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The RESILIENCE project aims at developing an integrated methodology for assessing the impact of climatic variations and changes on the intense precipitation and wind regimes, and on the consequent triggering of flash floods, debris-flows and wind-related forest damages. A significant increase of short and intense precipitation is expected in the next future due to global warming, with consequent impacts on flash floods and hydro-geomorphic hazards such as shallow landslides and debris flows. Despite their societal importance, only few studies have explored potential climate change effects on these hydrological and hydro-geological processes. In fact, no accepted estimates of such changes to be used in engineering practice or environmental management planning exist so far, nationally or regionally.The RESILIENCE project tries to address this specific knowledge gap. Two recent scientific advances are at the basis of the development of RESILIENCE. The first advance is the advent of high-resolution climate models, also called Convection-Permitting Climate Models (CPM), which improve the representation of both precipitation and wind field at the sub-daily scales compared to the standard coarser resolution Regional Climate Models. However, due to their computational costs, simulations are currently available for only short (typically ten years) time slices and few emission scenarios. These time series are too short to provide reliable statistics of extremes if analyzed using the classical extreme value methods. A second recent advance in the field of extreme value theory, the Metastatistical Extreme Value Distribution (MEVD), allows to overcome this limitation: it provides reliable extreme event probability estimates even from short time series, as in the case of CPM outputs, since it is based on all “ordinary events'' in the series instead of just yearly maxima or a few “peak-over-threshold” values per year as in the traditional methods.Given this background, and focusing on the Veneto region in Italy as a study area, the specific objectives of RESILIENCE are 1) to quantify near (2041-2050) and far (2090-2099) future changes in precipitation and wind extremes probability at sub-daily temporal scales with respect to the baseline (1996-2005) using the MEVD approach and high-resolution COSMO-CLM simulations, 2) to quantify the associated future impacts on flash floods, debris flows and forest damages, 3) to provide data and hazard models to support flood and forest risk management plans in the Italian North-East accounting for future climate changes.RESILIENCE brings together an interdisciplinary group of scientists, from hydrologists, to climate modelers, to statisticians, to forest science experts, and is based on the interaction with three key Project Stakeholders. The project results will be communicated and disseminated to a wide audience of residents in the Veneto region and beyond, through collaborations with Museums, Academies and Local Authorities.

Debris flow events, generated by surface runoff, occur with great frequency in the Dolomites (Northeastern Italian Alps) during the summer season. Summer thunderstorms, which are common in the region, can quickly generate runoff at the base of rocky cliffs, which then entrains and propagates downstream the underlying unconsolidated material. In the past, the main atmospheric feature considered in evaluating the initiation of debris flow events was rainfall. Observations led to the development of rainfall intensity–duration thresholds for sediment mobilization, which compared incoming severe rainfalls with the potential for triggering debris flows. This study works toward the examination of another characteristic of the atmosphere, the atmospheric electric field. In particular, the behavior of the electric field prior to convective rainfall is investigated as an indicator of rainfall intensities capable of triggering debris flows, in a basin near Cortina d’Ampezzo (Italy). Results suggest that prior to bursts of intense rainfall, the electric field derivative frequency distribution exhibits a recurrent pattern roughly half the time. When it occurs, the amplitude of derivative frequency distribution intersects the zero axis twice before rainfall reaches maximum intensity. A regression model is designed which considers the amplitude maximum and the difference in time between the crossings of the zero axis. The validation of this model suggests a mild relationship between electric field and rainfall intensity in an alpine environment.

Runoff-generated debris flows are a significant hazard in steep mountain ranges across the world. During intense rainfall storms, runoff can rapidly form in small steep basins and mobilise large volumes of sediment triggering debris flows which can damage infrastructure and endanger lives. A common method for forecasting debris flows is deriving empirical rainfall intensity–duration (ID) thresholds from previously recorded debris flow events in a given area. However, the storms which trigger debris flows usually are short and intense with high spatial variation making an accurate recording of the conditions responsible for initiation difficult.In this study, we investigate the impact of the spatial variability of rainfall on debris flow initiation in small, steep, and debris flow prone catchments in the eastern Italian Alps (Dolomites) using the SWEHR (Shallow Water Equation Hairsine-Rose) numerical model. The modelled catchments are monitored by multiple rain gages which we use to quantify the uncertainty of the rainfall ID thresholds due to the spatial variation of rainfall by comparing empirical and numerically modelled thresholds. We also compare simulated triggering discharges for debris flows with available field observations in the study area. This study will help to improve the quality of hazard forecasting of debris flows in mountainous regions

