Abstract
Four historically documented large and potentially dangerous lacustrine waves in Swiss lakes show that these waves have been seiches (standing waves) triggered by sublacustrine slides; a statement which is in accordance with the experience of seismologists who see earthquakes triggering seiches in lakes. Nevertheless, large historical waves in Switzerland have recently been modeled as progressive shallow water waves (tsunamis), probably because the slide/seiche dynamics are not known, and experiments with subaquatic slides fail to generate seiches in test–flumes. It appears that these tests exhibit a small shear–energy/slide–energy ratio ε, if compared with the situation in lakes. These facts incite a shear–stress lemma that states that ε is the constituent factor for the slide/seiche coupling. The structure of the subaqueous mass flow deposit (MFD) in lakes Lucerne and Geneva suggests the occurrence of subcritical and of supercritical slide flows. The former would generate a contortite, a MFD with contorted bedding, the latter a debrite (mudclast conglomerate). Potential slide energy considerations are used for an estimation of the amplitudes of large seiches produced by subaquatic slides, a proceeding that yields partly similar and partly very different results, as compared with numerical tsunami simulations.
Highlights
Four historically documented large and potentially dangerous lacustrine waves in Swiss lakes show that these waves have been seiches triggered by sublacustrine slides; a statement which is in accordance with the experience of seismologists who see earthquakes triggering seiches in lakes
An energy comparison is used for a rough estimation of the amplitudes of a seiche generated by a subaquatic slide, with the following assumptions: (i) The seiche/slide energy ratio of a specific basin is independent of the slide mass magnitude
6 Conclusions Archive studies show that historically documented large and potentially dangerous waves in Swiss lakes were seiches with periods of ~ 10 up to probably 70 min, accompanied by periodic river outflow interruptions, destructions of ships in harbors and large surges in the vicinity of the sublacustrine slides. These waves had been triggered by subaquatic slides caused by earthquakes
Summary
Tsunami waves are understood to be surface gravitational waves exhibiting periods within the range T ~ 102 – 1 04 [s]. The expression “Wasserbidem”, which was used until at least 1687 by the riparians of Lake Lucerne, is seen as an indication that the occurrence of large seiches was always present in language as a collective memory (Halbwachs, 1967) According to all these accounts the water level went quietly up and down, causing damages mainly to the fastened ships in harbors, phenomena typical for large seiches (Rabinovich, 2010). Especially 1601 and 1687 in Lake Lucerne and probably 1584 in Lake Geneva, occurred near the slides Such waves are considered to be large local surges and have been erroneously interpreted as tsunamis, e.g. by Siegenthaler and Sturm (1991). The slow vertical motion of the water-table at the shore, observed by the eyewitnesses, is a strong indication against a tsunami, a wave which may not occur in lake Brienz if the wave period is ≥ 10 min (Fig. 1). The following interpretation of the April 24 event is proposed. (i) The observed wave is a seiche. (ii) The eyewitnesses mistook an observed wave–period of 15–20 min for a half wave period, communicated ten years after the event. (iii) The seiche was a harbor wave with a 20 min period, a wave with this period a limnograph near the node at the lake outflow cannot register. (iv) The limnograph data indicate another event the same day, thirty minutes past noon
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