INTRUSIVE AND BOTTOM DENSITY CURRENTS AND INDUCED VERTICAL EXCHANGES IN A STRATIFIED LAKE

摘要

This paper describes the phenomenon of both intrusive and bottom turbidity currents in a stratified lake. The analysed behaviour is based on observations in nature and results of numerical simulations. The dynamic of the induced currents inside a temperature-stratified lake is presented and discussed for variable density of the tributary inflow due to suspended sediment load during a flood event. The developing interflow has a rather high Richardson number and behaves as a subcritical density current with negligible water entrainment or mixing through the interfaces. It spreads out slowly and almost radial along the thermocline. As soon as the density of the current exceeds the one of the thermocline in the stratified lake, it plunges on the bottom of the lake following the slope of the subwater topography. As the bottom slope is rather steep, some 26°, the bottom current behaves as a supercritical turbidity current with heavy water entrainment and mixing through the interface. This rapid current is also at the origin of a considerable vertical upward flux, lifting up oxygen-poor and phosphorus-rich deep waters, which is at the origin of surface water pollution during major flood events in Lake Lugano.