Spatial Distribution of Coastal Variables
Shoreline Change. The coastal dynamics across all three areas are moderate (with average rates of shoreline changes hovering around 0, slightly negative (Fig. 4a)). Generally, the coasts of the Ierapetra area demonstrate greater stability compared to those of the Koufonisi and Chrisi islands, albeit certain beaches are under severe erosion. The highest rates of retreat (about − 1 m/yr) are recorded at the northern cape of Koufonisi island (Fig. 4a). The most substantial advance of the coastal line took place at the areas of coastal protection construction in Ierapetra town (the port and the embankment, about 1 m/yr).
In the Ierapetra area, the maximal rates of retreat (up to -0.8 m/yr) are observed in the area of Stomio (Figs. 2 and 4a). Here, a formerly wide beach was reduced by nearly 50 m over a span of 44 years (from 1945 to 1989), followed by a continued but decelerated retreat. Such pronounced erosion could be triggered by the disturbance of sediment balance in the coastal zone resulting from the construction of the Bramiana (Mpramianon) dam in 1986 in the upper course of the river to the east of Gra Lygia for irrigation purposes. Considerable reduction of the water flow led to the reduced amount of sediments supplying the coastal zone, which apparently triggered coastal erosion in this area.
Intensive erosion (approximately − 0.5 m/yr) is also observed to the east of the Kathardes settlement in the western segment of Koutsounari/Megali beach (Fig. 4a). This phenomenon is likely related to the natural trend of coastal evolution. Also, it could be due to sediment excavation from the coastal zone for construction purposes during the land development in the period of study, which is suggested by the aerial/satellite images interpretation. The shoreline was retreating as well in the areas of the beaches to the west and to the east of Ierapetra. Most of the rocky coast remained stable or slightly (up to -0.1 m/yr) retreated. Among the high coasts the most actively retreated (about − 0.2 m/yr) were cliffs of marine conglomerates with arenaceous marls in the area of Limnaki and white/yellowish sandy marls of sandy texture close to Peristeras (Fig. 4b).
The most significant shoreline advancement (up to 2.2 m/yr) is observed in the vicinity of the Ierapetra port, primarily attributed to the construction of the pier between 1966 and 1989. Similarly, other cases of shoreline progradation are usually also linked to humans’ interventions such as coastal protection or beach nourishment projects: e.g. the expansion of Kakkos beach by approximately ~ 35 m during the period between 1989 and 2005, based on our analysis.
As previously noted, Koufonisi island is experiencing the most intensive erosion (-1 m/yr) along its northern coast, particularly near the Lefki archeological site (Figs. 3 and 4a). Here, a low cliff composed of soft white marls (Fig. 4b) is subject to erosion from high waves generated by the predominant northern winds. Sediment transport by these waves along the coast towards the southwest leads to slight progradation at Kamarelles beach (0.4–0.5 m/y). Other segments of the coastal line of the island are retreating as well: Anemertia beach at the NE of the island (-0.3 - -0.4 m/yr), the eastern cape of the island (-0.2 - -0.3 m/yr), Prosfora beach (~-0.2 m/yr), some of the western capes and beaches (-0.1 - -0.2 m/yr). However, most of the coasts of Koufonisi are very high and, accordingly, are harder to be destroyed by the waves, especially compared to the Chrisi coasts.
Across Chrisi island, the northern and eastern coasts predominantly exhibit retreat, while the southern coasts remain relatively stable (Fig. 4a). Notable erosion (-0.3 to -0.5 m/yr) is concentrated in the eastern part of the island, particularly south of Kataprosopo beach, with limited evidence of coastal advancement in this area.
Coastal Height. The average height of the coast is about 10 m at Koufonisi and in the Ierapetra area, and about 3 m at Chrisi (Fig. 5a). Notably, the highest coasts are situated in the southern part of Koufonisi, reaching elevations of 30–45 meters. In the Ierapetra area, the coast has a medium elevation (about 10–15 m) in the western part (to the west of Ierapetra), mainly low (about 5 m) – in its central part (excepting the high capes), and high (more than 20 m) – at the east (Fig. 5a). Koufonisi is highest at the south (30–45 m cliffs), medium at the west (10–15 m), and lowest at the north and north-east (2–5, up to10 m). Conversely, the coasts of Chrisi generally do not surpass 10 m, with the highest elevations observed in the northeast.
Coastal Slope. The slope of the coast varies substantially across the studied area, especially at Koufonisi. Koufonisi demonstrates the steepest slopes, averaging 44%, especially pronounced at its southern and western peninsulas, where slopes can reach up to 400%. Conversely, the northern coast of Chrisi island and the extensive beaches to the east of the Ierapetra area feature the lowest slopes, ranging from 1–5%.
