Stefan Boss

Das Schweizer Platten-Geophon-System (SPG) ist ein Messsystem für Geschiebetransport in Kiesflüssen. Es wurde bereits in 20 Gewässern, vor allem in den Alpen installiert und getestet. Die entsprechenden Studien ergaben im Allgemeinen recht robuste Kalibrierungsbeziehungen zwischen Signalimpulszahlen und transportierter Geschiebemasse. Hier vergleichen wir dieses System mit drei alternativen indirekten Messsystemen zum Monitoring des Geschiebetransports. Für den Vergleich der Messsysteme wurde das Geschiebe an den Gewässern auch direkt mit Körben und Geschiebenetzen eingefangen.

Le système suisses de plaques à géophones (SPG) est un système de mesure pour le charriage dans les rivières de montagne. Il a déjà été installé et testé dans 20 cours d'eau, principalement dans les Alpes. Les études correspondantes ont généralement révélé des relations de calibration assez robustes entre le nombre d'impulsions du signal et la masse de sédiments transportée. Nous comparons ici ce système avec trois systèmes de mesure indirects alternatifs pour le suivi du transport de sédiments. Pour comparer les systèmes de mesure, les sédiments ont également été directement collectés sur les cours d'eau à l'aide de paniers et de filets.

The performance of the Swiss Plate Geophone (SPG) system and three alternative indirect measurement systems is compared here based on calibration measurements made at the Swiss Erlenbach stream. Then we quantify the uncertainty of the SPG system using 308 calibration measurements at four channel sites in Switzerland. To determine total bedload transport, the SPG system allows robust calibration relationships to be established between the impulses (summary signal response) and the total transported bedload mass. Uncertainty calculations are also presented for the fractional bedload transport conversion of the SPG signal. Finally, we briefly introduce the new and relatively inexpensive Square Pipe System (SPS) prototype which is equipped with geophones, an accelerometer and a microphone, to combine the advantages of different systems.

Différents systèmes de surveillance sont utilisés dans le monde entier pour l'observation et la saisie de paramètres importants des laves torrentielles, mais ils ont tous une résolution limitée dans le temps et/ou dans l'espace. Dans cet article, nous présentons une nouvelle méthode de surveillance basée sur des scanners laser 3D (LiDAR), qui fournit des données à haute résolution temporelle et spatiale sur les laves torrentielles. Dans l'Illgraben, plusieurs scanners LiDAR de ce type ont été installés le long du chenal et ont déjà enregistré onze événements de laves torrentielles. Grâce à ces données 3D, nous avons pu mesurer avec une grande précision la vitesse d'objets individuels dans le mélange des laves torrentielles et déterminer les débits et les volumes. Nos analyses montrent que la technologie LiDAR peut apporter une contribution essentielle à une description plus détaillée de la dynamique des laves torrentielles, d'autant plus que les premiers résultats indiquent des différences significatives, par exemple dans la détermination du débit, entre cette méthode et les méthodes conventionnelles.

Zur Beobachtung und Erfassung wichtiger Kenngrössen von Murgängen kommen weltweit verschiedene Monitoring-Systeme zum Einsatz, die alle eine zeitlich und/oder räumlich beschränkte Auflösung haben. In diesem Artikel präsentieren wir eine neue Monitoring- Methode basierend auf 3D-Laserscannern (LiDAR), die zeitlich und räumlich hochaufgelöste Daten von Murgängen liefert. Im Illgraben wurden mehrere solche LiDAR-Scanner entlang des Gerinnes installiert und haben bereits elf Murgangereignisse erfasst. Anhand dieser 3D-Daten konnten wir Geschwindigkeiten einzelner Objekte in der Murgangmischung mit hoher Genauigkeit messen sowie Abflüsse und Volumina ermitteln. Unsere Analysen zeigen, dass die LiDAR-Technologie einen wesentlichen Beitrag zur detaillierteren Beschreibung der Murgangdynamik leisten kann, zumal erste Resultate auf massgebliche Unterschiede etwa in der Bestimmung des Abflusses zwischen dieser Methode und konventionellen Verfahren hindeuten.

The way Alpine rivers mobilize, convey and store coarse material during high-magnitude events is poorly understood, notably because it is difficult to obtain measurements of bedload transport at the watershed scale. Seismic sensor data, evaluated with appropriate seismic physical models, can provide that missing link by yielding time-varying estimates of bedload transport albeit with non-negligible uncertainty. Low cost and ease of installation allow for networks of sensors to be deployed, providing continuous, watershed-scale insights into bedload transport dynamics. Here, we deploy a network of 24 seismic sensors to estimate coarse material fluxes in a 13.4 km² Alpine watershed during a high-magnitude transport event. First, we benchmark the seismic inversion routine with an independent time-series of bedload transport obtained with a calibrated acoustic system. Then, we apply the procedure to the other seismic sensors across the watershed. Propagation velocities derived from cross-correlation analysis between spatially consecutive bedload transport time-series were too high with respect to typical bedload transport velocity suggesting that a faster-moving water wave (re-)mobilizes local coarse material. Spatially distributed estimates of bedload transport reveal a relative inefficiency of Alpine watersheds in evacuating coarse material, even during a relatively infrequent high-magnitude bedload transport event. Significant inputs estimated for some tributaries were rapidly attenuated as the main river crossed less hydraulically efficient reaches. Only a small proportion of the total amount of material mobilized in the watershed was exported at the outlet. Multiple periods of competent flows are likely necessary to evacuate coarse material mobilized throughout the watershed during individual bedload transport events.

