Quantifying the hydrodynamic impacts of cumulative sand mining on a large river-connected floodplain lake: Poyang Lake

Sand mining has significant impacts on the hydrological and hydrodynamic regimes of river-lake systems. Recently, continuous sand mining has occurred in Poyang Lake, a large floodplain lake connected to the Yangtze River. Previous studies on the impact of sand mining in Poyang Lake focused on water level variations. However, the influence of different magnitudes of sand mining on the hydrological and hydrodynamic regimes in such a complex river-lake floodplain system is unclear. The current study aims to address this issue by combining a hydrodynamic model and hydrological data. Based on average conditions (2000-2010), the influences of 1-4 m uniform sand mining in the northern lake on water levels and outflows were quantified, and the spatial and temporal impacts of cumulative sand mining (1998-2010) on current speeds and residence times were further assessed. The results showed that the elevations of the thalwegs in the northern outlet channels decreased unevenly from 1998 to 2010. The water levels decreased cumulatively but nonlinearly with increasing sand mining. Mining 1-4 m of sand caused cumulative water level decreases of 0.55-2.14 m in the northern channels when the water levels were lower than 14 m. Due to cumulative sand mining from 1998 to 2010, the current speeds in the northern mining channels generally decreased, and those in the central channels mostly increased, with the majority varying from - 0.4 to 0.4 m/s. The residence time changes were opposite to the current speed changes, with -4 to 10 day variations. The opposite outflow variations between uniform mining and realistic uneven mining indicated that outflow variations depended on the mining magnitudes and bed slope changes. The above results suggested that for this river-connected floodplain lake, sand mining in outlet channels enhanced the spatial and temporal heterogeneity of the lake hydrodynamics and that the blocking effect of the connected large river impacted the hydrodynamic variation patterns. This study leads to better understanding of the effects of different magnitudes of sand mining on the hydrologic and hydrodynamic regimes in Poyang Lake and provides a reference for quantifying similar human activities in complex floodplain lakes.