Quantifying channel development and sediment transfer following chute cutoff in a wandering gravel-bed river

Three-dimensional morphological adjustment in a chute cutoff (breach) alluvial channel is quantified using Digital Elevation Model (DEM) analysis for a ca. 0.7 km reach of the River Coquet, Northumberland, UK. Following cutoff in January 1999, channel and bar topography was surveyed using a Total Station on five occasions between February 1999 and December 2000. Analysis of planform change coupled with DEM differencing elucidates channel and barform development following cutoff, and enables quantification of sediment transfers associated with morphological adjustment within the reach. This exercise indicates an initial phase of bed scour, followed by a period characterised by extensive bank erosion and lateral channel migration where erosion (including bed scour) totalled some 15,000 m³ of sediment. The channel in the post-cutoff, disequilibrium state is highly sensitive to relatively low-magnitude floods, and provision of accommodation space by bank erosion encouraged extensive lateral bar development. Bar development was further facilitated by infilling of channels abandoned by repeated within-reach avulsion and large-scale aggradation of sediment lobes deposited by higher magnitude floods. Calculations indicate that at least 6600 m³ of sediment was deposited on emerging bars within the reach over the survey period, and >2300 m³ deposited within the channel. Sediment losses from the reach may have exceeded 6500 m³.     

长江上游重点产沙区产沙量对人类活动的响应

对金沙江屏山站、嘉陵江北碚站、宜昌站20世纪50年代以来的输沙量和含沙量进行分析。结果表明,在年径流量相同的情况下,由于大规模矿山、钢铁工业、交通建设增加水土流失,屏山站1976~1996年年产沙量要高于1954~1976年。与此相反,由于修筑水库、塘坝拦沙,修建梯田、恢复植被减少侵蚀,北碚站1983~1996年年产沙量要大大低于1954~1982年;宜昌站1985~1996年年产沙量也低于1954~1984年。北碚、宜昌两站都存在一个使水利、水土保持减沙量为0的年径流量临界值,北碚站和宜昌站这一临界值分别为1142×10⁸m³和4800×10⁸m³。     

Morphological dynamics of upland headwater streams in the southern North Island of New Zealand

Short-term channel dynamics of mountain stream reaches in the southern North Island of New Zealand were assessed over two successive 3-month periods using morphological budgeting. Response to floods varies between reaches, even when the catchments were located close to each other and had similar characteristics. The reaches on the Central Volcanic Plateau experienced least morphological change, while streams with steep catchments and migrating planform in the Tararua and Ruahine Ranges showed frequent channel adjustments. Channel response is conditioned by intrinsic variables rendering reaches responsive or robust to the effects of floods, and this is likely to reflect the degree of connectivity between slopes and channels, and reaches.     

长江宜昌-武汉河段泥沙年冲淤量对水沙变化的响应

运用泥沙收支平衡 (Sediment budget) 的概念确定长江中游宜昌-武汉河段的泥沙冲淤量,并运用数理统计方法,研究了泥沙冲淤过程对水沙变化的响应。研究表明,所研究河段的输沙具有“多来多排”的特性,在平均的意义上,年输入沙量为年输出沙量的1.1345倍,由此求得总净来沙中有11.85%淤积在河道中。河段出口输出沙量随时间而增大,大致在1980年达到峰值,然后再减小。1980年以前河段出口输出沙量的增大,与3口分沙减少 (等价于河段净来沙增多) 和下荆江人工裁弯 (使河道输沙能力增大,因而可以将更多的泥沙输送到河段出口以下) 有关,1980年以后的减少,则与宜昌站来沙量的显著减少有关。建立了1980~1997年间宜昌-汉口#河段年冲淤量与宜昌站年来沙量之间的回归方程,通过该方程估算出使宜昌-汉口河段不淤的宜昌站临界来沙量为3亿t/a。为了定量评价宜昌站的来水量和来沙量以及3口分水比和分沙比、宜昌站洪峰流量的变化对于河段冲淤量的相对贡献,我们以1980~1997年和1955~1997年两个时间系列的数据分别建立了多元回归方程。1980~1997年间的方程表明,宜昌站的来水量和来沙量以及3口分水比和分沙比、宜昌站洪峰流量的变化对宜昌-汉口河段年冲淤量的贡献率分别为6.23%、31.56%、25.77%、32.71%和3.73%。     

Assessment of downstream trends in channel gradient, total and specific stream power: a GIS approach

