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Modelling of erosional processes in the Ionian Islands (Greece)

Modelling of erosional processes in the Ionian Islands (Greece) Geomatics, Natural Hazards and Risk Vol. 3, No. 4, November 2012, 293–310 NIKI EVELPIDOU* Faculty of Geology and Geoenvironment, University of Athens, Panepistimiopolis, Zografou, 157-84, Athens, Greece (Received 5 April 2011; in final form 9 July 2011) This paper focuses on the study of the geomorphological processes and the impact of neotectonic deformation on the geomorphological structure. A vast database was developed, containing different kinds of information, concerning geology, topography, drainage characteristics, vegetation and land use of the Ionian Islands. A geographic information system platform was developed in order to analyse the data, and to model and finally map the prevailing geomorphological processes: the erosion and deposition zones. Erosion risk factors have been processed in order to develop erosion risk maps demonstrating vulnerable to erosion areas. This study also concludes that the Ionian Islands are strongly influenced by the neotectonic processes that have defined their current morphology. 1. Introduction For the analysis of problems related to erosion and other natural hazards, many different approaches have been applied (Brundsen et al. 1975, Carrara et al. 1977, Malgot and Mahr 1979, Ives and Mersserli 1981, Carrara 1983, Carrara et al. 1991). The purpose of this paper is to study the geomorphological processes and the impact of neotectonic deformation structures on the islands’ geomorphology and to model the prevailing geomorphological processes of erosion and deposition. The development of erosion risk maps involves a number of stages, within which the most crucial are the definition of the input and output variables, as well as the establishment of logical rules, and the analysis and visualization of the results. The study area is the Ionian Islands complex, in western Greece, comprising the islands of Zante, Kefallinia, Ithaca, Lefkas, Meganissi, Paxi and Corfu. The study of the Ionian Islands’ geomorphology was fundamental for understanding the various processes taking place in this area and the manner in which they interact with the erosion processes. Therefore, the major factors that affect erosion in the study area were detected and used as input variables in order to develop a model for the determination and mapping of the erosion risk zones. These factors include rocks’ susceptibility to erosion, morphology slope gradient and drainage density. The first variable (rocks’ susceptibility to erosion) is the most complicated, as it depends on many factors, such as the physical and chemical composition of the rock, *Email: evelpidou@geol.uoa.gr Geomatics, Natural Hazards and Risk ISSN 1947-5705 Print/ISSN 1947-5713 Online ª 2012 Taylor & Francis http://www.tandf.co.uk/journals http://dx.doi.org/10.1080/19475705.2011.604798 294 N. Evelpidou the existence of major (folds, faults) and minor (bedding, foliation and joints) tectonic structures and the geomorphological characteristics (e.g. karstification, planation surfaces, knick points, abrupt morphological changes, alluvial fans). Mineral composition is also a crucial factor. According to Sparks (1965), dark- coloured minerals (e.g. Olivine, Augite, Hornblende, etc.) are more vulnerable to weathering than light-coloured ones (e.g. Muscovite, Quartz, etc.). The first variable also depends on the lithology and permeability of the rock, with the last one being more or less defined by the land use. The lithology is associated with the hardness of the rock and its resistance to erosion. This variable is difficult to measure directly. Some observations on the rocks’ resistance to abrasion have resulted in a list of rocks, where they are presented in a decreasing row according to their resistance to erosion (Kuenen 1956). On the other hand, Selby (1980) has proposed a rock mass strength classification and rating, in order to express resistance to erosion. In the above-mentioned classification, limestone appears to be more resistant to erosion than schist. However, the first attempt to assign susceptibility values on different rock types was made by Jensen and Painter (1974). Permeability expresses the amount of water infiltrated into the ground and it is defined by several factors, such as lithology, land use and micro and macro structures of soil and rock (e.g. grain form, grain size or faulting zones, etc). Permeability directly affects the quantity of runoff water, which is the dominant erosion factor. Bolton (1979) has distinguished three categories of permeability –12 –8 –8 –5 –5 values: very low (10 –10 m/s), low to medium (10 –10 m/s) and high (10 – –2 10 m/s). Land use is also a factor that affects runoff and the amount of water infiltrated in the ground. For example, forest areas present low runoff phenomena, whereas in urban sealed areas infiltration is practically non-existent. Moreover, vegetation upholds the soil material and acts as a protective mechanism against soil and rock erosional processes. The last group of parameters, used in order to extract the susceptibility to the erosion parameter, are the geomorphological character- istics, which reflect a different degree of runoff. The second variable that was processed is the morphological slope gradient of each individual drainage basin. Schumm (1977) proved that there is an exponential relation between average slope and sediment yield. Apart from the slope gradient, the aspect and extent of the hill slopes were also taken into account. The next input variable is drainage density (ratio of the total stream length and the drainage basin area), which is highly defined by the runoff quantity of water and the substratum’s permeability. Drainage density is high in basins with weak imperme- able rocks and low in basins with resistant and permeable rocks. Furthermore, drainage density increases proportionally with the basin’s average slope (Gregory and Walling 1973). 2. Regional setting 2.1 Geographical setting of the study area The study area is the Ionian Islands complex, which is located in the Ionian Sea at the westernmost part of Greece. The complex includes the islands of Corfu, Paxi, Lefkas, Meganissi, Kefallinia, Ithaca and Zante (figure 1). The largest of the Ionian Islands is Kefallinia, covering an area of 779.3 km . The relief of the islands is mountainous with the highest peak (1628 m) on Mt. Ainos. Modelling of erosional processes in the Ionian Islands 295 Figure 1. Geographical setting of the study area. 2.2 Geological setting of the study area Geologically, the islands of the Ionian Sea are situated on the outer margin of the thrust blocks that occupy the Greek Territory. Their rocks belong to the Ionian and Paxi geotectonic zones, which represent the most recent evolutionary phases in Greece. The Ionian zone, as the outer large thrust block of the external Hellenides, is considered to have been overthrusting the autochthonous stable zone of Paxi during the latest Alpine events (Underhill 1985, 1989). Zante, Kefallinia, Ithaca, Lefkas and Paxi mainly consist of limestones, especially along their coasts (Bornovas 1964, Perry