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Subsidence of the Thessaloniki (northern Greece) coastal plain, 1960–1999
Engineering Geology 61 (2001) 243±256
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Subsidence of the Thessaloniki (northern Greece) coastal plain, 1960±1999 Stathis C. Stiros Applied Geodesy Laboratory, Department of Civil Engineering, University Campus, University of Patras, Patras 26500, Greece Received 11 May 2000; accepted for publication 8 February 2001
Abstract Comparison of aerial photos, maps and triangulation data reveal that in the last 40 yr a part of the Thessaloniki coastal plain, a delta formed in the last 2500 yr, subsided at a rate of up to 10 cm/yr. As a consequence the sea invaded up to 2 km inland; precious land in the suburbs of the city was lost, while a village and major industrial plants are in risk of ¯ooding. Part of the land was reclaimed thanks to barriers, pumping and arti®cial raising of the land surface. Yet, the situation is unstable and ¯ooding is not unusual. Ground water withdrawal for the needs of the Thessaloniki metropolitan complex has initially been regarded as the cause of the subsidence. However, the lack of correlation in space and in time between ¯uctuations of piezometric levels, topographic changes and pumping indicates that the observed subsidence should be regarded as the cumulative effect of several factors, including consolidation of near-surface sediments due to the decline of the piezometric level and the partial abandonment of the delta, oxidation of peat soils in the vadose zone, synsedimentary deformation (faulting and ¯ow) and loading-induced consolidation of deeper sediments. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Land subsidence; Delta; Sediment compaction; Pumping; Geodesy
1. Introduction The Kalochori area, in the suburbs of Thessaloniki, the second major city in Greece (Fig. 1), is located on the young (up to a few thousand years old) delta of the Axios (Vardar) and of other large rivers. The Kalochori area is currently subsiding and for this reason one of the major industrial zones in Greece is under the threat of marine invasion. In spite of its importance, this phenomenon has not been studied in detail and has been rather a priori assigned to compaction of sediments due to overpumping of ground water (Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). In this article we present geodetic constraints to the geographic distribution, timing and amplitude of the E-mail address: [email protected] (S.C. Stiros).
observed subsidence in the last 50 yr, and based on such evidence, we try to put some constraints on the history and causes of this phenomenon. 2. Previous studies and results Reliable historical evidence (Herodotus, VII 123± 124) suggests that in circa 500BC, Thermaikos Gulf, at the NW tip of the Aegean Sea (northern Greece), was much deeper and Pella, the capital of ancient Macedonia was a coastal town (Fig. 1). In the subsequent centuries deposition of deltaic sediments of four main rivers (Gallikos, Axios (Vardar), Loudias and Aliakmon) caused an approximately 80 km shift of the coastline leading to the partial closure of this gulf and the present-day coastal topography.
0013-7952/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0013-795 2(01)00027-8
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Fig. 1. The approximate shoreline of 500BC in the Thessaloniki plain is shown. Two dotted lines indicate the former beds of Axios River. The Kalochori area is the coastal area between the NW edge of Thessaloniki and the former Axios riverbeds. In the inset, location map.
The lowlands between the Gallikos River and Thessaloniki were ®rst occupied in 1922 by refugees from Minor Asia (Turkey) who established the Kalochori village, then in 1945 the Axios riverbed was redirected southwards in order to prevent the
Thessaloniki harbour, one of the major harbours in the Balkans, from silting. The Kalochori area obtained economic and social signi®cance in the 1960s, when major industries (re®neries etc.) were established there, taking advantage of cheap land in the suburbs
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Fig. 2. An example of subsidence and of well protrusion (approximately 80 cm) at a well SE of Kalochori. Photo taken in 1985, after the area was reclaimed. An arrow points to the upper limit of marine organisms marking sea level during the period of marine invasion.
of the major metropolitan centre of Thessaloniki. The increasing need for land inspired plans for development of the wider area, even the reclamation of wetlands south of Kalochori. The need for fresh water, on the other hand, led the Thessaloniki Water District to drill, since 1955, about 25 wells producing an average of 1600±1900 m 3/h for the period 1958± 1963. About 300 private wells providing smaller quantities of water for the needs of seasonal cultivation or small industries were in operation during this period (Andronopoulos et al., 1990). The ®rst signs of subsidence in the Thessaloniki plain were noticed in the 1960s in the Giannitsa area, close to Pella (Fig. 1); this subsidence followed major works for drainage of a nearly swampy plain, and has been attributed to decomposition of organic material (ETOME Co., 1974). In 1964 a progressive marine invasion was ®rst noticed, then in the subsequent years it was realized that discharge of waters after heavy rainfalls had become very dif®cult, leading to ¯ooding. In 1969, probably because of a combination of strong winds and a barometric peak, for the ®rst time seawater
reached the houses of Kalochori, an event, which forced the construction of a coastal dam. This dam was effective till 1973, but it was subsequently destroyed due to strong subsidence. The signs of this subsidence since at least 1973 were also visible in wells more than 100 m deep, which protruded by up to 0.9 m (Fig. 2), as well as in ¯ooded basements of a dozen houses and of the square around the church of Kalochori. As an attempted remedy, the ¯oor of the basements and of the village square were raised by approximately 1±1.2 m (Andronopoulos et al., 1990). A ®rst study of the subsidence was made in 1974, in the framework of plans for the development of the area, including the installation of the Thessaloniki International Fair in the Kalochori area. This preliminary study noticed the ongoing subsidence, but concluded that it could be controlled by a coastal barrier, pumping of the ponded water and arti®cial raising of the ground surface by 1±2.7 m; the necessary material to be deposited, about 7.6 £ 10 6 m 3, was to be quarried from the Gallikos riverbed, north of Kalochori (ETOME Co., 1974). A coastal barrier was constructed in 1976, approximately along the
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Fig. 3. Remains of electricity poles offshore, along a road constructed in 1975 south of Kalochori, but subsequently submerged. Photo taken in 1991.
