Impact of the channelization‐induced incision of the Skawa and Wisłoka Rivers,southern Poland,on the conditions of overbank deposition

The impact of river incision in response to channelization on the conditions of overbank deposition is shown by the study of two montane rivers from the upper Vistula drainage basin, southern Poland. The Wisl/oka River had insufficient energy to destroy the river‐control structures and remained laterally stable in the course of the channel downcutting. Under such conditions, the incision has raised the relative elevation of the floodplain above the river bed, thereby reducing considerably the frequency of overbank flows, and increasing concentration of suspended sediment transport within the incised channel. On the high‐energy Skawa, the long periods of incision of the channelized river alternated with the shorter periods of lateral channel migration over the twentieth century. This has led to the formation of an incised meander belt, within which flood flows are constricted, and where the high velocities of the floodplain flows inhibit overbank deposition. Field observations confirm an insignificant role played nowadays by floodplain sedimentation in the valleys of both rivers. This study shows that the potential of the floodplains of the Carpathian tributaries to the Vistula for sediment storage has been dramatically reduced over the few past decades as a result of the channelization‐induced incision of the rivers. The frequency of overbank flows has decreased considerably on the rivers draining the eastern part of the Polish Carpathians, and the majority of the suspended sediment is routed within the resultant enlarged channels. In the western part of the mountains, high velocities of the floodplain flows restrict overbank deposition on the narrow floodplains developed along incised channels. Copyright © 2001 John Wiley & Sons, Ltd.     

Role of the floodplain and riparian zone in suspended matter and nitrogen retention in the adour river,south-west France

The retention capacity of the floodplain and riparian zone for suspended matter and nitrogen has been investigated in the Adour River, a seventh order stream in south-west France. Suspended matter and nitrogen fluxes through a 25 km meandering stretch of the river were measured during two flood events and compared with the amount of sediment trapped in the riparian zone (1.1 km²) and the major floodplain (16.8 km²) of the studied area. It was estimated that the floodplain and the riparian zone together retained between 10 and 20% of the suspended matter entering the stretch under study during the two main floods (138 700 Mg). Moreover, they retained about 11% of the total particulate nitrogen fluxes (640 Mg). Although the riparian zones are 15 times smaller than the major floodplain, the total suspended matter and particulate nitrogen deposition were, 50 and 17 times, respectively, larger in the riparian zone. The results obtained on the Adour River floodplain show that large river systems should not be considered only as export systems as riparian zones can retain a significant amount of suspended organic and mineral matter during floods.     

Invasibility Drives Restoration of a Floodplain Plant Community

An understanding of the processes that determine plant community structure is a requisite for the planning and evaluation of restoration efforts on river floodplains. Variable disturbance regimes derived from flood pulses increase the susceptibility of river floodplains to colonizations by new species and establish invasibility as a potentially important factor in plant community assembly and dynamics. The role of invasibility in the restoration of a wet prairie community on the Kissimmee River floodplain in central Florida was evaluated by quantifying temporal species turnover rates during wet and dry season sampling over a 12‐year pre‐restoration and post‐restoration period. Turnover rates increased with reestablishment of annual inundation regimes and were significantly greater on the reflooded floodplain than on the drained, channelized floodplain. Recurrent periods of increased invasibility were associated with repeated high‐amplitude flood pulses and accompanied by increased diversity of plant communities within the wet prairie landscape. Neither invasibility nor beta diversity was strongly related to the variable hydroperiods or depths provided by local topography and restoration of seasonal hydrologic regimes. Results suggest that invasibility is a functional process by which the restored flood pulse has reestablished the structure and diversity of the wet prairie. Copyright © 2014 John Wiley & Sons, Ltd.     