et al. 1969, 1980, Stavropoulos 1991), while 296 N. Evelpidou Corfu is comprised mainly of Neogene and Quaternary formations (Maragkoudakis et al. 1970, Savoyat et al. 1970). In Zante marly limestones, sandstones, mudstones, marls and evaporites (mainly gypsum) may also be found, while the island’s main plain in the eastern part is dominated by alluvial depositions. In Kefallinia, limestones sometimes are found, along with shales, breccias, conglomerates and dolomites. The Quaternary depositions mainly consist of sandstones, conglomerates and alluvium. Paxi are entirely dominated by limestones (thin bedded in the NW and thick bedded in the SE) (Perry et al. 1969), while in Meganissi they are alternated with cherts. Lefkas is also, in great part, covered by limestones, granulated, microbreccian or in alternation with schists and dolomites. However, outcrops of flysch, marls, molasses and alluvial depositions can also be found (Bornovas 1964). Marls, marly sandstones and breccias are found on the western coast of Ithaca, but the rest of the island is covered by limestones, with the sole exception of some alluvial depositions of scree and talus (Stavropoulos 1991). Finally, Corfu is partially covered by limestones, schists and dolomites, mainly on the north-eastern part; otherwise the island is dominated by Neogene formations and Quaternary depositions (Koukouzas 1962, Maragkoudakis et al. 1970, Savoyat et al. 1970). Geomorphological observations in the area of the Ionian Sea (Gournelos et al. 1997, Vassilopoulos 2001) have provided the lithological structure of each island’s coastal zone: carbonate rocks are the predominant coastal formations, covering 79.34% of the coastal zone of Ithaca, 63.85% of Kefallinia, and 62.80% of Zante. A significant percentage of the coastal zone of Corfu (29.66%) and Zante (28.58%) is occupied by Neogene formations. The Quaternary formations cover almost half of Corfu’s coastal zone and 37.23% of Lefkas’. The Ionian Islands occupy a geodynamically important area in the north-western Hellenic Arc system, which is characterized by the subduction of the African plate beneath the Eurasian one. This subduction zone is active all along the south Ionian Sea, up to south of Kefallinia Island. The Kefallinia Transformation Fault (CTF), which is a dextral strike-slip fault, is located west of Kefallinia and Lefkas islands, while north of the Lefkas–Corfu zone is the northern extension of the Hellenic arc system, representing the collision zone of the Apulian Platform and the Hellenides (Anzidei et al. 1996, Lagios et al. 2007). The formation of the Ionian Islands took place in the Quaternary, as a result of intense compressive tectonism and uplift, which started in the Lower Pliocene. The intense seismicity, the focal mechanisms of earthquakes, the geodetic measurements and the coastal movements associated with strong earthquakes indicate that the compressive tension associated with uplift and the tectonic movements are still active (Mercier et al. 1972). This area is seismically considered among the most active in Greece and it is the area where the majority of the strongest (M 6.0) shallow earthquakes occur. Uplifted palaeo-shorelines are found in every island in the form of marine erosion marks, marine deposits or marine bioconstructions (Pirazzoli et al. 1994). Furthermore, several submerged archaeological sites, at depths of a few metres, are found off the islands’ coasts; however, these movements could be associated either with tectonics or eustatism. 2.3 Geomorphological setting of the study area Morphologically the Ionian complex is of NW–SE direction; it comprises part of the Dinaric Arc, with a length of 258.4 km and width of 60.3 km. The morphology of Modelling of erosional processes in the Ionian Islands 297 the islands has been strongly influenced by the recent geological status, the active tectonism, the differential lithology and the marine processes. In the study area many landforms have been formed and are still being developed on carbonate rocks, which constitute 52.36% of the Ionian Islands’ lithology. Particularly for Kefallinia Island, karstification is very intense and constitutes one of its principal geomorphological features. The karstic landforms of the Ionian Islands belong to the Dinaric karst type, which was developed during M. Pleistocene – Holocene. In some cases, the underground drainage network replaces the superficial runoff, even in areas that experience severe rainfalls. Thus, karstic regions appear as areas without vegetation due to the lack of superficial water. Superficial karstic forms, such as sculptures, dolines, poljes, karstic lakes and springs, as well as underground karstic forms (caves, sinkholes and submarine karstic springs), can be found. Most Ionian Islands’ plains are small and usually their formation is related to the fault lines that actually delimit them. The most characteristic examples are those of St. Eufimia plain in Kefallinia (figure 2), Vassiliki plain and Lagada Gria valley in Lefkas and the main plain of Zante. In Lefkas, knick points are related to differential erosion, but are mostly formed due to tectonics (Verikiou et al. 2000). Intense downcutting is also related to discontinuities, which is usually the case in Kefallinia (Vassilopoulos 2001). The same conditions stand in gorge formation, for example in Poros gorge in Kefallinia. In most cases fault lines lead to abrupt changes in morphological slope, a characteristic example of the island of Zante. In many cases alluvial fans are found on such slopes. During fieldwork, measurements of the directions of the main faults and discontinuities were performed. Analysis of the faults’ and discontinuities’ directions and their correlation with the directions of the hydrographical network revealed that the islands’ drainage network is affected by the tectonic processes. Figure 2. Most of the plains in the Ionian Islands are related to fault lines, such as in the St. Eufimia plain, in Kefallinia. 298 N. Evelpidou The coastlines of Kefallinia, Ithaca and Zante are practically defined by the presence of offshore normal faults and maintain abrupt slopes. Neogene coasts are locally very steep and retreat due to wave action. The most characteristic example of an Ionian Neogene shore is the northern part of Corfu, in the wider area of Sidari bay (figure 3) (Evelpidou et al. 2002b). On the other hand, Quaternary formations usually result to low relief coasts. The majority of the Ionian Islands is characterized by the presence of coastal caves. Their formation is the result of erosion caused by waves on tectonized rocks (mainly carbonate rocks but not exclusively), for example, Zante, Kefallinia, Meganissi and Figure 3. The existing discontinuities as a result of tectonics, in combination with the high susceptibility of the Neogene formations to erosion, leads to the rapid retreatment of Sidari area, with a mean annual erosion rate of 0.178 m (Evelpidou et al. 2002b). Modelling of erosional processes in the Ionian Islands 299 eastern Ithaca (figure 4). The existing discontinuities constitute the starting point of the rock’s dissolution that leads to the formation of the coastal cave. 3. Material and methods The first step of this study was the digitization of the geological, topographical and drainage system maps. The working scales for the topographical and geological maps are 1:25.000 and 1:50.000, respectively. Then the interpretation of aerial photos was achieved in a 1:33.000 scale and the collected geomorphological data were also imported into the geographic information system (GIS). A systematic survey took place during the springs and summers of 1998–2009 in order to map the Figure 4. Parallel caves on eastern part of Ithaca island. 