trace of the 1956 shoreline, permitting the reclamation of a part of the lost land, about 3±4 km 2, and a system of roads and an electricity network were established. Yet, after a storm in the winter of 1979±1980, the coastal dam failed, a major part of the area south of Kalochori, including the reclaimed land, was covered by waters, and the electricity poles were left offshore, as a reminder of the extent of the lost land (Fig. 3). The situation was partly stabilized after new coastal barriers were erected in the early 1980s, but the area remains vulnerable to ¯ooding. For instance, after the ¯ood of 23 October 1991, the coastal barrier west of Gallikos River failed, and an inhabited or cultivated area about 3 km 2 wide was covered with waters. Pumping in the Kalochori area was, on the other hand, drastically reduced in the early 1980s, and since then only small amounts of water, 10±15 000 m 3/day are estimated to have been extracted from mainly private wells, usually in summer (K. Miloglou, personal communication); the water table height which
had declined in the previous years as much as 25 m recovered to pre-pumping levels (Fig. 4), but no signi®cant rebound of the land surface was reported. A new study of the subsidence started in 1980 by the Institute of Geology and Mineral Exploration (IGME) was mostly based on three borehole data. The major conclusions of this study were: (1) that subsidence is con®ned to the area of Kalochori and Sindos; the ground is uniformly subsiding and no differential settlements have been noticed, but the greatest compaction varies between 2±3 m; (2) that the observed up to 40 m lowering of the piezometric surfaces of productive wells and the uniform lowering of the ground surface indicate that ground subsidence is caused by compaction of certain highly compressive layers following the decline of the ground water level; and (3) that ground subsidence, in particular, is related to reorientation of the abundant mica particles of highly compressive sandy horizons in two black silty clay layers producing extreme compaction
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Piezometric head decline (m)
0
-10
K6 -20
K11
K10
-30
K8 -40
1970
1975
1980
1985
Time
Fig. 4. Fluctuation of piezometric levels of four representative productive wells near Kalochori (for location see Fig. 7).
(Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). These conclusions were not without legal implications, for the community of Kalochori was accusing the Thessaloniki Water District Authority of causing the damage. 3. Geology The wider Kalochori area is located at the SE edge of the Thessaloniki plain. This plain corresponds to a complex delta formed in the last few thousand years (see above) in a more than 600 m deep Quaternary graben cut in Neogene, mainly sediments and ®lled with unconsolidated to partly consolidated deltaic sediments, mostly sand and silt (Fig. 5). Neogene formations, mostly sandstones, marls and clays of unknown thickness, overlain gneiss which crop out in the Thessaloniki area (Kockel et al., 1978). The relief in the wider Thessaloniki plain is very smooth while in the Kalochori area it is nearly ¯at, with a maximum elevation of 4±5 m above the water. Sandy formations dominate this area, and black silty clays SE of the village, along the coastal zone (Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). The subsurface geology of the area based on borehole data has been studied by Palasis (1972/73) and is shown in Figs. 5 and 6. However, the stratigraphy, of this recent complex delta is obviously not uniform,
but laterally variable, deviating from the above idealized scheme (Fig. 5). 4. Hydrology The ¯ow of four main rivers provides an intense and continuous recharge to aquifers of the area, which, however, are discontinuous due to the intense lithological variability of deltaic sediments. For this reason, pumping cones with radii of up to 600 m were observed in productive wells (ETOME Co., 1974). The most important productive aquifers identi®ed and exploited by the Thessaloniki Water District Authority are gravel and sand layers, up to a few meters thick, usually at the mean depth of 30, 60 and 90±120 m, and in many cases up to 220 m deep. These aquifers permitted a minimum pumping rate of 1600±1900 m 3/h for the Kalochori area and 1000±2100 m 3/h for the Sindos area (Andronopoulos et al., 1990); for location see Fig. 7. Groundwater piezometric level was initially at or close to the ground surface (0±3 m depth, measurements in 1974, ETOME Co., 1974), by declining to the maximum depth of 35 m in 1976, but recovering to 1970 levels in 1984, shortly after pumping was drastically reduced or even, since 1980, stopped (Fig. 4; Meladiotis and Demiris, 1988; Andronopoulos et al., 1990, 1991). The long-term exploitation of the Kalochori aquifers caused a small-scale encroachment of saline
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Fig. 5. Geotechnical classi®cation of the lithostratigraphic units in the Kalochori village area based on three boreholes (K1, K2, K3, based on Andronopoulos et al., 1990) in comparison with the idealized pro®le in the whole of the Delta (I, based on Palasis, 1972/73). Classi®cation according to the AUSCS system.
water up to a nearly E±W trending front passing from Kalochori village, as an increase of electric resistivity of sediments indicates (Nicolaou and Nicolaidis, 1987). This encroachment, however, did not affect productive wells which are located farther north (Fig. 7).
5. Geotechnical characteristics Andronopoulos et al. (1990) classi®ed the Quaternary formations in the Thessaloniki plain in three horizons, sandy, silty and black silty clays, and computed
Fig. 6. Two representative lithostratigraphic cross sections in the Thessaloniki Delta based on correlations of borehole data (modi®ed after Palasis, 1972/73). Location, map on top of ®gure. NM indicates Nea Magnesia; dotted line indicates boundary of Quaternary sediments. (1) Sand, (2) gravel and pebbles, (3) silt and clay, mainly terrestrial sediments, (4) silt, clay and ®ne sand, limnic sediments, (5) clay and sand, marine sediments, and (6) silt, terrestrial sediments.
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Fig. 7. Productive wells in the Kalochori area (solid squares), contours of depth of pre-Holocene basement (in m, thin lines), the northern limit of the saline water incursion deduced from geoelectric data (dotted line), and synsedimentary faults (solid lines). A solid triangle indicates geodetic station 64 used as reference in elevation changes of Fig. 7. A dotted zone indicates the Gallikos Riverbed, while the Neogene basement is shaded. Based on data from Meladiotis and Demiris (1988), Nicolaides and Nicolaou (1987), and Kockel et al. (1978).
