Evaluating the Differences of Gridding Techniques for Digital Elevation Models Generation and Their Influence on the Modeling of Stony Debris Flows Routing: A Case Study From Rovina di Cancia Basin (North-Eastern Italian Alps)

Debris \ufb02ows are among the most hazardous phenomena in mountain areas. To cope with debris \ufb02ow hazard, it is common to delineate the risk-prone areas through routing models. The most important input to debris \ufb02ow routing models are the topographic data, usually in the form of Digital Elevation Models (DEMs). The quality of DEMs depends on the accuracy, density, and spatial distribution of the sampled points; on the characteristics of the surface; and on the applied gridding methodology. Therefore, the choice of the interpolation method affects the realistic representation of the channel and fan morphology, and thus potentially the debris \ufb02ow routing modeling outcomes. In this paper, we initially investigate the performance of common interpolation methods (i.e., linear triangulation, natural neighbor, nearest neighbor, Inverse Distance to a Power, ANUDEM, Radial Basis Functions, and ordinary kriging) in building DEMs with the complex topography of a debris \ufb02ow channel located in the Venetian Dolomites (North-eastern Italian Alps), by using small footprint full- waveform Light Detection And Ranging (LiDAR) data. The investigation is carried out through a combination of statistical analysis of vertical accuracy, algorithm robustness, and spatial clustering of vertical errors, and multi-criteria shape reliability assessment. After that, we examine the in\ufb02uence of the tested interpolation algorithms on the performance of a Geographic Information System (GIS)-based cell model for simulating stony debris \ufb02ows routing. In detail, we investigate both the correlation between the DEMs heights uncertainty resulting from the gridding procedure and that on the corresponding simulated erosion/deposition depths, both the effect of interpolation algorithms on simulated areas, erosion and deposition volumes, solid-liquid discharges, and channel morphology after the event. The comparison among the tested interpolation methods highlights that the ANUDEM and ordinary kriging algorithms are not suitable for building DEMs with complex topography. Conversely, the linear triangulation, the natural neighbor algorithm, and the thin-plate spline plus tension and completely regularized spline functions ensure the best trade-off among accuracy and shape reliability. Anyway, the evaluation of the effects of gridding techniques on debris \ufb02ow routing modeling reveals that the choice of the interpolation algorithm does not signi\ufb01cantly affect the model outcomes

The debris flow occurred at ru secco creek, venetian dolomites, on 4 august 2015: Analysis of the phenomenon, its characteristics and reproduction by models

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 \ufb02ows 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 \ufb02ow 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\u2019Antrimoia inclined plateau at the base of the collapsed cliffs and, below it, on the Ru Salvela Creek, covering it from the head to the con\ufb02uence 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\u2019Antrimoia forming a debris \ufb02ow 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 \ufb02ow routed downstream the con\ufb02uence, \ufb02ooding 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 \ufb02ow 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 model-based early warning system for runoff-generated debris-flow occurrence: Preliminary results

Early warning systems for debris flows are low cost measures for mitigating this kind of hazard. The early warning systems provide a timely alert for upcoming events in order to take protective measures, such as closing railways-roads, evacuating people from the threatened areas, and put rescue forces into readiness. These systems usually are sensor-based, and the alert time is the interval between the timing of the first detachment of debris flow by a sensor and its arrival into the threatened area. At the purpose of increasing the alert time, we propose an early warning system based on a model-cascade: nowcasting, hydrological- and triggering models. Nowcasting anticipates rainfall pattern that is transformed into runoff by the hydrological model. The triggering model estimates the volume of sediments that the runoff can entrain, and compares it with a critical threshold. If this is exceeded the alert is launched. The proposed early warning system is tested against the available data of the Rovina di Cancia (Northeast Italy) site

Assessing different survey and gridding techniques for Digital Elevation Models generation and the related influence on stony debris flows modelling. A case study from Cancia basin (Venetian Dolomites, North-Eastern Italian Alps)

