长江荆江河段航道水深条件与河流演变的关系

Given the importance of waterway depths in river development,the effects of the evolution of bars and troughs on waterway expansion play an important role in river management and water depth conservation.This study aims to expand the waterway dimensions of the Jingjiang Reach of the Yangtze River.To achieve this objective,determining the relationship between river evolution processes and the potential for waterway depth improvement and navigation hindrances is vital.Therefore,the sedimentation,hydrological,and terrain data of the Jingjiang Reach from 1955 to 2020 are analysed to elucidate the above-mentioned relationship.Since the commissioning of the Three Gorges Dam,the scouring of the low-flow channel has accounted for 90%-95%of all scouring in the Jingjiang Reach.Furthermore,the central bars and beaches have shrunk by 9.4%and 24.9%,respectively,and 18.3%overall.Considering the bed scouring and waterway regulation projects in the Jingjiang Reach,we investigated the continuity of a 4.5 m×200 m×1050 m(depth×width×bend radius)waterway along the Jingjiang Reach,and find that navigation-hindering channels account for over 5.3%of the waterway length.Furthermore,part of the Jingjiang Reach is an important nature reserve and shelters numerous water-related facilities,which inhibits the implementation of waterway deepening projects.The findings of this study demonstrate that numerous challenges are associated with increasing the waterway depths of the Jingjiang Reach.

长江中游荆江河段同流量—水位演化特征及驱动成因

水库运行改变了坝下游水沙输移条件,在河道冲刷的同时,引起水位过程出现适应性调整。本文以长江中游荆江河段为对象,采用多项式拟合法,对比分析1991—2016年间分级流量—水位变化特征,采用基于河流动力学原理的分离变量法,识别河道冲淤、下游控制水位及河床综合糙率等变化对分级流量—水位变化的影响程度。研究表明:1991—2016年间,长江中游荆江河段同流量—枯水位呈下降趋势,2009年以来降幅增大;河道冲刷是引起同流量—枯水位下降的主控因素,河床综合糙率增加抑制了同流量—枯水位下降起到积极作用。1991—2016年间,荆江河段同流量—洪水位经历了先减小后增大的"凹"线型变化,2003年以前洪水特征为"高洪水流量—高水位",2009年以来逐渐演化为"中大洪水流量—高水位",同流量—洪水位特性发生转变;河床综合糙率增大是同流量—洪水位抬升的主控因素,河道冲刷抑制了同流量—洪水位的抬升态势。在航道条件及防洪情势上,应重点防控近坝段沙质河段冲刷引起的水位下降溯源传递作用,其洪水流量—水位特性的转变,不利于减缓荆江河段的防洪压力。

Quantitative relationship between channels and bars in a tidal reach of the lower Yangtze River:Implications for river management

Deep-water navigation channels in the tidal reaches of the lower Yangtze River are affected by water and sediment fluxes that produce complex shoals and unstable channel conditions.The Fujiangsha reach is particularly difficult to manage,as it has many braided channels within the tidal fluctuation zone.In this study,hydrologic and topographic data from the Fujiangsha reach from 2012 to 2017 were used to examine the variations in deposition and erosion,flow diversion,shoals,and channel conditions.Since the Three Gorges Dam became operational and water storage was initiated,the Fujiangsha reach has shown an overall tendency toward erosion.Channels deeper than 10 m accounted for 83.7% of the total erosion of the Fujiangsha reach during 2012-2017.Moreover,the dominant channel-forming sediments have gradually changed from suspended sediments to a mixed load of suspended and bed-load sediments.Deposition volumes of these sediments has varied significantly among different channels,but has mainly occurred in the Fubei channel.Furthermore,periodic variations in the Jingjiang point bar have followed a deposition-erosion-deposition pattern,and the downstream Shuangjian shoal mid-channel bar has been scoured and shortened.Approximately 44.0% of the bed load from the upstream Fujiangsha reach is deposited within the 12.5-m deep Fubei channel.The increased erosion and river flow from the Jingjiang point bar and the Shuangjian shoal during the flood season constituted 59.3% and 40.7%,respectively,of the total amount of siltation in the Fubei channel.

Evolution characteristics and drivers of the water level at an identical discharge in the Jingjiang reaches of the Yangtze River

The operation of large-scale reservoirs have modified water and sediment transport processes,resulting in adjustments to the river topography and water levels.The polynomial fitting method was applied to analyze the variation characteristics of water levels under different water discharge values in the Jingjiang reach of the Yangtze River from 1991–2016.The segregation variable method was used to estimate the contributions of the varied riverbed evaluation,the downstream-controlled water level,and the comprehensive roughness on the altered water level at an identical flow.We find that low water levels in the Jingjiang reach of the Yangtze River from 1991–2016 are characterized by a significant downward trend,which has intensified since 2009.Riverbed scouring has been the dominate factor causing the reduced low water level while increased roughness alleviated this reduction.From 1991–2016,there was first a decrease followed by an increase in the high water level.The variation characteristic in terms of the'high flood discharge at a high water level'before 2003 transformed into a'middle flood discharge at a high water level'since 2009.The increased comprehensive roughness was the main reason for the increased high water level,where river scouring alleviated this rise.For navigation conditions and flood control,intensified riverbed scouring of the sandy reaches downstream from dams enhanced the effects that the downstream water level has on the upstream water level.This has led to an insufficient water depth in the reaches below the dams,which should receive immediate attention.The alteredvariation characteristics of the high water level have also increased the flood pressure in the middle reaches of the Yangtze River.