min±max
0.10/0.68±0.47/1.33 0.12/0.62±0.29/1.00 0.08/0.41±1.10/2.05 0.03/0.46±0.38/0.80
min±max
5/7±80/28 1/28±50/30 3/1±170/37 43/8±170/58
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some of their geotechnical characteristics based on three drillings, 47±91 m deep. Their results are shown in Table 1 along with their results for the Neogene formations. As far as the black clay formation is concerned, Andronopoulos et al. (1990, Table 6.2, 1991) noticed that it is practically free of montmorillonite. They also reported that mica particles are abundant in both the clay fractions and the sandy intercalations of this formation. This is discussed below. 6. Topography changes
17±25 14±117 4±120 44±1340 12.3/16.5±15.2/19.9 11.8/14.8±18.4/21.7 8.7/14.8±18.2/23.0 6.3/10.6±21.1/13.8 silty horizon sandy horizon black clay horizon neogene sediments
21.0/17.5±49.0/32.2 8.7/11.8±39.0/21.4 17.8/10.0±94.8/38.0 20.0/8.0±75.1/26.6
23.2±53.0 3.1±39.1 4.0±69.4 8.0±44.6
min±max min±max min±max min±max
Dry and wet density (kN/m 3) Water content w (%)
Uncon®ned compression (kPa)
6.1. Shoreline shift
Atterberg limits wL/wF (%)
Mechanical properties Physical characteristics Formations
Table 1 Physical and mechanical properties of the formations in the Kalochori area, according to Andronopoulos et al. (1990)
Triaxial test (kPa/8) cu/ u
Consolidation test cc/e0
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For the period 1945±1980, just before the shoreline was arti®cially controlled by levees, seven series of aerial photos of various scales (1:16 000±1:45 000), covering the wider Kalochori area are available. The shoreline, which is observed in each of these series of aerial photos was transferred to a 1:20 000 scale map of the area (produced by magni®cation of the 1:50 000 scale map) using a monoscopic transferscope on the basis of characteristic and precisely identi®ed reference points common in at least two series of photos. Results are summarized in Fig. 8 and indicate a marine regression of the order of 1±2 km. The accuracy of these shorelines is de®ned by the accuracy of line drawing, typically better than 1 mm, and corresponds to an accuracy of a few tens of meters. Therefore, Fig. 8 is representative of the history of the shoreline changes in the Kalochori area for the period 1945±1980. 6.2. Comparison of 1:5000 scale maps Part of the study area was covered in 1955 by 1:5000 scale maps compiled in 1955 by the Ministry of Transportation and Public Works. An accuracy of 50±60 cm is expected for heights in this ¯at terrain computed on the basis of theodolite and stadia measurements. A second set of maps of the same scale, produced on the basis of aerial photos taken in 1980 and photogrammetric techniques by the Hellenic Military Geographic Service (HMGS) is also available. The accuracy in heights of the various points in these maps is better than 0.5 m.
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Fig. 8. Coastline shift in the Kalochori area between 1945 and 1980, based on comparison of aerial photos.
A comparison of the two sets of maps revealed that they contained common points which could be unambiguously compared to provide their height changes for the time lapse between the compilation of the two sets of maps. A common grid with a separation of 4 cm (corresponding to 200 m on the land surface) was plotted in each set of maps and the obtained differences in elevations of the corresponding points were estimated. The elevation changes obtained, ranging up to a few meters, are shown in contour form in Fig. 9. The accuracy of relative height differences of each point in this diagram can be estimated from
the law of propagation of errors for uncorrelated variables from their standard errors, 0.6 and 0.5 m, respectively. Hence, the accuracy of computed relative height changes is expected to be better than (0.6 2 1 0.5 2) 1/2 m, approximately 0.8 m. This indicates that the contours of diagram of Fig. 9 re¯ect, though with some noise (uncertainty), real changes in the ground elevation between 1955 and 1980. A large-scale subsidence of the order of 1± 3 m can therefore be deduced from this map. An extreme amount of subsidence (more than 5 m) is seen NW of Kalochori, next to the Gallikos riverbanks, but this area can be identi®ed as the site of
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Fig. 9. Contours of height changes in meters between 1955±1980 based on comparison of 1:5000 scale maps. Extreme subsidence (4 m contour) indicates quarrying, while small amounts of elevation change south of Kalochori (22, 21 and 0 contours) indicate waste in®ll or deposition for protection against marine water encroachment. The Gallikos riverbed barriers are shown by two sub-parallel gray, hatched lines.
quarrying for material used for the construction of coastal dams or the raising of the ground surface. On the contrary, uplift observed south of Kalochori (area de®ned by the 0 m contour) corresponds to an in®ll, mostly urban waste deposited before 1975. In summary, the comparison of 1955 and 1980 maps indicates that the amount of subsidence is of the order of 3±4 m, and is not con®ned to the Kalochori area, but probably covers a much wider region.
6.3. Triangulation levelling data The study area covered by the fourth order National Geodetic triangulation network measured in 1980 with techniques providing a typical accuracy of 15 cm for heights. In 1985, a number of stations were found and a local EDM triangulation network was re-measured by an IGME team. A part of the 1985 network was reoccupied in 1999, using similar techniques, providing an accuracy of 11±20 cm for relative heights. Due to the limited coverage of the
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Fig. 10. Contours of elevation changes between 1999 and 1980 and 1985 and 1980 in cm, based on comparison of elevation differences between triangulation stations marked with small triangles. Elevation changes are shown relative to point 64 (large triangle, also shown in Fig. 7) and are accurate to within 25 cm. Positive relative elevation changes SE of Kalochori do not re¯ect land uplift, but less intense subsidence than in the Sindos area.
1985 and 1999 networks, a comparison of the 1980, 1985 and 1999 surveys was made relative to benchmark 64 which is located in the subsiding area, and computed relative elevation changes in contour form are shown in Fig. 9. Since benchmark 64 is not located on stable ground, the apparent land uplift shown in this ®gure do not indicate a land uplift, but a rapid subsidence of the Sindos area (i.e. the area close to benchmark 64). 7. Discussion The history of shoreline retreat (Fig. 8) reveals a major phase of subsidence (about 500 m retreat of the shoreline) between 1960 and 1968, followed by a second one between 1972 and 1975±1976. This result is consistent with information conveyed by local authorities (last paragraph in Section 6.2) that the ®rst clear signs of marine invasion were noticed in 1964. The total amount of subsidence between 1955 and 1980 is at least of the order of 3 m, as the
