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三峡水库蓄水后荆江洲滩变化特征
薛兴华1,2,, 常胜1,2, 宋鄂平1,2
1. 湖北民族学院林学园艺学院,恩施 445000
2. 生物资源保护与利用湖北省重点实验室,恩施 445000

作者简介:薛兴华(1976-), 男, 湖北宣恩人, 博士, 副教授, 主要从事河流地貌与景观生态研究。E-mail: xinghua_xue@163.com

摘要

目前对三峡水库蓄水后荆江河段的洲滩演变还缺乏完整认识。基于三峡水库蓄水前后枯水期遥感影像,分析了荆江洲滩的冲淤变化与分布及形态演变。结果表明,蓄水后荆江洲滩总面积持续冲刷减小,累计冲刷4.56 km2,大部分发生在蓄水后前6年(冲刷速率0.55 km2/a)。上、下荆江洲滩的冲淤演变存在差异性。上荆江洲滩总面积一直处于冲刷萎缩中,且其强度明显大于下荆江,累计冲刷6.46 km2;下荆江前期(2002-2009年)冲刷、后期(2009-2015年)淤增,累计淤1.90 km2。在冲淤分布上,上荆江凸岸滩持续冲刷萎缩,凹岸滩前期冲刷、后期略有淤增,心滩(洲)前期淤积增长而后期冲刷萎缩;下荆江主要是凸岸滩冲刷,凹岸滩和心滩(洲)有所发展。根据滩体位置活动和冲淤动态性,荆江心滩(洲)演变被划分为8种典型类型。在形态演变上,上荆江以凸岸突出滩体和边滩发育的凹岸滩冲刷显著,形态变化不大。下荆江凸岸滩上游弯侧冲蚀后退、湾顶退缩、下游弯侧淤积伸长,形态趋向低弯扁平化,在高弯曲特定河湾平面形态格局下凸岸冲刷—淤积过程延伸到相邻河湾凹岸,成为下荆江凹岸滩和心滩淤积发展的重要因素,但淤积一般不越过凹岸湾顶。

关键词: 地貌效应; 洲滩; 冲淤动态; 形态演变; 荆江; 三峡水库;
Evolution of floodplains and bars at the Jingjiang reach of Yangtze River, China in response to Three Gorges Reservoir impoundment
XUE Xinghua1,2,, CHANG Sheng1,2, SONG Eping1,2
1. College of Forestry and Horticulture, Hubei University for Nationalities, Enshi 445000, Hubei, China
2. Key Laboratory of Biological Resources Protection and Utilization of Hubei Province, Enshi 445000, Hubei, China
Abstract

At present, there is a lack of sufficient understanding of the evolution of floodplains and bars (FB) at the Jingjiang reach of Yangtze River after Three Gorges Reservoir impoundment. The erosion/accretion pattern and morphological evolution of FB at the Jingjiang reach were studied using remote sensing images acquired in low water periods before and after Gorges Reservoir impoundment. The results showed that the total area of FB at the Jingjiang reach shrank continually after the reservoir impoundment. By 2015 an area of 4.56 km2 had been scoured away, and most of the scour occurred within the first 6 years after the impoundment at an erosion rate of 0.55 km2/a. There were evident differences in erosion/accretion pattern and in morphological evolution between the upper and lower sections of Jingjiang reach. The total area of FB at the upper Jingjiang has continually decreased due to scouring after the impoundment, and the scouring intensity was greater than that in the lower Jingjiang. In contrast, erosion of the lower Jingjiang took place in the early period after the impoundment (2002-2009), but accretion was observed in the later period (2009-2015). In erosion/accretion distribution, the floodplains of upper Jingjiang reach were continually scoured and shrunk while the bars at concave banks were scoured in the early period but were accreted slightly in the later period; the mid-channel bars of upper Jingjiang were accreted in the early period but were scoured in the later period. In contrast, erosion mainly occurred at the floodplains of lower Jingjiang, while the bars at its concave banks and mid-channel bars were accreted. The mid-channel bar evolution at the Jingjiang reach after the reservoir impoundment was classified into eight types according to their position movement and erosion/accretion dynamics. On morphological evolution, the protruding parts of floodplains and the bars at concave banks of the upper Jingjiang were visibly eroded, while the morphology changed little. At the lower Jingjiang, however, the floodplains evolution had a characteristic pattern that the upstream part was eroded back, with a shrinkage of the top, while the downstream part was accreted, resulting in the alteration of convex bank from a highly curved morphology to a flattening-curved morphology. This erosion-accretion process of floodplains at the upstream convex bank even extended to the adjacent downstream concave bank when there was no obvious straight section connecting the neighboring highly curved bends or when the upstream convex bank shared the same bank with the downstream concave bank. Extension of upstream floodplain accretion to the downstream concave bank was an important reason for the development of both the bars at concave banks and the mid-channel bars at the lower Jingjiang. Generally, the accretion at the concave bank did not span to the top of concave bank.

Keyword: geomorphic effects; floodplains and bars; erosion/accretion dynamics; morphological evolution; Jingjiang reach; Three Gorges Reservoir;
1 引言

大坝下游河道对水沙条件变动的地貌响应是河流地貌研究的重要课题[1],不仅影响大坝下游防洪、航运、农业等人类生产活动的进行,也涉及大坝下游河道演变的一些基本问题,得到国内外研究者广泛关注。目前,大坝下游地貌响应的研究,侧重于平面形态变化[2,3]、河床的冲刷与河床质粗化[4,5]、纵坡降变化及横断面形态调整[2, 4, 6]、河道摆动[2,3]以及河型转化[7]等河道演变基本问题。国内研究者针对黄河干流水库[8,9,10,11]、汉江丹江口水库[12,13]、长江三峡水库[14,15,16,17]等大型水利工程建成后的河床演变问题开展了大量研究。已有研究对洲滩等河床微地貌演变的讨论相对较少[18,19],洲滩作为河流地貌结构与格局的重要构成因素,对河流系统的完整性、复杂性及其生态服务功能具有重要意义[1,2]。大坝蓄水后河流系统都会迅速调整[1, 4, 6, 20],显示深刻的水文效应、地貌效应和生态效应。因此,人们十分关注大坝下游洲滩对拦蓄作用的响应特征。从平衡输沙理论出发,依据水沙条件和河道边界性质,可以区分不同类型的大坝下游地貌响应[1, 6],为理解大坝下游河流地貌演变提供了宏观的科学依据。相关的河流边滩冲淤变化实证研究[1],以及受调节河流与非调节河流的对比研究[21]表明,活动的河漫滩大幅度减少而非活动的显著增加,且地貌复杂度大大减小。大坝的河流地貌效应无论在结构上,还是在功能作用上,都有显著表现。据此,Legleiter[1]在分析大坝对下游河漫滩系统的影响时,将其区分为被动效应(连接性)和主动效应(过程功能)。从已有的研究中还应注意到,大坝下游河流地貌演变不仅受到不同水库运用方式下水沙条件变化的影响,而且也受到不同区域地质背景、河型等边界条件下响应差异性的影响[2, 6, 22],相应地也有必要开展这些方面针对性的研究。

