Mortality of riparian box elder from sediment mobilization and extended inundation

To explore how high flows limit the streamward extent of riparian vegetation we quantified the effects of sediment mobilization and extended inundation on box elder (Acer negundo) saplings along the cobble‐bed Gunnison River in Black Canyon of the Gunnison National Monument, Colorado, USA. We counted and aged box elders in 144 plots of 37.2 m², and combined a hydraulic model with the hydrologic record to determine the maximum shear stress and number of growing‐season days inundated for each plot in each year of the record. We quantified the effects of the two mortality factors by calculating the extreme values survived during the lifetime of trees sampled in 1994 and by recounting box elders in the plots following a high flow in 1995. Both mortality factors can be modeled as threshold functions; box elders are killed either by inundation for more than 85 days during the growing season or by shear stress that exceeds the critical value for mobilization of the underlying sediment particles. Construction of upstream reservoirs in the 1960s and 1970s reduced the proportion of the canyon bottom annually cleared of box elders by high flows. Furthermore, because the dams decreased the magnitude of high flows more than their duration, flow regulation has decreased the importance of sediment mobilization relative to extended inundation. We use the threshold functions and cross‐section data to develop a response surface predicting the proportion of the canyon bottom cleared at any combination of flow magnitude and duration. This response surface allows vegetation removal to be incorporated into quantitative multi‐objective water management decisions. Copyright © 1999 John Wiley & Sons, Ltd.     

Valley floor landscape change following almost 100 years of flood embankment abandonment on a wandering gravel‐bed river

Critical to restoring the nature conservation value of many river corridors is an understanding of how alluvial landscapes will respond to cessation of river management and land use practices that have previously degraded the environment. This paper analyses changes in valley floor landforms and vegetation patch dynamics, in relation to fluvial disturbance, over a period of almost 100 years following flood embankment abandonment on a wandering gravel‐bed river, namely the River Tummel, Scotland. Such rivers were once typical of many draining upland areas of northern maritime Europe. Prior to abandonment the valley floor landscape was agriculturally dominated and the river for the most part was single thread confined between flood embankments. The pattern of landform change and vegetation patch development over time following a decision in 1903 not to maintain embankments was tracked by geomorphic and land cover mapping utilizing successive sets of aerial photography for the period 1946 to 1994. A historical context for these changes was also feasible because the channel planform in 1900 and earlier channel planform changes dating back to 1753 were known due to the availability of old maps and earlier geomorphic studies. The land cover mapping was validated by comparison of results produced from the interpretation work on the 1994 aerial photographs with the field‐based UK National Vegetation Classification protocol. The findings of the study illustrate that bordering the River Tummel fluvial landforms and vegetation patch mosaics, presumably resembling those that occurred before valley floor land use intensification, evolved in less than 50 years after flood embankment abandonment with a resultant increase in habitat diversity. The change relates primarily to flood‐induced channel planform change and moderate levels of fluvial disturbance. The general significance of this change to plant species diversity on the valley floor of the River Tummel and elsewhere is discussed as is possible implications of the upstream impoundment and scenarios for climatically induced changes in flood frequency and magnitude. The overall outcome is the strong possibility that simple changes in river management and land use practices could result in re‐establishment of the nature conservation value of similar river corridors in Europe over the medium term without active restoration efforts. Copyright © 2002 John Wiley & Sons, Ltd.     

Two-Dimensional Composite Mathematical Alluvial Model for the Braided Reach in the Lower Yellow River

Abstract

The fluvial processes are considerably complicated in the braided reach of the Lower Yellow River, and the lateral or transverse channel deformation process is especially pronounced. The state-of-the-art of simulation techniques for the channel deformation in the Lower Yellow River was analyzed in this paper, and such a conclusion was drawn that existing mathematical alluvial models only can simulate the longitudinal channel deformation and cannot simulate such the lateral deformation as lateral erosion and failure of bank. Then a new two-dimensional (2D) composite model was developed herein, consisting of a 2D flow and sediment transport submodel and a riverbank-erosion submodel. This model not only can simulate the longitudinal and lateral channel deformations, but also can be used to simulate the fluvial processes of natural alluvial rivers with complex bed morphology. Through using the proposed composite model, the flood routing and channel deformation processes during the August 1982 flood were simulated. The maximum stage along the reach and the discharge hydrograph at the outlet so obtained were in good agreement with the observed data. Simulated results of the channel deformation processes showed that during the flood season, the main channel scours and the flood plain deposits; during the dry season, the main channel deposits and banks of the floodplain retreat due to lateral erosion by the flow and failure under gravity. Such results accord with the annual varying law of bed scouring and deposition in the braided reach.     