Geographic Information Systems (GIS) analyses of Digital Elevation Models (DEMs) coupled with catchment area based discharge estimation techniques provide a relatively simple means of modelling contiguous downstream trends in channel gradient, total stream power, and in riverscapes conducive to regime analysis, also specific stream power. For a small, high relief, coastal catchment in SE Australia, good agreement was obtained between channel gradients derived from a 25 m cell-size DEM and field survey equivalents over distances of several kilometres, indicating that channel gradients derived from DEMs can have a reasonable degree of absolute as well as relative accuracy over multi-kilometre reach scales. Assessment of downstream rates of change in channel gradient and specific stream power across four river systems suggests that some of the river reaches most responsive to high magnitude floods occur in zones where these variables rapidly decrease downstream. Modelling of downstream trends in channel gradient, total and specific stream power from catchment-wide DEMs has potential to provide a framework with which to investigate conceptual and empirical models between channel gradient, stream power and the form and dynamics of river systems.     

Planform characteristics and development of interchannel wetlands in a gravel-bed anastomosing river,Maqu Reach of the Upper Yellow River

Both interchannel wetlands and multi-channels are crucial geomorphologic units in an anastomosing river system. Planform characteristics and development of interchannel wetlands and multi-channels control the characteristics of anastomosing rivers. To understand the role that interchannel wetlands play in the development of anastomosing rivers, a study was conducted on the Maqu Reach of the Upper Yellow River (MRUYR), a gravel-bed anastomosing river characterized by highly developed interchannel wetlands and anabranches. Geomorphologic units in the studied reach were extracted from high resolution satellite imagery in Google Earth. The size distributions of interchannel wetlands and interchannel wetland clusters (IWCs), a special combination of interchannel wetlands and anabranches, were investigated. Geomorphologic parameters, including the ratio of interchannel wetland area to IWC area (P), shoreline density (D_l), and node density (D_n) were used to analyze planform characteristics of IWCs and the development of multi-channels in the studied reach. The results suggest that small or middle sized interchannel wetlands and large or mega sized IWCs are more common at the study site. The area of IWC (S_u) is highly correlated with other geomorphologic parameters. P increases with increasing S_u, and the upper limit is about 80%, which indicates that the development of interchannel wetlands and anabranches in the IWC is in the equilibrium stage. In contrast, D_l and D_n show a tendency to decrease with increasing S_u due to diverse evolution processes in IWCs with different sizes. There are three main reasons leading to the formation of IWCs: varying stream power due to the meandering principal channel; development of the river corridor due to the weakening of geologic structure control; and high stability of interchannel wetlands due to conservation by shoreline vegetation.     

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。     

Channel Form, Bed Material and Sediment Sources of the Sprongdøla, Southern Norway: Evidence for a Distinct Periglacio-Fluvial System

A geomorphological and sedimentological case study is made of the Sprongdøla, a medium-sized river flowing in a predominantly bedrock channel in the mountains of southern Norway. Particular attention is given to bed material characteristics and potential sediment sources. Downstream patterns in clast size and shape, including sphericity and roundness, are poorly developed in the Sprongdøla. There is an apparent absence of down-stream fining and a large proportion of bedload in the lower channel remains in very angular and angular roundness categories. Covariance analysis of clast shape and roundness allows this to be attributed to the widespread input of coarse, angular rock particles derived from snow-avalanche activity on the valley sides and especially from frost-shattering of bedrock in the channel, which produces characteristically slabby clasts. The latter process is important in channel widening and in steepening the channel walls. Strictly fluvial processes, avalanche activity and frost-shattering are controlled in different ways by related annual environmental cycles. Their interaction in the context of the periglacial river channel produces a unique process-sediment-landform association that distinguishes alpine periglacio-fluvial systems from those associated with glacio-fluvial rivers and fluvial landscapes in the temperate zone.     

Sediment budget analysis of slope–channel coupling and in-channel sediment storage in an upland catchment, southeastern Australia

Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km² gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using ¹³⁷Cs and ²¹⁰Pb_ex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.     

