Using flexible mesh modelling to simulate turbidity in a shallow, oligotrophic lake

Woods Lake is an oligotrophic, polymictic, impoundment in Tasmania. It hosts several endemic species, as well as native charophyte and macrophyte beds. Hydro Tasmania manipulates the water level which is linked to increased turbidity and changes to the trophic status of the lake that may have undesired consequences to the lake ecosystem. To facilitate proactive environmental management Hydro Tasmania aims identify the role of water level manipulation in driving turbidity and fostering biological resilience. While field monitoring and experimental work have provided valuable insight into some aspects of the lake’s response to water level manipulation, a modelling approach is essential to providing comprehensive picture of system dynamics.

We use a flexible mesh finite volume numerical model (TUFLOW-FV), coupled with an aquatic ecodynamics library (AED2) and surface wave model (SWAN) to simulate hydrodynamic, sediment transport and water quality processes. With a range of water levels and wind conditions we use the model to investigate sediment resuspension and consequent turbidity under different combinations of wind speed and water level.

Since we use a 3D mesh to define the lake area we are able to explore responses to water level and wind speed in different regions of the lake. In Woods Lake the critical areas of interest are littoral zones and vegetation beds and using the 3D mesh these areas can be specifically targeted with fine resolution modelling.

We simulated vegetation response to water circulation, sediment resuspension, and changing light climates, which allows us to explore the threshold where turbidity has a detrimental effect on ecosystem processes as a function of lake depth.

Outputs from the model are validated against values collected in the field. Overall, the model output provides further evidence about the depth thresholds where lake turbidities become excessive and can be used to refine operational risk bands.