196,009 research outputs found
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)
No full textIn 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
Probabilistic Identification of Debris‐Flow Pathways in Mountain Fans Within a Stochastic Framework
No full textModel-based approach for design and performance evaluation of works controlling stony debris flows with an application to a case study at Rovina di Cancia (Venetian Dolomites, Northeast Italy)
No full textThis 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 gen-
eration, 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 over-
flowed 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. Lon-
gitudinal 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 sec-
tional 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 trans-
ported 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
Deposition areas: An effective solution for the reduction of the sediment volume transported by stony debris flows on the high-sloping reach of channels incising fans and debris cones
No full textThe main approach to managing the volume of sediment transported by stony debris flows routing along channels is through retention basins and open check dams, usually built in the lower reach just upstream of inhabited areas where slopes are gentler. In some cases, these measures are not sufficient to retain all the volume of sediment transported by debris flows. Works for trapping the sediments should also be placed in the upper reach of debris-flow channels. In this area, where the channel bed is characterized by very high slopes and vertical variability over time, constructing and maintaining transversal embankments for sediment retention is difficult. In addition, they could also be susceptible to failure, potentially increasing the magnitude of the phenomenon instead of mitigating it. On the other hand, other types of works, such as solid-body check dams or net barriers, have a decreasing efficiency over time in reducing the volume of sediment transported by debris flows. An alternative solution could be a retention basin open on the downstream side, that is, without the ending transversal embankment or berm. Therefore, it can be designated as a deposition area because the retention effect of the downstream embankment is missing, and sediment deposition only occurs due to the flatness of the basin. For sizing purposes, here, we derive two relationships between the deposition area and the volume of the deposit. These two relationships are derived from a physically based geometrical approach and empirical approach, respectively. After analyzing the morphology of the depositional processes that occurred in an existing deposition area during debris-flow events, we test the validity of the two relationships. Overall, these can be used independently or in combination for designing a deposition area. At last, we introduce the necessary geomorphological conditions for the construction of the deposition area and guidance for its placement.Deposition areas are retention basins without a dyke on the downstream side, which are placed in the high-sloping reaches of the debris-flow channels. In the present work, we propose two relationships, the former derived by a physically based geometrical approach and the latter from an empirical approach, for the sizing of a deposition area. Both the relationships are successfully tested against data of sediment deposits occurred during debris-flow events on the existing deposition area of Rovina di Cancia (Dolomites, northeast Italy)
Observation of initiation conditions and role of sediment availability in runoff-generated debris flows at Cancia (North Eastern Italian Alps)
No full textIn alpine areas, 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. Runoff-generated debris flows are common in alpine areas but were observed also in recently burned steeplands, steep volcanic terrain and other environments. In any case, the process of channel infilling assumes notable relevance for debris flow initiation in the vast majority of basins, where sediment supply cannot be considered actually unlimited. Sediment recharge mechanisms include rock fall and dry ravell processes following debris flow, channel-bank failures, bed load by water flow and/or very small debris flows.
Here we document debris flow activity that took place in an active debris flow basin of the eastern Italian Alps 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 topographical surveys, distributed rainfall measurements, time-lapse (2 sec) 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. Given that events were closely spaced in time (16 days), we can document the impact of debris availability on flow dynamics and magnitude. Our data clearly illustrate how debris entrainmen t along the channel substantially contributes to the overall mobilized volume. The surging nature of the flow is observed at short distance from the initiation area where mobilized sediment is still limited compared to the event volume. Elevation-change models show that erosion is not evenly distributed along the flow path. It deepens the channel bed with maximum values of about 7 m and clearly dominates where the channel slope exceeds 20°. Further downstream, it is common to have sediment accumulation and depletion that occur alternately for the two successive events. This behavior indicates that sediment availability along the channel strongly influences the flow along the prevailing-transport reach. Most of the sediment deposited by the July event along this reach was entrained by the event that followed. Most of the sediment deposited by the August event was deposited where erosion by the previous event was more severe. Whenever deposition becomes dominant, the channel rapidly fills up with sediment producing potential flow avulsion. The comparison between the simulated water discharge and the overall volume of sediment mobilized by the two debris flows allows constraining their solid concentration
Runoff-generated debris flows: Observation of initiation conditions and erosion–deposition dynamics along the channel at Cancia (eastern Italian Alps)
No full textIn 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
Rainfall‐Runoff Modeling in Rocky Headwater Catchments for the Prediction of Debris Flow Occurrence
No full textIn the Dolomites, steep rocky cliffs are marked by numerous narrow gullies. When high-intensity short-duration precipitation occurs, these gullies concentrate and direct surface runoff to the screes at the foot of rock cliffs. Surface runoff mixes with loose sediments, creating a solid-liquid surge that, as it moves downhill, increases its volume entraining debris material and transforms into a granular debris flow. Given the ongoing challenge of modeling the relationship between intense rainfall, surface runoff, and debris flow initiation, we take advantage of data from three monitoring stations operating in distinct debris flow active catchments in our study area to make progress. These stations, strategically positioned close to debris flows initiation zones, record videos and different types of flow-stage data, helping us pinpoint the timing and form of incoming discharge hydrographs. Over a 15-year period of observation, we collected a comprehensive data set on runoff and mass mov..
GIS-based cell model for simulating debris flow runout on a fan
No full textA 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
Increment of stony debris flow events on the Boite Valley (Venetian Dolomites, Northeastern Italian Alps) and characterization of the extreme rainfalls triggering them
No full textAn alternative approach for the sediment control of in-channel stony debris flows with an application to the case study of the Ru Secco Creek (Venetian Dolomites, Northeast Italy) Italy
No full textThe sediment control for reducing the debris-flow hazard is generally approached using retention basins that can be closed or have an outlet structure, generally an open check dam. They are usually placed in mild slope zones that allow the minimum works for the excavation and the foundation of the outlet structure if present. Recently, it has been shown that the storage of sediments can also be achieved in the high-sloping reaches of debris-flow channels by means of deposition areas, basins that are open on the downstream side. Therefore, in this work, we propose an approach for the control of the sediment volume transported by debris flows consisting of a cascade of deposition areas and retention basins. We also include a framework for the planning, sizing and checking of the works. Two scenarios are considered, corresponding to the maximum values of the debris-flow peak discharge and volume, respectively. Moreover, the presence or absence of boulders is also considered. For this purpose, a method that evaluates the clogging of a single open check dam as a function of the coarse fraction of the sediment volume is simply extended to the case of multiple dams and implemented in a routing model. The proposed approach is applied along the Ru Secco Creek (northeast Italy) for the purpose of defending, from the debris-flow hazard, a resort area and a village hit in 2015 by a high-magnitude debris flow. After a careful survey and the study of the basin, a solution with a combination of deposition areas and retention basins is planned and sized. The validity and performance of the proposed solution are analyzed by means of debris-flow modelling for the two scenarios, considering both the absence and presence of boulders. Most of the sediment volume transported by debris flows is trapped and a small solid discharge flows downstream of the works
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