Illgraben Catchment Research Articles

A new method for detailed discharge and volume measurements of debris flows based on high-frequency 3D LiDAR point clouds; Illgraben, Switzerland

Debris flows are one of the most dangerous landslide types in mountainous regions. Their destructiveness is strongly controlled by their high peak discharge, which can be orders of magnitude larger than for floods. In order to reduce the associated hazards, detailed field measurements of natural debris flows are required to better constrain discharge and volume of these events. In this study, we used a high-frequency 3D LiDAR (light detection and ranging) scanner in combination with video cameras to measure key properties of a debris flow that occurred in the Illgraben catchment (Switzerland). Based on the LiDAR and video data, we directly measured i) front velocity ii) surge velocity iii) surface velocity and iv) cross-sectional area at sub-second intervals. We then estimated discharge and volume using these direct measurements and considering different channel bed geometry scenarios. We compared our results to estimates based on conventional methods and found that these more established methods substantially underestimate the (peak) discharge and volume for this event. Our results will be assessed in the future by analyzing more events, but our LiDAR-based method has the potential to provide much more detailed information on hazard-related debris-flow parameters, which will have important implications for understanding debris-flow processes and ultimately reducing their risk.

Inferring spatial variations in velocity profiles and bed geometry of natural debris flows based on discharge estimates from high-frequency 3D LiDAR point clouds; Illgraben, Switzerland

More detailed field measurements are required for a better understanding of surging debris flows. In this work, we analyze a debris flow at the field-scale using timelapse point clouds from a high-resolution, high-frequency 3D LiDAR sensor, which has been installed over a check dam on the fan of the Illgraben catchment in Switzerland. In our investigations, we manually measured the front velocity and tracked individual features such as large boulders and woody debris over a 25 m long channel segment. We observed a change in the front velocity as well as a difference in the velocity of large boulders and woody debris (vboulder≈ 0.6vwood) during the second surge of the event. We also estimated the discharge for different closely spaced channel sections based on automated measurements of the cross-sectional area and the surface velocity, which enabled us to infer spatial variations in the bed geometry and the velocity profile. From the discharge estimates, we then derived the volume of this event. Over the course of the next year, the amount of field-scale LiDAR data from the Illgraben will increase substantially and allow for an even more detailed analysis of fundamental debris-flow processes.

Infrasonic and Seismic Analysis of Debris‐Flow Events at Illgraben (Switzerland): Relating Signal Features to Flow Parameters and to the Seismo‐Acoustic Source Mechanism

AbstractDebris flows are among the most dangerous phenomena in mountain environment. Recently the use of seismic and infrasonic recordings has proven to be a powerful tool for studying and monitoring debris flows. However, open questions remain about how signal characteristics refllect flow parameters and fluid dynamic processes. We present a seismo‐acoustic analysis of the debris flow activity between 2017 and 2019 at the Illgraben catchment (Switzerland). Seismic and infrasonic amplitudes (maximum root mean square amplitude [RMSA]) and peak frequencies are compared with flow measurements (front velocity, maximum flow depth and density). Front velocity, maximum depth, peak discharge and peak mass flux show a positive correlation with both infrasonic and seismic maximum RMSA, suggesting that seismo‐acoustic amplitudes are influenced by these flow parameters. Comparison between seismo‐acoustic peak frequencies and flow parameters reveals that, unlike seismic signals, characterized by a constant peak frequency regardless of the magnitude of the flow, infrasound peak frequency decreases with increasing flow velocity, depth and discharge. Our results suggest that infrasound and seismic waves are generated by different source processes, acting at the flow free surface and at the channel bed respectively. We propose that infrasound is likely generated by waves and oscillations that develop at the surface of the flow, which, according to fluid dynamics, are mostly generated wherever the flow encounters significant channel irregularities. Furthermore, the observed positive correlations between seismo‐acoustic signal features and flow parameters highlight the potential to use infrasound and seismic measurements for debris‐flow monitoring and risk management.

Landslides and increased debris‐flow activity: A systematic comparison of six catchments in Switzerland