300 N. Evelpidou geomorphological characteristics of the studied islands. The mapping scale was 1:25.000 and a geomorphological map was produced for each studied island. Field work took place in two phases for each island: the inland part was first mapped and the coastal one followed. In the latter case the systematic survey was achieved by a boat in order to access all sites and to establish their continuity. In addition, during this stage measurements of the directions of the main faults and discontinuities were performed. In order to correlate runoff and tectonics, the distribution of discontinuities’ and streams’ directions was calculated. This variable was determined using the ‘Geoline Orientation Software’ algorithm (Vassilopoulos 1999, Evelpidou et al. 2002a), which calculates the azimuthal value of linear objects’ direction, such as discontinuities and streams. Results were statistically analysed and diagrams presenting the measured directions (rodograms) were produced. Two rodograms were produced, one for each set of data for discontinuities and streams, respectively. By creating the rodograms, all the directions were divided into 36 groups of a 108 range each. Therefore, the resulting rodograms were graphic depictions of the distribution of the disconti- nuities’ and streams’ directions. Through GIS analytical processes, the rocks’ susceptibility to erosion, the slope inclination and the drainage density values were calculated based on the initial data. The next step was to treat the previous variables as input factors and to assign membership functions to each one of them. In order to treat them as input factors, they needed to be transformed into units that are able to interact as variables of a function. This is carried out by splitting each information layer into areas of predefined size, in order to have a common benchmark. Consequently, the calculation of these factors (susceptibility to erosion, slope and drainage density) was accomplished by using a geographical grid (250 m 6 250 m), which was set as a new information layer within the GIS platform. In this way, a maximal area of 62,500 m is defined for each cell, except for the cells that coincide with the coastline and obviously do not maintain their square shape or their maximum size. After the development of this grid, each cell was updated with values derived from the three factors. The calculation of the input factors’ values was an automated process induced by the GIS, using algorithms that were developed for this study in the programming platform of MapBasic Software. In order to model erosion processes, fuzzy logic, dealing with reasoning that is fixed or approximate rather than fixed and exact, was adopted. Fuzzy logic variables may have a truth value that ranges in degree between 0 and 1, which is in contrast with crisp logic, where binary sets have only two values, that is, true or false. Fuzzy logic was used in this paper in order to handle the concept of partial truth, where the truth value may range between completely true and completely false. So, the fuzzy set theory was applied (Zadeh 1965, Dubois and Prade 1980, Yager et al. 1987, Zadeh 1987, Zimmermann 1991, Klir and Yuan 1995, Gournelos et al. 2004) in order to present and better analyse the boundary vagueness and the relative spatial imprecision of the most input variables. For this reason, the gradation of the input variables was ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1) and one category coincides with the other (for example, part of the low category is also included in the medium one and the same is the case between medium and high), following the triangular functions adopted from the fuzzy set theory. Based on the above variables, a fuzzy model was developed. The fuzzy inference process that was used is known as the Mamdani method (Mamdani and Assilian 1975) and is characterized Modelling of erosional processes in the Ionian Islands 301 by its fuzzy outputs. The last step was the defuzzification of fuzzy outputs, in order to map the results. The application of the rules that were developed (table 1) was accomplished with the use of Matlab software. Extensive fieldwork within the case study confirms, in many cases, the erosion risk map that was empirically created. 4. Discussion The rodograms that were created for the discontinuities’ and streams’ directions showed that the prevalent direction for the drainage system is N808E, which coincides with the discontinuities’ prevailing direction N758E (figure 5). The distribution of these directions shows that the neotectonic processes in the area have played a major role on the development of the drainage network. Tectonics has affected the main geomorphological features of the Ionian Islands. The formation of the St. Eufimia plain in Kefallinia seems to have been determined by tectonic lines (Vassilopoulos 2001). The same observation can be made for the formation of Argostoli bay (Vassilopoulos 2001), Vassiliki plain in Lefkas, the plain in eastern Zante, the coastal area in western Lefkas, the delimitation and morphological variation between the northern and southern Corfu, and the western Zante coastline (Gournelos et al. 2007). There are many examples, of major or minor importance or size, but as a whole they provide a clear view of the impact that tectonics and discontinuities have on the current morphology. The main geomorphic processes taking place on the Ionian Islands are related to coastal erosion, as well as inland erosion. These processes are primarily controlled by neotectonics, which is responsible for the existing discontinuities that control the direction of surface runoff. This was evident through the comparison of the drainage system’s prevailing directions with those of the discontinuities revealing that the prevalent direction of the drainage system coincides with the faults’ prevailing direction. Moreover, the main directions of the coastline are controlled by neotec- tonic faulting. For instance, the systematic study of the coastal caves, located in the southern part of the island, has shown that most of the caves are parallel to the discontinuities of the rocks (faults, joints, stratification), thus proving that their formation is mainly controlled by small- or large-scale faulting (Gournelos et al. 2009). Susceptibility to erosion is a complex factor, as previously mentioned, and various parameters were taken into account for its estimation, such as the type of lithology, the micro-tectonic structures of the rocks and the macro-tectonic structures of the area, the land use, the vegetation and soil cover and finally the geomorphological status. The lithology, which describes the rock’s hardness and resistance to erosion, along