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

Abstract

Rock falls and landslides plunging into lakes or small reservoirs can result in tsunamis with extreme wave run-ups. The occurrence of these natural hazards in populated areas have encouraged a recent sharp increase of studies that aim to mitigate their impact on human lives and assess infrastructure lost. This paper amalgamates in a novel fashion and at an unprecedented detail in situ historic measurements, geological data and numerical modeling of a rock fall event and associated tsunami wave that occurred in Lake Lovatnet (western Norway) in September 1936. Historical records report an event that released ca. 1 million m3 of rocks and debris from Ramnefjellet Mountain at an altitude of 800 m above Lake Lovatnet. The fragmented material plunged into the lake, causing a tsunami that reached a maximum run-up of 74 m and killed 74 people. In fact, the settlements of Bødal and Nesdal were wiped out as a result of the catastrophic wave. Sediments resulting from the 1936 rock fall and associated tsunami were identified in the subsurface of Lake Lovatnet by shallow geophysical investigations and were retrieved using gravity coring equipment. A set of high resolution physical and geochemical measurements were carried out on the cores with the aim of reproducing a highly detailed reconstruction of this catastrophic event in order to better understand and learn about the processes involved. The cores were retrieved in the northwestern sub-basin of the lake and its chronology was constrained by 210Pb and radiocarbon dating. A specially tailored physically based mathematical model was applied to better understand the tsunami event. Integration of the geophysical record, the sedimentological data and numerical modeling provide a comprehensive background to better understand the effects of such event in a deep fjord-like lacustrine basin and to generate information for better mitigation of similar events elsewhere.

From granular collapses to shallow water waves: A predictive model for tsunami generation

“…The line corresponds to Eqs. (8) for the three domains separated by the two vertical dotted lines at a = 0.93 and a = 3.…”

“…Tsunamis are among the most destructive natural disasters for human coastal settlements. While events generated by earthquakes have been extensively studied [1][2][3][4], several past or potential occurrences of high amplitude waves arising from large-scale landslides have also been reported in past decades [5][6][7][8][9], which constitutes a grand challenge in environmental fluid mechanics [10]. The 1958 Lituya Bay tsunami, featuring the highest recorded wave runup of 524 m [6], is reminiscent of the importance of understanding the physics underlying such events, for reliable hazard assessments.…”

“…Increasing the initial height H g of the column, while keeping the other initial parameters constant, systematically leads to higher values for the wave amplitude. In the following, the study focuses on shallow water waves (A m 0.45 h 0 ), whose understanding is of significant importance for risk assessments as bore waves are potentially very destructive in enclosed fjords scenarios [8], while at the same time solitary waves can propagate and distribute part of the collapse energy far from the source.…”

A Review of Event Deposits in Lake Sediments

“…In the nearshore (shallow) subbasins-that are distal from the tsunami source-of Swiss and Norwegian lakes, researchers identified graded deposits with a sandy (but up to pebble size) and sometimes erosional base, often including an organic-rich bed with plant debris and a fine-grained cap. These deposits were attributed to backwash [107,113] and/or erosion of nearshore sediments [113,114]. In all these studies, numerical tsunami modeling significantly aided the interpretation of the deposits.…”

Hazards from lakes and reservoirs: new interpretation of the Vaiont disaster

“…Tsunami hazards in lakes and fiords arising from subaerial landslides have received increasing attention in recent years: (i) major historical events are being re-analysed using the latest modelling and/or field geophysical and other techniques (e.g. Franco et al 2020;Waldmann et al 2021); (ii) evidence of previously unknown historical and prehistorical events continues to be found and assessed, such as in Swiss lakes (e.g. Spinney 2014;Strupler et al 2020); and (iii) recent events such as at Chehalis Lake, Canada in 2007 (Wang et al 2015) and Lake Askja, Iceland in 2014 (Gylfadóttir et al 2017) can be investigated almost immediately.…”