1990年以来黄河第一湾齐哈玛河段砾质网状河的演变特征

黄河第一湾的网状河型因其砾石质的河床质而与砂床质网状河明显不同,但是其具体的冲淤特性、河道与河间地的稳定性等是否与砂质网状河具有相似之处,尚待揭示。以齐哈玛乡主河道长约为12 km的砾石质网状河段为研究对象,利用1990年、2001年、2013年和2016年共4期Landsat遥感影像数据和2011年与2013年两期Google Earth高分辨率图像数据,结合野外采样观测分析其1990-2016年间的平面形态变化与沉积特征。结果表明：砾石质网状河整体具有很高的稳定性,众多分支河道与河间地无明显冲淤现象。网状带面积仅增加2.43%,陆地与水体面积比例接近1∶1;网状带部分小型河间湿地及河间岛屿呈现碎片化现象,导致河间湿地个数逐渐增加,最大增加率为62.16%。河道主流线长期左右迁移交替变化,且变化率相对稳定,受主流线迁移的影响,主河道内部河间岛屿形态变化较大,其河岸变化率为5 m/a。网状河众多支河道非常稳定,平均河宽变化率仅为1 m/a左右。河岸沉积物以细砂或粉砂为主,黏土含量较高,粒度分布曲线呈现多峰,这与砂质网状河流河岸以泥质沉积物为主略有不同,但河岸及河间湿地茂密的植被保护了众多分支河道免受侵蚀、维持了河道的稳定性,这也是砾石质网状河流体系具有高稳定性的重要原因。     

黄河玛曲砾质网状河段河间湿地形态特征及发育程度

河间湿地是网状河流体系中与多河道系统同等重要的地貌单元,其特征和发育程度与河道系统共同决定着网状河流的特性。本文以黄河玛曲网状河段作为研究对象,利用高分辨率Google Earth影像对研究河段河道与河间湿地等微地貌单元进行提取,利用湿地面积与湿地数对河间湿地的组成进行了分析,利用河间湿地面积占比、岸线密度、分汊点密度3个地貌参数,对河间湿地群体（河间湿地与分支河道特定组合体）的平面形态及多河道发育程度进行了研究。结果表明,河间湿地个体以中小型的为主,河间湿地群体则以大型和巨型的为主。河间湿地群体的发育程度与河间湿地群体面积（S_u）明显相关,河间湿地面积占比（P）随S_u的增大而增大,P值70%是河间湿地发育成熟的指标,而P值80%基本是河间湿地发育的上限值,此时河间湿地与分支河道基本达到动态平衡态。岸线密度、分汊点密度随S_u的增加呈减小趋势,这种变化主要是由于不同大小河间湿地群体中河道发育过程差异导致。黄河玛曲段河间湿地群体的形成在于弯曲主河道的主导作用、地形限制因素削弱导致的河道带的发育,以及植被维护的河岸使得河间湿地群体得以较长时间保存。     

网状河流多重河道形成过程的实验模拟

网状河流是受到人们关注的新型冲积河流,其水文特征、地貌特征和沉积特征已经不同程度地被揭示。然而,网状河流的水槽实验迄今仍是空白,而水槽模拟实验是在时间和空间都大大缩小之后的自然界河流演变过程的再现。本文报道的是在实验水槽中通过原河道决口后网状河流的发生和演变过程。实验初始条件为:水槽辅助区的待决口河道,目标区为轴部略微下凹的长方形泛滥平原(4.5 m×16.5 m),其上下层分别为1.5 cm厚、中值粒径为0.0132 mm的高岭土层和0.5 m厚、中值粒径为0.188mm的天然细砂层,从5.5 m-17 m区段的平均纵比降为0.0058,17m-22m区段的平均纵比降为0.0077。采用的定常流量为3 L/s,悬移质输沙量在前3小时为4.5g/min,其后为1.2 g/s。实验总历时50小时。实验初期,目标区的上游段以垂向加积作用为主,中游以随机侵蚀为主,下游以溯源侵蚀为主,在13.5小时左右,相互连通的多重河道的网状河流体系的雏形基本展现。此后至25.5小时,网状河道的演变以下蚀为主演变为以适度的侧蚀,但河岸的后退幅度很小,标志网状河道逐步过渡到成熟期。从25.5小时至50小时,个别河道的局部废弃和决口是该成熟期网状河道演变的新特点。实验成功地模拟了天然网状河流的形成、发展和演变过程,同时也证明了它是不同于分汉河流的河型。这不     

