“退田還湖” による洞庭湖の遊水池機能回復が長江中流域の洪水防御にもたらす効果

抄録

To evaluate the flood protection effect of the conversion of polders to retarding basin around the Dongting Lake in the middle region of the Changjiang River, China, we applied the integrated watershed hydrological model using gauged daily precipitation data of 1998 when the second largest flood occurred in the basin in the last century. While the estimated storage capacity of the Dongting Lake increased 11.5×10⁹ m³ by the conversion of polders in comparison with the estimated capacity at the flood period in 1998 at the average water level of 35.0 m, the simulated daily average water levels of the lake showed that the protection of the flood was impossible by only increasing capacity in the confluence between the Changjiang mainstream and the Dongting Lake in case of 1998 flood. The model also simulated that the flood protection was fully achieved at the confluence by making the volume diverging from the Changjiang mainstream to the Dongting lake increase with the diverging ratio at 1950s under the appropriate discharge control by the Three Gorges Dam. These results suggest that “return land to lake” policy around the Dongting Lake probably brings the best flood protection effect when the increased capacity of the lake uses for the control of the water level in the Jinjiang section of the Changjiang mainstream.