黄河下游游荡段排沙比对水沙条件与断面形态的响应

提高黄河下游游荡段的输沙能力是河道治理的主要任务,而河道输沙效率（排沙比）受到来水来沙条件和河床边界条件的共同影响。本文基于1971—2016年花园口—高村河段（简称花高段）的实测水沙及地形资料,计算了花高段的平均河相系数及水沙条件（来沙系数和水流冲刷强度）,从汛期和场次洪水2个时间尺度,定量分析了排沙比与水沙条件及前一年汛后主槽形态之间的响应关系。分析结果表明：① 汛期和场次洪水排沙比与来沙系数呈负相关,与水流冲刷强度呈正相关,临界的汛期不淤来沙系数为0.012 kg?s/m ⁶,场次洪水排沙比与来沙系数及水量比的决定系数为0.76;② 游荡段排沙比与河相系数呈负相关,当河相系数大于15 /m ^0.5时,河段排沙比基本小于1;③ 以来沙系数与河相系数为自变量的汛期排沙比计算式的决定系数为0.82,计算精度较高,对于场次洪水排沙比而言,断面形态的影响权重大于来沙系数。这些排沙比计算公式能够反映游荡段的输沙特点,有助于定量掌握断面形态及水沙条件对河道输沙能力的影响。     

黄河下游河道断面形态参数变化及其水沙过程响应

基于1965—2015年黄河下游花园口、高村、泺口站的逐年水文和汛前河道断面的实测资料,分析了河道断面形态参数(河道断面面积,河道宽深比等)的变化,以及对河道断面形态与来水来沙间的关系做出定量化分析。结果表明:主槽断面形态参数与水沙搭配以及前期断面形态密切相关,沿程3个断面形态参数调整方式存在显著差异。河宽调整幅度沿程减小,辫状河段变幅最大,尤其在1986—1999年,辫状河段萎缩程度最为严重,其次为弯曲河段,顺直河段横向调整幅度最小。受到前期断面形态的影响,辫状河段河道断面调整方式既有横向展宽(萎缩)又有垂直加深(淤积);弯曲河段河道宽深比与流量呈较弱的正相关关系,具有横向和垂向的调整方式;而顺直河段的宽深比与流量呈负相关关系,与来沙系数呈正相关关系,河道以垂直加深(淤积)为主。     

黄河河龙区间河流泥沙对相关重大事件与政策的响应

利用径流-输沙量双累积曲线与单位降雨侵蚀力输沙量(输沙量负荷)的分析表明,河龙区间泥沙变化与相关重大事件与政策密切相关。“大开荒” 时期输沙量负荷平均值最大,是输沙量负荷多年平均值的1.8倍;“大跃进”、“农业学大寨”和“文革”时期的输沙量负荷均值也较大,为输沙量负荷多年平均值的1.4~1.5倍。改革开放后,输沙量负荷下降明显,输沙量负荷平均值较改革开放前减少了55.7%,特别是在1999年实施“退耕还林(草)”后,输沙量负荷平均值下降到整个时期的最小值,仅为输沙量负荷多年平均值的33.3%。     

无定河流域侵蚀产沙过程对水土保持措施的响应

以黄河中游多沙粗沙区代表性支流无定河为例,研究了降水量变化的背景下水土保持措施对无定河流域侵蚀产沙的影响。从总体上看,年输沙量和年径流量都表现出随时间而减小的趋势,梯田、造林、种草和淤地坝拦沙造地4项水土保持措施面积随时间增大的趋势极其显著。全流域面平均年降水量和汛期 (6~9月) 降水量则略有减少的趋势,但其减少趋势的显著性远低于水土保持措施随时间变化的显著性。产沙模数与梯田、坝地、造林和种草面积的关系,均表现出明显的负相关。对基准期 (1956~1969年) 和措施期 (1970~1996) 进行了比较,结果表明,实施水土保持措施以后,产沙模数、径流系数和汛期径流能够被降水所解释的百分比,分别由69%、80%和77%,下降为26%、31%和54%。说明在后一时期中,水土保持措施起了很大作用,因而降水变化对于产沙模数、径流系数、汛期径流量变化的贡献率大大减小了。以1956~1996年间41年的资料为基础,建立了全沙产沙模数与径流系数、汛期降雨量、最大一日雨量、最大30日累积雨量、梯田林草面积和坝地面积之间的多元回归方程,该方程表明,全沙产沙模数随汛期降雨量、最大一日雨量、最大30日累积雨量的增大而增大,随梯田林草面积和坝地面积的增大而减小, 随径流系数的增大而增大。为了区分人为因素与气候变化对侵蚀产沙变化的影响,采用对数据进行标准化处理之后再进行多元回归分析的方法,来确定相对贡献率。结果表明,降水变化和水土保持措施变化对无定河全沙产沙模数变化的贡献率大致相等,前者占50.3%,后者占49.7%。文中还建立了粗泥沙产沙模数与上述影响因子之间的多元回归方程。     

Equal-mobility bed load transport in a small, step-pool channel in the Ouachita Mountains