AbstractAn increase in debris‐flow frequency is expected in steep Alpine catchments after the occurrence of a large landslide, such as a rock avalanche. Herein we describe changes in debris‐flow activity following increases in sediment availability due to landslides, or accelerated rock‐glacier movement, for five catchments in the Swiss Alps, the Spreitgraben, Schipfenbach, Bondasca, Riascio, and Dorfbach catchments. Documentation on debris‐flow activity is available from both before and after the landslide that generated the new sediment deposits. Data from nearby meteorological stations were used to explore possible changes in rainfall activity, and how the intensity and duration of rainfall events may have changed. In all cases there was a considerable increase in debris‐flows frequency for one to eight years following the landslide. The annual number of days with debris‐flow activity following the landslide was similar to that observed for the Illgraben catchment, where many such landslides occur annually. No clear change in precipitation totals preceding debris flows was apparent for the Riascio catchment, suggesting that the increase in frequency of debris flows is related to the increase in the amount of sediment that can be readily mobilized. In the two cases where rainfall data were available on an hourly basis, no systematic changes in the intensity or duration of rainfall related to debris‐flow triggering were apparent, as shown by the close‐clustering of storms on the intensity‐duration plots. Following the sediment‐generating event, an initial and sudden increase of the sediment yield was observed, followed by a decrease over time towards pre‐disturbance values. The response of the catchments appears to be related to the amount of debris‐flow activity prior to the landslide: sediment yield from catchments with frequent debris flows prior to the landslide activity did not increase as dramatically as in catchments where debris‐flow activity was less common prior to the landslide. © 2018 John Wiley & Sons, Ltd.

Testing seismic amplitude source location for fast debris-flow detection at Illgraben, Switzerland

Abstract. Heavy precipitation can mobilize tens to hundreds of thousands of cubic meters of sediment in steep Alpine torrents in a short time. The resulting debris flows (mixtures of water, sediment and boulders) move downstream with velocities of several meters per second and have a high destruction potential. Warning protocols for affected communities rely on raising awareness about the debris-flow threat, precipitation monitoring and rapid detection methods. The latter, in particular, is a challenge because debris-flow-prone torrents have their catchments in steep and inaccessible terrain, where instrumentation is difficult to install and maintain. Here we test amplitude source location (ASL) as a processing scheme for seismic network data for early warning purposes. We use debris-flow and noise seismograms from the Illgraben catchment, Switzerland, a torrent system which produces several debris-flow events per year. Automatic in situ detection is currently based on geophones mounted on concrete check dams and radar stage sensors suspended above the channel. The ASL approach has the advantage that it uses seismometers, which can be installed at more accessible locations where a stable connection to mobile phone networks is available for data communication. Our ASL processing uses time-averaged ground vibration amplitudes to estimate the location of the debris-flow front. Applied to continuous data streams, inversion of the seismic amplitude decay throughout the network is robust and efficient, requires no manual identification of seismic phase arrivals and eliminates the need for a local seismic velocity model. We apply the ASL technique to a small debris-flow event on 19 July 2011, which was captured with a temporary seismic monitoring network. The processing rapidly detects the debris-flow event half an hour before arrival at the outlet of the torrent and several minutes before detection by the in situ alarm system. An analysis of continuous seismic records furthermore indicates that detectability of Illgraben debris flows of this size is unaffected by changing environmental and anthropogenic seismic noise and that false detections can be greatly reduced with simple processing steps.

Debris-flow modeling at Meretschibach and Bondasca catchments, Switzerland: sensitivity testing of field-data-based entrainment model

Abstract. Debris-flow volumes can increase due to the incorporation of sediment into the flow as a consequence of channel-bed erosion along the flow path. This study describes a sensitivity analysis of the recently introduced RAMMS (Rapid Mass Movements) debris-flow entrainment model, which is intended to help solve problems related to predicting the runout of debris flows. The entrainment algorithm predicts the depth and rate of erosion as a function of basal shear stress based on an analysis of erosion measurements at the Illgraben catchment, Switzerland (Frank et al., 2015). Starting with a landslide-type initiation in the RAMMS model, the volume of entrained sediment was calculated for recent well-documented debris-flow events at the Bondasca and the Meretschibach catchments, Switzerland. The sensitivity to the initial landslide volume was investigated by systematically varying the initial landslide volume and comparing the resulting debris-flow volume with estimates from the field sites. In both cases, the friction coefficients in the RAMMS runout model were calibrated using the model, whereby the entrainment module was (1) inactivated to find plausible values for general flow properties by adjusting both coefficients (ξ and μ) and then (2) activated to further refine coefficient μ, which controls erosion (patterns). The results indicate that the model predicts plausible erosion volumes in comparison with field data. By including bulking due to entrainment in runout models, more realistic runout patterns are predicted in comparison to starting the model with the entire debris-flow volume (initial landslide plus entrained sediment). In particular, lateral bank overflow – not observed during these events – is prevented when using the sediment entrainment model, even in very steep (≈ 60–65 %) and narrow (4–6 m) torrent channels. Predicted sediment entrainment volumes are sensitive to the initial landslide volume, suggesting that the model may be useful for both reconstruction of historical events and the modeling of scenarios as part of a hazard analysis.