In the Ierapetra area the coast is steepest (up to 40%) on the capes of high rocky coasts of the eastern part, as well as on some narrow steep beaches apparently subjected to erosion in the western and central part of the area. In contrast, the flattest (up to 1%) coasts are the large beaches of the eastern area (Kathardes, Koutsounari/Megali). Koufonisi's coastlines vary significantly, characterized by sheer cliffs on its peninsulas and relatively flat, albeit not overly wide, beaches within its gulfs. The widest and flattest beaches are located to the southwest and the southeast of the northern peninsula. Overall, the coasts of Chrisi in general are not steeper than 30% with an occasional segment of 30–40% steep in the eastern and southeastern parts of the island.
Beach Width. The Ierapetra area has the widest beaches (averaging 18 meters in width) (Fig. 5b). Among these, the most expansive is Megali beach in the Koutsounari region, spanning 70–80 meters in width and extending 2.4 kilometers in length. The majority of the coasts of the Ierapetra area are presented by beaches with widths from 3 to 80 m (Fig. 5b). Though a considerable part (about 40%) have no clear beach zone: rocky coasts at the west and the east of the areas, capes in its central part, and technogenic coasts in the area of Ierapetra town. Notably, Megali beach stands out as the widest, while smaller pocket beaches, approximately 5 meters in width, are scattered between rocky capes in the eastern sector and eroded beaches in the western areas near Stomio and Gra Lygia. In contrast, Koufonisi exhibits the smallest percentage of beachy coasts (24%) and the smallest average width of the beaches (5 m). The largest beach on Koufonisi measures 50–100 meters in width and spans approximately 200 meters in length at Kamarelles beach, while other beaches are considerably smaller and narrower. Chrisi Island features a greater number of beaches, accounting for about 37% of its coastline, albeit they tend to be narrow and short. The largest among them, Golden beach, stretches 50 meters in width and spans 730 meters in length.
Lithology. The influence of the lithological composition of the coasts on their erodibility must be considered separately – for the rocky coasts, where the wave erosion affects the cliff; and for the flat beaches, due to conceptually different mechanisms of erosion. The destruction of unlithified mostly fine-grained sediments of the beaches is usually easier for the waves than for the solid rocky cliffs, which is why we related the first three ranks of vulnerability classification (1, 2, 3) to cliffs, and the last two (4, 5) to beaches. Following fieldwork inspections, correlation analysis with historical rates of retreat, and referencing relevant literature, we propose the following resistance levels to coastal erosion for the sediments comprising the area's cliffs.
Ierapetra. In the Ierapetra region, the Marine Upper Pleistocene terraces primarily conglomerates interspersed with arenaceous marls (Pts.tm), forming the capes west of the study area near Limnaki (Fig. 4b), exhibit a resistance level ranging from medium to low, indicating high vulnerability to erosion. Alluvial Pleistocene – Holocene loose sandy-clayed sediments (al), exposed at some cliffs to the west of Ierapetra, have mainly low resistance to erosion. Pliocene white or yellowish marls of sandy texture (Pl-ms) are the least resistant to erosion among all the cliffs of the area. However, in the eastern part of the area (close to Koutsounari), they have a more solid structure and were classified as medium-resistant. Marine Pleistocene conglomerates (dl-c1), presented at a very small section of the coast around the Peristeras beach, are also relatively low-resistant (closer to medium). The Lower - Middle Pliocene Finikia formation, comprising sea and brackish water deposits (interbedded marls, clays, sands, sandstones, conglomerates) (Pl), composing the most eastern part of the coast has high resistance. Fluvial torrential deposits of boulders, gravels, and pebbles, as well as red-gray sands, along with other materials of the Pleistocene age (dl-c3), exhibit the highest resistance among the studied sediments.
Building upon the findings of grain size analysis, field observations, and image interpretations, all beaches were categorized into two main types: predominantly gravel/pebble (more resistant) and predominantly sandy (less resistant). Approximately half of the beaches in the area consist mostly of sandy sediments (Stomio, Western and Eastern Ierapetra, Kathardes, Kakkos), while the remaining half feature gravel/pebble compositions (Gra Lygia-Potami, Koutsounari).
Koufonisi. The northern region of the island, as well as the eastern cape and two gulfs of the western coast (Fig. 4b) are predominantly composed of Pliocene white marls (of Koufonisi) (Pl-m), which are weak in terms of resistance to coastal erosion. It is these sediments that the Lefki site stands on (Fig. 3). Pliocene conglomerates within the aforementioned marls, pebbles mainly of limestone (Pl-c2) at the north-eastern part of the island have a medium strength. Holocene recent littoral deposits containing sand (al-s), outcropping in two segments of the coast, are softer, but still rather resistant (medium resistance, low vulnerability). Pliocene marls - marly limestones (thin to medium-bedded, white-yellow, homogeneous with locally travertine structure, Pl-mk) are mainly distributed on the island and considered as medium resistant to coastal erosion. A small section of Miocene white or gray marls, sometimes containing lenticular conglomerate intercalations (Mi-m) at the northeast of the island is probably the highest on the island, albeit still classified as medium resistance considering the lithological composition across all regions. The majority of the island's beaches consist of sandy sediments.
