Flash Flood Susceptibility Evaluation in Human-Affected Areas Using Geomorphological Methods—The Case of 9 August 2020, Euboea, Greece. A GIS-Based Approach.

Flash floods occur almost exclusively in small basins, and they are common in small Mediterranean catchments. They pose one of the most common natural disasters, as well as one of the most devastating. Such was the case of the recent flood in Euboea island, in Greece, in August 2020. A field survey was accomplished after the 2020 flash floods in order to record the main impacts of the event and identify the geomorphological and man-made causes. The flash flood susceptibility in the urbanized alluvial fans was further assessed using a Geographic Information System (GIS)-based approach. Our findings suggest that a large portion of the alluvial fans of Politika, Poros and Mantania streams are mainly characterized by high and very high hazard. In fact, ~27% of the alluvial fans of Politika and Poros streams are characterized with very high susceptibility, and ~54% of Psachna area. GIS results have been confirmed by field observations after the 2020 flash flood, with significant damages noted, such as debris flows and infrastructure damages, in buildings, bridges and the road networks. In addition, even though the adopted approach may be more time-consuming in comparison to purely computational methods, it has the potential of being more accurate as it combines field observations and the effect of past flooding events

Sea-level rise trends in the AtticoCycladic region (Aegean Sea) during the last 500 years

Sea-level change during the last 18,000 years is a combination of eustatic, isostatic and tectonic contributions. In an effort to minimize the tectonic contributions, our study of sea-level changes in the Aegean Sea within historical times is focused on the aseismic Attico-Cycladic geotectonic zone. On the basis of archaeological information and radiocarbon dating of coastal sedimentological formations, a sea-level curve for the Attico-Cycladic massif has been constructed for the past 5000 years and compared with existing curves. According to this curve, the rapid increase of sea level concluded prior to 5.5 ka and was followed by a slow steady rise at a rate of 0.9 mm/a up to its present stage. The latter is attributed primarily to the process of thermal expansion and secondarily to the residual melting of the glaciers and existing icecaps. By extrapolation of the curve, the sea level at the end of the 20th century is predicted to be about 9 cm higher than the present level; this value is much lower than the prediction of the last IPCC report (49 cm). If higher SLR rates are realised in the next few decades, then the excess 40 cm of the IPCC prediction can be attributed to human-induced global climatic change

The morphotectonic evolution of southern half of Kythira Island (Ionian sea, Greece) during the Quaternary

Kythira island is located between Peloponnese and Crete along the Hellenic island arc. The morpho tectonic study of this island could provide useful clues about the evolution of this area. The study focused on the southern part of the island where most of the characteristic landforms of uplift are found (terraces, gorges and notches). Large scale geomorphological mapping was performed in order to determine the most significant landforms of the area such as planation surfaces, marine terraces, gorges, knick points, cliffs and notches. It is concluded that the general morphology and evolution of the southern part of the island is depended primarily on the tectonic regime of the area which is exemplified by characteristic landforms in a series of well distinguished eight uplifted marine terraces on the eastern and six in the western part, marine notches and inclined planation surfaces, gorges and knick points indicating a continuous uplift of the island during the Quaternary. Finally, an average uplift rate of about 0.13 mm/yr for the Quaternary period is determined for the study area

Palaeogeographical Reconstruction of Ancient Diolkos Slipway by Using Beachrocks as Proxies, West Corinth Isthmus, Greece

Beachrocks are well known as significant proxies for paleoenvironmental analysis as they indicate the coastal evolution. The combination of geomorphological and archaeological sea level indicators has a significant contribution to the coastal paleogeographic reconstruction. In this study, we studied a beachrock from the Diolkos area (West Corinth canal, Greece) and remnants of Diolkos slipway to reconstruct the coastal evolution before Diolkos construction until today. We conducted detailed mapping of Diolkos beachrock using DGPS-GNSS, as well as mineralogical analysis and OSL dating of beachrock samples. The results showed that a beachrock slab was preserved before the construction of Diolkos below it, followed by its submergence by a co-seismic event after Diolkos abandonment during 146 B.C. Consequently, a new beachrock was developed on top of the submerged Diolkos around 120 ± 14 A.D. The RSL was stable until 1596 ± 57 A.D. when the beachrock developed even closer to the present-day coastline. After 1596 A.D., it was uplifted by 12 cm until it reached today’s condition

