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An increasing number of heavy precipitation events causing flash floods, debris flows and other types of landslides, as well as severe morphological effects on changes have affected Italy and Europe over the last years (Tarolli et al., 2012).  Italian territory is particularly prone to these types of phenomena, due to a combination of distinctive topographic and meteorological characteristics (Rinaldi et al., 2016). Extreme events induce physical impacts on rivers and valley bottoms, consisting in channel widening (due to bank erosion and overbank deposition), changes in bed level, avulsions, huge amount of sediment transport, as well as recruitment of large wood. On overall, channel widening is the dominant geomorphic effect observed in many rivers. If on one hand channel widening represents an essential process to sustain channel morphology and fluvial ecosystems, on the other hand, in highly populated fluvial corridors, it can result in the loss of agricultural land and damages to buildings and infrastructures; thus, it should be considered a natural hazard and a major management problem. Interests in geomorphic effects of large floods have been intensively increasing over the recent decades. At Italian level, several case study papers were published, and new integrated approaches were proposed (Rinaldi et al., 2016). Even, the Italian Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA) have proposed a method (the Event Dynamics Classification - EDC, Rinaldi et al., 2015) which provides tools to predict the geomorphic flood hazards, and supports the definition of morphological river corridors, meant to be complementary to inundation maps determined by “classical” hydraulic modelling. In light of this, an overview of recent studies on flood events at Italian level is presented, with the aims to highlight similar trends in channels changes induced by large floods and to assess the relative role of the different hydrological and morphological factors controlling the flood geomorphic effectiveness.

SGI 2021

Geomorphic effects of large floods: some examples from Italian rivers

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The National Institute of Oceanography and Experimental Geophysics - OGS and the Department of Mathematics and Geosciences of the University of Trieste are proud to host the 90th Congress of the Italian Geological Society in Trieste on 14-16 September 2021.  The Congress will be held entirely online.

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technical paper

The topic of my work is a seismic tomography which has as object the investigation of Southern Tyrrhenian. This tomography was obtained by means of inversion of teleseismic data and an iterative computation of them to obtain the so-called residual that is the difference between the observed travel times and these theoretical travel times. The final tridimensional velocity model corresponds to that having the minimum residual. The entire process of operation of a seismic tomography is very relevant in order to investigate subduction zones. This is the case of the Southern Tyrrhenian oceanic back-arc basin. The subducting lithosphere has been mostly consumed along the Tyrrhenian-Apennine system has been consumed with the exception of the Calabrian arc sector. The new seismic tomography, which derived from the inversion of travel times of teleseismic ray paths, travelling in the upper mantle at high depths. The adopted database consists of 1929 teleseisms  recorded in period 1990-2012 by 122 southern Italian seismic station directly connected to ISC (International Seismological Centre). The software FMTT was employed for the inversion of these arrival times. I have made 10 horizontal sections of final model from 50 km of depth to 500 km of depth, with an interval of 50 km of depth from each other. I have made 8 vertical sections and 3 transversal sections, choosing as traces the same illustrated by Montuori (2007), for her teleseismic tomography of Southern Tyrrhenian. This work, together with contributions of Chiarabba (2008) and Calò (2012). Summarising, the horizontal sections show an evolution of the high velocity body that represents the Ionian slab. At depth of 250 km, the tomography evidences a sort of “transition” due to the absence of the Southern Tyrrhenian HVA and the occurrence of a low velocity region with maximum of -0.5 km/s scattered between the Aeolian Islands and Calabria. This low velocity region practically splits the Tyrrhenian slab into two parts, in Neapolitan region and in the southern Calabria-northern Sicily region. The presence of this “window slab” could be interpreted as a tear in which unperturbed mantle insert itself.

Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

On 11 and 12 October 2018, Sardinia was hit by exceptional meteorological events that particularly affected the south-eastern area, in the Sarrabus, and in particular various road infrastructures under the responsibility of the Province of Southern Sardinia. The administrative procedure of the interventions originated from the Resolution of the Council of Ministers of 8 November 2018 which declared a state of national natural disaster. Subsequently, the Ordinance of the Head of the Civil Protection Department n ° 558 of 15 November 2018 was issued, which allowed the first management activities to be started for the planning of the first interventions, the rescue and assistance to the population and the restoration of functionalities of infrastructures affected by adverse phenomena. The Province, following an inspections carried out by the geologists and the engineers of the Traffic Service, identified 9 intervention areas, particularly affected by flood events. Among these, the one in correspondence with the bridge of the Camisa locality, at Km 23 + 250 of the provincial road n ° 20, was particularly critical. The procedures for safeguarding public safety and the structure concerned were immediately activated, in conjunction with the Prefecture, the Regional Civil Protection and the municipalities concerned. The temporary interruption of the road was arranged, which at the "Ponte Camisa" was bypassed by the large flow rates of the Rio Corr'e Pruna. The bridge was affected by an exceptional flood which led to the removal of a large part of the road foundation, embankments and escarpments. In the following days an initial census was made of the serious damage caused, which led to interruptions in the road network, narrowing of the carriageway and considerable risk conditions for public safety. An estimate of the damage was made and the request for funding was forwarded to the Civil Protection of the Sardinia Region. This estimate concerned the restoration of the work of art and the complete functional recovery and safety of the road artery. The works to be designed also include the verification of the residual functionality of the bridge in the riverbed, the previously installed gabions and the geological and hydraulic study of the erosive phenomena that have occurred. Following the granting of the requested funding, the restoration work was contracted out, the artifacts were rebuilt, the embankment works and the river bed were strengthened and restructured, able to better withstand any new calamitous events. Appropriate management procedures for the work were therefore provided in the event of an alert of adverse weather conditions. A first study was therefore carried out aimed at reducing the persistent risk on the area, for which the request for funding for the execution of the mitigation of the danger parameters of the area is being investigated.

Civil protection interventions - OCDPC n. 558 of 15.11.2018 - Functional recovery of the Provincial Road No. 20 - Municipality of Castiadas (Sardinia)

On 11 and 12 October 2018, Sardinia was hit by exceptional meteorological events that particularly affected the south-eastern area, in the Sarrabus, and in particular various road infrastructures under the responsibility of the Province of Southern Sardinia. The administrative procedure of the interventions originated from the Resolution of the Council of Ministers of 8 November 2018 which declared a state of national natural disaster. Subsequently, the Ordinance of the Head of the Civil Protection Department n ° 558 of 15 November 2018 was issued, which allowed the first management activities to be started for the planning of the first interventions, the rescue and assistance to the population and the restoration of functionalities. infrastructures affected by adverse phenomena. The Province, following an inspection carried out by the geologists and engineers of the Traffic Service, identified 9 intervention areas, particularly affected by flood events. The situation of the provincial road No. 97 was particularly critical, with seven road sections affected by erosive phenomena and a great state of alteration caused by the considerable outflows that occurred during the calamitous event. The procedures for safeguarding public safety and the structure concerned were immediately activated, in conjunction with the Prefecture, the Regional Civil Protection and the municipalities concerned. Various roadway narrowings have been prepared due to strong erosion phenomena which have led to a great increase in the danger to road users. The main hydro-geomorphological erosion phenomena concerned: - at Km 2 + 600, erosion of the embankment of the road embankment on the progressive right, for about 40 meters and a height of 2.5 meters, failure of the guard rail, anomalous accumulation of materials in the docks; - at Km 3 + 200, burying of the gutter for a length of 20 meters and of a “road manhole” crossing infrastructure; - at Km 3 + 850, strong erosion of the escarpment on the right side for about 75 meters in length and for heights ranging from 1 meter to 2.5 meters in height; failure of the guard rails on both sides of the road; - at Km 4 + 300, erosion of the right escarpment for a length of 16 meters and a depth of 1 meter and consequent erosion of the gutter, partially present also on the left side; - at Km 4 + 900, erosion of the right escarpment for 23 meters in length and 1 meter depth and consequent erosion of the quay, failure of the guard rail on both sides; - at Km 5 + 200, erosion of the embankment of the right road embankment for 14 meters in length and 1 meter in height, undermining of the crossing infrastructure and damage to underground services, failure of the guard-rail on the right side, erosion of the left embankment for about 23 meters of length and 1 meter in height, failure of the left guard rail; - at Km 5 + 400, erosion of the left gutter for 25 meters in length; An estimate of the damage was then made and the request for funding was forwarded to the Civil Protection of the Sardinia Region. This estimate concerned the restoration of the entire section of provincial road No. 97, and the complete functional recovery and safety of the road artery, both in shape and along the quays and appurtenances. The works to be designed also provide for the verification of the residual functionality of the bridge in the riverbed, the previously installed gabions and the geological and hydraulic study of the erosive phenomena that have occurred. Following the granting of the requested funding, the restoration work was contracted out, the artifacts were rebuilt, the hydraulic crossing works partially reinforced, able to better withstand any new calamitous events. A first study was therefore carried out aimed at reducing the risk persisting in the area, for which the request for a loan for the implementation of the intervention to mitigate the hazard parameters of the area is in the preliminary phase.

