Torrential Fans Research Articles

Vulnerability assessment for buildings exposed to torrential hazards at Sichuan-Tibet transportation corridor

The construction and operation of the Sichuan-Tibet railway require constructing extensive supporting facilities along its route. Sichuan-Tibet transportation corridor features high mountains and deep valleys where land resources suitable for such purposes are very limited. As a result, safe utilization of the torrential fan along the railway becomes essential. Focus on buildings at the torrential fan, this paper aims to develop a vulnerability model that can assess their maximum potential damage and support the planning and development of the torrential fan. The model was developed based on 51 building damage data collected at 11 torrential fans. The hazard intensity was defined by the geometric configuration of torrential fans and the location of assessed buildings. In addition, a physical experiment was carried out to study the shielding effect of building groups. The results show that the shielding effect range is about 25 times the building size, and the maximum reduction in impact force can reach 60%. The advantage of the proposed vulnerability assessment model is that it only requires the spatial information of the site that can be easily derived from the satellite image. In addition, the model can reflect the complex torrential process and the shielding effect of building groups. This study provides an alternative approach to building vulnerability assessment. It supports the planning and development of torrential fans when constructing the supporting facilities for the railway construction and operation.

Modeling and Classification of Alluvial Fans with DEMs and Machine Learning Methods: A Case Study of Slovenian Torrential Fans

Alluvial (torrential) fans, especially those created from debris-flow activity, often endanger built environments and human life. It is well known that these kinds of territories where human activities are favored are characterized by increasing instability and related hydrological risk; therefore, treating the problem of its assessment and management is becoming strongly relevant. The aim of this study was to analyze and model the geomorphological aspects and the physical processes of alluvial fans in relation to the environmental characteristics of the territory for classification and prediction purposes. The main geomorphometric parameters capable of describing complex properties, such as relative fan position depending on the neighborhood, which can affect their formation or shape, or properties delineating specific parts of fans, were identified and evaluated through digital elevation model (DEM) data. Five machine learning (ML) methods, including a hybrid Euler graph ML method, were compared to analyze the geomorphometric parameters and physical characteristics of alluvial fans. The results obtained in 14 case studies of Slovenian torrential fans, validated with data of the empirical model proposed by Bertrand et al. (2013), confirm the validity of the developed method and the possibility to identify alluvial fans that can be considered as debris-flow prone.

Traditional versus modern settlement on torrential alluvial fans considering the danger of debris flows: a case study of the Upper Sava Valley (NW Slovenia)

Abstract We analysed spatial development of traditional and modern settlements on active alluvial fans in the Upper Sava Valley (NW Slovenia), by using old cadastral data from the beginning of the 19th century, time series of aerial photographs from the middle of the 20th century and recent building cadastre. The valley is surrounded by the mountainous Julian Alps in the south and the Karavanke Mountains in the north where there is a lack of space for settlements due to steep slopes that are increasing the danger of slope processes, torrential processes and floods. By using a very high-resolution 1m LiDAR digital elevation model, we defined the morphometry of alluvial fans and the characteristics of the drainage system of contributing tributaries. We classified the areas according to the threat posed by the modelled torrents and debris flows. We analysed the resilience of settlement in different periods from this perspective and evaluated the integration of natural processes effects in modern spatial planning projects. We found that geomorphic processes threaten a relatively large proportion of some new and old settlements on alluvial fans and that safe planning of areas of settlement has been successful neither in the past nor in recent decades.

Geomorphic control on regional glacier lake outburst flood and debris flow activity over northern Tien Shan

Abstract Glacier lake outburst floods (GLOFs) and related debris flows (DF) are significant natural threats in the Tien Shan Mountains. Their occurrence is favoured by the formation of new glacier lakes and the destabilization of moraines and hillslopes due to climate warming. Understanding the frequency-magnitude of these processes is essential for the implementation of Disaster Risk Reduction strategies. Yet, long-term records of past GLOFs or DF in the region are almost completely missing, which renders rational hazard and risk assessments difficult. Here, we present a unique, multi-century dataset of regional GLOF-DF reconstruction for the Tien Shan based on tree-ring analyses from six different torrential fans, and provide insights on regional processes activity. Based on this dataset, we also test whether GLOF-DF activity is related to glacier degradation and changes in geomorphic characteristics at the catchment scale. Results from >430 disturbed trees growing on six different DF fans suggest frequent GLOF-DF activity since the 19th century, which is consistent with available historical records. We also observe an increase in process activity during the mid-20th century coinciding with phases of glacier stagnation or even slight glacier advances. This means GLOF-DF activity in northern Tien Shan is inversely related to moraine-glacier ratios as well as to glacier area shrinkage rates and fan characteristics (such as slope and depositional area). These findings imply that glaciological and geomorphic features could be used for regional susceptibility assessments in the future. Results presented here are the longest, annually resolved GLOF-DF series in the region, if not worldwide, and constitute a unique dataset to understand process variability. As such, they should be used for further climate change adaptation to mitigate natural hazard and risk in the region.