不同气候带的河道与沙丘分布格局及其类型划分

风水交互作用是干旱区常见的地貌现象和重要的地表过程,干旱区河流通过提供物源和场所控制沙漠分布的格局,沙漠分布与风沙活动制约河道发育和泥沙输移,在不同时空尺度表现不一,但关于二者交互作用的分类处于探索阶段,尚未形成分类体系,尤其在地貌格局分类上多为定性描述。因此,自西向东选择位于3个气候带的中国西部克里雅河、中部毛不拉格孔兑以及东部西拉木伦河,利用遥感影像解译获取河道与沙丘信息,探讨近源沙丘的分布与河型之间的组合关系。结果表明：河型、河流流向与风向之间的关系、水文以及距离河道的远近影响河道与沙丘组合的地貌格局;在河道-沙丘尺度上,划分为弯曲河道-对称式边滩沙丘、顺直河道-边滩沙丘、分汊河道-心滩式沙丘、网状河道-格状镶嵌式沙丘4种类型。     

基于遥感技术的黑龙江上中游河道特征研究

以2000年Landsat-7 ETM+影像、地形图、数字高程数据等为数据源,从河道平面形态及河流纵剖面形态对黑龙江上中游河道特征进行分析。从河流动力特征、流域自然环境条件及人类活动方面探讨其对河道形态特征形成的影响。研究发现:黑龙江上中游河道曲折系数均值为1.20,属于微弯顺直型河道,整体相对稳定,但局部河段河道形态复杂,河流纵剖面形态为下凹型曲线;河流形态的形成遵循一定的河流动力学规律,是水流和河床在自然因素与人类活动影响下相互作用结果。     

分汊型河床的形成与演变——以长江中下游为例

本文根据多年的地质、地貌,来水来沙等考察资料,河道变迁的历史记载以及实验研究成果,分析了分汊型河床的两个必要条件及三方面的演变特征,最后用河流力图使自己的能量损耗率达于最小的假说对汊河的形成和演变作了解释。     

珠江河网分形机理研究

以不同时期珠江网河历史地图及遥感影像为基础,进行了河网分形计算,并从河道发育、人为活动等多个角度分析了珠江网河分形机理。在发育过程中,1883-2001年间河网分维值逐渐减小;在空间上,正干水道中潮道分维值大于分汊河道。分析认为,人为活动是导致珠江网河分形变化的主要因素:联围筑闸消灭横向支汊、口门整治及河道固化减小了河道冲决的几率、河道挖沙引起的水沙分流比变化改变了河道冲淤特性。     

长江口长兴,横沙岛潮滩沉积特征及其影响机制

长兴、横沙潮滩是典型的河口潮滩。其沉积物粒度在总体上较四周河道细，且具有从潮下带向潮上带沉积物质逐渐变细的趋势，但由于风浪和植被等因素的影响，局部高程的变化往往较为复杂；在沿岸方向上，以面向口外的开敞岸段最粗，顶江迎流的洲头滩次之，顺江两岸物质相对较细。在此基础上，提出了分汊河口冲积岛潮滩的基本沉积模式和河口潮滩与正常海岸潮滩的区别。     

黄河青藏高原网状河段不同流量下活动河道的分布规律

以黄河青藏高原阿万仓和采日玛两段主河道50 km与65 km长的网状河段为研究对象,利用2013年3期遥感影像绘制了不同流量下（176 m³/s,978 m³/s,1 610 m³/s）网状河段活动河道的分布变化图,结合DEM数据分析了现有河道在流量增加过程中的空间分布格局和潜在定量关系。研究结果表明：根据流量大小划分出的3类活动河道,其分布规律基本是后者位于前者的两侧,据此可以推断,网状河分支河道基本是从主河道向两侧逐渐发展的。对于阿万仓河段,在河谷和河间地相对宽阔处,每个河道断面上3类活动河道数的比值为1:1.67:2.25;而在河谷较窄的地方,该比值为1:1.22:1.33。在采日玛河段,该比值分别为1:1.3:1.4和1:0.95:1.16。在宽阔的草原湿地河段,当网状河的主河道发生弯曲时,弯道内侧的活动河道数要大于弯道外侧的活动河道数。在平坦开阔的河谷地带,断面上的活动河道线密度与河谷宽度之间呈现出极好的线性负相关关系;而峡谷地带由于两侧高地形的限制,河谷宽窄不一,活动河道的线密度相对较大、且差别也较大。     