Understanding and predicting bedload transport is an important element of watershed management. Yet, predictions of bedload remain uncertain by up to several order(s) of magnitude. In this contribution, we use a five-year continuous time-series of streamflow and bedload transport monitoring in a 13.4 km² snow-dominated Alpine watershed in the Western Swiss Alps to investigate hydrological drivers of bedload transport. Following a calibration of the bedload sensors, and a quantification of the hydraulic forcing of streamflow upon bedload, a hydrological analysis is performed to identify daily flow hydrographs influenced by different hydrological drivers: rainfall, snowmelt, and combined rain and snowmelt events. We then quantify their respective contribution to bedload transport. Results emphasize the importance of combined rain and snowmelt events, for both annual bedload volumes (77% on average) and peaks in bedload transport rate. A non-negligible, but smaller, amount of bedload transport may occur during late summer and autumn storms, once the snowmelt contribution and baseflow have significantly decreased (9% of the annual volume on average). Although rainfall-driven changes in flow hydrographs are responsible for a large majority of the annual bedload volumes (86% on average), the identified melt-only events also represent a substantial contribution (14 % on average). The results of this study help to improve current predictions of bedload transport through a better understanding of the bedload magnitude-frequency relationship under different hydrological conditions. We further discuss how bedload transport could evolve under a changing climate through its effects on Alpine watershed hydrology.

The Swiss plate geophone (SPG) system has been installed and tested in more than 20 steep gravel-bed streams and rivers, and related studies generally resulted in rather robust calibration relations between signal impulse counts and transported bedload mass. Here, we compare this system with three alternative surrogate measuring systems. A variant of the SPG system uses the same frame (housing) set-up but with an accelerometer instead of a geophone sensor to measure the vibrations of the plate (GP-Acc, for geophone plate accelerometer). The miniplate accelerometer (MPA) system has a smaller dimension of the impact plate and is embedded in more elastomer material than the SPG system. The Japanese pipe microphone (JPM) is a 1 m long version of the system that has been installed in many streams in Japan. To compare the performance of the four systems, we used calibration measurements with direct bedload samples from three field sites and an outdoor flume facility with controlled sediment feed. At our field sites, the systems with an accelerometer and a microphone showed partly large temporal variations in the background noise level, which may have impaired the calibration measurements obtained during certain time periods. Excluding these periods, the SPG, GP-Acc, and JPM all resulted in robust calibration relations, whereas the calibration of the MPA system showed a poorer performance at all sites.

Acoustic sensors are increasingly used to measure bedload transport in Alpine streams, notably the Swiss plate geophone (SPG) system. An impact experiment using artificial weights is developed in this paper to assess the variability in individual plate response and to evaluate the extent to which calibration coefficients can be transferred from calibrated plates to non-calibrated plates at a given measuring site and/or to other measuring sites. Results of the experiment over 43 plates at four measuring sites have notably shown (a) that the maximum amplitude (V) recorded by individual plates tends to evolve as a power law function of the impact energy (J), with an exponent slightly larger than 1, for all the plates at all measuring sites; (b) that there is a substantial propagation of energy across plates that should be taken into account for a better understanding of the signal response; (c) that the response of individual plates is in most cases consistent, which suggests that calibration coefficients are comparable within and in between measuring sites, but site-specific and plate-specific variabilities in signal response have to be considered for a detailed comparison.

The Swiss plate geophone (SPG) system is an indirect bedload surrogale monitoring lechnique used to quantify bedload transport and retrieve information about the grain-size distribution from the recorded seismic signal. However, the PG system still requires direct measurements of bedload transport for calibration. In addition to a field calibration campaign carried out in 2019 at the Navisence stream in Switzerland, a series of prototype flume experiments was conducted to further investigate the variability of the SPG system for different bedload transport conditions. By reproducing natural flow conditions in the flume and using grain size distributions of bedload samples collected at the Navisence stream, we identified the importance of vibrations caused by larger grains impacting a neighboring plate or the surrounding concrete. This improves our understanding how the SPG signal response change with increasing grain sizes.

The Swiss Plate Geopbone (SPG) system is a surrogate monitoring technique which has been calibrated to determine bedload flux in the wiss mountain river Albula. The so-called impulse counts derived from the geophone signal are converted into bedload fluxes. The bedload transport measuring installation is located a few hundred meters upstream of a runoff gauging station in the village of Tiefencastel. The most important bedload transporting flows generally occur during the snowmelt period with additional runoff generation due to rainfall events. We present and discuss the variability of bedload transport during the sommer periods from 2015 to 2018. We also compare the annual bedload masses in the Albula river with measurements made with the SPG system in mountain rivers in Austria. Furthermore, we show how well averaged values of measured bedload transport can be predicted with bedload transport equations which were not calibrated with the measurements at the Albula River.