Impact of a Random Sequence of Debris Flows on Torrential Fan Formation

Debris flows with different magnitudes can have a large impact on debris fan characteristics such as height or slope. Moreover, knowledge about the impact of random sequences of debris flows of different magnitudes on debris fan properties is sparse in the literature and can be improved using numerical simulations of debris fan formation. Therefore, in this paper we present the results of numerical simulations wherein we investigated the impact of a random sequence of debris flows on torrential fan formation, where the total volume of transported debris was kept constant, but different rheological properties were used. Overall, 62 debris flow events with different magnitudes from 100 m3 to 20,000 m3 were selected, and the total volume was approximately 225,000 m3. The sequence of these debris flows was randomly generated, and selected debris fan characteristics after the 62 events were compared. For modeling purposes, we applied the Rapid Mass Movement Simulations (RAMMS) software and its debris flow module (RAMMS-DF). The modeling was carried out using (a) real fan topography from an alpine environment (i.e., an actual debris fan in north-west (NW) Slovenia formed by the Suhelj torrent) and (b) an artificial surface with a constant slope. Several RAMMS model parameters were tested. The simulation results confirm that the random sequence of debris flow events has only some minor effects on the fan formation (e.g., slope, maximum height), even when changing debris flow rheological properties in a wide range. After the 62 events, independent of the selected sequence of debris flows, the final fan characteristics were not significantly different from each other. Mann–Whitney (MW) tests and t-tests were used for this purpose, and the selected significance level was 0.05. Moreover, this conclusion applies for artificial and real terrain and for a wide range of tested RAMMS model rheological parameters. Further testing of the RAMMS-DF model in real situations is proposed in order to better understand its applicability and limitations under real conditions for debris flow hazard assessment or the planning of mitigation measures.

Understanding impact dynamics on buildings caused by fluviatile sediment transport

Understanding the impact dynamics on buildings caused by complex fluviatile sediment transport processes and the interaction with the exposed elements is still a major challenge. Especially flood hazards with large amount of mobilized sediments can cause severe damages to exposed building structures located within flood-prone torrential fans. Mitigation efforts, be they rooted in spatial planning or in local structural protection, may benefit from better insights into impact dynamics. In order to gain a better understanding of the interaction between torrential processes and buildings, a physical scale model was set up at a 1:30 scale to measure the impact forces on buildings and to comprehensively assess the determining factors. This model mirrors a part of the torrential fan of the Schnannerbach Torrent (Eastern European Alps, Austria). It includes its steep torrent channel and the adjacent floodplain, hosting complex building structures equipped with measurement devices on the wall elements to measure the three-dimensional impact forces of the torrential process. The experiments indicated that the impact forces depend mainly on the dynamics of the sediment deposition processes on the floodplain that are also influenced by the spatial distribution of the building structures, and less on the intensities of the analysed flood events and the grain size distributions of the transported and deposited sediments. The results of our study highlight the need to consider the interaction between the natural process and the built environment as well as the spatial distribution of buildings and their mutual interaction (shadowing vs. channelization) in flood risk management.

Experimental analyses of impact forces on buildings exposed to fluvial hazards

Fluvial processes have a significant impact on buildings and infrastructure and despite their local character they cause annually considerable costs in mountain areas worldwide.Vulnerability studies are based mainly on empirical data from past events whereas laboratory studies investigating the impact forces on buildings are still limited.The paper presents a study on impact forces resulting from complex flow processes on buildings, the influencing factors they depend on and the identification of process parameters they correlate with.A 1:30 physical scale model was built, representing the torrential fan of the Schnannerbach torrent (Austria). Measurement devices, installed on the building structures located on the torrential fan, recorded impact forces induced by a set of fluviatile flood scenarios. The measured impact forces were compared to computation results using recently developed calculation approaches and to measurements resulting from experiments with a simplified, homogeneous scale model set-up with a single wall element in a rectangular flume. The results show a clear correlation between the approaching flow heights and the impact forces on exposed buildings. Compared to clear water conditions, bed-load transport and deposition processes highly influence the impact forces due to sediment accumulations close to the buildings on the torrential fan. The gained insights contribute to a better understanding of potentially damage-causing impacts on buildings resulting from the interaction of flood processes and the exposed built environment.