Entrainment of riparian gravel and cobbles in an alluvial reach of a regulated canyon river

Many canyon rivers have channels and riparian zones composed of alluvial materials and these reaches, dominated by fluvial processes, are sensitive to alterations in streamflow regime. Prior to reservoir construction in the mid‐1960s, banks and bars in alluvial reaches of the Gunnison River in the Black Canyon National Monument, Colorado, USA, periodically were reworked and cleared of riparian vegetation by mainstem floods. Recent interest in maintaining near‐natural conditions in the Black Canyon using reservoir releases has created a need to estimate sediment‐entraining discharges for a variety of geomorphic surfaces composed of sediment ranging in size from gravel to small boulders. Sediment entrainment potential was studied at eight cross‐sections in an alluvial reach of the Gunnison River in the Black Canyon in 1994 and 1995. A one‐dimensional water‐surface profile model was used to estimate water‐surface elevations, flow depths, and hydraulic conditions on selected alluvial surfaces for discharges ranging from 57 to 570 m³/s. Onsite observations before and after a flood of 270 m³/s confirmed sediment entrainment on several surfaces inundated by the flood. Selective entrainment of all but the largest particle sizes on the surface occurred at some locations. Physical evidence of sediment entrainment, or absence of sediment entrainment, on inundated surfaces generally was consistent with critical shear stresses estimated with a dimensionless critical shear stress of 0.030. Sediment‐entrainment potential over a range of discharges was summarized by the ratio of the local boundary shear stress to the critical shear stress for d₅₀, given hydraulic geometry and sediment‐size characteristics. Differing entrainment potential for similar geomorphic surfaces indicates that estimation of minimum streamflow requirements based on sediment mobility is site‐specific and that there is no unique streamflow that will initiate movement of d₅₀ at every geomorphically similar location in the Black Canyon. Copyright © 2000 John Wiley & Sons, Ltd.     

A 12‐month sediment and nutrient budget in a floodplain reach of the River Adour,southwest France

The sediment and nutrient budget in the floodplain reach of the River Adour in southwest France has been examined over a 1‐year period (1991–1992). Outputs of total suspended matter, dissolved inorganic nitrogen (i.e. nitrate+ammonium ions) and dissolved inorganic phosphorus (DIP) and particulate phosphorus (PP) have been compared with inputs. Alternating phases of mobilization and retention are observed for all the elements considered. Low water periods in summer allowed retention of both soluble material (24–26% for inorganic nitrogen) and suspended matter (9–19%). However, mobilization of PP associated with suspended matter transport during these periods. Elevated discharges associated with increased rainfall during the autumn period induced significant mobilization for all elements. In contrast, the winter flood event gave rise to a retention of particulate material (i.e. suspended matter and PP, 11% and 26%, respectively), but a loss of soluble materials (5% for inorganic nitrogen and 186% for DIP). The spring flood event produced retention for particulate materials (26%) while inducing losses of inorganic nitrogen (9%) and gains of DIP (7.4%). These results are related to riparian vegetation and the dynamics of nutrient cycling within the floodplain. Copyright © 2000 John Wiley & Sons, Ltd.     

RIVERBED DIGITAL ELEVATION MODELS AS A TOOL FOR HOLISTIC RIVER MANAGEMENT: MOTUEKA RIVER,NELSON, NEW ZEALAND

River management in New Zealand's laterally active gravelly rivers has permitted floodplain development and protection of agricultural resources and infrastructure. Management of these dynamic systems has been hailed as a success for the approaches adopted, namely straightening and confining the river using bank protection and managing riverbed levels by gravel extraction. However, this activity also impacts river morphological/habitat diversity and potential gravel resource, by replacing broad riparian corridors with narrower channels and reducing lateral connectivity with the floodplain. This paper quantifies river behaviour in three laterally confined reaches in the upper Motueka River over a 7‐year period, using annual high‐resolution ground surveys to address the nature of morphological change and associated sediment flux in these reaches with a view to informing management of the gravel resource. Surveys between 2004 and 2010 acquired data to construct digital elevation models (DEMs) of the active riverbed in three ~1‐km‐long reaches. Morphological budgeting based on differencing between successive DEM surfaces reveals complex spatial and temporal patterns of erosion and deposition, demonstrating complex reach dynamics. Overall, volumetric changes suggest these narrowed reaches have been net exporters of sediment, associated with continued channel degradation. This has left bar features, traditionally the focus of gravel extraction in the reaches, relatively isolated from all but extreme flows, limiting replenishment of the gravel resource. The paper demonstrates the utility of riverbed DEMs as a potential tool to frame river character and behaviour at the reach scale in gravel‐bed rivers, thereby providing an important contribution to holistic river management in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Multi‐annual contemporary flood event overbank sedimentation within the vegetated lower Orinoco floodplain,Venezuela