Late Holocene palaeogeographical evolution of Paroikia Bay (Paros Island, Greece)

International audienceAlthough there is rich evidence for human occupation of Paros’ coastline, there is a dearth of data with regards to the evolution of the island's seaboard palaeoenvironments. In this paper, we use sedimentological and palaeontological proxies of late Holocene coastal deposits from lagoonal environment to reconstruct the evolution of coastal landscapes in Paroikia Bay (Paros Island, Greece). A semi-enclosed lagoon existed in the northeastern part of Paroikia from at least 2915–2551 BC, which was gradually infilled after around 780–436 BC. Although it was not possible to chronologically constrain the timing of the infill, it is most likely relatively young, indicating anthropogenic effects. A correlation of our chronostratigraphic data with archaeological remains and tidal notches in the study area suggests that the subsidence observed on Paros Island is linked to long-term subsidence in combination with vertical seismic displacements

Tracking shoreline evolution in central Cyclades (Greece) using beachrocks

International audienceBeachrocks represent a significant paleo-environmental proxy because they can record both the vertical and the horizontal evolution of the shoreline. They have often been used to assess Holocene shoreline evolution and crustally induced Relative Sea-Level (RSL) changes in the Mediterranean Sea. In this paper, we used submerged beachrocks from Paros and Naxos Islands (Aegean Sea, Greece) to reconstruct the shoreline modification and provide new insights on the RSL evolution in central Cyclades. Paros and Naxos Islands are of great importance in terms of archaeological evidence of coastal occupation. However, the evolution of their coastlines was seldom explored. In this study, we coupled detailed underwater surveys, analysis of aerial photogrammetry, microstratigraphic analysis and luminescence dating to study beachrock outcrops found down to about 6 m below the present sea-level. We, then, spatially and chronologically constrained some major palaeogeographical changes of a number of coastal sectors of the two islands. Furthermore, the multiple analyses of beachrocks, sediment coring and archaeological data suggested that RSL rose by at least 3.8 m in the last 4.0 ka and that RSL variation in the last 2.0 ka did not exceed 2 m with respect to the present mean sea level

Flash Flood Susceptibility Evaluation in Human-Affected Areas Using Geomorphological Methods—The Case of 9 August 2020, Euboea, Greece. A GIS-Based Approach

Flash floods occur almost exclusively in small basins, and they are common in small Mediterranean catchments. They pose one of the most common natural disasters, as well as one of the most devastating. Such was the case of the recent flood in Euboea island, in Greece, in August 2020. A field survey was accomplished after the 2020 flash floods in order to record the main impacts of the event and identify the geomorphological and man-made causes. The flash flood susceptibility in the urbanized alluvial fans was further assessed using a Geographic Information System (GIS)-based approach. Our findings suggest that a large portion of the alluvial fans of Politika, Poros and Mantania streams are mainly characterized by high and very high hazard. In fact, ~27% of the alluvial fans of Politika and Poros streams are characterized with very high susceptibility, and ~54% of Psachna area. GIS results have been confirmed by field observations after the 2020 flash flood, with significant damages noted, such as debris flows and infrastructure damages, in buildings, bridges and the road networks. In addition, even though the adopted approach may be more time-consuming in comparison to purely computational methods, it has the potential of being more accurate as it combines field observations and the effect of past flooding events

Palaeogeographic evolution of Alykes lagoon, Pydna, Northern Greece during the Holocene, based on geomorphological and sedimentological data

