*7.1. River Analysis*

The hydrographic network engraved by torrential watercourses possesses a tectonic control linked to the Cenozoic structural features. The main incisions that cross the basin of the Rio Pardu give rise to deep valleys with a mainly erosive and only locally depositional character. The Pardu Valley has a transverse "V" profile, which is more or less open depending on the evolutionary stage, the distance from the point of origin, and the competence of the lithotypes in which the river incision takes place. Sometimes, the profiles show marked asymmetries due to the different positions of the layers or the different exposure, which influences the vegetation. A lower steepness can be observed on the left side, which probably due to a lower vegetation cover, which favors erosion. The valley has developed in the formation of the Filladi grigie del Gennargentu. Only in the southeast is the formation of Monte Santa Vittoria affected.

As regards the evolutionary conditions of the Pardu Valley, considering the descriptive parameters, the geometric conditions, and the hypsometric curve, it is noted that was not able to develop to the point of acquiring characteristics that are attributable to a cycle of river erosion in an evolved phase, which suggests a relatively young age for engraving [4,5,25].

The longitudinal profile of the Rio Quirra differs from the normal profile of a river; it has an initial concave part with a strong steepness within the first kilometer from the head and a regular decrease in the slope along the rest of the watercourse. The evolutionary stage of the Quirra appears to have advanced; however, it must be considered that it represents the middle and final parts of the original Rio Pardu–Quirra, which are divided in two by the capture of the Rio Pelau. Currently, the Rio Quirra does not have a catchment basin at the head, and its feeding is mainly given by certain tributaries. The valley is oversized and over-flooded with respect to the current basin (Figure 13).

## *7.2. DGSD Dynamics*

The Rio Pardu and Quirra River represent two of the most susceptible areas to landslides in Sardinia, as well as to rockfalls and rainstorm-induced superficial landslides [4,5,26]. This sector is also interesting due to the fact that extreme rainfall over the last centuries has led to the evacuation and reconstruction of the towns of Osini and Gairo [5,7]. Recent studies have highlighted the presence of deep landslides with sackungtype kinematics and lateral spreads on the right side of the Rio Pardu [6].

In this paper, by analyzing integrated geomorphological, geo-structural, high-resolution topography and InSAR displacement data, we identified diffuse DGSDs on both sides of the valleys of the Pardu River and Quirra River, which are characterized by different kinematics.

DGSDs are commonly found in orogenetic environments with high tectonic and seismic activity and in areas affected by slope decompression due to post-deglaciation. The present work aimed to contribute to the knowledge on the influence of evolution of valleys—in particular, with high incision—on the triggering of large landslides or DGSDs in relation the Quaternary uplift.

Lateral spreads were developed at the edge of the plateau in relation to the favorable stratigraphy (dolostone on clays and altered metamorphites). The slope deformation generates vertical fractures in the carbonate and a zone of ductile basal deformation that affects the Genna Selole Formation and the summit, which thus altered the metamorphites (Bruncu Pranedda and San Giorgio DGSD). DGSDs with a higher vertical shift represent a more advanced stage with sackung features (Tisiddu Mountain and Tertenia DGSDs). The latter evolves in relation to the thrust that affects the median part of the slope. A large part of the deformation affects the Paleozoic basement, which was evidenced by the sinking of the carbonate sequence into the metamorphites.

On the left side of Rio Pardu, Gairo's DGSD shows a different behavior in relation to the different stratigraphic and structural setup. The DGSD has sackung-type kinematics with an important translational component linked to the thrust.

**Figure 13.** Map of the distribution of alluvial deposits in Rio Pardu and Rio Quirra.

From the structural viewpoint, the major faults in the NW–SE and NE–SW directions were in concordance with the main trenches and back-scarps in all sectors, indicating an important structural control. The secondary trenches and the joints did not exhibit a good correlation with the large-scale structures because they were associated with the features inside the deformation rock mass.

The Rio Pardu shows a straight valley with steep slopes, a valley bottom with a mainly erosive character, and two orders of terraces. This is linked to the intense erosive phase following the capture by the Rio Pelau. The valley of Rio Quirra shows a flat-bottomed valley with an actual riverbed of the braided channel type. The valley is over-sized and over-flooded with four orders of terraces, the result of an evolution prior to the capture of the Rio Pardu. The T3 terrace shows sedimentological characteristics related to a subtropical climate, which is probably linked to the warm climatic phase of MIS 5. The InSAR and morphostratigraphic analyses made it possible to define the state of activity of the DGSDs in the two hydrographic basins. In the valley of Rio Pardu, various areas of the slope that are affected by movements that can be classified as active DGSDs were identified, with movements of up to 2 cm/y on the left slope and up to 1 cm/y on the right slope. However, in the Quirra River, paleo-DGSD bodies are fossilized by the alluvial deposits of the T3 terrace. This indicates that the river capture led to an intense erosive phase in the Rio Pardu, leading to the recent instability of the slopes, thus justifying the active DGSD (Figure 14).

**Figure 14.** Relation between DGSD activity and river parameters.

These DGSDs were associated with numerous large collateral rockfalls and toppling landslides that affected the slopes. Dolomitic blocks with sizes of up to 30 m on each side were identified; these moved up to 900 m away from the detachment points, which were linked to mega-rockfall events with rock avalanche features. We also identified paleoDGSDs on the downslope that were associated with the collapsed slope side. Currently, a reactivation of quiescent DGSDs or an acceleration of movements can be triggered by extreme weather events or earthquakes.

Therefore, an acceleration of slope movements leading to a potential catastrophic failure poses a threat to communities, and the monitoring of these slopes is important for early warning and risk reduction. So, we studied the DGSDs and landslides in the inhabited areas of Pardu Valley in detail by using integrated remote sensing techniques, field mapping, and InSAR in order to understand the temporal evolution. The historical InSAR deformation rate supports our model of rock slope deformation. However, for risk reduction in a populated area, a 24/7 monitoring system could become an essential component of an early-warning system that is aimed at preparing evacuation protocols [55,111–118].
