Abstract
Geomorphological evolution, erosion and retreat processes that affect the rocky coasts of the mid-western Adriatic Sea (Abruzzo, Central Italy) are the subject of this research. This coastal sector, one of the few examples of clastic soft rock coasts in the Mediterranean Sea, is characterized by active, inactive and paleo cliffs, as well as coastal slopes, composed of the clayey-sandy-arenaceous-conglomeratic marine sequence (Early-Middle Pleistocene) covered by continental deposits (Late Pleistocene-Holocene). This study provides geomorphological and 3D modelling stability analyses of the cliffs of Torre Mucchia, Punta Lunga, Punta Ferruccio (Ortona, CH) and Punta Aderci (Vasto, CH), which are popular tourist sites included in natural reserve areas. They are representative of two main types of active cliffs on soft clastic rocks: cliffs on sandstone and cliffs on conglomerate with notches. In order to evaluate the processes and factors that induce cliffs to retreat and their recent evolution, the research was based on a DEM analysis (LIDAR 2 × 2 m data), aerial photos and an orthoimages interpretation, detailed geological–geomorphological surveys, and a structural analysis; field and remote investigations were combined with numerical modelling with a FLAC3D calculation code. Geological and geomorphological field data provided reliable 3D models, and FLAC3D numerical analyses allowed the definition of the most critical and/or failure areas, and the evaluation of the controlling factors, evolution mechanisms of the slopes and the sliding kinematics of gravitational instability phenomena. Different retreat mechanisms have been observed all along the investigated coastal sectors, induced by gravitational processes due to coastal erosion cycles at the foot of the cliffs, and controlled by lithological features and joints systems. The geomorphological analysis combined with the 3D modelling (i) showed that the retreat process of the cliffs is connected to translational slides and rockfalls (cliffs on sandstone), combined rockfalls, and topples (cliffs on conglomerate), largely controlled by main joints; (ii) defined the most critical areas along the cliffs. These results are of great interest in the assessment of hazard connected to potential sliding on the cliffs. Their implementation within Geographical Information Systems provides a valuable contribution to the integrated management of coastal areas, strongly improving the identification and prediction of landscape changes and supporting a new geomorphological hazards assessment, in areas of high tourism, as well as natural and cultural landscape value.
Highlights
Rock coasts on soft materials are the result of a complex combination of many endogenous and exogenous factors, such as the morphostructural and stratigraphical setting, lithology, climate, wave energy, tides, and long-term sea-level changes [1,2,3,4,5,6,7]
The landscape features a plateau and mesa relief, shaped by fluvial, slope and coastal processes. These sectors were affected by uplift since the Middle Pleistocene, up to 200 m above the present sea level, at a rate of ca. 0.2–0.4 mm/yr [29,30,31,32,33,34,35], and Middle Pleistocene-Holocene sea-level fluctuations
In this area, the joints show a minor control on the landslide mechanism: the displacement vectors originate from the innermost portions of the cliff, describing planar sliding surfaces (Figure 11b) connected to the translational slide, probably associated with rockfall
Summary
Rock coasts on soft materials are the result of a complex combination of many endogenous and exogenous factors, such as the morphostructural and stratigraphical setting, lithology, climate, wave energy, tides, and long-term sea-level changes [1,2,3,4,5,6,7]. They are examples of two main types of cliffs on soft clastic rocks developed on an uplifted clay-sand-sandstone-conglomerate Lower Pleistocene sequence and affected by active geomorphological processes: cliffs on sandstone and cliffs on conglomerate with notches [7] It is based on a combination of remote investigations (LIDAR-derived DEM processing; aerial photos and orthoimages interpretation), field investigations (detailed geological–geomorphological surveys, and a structural analysis), and FLAC3D numerical modelling. This work wants to provide a contribution to the identification, comprehension and prediction of the variable mechanisms of the fast retreat processes and the factors controlling these processes on specific soft rock cliff types (i.e., on sandstone and conglomerate) This can support the assessment of geomorphological hazards in these areas of high tourism, natural, and cultural landscape value, where coastal landslides are largely widespread and studied because of the risk connected to possible reactivations, damage to people and/or infrastructures
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