Quantitative reconstruction of late Holocene surface evolution on an alpine debris-flow fan

Debris-flow fans form a ubiquitous record of past debris-flow activity in mountainous areas, and may be useful for inferring past flow characteristics and consequent future hazard. Extracting information on past debris flows from fan records, however, requires an understanding of debris-flow deposition and fan surface evolution; field-scale studies of these processes have been very limited. In this paper, we document the patterns and timing of debris-flow deposition on the surface of the large and exceptionally active Illgraben fan in southwestern Switzerland. We use terrain analysis, radiocarbon dating of sediment fill in the Illgraben catchment, and cosmogenic 10Be and 36Cl exposure dating of debris-flow deposits on the fan to constrain the temporal evolution of the sediment routing system in the catchment and on the fan during the past 3200years. We show that the fan surface preserves a set of debris-flow lobes that were predominantly deposited after the occurrence of a large rock avalanche near the fan apex at about 3200years ago. This rock avalanche shifted the apex of the fan and impounded sediment within the Illgraben catchment. Subsequent evolution of the fan surface has been governed by both lateral and radial shifts in the active depositional lobe, revealed by the cosmogenic radionuclide dates and by cross-cutting geometrical relationships on the fan surface. This pattern of frequent avulsion and fan surface occupation provides field-scale evidence of the type of large-scale compensatory behavior observed in experimental sediment routing systems.

The importance of entrainment and bulking on debris flow runout modeling: examples from the Swiss Alps

Abstract. This study describes an investigation of channel-bed entrainment of sediment by debris flows. An entrainment model, developed using field data from debris flows at the Illgraben catchment, Switzerland, was incorporated into the existing RAMMS debris-flow model, which solves the 2-D shallow-water equations for granular flows. In the entrainment model, an empirical relationship between maximum shear stress and measured erosion is used to determine the maximum potential erosion depth. Additionally, the average rate of erosion, measured at the same field site, is used to constrain the erosion rate. The model predicts plausible erosion values in comparison with field data from highly erosive debris flow events at the Spreitgraben torrent channel, Switzerland in 2010, without any adjustment to the coefficients in the entrainment model. We find that by including bulking due to entrainment (e.g., by channel erosion) in runout models a more realistic flow pattern is produced than in simulations where entrainment is not included. In detail, simulations without entrainment show more lateral outflow from the channel where it has not been observed in the field. Therefore the entrainment model may be especially useful for practical applications such as hazard analysis and mapping, as well as scientific case studies of erosive debris flows.

Direct measurement of channel erosion by debris flows, Illgraben, Switzerland

[1] The timing and magnitude of channel bed erosion by three debris flows was measured in 2008 at the Illgraben catchment, Switzerland, using a scour sensor which consisted of a vertical array of erodible sensor elements. During the largest debris flow, sediment was entrained progressively and stepwise at the flow head within 20 s after front arrival, and onset of erosion started before maximum values for flow height and normal and shear stress, measured nearby, were reached. Erosion in one of the two smaller debris flows also occurred at the head of the flow, but the magnitude of erosion was at the detection limit for the sensor. For the other small debris flow, we were not able to determine the timing of erosion due to the presence of a sediment layer covering the sensors. Measurements of pressure fluctuations along the channel sidewall, which are produced by interparticle collisions within the flow, indicate that the entrainment of sediment is coincident with the largest mean and fluctuating pressures, suggesting that interparticle collisions may drive the erosion process at the front of debris flows. After erosion at the head of the debris flows, sediment was deposited on top of the erosion sensor columns, indicating that the bed was reworked to a larger depth than directly visible at the surface after the event. Observations from elsewhere along the channel support our measurements of the magnitude of net debris flow erosion and indicate that significant erosion can be expected on debris fans when the flow is confined to a channel. Debris flow entrainment and subsequent bulking of the flow influence the flow dynamics and therefore should be considered in debris flow models and hazard assessment.

Sediment transfer patterns at the Illgraben catchment, Switzerland: Implications for the time scales of debris flow activities

Debris flows strongly control sediment transfer patterns in mountainous catchments. We quantify sediment transfer at the Illgraben, Switzerland, where debris flows are frequent and geomorphic change is rapid. Four sequential aerial image series (from fall 2007 to fall 2009) were used to measure landscape change in relation to debris flows. The debris, often originating from bedrock landslides, was transported in patterns of erosion, storage, and remobilization. The landslides typically stopped on the downslope hillslope or in the channel, and they did not transform directly into debris flows. The magnitude and nature of sediment transfer patterns show large spatial and temporal variability, and the storage time of the deposits was shorter than one year. Landslide volumes were an order of magnitude smaller than the debris flows at the catchment outlet. While the mechanism of debris flow initiation could not be determined unambiguously, clearly debris flows must entrain substantial amounts of sediment along the flow path to attain the volumes estimated at the distal end of the fan.

