Greek case studies: Long term geomorphological changes in the coastal zone of the Thermaikos Gulf, Salonika Region, North Greece
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Department of Physical & Environmental Geography, School of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
During the 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 north-western parts of the area the coastal zone has been impacted by depositional processes of the Gallikos – Axios – Aliakmon rivers, which produced a very extensive deltaic complex. The 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 located. Generally, the morphology of the city’s surrounding area consists of a hilly terrain at the north and two lowland areas: the large Axios deltaic plain at the west and the Anthemuntas basin at the east, between the capes Mikro Emvolo and Megalo Emvolo.
The present morphology of both Thessaloniki Bay and Thermaikos Gulf is the result of numerous natural processes that have taken place during the past 10,000 years. The most important of these processes was the sea-level rise after the last glacial maximum (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 a new equilibrium was established in the area. Especially along the northern shore, numerous river deltas were formed. The marine transgression resulted in changing erosion/deposition conditions both on terrestrial and marine environments. Humans have appeared in the region since the past 7000 years (Grammenos, 1991; French, 1967). Many human settlements were buried under sediments delivered by the main rivers, whereas others were 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). For an overview of the recent evolution the reader is referred to the article Greek case studies: Geomorphological changes on the coastal area of the Inner Thermaikos Gulf over the past 150 years.
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 are characterized by rolling hills composed by Tertiary and Quaternary sediments. Erosional trends are ubiquitous with the only exception of the Anthemoundas River coastal zone.
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 the 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 taken place since the 1930’s (Albanakis et al., 1993). Most importantly, 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 because it reduced the siltation of Thessaloniki port (Evmorphopoulos, 1961). The low relief prograding coasts on the western part go along with 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 an artificialized coast.
Morphological changes in the coastal zone
The deltaic complex of the Aliakmon, Loudias, Axios, & Galikos Rivers
The study area comprises the lower part of a low lying river valley, which was drained at the end of the last glacial age by the Axios River and its tributaries Gallikos, Aliakmon and Anthemoundas rivers, together with other minor streams. The river mouth was located at the centre of the present-day Thermaikos Gulf and below the 100m-depth contour (Lykousis and Chronis, 1989). The rising sea flooded the river valleys and the coastline started to retreat to the north. Marine transgression 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).
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 sea in the actual alluvial plain;
- During the second stage, dated approximately from 3000 B.C. to 2700 B.C., we have 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 evidence of Roman constructions (Bridge) very close to the coastal zone;
- The last stage ends at the beginning of the 20th century. At the early 30’s we have the human interference with public works in the channels of the rivers and the coastal zone.
Erosional processes and shoreline migration
Along the eastern sides of northern Thermaikos Gulf and Thessaloniki Bay, sediments were deposited locally near the Anthemoundas river mouth. But as the sea was advancing and filling the Bay, the remaining beaches became subjected to the erosional action of waves. As beach formations and sediments were not cohesive enough to resist wave action, steep beaches and coastal terraces arose. The terrace front was subjected to continuous wave action, creating a scour at its base and hence leading to the collapse of the upper part. In this way the shoreline was gradually retreating backwards and the eroded sediments were deposited along the beach (Fig. 3).
The sea front of the Salonica city
Stratigraphic 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 anthropogenic debris up to 8 m thick above the marine sediments. It is remarkable that the marine sediments were covered immediately by anthropogenic 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 anthropogenic debris are lying 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. However, borehole data suggest that human activity over the centuries not only kept the seafront stable, but even extended the city and the harbour seawards in order to provide more space to the inhabitants. The Thessaloniki case provides a good example of how humans coped with an increasing sea level; this example is useful as a reference for the future response of coastal cities to sea level rise.
This article presents a description of the long-term geomorphological change of the coastal zone of the Thermaikos Gulf. The case study of the Thermaikos Gulf is a very typical example of the wide range of geomorphological changes that can take place in the coastal zone. Human occupation has adapted to the shoreline displacements since Neolithic times. During the historical period, Macedonian and Roman occupation left numerous archaeological remains in the coastal zone. According to the literature, we know that the configuration of the coastal zone changed frequently. The descriptions of the landscape from ancient authors confirm a strong spatial evolution and human adaptation to the changes. This contribution of the palaeogeographic reconstruction of the Thermaikos coastal zone provides insight that can be useful for coping with future changes in the coastal zone due to global warming.
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