreline displacements since Neolithic times. During the historical period, Macedonian and Roman occupation let in the coastal zone numerous archaeological remains. According to literature, we know that the configuration of the coastal zone changed frequently. The descriptions of the landscapes from ancient authors confirm a strong spatial evolution and a human adaptation to the changes. This contribution of the palaeogeographical reconstruction of the Thermaikos coastal zone, it will be a useful tool to predict the future changes in the coastal zone due to global warming.

References

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The main author of this article is Vouvalidis, Konstantinos Please note that others may also have edited the contents of this article. Citation: Vouvalidis, Konstantinos (2007): Greek case studies: Long term geomorphological changes in the coastal zone of the Thermaikos Gulf, Salonika Region, North Greece. Available from http://www.coastalwiki.org/wiki/Greek_case_studies:_Long_term_geomorphological_changes_in_the_coastal_zone_of_the_Thermaikos_Gulf,_Salonika_Region,_North_Greece [accessed on 27-04-2024] For other articles by this author see Category:Articles by Vouvalidis, Konstantinos For an overview of contributions by this author see Special:Contributions/Vouvalidis, Konstantinos