Over the last few decades, the need for landscape monitoring and assessment of changes in spatial patterns over time has grown. Landscapes sustain changes of various aspects, such as their structure, in response to continuous modifications in land cover, land management regimes and policy decisions. This study investigates changes in the landscape structure along the Raya graben bottom in northern Ethiopia from 1986 to 2017. Landsat imageries were used to analyse land cover. The spatial structure of the landscape for 3 decades was evaluated using FRAGSTATS. The number of patches increased at the class and landscape levels. At the landscape level, the patches totalled 8,147 in 1986 and increased to 886,893 in 2017. Similarly, the patch density (PD) in 1986 was 3.4 and reached 372.7 in 2017. Therefore, high landscape fragmentation occurred within the study area. The value of the Shannon diversity index is close to 1.1, and the diversity of patch types within the landscape is regular. The Shannon Evenness Index value ranges from 0.6 to 0.7. Moreover, the diversity of patches has shown an increment across the study period. Each land cover reduces in size and proximity. Those changes were caused by population size, infrastructural development and commercial agriculture. Thus, proper land use planning must be practiced to reduce fragmentation and environmental impacts in landscapes that face increasing population pressure and continuous land cover changes.

Il presente rapporto è stato prodotto nell'ambito dell'Accordo di collaborazione tecnico-scientifica per la definizione delle soglie di allarme e le conseguenti logiche di funzionamento del Sistema di monitoraggio ed allarme della colata detritica di Cancia (Borca di Cadore), tra l'Agenzia Regionale per la Prevenzione e Protezione Ambientale del Veneto (ARPAV), il Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica (CNR IRPI), l'Università degli Studi di Padova, Dipartimento TESAF (UNIPD) e la Libera Università di Bolzano (UNIBZ). Il rapporto, dopo aver richiamato le soglie di allertamento implementate nel 2020, presenta le risultanze delle osservazioni post-evento effettuate a seguito degli eventi verificatisi nel corso dello stesso anno e propone poi l'aggiornamento delle soglie pluviometriche, di livello e geofoniche, delle logiche di funzionamento del Sistema di monitoraggio ed allarme, precedentemente presentate nella relazione tecnica (rev. 1) del 15/04/2020 (Cavalli et al., 2020).

On 4 August 2015, a very high intensity storm, 31.5 mm in 20 min (94.5 mm/h), hit the massif of Mount Antelao on the Venetian Dolomites (eastern Italian Alps) triggering stony debris flow characterized by high magnitude. It routed along the Ru Secco Creek and progressively reached the resort area and the village of San Vito di Cadore, causing fatalities and damages. The aim of the present research is the study of this debris-flow event by means of pre and post-event topographic data derived by LiDAR (Light Detection and Ranging) and Structure-from-Motion (SfM) photogrammetry technique associated to its occurrence. This study analyzes the Digital Terrain Models (DTMs) derived from LiDAR survey carried out in July 2015 and UAV-SfM data obtained in September 2019. The most important step to compare these multi-temporal surveys was the co-registration process, fundamental to guarantee the coherence among the two different surveys. The post-event SfM-DTM of the area routed by debris flow subtracted to the pre-event LiDAR-DTM, provided a DoD (DTM of Difference) that was useful to assess the deposition-erosion patterns and estimate debris-flow volume. Multi-temporal topographical data are important to analyze the phenomenon and its characteristics. This allowed us to more in depth analyzed the debris-flow effects and provide valuable information for the planning of risk prevention measures.

Many sites of the Dolomites are threatened by channelized debris flows: solid-liquid surges initiated by the entrainment of large quantities of sediments into the abundant runoff at the head of channel incised on fans, can dramatically increase their volume along the downstream routing. This is the case of the Rovina di Cancia site where solid-liquid surges forming in the upper part of the basin can increase their volume up and over 50000 m3, seriously impacting the downstream village of Borca di Cadore. The debris-flow channel ends just upstream the village that in the past was hit by four debris flows (three in the recent years) that caused victims and destructions. Control works built until now are not sufficient to protect the village from high magnitude debris flows and a definitive solution calls to be planned. Present works are a flat deposition area, 300 m downstream the initiation area, an open dam under construction downstream it, and two retention basins at the end of the channel. Between the open dam and the upstream retention basin, there are the rest of eight check-dams made of gabions, built in the 60s and progressively damaged or destroyed by the debris flows occurred after their construction. This series of check-dams limited the entrainment of solid material and the occurrence of localized scours. The initial plan is the substitution of the check-dams with concrete structures and the widening of the dowsntream retention basin through the raising of high elevation embankment downstream it and the following demolition of the actual dyke. Finally, a channel crossing the village and national route on the valley bottom will deliver the fluid phase from the widened basin to the Boite river. All these control works have a very high cost for construction and maintenance and severely impact the village with the presence of a non-negligible residual risk. These drawbacks call for an alternative solution that is searched looking at to the morphology. Downstream of the open dam and on its right side, there is a deep impluvium that ends on a large grass sloping area. The novel solution requires the construction of a channel through the right high bank that deviates the debris flow into the impluvium. The impluvium, widened through the excavation of the surrounding slopes, is closed at the outlet by an open dam. Downstream the open dam, a channel will lead to a retention basin, where most of storage volume is obtained from the excavation of the grass sloping area, limiting the elevation of the dykes At the end of this basin an open dam will deliver the debris-flow fluid part to a channel passing under the national route and joining the Boite river. Such a solution composed of a deviatory channel, two retention basins (the deep impluvium and that excavated on the sloping grass area) and the channels between and downstream them, has quite a lower costs of construction and maintenance, eliminating the impact on the village because occupying uninhabited areas without interrupting the main roads.