Chrisi. The predominant geological composition of Chrisi island consists of robust Pre-middle Miocene flysch mélange sediments (fm). In the eastern part of the island, weaker Pleistocene – Holocene alluvial loose sandy-clayed deposits are evident in cliff exposures. Sandy beaches predominate on the island (northern, eastern, and south-eastern beaches), while western and south-western beaches comprise mostly gravel/pebble materials
Beachrocks. Approximately 90 beachrocks were identified within the study area. They are most prevalent along the coastlines of Chrisi, encompassing 69% of the island's shoreline, while their occurrence is least frequent along the coasts of Koufonisi, comprising only 8% of the island's coastal extent. Within the Ierapetra area, approximately 31% of the coastline features beachrocks. Notably, in the Ierapetra region, the majority of beachrocks, including the largest formations, were observed in the segment between Potami and Ierapetra, as well as along the eastern beaches of Ierapetra. Koufonisi exhibits several smaller beachrock formations along its eastern coastline. Conversely, Chrisi boasts numerous extensive beachrocks encircling its shores, with few exceptions along certain eastern coastal stretches.
Vertical Ground Motions. In general, the coast of Ierapetra has subsided during the last five years (European Ground Motion Service). The most intensive subsidence (up to 2 mm) occurred in the central part of the area (between Potami and Ierapetra, and in the area of Peristeras). The eastern portion of the area, particularly around Ferma, remained relatively stable or exhibited a slight uplift.
Wind speed. According to the Copernicus Reanalysis data, the average intensity of the winds grows gradually from the W (average multiannual speed is about 4 m/s) to the E (about 6 m/s). The strongest winds are the northern ones. The average wind speeds recorded at the coasts of the islands are higher than at the coasts of the Ierapetra area. Koufonisi is exposed to the strongest winds, compared to the other two areas.
Wave heights. The multi-annual average values of significant wave heights across the area rise evenly from the northwest to the southeast (Fig. 6a). The highest wave heights (about 0.87 m) are observed at the eastern coast of Koufonisi, exposed mostly to the waves from the open sea. Conversely, the Ierapetra area, particularly its western section, records the lowest average wave heights, measuring around 0.43 meters.
Wind Wave Effect. According to our calculations, WWE in the Ierapetra area is most pronounced on the capes (such as those at Ierapetra and Peristeras-Kathardes) and the coasts exposed to the S and SE winds: Eastern Ierapetra and Kousounari beaches (the highest values – up to 45,960 kt/yr – are in the eastern part of Koutsounari beach). The lowest WWE took place in the small closed gulf at the east of the area around Ferma and the area of Gra Lydia – Potami (these coasts are subjected to lower wave impact as far as they are particularly protected from the open sea waves by Chrisi island).
Sea level. According to the results of modeling of Copernicus Marine Service of mean sea level data based on the ICPP RCP45 scenario by the end of the century, sea level in the area will rise by approximately 39–40 cm (Fig. 6b). The highest rise is anticipated at Chrisi island, medium – at Koufonisi, the lowest – at Ierapetra. In the Ierapetra area, the capes shooting forth into the sea (Peristeras-Kathardes and Ierapetra port areas) will experience sea level rise the most. Conversely, the impact is expected to be relatively lower in the bays, including Stomio-Gra Lygia-Potami, Ierapetra East, and the eastern bays.
CVI
The CVI was calculated with use of the selected variables according to the following formula (9) (abbreviations are explained in the Table 4):
$$CVI= \sqrt{\frac{SH*CH*BW *L*WH*SL}{6}}$$
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According to the CVI assessment conducted in the study, the most sensitive to coastal erosion sites are located at the north, east, and south-east of Koufonisi island, at the north, east, and south-east of Chrisi. Conversely, the Ierapetra area is relatively safe in terms of coastal erosion (Fig. 8a).
