Morphological signatures of mass wasting and delta processes in a fjord-lake system: insights from Lovatnet, western Norway

Abstract

High-resolution bathymetrical data from the Lovatnet lake, western Norway, reveal a diverse morphology including traces from historic and prehistoric landslides. Onshore LiDAR and selected seismic profiles support the interpretation. Our data from the lake show significant debris lobes below Ramnefjellet that was the source of the well-known rock avalanche and tsunami disasters in 1905 and 1936 which caused a total of 134 casualties. The debris is divided into three main lobes reaching a total length of 1.8 km along the lake bottom from the impact site, and diffuse lobe features below the Bødalsdelta. Frontal deformations are minor. Lobes on the basin plain have a volume of more than 1.2 × 10 6 m 3 including debris entrained from the surrounding slopes and lake floor. The entire lake volume was set in motion during the catastrophic events. Returning bottom flows transported debris into the lake where lobes and minor, distinct mounds were deposited up to c. 1.5 km from the lake's outlet. The mounds could be anthropogenic, and possibly include a vessel that disappeared during the event. Erosion and smaller failures occurred elsewhere in the lake basin. Fan-delta morphologies are varied, with ridges and protuberances, reflecting deltaic sedimentation interrupted by major mass-wasting or other events in the deltas and/or their catchments. This is especially clear for the Raudi, Hellsete and Bødalen deltas, but minor protuberances are also found in other deltas. The data also reveal traces from large and previously undocumented landslide events. Gravitational deposits generally dominate the steep slopes and glacial landforms are scarce. The study demonstrates the importance of linking onshore and offshore morphological datasets in order to obtain an overview over process variability in fjord-type lakes of importance for further investigation and hazard assessment.

Anatomy of a Catastrophe: Reconstructing the 1936 Rock Fall and Tsunami Event in Lake Lovatnet, Western Norway

“…Previous works using a high-resolution sonar data show that Lake Lovatnet is divided into two sub-basins separated by a morphological high (Figure 1C; Hansen et al, 2016). The central part of the lake floor in each sub-basin is nearly planar, with 90 and 140 m water depth in the northern and southern subbasins, respectively.…”

“…1 km along the basin floor plain. Here, two major debris lobes make up a total volume of at least 1.2 × 10 6 m 3 (Hansen et al, 2016). Some debris are also present along the lateral slopes to the west and near Bødalsdelta to the east (Figure 5A).…”

“…Steep marginal slopes rim each sub-basin and are characterized by the total absence of sediments or by a thin sedimentary cover that is directly deposited over the metamorphic bedrock. High-resolution sonar reveals a lake-floor morphology characterized by signs of historic and prehistoric rock falls and landslides (Hansen et al, 2016), pointing to an active source-to-sink lacustrine system with vigorous mass wasting activity. A recent study carried out on a neighboring lake shows that suspended sedimentation with a singular main source (the adjacent glaciers) dominates the limnic depositional system (Storms et al, 2020).…”

“…The floor of Lake Lovatnet displays several debris lobes from failures along the lateral slopes (Hansen et al, 2016), with the most prominent identified around the area between Ramnefjellet and Nesodden (Figure 5A). The whole deposit appears to extend ca.…”

“…1.5 km. One of the southernmost debris mounds includes a 15 m long and 1 m high feature, which could potentially be the remains of the vessel Lodølen that disappeared during the 1936 event (Hansen et al, 2016).…”