冲积河流的汇合与分流

冲积河流具有一定的自动调整作用。遵循最小阻力原理，河道的汇合分流也应属于冲积河流的自动调整作用之列，事实上，在河道的汇合与分流处，支流与主流夹角的形成及其宽度比（或断面积比）的建立都不是随机的，它必须保证来自支流与主流的水和泥沙能在此通畅排泄。本文证明了水流在通过河道的汇合与分流处时所形成的夹角及其宽度比，对应于水头损失的最小值。河口段垂直于或趋向垂直于海岸线的特性，也是水流不断调整到最小阻力状态的结果。     

网状河流体系的基本特征及其影响因素

网状河流近年来受到较多关注,无论从沉积特征还是从地貌及水力学特征看,网状河流都是一种典型的河型,但目前对网状河流的认识还存在一定的局限性,从沉积特征,地貌及沉积微环境特征及形成条件等方面的对网状河流进行了系统的论述和总结。     

网状河流研究进展述评

网状河流作为一种新的冲积河流类型已经引起地貌学家、水利学家和沉积学家的关注 ,成为河流地貌领域、河流沉积领域以及河流水动力领域的研究热点之一。本文在介绍网状河流基本概念的基础上 ,综合国内外的研究成果 ,从河流的平面形态、边界条件、沉积特征、水动力条件以及在河型演化序列中的位置等方面 ,对网状河流的研究进展作一较全面的述评 ,并指出目前研究的薄弱环节 ,以利于研究者把握网状河流的研究现状 ,并推动对网状河流的进一步探讨。     

黄河流域河型转化现象初探

黄河以其高含沙水流以及下游河道的高沉积速率而著称于世。迄今的研究, 主要针对黄河中下游流域的 侵蚀、水文泥沙和河床演变方面的研究, 而对黄河流域主支流发生河型转化的现象关注不够。在黄河的不同河段, 河型的变化频繁, 类型多样, 现象复杂, 是研究者不可回避的科学问题。本文选取黄河上游第一弯的玛曲河段、黄河 上游末段托克托附近河段及黄河下游高村上下河段来研究河型转化的形式及影响因素。玛曲河段沿流向发生网状 河型→弯曲河型→辫状河型的转化现象, 该系列转化呈现出由极稳定河型向极不稳定河型的转化, 这与世界上通 常可以观察到的沿流向不稳定河型向稳定河型转化的情况完全相反。这主要受到地壳的抬升、上下峡谷卡口、水动 力特征、边界沉积物特征及植被的区域分布等因素的控制。托克托附近沿流向发生了弯曲河型→顺直河型转化的 现象, 这是较稳定河型向极稳定河型的转化, 主要受到边界沉积物、水动力等因素的控制。高村上下河段沿流向发 生的辫状河型→弯曲河型转化的现象, 是由极不稳定河型向较稳定河型转化的现象, 河道边界沉积物及水动力是 其主要控制因素, 人工大堤只是限制了河道摆动的最大幅度, 对河型的性质影响不大, 但其上游河段修筑的水库导 致下泻的水流在辫状河段的侵蚀能力增强而使其边界沉积物粗化, 并将泥质物大量沉积在弯曲河段, 客观上促进 了河型的转化。     

Human modifications to the sediment regime of the Lower Mississippi River flood plain

The Mississippi River is one of the most regulated rivers in the world. Human modifications constructed mainly after 1920 include dams and reservoirs, artificial levees, dikes, concrete revetments and a series of channel cutoffs. This paper examines some of the effects of these modifications on the channel and sediment budget of the river. In particular, the changes to the thalweg profile and the size of channel bars are examined in detail. It is concluded, that prior to the 1930s, when major modifications were introduced, the Lower Mississippi River was an aggrading meandering river. The role of the flood plain has also changed. Prior to modifications, the flood plain was the major sediment source as the result of bank caving. Today the flood plain provides only a minor amount of sediment. It can be shown that major degradation to the channel including the growth of channel bars has occurred as a result of these engineered modifications. The data also indicates that the different geomorphic regions respond to modifications in different ways.     

