Search

La connettività dei sedimenti, definita come il grado in cui un sistema favorisce il trasferimento di sedimenti attraverso i suoi diversi settori, è emersa recentemente come una proprietà fondamentale dei sistemi geomorfici. Il crescente interesse della comunità delle scienze della terra per la connettività ha portato questa proprietà a diventare un concetto chiave per quanto riguarda l'analisi dei processi di trasferimento dei sedimenti e uno degli elementi costitutivi della moderna geomorfologia. La crescente disponibilità di modelli digitali delle elevazioni (DEM) ad alta risoluzione derivati da diverse tecniche quali LiDAR e Structure from Motion (SfM) ha aperto la strada ad approcci quantitativi e semiquantitativi per valutare la connettività dei sedimenti. Nel corso degli ultimi anni è stato sviluppato e affinato un indice geomorfometrico di connettività dei sedimenti (Borselli et al., 2008; Cavalli et al., 2013), basato su parametri derivabili dal DEM come area drenata, pendenza, lunghezza dei percorsi di deflusso e scabrezza superficiale, insieme ad uno specifico strumento software stand alone, freeware e open source (SedInConnect, Crema & Cavalli, 2018). L'indice mira a rappresentare la connettività a scala di bacino per valutare il contributo di una data parte del bacino come sorgente di sedimenti e definire i relativi percorsi di trasferimento. Il crescente interesse per la caratterizzazione quantitativa dei legami tra le diverse unità del paesaggio e la semplice applicabilità di questo indice hanno portato a numerose applicazioni in diversi contesti (Cavalli et al., 2020). Il presente lavoro presenta e discute le principali applicazioni di questo indice che essendo basato sulla topografia, si concentra sugli aspetti strutturali della connettività tenendo in considerazione che qualità e risoluzione dei DEM possono avere un impatto significativo sui risultati. Lo sviluppo futuro dell'indice dovrebbe considerare un approccio più basato sui processi e incorporare la variabilità temporale direttamente nello schema di calcolo. Inoltre, questo lavoro dimostra che, se applicato con attenzione considerando i limiti intrinseci dell'approccio prevalentemente topografico, l'indice può fornire rapidamente una caratterizzazione spaziale della dinamica dei sedimenti, migliorando così la comprensione del comportamento del sistema geomorfologico e, di conseguenza, la valutazione della pericolosità e del rischio.

In very general terms, hypsometry is the measurement of land elevation. In geomorphology, hypsometry refers to the analysis of the cumulative distribution of the elevation, absolute or relative, in a given region. The distribution of elevation is a basic topographic feature of a territory, and its representation is useful for interpreting and characterizing geomorphic processes. Hypsometry can be computed in spatial domains of any shape and extent, including the whole earth surface; however, often the studies of hypsometry focus on drainage basins. Hypsometric curves, which represent the fraction of area above a given elevation, are a widely adopted tool for the graphical representation of hypsometry. Given the relationships between hypsometric characteristics and geomorphic processes and factors, it is not surprising the ample literature related to the exploitation of hypsometric-based indices in multiple research contexts, not limited to the analysis of drainage basins (e.g., morphology of glaciers, planetary morphology, etc.).

Sediment connectivity, defined as the degree to which a system facilitates the transfer of water and sediment through itself by means of coupling relationships between its components, has recently emerged as a paramount property of geomorphic systems. The growing interest of the earth sciences community in connectivity led this property becoming a key concept concerning sediment transfer processes analysis and one of the building blocks of modern geomorphology. The increasing availability of high-resolution Digital Elevation Models (DEMs) from different sources as LiDAR and Structure from Motion (SfM) paved the way to quantitative and semi-quantitative approaches for assessing sediment connectivity. A geomorphometric index of sediment connectivity (Borselli et al., 2008; Cavalli et al., 2013), based on DEM derivatives as drainage area, slope, flow length and surface roughness, has been developed along with related freeware software tool (SedInConnect). The index aims at depicting spatial connectivity patterns at the catchment scale to support the assessment of the contribution of a given part of the catchment as sediment source and define sediment transfer paths. The increasing interest in the quantitative characterization of the linkages between landscape units and the straightforward applicability of this index led to numerous applications in different contexts (Cavalli et al., 2020). This work presents and discusses the main applications of this sediment connectivity index. Being a topography-based index, it is focused on structural aspects of connectivity, and quality and resolution of DEMs may have significant impact on the results. Future development should consider process-based connectivity and incorporate temporal variability directly into the index. Moreover, this work demonstrate that, when carefully applied considering the intrinsic limitations of the mainly topographic-based approach, the index can rapidly provide a spatial characterization of sediment dynamics, thus improving the understanding of geomorphic system behavior and, consequently, hazard and risk assessment.

