Evolutions of rapids of creek-bar at Gezhouba Reservoir and their impacts on navigable channael in the Changjiang River

Abstract

葛洲坝水库位于长江三峡,库容15.8×10⁸ m³,水库长度188.1km,其中变动回水段占66%。溪口潍主要由粒径大于20mm的卵石推移质组成。葛洲坝库区的卵石推移质有65%来源于灰岩为主补给区,该补给区卵石推移质数量大,颗粒粗,易成滩,所以库区溪口滩有92%分布在以灰岩为主的补给区。葛洲坝库区溪口滩可分洪水急流滩、洪中水急流滩、中水急流滩、中枯水急流滩与枯水急流潍等5种类型,建库后库区回水变动段占总数50%的溪口滩得到改善。但是占总数50%的位于库尾段的溪口滩与变动回水段洪水急流滩溪口滩,未能得到改善。随着这些溪口滩得到卵石推移质补给,航道将会恶化。在三峡水库建成前应加强对溪口滩的观测、研究与整治。;Almost at every mouth of the tributary creeks of the Three Gorges in the Gezhouba Reservoir, the alluvial fans or alluvial cones formed the rapids called rapids of creek mouth-bar, which are composed of pebble bed load with d > 20mm. Based upon geological and geomorp ho logical investigations, the authors detect that the pebble bed load at the Gezhouba Reservoir comes from 2 sources, namely, region of main limestone and region of main pyrolith, and converges at the Gezhouba Project. After evaluating the petrography of the pebble bed load in the source regions and the converging region, the authors establish a model for calculating the converging percentage of the pebble bed load, 65% of which comes from the region of main limestone, and the rest from other regions. The region of main limistone had more pebbles, with coarser grain easier to deposit, resulting in the rapids of creek mouth-bed formed by those pebble bed load more abundant and stable than the ones in the region of main pyTolith. It is found that 11 of the rapids of creek month-bar are from the region of main limestone. Considering the water level fluctuations, the rapids of creek mouth-bar can be divided into 5 catagories, i. e. flood water ones, flood-medium water ones, medium water ones, medium-low water ones and low water ones. Due to the rise of water level after building the Gezhouba Reservoir, the navigable channels in the area of rapids of creek mouth-bar within the reach of varied backwater region are improved, taking up 50% of all 12 rapids. But in the tail reach where the water level rises less than 1 m, the flow of flood water rapids is identical even after the Reservoir is buiJt, the conditions of navigable channels are little improved and even under deterioration, because of the continuous supply of pebble bed load. Were some counter-measures on those rapids in the tail reach not employed before the Three Gorges Project comes into operation, the rapids will continue to interfere with the relevant navigable channels because of their constant replenishments.