A prolonged lack of rain and river discharge, combined with extended heat periods from 2016 to 2019 and increasing inputs of agriculture-based fertilizers, reduced dissolved oxygen levels in many Central European lakes to nearly zero. Without freshwater inflow and vertical mixing, this led to widespread die-off of flora and fauna, accompanied by characteristic decay odors.
Thermal stratification is highly sensitive to numerical instabilities and dispersion effects in CFD simulations. In this context, dispersion represents an artificial diffusion caused by linearization effects and shortcomings in numerical schemes.
In flowing waters, these effects are largely negligible for practical applications beyond basic research. A dispersion rate of 1 cm/h is insignificant in a main current of 2 m/s. However, after 20 days (480 h) without inflow or precipitation, the thermocline of a lake would be artificially smoothed over approximately 480 cm in depth in a numerical model.
using an academic water tank model.
Small-scale lake models are additionally affected by a high proportion of boundary nodes, which represent a major source of numerical instability.
The objective of the project was to identify advection schemes with minimal or no numerical dispersion and to locate and reduce additional dispersion sources within other components of Telemac 3D. These include turbulence models, wet/dry algorithms, and related modules.
The small “Lake Monsterloch” (English: “Monster’s Hole”) served as an ideal prototype for many Central European lakes that experienced similar conditions during the exceptionally hot summers of the past decade.
Telemac proved to be a suitable tool for resolving vertical heat exchange and simulating mixing processes under low-flow or stagnant conditions, provided that the valid parameter ranges of the core routines are well understood and a small number of minor code corrections are applied.
The project was carried out in cooperation with the BAW, which applies the developed methods within the “Blaues Band Deutschland – Monsterloch” project.
Finally, Telemac-3D was enhanced to simulate not only stagnant waters, but also long-term mixing effects induced by wind, groundwater, surface inflow, precipitation, and other external forces over periods of several months.
Read More:
MERKEL, U. 2019. Thermal Stratification in Small Lakes with TELEMAC-3D: Showcase “Lake Monsterloch”. In: Proceedings of the XXVIth TELEMAC-MASCARET User Conference, Paris, France ->Download<-