Within the Ierapetra area, the most vulnerable sites are the beaches of Stomio (where the Roman villa is located (Figs. 2a and 8a, Table 5) and Kathardes. Stomio beach has exhibited ongoing erosion over recent decades, a trend expected to persist due to its geomorphological characteristics such as low elevation, narrow beach width, and lithological composition. In the previous decades, Kathardes beach has not experienced severe erosion (Fig. 4a), however, in the coming decades, it will be subjected to the most intensive in the area sea level rise (Fig. 6b) and wave erosion (Fig. 6a). Moreover, geological criteria (low height and slope, as well as predominately sandy composition) make this beach especially vulnerable to coastal erosion in the near future. Also, Western Ierapetra, Eastern Ierapetra, Livadi, and Koutsonari beaches are at higher risk due to their historical trends, and geological, and climatic factors. The least vulnerable to erosion are high steep rocky coasts at the west and the east of the study area, technogenic coasts (with coastal protection structures), and beaches in closed bays (in the areas of Gra Lygia, Ierapetra, Ferma) (Fig. 8a). Archeological site Ferma Roman fish tank is located within the area of low CVI, while the Ierapetra Fort is located within the area with very low CVI (Table 5).
Table 5
Vulnerability of archeological sites of South Eastern Crete to coastal erosion
Archeological site
|
CVI
|
CVIw
|
Stomio Roman Villa
|
Low
|
High
|
Ierapetra Venetian Fort
|
Very Low
|
Very Low
|
Ferma Roman Fish Tank
|
Very Low
|
Low
|
Lefki Roman Town
|
Medium – High – Very High
|
High – Very High
|
Chrisi Minoan Settlement
|
Medium
|
Medium
|
The coasts of the islands are more susceptible to erosion, first of all, due to their exposure to the higher waves (Fig. 6a). There are also no wide beaches, which are common in the Ierapetra area (Fig. 5b). The northern and eastern capes of Koufonisi, composed of soft marls, relatively low-lying (especially, northern cape), and susceptible to strong waves are among the most vulnerable in the area. The Lefki archeological site is located in areas of medium risk (in the west), highly (in the middle), and very highly risky (in the east) in terms of coastal erosion (Table 5). Similarly, the south-eastern coast of Koufonisi, including cliffs, despite its high elevation, and Prosfora beach, are also very vulnerable due to their exposure to the highest waves in the area. The most stable (medium vulnerability) here is the north-western coast.
The coasts of Chrisi island are classified as highly vulnerable due to their low elevation and higher exposure to waves and sea-level rise. The northern coast of the island had tended to intensive retreat before (Fig. 4a), and seeing its flatness, exposure to northern waves, fine grain size of beaches, and sea level rise, the retreat may amplify in the future. The Chrisi Minoan settlement is located within an area classified as having medium coastal erosion risk (Table 5). Beaches located at the southeast of the island, such as Kataprosopo and Vages beaches, are also prone to retreat due to a combination of geomorphological and meteorological factors. The stability of the western and southern coasts, as well as the northeastern peninsula, can be attributed mainly to their lithological composition, consisting of strong flysch sediments.
CVI weighted
CVIw was calculated using the selected variables with the weights obtained through the AHP (Table 3), according to the following formula (10) (abbreviations are explained in the Table 4):
$${CVI}_{w} =0.360*SH+0.276*CH+0.110*BW+0.052*L+0.276*WH+0.035*SL$$
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Weighted CVI has also higher values at the islands rather than in the Ierapetra area (Fig. 8b). The spots with the highest vulnerability, in this case, are approximately the same as in the previous one (Stomio and Kathardes beaches in the Ierapetra area, northern, eastern, south-eastern coasts of Koufonisi, northern and eastern coasts of Chrisi – Fig. 8a), however, their areas are mainly larger (especially at Koufonisi), due to amplifications of contribution of main factors of erosion (waves height and coastal elevation), which have high vulnerability indexes here. The most vulnerable areas in the case of weighted CVI assessment are the northern and the eastern capes of Koufonisi and the eastern cape of Chrisi (around Kataprosporo beach). The least vulnerable segments are the central (protected coast within the area of Ierapetra town) and eastern parts of the Ierapetra area.
The amplification of geomorphological criteria, notably coastal elevation and beach width accentuates the vulnerability of several sites within the Ierapetra area, particularly Stomio, where the archaeological site is situated (Fig. 2a, Table 5). Additionally, Eastern Ierapetra, Koutsounari, and the west of Kathardes beaches exhibit an increased risk of coastal erosion due to their shoreline change history and exposure to the waves from the open sea. The least sensitive to erosion are again the coasts of the closed bays (Gra Lygia, Ierapetra, Ferma), technogenic coasts of Ierapetra town, and rocky cliffs to the east of Ferma.
Despite the high elevation of most of Koufonisi coasts, they are classified as highly vulnerable according to CVIw assessment, due to high values of vulnerability indexes of all the other variables (especially wave height, beach width, lithology, and sea level change). The vulnerability of the Chrisi coasts slightly decreases with the application of CVIw, due to the lowest historical rates of shoreline retreat here, and smaller wave impacts on this coast compare to Koufonisi. However, most of the coasts of the island are still moderately or highly sensitive to coastal erosion according to the CVIw assessment.