Examining sedimentological processes in a sublacustrine delta: from underflows to geomorphic changes (Lake Brienz, Switzerland)

Abstract

Slope failures within subaquatic deltas have the potential to induce underwater mass movements that can be tsunamigenic. Historical cases of subaquatic delta failures have been documented in marine contexts (Anthony & Julian, 1997; Bailey et al., 2021) and lacustrine settings (Girardclos et al., 2007; Hilbe & Anselmetti, 2015). However, the traces and failure planes of these mass movements are rapidly buried due to the high sedimentation rates caused by incoming rivers so that detailed process studies of such failures are challenging. Given the rise in population near shorelines, there is a need to gain a deeper understanding of this hazard. By monitoring present-day sedimentation processes, we can gain insights into the dynamics of erosion, deposition, and potential slope failures. As lakes are more accessible than marine settings yet share similar sedimentation processes, lacustrine deltas can serve as natural laboratories for any deltaic system. Our study employs a multi-method approach to monitor sedimentation processes within the Aare Delta of Lake Brienz, situated in a Swiss perialpine lake known for historical delta failures (Girardclos et al., 2007). This approach comprises (i) analyzing bottom currents derived from an Acoustic Doppler Current Profilers (ADCPs) campaign conducted from June to September 2022. These data are compared with river parameters (discharge, temperature, turbidity; from Federal Office of Environment) and meteorological data (rainfall, wind speed, directions; from Federal Office of Meteorology) to evaluate the governing processes of underflows, and (ii) examination of high-resolution bathymetric difference maps derived from two surveys conducted in 2018 and 2023. This assessment seeks to understand geomorphic changes over time and establish connections between these changes and the observed bottom currents. We show the results of these campaigns that offer valuable insights into sedimentation processes within lacustrine deltas. Repetitive bathymetric surveys highlight substantial geomorphic changes in submerged channels, while ADCPs moored in those areas reveal the presence of underflow currents. Yet, the exact triggers behind these events remain unclear, challenging our understanding of sediment-transport mechanisms within the Aare Delta.