Association Amelanchiero ovalis-Pinetum mugo in northwestern Slovenia

We conducted a phytosociological study of dwarf pine stands on torrential fans and glacial material (moraine, till) in cirques of several Alpine valleys (Tolminka, Možnica) and on steep, shady erosion slopes (Struje above the Zadlaščica valley) in the belt of montane beech forests in the Julian Alps. Based on comparisons with similar communities in northern Italy we classified these stands into the association Amelanchiero-Pinetum mugo and into the new geographical variant with Rhamnus fallax. Floristically slightly similar dwarf pine stands in the altimontane and subalpine belts on sunny slopes of the Tolmin-Bohinj Mts. are classified into the new variant Rhodothamno-Pinetum mugo typicum var. Genista radiata.

Geomorphology and kinematics of debris flows with high entrainment rates: A case study in the South French Alps

Debris flows consist of a fully saturated mixture of water, sediment and debris that can travel a distance of several kilometers. The entrainment processes occurring during the run-out phase are little discussed in the literature, especially from the discussion of observed events. In the early evening of 5 August 2003, a large debris flow occurred at the Faucon torrent in the Barcelonnette Basin (South French Alps). This debris flow caused substantial damage to several buildings located on the torrential fan. Some morphological aspects of this debris-flow event are presented through a careful field survey and the characterization of the transported sediment. First, the observations show that the triggering area corresponds to a strong incision in scree slopes; the grain-size distribution of the debris-flow deposits in this area is typical of granular debris flows. Second, the total volume of the debris-flow was estimated at 60,000 m 3. Channel scouring is responsible of the large difference between the mobilized volume and the deposited volume; an entrainment rate of 15 m 3.min −1 is estimated. Laboratory tests show that the 2003 debris-flow event started as a granular flow and then transformed into a muddy debris flow by incorporating clay-rich marly sediments along the flow track.

Estimation of magnitudes of debris flows in selected torrential watersheds in Slovenia

In this paper the application of different methods for estimation of magnitudes of rainfall-induced debris flows in 18 torrents in the Upper Sava River valley, NW Slovenia, and in 2 torrents in Pohorje, N Slovenia is described. Additional verification of the methods was performed in the torrential watersheds with active debris flows in the recent past (Predelica and Brusnik in the Soca River basin, W Slovenia). For some of the methods, the knowledge of morphometric characteristics of a torrential watershed, torrential channel and torrential fan is enough. For other methods, a mathematical tool (HEC-HMS) had to be applied in order to develop a hydrologic run-off model of precipitation that can trigger debris flows. Computed debris-flow magnitudes were of the order between 6,500 m3 and 340,000 m3. Their values are a function of torrential watershed parameters, such as: watershed area, Melton number, fan gradient, and torrential channel gradient. The investigated fans were classified into 3 groups with regard to the debris-flow hazard: debris-flow fans (hazard exists), torrential fans (no hazard), and transitional fans (debris flows are possible, but with low possibility). A limit between debris-flow fans and torrential fans is proposed: Melton number 0.3 and torrential fan gradient 4°, that is, 7%. Out of 24 investigated torrential fans, 13 fans were classified into the group of debris-flow fans, 5 fans were classified into the group of torrential fans, and the rest 6 fans were classified into the group of transitional fans.

