Marco Cavalli

We present an integrated approach that aims to: (i) document the spatial distribution of mass-wasting activity and sediment production in the Saldur River basin (97 km2); (ii) detect causal linkages between mass-wasting intensity and the potential spatial distribution of discontinuous permafrost; (iii) identify source-to-sink colluvial sedimentary pathways as modulated by the spatial organization of active and relict glacial and periglacial depositional landforms; and (iv) test the reliability of a geomorphometry-based index of sediment connectivity. In so doing we compare spatial patterns of process-based and structural geomorphic connectivity. To these ends, we map rock glaciers, protalus ramparts and moraines, and compile a field-and air photo-based multi-temporal) inventory of colluvial sediment sources. We then combine these data with two historical datasets of debris flow and landslide events (both implemented and maintained by the Autonomous Province of Bolzano) and analyse m...

ABSTRACT The role of sediment connectivity in controlling sediment fluxes, in particular between sediment sources and downstream areas, is a key issue in the study of sediment transfer processes. In mountain catchments, in which the degree of hillslope-channel coupling and decoupling is controlled by the rugged morphology and the distribution of sediment sources, the assessment of connectivity patterns is especially useful for giving watershed management priorities. In this work, the authors present the ongoing development and the first-stage testing phase of a free, open source and stand-alone application for the computation of the Index of Connectivity (IC), as expressed in Cavalli et al. (2013). The tool is intended to have a wide variety of users, both from the scientific community and from the authorities involved in the environmental planning. Due to its open source nature, the tool can be modified and/or integrated according to the users requirements. Being also a stand-alone, easy-to-use application, the tool can help management authorities in the quantitative estimation of sediment connectivity in the framework of hazard and risk assessment. First results of the testing phase are encouraging since the modelled connectivity appears suitable to detect the potential for the sediment to reach specific targets areas such as the main channel network.

ABSTRACT Morphological change evaluation of earth surface is an important task in environmental monitoring. Methods devoted to the assessment of geomorphic changes can be used to identify geomorphologically unstable areas, to quantify processes intensity and to compute sediment budgets. Digital elevation models (DEMs) built from repeated topographic surveys can be used to produce DEM of Difference (DoD) maps and to estimate volumetric changes through time. Nowadays LiDAR technology provides digital models representative of the bare earth surface (Digital Terrain Models - DTMs) at high spatial resolution and over large spatial extents, thus contributing to the increase of accuracy of morphometric and volumetric measurement of varying surfaces. In this study, high-resolution DTMs derived from airborne LiDAR data acquired in different years (2006 and 2011) were used in order to characterize sediment transport processes such as debris flows and bedload transport in two small Alpine basins. Two DTMs (2 m resolution) were derived for the Gadria and Strimm catchments (Vinschgau-Venosta valley, Autonomous Province of Bozen-Bolzano, Italy). These basins, which cover, respectively, areas of 6.3 and 8.5 km2, have been chosen due to their contrasting morphology and because they feature different types and intensity of sediment transfer processes: Gadria channel is characterized by frequent occurrence of debris flows (almost one debris flow per year), whereas Strimm is essentially a bedload stream. A method based on fuzzy logic (Wheaton et al., 2010), which takes into account DTM uncertainties, was used to derive the DoD of the study area. The comparison between the 2006 and 2011 DTMs permitted the assessment of morphometric changes at the basin scale over the 5 yrs period. The results of DoD analysis are consistent with field observations of erosion and sediment transport. Besides, the DoD proved useful to identify the relationship between erosion, deposition or no-change areas and geomorphometric parameters (e.g. slope, curvature, upslope area) relevant for topographic evolution of the landscape. References: Wheaton J.M., Brasington J., Darby S. E., Shear D. A., 2010. Accounting for uncertainty in DTMs from repeat topographic surveys: improved sediment budges. Earth Surface Processes and Landforms, 35, 136-156.

ABSTRACT Standardization of measurement procedures and their systematic performance are important goals in every field of science and are in general intensely pursued by scientists in many of their research activities. Certain phenomena, however, present particularly difficult challenges in this regard and many efforts are still needed to actually reach standardization and systematic performance of measurements. Debris flows are natural phenomena that certainly belong to this latter category. Due to their low frequency of occurrence, their short duration and their sudden and abrupt nature they are extremely difficult to be monitored. Only instrumented basins where debris flows occur with a sufficiently high frequency per year allow systematic monitoring activities. Even though during the last dec-ades several such basin have been instrumented, field measurement data are still scanty and methods of measurement are not yet sufficiently standardized. An European Territorial Cooperation project named “Sediment management in Alpine basins: integrating sediment continuum, risk mitigation and hydropower” (SedAlp) has been recently funded within the Alpine Space Programme that has, among its tasks, to make some advancement in this direction. One of the expected outputs of the SedAlp project is a protocol on debris-flow monitoring. In this paper, in particular, we will discuss some open issues regarding the debris-flow seismic monitoring. Keywords: debris flow, seismic monitoring, measurement standardization.