Civil protection interventions - OCDPC n. 558 of 15.11.2018 - Restoration and functional recovery of Provincial Road No. 97 - Municipalities of Castiadas and Muravera

Sediment cores and boreholes, high-resolution sub-bottom and multi-channel seismic reflection profiles allowed to study origin and recurrence of mass-transport deposits (MTDs) emplaced along the eastern slope of the Gela Basin in the Strait of Sicily. 27 stacked MTDs were mapped with volumes between < 1 km3 and 630 km3 and related to the different phases of margin evolution since the Pliocene. Sedimentary prograding wedges, fed from the north, developed from the late Pliocene and extended eastward. In the Pleistocene, progradation extended to the southern basin with oblique shelf-edge clinothems (Gauchery et al., 2021a). From the Pliocene to the Middle Pleistocene Transition (MPT, 0.8 Ma), the sediment wedge increased in thickness by 150 m/Myr, afterwards the margin developed an aggradational motif, with sediment rates of 900 m/Myr (Gauchery et al., 2021a). The MPT represented not only a drastic change in margin outbuilding, but also in the volume of submarine landslides, with large tectonic-related MTDs replaced by smaller contourite- and clinothem-related MTDs due to faster bottom currents and increased sedimentation rates. Contourite deposits formed in the Pliocene but grew significantly during MIS 5 and late MIS 2 (Gauchery et al., 2021a). The presence of erosional surfaces downcutting the contourite deposits favoured the emplacement of the South Gela Basin Slide (SGBS) around 17 kyr cal. BP (Gauchery et al., 2021b). Slope stability analysis indicates that even strong seismic shaking (Mw = 6.5) at the offshore continuation of the Scicli-Ragusa fault line, located < 10 km from the SGBS, was insufficient to mobilized it (Zaniboni et al., 2021). SGBS was promoted by high sedimentation rates (up to 1300 cm kyr-1) during the last phase of LGM and early phase of Heinrich Stadial 1 (HS1), driven by a combination of inner-mid shelf sediment input, lateral advection by strong Levantine bottom-current and an abrupt and brief intensification of the surface water speed. During the post-glacial sea-level rise pulses, salinity changes at the interface between surface and intermediate waters lead to enhanced bottom-current erosion and margin instability along the outer shelf and upper slope.

