GEOLOGICA BALCANICA, 23. 2, Sofia, April, 1993, p. 67-71

Coastal retreat in the plain of Marathon (East Attica), Greece: cause and effects

Hampik Maroukian, Athina Zamani, Kosmas Pavlopoulos

Departmellt of Geography and Climatology, University of Athens, Greece

(Received 28. 09. 1992; accepted 24. /2. /992)

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Abstract. The drainage network of lnois River is located in East Attica. The river has formed a del­ taic fan on which two main distributaries arc observed: the older western channel (Sehri Rema) and the younger eastern one (Kenourio Rema). The purpose of this paper is to study the rcxent evolution of lnois River on its deltaic fan, the chan­ ges that have taken place and the problems that have been created in property loss in the coastal environ­ ment near the mouth of Inois River in the last decades. The construction of Marathon dam in the 1920's resulted in a retreat of the coastline near the mouth of Kenourio Rema and this process is still going on today. The dam prevents the transportation and depo­ sition of sediments to the lower reaches of the drainage system of Inois River and results in the silting up of the artificial lake of Marathon. Following comparative observations of airphotos of various years (1938-1988) a retreat of the coast­ line of the order of 1m/year is observed in this area. The loss of land has created great concern to proper­ ty owners who have appealed to local administrative bodies to no avail. Various crude defensive structures constructed from time to time provide no solution to the problem of coastal erosion. If the anticipated rapid sea-level rise of the near future is confirmed, then the retreat of the coastline in this area is expected to accelerate in the coming decades and create even more serious problems to the coastal environment.

Introduction

The interference of man in the natural environment has been a major ecological issue in recent decades. This is especially true for the coastal environment which is particular­ ly sensitive to external factors that interfere in its complex interactions of land, sea and air.

67 A good example of such human interference is the construction of a dam along the river lnois ncar Marathon in 1929 (Fig. 1). This act caused the recession of the coast­ line near the mouth of Kenourio Rema, the last active distributary of Inois river to reach the sea.

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Fig. I. Location of study area

The purpose of this study is to trace the changes, both natural and artificial, that have taken place in the last fifty years in the area around the mouth of Kenourio Rema. The plain of Marathon is located in East Attica and is known by the ancient battle of Marathon fought between the Greeks and the Persians in 490 B. C. (Fig. 2). The plain is elongated today having a NE-SW direction. It is divided into two parts by the river Inois whose deposits form a deltaic fan with an elevation of twenty meters at its apex. Southwest of the fan, there was a marshy area (Vrexissa) which was drained some de­ cades ago. In the east extends the extensive marsh of Marathonas, separated from the sea by a barrier beach with low sand dunes on it (B a e t e man, 1985). The plain of Marathon is surrounded in the west by Pendeli mountain and in the north by the Strati heights. They are composed of Triassic and Jurassic marbles and cry-

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Fig. 2. Lo~.:ation of the river lnois in Eastern Attica stalline limestones. Tectonically, this area is affected by an older fault system having a NE-SW direction and a younger one having a NW-SE direction (Boote c h t e 1 & P a­ p a d e a s, 1968). The general direction of the drainage network of Inois river is NW-SE and its sour­ ces are found on the northern side of Mount Parnitha. The upper part of the drainage basin supplies with water the present artificial lake of Marathon. The dam was constructed in 1929 at the entrance of a two-kilometre long gorge having an east-west direction. The apex of the deltaic fan is located about two kilometres from the coast and is downcut about five meters. A little downstream, Jnois river bifurcates in the western Sehri Rema branch and the eastern Kenourio Rema. The channel of Sehri Rema has been inactive for several centuries and its remnants are easily visible today. Its depth is at no point more than two meters. It is mostly cultivated, filled up and subdivided into lots down to the coast. In a topographic sheet of 1889 (Curtius-Kaupert) Roman ruins and mosaic floors arc shown in the old channel of Sehri, which indicates that the channel was inactive in Ro­ man times. In airphotos of 1938 and 1945 one can see an even older channel further west which passes north of the Marathon earth tomb (hill) and flows into the sea following a south­ eastern direction. At the bifurcation point of Sehri Rema from the main channel, a new channel has formed in the east called Kenourio Rema, and follows an almost parallel path to Sehri. The new channel has, in the upper reaches of the deltaic fan, experienced intense down­ cutting of more than five metres at some points. The earth fills, sand and gravel quarrying and garbage dumping in Kenourio' s chan­ nel indicate that it has remained inactive in the last decades due to the construction of the dam of the artificial lake of Marathonas.

