Parameterisation of the vertical transport in a small thermally stratified lake

Abstract.

Vertical transport in a small eutrophic lake was investigated. The study site is a gravel pit lake in the Upper Rhine Graben near Ludwigshafen, Germany (800 × 250 m, mean depth 9 m, maximum depth 20 m). The governing mixing processes show patterns influenced by a strong density stratification. From weekly measured temperature profiles, we quantitatively estimated profiles of effective vertical transport coefficients K _z . In the stratified regions, K _z is at the level of heat conduction (~1.4·10^-7 m²/s). Towards the lake bottom, we found a characteristic increase of K _z to values up to 10⁻⁵ m²/s. Using a bottom-mounted 1,200 kHz-ADCP, we performed velocity measurements with a high vertical and temporal resolution (0.2 to 0.4 m and 5 min, respectively) of the horizontal currents. Using these horizontal flow data and stability profiles, Richardson numbers were analysed for a period of 5 weeks during summer stratification. The simultaneous measurements of currents and vertical diffusion coefficients allowed us to check a given parameterisation of K _z (Ri) and to discuss its applicability to the investigated system.