Assessment of hydraulic performance changes of Dongting Lake after the Three Gorges Reservoir impoundment by a novel semi-empirical approach

Floodplain lakes are important water storage areas in lowland regions that often undergo geomorphologic evolution, and timely topographic data are generally unavailable. In this study, to assess the impacts of lakebed deformation on hydraulic performance in Dongting Lake, a set of semi-empirical methods was proposed to establish performance graphs (PGs) using only hydrological data. These methods were used to evaluate the changes in water level, storage capacity, and flood detention ability in Dongting Lake caused by topographic adjustment after the Three Gorges Reservoir impoundment. These methods showed that PGs can effectively simulate the water level and outflow processes of Dongting Lake with Nash-Sutcliffe efficiency coefficients (NSEs) above 0.9. A comparison of the estimated water level and discharge using PGs from different periods suggested that bed erosion in Dongting Lake caused water level decreases of 0.18 m and 0.32 m during the flood and dry seasons, respectively. Because the magnitude of erosion at high elevations in the lake is small, the impacts of bed adjustment on the storage capacity and flood detention ability are not currently significant. This study showed that the hydraulic performance of a floodplain lake can be evaluated independently of topographic data under the condition of no reverse flows or negative water surface slopes.