comparison of maps indicates (Fig. 9), while triangulation data reveal that between 1980 and 1985 and 1985±1999 the rates of relative height changes are of the order of up to 8 and 10 cm/yr, respectively (Fig. 10). These data reveal that most surface subsidence and the related marine invasion seem to postdate the Kalochori aquifer exploitation, and continue for 20 yr after the latter practically stopped in the early 1980s and piezometric levels fully recovered (Fig. 4). This 20 yr-long period exceeds any possible hysterisis between piezometric level recovery and associated land surface rebound. Furthermore, 1. subsidence seems to pre-date aquifer exploitation and to correspond to a larger-scale effect, affecting the whole of the young delta and not only the area of important water extraction (see above); 2. no geographical correlation between pumping wells and subsidence, especially between 1980 and 1985 is evident; 3. protrusion in wells more than 100 m deep, exploiting
S.C. Stiros / Engineering Geology 61 (2001) 243±256
aquifers up to 50 m deep (see above), is less than 1m, roughly predicting a compaction of up to 2 m (AIT, 1981). These data and results cast doubts on whether the observed subsidence can be assigned to re-orientation of mica particles in sandy and silty beds following the decline of the underground water level, as proposed by Andronopoulos et al. (1990, 1991); Hadzinakos et al. (1990). The decline of the groundwater level could only have affected the upper 30±35 m of sediments, and among them sand and clay layers rich in mica with a thickness of 5±10 m in total. Since any rearrangement of mineral particles in clays can contribute to volume changes of the order of 15%, while the contribution of mica particles in such a process can only be minor (Mead, 1968), the expected compaction due to rearrangement of mineral particles can only account for a part of the observed ground subsidence. Possible, additional causes of ground subsidence are: 1. Consolidation of sediments due to oxidation of peat soils found in the vadose zone after the decline of the aquifer. This is highly possible for peat is abundant in certain layers (see Fig. 5) which were found above the groundwater piezometric surface for at least 10 or 20 yr. A similar effect was documented in the Giannitsa±Pella area in the 1960s (see above) and might be a larger scale effect associated with the partial abandonment of the delta after the deviation of the Axios ¯ow in 1945. 2. Synsedimentary deformation, which usually affects a large part of sediment supplied to deltas causing slumps, mud¯ows and synsedimentary faulting in deeper and shallower levels (cf. Elliott, 1986). Interestingly, the northern boundary of the subsiding, Kalochori area correlates with synsedimentary faults deduced from geophysical data (Fig. 7), while mud¯ows have been observed in the prodelta (in the Thermaikos Gulf marine basin; Ferentinos et al., 1981). 3. Sub-surface instability of delta-front muds produced by consolidation of relatively deep layers. This is due to the fact that many of the deltaic sediments often possess high pore pressure and low compaction values which cause them to be
255
extremely unstable as burial and loading continues (Elliott, 1986). All these effects produce a non-elastic (i.e. permanent) subsidence. Since, on the other hand, the observed subsidence is an effect localized in the Thessaloniki Delta, any regional-scale relative or absolute sea-level rise (for instance due to eustatic or tectonic effects; Pirazzoli, 1996) can be discarded as a possible cause of the observed subsidence. The amplitude of most of these events is at least one order of magnitude lower than that observed. The Thessaloniki coastal plain, therefore, does not seem to differ from other deltaic terrains in which land subsidence and subsequent encroachment of the marine environment are endemic conditions; for example the Mississippi delta in the USA (Kuecher, 1995) or the Po Delta in Italy (Carbognin et al., 1976; Bondesan et al., 1995). Thessaloniki on the other hand can be added to the list of other major coastal or nearcoastal towns, parts of which are under the threat of marine invasion and ¯ooding (for instance, Bangkok; Nutalaya and Rau, 1981; AIT, 1981; Venice, Carbognin et al., 1976; Bondesan et al., 1995). Acknowledgements Information and documentation on the Kalochori subsidence provided by K. Miloglou are greatly appreciated. I thank P. Giao for literature on the Bangkok subsidence. The 1985 triangulation survey was completed in the framework of the IGME Geotechnical study. The 1999 ®eld geodetic survey was made by D. Arapoglou, V. Chaitas and M. Konstantellos. This paper bene®ted from constructive comments by P. Rahn and E. de Mulder. References AIT, 1981. Investigation of land subsidence caused by deep well pumping in the Bangkok area. Comprehensive Report no 91, Asian Institute of Technology (AIT), Bangkok. Andronopoulos, V., Rozos, D., Hadzinakos, I., 1990. Geotechnical study of ground settlement in the Kalochori area, Thessaloniki District. Unpublished Report IGME E6319, p. 45. Andronopoulos, V., Rozos, D., Hadzinakos, I., 1991. Subsidence phenomena in the industrial area of Thessaloniki, Greece.
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In: Johnson, A. (Ed.). Land Subsidence, vol. 200. IAHS Publishers, pp. 59±69. Bondesan, M., Castiglioni, G., Elmi, C., Gabbianelli, G., Marocco, R., Pirazzoli, P., Tomasin, A., 1995. Coastal areas at risk from storm surges and sea level-rise in northeastern Italy. J. Coastal Res. 11, 1354±1379. Carbognin, L., Gatto, P., Mozzi, G., Gambolati, G., Ricceri, G., 1976. New trend in the subsidence of Venice. In: Proceedings of the Anaheim Symposium, Publ. No. 121, International Association of Hydrological Science, pp. 65±81. Elliott, T., 1986. In: Reading, H.G. (Ed.). Deltas. Second ed. Sedimentary Environments and Facies. Blackwell Scienti®c Publications, Oxford, pp. 95±154. ETOME Co., 1974. Preliminary study for development of the area of installation of the International Fair of Thessaloniki. Detailed Report. Ministry of Public Works/Hydraulic Works Service. pp. 96 and Annexes. Ferentinos, G., Brooks, M., Collins, M., 1981. Gravity induced deformation on the north ¯ank and ¯oor of the Sporades Basin of the north Aegean Sea trough. Mar. Geol. 44, 289±302. Hadzinakos, I., Rozos, D., Apostolidis, E., 1990. Engineering geological mapping and related geotechnical probelms in the wider industrial area of Thessaloniki, Greece. In: Price, D. (Ed.), Proceedings of Sixth International IAEG Congress, Amsterdam, Balkema, pp. 127±134.
Kockel, F., Antoniades, P., Ioannides, K., Lalechos, N., 1978. Geological map of Greece, 1:50 000 scale, Thessaloniki sheet. Institute of Geology and Mineral Exploration (IGME), Athens. Kuecher, G., 1995. dominant process responsible for subsidence of coastal wetlands in South Louisiana. In: Barends, F., Brouwer, F., Schroeder, F. (Eds.). Land subsidence, Natural causes, measuring techniques, the Groningen gas®elds. A. Balkema, Rotterdam, pp. 69±81. Mead, R., 1968. Compaction of sediments underlying areas of land subsidence in Central California. US Geol. Surv. Prof. Pap. 497D, 39. Meladiotis, J., Demiris, C., 1988. Variation du coef®cient d'emmagasinement de l'aquifeÁre strati®e de la plaine de Salonique (GreÁce). Revue FrancËaise de GeÂotechnique 45, 51±58. Nicolaou, S., Nicolaidis, M., 1987. Geoelectric study at Kalochori near Thessaloniki. Unpublished Report IGME. pp. 10 and 4 diagrams. Nutalaya, P., Rau, J., 1981. Bangkok: the sinking metropolis. Episodes 4, 3±8. È ber die eustatischen Schwankungen des Palasis, A., 1972/73. U Mittelmeerspiegels waÈhrend des PleistozaÈns im Raum des Thermaikos Golfes. QuartaÈr 23/24, 123±147. Pirazzoli, P.A., 1996. Sea-level changes. The Last 20 000-yr. Wiley, New York, p. 211.