Analysis of controls upon channel planform at the First Great Bend of the Upper Yellow River, Qinghai-Tibet Plateau

The 270 km long section of the Upper Yellow River at the First Great Bend is comprised of single channel and multiple channel systems that alternate among anastomosing, anabranching, meandering and braided reaches. The sequence of downstream pattern changes is characterized as： anastomosing-anabranching, anabranching-meandering, meandering-braided and braided-meandering. Remote sensing images, DEM data and field investigations are used to assess ahd interpret controls on these reach transitions. Channel slope and bed sediment size are key determinants of transitions in channel planform. Anas- tomosing reaches have a relatively high bed slope （0.86‰） and coarser sediment bed material （d50 = 3.5 mm）. In contrast, meandering reaches have a low slope （0.30‰） and fine sediment bed material （d50 = 0.036 mm）. The transition from a meandering to braided pattern is characterized by an increase in channel width-depth ratio, indicating the important role of bank strength （i.e. cohesive versus non-cohesive versus channel boundaries）. Interestingly, the braided-meandering and meandering-braided transitions are coincident with variable flow inputs from tributary rivers （Baihe and Heihe rivers respectively）. Theoretical analysis of the meandering-braided transition highlights the key control of channel width-depth ratio as a determinant of channel planform.     

Hydraulic and sedimentary processes causing anastomosing morphology of the upper Columbia River, British Columbia, Canada

The upper Columbia River, British Columbia, Canada, shows typical anastomosing morphology — multiple interconnected channels that enclose floodbasins — and lateral channel stability. We analysed field data on hydraulic and sedimentary processes and show that the anastomosing morphology of the upper Columbia River is caused by sediment (bedload) transport inefficiency, in combination with very limited potential for lateral bank erosion because of very low specific stream power (≤ 2.3 W/m²) and cohesive silty banks. In a diagram of channel type in relation to flow energy and median grain size of the bed material, data points for the straight upper Columbia River channels cluster separately from the data points for braided and meandering channels. Measurements and calculations indicate that bedload transport in the anastomosing reach of the upper Columbia River decreases downstream. Because of lateral channel stability no lateral storage capacity for bedload is created. Therefore, the surplus of bedload leads to channel bed aggradation, which outpaces levee accretion and causes avulsions because of loss of channel flow capacity. This avulsion mechanism applies only to the main channel of the system, which transports 87% of the water and > 90% of the sediment in the cross-valley transect studied. Because of very low sediment transport capacity, the morphological evolution of most secondary channels is slow. Measurements and calculations indicate that much more bedload is sequestered in the relatively steep upper anastomosing reach of the upper Columbia River than in the relatively gentle lower anastomosing reach. With anastomosing morphology and related processes (e.g., crevassing) being best developed in the upper reach, this confirms the notion of upstream rather than downstream control of upper Columbia River anastomosis.     

Fluvial processes of the downstream reaches of the reservoirs in the Lower Yellow River

Evolution of the river channel downstream of reservoirs is a complex process that is closely related to the operational mode of the reservoirs and the channel boundary conditions. Numerous studies have been carried out on the fluvial processes of downstream reservoirs. However, only a few of them have focused on the relationship between runoff-sediment conditions and channel pattern indicators. Also, the impacts of river training works on fluvial processes are seldom dealt with. In this paper, the evolutionary processes of three sections in the Lower Yellow River, including Tiexie-Yiluo River mouth reach, Huayuankou-Heigangkou reach and Jiahetan-Gaocun reach, were analyzed for variations in the channel boundary line and the mainstream between 1960 and 2015. Channel pattern indicators such as sinuosity, mainstream wandering range and width/depth ratio were analyzed based on field measurements obtained by the Hydrological Department of the Yellow River Conservancy Commission. The effects of river training works on the channel evolution are then described. Since 1960, numerous medium- and large-sized reservoirs have been built on the Yellow River, including Longyangxia Reservoir, Liujiaxia Reservoir and Xiaolangdi Reservoir. These reservoirs impound the runoff from upstream and retain the sediment, which changes the runoff and sediment conditions in the downstream reach. As a consequence, annual runoff and the frequency and peak of flooding have all decreased. As a result, the flow dynamics and their action on the river channel are also reduced, which changes the dynamic state of the river course. The discrimination results obtained using the single parameter discrimination rule and the discrimination equation show that the degree of wandering is weakened in the reaches studied. The variations in the channel pattern indicators show that the sinuosity increases and the wandering range decreases with a reduction in the total annual volume of water. However, the degree of wandering has little relationship to the sediment concentration. In addition, river training works play an important role in controlling the river course. Due to improvements in the river training works, the river course has become more stable under the same runoff and sediment conditions. A new discrimination rule that takes into account the impacts of the river training works is proposed. The discrimination results were found to fit well with the actual river pattern, which shows that the discrimination rule is applicable to the Lower Yellow River. The results show that the runoff and sediment conditions are the most important factors in the evolution of the river course. The river training works have at the same time limited the wandering range of the mainstream and played an important role in the fluvial processes. Both factors combined lead to the stabilization of the river.