Multi‐annual contemporary flood event overbank sedimentation rates were quantified on the World's third largest river in terms of discharge, the tropical lower Orinoco. We discuss the role of variables at the basin and reach scales that contributed to the complexity of spatio‐temporal overbank sediment deposition patterns. Monitored in situ plots were characterized by distance to the main channel, hydroperiod, different geomorphological units, and vegetation cover. Flood event sedimentation rates showed a high spatial variability ranging from the absence of sediment deposition up to 225.46 kg m^‐2 yr^‐1. Banks and levees received relatively high amounts of sediment (39.6 kg m^‐2 yr^‐1), whereas observed mean sedimentation rates on the more distant floodplain and backswamps tended to be lower (17.7 kg m^‐2 yr^‐1). Significant differences in sedimentation rates were observed in two major vegetation types: dense herbaceous and shrubby vegetation (42.2 kg m^‐2 yr^‐1) and floodplain forest (12.7 kg m^‐2 yr^‐1). However, overbank sedimentation patterns also reflected imbricated hydrosedimentary and biogeomorphological vegetation feedbacks that co‐construct fluvial landforms. The incidence of an El Niño–Southern Oscillation–La Niña episode during the study period on sediment availability and floodplain sedimentation suggests that within whitewater rivers, where suspended sediment concentrations are naturally high, hydrological connectivity seems to be more important for floodplain sedimentation than variations in suspended sediment concentrations. These results may provide a good basis for future biogeomorphological investigation projects using complementary methodologies, in order to better anticipate global change and fluctuations in the occurrence, strength or duration of El Niño–La Niña episodes in the tropical zone and their consequences for flood discharge and sediment dynamics during channel–floodplain exchanges.     

黄河下游泥沙沉积汇对入海悬移质泥沙粒度的影响

黄河下游泥沙沉积汇在黄河流域系统的泥沙收支平衡(Sediment budget)中起着重要的作用。通过河床主槽中泥沙的淤积和冲刷,运动泥沙的组成发生变化;通过滩地上泥沙淤积和河岸坍塌,河漫滩上前期淤积的泥沙与洪水所挟带的泥沙发生交换,使悬移质泥沙的组成发生变化。泥沙冲淤量对不同粒径组入海泥沙百分比的影响是不同的。小于0.01mm细泥沙的百分比与下游河道淤积量呈正相关,0.025~0.05mm和大于0.05mm的较粗泥沙的百分比与下游河道淤积量呈负相关。入海泥沙平均粒径与深泓年摆幅之间也存在着负相关关系。自20世纪60年代,中期以来,特别是自80年代中期以来,由于黄河下游径流量显著减小,黄河下游河道发生萎缩,河宽减小,因而主流线的摆动幅度也显著减小。这使得原来十分强烈的河岸侵蚀和滩槽泥沙交换强度大为减弱,主槽洪水得到的来自河岸侵蚀的细粒泥沙越来越少,因而使入海泥沙粒径变粗。     

黄河流域水沙资源量变化及其对泥沙资源化的影响

在分析黄河流域水沙资源区域分布的基础上,本文采用Mann-Kendall秩次相关检验法和水文量累计曲线法分析黄河水沙资源量的变化特征,进而探讨了黄河泥沙的资源化及水沙资源量变化对泥沙资源化的影响。黄河龙羊峡水库以上河段年径流量和输沙量没有明显变化,其他干流河段和中游大部分典型支流的径流量和输沙量随时间具有明显的减少趋势。黄河水沙资源量大幅减少对泥沙资源化产生不同程度的影响,水沙资源量大幅减少会降低引洪淤滩、淤临和淤背的效率,限制灌区黄河泥沙烧制建筑材料、淤改与稻改等途径的实施,减小黄河口泥沙造陆的速率,影响黄河与河口湿地质量与数量的塑造;而对灌区浑水灌溉、清淤泥沙利用等没有明显的影响。     