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Subsidence of the Thessaloniki (northern Greece) coastal plain, 1960±1999 Stathis C. Stiros Applied Geodesy Laboratory, Department of Civil Engineering, University Campus, University of Patras, Patras 26500, Greece Received 11 May 2000; accepted for publication 8 February 2001
Abstract Comparison of aerial photos, maps and triangulation data reveal that in the last 40 yr a part of the Thessaloniki coastal plain, a delta formed in the last 2500 yr, subsided at a rate of up to 10 cm/yr. As a consequence the sea invaded up to 2 km inland; precious land in the suburbs of the city was lost, while a village and major industrial plants are in risk of ¯ooding. Part of the land was reclaimed thanks to barriers, pumping and arti®cial raising of the land surface. Yet, the situation is unstable and ¯ooding is not unusual. Ground water withdrawal for the needs of the Thessaloniki metropolitan complex has initially been regarded as the cause of the subsidence. However, the lack of correlation in space and in time between ¯uctuations of piezometric levels, topographic changes and pumping indicates that the observed subsidence should be regarded as the cumulative effect of several factors, including consolidation of near-surface sediments due to the decline of the piezometric level and the partial abandonment of the delta, oxidation of peat soils in the vadose zone, synsedimentary deformation (faulting and ¯ow) and loading-induced consolidation of deeper sediments. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Land subsidence; Delta; Sediment compaction; Pumping; Geodesy
1. Introduction The Kalochori area, in the suburbs of Thessaloniki, the second major city in Greece (Fig. 1), is located on the young (up to a few thousand years old) delta of the Axios (Vardar) and of other large rivers. The Kalochori area is currently subsiding and for this reason one of the major industrial zones in Greece is under the threat of marine invasion. In spite of its importance, this phenomenon has not been studied in detail and has been rather a priori assigned to compaction of sediments due to overpumping of ground water (Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). In this article we present geodetic constraints to the geographic distribution, timing and amplitude of the E-mail address: [email protected] (S.C. Stiros).
observed subsidence in the last 50 yr, and based on such evidence, we try to put some constraints on the history and causes of this phenomenon. 2. Previous studies and results Reliable historical evidence (Herodotus, VII 123± 124) suggests that in circa 500BC, Thermaikos Gulf, at the NW tip of the Aegean Sea (northern Greece), was much deeper and Pella, the capital of ancient Macedonia was a coastal town (Fig. 1). In the subsequent centuries deposition of deltaic sediments of four main rivers (Gallikos, Axios (Vardar), Loudias and Aliakmon) caused an approximately 80 km shift of the coastline leading to the partial closure of this gulf and the present-day coastal topography.
0013-7952/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0013-795 2(01)00027-8
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Fig. 1. The approximate shoreline of 500BC in the Thessaloniki plain is shown. Two dotted lines indicate the former beds of Axios River. The Kalochori area is the coastal area between the NW edge of Thessaloniki and the former Axios riverbeds. In the inset, location map.
The lowlands between the Gallikos River and Thessaloniki were ®rst occupied in 1922 by refugees from Minor Asia (Turkey) who established the Kalochori village, then in 1945 the Axios riverbed was redirected southwards in order to prevent the
Thessaloniki harbour, one of the major harbours in the Balkans, from silting. The Kalochori area obtained economic and social signi®cance in the 1960s, when major industries (re®neries etc.) were established there, taking advantage of cheap land in the suburbs
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Fig. 2. An example of subsidence and of well protrusion (approximately 80 cm) at a well SE of Kalochori. Photo taken in 1985, after the area was reclaimed. An arrow points to the upper limit of marine organisms marking sea level during the period of marine invasion.
of the major metropolitan centre of Thessaloniki. The increasing need for land inspired plans for development of the wider area, even the reclamation of wetlands south of Kalochori. The need for fresh water, on the other hand, led the Thessaloniki Water District to drill, since 1955, about 25 wells producing an average of 1600±1900 m 3/h for the period 1958± 1963. About 300 private wells providing smaller quantities of water for the needs of seasonal cultivation or small industries were in operation during this period (Andronopoulos et al., 1990). The ®rst signs of subsidence in the Thessaloniki plain were noticed in the 1960s in the Giannitsa area, close to Pella (Fig. 1); this subsidence followed major works for drainage of a nearly swampy plain, and has been attributed to decomposition of organic material (ETOME Co., 1974). In 1964 a progressive marine invasion was ®rst noticed, then in the subsequent years it was realized that discharge of waters after heavy rainfalls had become very dif®cult, leading to ¯ooding. In 1969, probably because of a combination of strong winds and a barometric peak, for the ®rst time seawater
reached the houses of Kalochori, an event, which forced the construction of a coastal dam. This dam was effective till 1973, but it was subsequently destroyed due to strong subsidence. The signs of this subsidence since at least 1973 were also visible in wells more than 100 m deep, which protruded by up to 0.9 m (Fig. 2), as well as in ¯ooded basements of a dozen houses and of the square around the church of Kalochori. As an attempted remedy, the ¯oor of the basements and of the village square were raised by approximately 1±1.2 m (Andronopoulos et al., 1990). A ®rst study of the subsidence was made in 1974, in the framework of plans for the development of the area, including the installation of the Thessaloniki International Fair in the Kalochori area. This preliminary study noticed the ongoing subsidence, but concluded that it could be controlled by a coastal barrier, pumping of the ponded water and arti®cial raising of the ground surface by 1±2.7 m; the necessary material to be deposited, about 7.6 £ 10 6 m 3, was to be quarried from the Gallikos riverbed, north of Kalochori (ETOME Co., 1974). A coastal barrier was constructed in 1976, approximately along the
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Fig. 3. Remains of electricity poles offshore, along a road constructed in 1975 south of Kalochori, but subsequently submerged. Photo taken in 1991.