Submarine landslides in the Strait of Sicily: relation with tectonics and climate forcing, contourite deposits and high sedimentation rates, post-LGM sea-level fluctuations

In the last decade many studies have focussed on small-scale (≤1 km3 ) submarine landslide, since they are more frequent with respect to the larger ones and pose a potential threat for the coastal strip and infrastructures. The potential geohazard of a marine landslide is also related to the timing of the event and therefore is of paramount importance to define, as far as possible, its age. This study deals with the Taranto Landslide, an impressive landslide located 12 km off the coast, in the northeastern sector of the Gulf of Taranto, on the southwest-facing Apulian Continental Slope, with the aim to learn more on its geohazard potential. The dataset used in this study consists of multibeam echo sounder (MBES) data and high resolution Chirp Sonar profiles, dip- and strike-oriented which has served to define the morphometric parameters of the landslide and the thickness of the post slide sediment drape. The failure occurred across a slope gradient ranging between 3° and 4°. The evacuated scar surface extends from 400 m below sea level (mbsl) down to 1000 m deep and is bounded by a well-defined headscarp, up to 75 m high, and by two subparallel lateral scarps 4-6 km away from each other. The total inferred volume of the landslide was derived by the scar dimensions and resulted as being about 0,3 km3 (Meo et al., 2018). The slump exhibits a simple geometry, with a single arcuate headscarp and no evidence of retrogressive failure process - such as multiple scars merging or upslope modifications in the form of stepped profiles of the basal evacuation surface – was observed. Besides, the scar surface depicts a sub-parallel profile to the adjacent undisturbed slope, suggesting a single slope-parallel plane of failure along which the sediment and the cohesive blocks glided towards the base of the slope. Therefore a single failure event was inferred to the Taranto landslide.

The Taranto Landslide (North Ionian Sea): further morphological and stratigraphical observations and timing of the event by indirect stratigraphic evidences

The Research Institute for Geo-Hydrological Protection of the Italian National Research Council (IRPI CNR) has developed in cooperation with Civil Protection of Sardinia Region, a regional early warning system for the possible occurrence of rainfall-induced landslides (SARF Sardinia), which currently compares the national rainfall thresholds with the rainfall measured by national and regional rain gauge networks and with rainfall forecasts provided by COSMO-I5 and BOLAM models. In the framework of the current Collaboration Agreement between the Sardinia Region and IRPI CNR, a catalogue containing meteorological and hydrogeological information related to rainfall-induced landslides in Sardinia has been compiled. The catalogue lists 261 landslides that occurred in the period 2007-2020. The information includes: (i) event identifier; (ii) rainfall data (name and geographic coordinates of the selected rain gauge, duration, rainfall mean intensity, and cumulated rainfall responsible for triggering); (iii) landslide geographic data (region, province, municipality, location affected by the event, geographic coordinates of the landslide, spatial accuracy of the landslide); (iv) landslide type; (v) landslide temporal data (year, month, day, time, temporal accuracy of the landslide); and (vi) source of data/information. Rainfall series used to reconstruct the landslide triggering conditions include measurements of 186 rain gauges from a Regional Agency network (SASI), 186 additional stations provided by the Environmental Protection Agency of the Region of Sardinia (ARPAS) and the former Hydrological Service (ExSI), and 96 rain gauges from the National Department for Civil Protection network. Information on rainfall-induced landslides comes from digital and printed newspapers, blogs, technical documents, landslide event reports, and fire fighter reports. The information in the catalogue is used to define regional rainfall thresholds to be implemented in the SARF Sardinia. Thresholds are defined using the frequentist method, which allows the calculation of cumulated event rainfall–rainfall duration (ED) threshold curves corresponding to different non-exceedance probability levels and including uncertainty in the parameters that define the thresholds. Currently, the Civil Protection of Sardinia Region is using the implemented system to support decisions in order to issue warning advices.