荆江是长江中游河流地貌过程颇为活跃和动态演变相当敏感的河段,对三峡水库运行后水沙条件变化的响应显著。历史上,荆江河段受到了自然和人工裁弯、葛洲坝等干扰的影响,一般认为,至三峡水库运行前的2002年荆江河段已处于冲淤动态平衡[23,24],自2003年三峡水库蓄水以来荆江河段又处于快速调整期[15, 23, 25-26]。目前,对荆江的河床冲刷[14-15, 27]、断面形态演变[27]和平面形态变化[16]等取得了较丰富的认识。研究表明,蓄水后荆江河段冲淤显著变大、加快,且冲淤部位有所变化,由蓄水前的槽冲滩淤转为滩槽均发生冲刷[23, 25],并出现一些新的演变特征,如下荆江河段凸岸冲刷、凹岸淤积的问题[25, 27-29]。对于三峡水库蓄水后荆江的洲滩演变,已有的研究,大多是针对小尺度特定河湾洲滩形态变化[30,31]或是对特定冲淤现象的讨论[19, 28-29],对全河段河型演变规律及驱动机制有所研究[32],但缺乏对荆江全河段洲滩演变格局的完整认识。为此,本文以荆江全河段为研究对象,利用遥感影像资料,三峡水库蓄水前后洲滩分布、面积及发育程度的变化情况,定量描述蓄水后洲滩的冲淤变化及其空间分布特性,并揭示洲滩形态的演变特征,以期深化对三峡水库下游地貌效应的认识,为荆江河段的管理提供参考依据。

2 研究区与方法
2.1 研究区简况

长江中游荆江河段自湖北省枝江市(枝江大桥)至湖南省城陵矶(图1),全长约327 km,以藕池口为界分为上、下荆江,上荆江为微弯分叉型河段,江心洲滩发育;下荆江为蜿蜒型河段,边滩发育。江口以上两岸有低山、丘陵,河床为砂卵石质;江口以下是冲积平原,为砂质河段[15, 33]。在河湾分布上[16],荆江河段包括23个河湾,上荆江由规模较大、弯曲率较小的6个河湾组成,在本文中为便于分析讨论,自上游而下依次编号为101~106,下荆江由17个弯曲率较大的河湾(编号207~223)组成。

图1 研究区及基于遥感资料提取的2002-2015年洲滩分布 Fig. 1 Distribution of floodplains and bars based on the data extracted from remote sensing images in the study area from 2002 to 2015

2.2 遥感影像选取及洲滩分布信息提取

三峡水库自2003年蓄水以来已运行10余年,为更好地呈现研究区的洲滩分布,避免洪水淹没对洲滩识别的影响,再者蓄水后荆江枯水河床洲滩冲淤较明显[24,25],本项研究选取了蓄水前后3期枯水期Landsat ETM+影像(获取时间均为1月份)。至2002年荆江已处于总体冲淤平衡[23,24],故以2002年作为蓄水前的参照状态,以2009年、2015年反映蓄水后6年、12年的洲滩分布状态。遥感影像资料源于美国地质调查局(https://glovis.usgs.gov/),并通过影像融合技术,得到15 m多光谱影像。经空间纠正、配准后,综合采用遥感指数法[34]和地理相关法,将洲滩地貌单元分类为凸岸滩、凹岸滩、心滩(洲),得到研究区不同时期的洲滩分布(图1),在GIS软件中计算河段、河湾、洲滩单元等尺度上的面积变化与发育度变化,并制作洲滩形态分布图,以刻画洲滩形态演变。

荆江是限制性河道,其横向发展受荆江大堤控制,因此以荆江大堤为河道边界,对于上荆江少部分无堤段,借鉴相关研究普遍采用的方法[35],以河岸植被为边界。洲滩演变分析除了采用面积测算外,为增加不同规模河湾(河段)的可比性,还构建了洲滩发育度指数(Development Degree of floodplains and Bars, DDB),即单位河长的洲滩面积(hm2/km):

DDB = S / L (1)

式中:S为洲滩面积(hm2);L为河道中心线长度(km)。

3 结果与分析
3.1 荆江河段洲滩基准结构特征

以蓄水前的2002年为基准(图2),荆江的洲滩发育度高、面积宽广,总洲滩面积590.2 km2,洲滩总体发育度为180.6 hm2/km,其中凸岸滩最为发育,其次为凹岸滩,心滩(洲)的规模和发育度最小。凸岸滩、凹岸滩、心滩(洲)的总面积依次为387.6 km2、146.7 km2、52.5 km2,其对应的发育程度分别为118.6 hm2/km、44.9 hm2/km、16.1 hm2/km。上、下荆江在洲滩结构上存在差异性(图2),总体上,下荆江洲滩发育度较上荆江大得多,主要是凸、凹岸滩的面积和发育度明显高于上荆江,而上荆江心滩(洲)发育程度较下荆江大,这种洲滩结构与上荆江微弯分汊、下荆江高弯曲蜿蜒的平面形态结构[16]是吻合的。

图2 2002年荆江河段洲滩基准组成结构 Fig. 2 Reference geomorphic structure of the Jingjiang reach in Yangtze River before Three Gorges Reservoir impoundment in 2002

3.2 荆江河段洲滩总体变化特征

在洲滩总体变化上,三峡水库蓄水后,荆江河段洲滩总面积持续减少,且大部分发生在蓄水后的前6年(2002-2009年),约占总冲刷量的83.7%,期间冲刷速率为0.55 km2/a,后期减弱,与蓄水前的2002年相比,累计冲刷减少4.56 km2图3a)。相关研究也认为,大坝下游的冲刷调整大部分发生在蓄水后约10年甚至几年内[4, 6]。在冲淤分布上,总体看具有凸岸滩明显冲刷减小、凹岸滩及心滩(洲)有所淤增的特点(图3c~3h)。相较于2002年,至2015年荆江河段凸岸滩累计冲刷减少6.22 km2,前期冲刷强烈(减小7.03 km2),后期略有淤增,主要与下荆江少部分河湾的明显淤增有关。

图3 三峡水库蓄水后不同时期内荆江河段的洲滩变化 Fig. 3 Changes in the acreage and development of floodplains and bars at the Jingjiang reach in different periods after Three Gorges Reservoir impoundment