In the Dolomites area (North-Eastern Italian Alps), debris flows can be regarded as one of the most hazardous geomorphological processes. In the last few years, these natural phenomena amplified their occurrence rate due to the rise of extreme rainfall events, and the increasing availability of debris material yielded by the retreat of the glaciers and the permafrost areas to higher elevations. In order to cope with debris flow hazard, it is common to couple structural and non-structural measurements, such as the zoning of risk prone areas by means of routing models. Since the motion of gravity-driven flows is extremely sensitive to surface morphology, topographic data in the form of Digital Elevation Models (DEMs) represent the most important input in debris flow routing models. As a matter of fact, a DEM can be regarded as a mathematical representation of the bare earth in digital form, and it is commonly used to represent the surface morphology in three dimensions. The “quality” of DEMs depends on the accuracy, density, and spatial distribution of the topographic data (i.e., on the employed survey technique); on the characteristics of the surveyed surface; and on the applied gridding methodology. Therefore, the choice of both the survey technique and the gridding methodology might represent a critical concern for the reliability of routing modeling outcomes. In order to advance in the knowledge regarding the influence of geomatic techniques on the numerical modeling of stony debris flows routing, in the present research we initially assessed the performances of common digital terrain modelling algorithms (i.e., linear triangulation, natural neighbor, nearest neighbor, inverse distance to a power, ANUDEM, completely regularize spline function, thin-plate spline function, thin-plate spline plus tension function, multi-quadratic function, inverse multi-quadratic function, point ordinary kriging, and block ordinary kriging) and survey techniques (i.e., full-waveform Light Detection And Ranging, LiDAR; and Global Navigation Satellite System, GNSS) in characterizing the complex topography of a debris flow channel located in the Venetian Dolomites. After that, their inherent influence on the results of a Geographic Information System (GIS)-based cell model for simulating stony debris flows routing is investigated through a combination of statistical and visual techniques, by considering both high- and low-magnitude flow conditions. On one and, the research points out that the linear triangulation, the natural neighbor algorithm, and the thin-plate spline plus tension and completely regularized spline basis functions could represent the best choice for applications relying on the proper representation of the surface shape (e.g., hydraulic and hydrological modeling). In fact, these gridding algorithms proved to ensure the best trade-off between interpolation accuracy and shape reliability. However, the research also shows that the choice of the gridding methodology actually does not represent a determining factor in debris flows routing modeling. On the other hand, the analysis carried out on the capability of the two tested survey techniques in characterizing the topography of the studied debris flow channel, highlights a high degree of interoperability, since both of them could be used to generate bias-free and accurate high-resolution DEMs of morphologically complex areas. However, the pairwise comparison of the GNSS- and LiDAR-derived DEMs reveals that, although the two investigated survey techniques provide a comparable (i.e., not statistically different) topographic characterization of the channel bathymetry, meaningful vertical discrepancies could be detected in correspondence of morphologically complex channel features (e.g., channel banks and longitudinal/transversal slope discontinuities). Furthermore, the detected discrepancies proved to be able to affect the cell routing model behavior, thus leading to the conclusion that the choice of the survey technique could represent a critical concern for the reliability of routing modeling outcomes

Does the topographic data source truly influence the routing modelling of debris flows in a torrent catchment?

To cope with debris-flow hazards, a common practice is the mapping of threatenedareas through routing models. Considering the primary role of topography in affect-ing the mobility of gravity-driven flows, its proper representation through digital ele-vation models (DEMs) is a requirement in routing modelling applications. The‘quality’of DEMs mainly depends on the quality, resolution and spatial arrangementof the topographic measurements (i.e. on the employed survey technology). Never-theless, no attempt to systematically evaluate the influence of the topographic datasource on the behaviour of routing models has been carried out. To address this, weinitially assess the performances of both terrestrial- (i.e. global navigation satellitesystem, GNSS) and airborne-based (i.e. full-waveform LiDAR and structure-from-motion, SfM digital photogrammetry) survey technologies in characterizing thetopography of a debris-flow channel. Afterwards, we investigate whether the topo-graphic data source can effectively influence the behaviour of a geographic informa-tion system (GIS)-based cell routing model. Regarding the assessment of the surveytechnologies performances, the‘standard’statistic-based approach indicated thatGNSS and full-waveform LiDAR can provide an accurate digital representation of thegully. However, the analysis of the shapes stressed that the most faithful and finerreproduction of the topographic singularities is yielded by the photogrammetricallyreconstructed surface due to the extremely high data source resolution. Furthermore,the pairwise comparison of derived elevation models pointed out that meaningfuldiscrepancies among tested survey technologies can be detected in morphologicallycomplex areas because of the inherent limits of the terrestrial-based method. Here,this research showed how these discrepancies have the potential to affect simulatedflow dynamics, even if not in a meaningful way from a risk planning and managementpoint of view. Overall, it appears that the topographic data source does not truly rep-resent a determining factor in modelling applications of channelized debris-flowrouting

10th Alexander von Humboldt Conference 2015

Four occurred events of debris flows are documented and modeled by back-analysis. The four debris flows events are those occurred at Rio Lazer on the 4th of November 1966, at Fiames on the 5th of July 2006, at Rovina di Cancia on the 18th of July 2009 and at Baselga di Pin\ue8 on the 15th of August 2010. All the four sites are located in the North-Eastern Italian Alps. In all the events, runoff entrained sediments present on natural channels and formed a solid-liquid wave that routed downstream. The first event concerns the routing of debris flow on an inhabited fan. Map of deposition pattern of sediments is built by using post-events photos through stereoscopy techniques. The second event concerns the routing of debris flow along the main channel descending from Pomagagnon Fork. Due to the obstruction of the cross-section debris flow deviated from the original path on the left side and routed downstream by cutting a new channel on the fan. It dispersed in multiple paths when met the wooden area. Map of erosion and deposition depths is built after using a combination of LiDAR and GPS data. The third event concerns the routing of debris flow in the Rovina di Cancia channel that filled the reservoir built at the end of the channel and locally overtopped the retaining wall on the left side. A wave of mud and debris inundated the area downstream the overtopping point. Map of erosion and deposition depths is obtained by subtracting two GPS surveys, pre and post event. The fourth event is that of debris flow occurred along Rio Val Molinara where runoff entrained bed sediments and routed downstream flooding the village of Baselga di Pin\ue8. The map of erosion and deposition depths of Rio Val Molinara is obtained after comparing two LiDAR data corresponding to the pre and post situation while the map of deposition depth of the village is built through carefully analysis of the photo taken after the event with field measurements. All the four occurred debris flows are simulated by modeling runoff that entrained debris flow for determining the solid-liquid hydrograph downstream the triggering areas. The routing of the solid-liquid hydrograph is simulated by a bi-phase cell model based on the kinematic approach. The comparison between simulated and measured erosion and deposition depths is satisfactory. Nearly the same parameters for computing erosion and deposition are used for all the four occurred events

GIS-based cell model for simulating debris flow runout on a fan

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