Rainfall thresholds for the occurrence of debris flows are commonly defined by Intensity-Duration curves (ID thresholds). Interestingly, many empirical ID thresholds show up as straight lines in a log-log plot and therefore can be expressed by power-law functions of the form I=α⋅Dβ, where α is the scaling coefficient and β is the exponent of the power function. The different values of α and β reflect the variability of geological and hydrological conditions in the different areas. In most cases, however, field conditions are so complex that a quantitative interpretation of the empirical rainfall threshold is impossible and the physical meaning of α and β remains obscure. In this work, we provide a physical interpretation of the rainfall thresholds that characterize an active debris flow catchment in the Eastern Italian Alps (the Dimai Basin, Belluno Province). The catchment is affected by frequent debris flows generated by surface-water runoff and has been monitored since 2010 to investigate the initiation process. Monitoring data allowed for the detection of two rainfall thresholds: a lower one that identifies the arrival of water in the initiation area (Catchment Outflow Threshold) and an upper one that identifies the initiation of debris flows by channel runoff (Debris Flow Threshold). We demonstrate that these two thresholds can be satisfactorily reproduced by a simple physically based model in which the excess rainfall is routed over the catchment using a kinematic-wave scheme. This simple analysis provides a sound explanation of the observed thresholds and can be used to develop physically based thresholds for runoff-generated debris flows.

In the Dolomitic region, abundant coarse hillslope sediment is commonly found at the toe of rocky cliffs. Ephemeral channels originate where lower permeability bedrock surfaces concentrate surface runoff. Debris flows initiate along such channels following intense rainfall and determine the progressive erosion and deepening of the channels. Sediment recharge mechanisms include rock fall, dry ravel processes and channel-bank failures. Here we document debris flow activity that took place in an active debris flow basin during the year 2015. The Cancia basin is located on the southwestern slope of Mount Antelao (3264 m a.s.l.) in the dolomitic region of the eastern Italian Alps. The 2.5 km2 basin is incised in dolomitic limestone rocks. The data consist of repeated topographic surveys, distributed rainfall measurements, time-lapse (2 s) videos of two events and pore pressure measurements in the channel bed. During July and August 2015, two debris flow events occurred, following similarly intense rainstorms. We compared rainfall data to existing rainfall triggering thresholds and simulated the hydrological response of the headwater catchment with a distributed model in order to estimate the total and peak water discharge. Our data clearly illustrate how debris entrainment along the channel is the main contributor to the overall mobilized volume and that erosion is dominant when the channel slope exceeds 16°. Further downstream, sediment accumulation and depletion occurred alternately for the two successive events, indicating that sediment availability along the channel also influences the flow behaviour along the prevailing-transport reach. The comparison between monitoring data, topographical analysis and hydrological simulation allows the estimation of the average solid concentration of the two events and suggests that debris availability has a significant influence on the debris flow volume. © 2020 John Wiley & Sons, Ltd.