三峡水库蓄水后,上、下荆江洲滩冲淤变化的时空格局存在差异性,显示出对水沙调节的响应复杂性,这种复杂性在其他河流的研究中也可以观察到[8, 13]。在洲滩总体冲淤变化上,无论是从面积变化还是发育度变化上看,上荆江洲滩的冲刷萎缩都要强于下荆江(图3a、3b)。断面冲淤研究也表明,上荆江冲刷强度较下荆江大[27]。蓄水后上荆江洲滩一直处于冲刷萎缩中,并有加剧之势,而下荆江表现为前期(2002-2009年)明显冲刷减小、后期(2009-2015年)淤积增长,即存在上荆江洲滩冲刷、泥沙向下游输移、部分在下荆江淤积的特点,这种上游冲刷、泥沙输移至下游方向淤积的冲淤分布特征在河湾尺度上表现得更为直接、明显。2002-2009年、2009-2015年上荆江洲滩冲刷分别减小2.39 km2、4.07 km2,累计减小6.48 km2,冲刷速率为0.50 km2/a,洲滩发育度较蓄水前的2002年减小4.24 hm2/km。2002-2009年下荆江洲滩冲刷减小1.43 km2,2009-2015年淤增3.33 km2

在冲淤分布上(图3c~3h),蓄水后的前期阶段(2002-2009年),上荆江凸、凹岸滩冲刷萎缩,心滩(洲)有淤积增长;后期(2009-2015年)凸岸滩和心滩(洲)冲刷萎缩明显,凹岸滩有所淤积增长(与部分河湾心滩并岸有关)。至2015年,上荆江凸、凹岸滩累计分别冲减5.35 km2、1.17 km2,心滩(洲)淤增0.71 km2。心滩(洲)的淤增在成因上可能是,该河段涨淤落冲,枯水河槽冲刷严重、蚀深下切[27, 33],乃至枯水位下降[36],三峡水库蓄水后退水快、汛后冲刷不足[26]。下荆江,在前期阶段,凸岸滩冲刷减小、凹岸滩淤积增长,凸岸滩冲刷强度甚至超过上荆江,冲刷速率达0.57 km2/a,凸、凹岸的冲淤对比关系已有报道[25, 28-29, 31],其成因可能与后文所述高蜿蜒河湾平面形态下凸、凹岸的互动演变关系有关。后期阶段,下荆江凸岸滩明显淤积增长,凹岸滩变化不大。下荆江心滩(洲)在蓄水后持续淤积发展,累计淤增0.63 km2

3.3 荆江河段洲滩冲淤分布与形态演变

3.3.1 荆江河段心滩(洲)演变特征 上荆江各河湾均发育有规模较大的心滩(洲),且分布位置大体稳定,其中在跨度较大的河湾102、103和104分别存在4、2、3个心滩(洲)体(图4)。在下荆江除了石首和监利2个河湾(编号208、214)存在分布稳定的心滩(洲)外,其他5个河湾处的心滩(洲)存在明显的时空动态性,其中藕池口分流(河湾编号207)心滩在蓄水后,伴随分流口滩的迅速扩张而并滩,调关、七弓岭、观音洲河湾(编号211、221、222)的心滩则是蓄水后的新生滩。

图4 三峡水库蓄水后荆江心滩(洲)的变化分布 Fig. 4 Changes in the acreage and development of mid-channel bars/islands at the Jingjiang reach after Three Gorges Reservoir impoundment

三峡水库蓄水后的前期阶段(2002-2009年),上荆江心滩(洲)淤增3.23 km2,后期(2009-2015年)冲刷萎缩,减小2.52 km2。前期主要是沙市、公安和郝穴3河湾(编号104、105、106)处心滩(洲)的淤积增长(图4a、4b),尤其是前二者中的太平口心滩(编号104101)和南星洲分别淤增1.73 km2、2.09 km2,其他心滩(洲)冲淤变化不大。后期,除公安河湾处的南星洲继续淤增外,其他河湾的心滩(洲)皆冲刷萎缩。长江中游心滩(洲)冲淤的时空变异性在相关研究中也有报道[19],在成因上,部分河湾及前期的淤增,如前文所述,是河段自身冲淤性质与水文情势调整共同作用的结果[26-27, 33, 36],后期的普遍冲刷表明水文情势变化引起的清水下泄、冲刷仍占主导。对于同一河湾内的多个心滩(洲)体,河湾102、103和104均表现出上游心滩(洲)冲淤动态变化大、下游心滩(洲)动态变化较小的特征(图4c、4d)。

比较而言,下荆江心滩(洲)在三峡水库蓄水后具有总体淤积增长之势(图4a、4b),除藕池口分流处(河湾207)心滩并滩、监利河湾(编号214)乌龟洲有所冲刷缩小外,其他5个河湾的心滩(洲)均具累计淤增的特征,甚至在河湾211、222新成心滩。心滩(洲)淤增、新成心滩并不一定意味着河型调整,目前来看,大多认为三峡水库蓄水后荆江河段还不存在河型调整[16, 37],其成因可能是,河流泥沙沿程得到恢复[38],在高蜿蜒河湾平面形态控制、洞庭湖顶托作用下,上游冲刷泥沙在向下游输移中的暂时性部分淤积。

三峡水库蓄水后,荆江河段心滩(洲)除了面积冲淤增减外,还存在滩体位置活动现象,综合二者可将心滩(洲)演变区分为8种类型,如表1所示。

表1 三峡水库蓄水后荆江心滩(洲)演变的类型划分 Tab. 1 Classification of the mid-channel bars/islands evolution at the Jingjiang reach after Three Gorges Reservoir impoundment (according to position movement and erosion/accretion dynamics)

首先,按滩体位置活动趋向,区分为定位型、变位型、后退型、前进型和合滩型等5个第一级类;然后,按滩体冲淤变化情况,区分了6个第二级亚类。对于后退型和合滩型,考虑到研究期内冲淤变化方式较简,未细分第二级亚类,即存在8种典型心滩(洲)演变类型:① 定位平衡型,如上荆江河湾102的水陆洲和柳条洲(图5a),位置分布稳定,冲淤变化不大;② 定位冲淤交替型,如上荆江河湾102的芦家河浅滩、河湾104的金城洲(图5b),下荆江河湾218的广兴洲心滩,其位置分布稳定,但时冲时淤,冲淤变化皆较大;③ 变位冲淤交替型,如上荆江的三八滩(图5c),位置分布明显向凹岸移动,冲减和淤增变化皆较大;④ 变位淤扩型,如下荆江石首(河湾208)倒口窑心滩、河湾221处的七弓岭心滩(图5d),表现为滩体明显向凹岸移动,并淤增扩大;⑤ 后退型,如上荆江河湾101的关洲(图5e)、河湾103的火箭洲和马羊洲,其主体位置不变,局部侵蚀后退;⑥ 整体前进型,如下荆江河湾222的观音洲新成心滩(图5f),表现为滩体主体位置不变,整体淤积前进;⑦ 洲头前进型,如上荆江太平口心滩、公安河湾(编号105)的南星洲(图5g)和下荆江的乌龟洲,其主体位置不变,洲(滩)头淤积前进;⑧ 合滩型,如上荆江河湾106的颜家台心滩、藕池口分流段(编号207)的茅林口心滩(图5h),表现为心滩靠岸并滩。

