Hydro- and morphodynamics of turbidity currents induced by river inflows into lakes

Abstract

Turbidity currents are gravity currents driven by their sediment load. They occur in oceans and connect terrestrial sediment sources to deep-sea sediment sinks [1]. They also occur in lakes [2] and are a major cause of sedimentation and capacity loss of reservoirs [3]. Although the existence of turbidity currents was inferred in the 19th century [4], field measurements of turbidity currents remain scarce due to their notorious reputation of damaging instruments along their path [5]. Recently, ADCP-based measurements of turbidity currents in ocean canyons were reported [1]. Here, we present the first ADCP-based measurements of a turbidity current in a lake (Lake Geneva) and its interaction with the lake bottom. On 3 July 2018, a convective storm occurred in the high-mountain watershed of the Navisence, which is a tributary of the Rhône River situated about 80 km upstream of Lake Geneva. As a result, the Navisence supplied a high suspended sediment load to the Rhône River. Maximum sediment concentrations of O(10 kg m-3) were measured at the hydrological measurement station at Pt du Scex, located 6 km upstream of the Rhône River mouth. Since the sediment-laden Rhône waters were denser than the lake water, they produced a turbidity current in the canyon carved by the Rhône inflow on the bottom of Lake Geneva (Fig. 1). The characteristics of the turbidity current were measured with moored ADCPs at three locations along the canyon: at 1400 m, 2250 m and 3500 m from the river mouth. Figure 1 shows the measurement site at 1400 m where the ADCP measurements were taken.

Fig. 1: (top left) Lake Geneva, the Rhône River and its 14 km long canyon at the lake bottom. (top right) Longitudinal bottom profile along the initial part of canyon and cross-sectional profile of the measurement site at 1400 m from the river mouth. (bottom) ADCP-based velocity pattern during the passage of the turbidity current. The measurements revealed remarkable features. First, the peak discharge in the turbidity current at 1400 m from the mouth was more than an order of magnitude larger than the Rhône discharge at the mouth, which suggests a massive pick-up of sediment from the lake bottom. Second, the maximum velocities in the turbidity current were more than 2 m s-1 (Fig. 1), which is comparable to turbidity current velocities in much larger ocean canyons [6,7]. Third, the turbidity current weakened between the measurement sites at 2250 m and 3500 m from the mouth. This weakening was accompanied by a significant sediment deposition on the canyon bottom (not shown). [1] Talling PJ et al. Nature Communications 13(1): 4193, 2022. [2] Lambert A & Giovanoli F. L&O, 33(3): 458-468, 1988. [3] De Cesare G et al. J. Hydraul. Eng. 127(1): 6-16, 2001. [4] Forel FA. Le Léman : Monographie limnologique, Vol. 1, F. Rouge Ed., 1892. [5] Talling PJ. Marine Geology, 352: 155-182, 2014. [6] Azpiroz-Zabal M et al. Science Advances 3(10), 1700200, 2017. [7] Paull CK et al. Nature Communications 9(1), 4114, 2018.