Ecological connectivity in alluvial river ecosystems and its disruption by flow regulation

The dynamic nature of alluvial floodplain rivers is a function of flow and sediment regimes interacting with the physiographic features and vegetation cover of the landscape. During seasonal inundation, the flood pulse forms a ‘moving littoral’ that traverses the plain, increasing productivity and enhancing connectivity. The range of spatio-temporal connectivity between different biotopes, coupled with variable levels of natural disturbance, determine successional patterns and habitat heterogeneity that are responsible for maintaining the ecological integrity of floodplain river systems. Flow regulation by dams, often compounded by other modifications such as levee construction, normally results in reduced connectivity and altered successional trajectories in downstream reaches. Flood peaks are typically reduced by river regulation, which reduces the frequency and extent of floodplain inundation. A reduction in channel-forming flows reduces channel migration, an important phenomenon in maintaining high levels of habitat diversity across floodplains. The seasonal timing of floods may be shifted by flow regulation, with major ramifications for aquatic and terrestrial biota. Truncation of sediment transport may result in channel degradation for many kilometres downstream from a dam. Deepening of the channel lowers the water-table, which affects riparian vegetation dynamics and reduces the effective base level of tributaries, which results in rejuvenation and erosion. Ecological integrity in floodplain rivers is based in part on a diversity of water bodies with differing degrees of connectivity with the main river channel. Collectively, these water bodies occupy a wide range of successional stages, thereby forming a mosaic of habitat patches across the floodplain, This diversity is maintained by a balance between the trend toward terrestrialization and flow disturbances that renew connectivity and reset successional sequences. To counter the influence of river regulation, restoration efforts should focus on reestablishing dynamic connectivity between the channel and floodplain water bodies.     

三峡水库库尾重庆主城区河段冲淤特性

重庆主城区河段是西南地区水陆交通枢纽,其河道泥沙冲淤关系到防洪、航运及码头作业等一系列问题,是三峡水库泥沙问题的重点内容之一。三峡水库175 m试验性蓄水后,该河段位于库尾段,年内水力特性显著改变,河床冲淤规律相应发生变化。依据实测水沙及河道原型观测资料,对比研究了近15 a天然状态下和三峡水库175 m水位试验性蓄水后河段的泥沙冲淤变化。结果表明:自然状态下重庆主城区河段以冲刷为主,汛后走沙、汛期淤积规律明显,且汛后为主走沙期;三峡水库175 m水位试验性蓄水后至2012年河床冲刷强度下降,主走沙期推迟至汛前消落期;2013年以来上游来沙大幅减少使得河段冲刷强度增大。研究成果可为三峡水库优化调度和重庆主城区河段的规划治理提供参考。     

分汊河道分流比估算方法

利用冲淤平衡的多分汊河道各分汊含沙量基本相等和冲积分汊河流在分汊口两支汊水流满足动量守恒条件的特点,推导出2种计算分汊河道分流比的简便方法,即等含沙量法和动量平衡法。利用珠江河网共8处的分汊口分流资料进行验证,结果表明,与实际分流比相比,等含沙量法计算的分流比平均相对误差为21%,动量平衡法计算的分流比平均相对误差为6.4%,认为所提出的等含沙量法和动量平衡法适用于分汊河道分流比的估算。     

Integrating remotely acquired and field data to assess effects of setback levees on riparian and aquatic habitats in glacial‐melt water rivers

