Lai Xijun

The characteristics of flood responses to the restoration of polders on Dongting Lake, China

Abstract The serious shrinkage of Dongting Lake in China has led to the deterioration of its flood diversion and storage capacity, and now flood disasters have become more frequent. A lake restoration strategy is proposed in order to relieve flood-prone areas. However, flood movement is a dynamic process, so that approaches neglecting the hydrodynamics cannot accurately represent flood responses to restoring polders to a lake in a complex river basin. Here, a coupled one-dimensional and two-dimensional hydrodynamic model was applied to investigate flood responses to restoring polders to Dongting Lake under the conditions of the 1998 flood event. Quantified flood reduction effects show that the characteristics of flood responses vary significantly with the management option chosen. Appropriate strategies of lake restoration are suggested according to different flood responses. Polders of type II, which favour flood discharge, should be restored in order to streamline flood conveyance. Considering the ecological requirements of the lakes and wetlands, type I polders far from main flows can be partially restored, resulting in evacuation of residents to safe areas but keeping the cultivation.

Numerical simulation of thermal discharge based on FVM method

A two-dimensional numerical model is proposed to simulate the thermal discharge from a power plant in Jiangsu Province. The equations in the model consist of two-dimensional non-steady shallow water equations and thermal waste transport equations. Finite volume method (FVM) is used to discretize the shallow water equations, and flux difference splitting (FDS) scheme is applied. The calculated area with the same temperature increment shows the effect of thermal discharge on sea water. A comparison between simulated results and the experimental data shows good agreement. It indicates that this method can give high precision in the heat transfer simulation in coastal areas.

Two-dimensional numerical simulation of hydrodynamic and pollutant transport for Lake Poyang

The present study aims to develop a mathematical model for analyzing and predicting the hydrodynamic and water quality in one large lake(Lake Poyang) with great and rapid stage fluctuation,the complex topography and geometry,the complicated connectivity of narrow channels and depressions inside the lake,the frequently switch between exposed grass beach and water.Based on the two-dimensional shallow water equations and pollutant transport equations,the coupled two-dimensional hydrodynamic and pollutant transport model is developed.The unstructured finite volume method is applied to discretize the equations.The normal water mass,momentum and pollutant fluxes are computed by the HLLC solver.The water front moving boundary is identified automatically by the judgment of the latest water stage.It makes the model be capable of simulating the complex flow regime and the dynamic of pollutant transport in the lake with drying and wetting processes.The hydrodynamic model is calibrated by the water regime of the year 1998.Based on the calibrated hydraulic parameters,the hydrodynamic and water quality(CODMn and ammonia) from January to October in 2008 are simulated by the proposed model.The time series data of measured water stage,CODMn and ammonia,and remotely water extent of CBERS data are both used to validate the results of computation in time and space.The results indicate that the model has the capability to model the flows dynamic and the pollutant transport in this kind of lake.

Pattern of impoundment effects and influencing mechanism of Three Gorges Project on water regime of Lake Dongting

With the operation of Three Gorges Project(TGP),water regime of rivers and lakes downstream of the dam changed with the reservoir regulation.The lowering of water level becomes normal after flood season compared with the years before TGP.Here,the water storage of TGP in 2006 was selected as a case to quantitatively investigate the effects of TGP on water regime of Lake Dongting.We applied the coupled hydrodynamic analysis model for the middle Yangtze River to compute the variation of water regime induced by the water storage.During impoundment,the reduction of the water level and discharge are 1.32m and 466m3/s,respectively,at Chenglingji,the outlet of Lake Dongting.The bottomlands inside the Lake Dongting are exposed ahead of time and the lake area with over 5 inundation days reach to 287 km2.Results also indicate that: 1) the impact of TGP on water regime varied with locations: east Lake Dongting takes the first place,the east of South Lake Dongting and the north of West Lake Dongting second,and the west of South Lake Dongting and the south of West Lake Dongting third.2) The bottomlands which strongly affected by the impoundment are mainly located at East Lake Dongting,South Lake Dongting and the both sides of flood ways.3) Two influencing mechanisms on water regime are identified: one is the increase of the lake discharge induced by the rapid drawdown of the mainstream of the Yangtze River;the other is the reduction of inflow from the Yangtze River via Sankou distributary channels.

Discharge capacity analysis on Lake Poyang

The recently frequent low water level in Lake Poyang attracted wide public attention. As an open lake,Lake Poyang is naturally connected with the Yangtze River via the Hukou Waterway. The interactions between the Yangtze River and the Lake Poyang are directly affected by the flow characteristics of the Hukou Waterway. The discharge capacity is introduced to represent the ability to convey water away using the hydraulic approach. The discharge capacity and its change is investigated here. Results show that: The discharge capacity was stable before 2000s and thereafter it rapidly increased. Its remarkable increase altered the interaction between the Yangtze River and Lake Poyang,leading to the recent severe drought regime. Large-scale sand dredging in Lake Poyang is attributed to be the cause of this increase. The management for sand dredging should be strengthened for conserving the water resources in Lake Poyang.