图5 三峡水库蓄水后荆江心滩(洲)演变的8种典型类型 Fig. 5 Eight typical types of mid-channel bars/islands evolution at the Jingjiang reach after Three Gorges Reservoir impoundment

3.3.2 荆江河段凸、凹岸滩演变特征 荆江各河湾的冲淤分布亦表明,凸岸滩的冲淤变化明显大于凹岸滩,下荆江较上荆江具有更复杂的冲淤演变特征。上荆江各河湾凸岸滩都在不同时期内存在不同程度的冲刷萎缩,其中较显著的有河湾101、104、105、106,其凸岸滩面积缩小超过0.5 km2图6a)。这些河湾凸岸滩较为发育,在三峡水库蓄水前存在明显向河突出的滩体,如河湾101的同济垸边滩(图7a)、河湾104的腊林洲边滩(图7b)和陈家台边滩(图7c)、河湾105的文村边滩(图7d)和周家台边滩(图7e)、河湾106的林家台边滩(图7f),滩体突出部在蓄水后大都被冲蚀。下荆江凸岸滩的冲淤变化并非完全一致(图6),尽管大多数河湾存在不同程度的冲刷萎缩,但藕池分流口河湾、石首河湾和监利河湾(编号207、208、214)等明显淤积增长,部分前期(2002-2009年)冲蚀的河湾在后期(2009-2015年)淤增(河湾211、220)。可见,下荆江凸岸滩并非完全是“凸岸冲刷”[25, 28-29, 31],而是有冲有淤,冲刷为主。

图6 三峡水库蓄水后荆江凸、凹岸滩冲淤面积分布 Fig. 6 Erosion/accretion distribution of floodplains and bars at the convex and concave banks of the Jingjiang reach after Three Gorges Reservoir impoundment

图7 三峡水库蓄水后上荆江凸、凹岸滩冲淤形态演变 Fig. 7 Morphological evolution of floodplains and bars at the convex and concave banks of upper Jingjiang reach after Three Gorges Reservoir impoundment

凹岸滩冲淤变化大多不超过0.5 km2,冲淤较大者有上荆江的河湾102、106和下荆江的河湾208、214、207(图6)。上荆江凹岸的冲淤分布与形态演变与凸岸滩相似,在沿程冲刷的背景上,凹岸边滩发育处明显冲蚀,岸滩形态复杂性减小,如河湾102凹岸的江口边滩(图7g)和河湾105凹岸的雷家洲边滩(图7h),河湾106凹岸滩后期的面积增长则与颜家台心滩的靠岸并滩有关(图7f)。即,上荆江凸岸突出滩体和凹岸发育的边滩冲刷显著,但形态变化不大,其成因可能与其河岸边界条件有关,该河段江口以上两岸为低山、丘陵控制[15, 39],江口以下两岸冲积平原,上层粘土层厚[39],抗冲蚀力强,而蓄水前形成的砂质边滩则易于冲刷。

在冲淤分布与形态演变上,相较于上荆江,下荆江凸岸滩和凹岸滩的演变均表现出一定的变异性。下荆江凸岸滩演变的一个重要特点是,凸岸滩上游弯侧冲蚀后退,湾顶退缩,过湾顶后,下游弯侧淤积伸长,反映出滩体向下游渐次冲刷输运的特征,河湾弯曲率减小[16],凸岸高弯异化形态存在向正弦化形态[40]转变的趋势,典型的如石首河湾的向家洲边滩、碾子湾河湾的南碾子湾边滩(图8a)。这种冲淤分布与形态演变过程在下荆江17个河湾中的13个均有表现。下荆江凸岸滩上冲下淤的演变特征,可能是三峡水库蓄水后沿程冲刷,至下荆江河流泥沙恢复达到一定量[38]并部分淤落的结果,尤其后期下荆江洲滩总体明显淤增即是河流泥沙得到有效恢复的一个重要表现。

图8 三峡水库蓄水后下荆江凸、凹岸滩冲淤演变 Fig. 8 Morphological evolution of floodplains and bars at the convex and concave banks of lower Jingjiang reach after Three Gorges Reservoir impoundment

另外,下荆江属高蜿蜒河段,相邻河湾过渡段不明显,凸、凹岸紧密相连,如寡妇夹河湾凸岸与调关河湾凹岸,以及后者的凸岸与中洲子河湾的凹岸(图8b),在急弯段甚至出现上游河湾凸岸与下游河湾凹岸共用的情形,如七弓岭河湾的凸岸与观音洲河湾的凹岸(图8d),使得上一河湾凸岸滩的上冲下淤过程,延伸到下一河湾凹岸,引起下游河湾凹岸的淤增,但淤增一般未延伸到凹岸的下半段,下半段一般仍处冲刷后退中。如寡妇夹凸岸边滩的冲淤延伸到调关河湾的凹岸、调关凸岸张智垸边滩的上冲下淤延伸到中洲子河湾凹岸(图8b),河湾215凸岸大马洲冲刷而在河湾216凹岸显著淤增(图8e),城陵矶河湾(编号223)七姓洲凹岸边滩生长发展(图8c)。即,下荆江相邻河湾凸、凹岸滩演变存在纵向互动关系,甚至促使凹岸侧心滩的发展,如图8b中的调关新成心滩,以及图8d中七弓岭心滩向凹岸的移动与增长、观音洲凹岸侧新成心滩,这可能是三峡水库蓄水后下荆江心滩有所发展的原因之一。

当相邻河湾过渡段较明显或同向弯曲时,下游河湾凹岸则处“正常”冲蚀状态。如荆江门河湾(编号219)上游为长顺直过渡段,荆江门河湾与其下游河湾220间亦有顺直段过渡,碾子湾河湾(编号209)和寡妇夹河湾为同向河湾,可以看到寡妇夹河湾凹岸边滩(图8b)、荆江门河湾凹岸滩、河湾220的凹岸滩(图8d)都处于与上荆江凹岸类似的冲刷蚀退过程中。可见,高弯曲度、过渡段不明显乃至上、下游凸、凹岸共用的河湾平面形态格局是下荆江相邻凸凹岸滩的互动演变关系的一个重要条件。

