Subaquatic slope instabilities: The aftermath of river correction and artificial dumps in Lake Biel (Switzerland)

River engineering projects are developing rapidly across the globe, drastically modifying water courses and sediment transfer. Investigation of the impact of engineering works focuses usually on short-term impacts, thus a longer-term perspective is still missing on the effects that such projects have. The ‘Jura Water Corrections’ – the largest river engineering project ever undertaken in Switzerland – radically modified the hydrological system of Lake Biel in the 19th and 20th Century. The deviation of the Aare River into Lake Biel more than 140 years ago, in 1878, thus represents an ideal case study to investigate the long-term sedimentological impacts of such large-scale river rerouting. Sediment cores, along with new high-resolution bathymetric and seismic reflection datasets were acquired in Lake Biel to document the consequences of the Jura Water Corrections on the sedimentation history of Lake Biel. Numerous subaquatic mass transport structures were detected on all of the slopes of the lake. Notably, a relatively large mass transport complex (0·86 km²) was observed on the eastern shore, along the path of the Aare River intrusion. The large amount of sediment delivered by the Aare River since its deviation into the lake likely caused sediment overloading resulting in subaquatic mass transport. Alternatively, the dumping since 1963 in a subaquatic landfill of material excavated during the second phase of river engineering, when the channels flowing into and out of Lake Biel were widened and deepened, might have triggered the largest mass transport, dated to 1964 or 1965. Additional potential triggers include two nearby small earthquakes in 1964 and 1965 (M_W 3·9 and 3·2, respectively). The data for this study indicate that relatively large mass transports have become recurrent in Lake Biel following the deviation of the Aare River, thus modifying hazard frequency for the neighbouring communities and infrastructure.     

Mapping basin-wide subaquatic slope failure susceptibility as a tool to assess regional seismic and tsunami hazards

Marine Geophysical Research - This study of subaquatic slope failures in Lake Lucerne, central Switzerland, presents a new concept for evaluating basin-wide slope stability through time as a...     

Subaqueous landslide-triggered tsunami hazard for Lake Zurich,Switzerland

Subaqueous landslides can induce potentially damaging tsunamis. Tsunamis are not restricted to the marine environment, but have also been documented on lakes in Switzerland and worldwide. For Lake Zurich (central Switzerland), previous work documented multiple, assumedly earthquake-triggered landslides. However, no information about past tsunamis is available for Lake Zurich. In a back-analysis, we model tsunami scenarios as a consequence of the earthquake-triggered landslides in the past. Furthermore, on the basis of a recent map of the earthquake-triggered subaqueous landslide hazard, we present results of a tsunami hazard assessment. The subaqueous landslide progression, wave propagation and inundation are calculated with a combination of open source codes. Although no historic evidence of past tsunamis has been documented for Lake Zurich, a tsunami hazard exists. However, only earthquakes with long return periods are assumed to cause considerable tsunamis. An earthquake with an exceedance probability of 0.5% in 50 years (corresponding to an earthquake with a return period of 9975 years) is assumed to cause tsunamigenic landslides on most lateral slopes of Lake Zurich. A hypothetical tsunami for such an event would create damage especially along the shores of the central basin of Lake Zurich with estimated peak flow depths of up to ~?4.6 m. Our results suggest that for an earthquake with an exceedance probability of 10% in 50 years (i.e., mean return period of 475 years), no considerable tsunami hazard is estimated. Even for a worst-case scenario, the cities of Zurich and Rapperswil, located at the northern and southern ends of the lake, respectively, are assumed to experience very little damage. The presented first-order results of estimated wave heights and inundated zones provide valuable information on tsunami-prone areas that can be used for further investigations and mitigation measures.     

Reconstructing 4000 years of mass movement and tsunami history in a deep peri‐Alpine lake (Lake Geneva,France‐Switzerland)

The study of mass movements in lake sediments provides insights into past natural hazards at historic and prehistoric timescales. Sediments from the deep basin of Lake Geneva reveal a succession of six large‐scale (volumes of 22 × 10⁶ to 250 × 10⁶ m³) mass‐transport deposits, associated with five mass‐movement events within 2600 years (4000 cal bp to 563 ad ). The mass‐transport deposits result from: (i) lateral slope failures (mass‐transport deposit B at 3895 ± 225 cal bp and mass‐transport deposits A and C at 3683 ± 128 cal bp ); and (ii) Rhône delta collapses (mass‐transport deposits D to G dated at 2650 ± 150 cal bp , 2185 ± 85 cal bp , 1920 ± 120 cal bp and 563 ad , respectively). Mass‐transport deposits A and C were most probably triggered by an earthquake, whereas the Rhône delta collapses were likely to be due to sediment overload with a rockfall as the external trigger (mass‐transport deposit G, the Tauredunum event in 563 ad known from historical records), an earthquake (mass‐transport deposit E) or unknown external triggers (mass‐transport deposits D and F). Independent of their origin and trigger mechanisms, numerical simulations show that all of these recorded mass‐transport deposits are large enough to have generated at least metre‐scale tsunamis during mass movement initiation. Since the Tauredunum event in 563 ad , two small‐scale (volumes of 1 to 2 × 10⁶ m³) mass‐transport deposits (H and I) are present in the seismic record, both of which are associated with small lateral slope failures. Mass‐transport deposits H and I might be related to earthquakes in Lausanne/Geneva (possibly) 1322 ad and Aigle 1584 ad , respectively. The sedimentary record of the deep basin of Lake Geneva, in combination with the historical record, show that during the past 3695 years, at least six tsunamis were generated by mass movements, indicating that the tsunami hazard in the Lake Geneva region should not be neglected, although such events are not frequent with a recurrence time of 0·0016 yr^?1.     