Paleoecological constraints on reef-coral morphologies in the Tortonian–early Messinian of the Lorca Basin, SE Spain

Abstract Coral reefs represent one of the main carbonate factories that contributed to the control of the stratigraphic architecture of carbonate platforms, which had a widespread development during the late Miocene in the paleo-Mediterranean area. The late Miocene reef complexes of the Lorca Basin in southeastern Spain are composed of five mixed siliciclastic/carbonate units, middle Tortonian to early Messinian in age. The development of coral reefs probably ceased when the first evaporitic event occurred in the basin centre in the early Messinian. This study mainly focuses on the response of reef communities and the modifications of reef organisation to global and regional parameters. At the platform scale, the carbonates are intermixed with terrigenous deposits related to two main types of clastic systems: torrential fans and fluvial to deltaic systems. The amount of clastic input greatly affected reef growth and coral morphologies. Three different types of stratal geometries were delineated in the reef complex: sigmoids, bioherms, and patches and carpets. The reef frameworks are mainly constructed by a poorly diversified assemblage of corals composed of poritids, faviids, and mussids. Porites is the principal reef builder of the sigmoids and carpets where it is widely distributed. Tarbellastraea is common in bioherms and Acanthrastraea appears generally associated with Porites in patches. Five basic growth forms of Porites are observed: thin branching or “finger-shaped”, thick branching to columnar, domed to hemispheric, encrusting, and platy to dish. Differences in coral morphology are used to define a relative water depth zonation in monogeneric reefs. The distribution of these growth forms was principally controlled by water depth. The reef flat is dominated by small thin branching or finger-shaped corals that are replaced towards the reef front by domed to hemispheric corals commonly encrusted by coralline algae. Downslope, columnar morphologies grade into thin branching shapes. The reef morphologies are variable throughout the five mixed siliciclastic/carbonate units at the platform scale. The first and oldest unit is dominated by bioclasts, whereas units 2, 3, and 5 are Porites-dominated, sigmoid complexes. Unit 4 is a well-developed biohermal complex mainly composed of Tarbellastraea. These units started to develop as early as middle Tortonian and stopped as late as early Messinian, and show a progradational trend, where the two latest units are well developed. Thus, carbonate production changed from grain-producing biota in the basal unit to framework-producing biota in the overlying units, consistent with evolution from a distally steepened ramp to a reef-rimmed shelf. At the scale of individual reef units, the relative water depth zonation of the corals is controlled by ecological changes (substrate, nutrients, synecologic relations, and diversification of coral species). In the transects across the carbonate platform related to the different units, the coral zonation records changes in spatial distribution of corals in response to ecological stresses and changes in regional and global environments (tectonic, relative sea-level changes, and runoff).

Paleoecological constraints on reef-coral morphologies in the Tortonian–early Messinian of the Lorca Basin, SE Spain

Coral reefs represent one of the main carbonate factories that contributed to the control of the stratigraphic architecture of carbonate platforms, which had a widespread development during the late Miocene in the paleo-Mediterranean area. The late Miocene reef complexes of the Lorca Basin in southeastern Spain are composed of five mixed siliciclastic/carbonate units, middle Tortonian to early Messinian in age. The development of coral reefs probably ceased when the first evaporitic event occurred in the basin centre in the early Messinian. This study mainly focuses on the response of reef communities and the modifications of reef organisation to global and regional parameters. At the platform scale, the carbonates are intermixed with terrigenous deposits related to two main types of clastic systems: torrential fans and fluvial to deltaic systems. The amount of clastic input greatly affected reef growth and coral morphologies. Three different types of stratal geometries were delineated in the reef complex: sigmoids, bioherms, and patches and carpets. The reef frameworks are mainly constructed by a poorly diversified assemblage of corals composed of poritids, faviids, and mussids. Porites is the principal reef builder of the sigmoids and carpets where it is widely distributed. Tarbellastraea is common in bioherms and Acanthrastraea appears generally associated with Porites in patches. Five basic growth forms of Porites are observed: thin branching or “finger-shaped”, thick branching to columnar, domed to hemispheric, encrusting, and platy to dish. Differences in coral morphology are used to define a relative water depth zonation in monogeneric reefs. The distribution of these growth forms was principally controlled by water depth. The reef flat is dominated by small thin branching or finger-shaped corals that are replaced towards the reef front by domed to hemispheric corals commonly encrusted by coralline algae. Downslope, columnar morphologies grade into thin branching shapes. The reef morphologies are variable throughout the five mixed siliciclastic/carbonate units at the platform scale. The first and oldest unit is dominated by bioclasts, whereas units 2, 3, and 5 are Porites-dominated, sigmoid complexes. Unit 4 is a well-developed biohermal complex mainly composed of Tarbellastraea. These units started to develop as early as middle Tortonian and stopped as late as early Messinian, and show a progradational trend, where the two latest units are well developed. Thus, carbonate production changed from grain-producing biota in the basal unit to framework-producing biota in the overlying units, consistent with evolution from a distally steepened ramp to a reef-rimmed shelf. At the scale of individual reef units, the relative water depth zonation of the corals is controlled by ecological changes (substrate, nutrients, synecologic relations, and diversification of coral species). In the transects across the carbonate platform related to the different units, the coral zonation records changes in spatial distribution of corals in response to ecological stresses and changes in regional and global environments (tectonic, relative sea-level changes, and runoff).