下荆江监利河湾的凸岸演变略显不同,但其凹岸仍然受上游河湾凸岸冲刷—淤积延伸的影响(图8e),窑圻脑边滩淤增延伸至乌龟洲头部一带,甚至乌龟洲头部心滩得到了显著发展,而凹岸下半部至太和岭边滩都处冲蚀后退中。对于监利河湾凸岸,一方面其上游弯侧受护岸工程保护冲刷较弱,再者顶部由于乌龟洲南部心滩靠岸并滩而呈淤增状态,凸岸下游半段丙寅洲边滩则处于冲蚀中,冲刷一直延伸到下一河湾凹岸顶部的天子一号附近,此时,河湾215凹岸呈“正常”冲蚀状态。另外,石首河湾208(图8a)上游河湾藕池口一直处于迅速向上游方向淤积伸展中,但滩体左侧(与石首河湾凹岸同侧)严重冲刷(图9c),石首河湾凹岸亦处于“正常”冲蚀中。可见,上游河湾凸岸上冲下淤亦应是下荆江河段相邻河湾凸、凹岸演变纵向互动关系的一个重要前提。

图9 三峡水库蓄水后荆江分流口洲滩演变 Fig. 9 Evolution of the floodplains and bars around the three bifurcations of Jingjiang reach after Three Gorges Reservoir impoundment

综上所述,下荆江洲滩演变以凸岸冲淤演变为主导,河湾凸岸滩的上游弯侧冲蚀、湾顶退缩、下游弯侧淤增延伸,淤伸过程甚至可达相邻下游河湾凹岸,使得该凹岸上半部边滩淤积发展,乃至促使凹岸侧心滩的发展,但相邻河湾凸、凹岸演变的纵向互动关系一般不越过凹岸湾顶,凹岸下半部一般仍处于“正常”冲蚀状态。相关研究中“凸岸冲刷、凹岸淤积”[25, 28-29, 31]即是相邻河湾凸、凹岸互动演变关系的一种体现,但下荆江岸滩演变并不严格地凸岸冲刷、凹岸淤积,凸岸滩并非完全是冲刷,而是冲刷为主、有冲有淤,凹岸亦非完全是淤积,而是上半部淤积、下半部冲刷,还存在凹岸“正常”冲刷状态下的河湾。下荆江相邻河湾凸、凹岸滩演变的纵向互动关系,存在于以下3个方面的主要基础:① 三峡水库蓄水后清水下泄、沿程冲刷,下荆江河流泥沙得到有效恢复;② 凸岸上游侧冲蚀、下游弯侧淤伸;③ 高蜿蜒河湾平面形态格局,相邻河湾过度段不明显,凸凹岸紧连甚至共用;④ 还有洞庭湖的顶托作用。

3.3.3 荆江河段分流口洲滩演变 三峡水库蓄水后,三口分流处洲滩演变存在差异性,松滋口分流处两岸边滩强烈冲蚀萎缩,太平口分流处岸滩变化不大而心滩淤增发展,藕池口分流处洲滩迅猛淤积扩大并向上游延伸。松滋河分流口两岸的羊角洲和陈二口边滩都强烈冲刷后退,分流口下游处的心滩处于动态平衡中(图9b)。太平口分流处,分流口两侧岸滩冲淤变化不大,但太平口心滩明显淤积增长(图9a)。藕池河分流口滩体迅猛淤积延伸,至2009年已与分流口心滩白洲连为一体(图9c),分流口淤积严重,枯水期断流。

三分流口所在河段平面形态相似,均为顺直微弯段,三者冲淤演变差异与荆江河段宏观冲淤格局是一致的,在三峡水库蓄水后清水下泄、沿程冲刷的背景下,河流泥沙沿程恢复状况不同,还受到局地洲滩形态的水力作用关系及其物质组成差异性的影响。松滋口位于荆江起始段,处于山区型向平原型河流过度的河段,该河段砂卵石河床抗冲蚀力强[36],至松滋口沿程泥沙恢复有限,河流冲刷力强,但分流口洲滩为易于冲刷的砂质滩[41],再加上羊角洲顶冲上游来水,洲滩头部冲刷严重,萎缩后退。太平口位于上荆江中部的沙市河湾,分流处岸滩为粘土/中细沙/卵石层三元结构,上部粘土层较厚[39],抗冲蚀力强,分流口两侧岸滩冲刷较弱;而太平口心滩由于河流泥沙沿程冲刷得到一定恢复,太平口河道顺直放宽,再者该河段洪淤枯冲且枯水河槽强烈冲蚀下切[14],三峡水库蓄水后退水快,汛后心滩体冲刷不足[28],皆有利于心滩淤积发展。藕池口正处上、下荆江过渡处,三峡水库蓄水后,上荆江河段冲刷量巨大[14],至下荆江河流泥沙得到较好的恢复[38],藕池分流口河段的入口和出口窄、中间放宽,中部雍水、分散,天星洲顶托,分流口洲滩淤积扩大,滩体甚至已延伸到该河湾入口。

4 结论

(1)三峡水库蓄水后至2015年荆江洲滩总面积持续冲刷减小,累计冲减4.56 km2,大部分发生在蓄水后的前6年(冲刷速率0.55 km2/a);洲滩演变以凸岸滩冲刷萎缩占主导,累计冲刷6.22 km2;凹岸滩和心滩(洲)有所淤积,分别淤增0.98 km2、1.33 km2;上、下荆江在冲淤动态、分布和形态演变上存在差异性。

(2)在冲淤动态上,上荆江洲滩总面积一直处于冲刷萎缩中,且其强度明显大于下荆江,累计冲刷6.46 km2;下荆江前期(2002-2009年)冲刷、后期(2009-2015年)淤增,累计淤1.90 km2

(3)在冲淤分布上,上荆江凸岸滩持续冲刷萎缩,凹岸滩前期冲刷明显、后期略有淤增,心滩(洲)前期淤增、后期冲蚀,至2015年凸、凹岸滩累计分别冲减5.35 km2、1.17 km2,心滩(洲)淤增0.71 km2。下荆江冲刷主要发生在凸岸滩,至2015年累计冲刷0.87 km2,凹岸滩和心滩(洲)有所发展,分别累计淤增2.15 km2、0.63 km2

(4)根据滩体的位置活动和冲淤动态性,荆江心滩(洲)演变被区分为8种典型类型,包括定位平衡型、定位冲淤交替型、变位冲淤交替型、变位淤扩型、后退型、洲头前进型、整体前进型和合滩型。

(5)在形态演变上,上荆江凸岸突出滩体和凹岸发育的边滩冲刷显著,凸、凹岸滩形态变化不大。下荆江凸岸滩上游弯侧冲蚀后退、湾顶退缩、下游弯侧淤积伸长,高弯异化形态向正弦化形态转变,形态趋于低弯扁平化;在连续高弯曲、过渡段不明显甚至相邻凸、凹岸共用的河湾平面形态格局下,上游河湾凸岸冲刷—淤积过程延伸到相邻河湾凹岸,成为下荆江凹岸滩和心滩发展的重要因素,但凹岸淤积一般不越过湾顶。

The authors have declared that no competing interests exist.