Debris flows are a widespread natural hazard in mountain catchments causing major damage when they intersect transport vulnerable elements at risk, including transport routes and urban areas. Climate change is expected to lead to an increase in the frequency and magnitude of those phenomena. It is thus fundamental to extend the collection of primary data on debris-flows occurrence and volume in order to evaluate the variations of debris-flow frequency and magnitude with time and, at the same time, provide new tools for the assessment of areas affected by debris flows and their spatial characterization. This work presents the results of the analysis of date of occurrence and volume of a large dataset of debris-flows collected in the eastern Italian Alps. The occurrence, in the last forty years, of some high-magnitude debris flows characterized by the largest unit volumes of the dataset might be associated with current climate changes. In order to deal with the observed increase in this processes in mountain environments, a simple and fast semi-automated procedure is proposed for the regional-scale identification of debris-flow prone channels and alluvial fans. The main aim is to develop a low data-demanding method for a preliminary mapping of potentially debris-flow affected areas to enable priority ranking of channels and alluvial fans at risk by debris flows. This approach has been validated by means of field checks and through its extensive application in the eastern Italian Alps.

Debris flows are widespread phenomena in mountain catchments that often cause damage to urbanized areas and transport routes. The spatial characterization of the affected areas is a major issue in the framework of watershed management. We developed a simple and fast semi-automated and low data-demanding procedure for regional-scale identification of debris-flow prone channels and alluvial fans. A GIS-based approach enables a preliminary mapping of potentially debris-flow affected areas and provides information for the priority ranking of channels and alluvial fans exposed to debris flows. The methodology exploits Digital Elevation Models (DEMs) to derive geomorphometric parameters analyzed for the identification of debris-flow triggering areas and their propagation along the channel. Potential initiation sites of debris flows are identified as those exceeding a threshold of local slope versus contributing area, while channel reaches corresponding to debris flows propagation, deceleration and halting conditions are derived from thresholds of local slope. An analysis of longitudinal channel profiles, which considers the traveled distance and the local slope, is used for the computation of the debris-flow runout. The procedure takes into account the presence of hydraulic control works (i.e. check dams) along with information on erosion-resistant bedrock channels and sediment availability. This approach has been validated by means of field checks and through its extensive application in the eastern Italian Alps. The developed methodology has been implemented in a set of freely-available software tools (https://github.com/HydrogeomorphologyTools) in order to facilitate its application and further validation in different environments.

Debris flows are widespread phenomena in mountain catchments causing major damage when they intersect transport routes or urban areas. The assessment of areas affected by debris flows and their spatial characterization is thus a major issue in the framework of watershed management. This work presents a simple and fast semi-automated procedure developed for a regional-scale identification of debris-flow prone channels and related downstream alluvial fans. The main aim is to develop a low data-demanding method for a preliminary mapping of potentially debris-flow affected areas at the regional scale to enable priority ranking of channels and alluvial fans at risk by debris flows. The methodology is based on a geomorphometric approach that involves the analysis of morphometric parameters derived from Digital Elevation Models (DEMs) for the identification of debris-flow triggering areas and their kinematic along the channel. Potential initiation sites of debris flows are identified as those exceeding a threshold identified on a plot of local slope versus contributing area whereas channel reaches corresponding to debris flows deposition, deceleration and halting conditions are derived from thresholds of local slope. An empirical method, which considers the traveled distance and the local slope along the channel profile, is used for the computation of the debris-flow runout. The presence of hydraulic control works (i.e. check dams) along with information on erosion-resistant bedrock channels and sediment availability are taken into account within the procedure. This approach has been validated by means of field checks and through its extensive application in the eastern Italian Alps. The developed methodology has been implemented in a set of freely-available software tools (https://github.com/HydrogeomorphologyTools) in order to facilitate its application and further validation in different environments. Potential and limitations of the method will be also presented.

