Study On The Influence Factors Of Transparency By Experiment And Numerical Model In Lake Taihu

Water transparency is an important parameter characterizing lake optics, which can directly reflect light field distribution below water surface of lakes. Light field below water surface is a significant factor of lake ecosystem, which has direct effects on the growth and propagation of aquatic plants. Therefore, the basis and prerequisite for aquatic plant restoration is to understand the main factors affecting water transparency and study on efficient control measures. Water transparency of Lake Taihu is mainly affected by suspended solids (SS), concentrations of algae and dissoluble organic matter, all of which are closely related to hydrodynamic processes of the lake. In this paper, the characteristics and changes of the effecting factors of water transparency under different hydrodynamic conditions were studied by lab and field experiments. The data from these experiments were presented in models for respective effecting factor. The water transparency model of Lake Taihu under hydrodynamic conditions was developed by coupling the models of each effecting factor. This coupling model provides scientific basis for restoration of aquatic plants and rehabilitation of healthy ecosystem of Lake Taihu.The effecting factors of water transparency of Lake Taihu are related to hydrodynamic conditions. Thus, study on hydrodynamic characteristics of Lake Taihu is the basis and prerequisite for water transparency study. In this paper, we analyzed the basic characteristics of lake current and wave of Lake Taihu based on measured data. Respective numerical models were developed. Lake current model and wave model (SWAN model) were coupled to analyze the effects of wave on lake current. In swan the evolution of the wave spectrum is described by the spectral action balance equation. Considering conditions such as wind input, bottom friction, bottom topography, whitecapping, depth-induced wave breaking, wave refraction, nonlinear wave-wave interaction, lake current etc., wave parameters (wave height, wave period, wave length, wave steepness and the orbital velocity near the bottom, etc.) under the condition of dominant wind at Lake Taihu area were simulated using this model. The effecting factors of wave of Lake Taihu were also studied.SS is the main factor affecting water transparency of Lake Taihu. Lab and field experiments were conducted to study on the resuspension and sedimentation characteristics of Lake Taihu sediments under different hydrodynamic conditions, and then develop SS model of Lake Taihu. By using bench-scale circular water tank, the starting pattern of Lake Taihu sediment was simulated. From this experiment, the incipient velocities under different starting standards were tested and then the quantitative correlations betweennutrient (nitrogen and phosphorus) release flux and flow velocity were developed. By field experiments, resuspension and sedimentation patterns of sediment, changes of nutrients induced by the process of sediment resuspension and sedimentation under different hydrodynamic conditions were mainly studied. The calculation method of resuspension flux was discussed, and then the correlations of resuspension flux of Lake Taihu sediments, SS sedimentation flux and hydrodynamic conditions were established. Additionally, settling experiment under static condition was conducted to calculate the settling velocity of SS under different concentrations. The hydrodynamic SS model of Lake Taihu was developed based on these experimental results and SS characteristics. In the model, the disturbance of wave and lake current to sediments was considered. The suspension amount and sedimentation amount of sediments close to bed surface were treated separately, and starting conditions of sediment suspension were introduced in the model. Dynamic simulation of SS changes under different hydrodynamic conditions was conducted.Additionally, algae and dissoluble organic matter are the minor factors affecting water transparency, because they can absorb and disperse light. However, the effects of algae and organic matter on water transparency are relatively uncertain. Effects of algae are different by seasons. The effects are more significant in summer. Effects of organic matter are not distinct, but cannot be overlooked. In this paper, eutrophication model and water quality model of Lake Taihu were developed according to measured data, to simulate the transportation and diffusion patterns of algae and dissoluble organic matter under hydrodynamic conditions.Temporal and spatial distribution of water transparency of Lake Taihu was analyzed based on measured data. And the multiple regression equations for Lake Taihu water transparency and three effecting factors (SS, algae, CODmr) for different seasons and lake parts were deduced. Numerical models for lake current, wave, SS, water quality and algae were coupled to simulate water transparency of Lake Taihu based on the multiple regression equations.