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<正> 水库下游河床演变的研究虽已有数十年的历史,但对水库下游河床调整的宏观趋势及河型转化问题,至今尚未取得一致的意见。这表明,在这方面的研究中,我们所用的概念和方法还有待于更新。鉴于此,本文试图运用系统论中的复杂响应原理,对丹江口水库下游汉江中游段(丹江口至钟祥)的河床调整过程进行分析。系统复杂响应原理用于地貌系统的阐释,始于美国地貌学家Schumm。本文则将这一原理进一步用于水库下游河床调整过程。
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本文根据三峡水库蓄水以来荆江河段实测水沙、河道冲淤等实测资料,统 计并分析了蓄水以来荆江河段河床冲淤变化及水位变化特点。分析指出,三峡水库蓄水运用后,荆江河段来沙量大幅减少,总体冲刷量较蓄水前有所增大,且主要集 中在枯水河槽,与航道条件密切相关的枯水河槽以上的滩地部分冲刷也有所增大;同流量下沿程水位均有发生不同程度的下降,其中,砂卵石河床段、临近城陵矶的 荆江河段尾端,水位下降幅度较小,而紧邻砂卵石河床段的沙市附近水位下降幅度较大。
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<p>为确定三峡水库蓄水后荆江河段平滩河槽形态调整特点,提出基于河段尺度的平滩河槽形态参数的计算方法,计算上、下荆江段2002&mdash;2013年汛后河段平均的平滩河槽形态参数.结果表明:三峡工程运用后,坝下游河床冲刷加剧,个别河段河势变化剧烈,但总体河势仍基本稳定;尽管局部河段的崩岸现象较为突出,但河段平滩宽度总体变化不大,上、下荆江平均河宽分别为1 388及1 305 m,而河段平滩水深平均增加1.6及1.0 m,故荆江河段的平滩面积在持续增加.最后建立河段尺度的平滩河槽形态参数与前期5 a平均的汛期水流冲刷强度之间的计算关系,用于预测该河段平滩河槽形态随水沙条件的变化趋势.</p>
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目前对三峡水库运行后下游水文泥沙特性变化、河道冲刷与崩岸等问题已有认识,但对河湾平面形态演变还少见论述。本项研究基于研究区枯水期遥感影像,分析了三峡水库运行10余年来荆江段枯水河湾平面形态的演变特征。结果表明,枯水河湾的中心线长度、凸岸长度、凹岸长度、弯曲率和中心轴长度存在显著变异,以下荆江段的变异程度更大,凸、凹岸及上、下荆江段在演变过程上存在非同步性。上荆江段凹岸先于凸岸发生显著变异,且平面形态演变以凹岸冲刷延伸占主导作用,下荆江段凸岸先于凹岸显著变异,并以凸岸冲刷收缩占主导。大部分河湾尤其在下荆江,表现为面积、平均宽度明显增加,而中心线长度、弯曲率与中心轴长度减小,河湾发展具有河道冲刷展宽但弯曲退缩的特征。还讨论了平面形态演变与凸、凹岸冲淤变化的关系。
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2003年三峡水库蓄水运行以来,进入长江中下游的年平均径流量变化不大,但输沙量锐减.分汊河段的演变对水沙条件的变化十分敏感,本文对长江中下游鹅头型、弯曲型、顺直型分汊河段的演变与调整进行了系统分析,分流比及汊道冲淤情况实测资料表明,长江中下游各种分汊河段的演变调整具有较好的一致性,比降较大的汊道趋于发展,与汊道分流比的大小关系不大.对此,分析认为主要是因为来沙大幅度减少后,河道由淤积型转换为冲刷型,水流条件的作用凸显,比降较大的汊道易于多冲或是少淤,随之呈现出发展态势.
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心滩作为辫状河道最主要的微地貌特征,其变化特征的研究对于理解辫状河道的演变具有重要意义.以黄河临河段213 km长的辫状河道为研究对象,基于1988-2013年的遥感图像绘制了相应日期的河道平面形态图,分析了该河段心滩数量和心滩面积变化;利用2013年内6幅河道平面图建立了心滩面积与巴彦高勒站水位之间的线性关系式,利用该关系式求取了其他年份1 050m参考水位下的心滩面积并进行心滩冲淤变化对比.结果表明:黄河临河段心滩总个数、心滩总面积、心滩平均面积、中位数心滩面积和最大心滩面积总体上都具有减小的趋势.在1 050 m参考水位下,黄河临河段心滩面积在1988-2013年具有不同的冲淤变化特征,其中在2000年以前的变化幅度大,趋势不明显,2000年以后则分布较为集中,且有减小的趋势,并在1990-2000年大幅减小.在龙羊峡、刘家峡等水库运行影响下汛期黄河临河段水量和输沙量占全年的比例在减小,年内分配趋于均衡化,并且输沙量的减幅要大于水量的减幅,这对于研究区心滩的冲淤有重要影响.同时,上游流域来沙系数的变化也在一定程度上影响着该河段心滩的冲淤变化.
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<p>长江中下游河道存在数量众多的江心洲,三峡水库蓄水运行后这些江心洲的演变深为人们关注。选取2000~2011年宜昌、汉口、大通站的年均流量和输沙量,1999、2001、2003、2005、2007、2009和2011年长江中下游20个代表性江心洲的遥感影像,分析三峡水库蓄水前后水沙变化及江心洲面积变化过程,同时利用概念模型分析江心洲面积变化与流量和含沙量之间的关系。研究表明:(1)三峡水库蓄水后(2003~2011年)宜昌、汉口、大通站的年均径流量较蓄水前分别减少61%、73%、95%,输沙量分别减小903%、721%、669%,平均含沙量分别减少了896%、708%、648%;(2)2003年蓄水前,12个(60%)江心洲面积逐年增加,相对于1999年其平均变化率为268%;2003年蓄水后,只有9个(45%)江心洲面积逐年也增加但是平均变化率减少至221%,11个(55%)江心洲面积逐年减少,相对于1999年其平均变化率为194%;(3)概念模型分析表明:江心洲面积变化量跟水位、冲蚀量成正相关,而水位、冲蚀量跟流量、悬沙减少量成正相关。江心洲面积变化量主要受水位变化控制,这种关系导致长江中下游江心洲演变的一种假象:既蓄水后面积不断增加的少数江心洲实际上也处于不断冲蚀的过程</p>
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[21] Graf W L.Downstream hydrologic and geomorphic effects of large dams on American rivers. Geomorphology, 2006, 79(3): 336-360.
The hydrology and geomorphology of large rivers in America reflect the pervasive influence of an extensive water control infrastructure including more than 75,000 dams. One hundred thirty-seven of the very large dams, each storing 1.2 km 3 (10 6 acre feet) of water or more, alter the flows of every large river in the country. The hydrologic effects of these very large dams emerge from an analysis of the stream gage records of 72 river reaches organized into 36 pairs. One member of each pair is an unregulated reach above a dam, whereas the other is a regulated reach downstream from the same structure. Comparison of the regulated and unregulated reaches shows that very large dams, on average, reduce annual peak discharges 67% (in some individual cases up to 90%), decrease the ratio of annual maximum/mean flow 60%, decrease the range of daily discharges 64%, increase the number of reversals in discharge by 34%, and reduce the daily rates of ramping as much as 60%. Dams alter the timing of high and low flows and change the timing of the yearly maximum and minimum flows, in some cases by as much as half a year. Regional variation in rivers, dams, and responses are substantial: rivers in the Great Plains and Ozark/Ouachita regions have annual maximum/mean flow ratios that are 7 times greater than ratios for rivers in the Pacific Northwest. At the same time, the ratio of storage capacity/mean annual water yield for dams is greatest for Interior Western, Ozark/Ouachita and Great Plains