ABSTRACT Geomorphic changes caused by three debris-flow events in a small mountain catchment (Moscardo Torrent, Eastern Italian Alps) were investigated by means of multitemporal terrestrial laser scanning (TLS) surveys over three areas exposed to debris-flow dynamics. For these areas, DEM of Differences (DoDs) with a 0.2 m spatial resolution allowed the assessment of elevation uncertainty as spatially variable by applying the fuzzy inference system (FIS) proposed by Wheaton et al. (2010). Along with two commonly used parameters affecting Digital Elevation Model (DEM) accuracy, such as terrain slope and point density, we developed and employed in the FIS a new index, named Vegetation Noise, that takes into account the disturbance caused by the vegetation cover on the DEM accuracy. The results of volumetric budgets of the surveyed sediment source areas derived from the DoD analysis were compared with debris-flow volumes estimated from flow stage measurements at the instrumented channel reach. On the one hand, the discrepancies arising from this comparison underline the limitations of TLS surveys over morphologically complex areas such as debris-flow catchments. On the other hand, the TLS unveils the geomorphic changes at the scale of the single event, because a terrestrial survey is usually easier to carry out than an aerial one. The analysis of DEM quality and uncertainty indexes correlation may help to refine methods for spatializing elevation errors and improving the reliability of the models.

ABSTRACT Understanding and modelling the dynamics of large wood (LW) in rivers during flood events has spurred a great deal of research in recent years. Whereas most of the research on LW has focused on its spatial distribution and geomorphologic role at longer time scales, only few studies have documented the effect of high-magnitude flash floods on LW recruitment, transport and deposition. On October 25th2011, the Magra river basin (North-western Italy) was hit by an extreme meteorological event, with hourly rainrates up to 130 mm h-1 and event rain accumulations up to 540 mm. Such large rainfall intensities originated flash floods in the main river channels and in some of the tributaries, with unit peak discharges up to around 20 m3s-1km-2 in catchments of 10-20 km2, causing severe damages and loss of lives. Numerous landslides were triggered and the morphology of the tributaries was highly affected in response to intense lateral and vertical channel dynamics. Besides, many bridges were partly or fully clogged by LW jams. A post-floodsurvey was carried out in November 2011 and February 2012 along four of the tributaries most severely affected by this event. The total length of surveyed channels is 9.5 km. These channels were divided into reaches of similar morphological characteristics (slope, width, vegetation cover), and in every reach the volume of LW deposited was estimated by a combination of field surveys and interpretation of aerial photos. In addition, LW recruited from hillslopes and floodplains was estimated by comparing pre and post-event orthophotos. Preliminary results show very high rates of LW recruitment (1127 m3km-1 on average ranging from 569-2001 m3km-1) along the analysed channels, the majority (about 80%) stemming from floodplain erosion and the rest from the colluvial processes (predominantly landslides). Channels width has increased on average 10 times (ranging from 4 to 23 times). Despite the large variability among the channels, LW dynamics seems to be related to reach characteristics. In fact, LW deposition was higher - and LW recruitment was smaller - in the wider reaches, and the presence of newly-formed islands or even of single standing trees was key in increasing the trapping efficiency of a reach. Also, the steeper the channel, the lesser is LW deposition and the larger is LW supply. Finally both LW recruitment and deposition show a positive relation with drainage area (for basins > 10 km2). These observations suggest a conceptual model for LW dynamics in mountain rivers different from others recently put forward that have analysed LW storage data alone, at few snapshots in time long after the occurrence of extreme events like the one described here. The lessons learned from this event may be useful for a management of mountain catchments which should explicitly take into account the crucial role of LW during extreme events, especially for its interaction with infrastructures.

ABSTRACT Debris-flow monitoring in instrumented areas is an invaluable way to gather field data that may improve the understanding of these hazardous phenomena. A new experimental site has been equipped in the Autonomous Province of Bozen-Bolzano (Eastern Alps, Italy) for both monitoring purposes and testing early warning systems. The study site (Gadria basin) is a 6.3 km 2 catchment subjected to frequent debris flows. The monitoring system in the Gadria basin consists of rain gauges, radar sensors, geophones, video cameras, piezometers and soil moisture probes. Transmission of data and alerts from the instruments exploits in part radio technology. The paper presents the data gathered Electronic supplementary material The online version of this article (doi:10.1007/s11069-014-1088-5) contains supplementary material, which is available to authorized users. during the first three years of activity, with two debris-flow events recorded at the station varying in magnitude and characteristics, and discusses the perspectives of debris-flow monitoring and related research.