Coastal geomorphology The coastline of the study area is almost straight except for an area just east of the mouth of Kenourio Rema where it moves inland due to a change of sediment particle size (Fig. 3). At this point the apron of the deltaic fan of Inois river extends to the northeast, away from the coast. The slope of the coast in the whole study area is small ( < 20 %) and comprises part of the apron of the deltaic fan of Jnois river. The particle size of the coastal sediments is mostly coarse composed of gravels and cobbles whose diameter (L) rarely exceeds 20 em at the mouth of Kenourio Rema. Fur­ ther east the sediment is mostly sand, mixed with gravels and cobbles. In the west, the proportion of sand increases towards the old mouth of Sehri Rema. Coastal dunes are observed all along the coast of the study area, except around the old mouth of Kenourio Rema. They do not exceed a height of 1.5 meters and are old, stabilized dunes usually covered by shrubs. Some are eroding today due to coastal retreat. There are at least seven older river mouths in the area of the pres_ently blocked mouth of Inois river (Kenourio Rema). Comparative observations of airphotos of 1938 and 1988 show a maximum retreat of the coastline of more than 100 metres at the mouth of Kenourio Rema, a retreat of 2 mfyr. If we consider the construction of the dam of Marathonas lake (completed in 1929), the deposition of fluvial sediments behind it and the very limited river flow fur­ ther downstream, then the observed rapid coastal retreat is "natural". We should also add here the extensive sand and gravel quarrying that is taking place in Kenourio chan­ nel in recent years, thus rendering it completely inactive, even when heavy thunderstorms occur in the lower parts of the drainage basin of Inois river below the dam. Comparing the limit of the coastline on maps of Curtius and Kaupert of 1889 at a scale of 1:25 000 with airphotos of 1938, no significant change is observed.

69 Naturally, the coastal retreat of the last fifty years is not expected to continue at the same rate. Under today's conditions and bearing in mind that the old channel of Kenou­ rio distributary is almost parallel to the coastline near its mouth, then the rapid retreat is expected to continue for several years until the old channel is consumed by the sea.

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LEGEND - LA. NO S£4 + Stable coaPt in• LAND SEA RrtrHt.ing ~tl i n• -- Sandy beach Wotertront ,Cl\JO Y 0 0 Shingl• beoch Damaged or ct.ttroyed MQWOIIs or pMrn 9 &eoch with m ixed Mdimeornts Destroyed or1.if iciol structures 9 t Pre'Yaifi ng klng•~ drift S..er 11 StabiliZed dune~ w~ u HEIGHTS IH METERS 0 Coast com~ of terr•t,... 0 0 50 100 150 20J 2!0 m 0 uncon.otid~Mad sedinwnb

Fig. 3. Coastal geomorphological map of Sehri Rema and Kenourio Rema

Artificial structures

According to recent scientific studies of climatic change however, a sea level rise of up to one meter is expected to occur in the next 100 years (P a r k et al., 1989). Even if we have a rise of a few tens of centimeters, it will cause an accelerated retreat of the coastline for many years. Even though only 18 % of the winds blow from the southeastern quartile (the longest fetch), the waves exceed a height of two meters with 7 BF force crea­ ting an eastely longshore drift and transporting all the finer materials from the area around the mouth of Kenourio Rema towards the barrier beach to the east. A secondary, weaker curr carries sediment to the we st.