trace of the 1956 shoreline, permitting the reclamation of a part of the lost land, about 3±4 km 2, and a system of roads and an electricity network were established. Yet, after a storm in the winter of 1979±1980, the coastal dam failed, a major part of the area south of Kalochori, including the reclaimed land, was covered by waters, and the electricity poles were left offshore, as a reminder of the extent of the lost land (Fig. 3). The situation was partly stabilized after new coastal barriers were erected in the early 1980s, but the area remains vulnerable to ¯ooding. For instance, after the ¯ood of 23 October 1991, the coastal barrier west of Gallikos River failed, and an inhabited or cultivated area about 3 km 2 wide was covered with waters. Pumping in the Kalochori area was, on the other hand, drastically reduced in the early 1980s, and since then only small amounts of water, 10±15 000 m 3/day are estimated to have been extracted from mainly private wells, usually in summer (K. Miloglou, personal communication); the water table height which
had declined in the previous years as much as 25 m recovered to pre-pumping levels (Fig. 4), but no signi®cant rebound of the land surface was reported. A new study of the subsidence started in 1980 by the Institute of Geology and Mineral Exploration (IGME) was mostly based on three borehole data. The major conclusions of this study were: (1) that subsidence is con®ned to the area of Kalochori and Sindos; the ground is uniformly subsiding and no differential settlements have been noticed, but the greatest compaction varies between 2±3 m; (2) that the observed up to 40 m lowering of the piezometric surfaces of productive wells and the uniform lowering of the ground surface indicate that ground subsidence is caused by compaction of certain highly compressive layers following the decline of the ground water level; and (3) that ground subsidence, in particular, is related to reorientation of the abundant mica particles of highly compressive sandy horizons in two black silty clay layers producing extreme compaction
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Piezometric head decline (m)
0
-10
K6 -20
K11
K10
-30
K8 -40
1970
1975
1980
1985
Time
Fig. 4. Fluctuation of piezometric levels of four representative productive wells near Kalochori (for location see Fig. 7).
(Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). These conclusions were not without legal implications, for the community of Kalochori was accusing the Thessaloniki Water District Authority of causing the damage. 3. Geology The wider Kalochori area is located at the SE edge of the Thessaloniki plain. This plain corresponds to a complex delta formed in the last few thousand years (see above) in a more than 600 m deep Quaternary graben cut in Neogene, mainly sediments and ®lled with unconsolidated to partly consolidated deltaic sediments, mostly sand and silt (Fig. 5). Neogene formations, mostly sandstones, marls and clays of unknown thickness, overlain gneiss which crop out in the Thessaloniki area (Kockel et al., 1978). The relief in the wider Thessaloniki plain is very smooth while in the Kalochori area it is nearly ¯at, with a maximum elevation of 4±5 m above the water. Sandy formations dominate this area, and black silty clays SE of the village, along the coastal zone (Andronopoulos et al., 1990, 1991; Hadzinakos et al., 1990). The subsurface geology of the area based on borehole data has been studied by Palasis (1972/73) and is shown in Figs. 5 and 6. However, the stratigraphy, of this recent complex delta is obviously not uniform,
but laterally variable, deviating from the above idealized scheme (Fig. 5). 4. Hydrology The ¯ow of four main rivers provides an intense and continuous recharge to aquifers of the area, which, however, are discontinuous due to the intense lithological variability of deltaic sediments. For this reason, pumping cones with radii of up to 600 m were observed in productive wells (ETOME Co., 1974). The most important productive aquifers identi®ed and exploited by the Thessaloniki Water District Authority are gravel and sand layers, up to a few meters thick, usually at the mean depth of 30, 60 and 90±120 m, and in many cases up to 220 m deep. These aquifers permitted a minimum pumping rate of 1600±1900 m 3/h for the Kalochori area and 1000±2100 m 3/h for the Sindos area (Andronopoulos et al., 1990); for location see Fig. 7. Groundwater piezometric level was initially at or close to the ground surface (0±3 m depth, measurements in 1974, ETOME Co., 1974), by declining to the maximum depth of 35 m in 1976, but recovering to 1970 levels in 1984, shortly after pumping was drastically reduced or even, since 1980, stopped (Fig. 4; Meladiotis and Demiris, 1988; Andronopoulos et al., 1990, 1991). The long-term exploitation of the Kalochori aquifers caused a small-scale encroachment of saline
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Fig. 5. Geotechnical classi®cation of the lithostratigraphic units in the Kalochori village area based on three boreholes (K1, K2, K3, based on Andronopoulos et al., 1990) in comparison with the idealized pro®le in the whole of the Delta (I, based on Palasis, 1972/73). Classi®cation according to the AUSCS system.
water up to a nearly E±W trending front passing from Kalochori village, as an increase of electric resistivity of sediments indicates (Nicolaou and Nicolaidis, 1987). This encroachment, however, did not affect productive wells which are located farther north (Fig. 7).
5. Geotechnical characteristics Andronopoulos et al. (1990) classi®ed the Quaternary formations in the Thessaloniki plain in three horizons, sandy, silty and black silty clays, and computed
Fig. 6. Two representative lithostratigraphic cross sections in the Thessaloniki Delta based on correlations of borehole data (modi®ed after Palasis, 1972/73). Location, map on top of ®gure. NM indicates Nea Magnesia; dotted line indicates boundary of Quaternary sediments. (1) Sand, (2) gravel and pebbles, (3) silt and clay, mainly terrestrial sediments, (4) silt, clay and ®ne sand, limnic sediments, (5) clay and sand, marine sediments, and (6) silt, terrestrial sediments.
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Fig. 7. Productive wells in the Kalochori area (solid squares), contours of depth of pre-Holocene basement (in m, thin lines), the northern limit of the saline water incursion deduced from geoelectric data (dotted line), and synsedimentary faults (solid lines). A solid triangle indicates geodetic station 64 used as reference in elevation changes of Fig. 7. A dotted zone indicates the Gallikos Riverbed, while the Neogene basement is shaded. Based on data from Meladiotis and Demiris (1988), Nicolaides and Nicolaou (1987), and Kockel et al. (1978).