In this study, we investigate the surface flow time of rise in response to rainfall and snowmelt events at different spatial scales and the main sources originating channel runoff and spring water in a steep nested headwater catchment (Rio Vauz, Italian Dolomites), characterized by a marked elevation gradient. We monitored precipitation at different elevations and measured water stage/streamflow at the outlet of two rocky subcatchments of the same size, representative of the upper part of the catchment dominated by outcropping bedrock, at the outlet of a soil-mantled and vegetated subcatchment of similar size but different morphology, and at the outlet of the main catchment. Hydrometric data are coupled with stable isotopes and electrical conductivity sampled from different water sources during five years, and used as tracers in end-member mixing analysis, application of two component mixing models and analysis of the slope of the dual-isotope regression line. Results reveal that times of rise are slightly shorter for the two rocky subcatchments, particularly for snowmelt and mixed rainfall/snowmelt events, compared to the soil-mantled catchment and the entire Rio Vauz Catchment. The highly-variable tracer signature of the different water sources reflects the geomorphological and geological complexity of the study area. The principal end-members for channel runoff and spring water are identified in rainfall and snowmelt, which are the dominant water sources in the rocky upper part of the study catchment, and soil water and shallow groundwater, which play a relevant role in originating baseflow and spring water in the soil-mantled and vegetated lower part of the catchment. Particularly, snowmelt contributes up to 64 ± 8% to spring water in the concave upper parts of the catchment and up to 62 ± 11% to channel runoff in the lower part of the catchment. These results offer new experimental evidences on how Dolomitic catchments capture and store rain water and meltwater, releasing it through a complex network of surface and subsurface flow pathways, and allow for the construction of a preliminary conceptual model on water transmission in snowmelt-dominated catchments featuring marked elevation gradients.

Debris flows are one of the most relevant natural hazards in mountain regions. A combination of structural (e.g. check-dams) and non structural (e.g. warning systems) measures is needed in most cases to reduce debris flow risks, and both benefit from experimental data on debris-flow characteristics. A new experimental site is being equipped in the Autonomous Province of Bolzano (Italy) for both monitoring purposes and testing warning systems. The study site (Gadria basin) is a small channel subjected to frequent debris flows. The overall system includes the following components: 4 rain gauges located at different elevations, 5 radar sensors and 5 geophones, and 3 video cameras and flash lights to record the propagation and the deposition of the debris flow. Transmission of data and alerts from the instruments will use radio technology because GSM coverage is not available in the basin.

Airborne Laser Scanner (ALS) surveys are widely used to obtain high-resolution DTMs to be used in natural hazards preventive analysis, e.g. flood and debris-flow modelling, as well as to reconstruct sediment budget through DoD (DEM of Difference) analysis, in particular in mountain basins after large events . In contrast, the use of ALS to capture the morphometric features of stream channels is less favorable as the infrared signal, characteristic of the most common LiDAR instruments, do not penetrate deep water bodies and thus do no permit to analyze the wet portion of channels. Yet, surveys carried out by these instruments can provide sufficient data to characterize and analyze morphometrically stream channels featuring either very shallow flows relative to bed roughness(e.g. steep mountain channels) or relatively limited wet areas (e.g. braided rivers). So far, very few investigations have deployed bathymetric LiDAR surveys, also known as green LiDAR, to map river systems. Bathymetric LiDAR has the potential to offer high-resolution DTMs for morphometric analysis in river systems complementary to those mentioned above (i.e. channels with perennial and substantial wet areas) and where bathymetric surveys using boats are not easily carried out due to fast and turbulent flows. These conditions are typical of mild-sloping, single-thread or wandering mountain rivers, widely distributed worldwide, in which detailed morphometric analysis are thus very challenging when carried out through traditional approaches. We will present a state of the art of morphometric analyses used to characterize channel morphology, including published and unpublished examples from step-pool channels as well as from sinuous-meandering and braided rivers, based on both infrared and green LiDAR data.