Signatures of slope failures and river‐delta collapses in a perialpine lake (Lake Lucerne,Switzerland)

Historical reports from the 17th Century document two destructive tsunamis with run‐ups exceeding 5 m, affecting proximal basins of Lake Lucerne (Switzerland). One event in ad 1601 is coeval with a strong nearby earthquake (M_W ca 5·9) which caused extensive slope failures in many parts of the lake. The second event in ad 1687 is associated with an apparently spontaneous partial collapse of the Muota river delta. This study combines high‐resolution bathymetry, reflection seismic and lithological data to document the sedimentary and morphological signatures of the two subaqueous mass movements that probably generated the observed tsunamis. Such mass movements are significant as a common sedimentation process and as a natural hazard in fjord‐type lakes and similar environments. The deposits, covering large parts of the basins with thicknesses reaching >10 m, consist of two subunits: A lower ‘massflow deposit’ contains variably deformed sediments from the source areas. Its emplacement affected pre‐existing sediments, incorporating thin sediment slices into the deposit and increasing its volume. Deep‐reaching deformation near basin margins is expressed as bulges on the lake floor. An overlying ‘megaturbidite’, featuring a graded, sandy base and a thick homogeneous muddy part, was deposited from suspended particles. The source area for the ad 1601 event, gently dipping lateral slopes with an unconsolidated hemipelagic sediment cover, hosts a pronounced slide scar with sharp escarpments and sliding surfaces. The source area for the ad 1687 event on an active delta slope has been overprinted by continued sedimentation and does not show an unambiguous scar. The case studies are exemplary for end‐member types of source areas (lateral versus delta slopes) and trigger mechanisms (seismic versus aseismic); they show that morphological mapping and reconstructions of past events are key components of a hazard assessment for mass movement‐generated tsunamis.     

Subaqueous morphology of Lake Lucerne (Central Switzerland): implications for mass movements and glacial history

Bathymetric data available for Swiss lakes have typically only low to moderate resolution and variable quality, making them insufficient for detailed underwater geomorphological studies. This article presents results of a new bathymetric survey in perialpine Lake Lucerne using modern hydrographic equipment. A digital terrain model (DTM) of the lake floor (raster dataset with 1 m cell size) covering the Chrüztrichter and Vitznau basins documents signatures of major Holocene mass movements and relics from the glacial history of the lake. Combining the bathymetry data with reflection seismic profiles and an existing event chronology allows investigating the morphology in its geological context. Subaqueous sediment slide scars with sharp headwalls cover large areas on moderately inclined slopes. The particularly large Weggis slide complex, correlated with an historical earthquake (ad 1601), features a ~9 km long and 4–7 m high headwall and covers an area of several square kilometers. Large debris cones of prehistoric rockfalls and the deposits of recent rockfall events imaged on the almost flat basin plain document mass-movement activity on steep slopes above the lake. Six transverse moraines, visible as subaqueous ridges, as lake-floor lineaments, or only imaged on reflection seismic profiles, indicate a complex glacial-inherited morphology. As many of the documented features result from potentially catastrophic events, high-resolution bathymetry can significantly improve natural hazard assessment for lakeshore communities by extending classical hazard maps to the subaqueous domain.     

A subaqueous hazard map for earthquake-triggered landslides in Lake Zurich,Switzerland

The awareness of geohazards in the subaqueous environment has steadily increased in the past years and there is an increased need to assess these hazards in a quantitative sense. Prime examples are subaqueous landslides, which can be triggered by a number of processes including earthquakes or human activities, and which may impact offshore and onshore infrastructure and communities. In the literature, a plenitude of subaqueous landslide events are related to historical earthquakes, including cases from lakes in Switzerland. Here, we present an approach for a basin-wide earthquake-triggered subaquatic landslide hazard assessment for Lake Zurich, which is surrounded by a densely populated shoreline. Our analysis is based on high-resolution sediment-mechanical and geophysical input data. Slope stabilities are calculated with a grid-based limit equilibrium model on an infinite slope, which uses Monte Carlo sampled input data from a sediment-mechanical stratigraphy of the lateral slopes. Combined with probabilistic ground-shaking forecasts from a recent national seismic hazard analysis, subaquatic earthquake-triggered landslide hazard maps are constructed for different mean return periods, ranging from 475 to 9975 years. Our results provide a first quantitative landslide hazard estimation for the lateral slopes in Lake Zurich. Furthermore, a back-analysis of a case-study site indicates that pseudostatic accelerations in the range between 0.04 and 0.08 g were needed to trigger a well-investigated subaqueous landslide, dated to ~2210 cal. years B.P.