This study deals with the palaeogeographic evolution of the Alykes lagoon, located on the western coast of the Thermaikos Gulf in northern Greece. For this purpose, detailed geomorphological mapping was conducted and two shallow boreholes were drilled in an attempt to recognize the late Holocene palaeoenvironmental changes. In total 64 sediment samples collected from the cores were granulometrically and palaeontologically analysed. Moreover, two shell samples were 14C dated and provided the chronostratigraphy of the cores. Four biosedimentary units were identified corresponding to different depositional palaeoenvironments including coastal shallow marine, lagoonal, mesohaline to oligohaline and brackish mesohaline respectively. A shallowing upwards sedimentary sequence was recorded with the lower unit corresponding to a shallow marine environment which progressively changes to a more protected lagoonal one, with restricted communication to the open sea. Based on the radiocarbon dates it is evident that the coastline was located a few hundreds of meters west of the present-day one by the end of the seventh millennium BP. Furthermore, we concluded that the establishment of the lagoon can be safely put around the sixth millennium BP and that its confinement can be attributed to the formation and evolution of two groups of beach ridges located at the eastern part of the lagoon. The fining upwards sequence of the sedimentary record of the cores is interrupted by a coarse-grained layer corresponding to a high energy marine inundation event attributed probably to a palaeotsunami which occurred not long before 4330-3920 cal BP. No, or negligible, vertical tectonic displacement can be concluded, for the past 4330-3920 cal BP

Palaeogeographic evolution of Alykes lagoon, Pydna, Northern Greece during the Holocene, based on geomorphological and sedimentological data

This study deals with the palaeogeographic evolution of the Alykes lagoon, located on the western coast of the Thermaikos Gulf in northern Greece. For this purpose, detailed geomorphological mapping was conducted and two shallow boreholes were drilled in an attempt to recognize the late Holocene palaeoenvironmental changes. In total 64 sediment samples collected from the cores were granulometrically and palaeontologically analysed. Moreover, two shell samples were 14C dated and provided the chronostratigraphy of the cores. Four biosedimentary units were identified corresponding to different depositional palaeoenvironments including coastal shallow marine, lagoonal, mesohaline to oligohaline and brackish mesohaline respectively. A shallowing upwards sedimentary sequence was recorded with the lower unit corresponding to a shallow marine environment which progressively changes to a more protected lagoonal one, with restricted communication to the open sea. Based on the radiocarbon dates it is evident that the coastline was located a few hundreds of meters west of the present-day one by the end of the seventh millennium BP. Furthermore, we concluded that the establishment of the lagoon can be safely put around the sixth millennium BP and that its confinement can be attributed to the formation and evolution of two groups of beach ridges located at the eastern part of the lagoon. The fining upwards sequence of the sedimentary record of the cores is interrupted by a coarse-grained layer corresponding to a high energy marine inundation event attributed probably to a palaeotsunami which occurred not long before 4330–3920 cal BP. No, or negligible, vertical tectonic displacement can be concluded, for the past 4330–3920 cal BP

Long-term spatial and temporal shoreline changes of the Evinos River delta, Gulf of Patras, Western Greece

This study deals with the long-term shoreline displacement of the Evinos River delta in Western Greece using various geospatial data sets from different sources. The comparison between the extracted coastlines allowed us to identify segments where the delta progrades and parts of the delta that are being eroded over the periods 1945–1969 and 1969–2015. Coastal area variations giving land losses or gains of the most active sectors of the delta associated with changes in shoreline position were calculated over the periods 1993–2002 and 2002–2018 using Landsat TM satellite images. The results showed that nearly 46% of the Evinos delta is in retreat. Erosion is essentially affecting the eastern deltaic shoreline with retreat rates commonly reaching up to 8 m/yr in places. In addition, the area of the artificially closed pre-1959 mouth of the river has retreated at an average rate of 14 m/yr over the past 57 years. The old natural mouth of the river (depicted in a map of 1885) at Akra Evinou (Cape Evinos) has retreated up to 350 m (5 m/yr) over the last 71 years. West of the active river mouth, mobile elongated features such as sand spits and barriers are formed by deposition of sediment produced by the erosion of the abandoned eastern delta coastline. The main natural cause of the long-term Evinos River delta coastline displacements is the nearshore current activity induced by the dominant incoming wind generated waves. Land loss of the active part of the delta (at rates up to 16,797 m 2/yr) has been recorded over the period 2002–2018. This generalized erosion can be attributed to human activities that include: the construction of a dam in the upper reaches of the catchment that dramatically decreased the suspended sediment concentrations in the delta, the extensive sand and gravel mining in the distributary channel, and a small earth dam constructed at the apex of the delta for irrigation purposes. </p