长江科学院长江泥沙若干问题研究与实践

简要介绍了近50年来,长江河道水流泥沙运动、河道演变、河道整治及水利枢纽泥沙问题研究情况与所取得的进展,提出了需要继续深入研究的若干重大问题,包括长江水土资源综合利用、干支流水库群联合运用对水沙及江湖演变的影响与对策、长江中下游河道和湖泊及长江口治理研究等。     

黄河下游河道萎缩模式研究

依据河工模型试验和原型定位观测资料分析,研究了黄河下游河道萎缩的基本模式及河道萎缩与水沙过程的跟随性。结果表明,河道萎缩模式主要有“滩槽并淤”和“集中淤槽”两大类,同时,还存在着“淤积不萎缩”的造床现象;对于某一变异水沙过程,河道萎缩具有“前置”性,而且,河道萎缩并不需要在很长的时期内才可完成,一个汛期内就可能发生明显萎缩;主河槽过流面积、主河槽河床平均高程是判别河道萎缩的必要因子,而断面宽深比不是必要因子。     

新情势下黄河口演变与整治研究思路

基于黄河口水沙变异的新情况以及经济发展和生态环境建设的新需求,提出新情势下黄河口演变与整治的研究思路。针对目前研究中需要突破的薄弱环节,瞄准国际河口学领域中的前沿问题,采用多学科交叉和多种研究手段,对黄河口开展创新研究。主要研究:黄河口水沙变异特征,尾闾河道枯萎机制及其演变响应,拦门沙演变响应过程及其反馈影响;河口泥沙通量,最大浑浊带、泥沙异重流形成机理,河口海岸淤进、蚀退及其动力平衡机制;黄河口陆海相互作用的整体数模,河口演变趋势与入海流路使用年限;新情势下黄河口治理对策及水土资源优化配置。     

Species composition and plant traits: Characterization of the biogeomorphological succession within contrasting river corridors

Plant communities and dynamics can be characterized according to species composition or plant traits. Here, we used species composition and plant traits to compare their effectiveness in discriminating the biogeomorphological (involving reciprocal feedbacks between physical and biological processes) and ecological (mainly biologically driven) phases of the fluvial biogeomorphological succession (FBS) model. The comparison was done between two French rivers, the largely unchannelized lower Allier and the channelized middle Garonne. One reach representative of each river section was selected for the study. Within each river reach, we chose two contrasted study sites in terms of channel and floodplain dynamics: a reference site (least altered channel and floodplain dynamics) and an altered site (laterally stabilized by riprap and constrained). In the four study sites, we sampled vegetation in 402 plots of 4 m². The 512 species identified in the plots were characterized in terms of plant traits (20) from a literature review. When comparing reaches in unconstrained ordinations and permutational multivariate analyses of variance, both species composition and plant traits led to a similar identification of the biogeomorphological and the ecological successional trajectories. Nevertheless, the trait approach was less influenced by local and regional bioclimatic, hydrogeomorphological, and anthropogenic settings and thus produced a more comprehensive and general classification of the biogeomorphological and ecological phases of the FBS model. A lower than expected contrast between the four sites was found, because neither species composition nor plant traits could entirely characterize distinct successional trajectories occurring in our reference or altered sites. Furthermore, our results contributed to a better understanding of the multiple successional trajectories that can occur in midlatitude river corridors. It also showed that relating plant traits to their effects on fluvial landform dynamics remains a core challenge in explaining succession including feedback mechanisms between hydrology, morphodynamics, and vegetation dynamics.     

略论二级悬河

悬河是指河床明显高出河流堤防背河侧地面的河流(或河段),又称地上河,它具有水位常年高于背河侧地面、河势多变、防洪工程自行降低标准、堤防决口后灾害严重、一旦堤防决口可能造成改道等特点。二级悬河是指河槽平均河底高程高于滩地平均河底高程的河流(或河段),黄河下游20世纪70年代初开始形成二级悬河。平原堆积性河流,河床不断淤积抬高,存在滩面横比降;悬河,具有平原堆积性河流的特点,且河床明显高于河流堤防背河侧地面;二级悬河由悬河淤积而成,除具有堆积性河道、悬河的特点外,就断面形态而言,还应同时具有滩面横比降陡、滩唇高程远高于堤防临河侧附近滩面高程、河槽平均河底高程高于滩地平均河底高程的特点。     

汉江河口段航道整治工程效果与影响的计算分析

汉江河口段航道整治工程以疏浚、清障为主,辅以必要的护岸、护滩、填挖槽等工程措施,本文采用河道演变分析和一、二维水流数学模型计算,研究了该航道整治工程的效果及其对河道行洪的影响,并优化了工程方案.研究表明:(1)丹江口水库自1968年正式蓄水以来,汉江河口段河道总体冲刷下切,年内洪淤枯冲;影响河道演变的主要因素有:丹江口水库、长江与汉江顶托和天然节点与人工护岸等.(2)航道整治工程实施后.汉江河口段上段流速趋于减小,下段流速趋于增加;河道流速分布趋于均化.(3)航道整治工程对河道行洪的影响有限,最大洪水壅高9.4cm,优化方案的最大洪水壅高7.0cm.