Il presente rapporto presenta le attività della dell'Accordo di collaborazione tecnico-scientifica (nel seguito: Accordo) tra l'Agenzia Regionale per la Prevenzione e Protezione Ambientale del Veneto (ARPAV), il Consiglio Nazionale delle Ricerche - Istituto di Ricerca per la Protezione Idrogeologica (CNR IRPI), l'Università degli Studi di Padova Dipartimento TESAF (UNIPD) e la Libera Università di Bolzano (UNIBZ) nell'ambito della definizione delle soglie di preallarme e allarme e delle logiche di funzionamento del Sistema di monitoraggio e allarme di Cancia. Sulla base di quanto previsto dall'Art. 1 dell'Accordo, il presente documento descrive le attività condotte e presenta i risultati ottenuti relativamente a: I. Definizione di soglie pluviometriche di preallarme; II. Definizione di soglie di allarme legate ai sensori di livello; III. Definizione di soglie di allarme legate ai geofoni; IV. Definizione di logiche di funzionamento del Sistema e periodo di attivazione; V. Valutazione e implementazione del criterio volumetrico. Come previsto nell'Art. 2 dell'Accordo, il CNR IRPI ha svolto il ruolo di coordinatore delle attività tecniche effettuate in collaborazione con UNIPD e UNIBZ e ha analizzato i dati pregressi raccolti dal Sistema di allerta e monitoraggio di Cancia, nonché i dati idrologici storici ARPAV della zona per la definizione delle soglie pluviometriche di preallarme del Sistema. UNIPD ha analizzato i dati pregressi raccolti dal Sistema relativi ai sensori di livello ed ai cavi a strappo per la definizione delle soglie di allarme legate ai sensori di livello. Le analisi condotte da UNIBZ hanno riguardato i dati sismici raccolti del Sistema per la definizione soglie di allarme legate ai segnali forniti dai geofoni e l'implementazione di un criterio volumetrico per la definizione delle logiche di funzionamento del Sistema tale da discriminare, attraverso i sensori a disposizione, le colate di magnitudo ridotta, destinate a fermarsi nella parte alta del bacino, da quelle in grado di raggiungere le due piazze di deposito di Cancia. CNR IRPI, UNIPD e UNIBZ hanno lavorato congiuntamente, ed in stretta collaborazione con ARPAV, alla definizione di logiche di funzionamento del sistema basate su differenti scenari, periodo di attivazione e alternative di gestione in caso di malfunzionamento di uno o più sensori.

This paper proposes a methodology for the design and performance evaluation of debris-flow control works based on modeling the phenomenon and its interaction with the works. The reliability of such an approach depends on the trustworthy reproduction of both the phenomenon and its interaction with the structures: the former is provided by simulating all the physical processes concerning the phenomenon (runoff generation, initiation and propagation of a solid-liquid mixture) by models tested against field measurements; the latter by upgrading the propagation model for considering the non-erosive surfaces of the works overflowed or hit by debris flows and computing the forces exerted on them. This methodology is applied to a case study on the Rovina di Cancia channel (northeast Italy), frequently affected by stony debris flows. Longitudinal and transversal works are planned in a reach of the channel subject to high erosion because of its high slope and the supply of water discharge by the Bus de Diau Creek tributary. The works consist of a sectional dam at the end of the reach and in moving there the mouth of the Bus de Diau Creek. Both of them contribute to decrease bed erosion along the reach and to reduce the peak discharge and the transported sediment volume. The proposed methodology is used in the design phase to explore different geometries of the dam opening and choose its transversal position through the analysis of flow (velocity and free surface), whereas, in the evaluation phase, to estimate the reduction in the bed erosion, peak discharge and transported sediment volume provided by the works (i.e. the performance evaluation). Finally, the plot of the forces acting on the dam breakers and bottom over time allows their static dimensioning.

A storm, composed of two delayed cells, hit the Monte Antelao slopes (Dolomites, North Eastern Italy) in the early morning of July 18th, 2009. The resulting runoff triggered two consecutive debris flows along the Rovina di Cancia channel. The detailed topographic data collected before and just after this event allowed an accurate reconstruction of the morphological changes experienced by the channel bed. These data are here used as benchmark to test the ability of numerical models to reproduce the dynamics of a real event, taking into account the morphology changes of the channel bed. The aim is to provide an efficient model for engineering applications on large scales, such as those required by debris flow hazard assessment. A rainfall-runoff transformation is applied to reconstruct the solid-liquid hydrograph needed for computing the debris flow propagation. Two routing models are used: a GIS-based movable bed model, and a widely used fixed bed model (FLO-2D). Although similar results are obtained in terms of areas subjected to deposition, significant differences emerge in terms of mobilized volumes. Only the simulation of both the deposition and entrainment processes allows to reliably reproduce the sediment volumes estimated from the pre- and post-event topographic data. This information is fundamental in any hazard assessment because the volume of sediment mobilized by debris flow events exerts a fundamental control on the extension of areas subjected to inundation and on the thickness of sediment deposits. The capability to reproduce correctly the mobilized volumes also entails a more reliable simulation of the evolution of the peak and volume of the solid-liquid hydrograph as the debris flow propagates downstream, allowing the identification of the channel reach where banks could be overflowed. Conversely, adopting a fixed bed model leads mainly to an underestimation of the both the transported sediments volumes and the area subjected to deposition. As a consequence, the maximum debris flow depth in the portion of the channel subjected to erosion is underestimated and that in the portion of the channel subjected to deposition is overestimated. All these types of information are of great importance for an effective hazard assessment.