min±max
0.10/0.68±0.47/1.33 0.12/0.62±0.29/1.00 0.08/0.41±1.10/2.05 0.03/0.46±0.38/0.80
min±max
5/7±80/28 1/28±50/30 3/1±170/37 43/8±170/58
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some of their geotechnical characteristics based on three drillings, 47±91 m deep. Their results are shown in Table 1 along with their results for the Neogene formations. As far as the black clay formation is concerned, Andronopoulos et al. (1990, Table 6.2, 1991) noticed that it is practically free of montmorillonite. They also reported that mica particles are abundant in both the clay fractions and the sandy intercalations of this formation. This is discussed below. 6. Topography changes
17±25 14±117 4±120 44±1340 12.3/16.5±15.2/19.9 11.8/14.8±18.4/21.7 8.7/14.8±18.2/23.0 6.3/10.6±21.1/13.8 silty horizon sandy horizon black clay horizon neogene sediments
21.0/17.5±49.0/32.2 8.7/11.8±39.0/21.4 17.8/10.0±94.8/38.0 20.0/8.0±75.1/26.6
23.2±53.0 3.1±39.1 4.0±69.4 8.0±44.6
min±max min±max min±max min±max
Dry and wet density (kN/m 3) Water content w (%)
Uncon®ned compression (kPa)
6.1. Shoreline shift
Atterberg limits wL/wF (%)
Mechanical properties Physical characteristics Formations
Table 1 Physical and mechanical properties of the formations in the Kalochori area, according to Andronopoulos et al. (1990)
Triaxial test (kPa/8) cu/ u
Consolidation test cc/e0
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For the period 1945±1980, just before the shoreline was arti®cially controlled by levees, seven series of aerial photos of various scales (1:16 000±1:45 000), covering the wider Kalochori area are available. The shoreline, which is observed in each of these series of aerial photos was transferred to a 1:20 000 scale map of the area (produced by magni®cation of the 1:50 000 scale map) using a monoscopic transferscope on the basis of characteristic and precisely identi®ed reference points common in at least two series of photos. Results are summarized in Fig. 8 and indicate a marine regression of the order of 1±2 km. The accuracy of these shorelines is de®ned by the accuracy of line drawing, typically better than 1 mm, and corresponds to an accuracy of a few tens of meters. Therefore, Fig. 8 is representative of the history of the shoreline changes in the Kalochori area for the period 1945±1980. 6.2. Comparison of 1:5000 scale maps Part of the study area was covered in 1955 by 1:5000 scale maps compiled in 1955 by the Ministry of Transportation and Public Works. An accuracy of 50±60 cm is expected for heights in this ¯at terrain computed on the basis of theodolite and stadia measurements. A second set of maps of the same scale, produced on the basis of aerial photos taken in 1980 and photogrammetric techniques by the Hellenic Military Geographic Service (HMGS) is also available. The accuracy in heights of the various points in these maps is better than 0.5 m.
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Fig. 8. Coastline shift in the Kalochori area between 1945 and 1980, based on comparison of aerial photos.
A comparison of the two sets of maps revealed that they contained common points which could be unambiguously compared to provide their height changes for the time lapse between the compilation of the two sets of maps. A common grid with a separation of 4 cm (corresponding to 200 m on the land surface) was plotted in each set of maps and the obtained differences in elevations of the corresponding points were estimated. The elevation changes obtained, ranging up to a few meters, are shown in contour form in Fig. 9. The accuracy of relative height differences of each point in this diagram can be estimated from
the law of propagation of errors for uncorrelated variables from their standard errors, 0.6 and 0.5 m, respectively. Hence, the accuracy of computed relative height changes is expected to be better than (0.6 2 1 0.5 2) 1/2 m, approximately 0.8 m. This indicates that the contours of diagram of Fig. 9 re¯ect, though with some noise (uncertainty), real changes in the ground elevation between 1955 and 1980. A large-scale subsidence of the order of 1± 3 m can therefore be deduced from this map. An extreme amount of subsidence (more than 5 m) is seen NW of Kalochori, next to the Gallikos riverbanks, but this area can be identi®ed as the site of
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Fig. 9. Contours of height changes in meters between 1955±1980 based on comparison of 1:5000 scale maps. Extreme subsidence (4 m contour) indicates quarrying, while small amounts of elevation change south of Kalochori (22, 21 and 0 contours) indicate waste in®ll or deposition for protection against marine water encroachment. The Gallikos riverbed barriers are shown by two sub-parallel gray, hatched lines.
quarrying for material used for the construction of coastal dams or the raising of the ground surface. On the contrary, uplift observed south of Kalochori (area de®ned by the 0 m contour) corresponds to an in®ll, mostly urban waste deposited before 1975. In summary, the comparison of 1955 and 1980 maps indicates that the amount of subsidence is of the order of 3±4 m, and is not con®ned to the Kalochori area, but probably covers a much wider region.
6.3. Triangulation levelling data The study area covered by the fourth order National Geodetic triangulation network measured in 1980 with techniques providing a typical accuracy of 15 cm for heights. In 1985, a number of stations were found and a local EDM triangulation network was re-measured by an IGME team. A part of the 1985 network was reoccupied in 1999, using similar techniques, providing an accuracy of 11±20 cm for relative heights. Due to the limited coverage of the
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Fig. 10. Contours of elevation changes between 1999 and 1980 and 1985 and 1980 in cm, based on comparison of elevation differences between triangulation stations marked with small triangles. Elevation changes are shown relative to point 64 (large triangle, also shown in Fig. 7) and are accurate to within 25 cm. Positive relative elevation changes SE of Kalochori do not re¯ect land uplift, but less intense subsidence than in the Sindos area.
1985 and 1999 networks, a comparison of the 1980, 1985 and 1999 surveys was made relative to benchmark 64 which is located in the subsiding area, and computed relative elevation changes in contour form are shown in Fig. 9. Since benchmark 64 is not located on stable ground, the apparent land uplift shown in this ®gure do not indicate a land uplift, but a rapid subsidence of the Sindos area (i.e. the area close to benchmark 64). 7. Discussion The history of shoreline retreat (Fig. 8) reveals a major phase of subsidence (about 500 m retreat of the shoreline) between 1960 and 1968, followed by a second one between 1972 and 1975±1976. This result is consistent with information conveyed by local authorities (last paragraph in Section 6.2) that the ®rst clear signs of marine invasion were noticed in 1964. The total amount of subsidence between 1955 and 1980 is at least of the order of 3 m, as the