Setback levees, in which levees are reconstructed at a greater distance from a river channel, are a promising restoration technique particularly for alluvial rivers with broad floodplains where river‐floodplain connectivity is essential to ecological processes. Documenting the ecological outcomes of restoration activities is essential for assessing the comparative benefits of different restoration approaches and for justifying new restoration projects. Remote sensing of aquatic habitats offers one approach for comprehensive, objective documentation of river and floodplain habitats, but is difficult in glacial rivers because of high suspended‐sediment concentrations, braiding and a lack of large, well‐differentiated channel forms such as riffles and pools. Remote imagery and field surveys were used to assess the effects of recent and planned setback levees along the Puyallup River and, more generally, the application of multispectral imagery for classifying aquatic and riparian habitats in glacial‐melt water rivers. Airborne images were acquired with a horizontal ground resolution of 0.5 m in three spectral bands (0.545–0.555, 0.665–0.675 and 0.790–0.810 µm) spanning from green to near infrared (NIR) wavelengths. Field surveys identified river and floodplain habitat features and provided the basis for a comparative hydraulic analysis. Broad categories of aquatic habitat (smooth and rough water surface), exposed sediment (sand and boulder) and vegetated surfaces (herbaceous and deciduous shrub/forest) were classified accurately using the airborne images. Other categories [e.g. conifers, boulder, large woody debris (LWD)] and subdivisions of broad categories (e.g. riffles and runs) were not successfully classified either because these features did not form large patches that could be identified on the imagery or their spectral reflectances were not distinct from those of other habitat types. Airborne imagery was critical for assessing fine‐scale aquatic habitat heterogeneity including shallow, low‐velocity regions that were not feasible or practical to map in the field in many cases due to their widespread distribution, small size and poorly defined boundaries with other habitat types. At the reach‐scale, the setback levee affected the amount and distribution of riparian and aquatic habitats: (1) the area of all habitats was greater where levees had been set back and with relatively more vegetated floodplain habitat and relatively less exposed sediment and aquatic habitat, (2) where levees confine the river, less low‐velocity aquatic habitat is present over a range of flows with a higher degree of bed instability during high flows. As river restoration proceeds in the Pacific Northwest and elsewhere, remotely acquired imagery will be important for documenting its effects on the amount and distribution of aquatic and floodplain habitats, complimenting field data as a quantitative basis for evaluating project efficacy. Copyright © 2008 John Wiley & Sons, Ltd.     

美国基西米河渠化工程对河流生态环境的影响

位于美国佛罗里达州的基西米河，所处流域自然条件优越，季节性的水流变化较大，河流沿岸湿地资源丰富，为各种水生生物提供了适宜的生存环境，构成了流域内丰富多彩的生态系统．由于防洪排涝的需要，1961年开展了大规模的渠化工程，将蜿蜒的自然河流变成了几段较为顺直的人工运河，提高了河道的防洪能力，然而渠化后的河道及其下游流域的生态环境也随之出现了一系列负面反应，原有的生态平衡受到破坏，教训是非常深刻的．文中针对该渠化工程的后果及教训进行了总结和介绍。     

长江口深水航道治理一期工程前后泥沙运动特性初步分析

利用长江口徐六泾、横沙和佘山三站三年逐日表层含沙量、盐度和气象观测资料，以及北槽河段沿线含沙量洪、枯季准同步观测资料，对长江口深水航道治理一期工程实施前后北槽河段含沙量沿程分布规律和垂向变化特征，以及滩槽泥沙交换的机理进行了分析。结果表明：北槽河段高含沙区分布位置在一期工程后适量下移；风浪对浅滩泥沙的掀动作用和滩槽问泥沙交换是造成工程区河段含沙量增加的主要动力因素；滩槽泥沙交换及输移途径取决于滩槽间流场变化，南北导堤在二期工程继续下延后，将进一步削弱滩槽泥沙交换量，有利于对深水航道的维护。     

渭河下游近期淤积发展情况的分析研究

三门峡水库于１９６０年９月建成并投入运用已３８年。本文分析了渭河下游在近期的淤积发展情况，潼关高程一直居高不下，渭河下游泥沙淤积急剧增加，主槽过洪能力锐减，同流量洪水位迅速抬升，洪水灾害日趋频繁，防洪形势十分严峻，本文并对泥沙淤积发展的原因进行了分析研究，为渭河下游的防洪和工程治理提供科学的依据。     

