Relationship between the geomorphological evolution of the Palatine western slope and the fluvial dynamics of the Velabrum Minus Valley (Rome, Italy) by means of numerical modelling constrained to geological data.

The Palatino Hill is located in downtown Rome and is among the main archaeological sites of the Roman antiquity. Despite the almost 3000 years of occupation and anthropic modifications the original morphology of the hill is still partly preserved, and is featured by an asymmetric profile with a western slope steeper than the eastern one. The geological substratum of the Palatine western slope is characterized by jointed tuff rock masses inter-layered with sedimentary continental (fluvio-lacustrine) deposits, both of Middle Pleistocene age. Records of slope instability are documented in historical times and fallen rock blocks can be still observed in the area. Moreover, the remnants of several orders of restraining walls of Royal, Republican and Imperial ages testify to a long-lasting work for the stabilization of the rock cliffs in an area that hosted important civil and religious buildings through the ages. The hillslope is delimited by the "Velaber Minus" valley, a narrow morphological depression separating the Palatine Hill from the Campidoglio Hill, where a similar geological multilayer is partly exposed. The valley presently hosts a decametric pile of Holocene alluvial deposits linked to the Tiber River fluvial network, flowing into the Tyrrhenian Sea. Holocene deposits lay on an erosive basal surface shaped during the last glacial maximum and carved within Upper Pliocene marine clays, these last being the geological bedrock of the entire Rome municipality. This paper wants to investigate the possible relationships between the geomorphological evolution (and stability conditions) of the Palatine western slope and the Middle Pleistocene-Holocene dynamics of the "Velaber Minus" Valley, featured by alternating episodes of fluvial erosion and deposition. Such analysis was performed by means of several stress-strain numerical models carried out through the FDM code FLAC 5.0 (Itasca, 2005). The numerical models were based on a sequence of geological reconstructions of the valley-slope system constrained to the analysis of local outcrops and well data. Different steps illustrate several cycles of valley erosion during glacial periods (and relative sea level lowstands), and valley infilling during inter-glacial periods (and sea level rise). The fluvial dynamics influenced the stress-strain conditions of the geological formations on the valley sides, because it implied corresponding cycles of lateral loading and unloading. For each step, a numerical model illustrates the stress variations and related deformation patterns within the Middle Pleistocene substratum and the tuff rock masses in particular. The numerical modelling takes also into consideration the stress-strain conditions under the present geomorphological setting in a sector of the Palatine western slope located right beneath the foundation of the Magna Mater Temple (Republican age). Such last effort, integrated with a dedicated analysis of the jointed tuff rock cliffs according to the Markland criteria, was issued to the evaluation of the stability condition of an area nowadays potentially prone to rock failures. Finally, results of these analyses were used to define and realize a proper plane for site remediation.