大通至徐六泾一维水沙数值模拟的下边界问题研究

Study on tidal boundary conditions in 1-D numerical modeling of hydrodynamic and sediment transport in Datong to Xuliujing reach of the Yangtze River

在仅有进口来水来沙过程而缺乏配套的出口潮位和含沙量情况下,下边界的确定是实施感潮河段水沙数值模拟的难点之一。针对该问题,以长江口徐六泾断面为例,采用理论分析与观测资料统计相结合,提出了简捷的下边界处理模式。结果表明:在徐六泾断面附近,径流和潮汐对潮位的影响可线性迭加,据此可建立潮位的经验计算模式;根据断面含沙量变化由时间所主导的特点简化悬沙输移方程,可导出下边界海向来沙计算模式。将下边界的概化计算处理模式纳入大通至徐六泾一维水沙数学模型,计算的下边界逐时潮位误差绝对值小于0.3 m的占比达72.3%,计算的逐时输沙率与实测值之间决定系数达0.83,河段内沿程潮位和输沙量也能取得较满意的模拟效果。

Because detailed data of tidal level and sediment concentration is often unavailable,downstream boundary conditions are difficult to develop in numerical modeling of hydrodynamic and sediment transport in tidal reach.As a case study,the gauged data of tidal level and sediment concentration at Xuliujing Station in the Yangtze Estuary was analyzed and a simple mode to build boundary conditions was proposed.The results show that,tidal level can be calculated by the addition of a runoff related water level and the astronomic tidal in the open estuary downstream of Xuliujing.Based on the characteristics of sediment transportation in estuary,the suspended sediment transport equation was simplified and a relation for sediment boundary condition were derived.Result shows that the sediment concentration at the downstream boundary was determined by the weighted sum of the present sediment transport capacity and the preceding sediment concentration.By the downstream boundary conditions,1 D numerical simulations of flow and sediment processes in the Datong to Xuliujing reach were carried out.The calculated results show that hourly tidal level with error less than 0.3 m accounted for 72.3%of the total data,and the determination coefficient between the calculated hourly sediment transport rate and the measured value was 0.83 at the downstream boundary.Furthermore,calculated results inside the long reach agreed well with the gauged data.