comparison of maps indicates (Fig. 9), while triangulation data reveal that between 1980 and 1985 and 1985±1999 the rates of relative height changes are of the order of up to 8 and 10 cm/yr, respectively (Fig. 10). These data reveal that most surface subsidence and the related marine invasion seem to postdate the Kalochori aquifer exploitation, and continue for 20 yr after the latter practically stopped in the early 1980s and piezometric levels fully recovered (Fig. 4). This 20 yr-long period exceeds any possible hysterisis between piezometric level recovery and associated land surface rebound. Furthermore, 1. subsidence seems to pre-date aquifer exploitation and to correspond to a larger-scale effect, affecting the whole of the young delta and not only the area of important water extraction (see above); 2. no geographical correlation between pumping wells and subsidence, especially between 1980 and 1985 is evident; 3. protrusion in wells more than 100 m deep, exploiting
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aquifers up to 50 m deep (see above), is less than 1m, roughly predicting a compaction of up to 2 m (AIT, 1981). These data and results cast doubts on whether the observed subsidence can be assigned to re-orientation of mica particles in sandy and silty beds following the decline of the underground water level, as proposed by Andronopoulos et al. (1990, 1991); Hadzinakos et al. (1990). The decline of the groundwater level could only have affected the upper 30±35 m of sediments, and among them sand and clay layers rich in mica with a thickness of 5±10 m in total. Since any rearrangement of mineral particles in clays can contribute to volume changes of the order of 15%, while the contribution of mica particles in such a process can only be minor (Mead, 1968), the expected compaction due to rearrangement of mineral particles can only account for a part of the observed ground subsidence. Possible, additional causes of ground subsidence are: 1. Consolidation of sediments due to oxidation of peat soils found in the vadose zone after the decline of the aquifer. This is highly possible for peat is abundant in certain layers (see Fig. 5) which were found above the groundwater piezometric surface for at least 10 or 20 yr. A similar effect was documented in the Giannitsa±Pella area in the 1960s (see above) and might be a larger scale effect associated with the partial abandonment of the delta after the deviation of the Axios ¯ow in 1945. 2. Synsedimentary deformation, which usually affects a large part of sediment supplied to deltas causing slumps, mud¯ows and synsedimentary faulting in deeper and shallower levels (cf. Elliott, 1986). Interestingly, the northern boundary of the subsiding, Kalochori area correlates with synsedimentary faults deduced from geophysical data (Fig. 7), while mud¯ows have been observed in the prodelta (in the Thermaikos Gulf marine basin; Ferentinos et al., 1981). 3. Sub-surface instability of delta-front muds produced by consolidation of relatively deep layers. This is due to the fact that many of the deltaic sediments often possess high pore pressure and low compaction values which cause them to be
255
extremely unstable as burial and loading continues (Elliott, 1986). All these effects produce a non-elastic (i.e. permanent) subsidence. Since, on the other hand, the observed subsidence is an effect localized in the Thessaloniki Delta, any regional-scale relative or absolute sea-level rise (for instance due to eustatic or tectonic effects; Pirazzoli, 1996) can be discarded as a possible cause of the observed subsidence. The amplitude of most of these events is at least one order of magnitude lower than that observed. The Thessaloniki coastal plain, therefore, does not seem to differ from other deltaic terrains in which land subsidence and subsequent encroachment of the marine environment are endemic conditions; for example the Mississippi delta in the USA (Kuecher, 1995) or the Po Delta in Italy (Carbognin et al., 1976; Bondesan et al., 1995). Thessaloniki on the other hand can be added to the list of other major coastal or nearcoastal towns, parts of which are under the threat of marine invasion and ¯ooding (for instance, Bangkok; Nutalaya and Rau, 1981; AIT, 1981; Venice, Carbognin et al., 1976; Bondesan et al., 1995). Acknowledgements Information and documentation on the Kalochori subsidence provided by K. Miloglou are greatly appreciated. I thank P. Giao for literature on the Bangkok subsidence. The 1985 triangulation survey was completed in the framework of the IGME Geotechnical study. The 1999 ®eld geodetic survey was made by D. Arapoglou, V. Chaitas and M. Konstantellos. This paper bene®ted from constructive comments by P. Rahn and E. de Mulder. References AIT, 1981. Investigation of land subsidence caused by deep well pumping in the Bangkok area. Comprehensive Report no 91, Asian Institute of Technology (AIT), Bangkok. Andronopoulos, V., Rozos, D., Hadzinakos, I., 1990. Geotechnical study of ground settlement in the Kalochori area, Thessaloniki District. Unpublished Report IGME E6319, p. 45. Andronopoulos, V., Rozos, D., Hadzinakos, I., 1991. Subsidence phenomena in the industrial area of Thessaloniki, Greece.
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In: Johnson, A. (Ed.). Land Subsidence, vol. 200. IAHS Publishers, pp. 59±69. Bondesan, M., Castiglioni, G., Elmi, C., Gabbianelli, G., Marocco, R., Pirazzoli, P., Tomasin, A., 1995. Coastal areas at risk from storm surges and sea level-rise in northeastern Italy. J. Coastal Res. 11, 1354±1379. Carbognin, L., Gatto, P., Mozzi, G., Gambolati, G., Ricceri, G., 1976. New trend in the subsidence of Venice. In: Proceedings of the Anaheim Symposium, Publ. No. 121, International Association of Hydrological Science, pp. 65±81. Elliott, T., 1986. In: Reading, H.G. (Ed.). Deltas. Second ed. Sedimentary Environments and Facies. Blackwell Scienti®c Publications, Oxford, pp. 95±154. ETOME Co., 1974. Preliminary study for development of the area of installation of the International Fair of Thessaloniki. Detailed Report. Ministry of Public Works/Hydraulic Works Service. pp. 96 and Annexes. Ferentinos, G., Brooks, M., Collins, M., 1981. Gravity induced deformation on the north ¯ank and ¯oor of the Sporades Basin of the north Aegean Sea trough. Mar. Geol. 44, 289±302. Hadzinakos, I., Rozos, D., Apostolidis, E., 1990. Engineering geological mapping and related geotechnical probelms in the wider industrial area of Thessaloniki, Greece. In: Price, D. (Ed.), Proceedings of Sixth International IAEG Congress, Amsterdam, Balkema, pp. 127±134.
Kockel, F., Antoniades, P., Ioannides, K., Lalechos, N., 1978. Geological map of Greece, 1:50 000 scale, Thessaloniki sheet. Institute of Geology and Mineral Exploration (IGME), Athens. Kuecher, G., 1995. dominant process responsible for subsidence of coastal wetlands in South Louisiana. In: Barends, F., Brouwer, F., Schroeder, F. (Eds.). Land subsidence, Natural causes, measuring techniques, the Groningen gas®elds. A. Balkema, Rotterdam, pp. 69±81. Mead, R., 1968. Compaction of sediments underlying areas of land subsidence in Central California. US Geol. Surv. Prof. Pap. 497D, 39. Meladiotis, J., Demiris, C., 1988. Variation du coef®cient d'emmagasinement de l'aquifeÁre strati®e de la plaine de Salonique (GreÁce). Revue FrancËaise de GeÂotechnique 45, 51±58. Nicolaou, S., Nicolaidis, M., 1987. Geoelectric study at Kalochori near Thessaloniki. Unpublished Report IGME. pp. 10 and 4 diagrams. Nutalaya, P., Rau, J., 1981. Bangkok: the sinking metropolis. Episodes 4, 3±8. È ber die eustatischen Schwankungen des Palasis, A., 1972/73. U Mittelmeerspiegels waÈhrend des PleistozaÈns im Raum des Thermaikos Golfes. QuartaÈr 23/24, 123±147. Pirazzoli, P.A., 1996. Sea-level changes. The Last 20 000-yr. Wiley, New York, p. 211.