In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼ 0.032 km2, average slope ∼ 320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp-crested weir. Six runoff events were recorded in the period 2011-2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve-Number (SCS-CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape including peak value and timing is obtained only by combining the SCS-CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins. This article is protected by copyright. All rights reserved.

The Dolomite Alps of northeastern Italy experience debris flows with great frequency during the summer months. An ample supply of unconsolidated material on steep slopes and a summer season climate regime characterized by recurrent thunderstorms combine to produce an abundance of these destructive hydrogeologic events. In the past debris flow events have been studied primarily in the context of their geologic and geomorphic characteristics. The atmospheric contribution to these mass wasting events has been limited to recording rainfall and developing intensity thresholds for debris mobilization. This study aims to expand the examination of atmospheric processes that preceded both locally intense convective rainfall (LICR) and debris flows in the Dolomite region. 500 hPa pressure level plots of geopotential heights were constructed for a period of three days prior to debris flow events to gain insight into the synoptic scale processes which provide an environment conducive to LICR in the Dolomites. Cloud-to-ground (CG) lightning flash data recorded at the meso-scale were incorporated to assess the convective environment proximal to debris flow source regions. Twelve events were analyzed and from this analysis three common synoptic scale circulation patterns were identified. Evaluation of CG flashes at smaller spatial and temporal scales illustrated that convective processes vary in their production of CG flashes (total number) and the spatial distribution of flashes can also be quite different between events over longer periods. During the 60 min interval immediately preceding debris flow a majority of cases exhibited spatial and temporal collocation of LICR and CG flashes. Also a number of CG flash parameters were found to be significantly correlated to rainfall intensity prior to debris flow initiation.

On 4 August 2015, a very high intensity storm, 31.5 mm in 20 min (94.5 mm/h), hit the massif of Mount Antelao on the Venetian Dolomites triggering three stony debris flows characterized by high magnitude. Two of them occurred in the historical sites of Rovina di Cancia and Rudan Creek and were stopped by the retaining works upstream the inhabited areas, while the third routed along the Ru Secco Creek and progressively reached the resort area and the village of San Vito di Cadore, causing fatalities and damages. The main triggering factor of the Ru Secco debris flow was a large rock collapse on the northern cliffs of Mount Antelao occurred the previous autumn. The fallen debris material deposited on the Vallon d'Antrimoia inclined plateau at the base of the collapsed cliffs and, below it, on the Ru Salvela Creek, covering it from the head to the confluence with the Ru Secco Creek. The abundant runoff, caused by the high intensity rainfall on 4 August 2015, entrained about 52,500 m3 of the debris material laying on the Vallon d'Antrimoia forming a debris flow surge that hit and eroded the debris deposit covering the downstream Ru Salvela Creek, increasing its volume, about 110,000 m3 of mobilized sediments. This debris flow routed downstream the confluence, flooding the parking of a resort area where three people died, and reached the village downstream damaging some buildings. A geomorphological analysis was initially carried out after surveying the whole basin. All liquid and solid-liquid contributions to the phenomenon were recognized together with the areas subjected to erosion and deposition. The elaboration of pre and post-event topographical surveys provided the map of deposition-erosion depths. Using the rainfall estimated by weather radar and corrected by the nearest rain gauge, about 0.8 km far, we estimated runoff by using a rainfall-runoff model designed for the headwater rocky basins of Dolomites. A triggering model provided the debris flow hydrographs in the initiation areas, after using the simulated runoff. The initial solid-liquid surge hydrographs were, then, routed downstream by means of a cell model. The comparison between the simulated and estimated deposition-erosion pattern resulted satisfactory. The results of the simulation captured, in fact, the main features of the occurred phenomenon.