Equal-mobility transport (EMT) of bed load is more evident than size-selective transport during near-bankfull flow events in a small, step-pool channel in the Ouachita Mountains of central Arkansas. Bed load transport modes were studied by simulating five separate runoff events with peak discharges between 0.25 and 1.34 m³/s (1.0- to 1.6-year recurrence intervals) in a natural channel using controlled releases from a storage tank. EMT occurrence was investigated using four different bed load relationships suggested by previous research. With each of these approaches, the relationship of a given bed load characteristic (D_max, distribution percentile, displacement distance and skewness) to some independent factor (τ_c*, τ and grain size) was assessed to determine which transport mode was evident. Regression models derived using combinations of these four relationships with different datasets provide seven separate tests. Five of the seven tests indicate that EMT occurred or was predominant. Several reasons may explain the apparent contradictory results, but the confounding effects of changes in the structural arrangements of bed material prior to or during the events seem particularly important.     

废黄河沉积记录中来自不同河流物质的信息——基于重矿物与地球化学元素分析

在全新世时间尺度内,沉积记录主要受控于沉积物供给、输运和堆积过程等因素。对废黄河沉积记录的深入分析可以反演黄河南流夺淮过程中沉积物的“源”-“汇”关系。基于废黄河故道沉积记录中重矿物和地球化学元素特征,并结合与现代黄河沉积物的对比,揭示废黄河沉积记录中物质来源的信息。结果表明,废黄河故道沉积物中重矿物平均含量为0.19%,种类多达15余种,以稳定矿物为主,不同矿物的时空变化差异较大。废黄河故道泥质和砂质沉积物中,地球化学元素表现出明显的时空变化规律,随深度递增元素含量可分为降低、稳定和增加趋势。对比现代黄河沉积物,发现废黄河故道沉积物中稳定和极稳定矿物相对含量较高,泥质沉积物中更多地富集Zn,Ni,Al,Ba,Cu,Li,K等元素。这一差异性反映出其它不同河流物质补给而产生的影响,淮河沉积物中重矿物和地球化学元素的研究为该论断提供了佐证。     

Simulating cross-sectional geometry of the main channel in response to changes in water and sediment in Lower Yellow River

Journal of Geographical Sciences - To understand the non-equilibrium morphological adjustment of a river in response to environmental changes, it is essential to (i) accurately identify how past...     

三角洲前缘岸滩对河流来沙减少响应的敏感性探讨——以长江口门区崇明岛向海侧岸滩为例

利用ArcGIS对1977—2004年不同时段的长江口崇明岛向海侧水下地形测量资料进行处理,并结合该区的潮滩历史遥感图片、现场高程测量以及同期大通站输沙率资料,探讨三角洲前缘岸滩冲淤演变对河流来沙减少响应的敏感性。结果表明:研究区岸滩的淤涨速率总体上在年代以上时间尺度随着长江来沙减少而下降,但由于局部滩—槽演变和海洋动力条件的影响,某些年代内和某些部位可能出现相反的趋势,使岸滩演变对河流来沙减少的响应变得复杂化。在此基础上预测今后几十年研究区的冲淤趋势。     

Land-use/cover changes in relation to stream dynamics in a marginal graben along the northern Ethiopian Rift Valley

This paper investigates land-use/cover changes related to river dynamics in northern Ethiopia. Aerial photographs from 1965 to 1986, and SPOT images of 2007 and 2014 were used to extract land units. Land-use/cover changes took place in 48% of the entire landscape around the river across the last five decades. Changes related to swap accounted for 37%, whereas net changes accounted for 11%. The most systematic transitions in terms of gain were from shrubland to farmland, alluvial deposit to settlement, and alluvial deposit to active channel and settlement. Most of these transitions were related to the river dynamics and depict cyclic transitions: farmland → active channel → alluvial deposits → grassland/shrubland → farmland. Human interventions and natural vegetation succession were also very important. The study concludes that river systems have considerable impact on livelihood and need attention in land management undertakings in graben bottoms.     

Limited Downstream Response of Stream Channel Size to Urbanization in a Humid Tropical Basin*

Stream channel changes are often assumed to propagate downstream of the disturbed or urbanized reach, especially where the main controlling variables of discharge and sediment load vary regularly in the downstream direction. A survey of bankfull cross-sectional sizes at 10 stations along a tropical stream shows that downstream propagation of the disturbance may not always occur. Serial regression analysis suggests that a reduction in channel size induced by urbanization did not extend beyond the urbanized reach. Furthermore, local channel change did not alter the downstream log linear trend between channel size and basin area. Apparently, the storage of floodwaters and sediments on floodplains reduced the downstream effects of urbanization.