with discontinuities, vegetation and soil cover, defines the permeability of the rocks. The micro-tectonic structures also influence the rock’s resistance to erosion and, therefore, make a strongly deformed formation, which is more vulnerable to erosion than a healthy one. The macro-tectonic structures or even the path of water runoff create weakness zones, altering the susceptibility to erosion even within the body of the same lithological formations. The geomorphological status of the region is also significant since, according to the geomorphological features, the amount of water runoff is changing. For instance, in areas of intense karstification, the infiltration rates are higher, thus the superficial erosion is decreased, which is important for this study. In addition, in planation 302 N. Evelpidou Table 1. The logical rules used to derive the erosion risk index. If SUSCEPTIBILITY IS High & SLOPE IS High Then Erosion risk index Is High If SUSCEPTIBILITY IS High & SLOPE IS Medium & Drainage Density IS High Then Erosion risk index Is High If SUSCEPTIBILITY IS High & SLOPE IS Low Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS High Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS Medium & Drainage Density IS High Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS Low & Drainage Density IS High Then Erosion risk index Is Low If SUSCEPTIBILITY IS Low & SLOPE IS Low Then Erosion risk index Is Very low Modelling of erosional processes in the Ionian Islands 303 Figure 5. Rodograms showing that the prevailing direction of discontinuities and streams coincides. surfaces the water runoff occurs in sheets rather than in streams, favouring uniform erosion. Vegetation cover or land use are responsible either for the change of infiltration rates (e.g. the sealed surface of an urban region has almost zero infiltration, favouring runoff) or the alteration of runoff speed (e.g. an area covered with grass has slower runoff, and thus lower sediment transfer capacity, than an area with bare rocks). This particular factor (figure 6) was extracted by taking into account all the above sub-parameters, which were scaled according to their influence on susceptibility to erosion. The susceptibility to erosion factor was normalized in values from 0 to 1, with a value of 0 corresponding to non-vulnerable rocks, and a value of 1 corresponding to the formations that are significantly prone to erosion. The values that were applied for the development of the susceptibility to erosion map were grouped into three categories: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The slope factor is associated with the inclinations of the topographical relief. Water runoff depends strongly on the form (convex or concave) and the aspect of the inclination, and on the extent of the slope. Of course, the most significant factor is the slope steepness. Both inclination (figure 7) and aspect were used as a scaled parameter for the final map, while the slope factor was grouped into three categories of normalized value ranges: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The drainage density factor is related to the length of river branches that cross a specific area, and it is normally calculated as the ratio of total stream length to drainage basin area. This factor (figure 8) was also categorized into three groups of normalized value ranges: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The formulation of logical rules (table 1) took place for estimating the erosion risk values and creating the corresponding thematic map with the grid based on the geographical distribution of the values (figure 9). As shown on the final erosion risk map (figure 9), the study area is regarded as of medium erosion risk. The erosion risk model that was developed for the area of the Ionian Islands, based on logical rules, resulted in the erosion risk map. However, erosion risk maps deriving from analytical methods should not be considered as final, because they 304 N. Evelpidou Figure 6. Geographical distribution of the susceptibility to erosion factor in the study area. Modelling of erosional processes in the Ionian Islands 305 Figure 7. Geographical distribution of the slope gradient in the study area. should be cross checked through field work in order to correct the gravity by which each parameter influences the model. For this reason, extensive field work took place and the model was reclassified in parts. The final map that derived from the 306 N. Evelpidou Figure 8. Geographical distribution of the drainage density parameter in the study area. Modelling of erosional processes in the Ionian Islands 307 Figure 9. Geographical distribution of erosion risk in the study area. reclassified model (figure 9) showed that the most dangerous zones appear to be the areas of high slopes, as happens in the case of Kefallinia and Ithaca. The highest rates of erosion risk occur in the islands of Corfu and Zante. In Corfu, the high rates 308 N. Evelpidou are explained by the vulnerable Quaternary and Neogene depositions that comprise accordingly 28.7% and 38.4%, respectively, of the lithology. In Zante, a high-risk zone is observed along the faulting zone of the NE–SW direction. This faulting zone provokes an abrupt change of the topographic grade between the carbonate rocks of the island’s western part and the Quaternary and Neogene depositions of the eastern part. Along this zone many alluvial fans are found. The behaviour of the model to minimal changes of the input variables was tested by a sensitivity analysis. As expected, small perturbation of the values of the three input variables contribute to insignificant changes of the output variables (erosion risk index). 5. Conclusions The Ionian Islands are strongly affected by the neotectonic processes that have defined their current morphology. A statistical analysis of the discontinuities in comparison to the stream directions showed that neotectonic processes in the area have contributed significantly to the development of the drainage network, and inland discontinuities have affected the stream directions. Moreover, the direction of the current coastlines coincides very often with the directions of the major discontinuities. Specifically, the faulting processes were the predefining factor for the water flow directions, as was demonstrated by the rodograms. All related factors, such as topography, morphology, geology, climatology, land use–land cover and tectonic features were combined to develop erosion risk maps that show the areas vulnerable to erosion. The final map (figure 9) reveals that the highest rates of erosion risk occur in the islands of Corfu and Zante. In Corfu, the high rates are attributed to the vulnerable Quaternary and Neogene depositions, comprising 28.7% and 38.4% of the lithology, respectively. In Zante, a high-risk zone is observed along the faulting zone of the NE–SW direction. 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ZIMMERMANN, H.J., 1991, Fuzzy Set Theory and Its Application, 2nd edition, p. 435 (Norwell, MA: Kluwer Academic). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Geomatics, Natural Hazards and Risk" Taylor & Francis

Modelling of erosional processes in the Ionian Islands (Greece)