A systematic set of flume experiments is used to investigate the features of velocity profiles within the body of coarse-grained debris flows and the dependence of the transport sediment concentration on the relevant parameters (runoff discharge, bed slope, grain size, and form). The flows are generated in a 10 m long laboratory flume, initially filled with a layer consisting of loose debris. After saturation, a prescribed water discharge is suddenly supplied over the granular bed, and the runoff triggers a debris flow wave that reaches nearly steady conditions. Three types of material have been used in the tests: gravel with mean grain size of 3 and 5 mm, and 3 mm glass spheres. Measured parameters included: triggering water discharge, volumetric sediment discharge, sediment concentration, flow depth, and velocity profiles. The dynamic similarity with full-sized debris flows is discussed on the basis of the relevant dimensionless parameters. Concentration data highlight the dependence on the slope angle and the importance of the quasi-static friction angle. The effects of flow rheology on the shape of velocity profiles are analyzed with attention to the role of different stress-generating mechanisms. A remarkable collapse of the dimensionless profiles is obtained by scaling the debris flow velocity with the runoff velocity, and a power law characterization is proposed following a heuristic approach. The shape of the profiles suggests a smooth transition between the different rheological regimes (collisional and frictional) that establish in the upper and lower regions of the flow and is compatible with the presence of multiple length scales dictated by the type of contacts (instantaneous or long lasting) between grains.

A formulation of the two-dimensional shallow water equations adapted to general and complex terrains is proposed. Its derivation starts from the observation that the typical approach of depth integrating the Navier–Stokes equations along the direction of gravity forces is not exact in the general case of a tilted curved bottom. We claim that an integration path that better adapts to the shallow water hypotheses follows the “cross-flow” surface, i.e., a surface that is normal to the velocity field at any point of the domain. Because of the implicitness of this definition, we approximate this “cross-flow” path by performing depth integration along a local direction normal to the bottom surface, and propose a rigorous derivation of this approximation and its numerical solution as an essential step for the future development of the full “cross-flow” integration procedure. We start by defining a local coordinate system, anchored on the bottom surface to derive a covariant form of the Navier–Stokes equations. Depth integration along the local normals yields a covariant version of the shallow water equations, which is characterized by flux functions and source terms that vary in space because of the surface metric coefficients and related derivatives. The proposed model is numerically discretized with a first order FORCE-type Godunov Finite Volume scheme that allows straight forward implementation of spatially variable fluxes. We investigate the validity of our SW model and the effects of the geometrical characteristics of the bottom surface by means of three synthetic test cases that exhibit non negligible slopes and surface curvatures. The results show the importance of taking into consideration bottom geometry even for relatively mild and slowly varying curvatures. By comparison with the numerical solution of vertically integrated models, we observe differences of almost 20%, in particular for the peak values and the shape of the hydrographs calculated at given sections of the fluid domain.

During intense rainstorms, runoff plays a major role in triggering debris flows in the Dolomite region of Italy. Here the landscape is dominated by steep dolomite massifs where rainfall rapidly concentrates in steep headwater catchments. Debris flow channels originate at the outlet of headwater catchments and erode thick talus deposits delivering sediments to the fan. Headwater catchments are typically very steep (45°-60° on the average) and mostly consist of exposed bedrock with no vegetation and sparse to none soil cover. Debris flows occur in the summer season during short and intense thunderstorms. In order to gain a better understanding of debris flow initiation by surface water runoff, we monitored rainfalls and pore pressures in the source area of a small catchment. We also obtained video footage of surface runoff and debris flow initiation from two cameras. Observations show that incoming water infiltrates in highly porous channel bed debris flowing downstream as subsurface stormflow. Only when increasing discharge exceeds the capacity of the channel bed, surface flow appears and debris flows are triggered by progressive erosion. Although we don’t have a measure of water discharge, we can distinguish three types of hydrologic response in the initiation area: subsurface runoff; surface runoff and debris flow occurrence. Data are sufficient to empirically define the debris flow rainfall triggering threshold in the duration-intensity space. By analyzing type and timing of the response in relation to rainfall forcing, we can calibrate a simple hydrologic model based on SCS method to reproduce our observations. The model is well suited to describe the empirical rainfall triggering threshold. Even more simple estimates of concentration time and initial loss would be sufficient to explain the fundamental processes ruling the shape and slope of the rainfall threshold. The relative simplicity of the mechanisms at the base of the hydrologic response of debris flow catchments in the Dolomites suggests that a regionalization of debris flow rainfall threshold should be possible.

