Using water age to quantify the hydrodynamic circulation in Lake Taihu using D-Flow Flexible Mesh.

Sien Liu, Qinghua Ye, Marcel Stive

Friday 3 july 2015

12:45 - 12:48h at Oceania (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: Poster pitches: 15K. Deltas - Wetland

Being the third largest freshwater lake and located in one of the most prosperous areas in China, Lake Taihu is suffering from cyanobacteria blooms and severe eutrophication problems. The local government has taken steps to both restrict the sewage effluent of nearby plants and transfer water from Yangtze River to flush Lake Taihu. However, the current ecological condition of Lake Taihu still needs improvement. The lack of knowledge on the lake's hydrodynamic circulation dynamics is a main reason for the deficiency of present improvement measures. The intention of this study is to investigate the hydrodynamic circulation and water exchange in Lake Taihu with the three-dimensional hydrodynamic model, D-Flow Flexible Mesh (D-Flow FM). D-Flow FM is the new computational engine for the unstructured grid in 1D, 2D and 3D. Using the concept of water age, hydrodynamic circulation indices are applied to indicate the circulation efficiency and evaluate the sensitivity of the environmental conditions. Although the hydrodynamics of the lake has been studied quite extensively, virtually no attention has been given to the river network between Lake Taihu and Yangtze River. The hypothesis that the river network is important is supported by the fact that Lake Taihu’s hydrodynamics is strongly influenced by river inflow and outflow discharge from the river network. Thus, the entire Taihu Basin river network is included in the model using D-Flow FM, with the river branches and Lake Taihu in 1D and 2D model. A hydrodynamic model is set up and calibrated with measured data. The water age as the circulation indices is coupled in the hydrodynamic model to quantify the hydrodynamic circulation. Scenarios are designed to test the influence of wind direction, wind speed and evaluate the water transfer plans with the circulation indices. The model results show the significance of wind in the hydrodynamic circulation and its dynamic effects for each subzone in the Lake Taihu. The large scale water transfer from Yangtze River project also helps to improve the water quality to decrease the occurrence of algae bloom in certain subzones of the lake. This study quantifies the water circulation and water exchange by using the hydrodynamic circulation indices as indicators, thus improving the understanding of water circulation of the entire river network. The conclusion of this study will help to understand similar large, shallow lake systems with complicated river network.