In high elevation alpine catchments, first-order streams are often constituted by steep and narrow channels bound by cliffs. These channels frequently have a structural control imposed by fractures and faults in bedrock and typically constitute temporary streams where snowmelt processes strongly influence runoff. Rocky headwater basins show a different hydrological response with respect to soil-mantled basins and their hydrology is poorly known due to the lack of widespread monitoring sites. Herein we present the preliminary results achieved through an experimental hydro-meteorological monitoring network setup in a 0.1 km2 rocky headwater basin located on the southern flank of the Sella Group in the Dolomites (North-Eastern Italy). Elevation ranges between 2700 m, at the outlet, and 3174 m, with an average value of 2950 m. Geology is constituted mainly by the Norian "Dolomia Principale" (Dolomite) featuring a complex structural setting. The monitoring network, active since 2009, is designed with three rain gauges with a time rate of 5 minutes. Two are located at 2609 and 2597 m (close to the outlet), and the third is located on the divide in the central part of the basin (2911 m). Runoff at the outlet is monitored by a pressure transducer. The time interval was set at 5 minutes in the summer months in order to capture the stream response due to intense and spatially-concentrated rainfall events. In winter the pressure transducer is maintained active with a time rate of 30 minutes so as to be ready to register in spring the stream response due to snowmelt. In the summer 2011, some precipitation, runoff, snow and spring water samples for isotopic analysis ( 18 O and 2 H) were collected aiming to better characterize the origin of subsurface water and the main sources to runoff. Preliminary results show ephemeral presence of runoff, mostly occurring during snowmelt (from May to early July) and after intense summer rainstorms. The lag-time from precipitation centroid to runoff peak (on the order of about 1 hour) is rather long for a very small and steep basin. This is likely due to the presence of fractured bedrock that creates a shallow, subsurface reservoir and increases the catchment response time. Isotopic data of water samples well reflect the precipitation signal indicating reduced evaporation processes during the transfer of the water input to the outlet. Moreover, the isotopic composition of streamflow is close to that of the sampled springs but less depleted than snowmelt samples, revealing a possible contribution to runoff of snowmelt water mixed with subsurface water previously stored in the system.

The mapping of the channel network plays a fundamental role in manifold landscape management issues, such as geo-hydrological risk analysis, legal matters related to land use, technical cartography and water resources management. Human activities in mountain regions results in a wide variety of environmental impacts. Human activities that influence runoff generation and drainage pattern have reflections in the context of hydro-geological hazard and water resources management. In some cases, channel network has been modified for drainage and irrigation purposes, in other the alteration of drainage patterns is due to the presence of man-made features, such as roads and new urbanized areas. The last 40 years have seen relevant changes in land use in mountain regions, with an important increase of urbanized areas and road network, and consequent modifications of the hydrographic network. Accordingly, an up-to-date detailed recognition of the channel network is needed for landscape management purposes. The availability of high-resolution digital terrain models (HR-DTMs) of regional coverage opens interesting prospects for the analysis and the definition of the channel network. These terrain models offer an unprecedented capability to interpret surface morphology and the related geomorphic and hydrological processes. Moreover, the availability of regional HR-DTMs is increasing, thanks to technological developments and decreasing costs of data acquisition and processing. In this study, we present our experience in the derivation of channel network from regional HR-DTM for an alpine region (Autonomous Province of Trento, Northern Italy), covering an area of 6500 km2. The derivation of the channel network is conducted via a geomorphometric approach. Moreover, we analyze the interaction between human activities and the channel network by comparing the channel network derived from the HR-DTM with field evidences collected during extensive field surveys.