Summary A GIS-based cell model, based on a kinematic approach is proposed to simulate debris flow routing on a fan. The sediment–water mixture is modeled as a monophasic continuum, and the flow pattern is discretized by square cells, 1 m in size, that coincide with the DEM cells. Flow occurs from cells with a higher mixture free surface to those with a lower mixture free surface. A uniform-flow law is used if the elevation of the former cell is higher than that of the latter; otherwise, the flow is simulated using the broad-crested weir law. Erosion and deposition are simulated using an empirical law that is adjusted for a monophasic continuum. The sediment concentration in the routing volume is computed at each time step and controls both erosion and deposition. The cell model is used to simulate a debris flow that occurred on the Rio Lazer (Dolomites, North-Eastern Italian Alps) on November 4th, 1966. Furthermore, the hydrologic and the hydraulic conditions for the initiation of debris flow are simulated, providing the solid–liquid hydrograph of the resulting debris flow. A number of simulations has been carried out with physically reasonable parameters. The results are compared with the extension of the debris-flow deposition area and the map of observed depths of deposited sediments. This comparison shows that the proposed model provides good performance. The analysis of sensitivity carried out by systematically varying the model parameters shows that lower performances are associated with parameter values that are not physically reasonable. The same event is also simulated using a cellular automata model and a finite volume two-dimensional model. The results show that the two models provide a sediment deposition pattern less accurate than that provided by the present cell model.

We present the results of a systematic series of 168 laboratory experiments that examine the critical conditions for the failure of landslide dams, which obstruct the course of mountain rivers. The experiments were carried out by using three different sediment materials with a quasiuniform grain size distribution and with a flume bed slope angles that ranged between 0° and 5.71° (0–10%). Three main typologies of dam failure were observed for increasing values of the dam’s downstream-face angle: (1) overtopping; (2) headcutting, which led to the formation of an erosion channel on the dam’s downstream face that progressively migrated up to the dam crest; and (3) initial slide of large part of superficial layer of the dam’s downstream face, which was followed by headcutting. The experiments focused on the second type of failure to provide a safety criterion based on the upstream reservoir level. The quantities that govern this phenomenon have been identified and a functional relationship is proposed based on...

The debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff, triggering a water-sediment mixture flow known as debris flow. The routing of debris flow through human settlements can cause damage to civil structures and loss of human lives. The prediction of such an event, or the runoff discharge that triggers it, assumes an interest in risk analyses and the planning of defence measures. The object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel-bed failure. Historical and rainfall data on 30 debris flows that occurred in six watersheds of the Dolomites (north-eastern Italian Alps) were collected from different sources. Field investigations at the six sites, together with the hydrologic response to the rainfalls that triggered the events, were performed to obtain a realistic scenario of the formation of the debris flow there occurred. Field observations include a survey along the channel of the triggering reach of debris flow, with measurements of the channel slope and cross-section and sampling of debris deposits for grain size distribution. Simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure. The results are discussed to provide a plausible physical-based method for the prediction of the triggering of debris flow by channel-bed failure.

Present work aims to explore the differences in hydrological modeling when using digital elevation models (DEMs) generated by points from LiDAR surveys and those digitized on the contour lines of the regional technical map (RTM) and their relevance for the simulation of debris flow triggering. Hydrological models for mountainous areas are usually based on DEMs. DEMs are used to determine the flow path from each pixel, by which the basin is discretized, to the outlet. Hydrological simulations of runoff that triggered debris flows occurred in two rocky headwater basins of Dolomites, Fiames Dimai (area approximately 0.03 km2), and Cancia (area approximately 0.7 km2) are carried out using a DEM-based model designed for simulating runoff that descends from headwater areas. For each basin, the runoff is simulated using DEMs that are generated using points from LiDAR, and those digitized on the contour lines of the regional technical map, respectively. The results show that the peak discharge values corresponding to the simulations carried out using the LiDAR-based DEMs are higher than those corresponding to the simulations carried out using the RTM-based DEMs. Larger differences are observed for the Dimai basin, where the area corresponding to the RTM-based DEM is markedly smaller than the area corresponding to LiDAR-based DEM, whereas for the Cancia basin, the two areas are similar. Both the differences in the peak discharge and the basin area are due to the poor accuracy of the contour-based DEM (i.e., elevation accuracy), that is, a poor representation of the morphological features that leads to errors on the watershed divide and simplifications of the flow paths from each cell to the outlet. This result is highly relevant for estimating the triggering conditions of runoff generated debris flows. An incorrect simulated value of peak discharge can lead to errors both in planning countermeasures against debris flows and in predicting their occurrence.