Artifical structures

In 1938, there was no man-made structure near the coast of the study area. The whole area was cultivated in small, usually elongated plots. In the first 300 meters of the coast­ al zone the plots are situated perpendicular to the shoreline while those further inland

70 are parallel to it. It is believed that this difference in the position of plots is due to land accretion owed to deposition by the old Sehri Rema and the younger Kenourio Rema in the last centuries. Kenourio Rema remained active for some years, even after the construction of Ma­ rathonas dam in 1929, because of limited water use by the city of Athens which was much smaller in those years. Between 1945 and 1960 the first structures (eight, mostly summer cottages) appear in a 100 meter-wide coastal zone between Sehri Rema and Kenourio Rema. At the same time we have an acceleration of the rate of coastal retreat (L e o n t a r i s & M a­ r o u k i a n, 1988; M o u t z o u r i s & M a r o u k i a n, 1988). Intensive house development occurs in the 1960s with the construction of at least twen­ ty more cottages in the same coastal zone. Coastal erosion is still very active. In the 1970s, a coastal asphalt road was constructed near Sehri Rema, going westwards. An old dirt road crossing the blocked mouth of Kenourio Rema is eroded in the late 70s. An earthen quay is constructed west of Kenourio Rema. In 1984 the number of cottages has increased to 48 in the same area. Two small quays have already been constructed before 1984 on both sides of the mouth of Kenourio Rema. The eastern one is earthen and lasts only a few years. The western is a concrete structure, about ten meters long. Coastal retreat is about a meter a year, slower than previous de­ cades. By 1988 the houses have increased to about 65. The western concrete quay is being destroyed. In recent years, attempts have been made by local land owners to stop the ad­ vancing sea by throwing rubble and covering the shore with concrete, to no avail (Plate I, Fig. 4).

Conclusions

It is evident that the deltaic fan of Inois river was more extensive towards the sea in re­ cent times. Before the construction of the dam in 1929, the area around the mouth of Kenourio Rema was in a dynamic equilibrium or even was advancing into the sea. After 1929, this equilibrium was upset and some years later coastal retreat started reaching up to 2 mfyr in the 1950s and 1960s and slowing down to I mfyr in recent years near the mouth of Kenourio Rema (Plate I, Fig. 5). It is expected that the shoreline retreat will continue in the near future. If the anti­ cipated sea level rise of the next decades due to the greenhouse effect is confirmed, then we may expect an accelerated coastal erosion in the area between Sehri and Kenourio Rema.

References

8 a e t em an, C. 1985. Late Holocene geology of tile Marathon plain (Greece).- Journal of Coastal Research, 1, 2; 173-185. B o e t e c 11 t e I, J., Pap ad e as, G. 1968. Tectonic aerial interpretation in tile Mediterranean region exem­ plified by the metamorphic series at Eastern Greece, near Marathon.- Plzotogrammetria, 26, 6; 201-210. Leon tar is, S., Mar o u k ian, H. 1988. Coasts problematic to man in tile South Euboic Gulf, Faros Avlidas-Skala Oropou area.- Greek Geological Society, Bulletin, 20; 383-398 (in Greek). M out z our is, C. I., Mar o u k ian, H. 1988. Greece. In: Walker, H. J. (ed.), Artificial structures and shorelines, Kluwer Academic Publishers; 207-215 . Park, R. A., T r e ham, M. S., Manse I, P. W., Howe, R. C. 1989. Coastal wetlands in the twenty first century: Profound alterations due to rising sea level. In: Wetlands Concerns and successes. American Water Resources Association; 71-80.

71 EXPLANATION TO THE PLATE

Fig. 4. Coastal retreat few meters west of the mouth of Kenourio Rema Fig. 5. The blocked mouth of Kenourio Rema (foreground) and coastal retreat east of it

72 PLATE

5 Geologica Balcanica 23.2 Ham pi k Mar o u k ian, At hi n a Zaman i. K o s mas P a vI o p o u I o s- Coastal retreat. ..