rivers and least for Pacific Northwest streams. Thus, in many cases those rivers with the highest annual variability have the greatest potential impact from dams because structures can exert substantial control over downstream hydrology. The hydrologic changes by dams have fostered dramatic geomorphic differences between regulated and unregulated reaches. When compared to similar unregulated reaches, regulated reaches have 32% larger low flow channels, 50% smaller high flow channels, 79% less active flood plain area, and 3.6 times more inactive flood plain area. Dams also affect the area of active areas, the functional surfaces that are functionally connected to the present regime of the river. Regulated reaches have active areas that are 72 smaller than the active areas of similar unregulated reaches. The geomorphic complexity (number of separate functional surfaces per unit of channel length) is 37% less in regulated reaches. Reductions in the size of hydrologically active functional surfaces are greatest in rivers in the Great Plains and least in Eastern streams. The largest differences in geomorphic complexity are in interior western rivers. The shrunken, simplified geomorphology of regulated large rivers has had direct effects on riparian ecology, producing spatially smaller, less diverse riparian ecosystems compared to the larger, more complex ecosystems along unregulated reaches of rivers.
DOI:10.1016/j.geomorph.2006.06.022      [本文引用:1]
[22] Grant G E, Schmidt J C, Lewis S L.A geological framework for interpreting downstream effects of dams on rivers//O'Connor J E, Grant G E. A Peculiar River: Geology, Geomorphology, and Hydrology of the Deschutes River, Oregon. Washington DC: American Geophysical Union, 2003: 203-219.
Summary This chapter contains sections titled: Introduction A Geologic Framework for Interpreting Geomorphic Effects of Dams Combining Hydrogeomorphic and Geologic Controls to Predict Downstream Impacts of Dams Examples of Downstream Response to Dams: The Role of Geology Implications for Interpreting Downstream Responses to Dams: An Example from the Oregon Cascades Conclusions
DOI:10.1029/007WS13      [本文引用:1]
[23] Xu Quanxi, Yuan Jing, Wu Wenjun, et al.Fluvial processes in middle Yangtze River after impoundment of Three Gorges Project. Journal of Sediment Research, 2011, 36(2): 38-46.
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[25] He Guangshui, Yao Shiming, Jin Zhongwu.Study of atypical erosion of convex bank of river bend in Jingjiang Reach of Yangtze River. Yangtze River, 2011, 42(17): 1-3.
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[何广水, 姚仕明, 金中武. 长江荆江河段弯道凸岸边滩非典型冲刷研究. 人民长江, 2011, 42(17): 1-3.]
[26] Xu Quanxi, Zhu Lingling, Yuan Jing.Research on water-sediment variation and deposition-erosion in middle and lower Yangtze River. Yangtze River, 2013, 44(23): 16-21.
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[许全喜, 朱玲玲, 袁晶. 长江中下游水沙与河床冲淤变化特性研究. 人民长江, 2013, 44(23): 16-21.]
利用长江干、支流水文和河道地形实测资料,研究了三峡工程运用前后坝下游水沙输移特性与河湖泥沙冲淤的时空格局和变化特征。结果表明:①三峡工程建成运行后,长江中游洪峰流量减小,中水时间延长,汛后退水时间缩短,干流输沙量大幅减少,泥沙来源发生新变化,荆江三口分流分沙量继续减少;②长江中游干流河势总体稳定,但河床沿程纵向冲刷强度与三峡水库运用前相比明显增大,且冲刷强度和发展速度均大于原预测值;③荆江三口洪道由蓄水前的淤积转为冲刷;④洞庭湖淤积速度大为减缓,鄱阳湖受河道采砂等影响,总体由淤积转为冲刷;⑤长江中游河、湖泥沙冲淤格局发生调整,三峡水库蓄水前长江中游河、湖呈淤积状态,蓄水后则呈冲刷状态。
[27] Qu Geng, Xu Hui, Tang Wenjian, et al.Morphology variation of different sections in Jingjiang River and its impact on Jingjiang channel condition after TGR operation. Port & Waterway Engineering, 2011(12): 117-122.
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[渠庚, 许辉, 唐文坚, . 三峡水库运用后荆江河道断面形态变化及对航道条件的影响. 水运工程, 2011(12): 117-122.]
This paper utilizes the data of water and sediment and river topography after the impoundment of the Three Gorges reservoir to analyze the channel and sediment of Jingjiang river, channel scouring and siltation,width-to-depth ratio and change of riverbed profile. On this base, this thesis studies the impact of the change riverbed profile on the Jingjiang channel condition. The laws of morphology variation of different sections in Jingjiang river are proved to be different after TGR operation, yet common features among these laws are clearly represented by upstream water sediment variation-deduced point bar scouring, river-width expansion, and contraction decline,as well as the decrease of degree of curve under the same flow. Under the impact of release of water these changes will be sustainable development. The nearshore scouring and the width of river channel increase are still themain forms and features. Moreover, these changes may bring about some unfavorable effects on Jingjiang channel condition.