Rainfall thresholds for landslide early warning system in Sardinia

Landslides are frequent and destructive geo-hydrological events that cause harm to people every year worldwide (Petley, 2012). In the 13-year period 2004-2016, Froude and Petley (2018) recorded 55, 997 landslide fatalities in 4,862 distinct landslide events at the global scale. Catalogues of landslide fatalities are also available at national and regional level. Nevertheless, a small number of studies have investigated the causes and circumstances that have led to the loss of lives. In Italy, a country where landslides cause significant societal and economic damage every year, a catalogue of landslide events with human consequences is continually updated, and used to determine the individual and the societal risk posed by landslides (Guzzetti 2000; Salvati et al., 2010, 2013; Rossi et al., 2019). Despite the efforts, little is known on the circumstances of landslides fatalities, and their dependence on the age, gender and time of the day. For this purpose, efforts were spent to update the national catalogue (Salvati et al., 2003; 2017) for the 51-year period 1970-2020. We looked specifically to the hour of occurrence grouping them in by day or by night and considering the daytime duration differences during the year.  The new version of the catalogue lists 336 fatal landslides that have caused 1099 fatalities. We determined the gender (from the name) of 1,040 landslide fatalities (94.5%), and the age for 1,027 fatalities (93.5%) occurred in Italy in the period 1970 and 2020. Overall, males (587) account for 56.7% and females (453) for 43.3% of the landslide fatalities for which the gender is known in the catalogue. The new collected data allowed determining possible relations between the light condition (day or night), the places (indoor or outdoor), the gender (male, female) and the age (using 4 age categories) of landslide fatalities. We were able to identify the exact times for 948 fatalities (91%) and the exact place indoor or outdoor for 649 fatalities (62.4%).  Then, for each fatality, we related the times of occurrence with the place (outdoor and indoor) were people lost their life and with the gender and age. Males lost their life frequently outdoor, mainly inside their vehicles, representing a dangerous death contingence, most of them occurring in daylight. Differently, female landslide fatalities occurred more frequently indoor and in the majority by night revealing for them, the dangerous condition of darkness. Using the national census data and a multinomial distribution we estimated the expected fatalities by gender and age and we compared the results with the observed landslide fatalities distribution. Landslide male fatalities compared to the female ones are significantly higher than those expected by census data, indicating both a diverse propensity towards the risk taking and a different degree of exposure between males and females.

Conditions and behaviours influencing people vulnerability to landslides

Shallow landslides susceptibility assessment usually relies on the parametrization of hydraulic and geotechnical features of soils. Among these, saturated hydraulic conductivity (Ks) represents one of the most important soil properties that influences infiltration rates, runoff, groundwater recharge and drainage processes, which makes it of particular concern in the prediction of natural hazards including catastrophic floods and shallow landslides. On the other hand, it is well known that vegetation may play a positive role towards slope stability and over the last decades many efforts have been done in order to quantify the role of the vegetation towards protection from shallow landsliding (Giadrossich et al., 2017). Basically, there are two main vegetation effects: hydrological (e.g., reduction of the water pore pressure through tree rainfall interception) and mechanical ones (increase of the soil strength due to the presence of roots and increase of the soil shear stress due to the vegetation load). Soil Ks is expected to be an important factor that influences plant growth, plant-available water, root system and root biomass and, therefore, this work focuses on the quantitative assessment of the relationship between vegetation characters and hydraulic conductivity. Study areas affected by shallow landslides were chosen in the Garfagnana and Cardoso basin (Northern Apennines, Italy) and in Mt. Amiata volcano (Southern Tuscany, Italy) where field measurements (about 200) of below-ground vegetation (Root Area Ratio - RAR), above-ground vegetation (Leaf Area Index-LAI and vegetation load) and Ks were carried out inside, in the neighbour and far outside shallow landslide locations. Acquisition of data within landslides areas was supported by means of a multi-temporal landslide inventory already available for study areas. Below-ground vegetation data were collected in trench profiles, while above-ground vegetation data were acquired using digital relascope and digital cover photography. Measurements of Ks were carried out by means of both constant and falling head permeameters. Results showed that RAR and soil depth affected Ks, with a decrease of Ks as the depth increases and roots decrease. Moreover, Ks varied for different vegetation types, indicating that shifts in aboveground vegetation features may strongly impact the water dynamics of soil. Above-ground vegetation plays a slight negative role on slope stability. Instead, root reinforcement to soil in terms of a root-related cohesion parameter plays a relevant role for depths involved in shallow landslides.