"Geomatics, Natural Hazards and Risk" , Volume 3 (4): 18 – Nov 1, 2012

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1947-5713
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1947-5705
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10.1080/19475705.2011.604798
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Abstract

Geomatics, Natural Hazards and Risk Vol. 3, No. 4, November 2012, 293–310 NIKI EVELPIDOU* Faculty of Geology and Geoenvironment, University of Athens, Panepistimiopolis, Zografou, 157-84, Athens, Greece (Received 5 April 2011; in final form 9 July 2011) This paper focuses on the study of the geomorphological processes and the impact of neotectonic deformation on the geomorphological structure. A vast database was developed, containing different kinds of information, concerning geology, topography, drainage characteristics, vegetation and land use of the Ionian Islands. A geographic information system platform was developed in order to analyse the data, and to model and finally map the prevailing geomorphological processes: the erosion and deposition zones. Erosion risk factors have been processed in order to develop erosion risk maps demonstrating vulnerable to erosion areas. This study also concludes that the Ionian Islands are strongly influenced by the neotectonic processes that have defined their current morphology. 1. Introduction For the analysis of problems related to erosion and other natural hazards, many different approaches have been applied (Brundsen et al. 1975, Carrara et al. 1977, Malgot and Mahr 1979, Ives and Mersserli 1981, Carrara 1983, Carrara et al. 1991). The purpose of this paper is to study the geomorphological processes and the impact of neotectonic deformation structures on the islands’ geomorphology and to model the prevailing geomorphological processes of erosion and deposition. The development of erosion risk maps involves a number of stages, within which the most crucial are the definition of the input and output variables, as well as the establishment of logical rules, and the analysis and visualization of the results. The study area is the Ionian Islands complex, in western Greece, comprising the islands of Zante, Kefallinia, Ithaca, Lefkas, Meganissi, Paxi and Corfu. The study of the Ionian Islands’ geomorphology was fundamental for understanding the various processes taking place in this area and the manner in which they interact with the erosion processes. Therefore, the major factors that affect erosion in the study area were detected and used as input variables in order to develop a model for the determination and mapping of the erosion risk zones. These factors include rocks’ susceptibility to erosion, morphology slope gradient and drainage density. The first variable (rocks’ susceptibility to erosion) is the most complicated, as it depends on many factors, such as the physical and chemical composition of the rock, *Email: evelpidou@geol.uoa.gr Geomatics, Natural Hazards and Risk ISSN 1947-5705 Print/ISSN 1947-5713 Online ª 2012 Taylor & Francis http://www.tandf.co.uk/journals http://dx.doi.org/10.1080/19475705.2011.604798 294 N. Evelpidou the existence of major (folds, faults) and minor (bedding, foliation and joints) tectonic structures and the geomorphological characteristics (e.g. karstification, planation surfaces, knick points, abrupt morphological changes, alluvial fans). Mineral composition is also a crucial factor. According to Sparks (1965), dark- coloured minerals (e.g. Olivine, Augite, Hornblende, etc.) are more vulnerable to weathering than light-coloured ones (e.g. Muscovite, Quartz, etc.). The first variable also depends on the lithology and permeability of the rock, with the last one being more or less defined by the land use. The lithology is associated with the hardness of the rock and its resistance to erosion. This variable is difficult to measure directly. Some observations on the rocks’ resistance to abrasion have resulted in a list of rocks, where they are presented in a decreasing row according to their resistance to erosion (Kuenen 1956). On the other hand, Selby (1980) has proposed a rock mass strength classification and rating, in order to express resistance to erosion. In the above-mentioned classification, limestone appears to be more resistant to erosion than schist. However, the first attempt to assign susceptibility values on different rock types was made by Jensen and Painter (1974). Permeability expresses the amount of water infiltrated into the ground and it is defined by several factors, such as lithology, land use and micro and macro structures of soil and rock (e.g. grain form, grain size or faulting zones, etc). Permeability directly affects the quantity of runoff water, which is the dominant erosion factor. Bolton (1979) has distinguished three categories of permeability –12 –8 –8 –5 –5 values: very low (10 –10 m/s), low to medium (10 –10 m/s) and high (10 – –2 10 m/s). Land use is also a factor that affects runoff and the amount of water infiltrated in the ground. For example, forest areas present low runoff phenomena, whereas in urban sealed areas infiltration is practically non-existent. Moreover, vegetation upholds the soil material and acts as a protective mechanism against soil and rock erosional processes. The last group of parameters, used in order to extract the susceptibility to the erosion parameter, are the geomorphological character- istics, which reflect a different degree of runoff. The second variable that was processed is the morphological slope gradient of each individual drainage basin. Schumm (1977) proved that there is an exponential relation between average slope and sediment yield. Apart from the slope gradient, the aspect and extent of the hill slopes were also taken into account. The next input variable is drainage density (ratio of the total stream length and the drainage basin area), which is highly defined by the runoff quantity of water and the substratum’s permeability. Drainage density is high in basins with weak imperme- able rocks and low in basins with resistant and permeable rocks. Furthermore, drainage density increases proportionally with the basin’s average slope (Gregory and Walling 1973). 