[28] Zhang Weijun, Wei Lipeng, Qu Geng.Channel evolution of different river patterns in Jingjiang after Three Gorges Project operation. Water Conservancy Science and Technology and Economy, 2013, 19(11): 56-59.
[本文引用:5]
[张卫军, 魏立鹏, 渠庚. 三峡工程运用后荆江不同河型河道演变分析. 水利科技与经济, 2013, 19(11): 56-59.]
利用三峡工程运用以来实测水沙和地形资料,分析荆江河道分汊、弯曲、顺直等河型调整的规律,并对其变化原因进行探讨。三峡工程运用以后,分汊型河道变化主要表现为凸岸支汊发展;弯曲型河道演变特点为凹岸深槽淤积,凸岸边滩冲刷,断面形态由偏"V"单槽向"W"型双槽转化;顺直型河道变化特点是两岸交错边滩冲刷,深槽淤积,河道断面向宽平方向发展,深泓年内变化频繁。从水流能量的角度,探讨了荆江不同河型河道调整的原因。由于三峡工程运用后荆江特别是下荆江河段水流输沙所需要的弯曲度明显较建库前小,因此弯曲河段凸岸边滩冲刷以减小水流弯曲度,是响应上游水沙变化的重要方式。
[29] Zhu Lingling, Xu Quanxi, Xiong Ming.Fluvial processes of meandering channels in the lower Jingjiang river reach after the impoundment of Three Gorges Reservior. Advances in Water Science, 2017, 28(2): 193-202.
[本文引用:5]
[朱玲玲, 许全喜, 熊明. 三峡水库蓄水后下荆江急弯河道凸冲凹淤成因. 水科学进展, 2017, 28(2): 193-202.]
三峡水库蓄水后沙量骤减使得荆江河段河床剧烈冲刷调整,下荆江急弯段"凸冲凹淤",部分弯道凹岸侧淤积形成水下潜洲。这种不同于弯曲河道一般性演变规律的异常现象势必给下荆江的行洪、航运及江湖关系变化等带来新的影响,其内在成因急需揭示。基于原型观测资料分析,通过研究滩槽冲淤量及分布、滩体及河道形态特征等,全面揭示了近期下荆江急弯段"凸冲凹淤"的演变特征及河道形态响应。结果表明,流量年内过程重分配、来沙量及组成突变是下荆江急弯段"凸冲凹淤"的决定性因素。
[30] Duan Guanglei, Peng Yanbo, Xiao Hucheng, et al.Preliminary probe into evolvement mechanism of typical shoals at Jingjiang reach of Yangtze River. Hydro-Science and Engineering, 2008(2): 10-15.
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[31] Li Ningbo, Zeng Yong, Wu Zhongming.Preliminary study of causes of mainstream bending of Qigongling bend in Jingjiang reach of Yangtze River. Yangtze River, 2013, 44(1): 22-25.
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[32] Fan Yongyang, Zhang Wei, Han Jianqiao, et al.The typical meandering river evolution adjustment and its driving mechanism in the downstream reach of TGR. Acta Geographica Sinica, 2017, 72(3): 420-431.
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[樊咏阳, 张为, 韩剑桥, . 三峡水库下游弯曲河型演变规律调整及其驱动机制. 地理学报, 2017, 72(3): 420-431.]
大型水库的兴建深刻改变了下游水沙输移特点,进而导致河床演变规律显著调整,水库下游弯曲河型对水沙过程改变响应敏感,是水库下游河床演变、航道整治、河势控制等方面研究的关键区域.本文基于1996-2016年的实测水文、地形资料,对长江三峡水库下游弯曲河型的演变规律及其驱动机制开展研究,结果表明:①三峡水库蓄水前,下荆江存在“凸淤凹冲”、“凸冲凹淤”两类弯曲河型,而三峡水库蓄水后均表现为“凸冲凹淤”的一致性规律;②在水库拦沙作用的影响下,下荆江河段平滩河槽存在累积性冲刷现象,冲刷部位集中于枯水河槽与基本河槽之间的低滩,冲淤部位调整主要由变化的流量过程所驱动,上游河势、河道边界以及支流入汇等因素均有一定驱动作用;③在三峡水库蓄水后缺乏大洪水的情况下,凸岸水流挟沙力随流量增加逐渐增强,水流对凸岸冲蚀力度在平滩流量级附近(20000~25000m3/s)达到最强,平滩流量附近流量级的持续时间超过20天时,弯曲河道发生凸冲凹淤现象.而悬沙中造床粗沙的减少,增强了水流冲刷强度,加剧了凸岸的冲蚀程度.
DOI:10.11821/dlxb201703005     
[33] Yang Fangli, Huang Wei, Fu Zhongmin, et al.River evolution and waterway regulation of Zhijiang-Jiangkou section at midstream of the Yangtze River. Port & Waterway Engineering, 2012(10): 24-29.
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Based on the existed research results and field data, this paper analyzes the recent evolution regularity, navigation-obstruction characteristics, as well as main problems of the waterway, and then predicts the river evolution trend according to the model results. Firstly, the necessity of waterway regulation project is analyzed.According to the development demands of the reach, this paper discusses the necessity of waterway regulation in the reach, and then proposes the regulation ideas and tentative scheme under consideration of the navigation-obstruction characteristics and waterway regulation targets. The research results concerning the bed evolution, navigationobstruction characteristics and scheme comparison and selection may serve as reference for the follow-up systematic regulation engineering.
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流域水库工程的修建,将改变坝下游原有的水沙输移过程,三峡水库蓄水作用对坝下游水沙输移的影响已初步显现。具体表现为:(1)三峡水库坝下游洪水持续时间和流量被削减,下泄沙量大幅减少,沿程上输沙量虽得到一定恢复,但总量仍未超过蓄水前多年均值;(2)2003-2014年d0.125 mm(粗)输沙量得到一定恢复,至监利站恢复程度最大,基本达到蓄水前均值,在恢复后其下游该组分泥沙冲淤特性与蓄水前一致,其中2008-2014年恢复程度弱于2003-2007年;(3)三峡水库蓄水后坝下游d0.125 mm(细)输沙量沿程上得到一定程度恢复,但总量仍小于蓄水前均值;(4)三峡水库蓄水后坝下游d0.125 mm泥沙输移量因河床补给作用,沿程上得到恢复,但补给量将不超过0.44亿t/y,主要受制于洪水持续时间及流量均值,而上游干流、河段间支流和湖泊分汇作用占次要地位,而d0.125 mm悬沙恢复受上游干流、区间支流和湖泊分汇及河床补给控制,因河床粗化使得床沙对细颗粒悬沙的补给作用减弱;(5)2003-2007年和2008-2014年两时段间宜昌至枝城、上荆江为粗细均冲,下荆江为淤粗冲细,汉口至大通河段为淤粗冲细,城陵矶至汉口河段2003-2007年为淤粗冲细,2008-2014年为粗细均冲,这一差异受控于螺山站洪水流量持续时间和量值。
DOI:10.11821/dlxb201607012     
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