Sediment chemistry and nutrient influx in a hydrologically restored bottomland hardwood forest in Midwestern USA

Exchanges of total N and C between a river and its floodplain forest have been enhanced by sedimentation processes during flood pulses at a 5‐ha bottomland hardwood forest located at the Olentangy River Wetland Research Park in central Ohio. In the spring of 2000, the forest was hydrologically restored by notching an artificial levee that had separated the Olentangy River and its floodplain forest for 100 years. One‐hundred flat sediment traps (30 cm × 30 cm) were used to collect sediment samples during spring/summer flooding events from 2003 to 2005. Results showed that sediment deposition is determined by the landscape variability during flooding events, Net sediment deposit in the wettest area averaged 134 ± 12 g‐dry wt m^?2 in 2003, 127 ± 17 g‐dry wt m^?2 in 2004 and 149 ± 23 g‐dry wt m^?2 in 2005. Total N and C sedimentation ranged from 0.49 to 0.92 g‐N m^?2 and 5.2 to 19.9 g‐C m^?2. Fe dominates the sediment chemistry and results show the elemental abundance in the order of Al > Fe > Ca > K > Mg > S > P > Na > Mn > Zn > B > Cu > Mo. A hydrologic pulsing index (HPI) could be used as an indicator for mass changes of energies with hydrologic pulsing events. Restoration of seasonally flooded bottomland forests could stimulate potentially large nutrient and Fe releases, which would eventually lead to an enhanced forest productivity and biodiversity. Copyright © 2007 John Wiley & Sons, Ltd.     

Geomorphic response to river flow regulation: Case studies and time-scales

River regulation imposes primary changes on flow and sediment transfer, the principal factors governing the alluvial channel regime. In this study, the effect of flow regulation is isolated from sediment delivery. Peace River (Q? = 1080m³s^?1, increasing to 2110m³s^?1 downstream) was regulated in 1967 for hydropower. The gravel-bed reach immediately downstream from the dam has become stable. Gravel accumulates at major tributary junctions, so the river profile is becoming stepped. Further downstream, the river has a sand bed. It can still transport sand, so morphological changes along the channel include both aggradation and channel narrowing by lateral accretation. In the gravel-bed Kemano River (Q? = 150m³s^?1), the addition of water by diversion from another river caused degradation when additional bed material was entrained below the inflow point. However, the effect became evident only after many years, when a competent flood occurred. The short-term response was channel widening. The time-scale for the response depends on the size of the river and the nature and severity of regulation. In both rivers, significant adjustment will require centuries and will intimately involve the riparian forest.     

对于黄河下游治理方略的几点建议

二级悬河是黄河下游防洪的心腹之患，必须改变当前槽高滩低的局面，消除二级悬河，才能使黄河下游成为一条有健康生命的河流。只有通过废除生产堤，才能利用大自然的力量，较快、较好地消除二级悬河的严峻现状。要废除生产堤，首先要搞好滩区的安全建设。在此前提下，建议改变小浪底水库的运用方式：拦蓄大洪水，不拦蓄中小洪水；排泄浑水，不排泄清水；汛期水库少量蓄水，做到拦粗排细；允许洪水漫滩，实现漫滩刷槽，利用滩地处理大部分来沙。     

黄河下游河床演变数学模型和初步研究及其应用

以黄河下游河床演变特性为基础，提出了漫滩水沙运动及水沙交换的处理方法，探讨了冲淤量沿河宽分布的准二维计算方法，分析了河床横向变化及其对纵向输沙的影响，并提出了相应的模拟方法，在已用实测资料对这几项研究率定的基础上，建立了适用于黄河下游的准二维数学模型，并用黄河１９６９－１９７８年实测资料进行了验算，验证成果表明，计算与实测基本一致，最后计算和比较了不同来水来沙方案对下游的影响。