April 24, 1996 Tsunami: Lake Brienz, Switzerland

"In spring 1996 AD, the occurrence of a large mass-transport was detected by a series of events, which happened in Lake Brienz, Switzerland: turbidity increase and oxygen depletion in deep waters, release of an old corpse into surface waters and occurrence of a small tsunami-like wave." On April 24th 1996, a sunny and windless day, workers from 'Aarekies AG' company were on a floating bagger anchored 130-140 m offshore 'Alter Aaregg' (old Aare delta; Fig. 1) performing dredging at 8-10 m water depth. The ∼50 t dredging bagger was tightly anchored with 5 wire cables, 2 of them attached onshore and 3 anchored offshore. At about 8:00 GMT, the cable winches went surprisingly a bit backwards despite their brakes, and a loud noise raised the team's attention. Immediately after, two wire cables, one onshore and one offshore (16 mm and 19 mm diameter, respectively), broke at once. After this incident, upon reaching the harbor, the team reported that the boat was staying lower on the quay than normal. Some time later, the workers noted that the boat was then staying half a metre higher than its former position and deduced that the lake level was greatly fluctuating without generating any noticeable surface waves. The time period between the highest and lowest lake level (with approximately half metre amplitude) was 15-20 min (pers. comm. Jürg Gruber and Mr. Moser) and is identified as a small tsunami wave […]This incident combined with the small tsunami wave observed by Aarekies AG workers can be related to the starting of an underwater mass movement/flow, allowing to further refine the dating of the event starting time to April 24th 1996 around 8:00 GMT […] The occurrence of the tsunami wave, together with the release of the ‘Brienzi’ corpse into the lake waters, point to a slope failure in the proximal Aare delta area, which induced the tsunami wave, and ultimately developed into the final turbidite deposit. Under these conditions, the steep upper Aare delta slope angle (maximum 17°; Adams et al., 2001) could easily provide the necessary acceleration to transform the slide into a turbidity current or debris flow, and to travel across the low gradient Lake Brienz deep basin. (reference #6690)

Reference #12345 lists as questionable tsunami (accessed July 2024)