2. Regional setting 2.1 Geographical setting of the study area The study area is the Ionian Islands complex, which is located in the Ionian Sea at the westernmost part of Greece. The complex includes the islands of Corfu, Paxi, Lefkas, Meganissi, Kefallinia, Ithaca and Zante (figure 1). The largest of the Ionian Islands is Kefallinia, covering an area of 779.3 km . The relief of the islands is mountainous with the highest peak (1628 m) on Mt. Ainos. Modelling of erosional processes in the Ionian Islands 295 Figure 1. Geographical setting of the study area. 2.2 Geological setting of the study area Geologically, the islands of the Ionian Sea are situated on the outer margin of the thrust blocks that occupy the Greek Territory. Their rocks belong to the Ionian and Paxi geotectonic zones, which represent the most recent evolutionary phases in Greece. The Ionian zone, as the outer large thrust block of the external Hellenides, is considered to have been overthrusting the autochthonous stable zone of Paxi during the latest Alpine events (Underhill 1985, 1989). Zante, Kefallinia, Ithaca, Lefkas and Paxi mainly consist of limestones, especially along their coasts (Bornovas 1964, Perry et al. 1969, 1980, Stavropoulos 1991), while 296 N. Evelpidou Corfu is comprised mainly of Neogene and Quaternary formations (Maragkoudakis et al. 1970, Savoyat et al. 1970). In Zante marly limestones, sandstones, mudstones, marls and evaporites (mainly gypsum) may also be found, while the island’s main plain in the eastern part is dominated by alluvial depositions. In Kefallinia, limestones sometimes are found, along with shales, breccias, conglomerates and dolomites. The Quaternary depositions mainly consist of sandstones, conglomerates and alluvium. Paxi are entirely dominated by limestones (thin bedded in the NW and thick bedded in the SE) (Perry et al. 1969), while in Meganissi they are alternated with cherts. Lefkas is also, in great part, covered by limestones, granulated, microbreccian or in alternation with schists and dolomites. However, outcrops of flysch, marls, molasses and alluvial depositions can also be found (Bornovas 1964). Marls, marly sandstones and breccias are found on the western coast of Ithaca, but the rest of the island is covered by limestones, with the sole exception of some alluvial depositions of scree and talus (Stavropoulos 1991). Finally, Corfu is partially covered by limestones, schists and dolomites, mainly on the north-eastern part; otherwise the island is dominated by Neogene formations and Quaternary depositions (Koukouzas 1962, Maragkoudakis et al. 1970, Savoyat et al. 1970). Geomorphological observations in the area of the Ionian Sea (Gournelos et al. 1997, Vassilopoulos 2001) have provided the lithological structure of each island’s coastal zone: carbonate rocks are the predominant coastal formations, covering 79.34% of the coastal zone of Ithaca, 63.85% of Kefallinia, and 62.80% of Zante. A significant percentage of the coastal zone of Corfu (29.66%) and Zante (28.58%) is occupied by Neogene formations. The Quaternary formations cover almost half of Corfu’s coastal zone and 37.23% of Lefkas’. The Ionian Islands occupy a geodynamically important area in the north-western Hellenic Arc system, which is characterized by the subduction of the African plate beneath the Eurasian one. This subduction zone is active all along the south Ionian Sea, up to south of Kefallinia Island. The Kefallinia Transformation Fault (CTF), which is a dextral strike-slip fault, is located west of Kefallinia and Lefkas islands, while north of the Lefkas–Corfu zone is the northern extension of the Hellenic arc system, representing the collision zone of the Apulian Platform and the Hellenides (Anzidei et al. 1996, Lagios et al. 2007). The formation of the Ionian Islands took place in the Quaternary, as a result of intense compressive tectonism and uplift, which started in the Lower Pliocene. The intense seismicity, the focal mechanisms of earthquakes, the geodetic measurements and the coastal movements associated with strong earthquakes indicate that the compressive tension associated with uplift and the tectonic movements are still active (Mercier et al. 1972). This area is seismically considered among the most active in Greece and it is the area where the majority of the strongest (M 6.0) shallow earthquakes occur. Uplifted palaeo-shorelines are found in every island in the form of marine erosion marks, marine deposits or marine bioconstructions (Pirazzoli et al. 1994). Furthermore, several submerged archaeological sites, at depths of a few metres, are found off the islands’ coasts; however, these movements could be associated either with tectonics or eustatism. 2.3 Geomorphological setting of the study area Morphologically the Ionian complex is of NW–SE direction; it comprises part of the Dinaric Arc, with a length of 258.4 km and width of 60.3 km. The morphology of Modelling of erosional processes in the Ionian Islands 297 the islands has been strongly influenced by the recent geological status, the active tectonism, the differential lithology and the marine processes. In the study area many landforms have been formed and are still being developed on carbonate rocks, which constitute 52.36% of the Ionian Islands’ lithology. Particularly for Kefallinia Island, karstification is very intense and constitutes one of its principal geomorphological features. The karstic landforms of the Ionian Islands belong to the Dinaric karst type, which was developed during M. Pleistocene – Holocene. In some cases, the underground drainage network replaces the superficial runoff, even in areas that experience severe rainfalls. Thus, karstic regions appear as areas without vegetation due to the lack of superficial water. Superficial karstic forms, such as sculptures, dolines, poljes, karstic lakes and springs, as well as underground karstic forms (caves, sinkholes and submarine karstic springs), can be found. Most Ionian Islands’ plains are small and usually their formation is related to the fault lines that actually delimit them. The most characteristic examples are those of St. Eufimia plain in Kefallinia (figure 2), Vassiliki plain and Lagada Gria valley in Lefkas and the main plain of Zante. In Lefkas, knick points are related to differential erosion, but are mostly formed due to tectonics (Verikiou et al. 2000). Intense downcutting is also related to discontinuities, which is usually the case in Kefallinia (Vassilopoulos 2001). The same conditions stand in gorge formation, for example in Poros gorge in Kefallinia. In most cases fault lines lead to abrupt changes in morphological slope, a characteristic example of the island of Zante. In many cases alluvial fans are found on such slopes. During fieldwork, measurements of the directions of the main faults and discontinuities were performed. Analysis of the faults’ and discontinuities’ directions and their correlation with the directions of the hydrographical network revealed that the islands’ drainage network is affected by the tectonic processes. Figure 2. Most of the plains in the Ionian Islands are related to fault lines, such as in the St. Eufimia plain, in Kefallinia. 298 N. Evelpidou The coastlines of Kefallinia, Ithaca and Zante are practically defined by the presence of offshore normal faults and maintain abrupt slopes. Neogene coasts are locally very steep and retreat due to wave action. The most characteristic example of an Ionian Neogene shore is the northern part of Corfu, in the wider area of Sidari bay (figure 3) (Evelpidou et al. 2002b). On the other hand, Quaternary formations usually result to low relief coasts. The majority of the Ionian Islands is characterized by the presence of coastal caves. Their formation is the result of erosion caused by waves on tectonized rocks (mainly carbonate rocks but not exclusively), for example, Zante, Kefallinia, Meganissi and Figure 3. The existing discontinuities as a result of tectonics, in combination with the high susceptibility of the Neogene formations to erosion, leads to the rapid retreatment of Sidari area, with a mean annual erosion rate of 0.178 m (Evelpidou et al. 2002b). Modelling of erosional processes in the Ionian Islands 299 eastern Ithaca (figure 4). The existing discontinuities constitute the starting point of the rock’s dissolution that leads to the formation of the coastal cave. 3. Material and methods The first step of this study was the digitization of the geological, topographical and drainage system maps. The working scales for the topographical and geological maps are 1:25.000 and 1:50.000, respectively. Then the interpretation of aerial photos was achieved in a 1:33.000 scale and the collected geomorphological data were also imported into the geographic information system (GIS). A systematic survey took place during the springs and summers of 1998–2009 in order to map the Figure 4. Parallel caves on eastern part of Ithaca island. 