In this chapter, we evaluate the movement of 6 points near a landslide body, which were surveyed with GNSS receivers over time. We apply Bayesian inference to identify the areas on the ground with statistically significant vertical (downwards) shifts. Traditional statistical methods work well only when point displacements between different survey epochs are sufficiently large compared to the standard deviations of related coordinates. In such cases, coordinate differences of some points can be marked as potential displacements. The Bayesian analysis can help to improve discrimination when height differences, computed with respect to the first measurement epoch, are at the same order of magnitude as the uncertainties of the measures. After the application of the classical statistical test, one network point, close to the upper part of the landslide area, seemed to be more unstable than the remainder. In order to remove or validate the hypothesis of instability, the Bayesian statistical inference was applied, and all three of the upper group of points show significant shift, depending on the data prior parameters. This application shows that the Bayesian approach can be considered as an integration to classical statistical significance testing (e.g. z-test), reliably showing significance in vertical directional (i.e., downwards) coordinate shifts, thus supporting detection of movements having lower magnitude.

A model for the simulation of shallow landsliding triggered by heavy rainstorms is analysed and discussed. The model is applied in two mountainous catchments in the Dolomites (Eastern Italian Alps): the Cordon catchment (5 km2) and the Vauz catchment (1·9 km2), where field surveys provided a description of hydraulic and geotechnical properties of soils and an inventory of landslide scars is available. The stability mapping procedure, which is similar to that proposed by Montgomery and Dietrich (1994 Water Resources Research30: 1153), combines steady-state hydrologic concepts with the infinite slope stability model. The model provides an estimate of the spatial distribution of the critical rainfall, which is the minimum steady-state rainfall predicted to cause instability. The comparison of the landslides observed in the study basins with model predictions shows that the distribution of critical rainfall obtained from the model provides a surrogate for failure initiation probability as a function of topographic location. Copyright © 2002 John Wiley & Sons, Ltd. Language: en

The instability of a granular slope, with seepage flow and low tail downstream water level, has been experimentally investigated to point out the conditions associated to the formation of a debris flow, excluding the case of the debris flow triggered by a failure due to the toe erosion. The materials employed in the experiments are uniform glass spheres (d = 0.003 m) and two different gravel (d50 = 0.007 m and d50 = 0.0022 m). Experimental runs show that the triggering of debris flow is due to the overland flow.

Laser scanner technology can be very useful for applications relative to surveying river-beds with high and very high resolution (sampling heights at one meter spacing or less). A 3D model of structures including the river-bed, sides and contiguous areas allows to develop rigorous hydraulic models relative to flooding predictions and to the estimation of relative damages. In the field of hydraulic modelling, an important role has been played by 3D modelling when the flooding of the Bacchiglione River in 2010. Laser scanner techniques have been used for the rapid and accurate definition of which areas are prone to high risk and to potential damage in the case of bursting of river banks. The results allow estimating the amount of safety in particular cases which depend on the presence of natural and/or artificial protection structures which prevent devastation of houses or other human-made artefacts by acting as an obstacle to the water flow.

This study proposes relationships between mean streamflow depth and cohesionless particle bed characteristics at the beginning of the sediment transport at high slopes in low submergence conditions. The inception of sediment transport is theoretically approached by the incipient motion condition. A previous theoretical analysis of the equilibrium of a gravel particle based on the exposure of the particle itself to the streamflow is generalized by introducing the relative position of the exposed part of the particle with respect to the bottom and by considering the case of a partially submerged particle. Using a low-submergence flow velocity profile in the particle balance of forces at the limit of equilibrium leads to a relationship between the mean flow depth and the sediment size. Another velocity profile of low-submergence flow is added to that used in the previous study, and two analytical expressions of flow depths corresponding to the beginning of sediment transport are obtained for both fully submerged particles and partially submerged particles. The theoretical flow depths given by the analytical expressions are then compared with those from previous experimental tests at the beginning of sediment transport at high slopes with fully submerged particles. The comparison shows a satisfactory agreement between theoretical and experimental values of flow depth corresponding to the degree of exposure and the relative position of the exposed part of the surface layer bed particles.

[1] This paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream. A theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow. The parameter, relative degree of exposure, relates the positioning of a particle with the upstream one. The problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence. The results of the theory are compared with a series of ad hoc experiments performed by C. Gregoretti.

Debris deposit laying over an impermeable high slope surface (e.g. in steep streams and gullies) could develop a water sediment mixture flow (usually known as debris flow) when a high enough water flow is flowing over the sediment bed. In particular the rainfall first saturates the debris deposit and then mobilizes it by an overland flow. The mechanisms which make unstable the granular material layer in such a case were studied by experimental tests performed in a tilting flume filled with uniformly distributed granular material. The experimental results are compared with theoretical relationships on the occurrence of debris flow and with experimental findings pertinent to sediment transport phenomena. The material employed in the experiments was nearly uniform gravel of three different sizes with mean diameters respectively equal to d = 0.023, 0.029 and 0.034 m.