The contribution of snow meltwater to catchment streamflow can be quantified through hydrograph separation analyses for which stable water isotopes (¹⁸O, ²H) are used as environmental tracers. For this, the spatial and temporal variability of the isotopic composition of meltwater needs to be captured by the sampling method. This study compares an optimized snowmelt lysimeter system and an unheated precipitation collector with focus on their ability to capture snowmelt rates and the isotopic composition of snowmelt. The snowmelt lysimeter system consists of three individual unenclosed lysimeters at ground level with a surface of 0.14 m² each. The unheated precipitation collector consists of a 30 cm-long, extended funnel with its orifice at 2.3 m above ground. Daily snowmelt samples were collected with both systems during two snowfall-snowmelt periods in 2016. The snowmelt lysimeter system provided more accurate measurements of natural melt rates and allowed for capturing the small-scale variability of snowmelt process at the plot scale, such as lateral meltwater flow from the surrounding snowpack. Because of the restricted volume of the extended funnel, daily melt rates from the unheated precipitation collector were up to 43% smaller compared to the snowmelt lysimeter system. Overall, both snowmelt collection methods captured the general temporal evolution of the isotopic signature in snowmelt.

Rain-on-snow (ROS) events in mountainous catchments can cause enhanced snowmelt, leading to an increased risk of destructive winter floods. However, due to differences in topography and forest cover, the generation of snowpack outflow volumes and their contribution to streamflow are spatially and temporally variable during ROS events. In order to adequately predict such flood events with hydrological models, an enhanced process understanding of the contribution of rainwater and snowmelt to stream water is needed.
In this study, we monitored and sampled snowpack outflow with fully automated snowmelt lysimeter systems installed at three different elevations in a pre-Alpine catchment in central Switzerland. We measured snowpack outflow volumes during the winters of 2017 and 2018, as well as snowpack outflow isotopic compositions in winter 2017. Snowpack outflow volumes were highly variable in time and space, reflecting differences in snow accumulation and melt. In winter 2017, around 815 mm of snowpack outflow occurred at our reference site (grassland 1220 m a.s.l. – metres above sea level), whereas snowpack outflow was 16 % less at the nearby forest site (1185 m a.s.l.), and 62 % greater at another grassland site located 200 m higher (1420 m a.s.l.). A detailed analysis of 10 ROS events showed that the differences in snowpack outflow volumes could be explained mainly by rainfall volumes and initial snow depths.
The isotope signals of snowpack outflow were more damped than those of incoming rainwater at all three sites, with the most damped signal at the highest elevation site because its snowpack was the thickest and the residence times of liquid water in its snowpack were the longest, thus enhancing isotopic mixing in the snowpack. The contribution of snowpack outflow to streamflow, estimated with an isotope-based two-component end-member mixing model, differed substantially among the three lysimeter sites (i.e. between 7±4 and 91±21 %). Because the vegetation in our study catchment is a mixture of grassland and forest, with elevations ranging from 1000 to 1500 m a.s.l., our site-specific hydrograph separation estimates can only provide a range of snowpack outflow contributions to discharge from different parts of the study area. Thus, the catchment-average contribution of snowpack outflow to stream discharge is likely to lie between the end-member mixing estimates derived from the three site-specific data sets. This information may be useful for improving hydrological models in snow-dominated catchments.

Im Illgraben bei Leuk (VS) wurde 2003 die weltweit erste Murgangwaage gebaut. Sie lieferte ab Sommer 2004 bis zu ihrer Zerstörung im Juli 2016 wertvolle Daten über die dynamischen Kräfte von Murgängen. Im Rahmen eines Investitionsprojektes der Eidg. Forschungsanstalt WSL wird nun eine ähnlich konzipierte Messeinrichtung am selben Standort unter der Kantonsstrassenbrücke über den Illgraben gebaut. Erfahrungen aus dem bisherigen Betrieb fliessen zusammen mit den Ergebnissen aus den Tests im Labor in eine optimierte optimierte Version ein, welche uns ab Mai 2019 weitere interessante Einblicke in die Fliessdynamik von Murgängen ermöglichen wird.

Bedload transport measurements with acoustic sensors were obtained during summer 2015 in the Albula River in Switzerland. An impact plate measuring system was used with geophone and accelerometer sensors. This system provides indirect estimations of bedload transport in water courses. In April 2015, 30 impact sensors were installed in a new permanent measuring station to monitor continuously bedload transport in a mountain river with a large annual rate of sediment transport (around 90 000 m³ yr⁻¹). Records of the first year of measurement showed that (i) the signal response in terms of geophone and accelerometer impulses is comparable for both types of sensors; (ii) there is a good correlation between discharge data and impulses recorded by both types of sensors; (iii) the critical discharge at the start of bedload transport is around 5 m³ s⁻¹; (iv) a mean calibration factor for the geophone impulses can be estimated which is in a similar range as values determined from other sites with field calibration measurements.