300 N. Evelpidou geomorphological characteristics of the studied islands. The mapping scale was 1:25.000 and a geomorphological map was produced for each studied island. Field work took place in two phases for each island: the inland part was first mapped and the coastal one followed. In the latter case the systematic survey was achieved by a boat in order to access all sites and to establish their continuity. In addition, during this stage measurements of the directions of the main faults and discontinuities were performed. In order to correlate runoff and tectonics, the distribution of discontinuities’ and streams’ directions was calculated. This variable was determined using the ‘Geoline Orientation Software’ algorithm (Vassilopoulos 1999, Evelpidou et al. 2002a), which calculates the azimuthal value of linear objects’ direction, such as discontinuities and streams. Results were statistically analysed and diagrams presenting the measured directions (rodograms) were produced. Two rodograms were produced, one for each set of data for discontinuities and streams, respectively. By creating the rodograms, all the directions were divided into 36 groups of a 108 range each. Therefore, the resulting rodograms were graphic depictions of the distribution of the disconti- nuities’ and streams’ directions. Through GIS analytical processes, the rocks’ susceptibility to erosion, the slope inclination and the drainage density values were calculated based on the initial data. The next step was to treat the previous variables as input factors and to assign membership functions to each one of them. In order to treat them as input factors, they needed to be transformed into units that are able to interact as variables of a function. This is carried out by splitting each information layer into areas of predefined size, in order to have a common benchmark. Consequently, the calculation of these factors (susceptibility to erosion, slope and drainage density) was accomplished by using a geographical grid (250 m 6 250 m), which was set as a new information layer within the GIS platform. In this way, a maximal area of 62,500 m is defined for each cell, except for the cells that coincide with the coastline and obviously do not maintain their square shape or their maximum size. After the development of this grid, each cell was updated with values derived from the three factors. The calculation of the input factors’ values was an automated process induced by the GIS, using algorithms that were developed for this study in the programming platform of MapBasic Software. In order to model erosion processes, fuzzy logic, dealing with reasoning that is fixed or approximate rather than fixed and exact, was adopted. Fuzzy logic variables may have a truth value that ranges in degree between 0 and 1, which is in contrast with crisp logic, where binary sets have only two values, that is, true or false. Fuzzy logic was used in this paper in order to handle the concept of partial truth, where the truth value may range between completely true and completely false. So, the fuzzy set theory was applied (Zadeh 1965, Dubois and Prade 1980, Yager et al. 1987, Zadeh 1987, Zimmermann 1991, Klir and Yuan 1995, Gournelos et al. 2004) in order to present and better analyse the boundary vagueness and the relative spatial imprecision of the most input variables. For this reason, the gradation of the input variables was ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1) and one category coincides with the other (for example, part of the low category is also included in the medium one and the same is the case between medium and high), following the triangular functions adopted from the fuzzy set theory. Based on the above variables, a fuzzy model was developed. The fuzzy inference process that was used is known as the Mamdani method (Mamdani and Assilian 1975) and is characterized Modelling of erosional processes in the Ionian Islands 301 by its fuzzy outputs. The last step was the defuzzification of fuzzy outputs, in order to map the results. The application of the rules that were developed (table 1) was accomplished with the use of Matlab software. Extensive fieldwork within the case study confirms, in many cases, the erosion risk map that was empirically created. 4. Discussion The rodograms that were created for the discontinuities’ and streams’ directions showed that the prevalent direction for the drainage system is N808E, which coincides with the discontinuities’ prevailing direction N758E (figure 5). The distribution of these directions shows that the neotectonic processes in the area have played a major role on the development of the drainage network. Tectonics has affected the main geomorphological features of the Ionian Islands. The formation of the St. Eufimia plain in Kefallinia seems to have been determined by tectonic lines (Vassilopoulos 2001). The same observation can be made for the formation of Argostoli bay (Vassilopoulos 2001), Vassiliki plain in Lefkas, the plain in eastern Zante, the coastal area in western Lefkas, the delimitation and morphological variation between the northern and southern Corfu, and the western Zante coastline (Gournelos et al. 2007). There are many examples, of major or minor importance or size, but as a whole they provide a clear view of the impact that tectonics and discontinuities have on the current morphology. The main geomorphic processes taking place on the Ionian Islands are related to coastal erosion, as well as inland erosion. These processes are primarily controlled by neotectonics, which is responsible for the existing discontinuities that control the direction of surface runoff. This was evident through the comparison of the drainage system’s prevailing directions with those of the discontinuities revealing that the prevalent direction of the drainage system coincides with the faults’ prevailing direction. Moreover, the main directions of the coastline are controlled by neotec- tonic faulting. For instance, the systematic study of the coastal caves, located in the southern part of the island, has shown that most of the caves are parallel to the discontinuities of the rocks (faults, joints, stratification), thus proving that their formation is mainly controlled by small- or large-scale faulting (Gournelos et al. 2009). Susceptibility to erosion is a complex factor, as previously mentioned, and various parameters were taken into account for its estimation, such as the type of lithology, the micro-tectonic structures of the rocks and the macro-tectonic structures of the area, the land