江湖流量分配变化导致长江中游新的洪水形势

Changes of Flood Situation Along the Middle Yangtze River

本文全面论述了江湖流量分配变化导致长江中游出现的新的防洪形势。文中首先举出江湖流量分配的巨大变化。大约在３０年代，四口分流处于鼎盛时期的尾声，尚能分走荆江年径流量约４３１％，洪峰流量约５２７％，以后逐渐衰减，目前（８０年代至９０年代），仅能分走年径流量１５５％，洪峰流量约２９４％。从而使下荆江径流量较３０年代加大约１２３９×１０８ｍ３，洪峰流量加大约１７２００ｍ３／ｓ。可见这种变化是很惊人的。其次全面论述了这种变化对荆江与洞庭湖防洪的影响。仅与５０年代相比，防洪形势有下述变化。第一，荆江特别是下荆江因流量加大抬高的水位大于冲刷（包括裁弯冲刷）降低的水位，从而使洪水位抬高１～２ｍ。第二，莲花塘至武汉因含沙量加大约２５％而淤积，洪水位也有明显的抬高。第三，三口分流河道因径流量减少，加速了衰退与萎缩，淤积比增大，有的河道水环境趋于恶化，严峻的防洪形势并未缓解。第四，洞庭湖泥沙淤积减缓，对保留更多的湖容颇为有利，但湖口城陵矶水位抬高，则对东洞庭湖以至南洞庭湖不利。从总体看，目前这种变化对长江中游是不利的，这种不利突出反映在１９９８年的洪水中。文中在论述这种变化时特别以１９９８年为例予以阐明。

The new situation of flood control along middle reach of the Yangtze River resulted from the variation of the relationship between the Yangtze River and the Dongting Lake is comprehensively revealed in this paper. First, the variation of the discharge rate distributed to the River and the Lake separately is concerned. The diverted discharge from the Jingjiang River to Dongting Lake at four outlet with 43.1% of annual amount and 52.7% of peak amount in 1930's was reduced gradually to 15.5% of annul amount and 29.4% of peak amount in 80's and 90's, and thus at Jingjiang River the annual discharge is increased by 1239×10 8 m 3 and the peak discharge by 17,200m 3. This variation is tremendous. Second, the influence of the discharge variation on flood control of Jingjiang River and Dongting Lake is described in the following aspects: 1.The enlarged discharge at Jingjiang River, especially at its lower part, as well as the cut-off project led to a serious scouring of the river bed, but the raised level because of the increased discharge is larger than the reduced level caused by scouring and results in 1～2m of level increment in flood season; 2.The deposition at the river reach from Lianhuatang to Wuhan resulted from 25% increment of concentration leads to a raise of flood level; 3.The branches from the diverted outlet to Dongting Lake are decaying more rapidly due to the higher rate of sedimentation, the water environment turns worse and the flood control problems are still serious; 4.The storage reservation of Dongting Lake is benefit from less sedimentation, but at the outlet location Chenglingji the level raised due to the increase level at lower Jingjiang will not benefit the east and south lake; Therefore, from an overall point of view,the change of the relationship between the river and the lake makes the flood control problem at the middle reach of the Yangtze River more seriously. The flood level from Shashi to Wuhan in 1998 is taken as a typical example in this paper to show the effect of the change.