Coastal instability associated to gravitational processes in large Patagonian lakes: a study of recent events

BEIGT, DÉBORA; VILLAROSA, GUSTAVO; DE LUCA, LAUTARO; BARBOSA HETHERINGTON, ANDRES; GÓMEZ, EDUARDO ALBERTO; ARIEL RANIOLO

Bariloche

Congreso; IAL IPA 2022; 2022

International Paleolimnology Association - International Association of Limnogeology

Subaqueous mass-wasting involving coastal areas of large Patagonian lakes is a littleknownand almost unexplored phenomenon. However, its impacts over people and coastalinfrastructure are increasingly reported. We study the characteristics, conditioning factorsand probable triggers of a series of recent events that occurred at lakes Nahuel Huapi(2005), Huechulafquen (2010), Lacar (2015) and Espejo (2021), affecting coastal propertiesand public recreational beaches. Available information was collected through interviewswith witnesses, municipal government officials, national parks staff, landowners andconcessionaires of lacustrine coasts. Geophysical equipment (GPR, side scan sonar, phasemeasuringbathymetric sounder, sub-bottom profiler and dual-frequency echosounder)were used to survey the lake floor. Drone and satellite imagery were visually interpreted. Acoastal sublacustrine slope located immediately below the wave-base depth (locally called“veril”) is a typical feature of Patagonian lakes, due to their glacially-carved basins and thereworking of coastal deposits by waves. These results show the occurrence of rotationalslumps (with headscarps of up to 15 m height and 500 m length) that initiate at the coastalsubaqueous slopes (18° - 32°) and may affect the shore either by subsidence or erosion bythe original failure or headscarp retreat. The slumps subsequently evolve into translationalslides or channelized flows that mobilize the sediment masses hundreds of meters into thelake basins. From sedimentological and morphological analysis we conclude that coastalareas with high sedimentation rates (e.g. lacustrine deltas or windward-facing beacheswhere thick remobilizedtephra deposits were accumulated) show the highest susceptibility,especially with recent anthropic intervention on the subaqueous slopes. A clear link betweenthe occurrence of these events and concomitant regional or local seismic events were found.In order to reduce the exposure of vulnerable coasts, coastal gravitational processes shouldbe considered in territorial management, especially when addressing secondary hazardsderived from seismic and volcanic events in Patagonia.