ABSTRACT Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes and by the influence of the riparian vegetation and disturbances such as floods. In particular, the balance between sediment supply and transport capacity can determine the morphological evolution of a river. For instance, aggrading and widening trends are distinctive of reaches where sediment supply is higher than transport capacity. In contrast, incising and narrowing tendencies are dominant. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a small reach of a relatively unimpacted gravel-bed braided river (Tagliamento River, northeast Italy) using a terrestrial laser scanner (TLS). The study area is around 23 ha and has been surveyed before after two periods with relevant flood events, two of which were higher than the bankfull level and occurred between September 2010 and September 2011. The very high point clouds density allowed us to derive three high resolution digital elevation models (DEMs) with 0.125 × 0.125 m pixel size. Scan data cloud merging was achieved with an overall high degree of accuracy and resolution (subcentimeter). Topographic data were more accurate for exposed surfaces than those collected in wet areas. A detailed net of dGPS control points allowed us to verify the high quality of the DEMs derived from the surveys (RMSE of about 5 cm). Two DEMs of difference (DoD) were computed, revealing different and consistent episodes of erosion and deposition within the analyzed area, and changes in morphology of channel and bars could also be detected, such as bar edge accretion and bank erosion demonstrating a strong dynamicity of the Tagliamento River. Moreover, a very detailed estimation of the surface roughness in the study area has been carried out, permitting a large-scale analysis of the roughness values distribution. The results of the analysis on the TLS collected data show that along a river with a high natural character (i.e., Tagliamento River), the dynamic processes are also common during low magnitude events.

ABSTRACT In the Small Dolomites group (Eastern Italian Alps), the Rotolon catchment is affected by a landslide that historically threatened the nearby village of Recoaro Terme. After the last re-activation on November 2010, the need to deploy devices to monitor deformations on the unstable slope became of paramount importance. This paper deals with the methodology, the techniques and the integrated services adopted for the design and the realization of a web-based platform for automatic and continuous monitoring of the Rotolon landslide. The choice of a web environment simplifies data collection, while a remote control permits technical maintenance and calibration on instruments and sensors in the field. Data management is straightforward on a single server, with the dataset being continuously updated. There is a user-friendly web interface which allows a practical up-to-date solution for decision-makers. This web-based monitoring platform represents the first step in the implementation of a complete early warning system.

Alluvial fans of alpine torrents are both natural deposition areas for sediment discharged by floods and debris flows and preferred areas for agriculture and settlements. Hazard assessment on alluvial fans depends on proper identification of flow processes and their potential intensity. In this context, the recognition of surficial features in various parts of the alluvial fan is essential to outline areas with evidence of debris flows and areas affected by floods with bed-load. The recognition of man-made features due to human activity ...

Stream power is a key variable for the analysis of landforms associated to the fluvial systems. Several studies on stream power related to bankfull or near-bankfull discharges, which are regarded as morphologically significant. The analysis of stream power for bankfull discharges makes it possible homogeneous comparisons of stream power between different parts of a channel network. However, it does not permit depicting patterns of stream power resulting from larger floods that hit various parts of a basin with different intensity. The ...

Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes, and the influence of the riparian vegetation and disturbances such as floods. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a relatively un-impacted gravel-bed braided river (Tagliamento river, Northeast Italy) using a Terrestrial Laser Scanner (TLS). The device used in this study (Leica Scan Station 2) is a ...

Monitoring of debris flows in instrumented catchments permits collection of data on these phenomena and provides a valuable link with geomorphological and topographical observations of erosion, sediment supply and channel bed evolution. Numerous sites recently instrumented in various geographical regions show that field monitoring is receiving increasing attention in debris-flow research worldwide. This poster presents a new system for debris-flow monitoring in the Gadria catchment (Eastern Alps), installed and managed ...

Surface texture analysis applied to high-resolution digital terrain models (HRDTMs) is a promising approach for extracting useful fine-scale morphological information. Surface roughness, considered here as a synonym of surface texture, can have a discriminant role in the detection of different geomorphic processes and factors. Very often, the local morphology presents, at different scales, anisotropic characteristics that could be taken into account when calculating or measuring surface roughness. The high morphological detail of HRDTMs permits the description of different aspects of surface roughness, beyond an evaluation limited to isotropic measures of surface roughness. The generalization of the concept of roughness implies the need to refer to a family of specific roughness indices capable of capturing specific multiscale and anisotropic aspects of surface morphology. An interesting set of roughness indices is represented by directional measures of roughness that can be meaningful in the context of analyzed and modeled flow processes. Accordingly, we test the application of a flow-oriented directional measure of roughness based on the geostatistical bivariate index MAD (median of absolute directional differences), which is computed considering surface gravity-driven flow direction. MAD is derived from a modification of a variogram and is specifically designed for the geomorphometric analysis of HRDTMs. The presented approach shows the potential impact of considering directionality in the calculation of roughness indices. The results demonstrate that the use of flow-directional roughness can improve geomorpho-metric modeling (e.g., sediment connectivity and surface texture modeling) and the interpretation of landscape morphology.