Influences of vegetation characters on saturated hydraulic conductivity at catchment scale

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can range a few thousand meters below sea level. Despite their small area, the erosional, depositional, tectonic and volcanic processes affecting the subaerial island leave a significant imprint on the shelf morphology when compared to the slope or the submarine base of the islands (Quartau et al., 2014). Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection data of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge (~100m b.s.l.), aiming to better understand the evolution of mass-movements affecting the area. In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor. The sedimentary cover on the shelf is highly variable both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as present-day highstand deposits. Based on the morphological and seismo-stratigraphic differences the study area has been divided in three sectors (Western, Central and Eastern). On the Western and Eastern sectors two sets of submerged erosional terraces have been mapped at different depths (35-45 m, 50-75 m b.s.l.). The overall arcuate shape of the southern shelf edge of Porto Santo Island strongly matches the coastline shape, suggesting the occurrence of an old large-scale landslide event (LS1),  prior to the shelf formation (Quartau et al., 2018a). Further incisions on the shelf edge of the Central sector, mapped at and upslope the shelf edge, imply more recent mass-movement processes, LS2 at least older than seismic unit U1 and LS3 formed on the late Holocene, indicating a retrogressive evolution of the instability process. The absence of erosional terraces in the Central sector supports this hypothesis, being probably removed by the LS2 event. The terraces depths and their destruction by the LS2 event suggests that these were formed/modified at least by the MIS 5a-5d stillstands or even by older ones. The estimated volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. In summary, three different mass-movements were defined according to their dimension and position on the shelf: from LS1 to LS3, the scars are increasingly smaller and further upslope. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes.

Small-scale instability processes affecting volcanic island shelves: the case study of the southern shelf of Porto Santo Island (Madeira Archipelago)

Assessment of the stability of rock masses is crucial in the process of risk mitigation. Among the many factors predisposing the Italian territory to geohazards, it has to be mentioned the geological and geomorphological setting, often characterized by karst processes and widespread presence of cavities, of both natural and anthropogenic origin, potentially leading to sinkholes and other instability mechanisms. For these reasons, the study of rock masses has always been of primary importance, as a primary tool to define the susceptibility to geological instability and to safeguard the environment. To solve the problems related to traditional analyses, in many cases too expensive and difficult to carry out, over the last decade the implementation of new digitalized methods, such as close- and long-range remote sensing techniques, has become essential to quantitatively describe the structural arrangement of rock masses. This necessarily requires robust and reliable methods dedicated to extracting the primitive geometries representing the discontinuities on a rock outcrop. A novel approach, combining observations made in situ with digital results, has been recently proposed (Cardia et al., 2021). This method is able to extract discontinuities clusters in a fully supervised way, thus allowing the user to evaluate every situation by determining specified tolerance angle ranges for both dip/dip direction. We provide here an ongoing development of the method, based on an algorithm that has been implemented in the software GeoDS. The algorithm takes as input a K value, representing a density threshold, from which it will start searching for coplanar points in a given range of 10° for both dip and dip direction, and a seed range value, that is the number of sample points that the system chooses randomly to evaluate each cluster. The greater this value is set, the more successful is going to be the result; however, this comes with a computation time cost, which extent depends on the computer capability. This new algorithm, called IPDE (Iterative Pole Density Estimation) is then coupled with a KDE (Kernel Density Estimation), extremely useful to plot the graphic projection, on which the user can choose to manually select points or to evaluate the sets with two different automatic clustering techniques, K-means and Gaussian Mixture. The proposed method aims to improve the evaluation of discontinuities on a rock outcrop in a new and original way.

Geomorphic effects of large floods: some examples from Italian rivers

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Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

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Geomorphic effects of large floods: some examples from Italian rivers

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Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

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Short-term effects of storm events on the coastline morphology of a Mediterranean microtidal wave-dominated beach (Piscinnì, SW Sardinia)

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