Water depth resources and driving mechanism of navigation hindrance of Jingjiang Reach watershed after Three Gorges Project operation

Abstract

系统解析长江中游河湖交汇区域航道水深资源的变化,可支撑“黄金航道”的可持续发展。本研究以洞庭湖-荆江交汇段为对象,研究洞庭湖分汇区域水沙条件、荆江河段滩槽演变与河湖交汇段航道水深资源的关系。研究表明:三峡工程运行以来,洞庭湖三口分流量和分沙量延续了三峡工程运行前的减少态势,伴随洞庭湖分流量减少,相对增加的长江干流径流量增强了河床冲刷强度,反馈使得洞庭湖三口分流量和分沙量均处于较低值;荆江河段河床冲刷给航道尺度提升奠定了有利基础,河湖分汇区域不满足4.5 m×200 m(水深×宽度)的长度为12.6 km,占荆江河段碍航总长度的68.35%。碍航驱动机制上:松滋口分流区段(枝城-昌门溪)的4.5 m水槽贯通但宽度不足200 m,汊道分流关系不稳定及洲滩萎缩制约航道条件稳定,枯水位下降及“坡陡流急”现象仍然严峻,不利于航道条件保障及船舶安全航行;太平口分流区段水位下降、洲滩萎缩、汊道交替发展使得枯水航路不稳定或水深不足4.5 m;藕池口分流区域的洲滩崩退、洲滩冲刷引起的向下游泥沙输移,碍航表现为航宽不足200 m或出现水深不足4.5 m的浅滩;洞庭湖入汇影响区段(熊家洲-城陵矶)受弯道冲淤特性、湖泊出流顶托作用等影响,航槽内出现水深不足4.5 m的浅滩,但顶托作用存在趋缓趋势,有利于航道条件的改善。;A systematic analysis on the effects of the changed interactive relationships between rivers and lakes on fairway depth can help to support the sustainable development of the “Gold waterway” of the Yangtze River. In this study, we investigate the variation characteristics of water and sediment, and the relationships between the evolution of shoals and channels and the fairway depth, in the Lake Dongting-Jingjiang Reach confluence of the Yangtze River. The results indicate that since the operation of the Three Gorges Project, the split ratio of watersplit and the sediment distribution of the three outlets of Lake Dongting continued to decrease. Along with the decreasing flow of Lake Dongting, the relative increase in the runoff of the Jingjiang Reach increases the riverbed erosion scouring intensity, which keep the split ratio of water split and the sediment distribution of the three outlets at a lower value. The riverbed erosion scouring of the Jingjiang Reach has created a promising foundation for an increase in the waterway dimensions. However, the length of the reach fail to meet the requirement of 4.5 m× 200 m (water depth × width) is 12.6 km in the river-lake confluence, accounting for 68.35% of the total length of obstructed navigation. The mechanisms of the obstructed navigation are as follow: at the reach affected by the first outlet (Songzikou), a 4.5 m flume has been formed, but its width is less than 200 m. The instability of water split and the shrinkage of bottomland restrict the stability of channel conditions. Moreover, the reductions of low-flow water level and the phenomenon of “steep slope and rapid current” affect the ships' safety. At the reach (Taipingkou) affected by the second outlet, the shrinkage of the bottomland, and the intersecting braided river channels make the low-flow routes unstable, the water depth being less than 4.5 m. At the reach (Ouchikou) affected by the third outlet, the shoreline of the reach has collapsed and retreated significantly. Additionally, point bar beach erosion scouring has led to the downstream movement of sediment, which have caused the downstream waterway width at the reach to be reduced to less than 200 m or formed shoals with a water depth of less than 4.5 m. Owing to the characteristics of scouring and deposition in the bends reaches and the jacking effect of the lake's outflow backwater, the water depth of some shoals are less than 4.5 m in the navigation waterway. However, the jacking effect of the lake's outflow backwater is weakening, which benefits the improvement of channel conditions.