Sur l'hydrogéomorphologie d'un espace à forte anthropisation urbaine : le site de Nı̂mes (Languedoc, France) du Pléistocène supérieur à l'Antiquité ; impacts postérieurs

Abstract Hydrogeomorphological, archaeological and geohistorical integrated studies of the very urbanised Nimes area attest of piedmont slopes and torrential alluvial fan existence from the beginning of digging of the Vistre valley since the end of the ancient Pleistocene. The urbanisation is situated on top of a major superior Pleistocene–Holocene alluvial fan. This fan existed during the old Antiquity, and has been widely truncated by the anthropisation during the High Empire. This fan is connected to other little ones, and becomes a colluvial–alluvial fan landform connected to the present flood plain of the Vistre River. It is still functioning now, as during the recent flash flood disaster of 3 October 1988.

Plio-Quaternary geodynamic evolution of a segment of the Peruvian Andean Cordillera located above the change in the subduction geometry: the Cuzco region

The Cuzco region, which is located above a change in subduction geometry, appears to be characterized by a variable Plio-Quaternary tectono-sedimentary evolution essentially located along the major fault system that separates the High Plateaux from the Eastern Cordillera. After the higher surface formation of the High Plateaux, a set of Neogene basins were filled by Miocene “ fluvio-torrential” series and by Plio-Pleistocene fluvio-lacustrine deposits. The Neogene series have been affected by compressional tectonic forces attributed to the Late Miocene. This compression is followed by roughly E-W trending syn-sedimentary extensional tectonics attributed to the Pliocene; it is related to reactivation of the pre-existing major faults, basin evolution, and volcanic activity concentrated along the faults. In the Early Pleistocene, fluvio-lacustrine deposits are affected by syn- and post-sedimentary compressional tectonism it is characterized by shortening that trends both N-S and E-W and produces folding and faulting of the sedimentary cover. Extensional tectonism trending roughly N-S has been taking place from the Middle Pleistocene to the Present; it is coeval with shoshonitic volcanic activity, and with sedimentation of fluvio-lacustrine terraces, torrential fans and moraines. Quaternary and active normal faults due to this tectonism, are located in a narrow zone more than 100 km-long between the High Plateaux and the Eastern Cordillera, and two 15 km-long fault sectors in the Eastern Cordillera. Characteristic Pleistocene scarps, 400 m or more high, are due to the cumulative normal offset, and there are also little scarps, with heights ranging between 2 and 20 m, which are related to Holocene fault reactivations. Recent fault reactivation on the Cuzco fault system, during the April 5, 1986 earthquake ( m b = 5.3), is due to the N-S trending extension. This state of stress, located at a mean elevation of roughly 3730 m, is generally homogeneous to different scales. The active Cuzco normal faults may be a consequence of adjustment between the compensated Western Cordillera and the undercompensated Eastern Cordillera, this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge. The variation of the state of stress, during the Plio-Quaternary is in agreement with the variations of the compressional boundary forces. It may be explained by variation of the convergence rate or by the variation of pull-slab forces.

Engineering geology and mapping of the Quaternary deposits of the city of Xanthi, N. Greece

Abstract The city of Xanthi is situated on the border between the crystalline massif of Rodope and the extended coastal plain of Thrace. The broader urban area is covered mainly by Quaternary formations of screes and talus deposits from the mountain slopes, various generations of alluvial deposits in the form of torrential fans, recent fluvio-torrential deposits and eluvial sheets on marly slopes. From data collected from natural ground cuts, boreholes, excavations for foundations and some laboratory testing, the area has been divided into four main geotechnical zones and a number of subzones. The soils in the main zones have similar or analogous geotechnical behaviour. A folio of engineering geology and hydrogeology maps was produced. This study shows how a reliable geotechnical evaluation and zoning in Quaternary formations can be approached by detailed engineering geological mapping, mainly for the types of terrain found in the Xanthi area, where coarse materials are predominant.