use, the vegetation and soil cover and finally the geomorphological status. The lithology, which describes the rock’s hardness and resistance to erosion, along with discontinuities, vegetation and soil cover, defines the permeability of the rocks. The micro-tectonic structures also influence the rock’s resistance to erosion and, therefore, make a strongly deformed formation, which is more vulnerable to erosion than a healthy one. The macro-tectonic structures or even the path of water runoff create weakness zones, altering the susceptibility to erosion even within the body of the same lithological formations. The geomorphological status of the region is also significant since, according to the geomorphological features, the amount of water runoff is changing. For instance, in areas of intense karstification, the infiltration rates are higher, thus the superficial erosion is decreased, which is important for this study. In addition, in planation 302 N. Evelpidou Table 1. The logical rules used to derive the erosion risk index. If SUSCEPTIBILITY IS High & SLOPE IS High Then Erosion risk index Is High If SUSCEPTIBILITY IS High & SLOPE IS Medium & Drainage Density IS High Then Erosion risk index Is High If SUSCEPTIBILITY IS High & SLOPE IS Low Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS High Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS Medium & Drainage Density IS High Then Erosion risk index Is Medium If SUSCEPTIBILITY IS Medium & SLOPE IS Low & Drainage Density IS High Then Erosion risk index Is Low If SUSCEPTIBILITY IS Low & SLOPE IS Low Then Erosion risk index Is Very low Modelling of erosional processes in the Ionian Islands 303 Figure 5. Rodograms showing that the prevailing direction of discontinuities and streams coincides. surfaces the water runoff occurs in sheets rather than in streams, favouring uniform erosion. Vegetation cover or land use are responsible either for the change of infiltration rates (e.g. the sealed surface of an urban region has almost zero infiltration, favouring runoff) or the alteration of runoff speed (e.g. an area covered with grass has slower runoff, and thus lower sediment transfer capacity, than an area with bare rocks). This particular factor (figure 6) was extracted by taking into account all the above sub-parameters, which were scaled according to their influence on susceptibility to erosion. The susceptibility to erosion factor was normalized in values from 0 to 1, with a value of 0 corresponding to non-vulnerable rocks, and a value of 1 corresponding to the formations that are significantly prone to erosion. The values that were applied for the development of the susceptibility to erosion map were grouped into three categories: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The slope factor is associated with the inclinations of the topographical relief. Water runoff depends strongly on the form (convex or concave) and the aspect of the inclination, and on the extent of the slope. Of course, the most significant factor is the slope steepness. Both inclination (figure 7) and aspect were used as a scaled parameter for the final map, while the slope factor was grouped into three categories of normalized value ranges: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The drainage density factor is related to the length of river branches that cross a specific area, and it is normally calculated as the ratio of total stream length to drainage basin area. This factor (figure 8) was also categorized into three groups of normalized value ranges: ‘low’ (0–0.5), ‘medium’ (0.25–0.75) and ‘high’ (0.5–1). The formulation of logical rules (table 1) took place for estimating the erosion risk values and creating the corresponding thematic map with the grid based on the geographical distribution of the values (figure 9). As shown on the final erosion risk map (figure 9), the study area is regarded as of medium erosion risk. The erosion risk model that was developed for the area of the Ionian Islands, based on logical rules, resulted in the erosion risk map. However, erosion risk maps deriving from analytical methods should not be considered as final, because they 304 N. Evelpidou Figure 6. Geographical distribution of the susceptibility to erosion factor in the study area. Modelling of erosional processes in the Ionian Islands 305 Figure 7. Geographical distribution of the slope gradient in the study area. should be cross checked through field work in order to correct the gravity by which each parameter influences the model. For this reason, extensive field work took place and the model was reclassified in parts. The final map that derived from the 306 N. Evelpidou Figure 8. Geographical distribution of the drainage density parameter in the study area. Modelling of erosional processes in the Ionian Islands 307 Figure 9. Geographical distribution of erosion risk in the study area. reclassified model (figure 9) showed that the most dangerous zones appear to be the areas of high slopes, as happens in the case of Kefallinia and Ithaca. The highest rates of erosion risk occur in the islands of Corfu and Zante. In Corfu, the high rates 308 N. Evelpidou are explained by the vulnerable Quaternary and Neogene depositions that comprise accordingly 28.7% and 38.4%, respectively, of the lithology. In Zante, a high-risk zone is observed along the faulting zone of the NE–SW direction. This faulting zone provokes an abrupt change of the topographic grade between the carbonate rocks of the island’s western part and the Quaternary and Neogene depositions of the eastern part. Along this zone many alluvial fans are found. The behaviour of the model to minimal changes of the input variables was tested by a sensitivity analysis. As expected, small perturbation of the values of the three input variables contribute to insignificant changes of the output variables (erosion risk index). 5. Conclusions The Ionian Islands are strongly affected by the neotectonic processes that have defined their current morphology. A statistical analysis of the discontinuities in comparison to the stream directions showed that neotectonic processes in the area have contributed significantly to the development of the drainage network, and inland discontinuities have affected the stream directions. Moreover, the direction of the current coastlines coincides very often with the directions of the major discontinuities. Specifically, the faulting processes were the predefining factor for the water flow directions, as was demonstrated by the rodograms. All related factors, such as topography, morphology, geology, climatology, land use–land cover and tectonic features were combined to develop erosion risk maps that show the areas vulnerable to erosion. The final map (figure 9) reveals that the highest rates of erosion risk occur in the islands of Corfu and Zante. In Corfu, the high rates are attributed to the vulnerable Quaternary and Neogene depositions, comprising 28.7% and 38.4% of the lithology, respectively. In Zante, a high-risk zone is observed along the faulting zone of the NE–SW direction. 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"Geomatics, Natural Hazards and Risk"Taylor & Francis

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