Limits of sediment transfer in an alpine debris-flow catchment, Illgraben, Switzerland

The 9.5 km 2 Illgraben catchment, located in the Rhône valley in the Central Alps of Switzerland, is one of the most active debris flow torrents in the Alps. In this paper we present sediment yield data collected in 2006 for segments where hillslopes and channels form a fully connected network and contrast these with sediment yields measured for disconnected hillslopes. The data reveal that sediment yields are 1–2 orders of magnitude larger in segments where hillslopes are connected with the channel network than on disconnected hillslopes. Support for this conclusion is provided by observations made on 1959, 1999 and 2004 aerial photographs that the vegetation cover in the disconnected segments is still intact, whereas denudation rates of several centimeters per year in the connected segments have inhibited the establishment of a stable vegetation cover. Furthermore, sediment supplied from hillslopes during the past 40 years has temporarily accumulated along the Illgraben channel, indicating that the channel aggraded over this period and has not yet recovered. An implication of this observation is that initiation of debris flows in the Illgraben catchment is limited more by the availability of intense rainfall than sediment. In contrast, on disconnected hillslopes, sediment flux does not appear to be driven by precipitation. The petrographic composition of the Illgraben fan deposits indicates two distinct sediment sources, one related to rockfall and the other to landslides and debris flows. The presence of clasts from both sources implies multiple processes of erosion, deposition, mixing and re-entrainment in the catchment before the material is exported to the Illgraben fan and to the Rhône River. In addition, delivery of large amounts of coarse-grained sediment to the Rhône causes a modification of the flow pattern from meandering or anastomosing upstream to braided downstream. Hence, the direct connectivity between hillslope and channelized processes in the Illgraben catchment causes not only rapid topographic modifications in the catchment, but also morphologic adjustment in the Rhône valley downstream.

A debris-flow alarm system for the Alpine Illgraben catchment: design and performance

We describe the development, implementation, and first analyses of the performance of a debris-flow warning system for the Illgraben catchment and debris fan area. The Illgraben catchment (9.5 km2), located in the Canton of Valais, Switzerland, in the Rhone River valley, is characterized by frequent and voluminous sediment transport and debris-flow activity, and is one of the most active debris-flow catchments in the Alps. It is the site of an instrumented debris-flow observation station in operation since the year 2000. The residents in Susten (municipality Leuk), tourists, and other land users, are exposed to a significant hazard. The warning system consists of four modules: community organizational planning (hazard awareness and preparedness), event detection and alerting, geomorphic catchment observation, and applied research to facilitate the development of an early warning system based on weather forecasting. The system presently provides automated alert signals near the active channel prior to (5–15 min) the arrival of a debris flow or flash flood at the uppermost frequently used channel crossing. It is intended to provide data to support decision-making for warning and evacuation, especially when unusually large debris flows are expected to leave the channel near populated areas. First-year results of the detection and alert module in comparison with the data from the independent debris-flow observation station are generally favorable. Twenty automated alerts (alarms) were issued, which triggered flashing lights and sirens at all major footpaths crossing the channel bed, for three debris flows and 16 flood flows. Only one false alarm was issued. The major difficulty we encountered is related to the variability and complexity of the events (e.g., events consisting of multiple surges) and can be largely solved by increasing the duration of the alarm. All of the alarms for hazardous events were produced by storms with a rainfall duration and intensity larger than the threshold for debris-flow activity that was defined in an earlier study, supporting our intention to investigate the use of rainfall forecasts to increase the time available for warning and implementation of active countermeasures.

Numerical modelling of debris surges based on shallow-water and homogeneous material approximations

The objective of this contribution is to analyse the formation of debris waves in natural channels. Numerical simulations are carried out with a ID code based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behaviour alternatively follows Herschel-BulkJey, Bingham or generalised visco-plastic models. The code is applied to real debris flow events that consisted of a single wave and multiple surges, in particular in the Illgraben catchment (Switzerland) and in the Cortina d’Ampezzo area (Dolomites). Numerical results are presented and compared with available flow depth registrations. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibration of rheological parameters. Wave propagation in time along the channel is investigated to verify if overtaking phenomena like in water roll waves do develop. Finally, an original interpretation for the formation of finite amplitude waves in a transient unstable flow is provided, based both on field data and numerical simulations.

Numerical modelling of debris surges based on shallow-water and homogeneous material approximations

The objective of this contribution is to analyse the formation of debris waves in natural channels. Numerical simulations are carried out with a 1D code based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behaviour alternatively follows Herschel-Bulkley, Bingham or generalised visco-plastic models. The code is applied to real debris flow events that consisted of a single wave and multiple surges, in particular in the Illgraben catchment (Switzerland) and in the Cortina d'Ampezzo area (Dolomites). Numerical results are presented and compared with available flow depth registrations. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibration of r...