摘要
受三峡工程运用与洞庭湖出流顶托影响,下荆江河段的进口水沙条件及其出口侵蚀基准面发生改变,河床冲淤调整剧烈,平滩河槽形态及过流能力变化显著,威胁到两岸的防洪安全。首先采用一维水动力学模型及河段平均的河床演变分析方法,计算了下荆江2002—2016年河段尺度的平滩特征值(平滩宽度■、水深■、面积■及流量■),以及石首与监利两站相应警戒水位下的过流流量(Q_(wn)~(SS)、Q_(wn)~(JL))。其次分析了上述参数对上下游边界条件的响应,并建立了这些参数与前5年汛期平均水流冲刷强度■(上边界)、当年上下游汛期平均水位差■(下边界)之间的单因素及多因素响应关系,并对综合关系式进行了率定与验证。结果表明:(1)受大规模护岸工程控制,下荆江河床调整以河段平滩水深增加为主,增幅为8.8%,相应宽深比减小8%,河床趋于窄深;(2)河道过流能力年际间变化较大,无明显单向增加或减少趋势,其中河段平滩流量介于27 401~34 548 m~3/s之间,多年平均值为31 335 m~3/s,而石首及监利两站警戒水位下过流流量的多年平均值分别为36 976、34 381 m~3/s;(3)在综合关系式中,对于河段平滩面积■而言,上边界■占比的平均值约为96%,且■随■的增加而增大;对于河道过流能力(■、Q_(wn)~(SS)、Q_(wn)~(JL))而言,下边界■占比的平均值约为86%,且■、QwnSS和QwnL均随■的增加而增大。故下荆江河段平滩河槽形态调整主要与进口水沙条件有关,而过流能力调整主要受出口侵蚀基准面条件(洞庭湖出流顶托)控制。
The Lower Jingjiang Reach(LJR)underwent remarkable morphodynamic evolution owing to the combined effects of the altered flow-sediment regimes caused by the upstream operation of the Three Gorges Project(TGP),and the local base-level changes because of the downstream confluence from the Dongting Lake.Variations in bankfull channel geometry and flood-discharge capacity of this reach were investigated by a one-dimensional hydrodynamic model and a reach-averaged method,covering the changes in reach-scale bankfull geometry(width ■,depth ■ and area ■)and discharge(■),as well as thespecified discharge under the warning levels at the water gauge stations of Shishou(Q_(wn)~(SS))and Jianli(Q_(wn)~(JL))during the period 2002-2016.Furthermore,these variables were represented by empirical functions of two key hydrodynamic parameters,covering the previous five-year average fluvial erosion intensity during floodseasons at Jianli(upstream boundary ■),and the corresponding difference between the average water stages at Xinchang and Lianhuatang(downstream boundary ■).Calculated results indicate that:(i)owing to the impacts of various river regulation engineering,channel evolution in the LJR was mainly characterised by the variation in bankfull depth,with an increase of 8.8%and 9.9%in the reach-scale bankfull depthand area from 2002 to 2016,respectively;(ii)the flood-discharge capacity of this reach varied greatly in different years after the TGP operation,showing no monotone increasing or decreasing trend,with the average values of ■,Q_(wn)~(SS),Q_(wn)~(JL) being 31 335,36 976 and 34 381 m~3/s,respectively;and(iii)the adjustments in the channel geometry were mainly controlled by the upstream boundary condition,with the value of ■ increasing with a larger value of ■,but the variation in the flood-discharge capacity was mainly influenced by the downstream boundary condition,with the values of ■,Q_(wn)~(SS) and Q_(wn)~(JL) increasing with a larger value of ■.
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