Morphology and sedimentology of a tributary confluence bar in a regulated river: North Tyne,U.K.

The form and structure of a tributary confluence bar in a regulated channel are analysed and related to channel adjustment to the imposed flow regime. The bar is characterized by a tripartite structure with two facies, and by marked lateral coarsening, characteristics which relate to the changing hydraulic conditions within the narrowing channel form.     

A practical method for analysis of river waves and for kinematic wave routing in natural channel networks

The behaviour of river waves is described using a simplified dimensionless form of the momentum equation in conjunction with the continuity equation. Three dimensionless parameters were derived based on a quantitative linear analysis. These parameters, which depend on the Froude number of the steady uniform flow and the geometric characteristics of the river, permit quantification of the influence of inertia and pressure in the momentum equation. It was found that dynamic and diffusion waves occur mainly on gentle channel slopes and the transition between them is characterized by the Froude number. On the other hand, the kinematic wave has a wide range of applications. If the channel slope is greater than 1%, the kinematic wave is particularly suitable for describing the hydraulics of flow. Since slopes in natural channel networks are often greater than 1%, an analytical solution of the linearized kinematic wave equation with lateral inflow uniformly distributed along the channel is desirable and was therefore derived. The analytical solution was then implemented in a channel routing module of an existing simple rainfall–runoff model. The results obtained using the analytical solution compared well with those obtained from a non‐linear kinematic wave model. Copyright © 2000 John Wiley & Sons, Ltd.     

Particle size distribution of bed materials in the sandy river bed of alluvial rivers

The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The for-mula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers.     

Experimental study on phosphorus release from sediments of shallow lake in wave flume

Influence of wave on sediment resuspension and nutrients release from sediments, collected from Lake Taihu and Lake Chaohu, was studied in flume experiments. Under strong-wave conditions, concentrations of suspended solids (SS), total phosphorus (TP) and dissolved total phosphorus (DTP) in overlying water were increased significantly following the sediments re-suspension. During the experiments on sediments of Lake Taihu and Lake Chaohu, TP concentrations increased 6 times and 3 times, and DTP concentration increased 100% and 70% more than it in presuspension, respectively. Concentration of soluble reactive phosphorus (SRP) of experiment on sediment of Lake Taihu increased 25%. During the massive sediment suspension, the dissolved phosphorus in pore water and much of the phosphorus adsorbed by the sediment particles were released into overlying water. The phenomena in this wave flume experiment are quite similar to the situation observed in situ of Lake Taihu. The critical wave stresses of sediment re-suspension are nearly equal. The change of concentrations of SS, TP, and SRP was the same as that in situ situation. This study showed that concentrations of TP and SRP in lake water could be increased significantly by wave disturbance. Phosphorus release was significantly enhanced by wave disturbance at the beginning of massive sediment re-suspension, but decreased later.     

辽宁省大伙房水库及入库河流水质空间特征与河库水质关系

入库河流与水库存在空间上的连续性,河流污染物输入是水库水质恶化的主要原因,对大伙房水库及其入库支流61个采样点的水质状况进行调查,并运用聚类分析和主成分分析对大伙房水库及入库支流的水质空间特性和主要污染物进行分析.聚类分析显示,按照水质相似性将大伙房水库及入库支流水质可分为上游区、下游区和库区3个典型空间区域.分别对3个区域进行主成分分析,结果显示:入库支流上游区和下游区水质主要影响因素为氨氮、总氮和化学需氧量,库区影响水质的主要因素为温度、p H值、浊度、溶解氧、电导率、氨氮和总氮.对上游、下游和库区水质均有显著影响的因子为氨氮和总氮,上游区、下游区和库区氨氮浓度均值分别为0.06、0.10和0.19 mg/L,总氮浓度均值分别为0.13、0.16和0.26 mg/L.入库河流下游区对水库水质影响较大,受社河和浑河污染物输入的影响,大伙房水库水质在空间上呈现社河入库区水质优于浑河入库区水质.并且库区氨氮和总氮浓度均与距岸边距离呈负相关,溶解氧和p H值均与距入库口距离呈负相关,表明入库河流污染物输入和环库区面源污染均对大伙房水库水质产生一定影响.     

Computational fluid dynamics modelling of boundary roughness in gravel‐bed rivers: an investigation of the effects of random variability in bed elevation

Results from a series of numerical simulations of two‐dimensional open‐channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open‐channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra‐grid‐scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h = 0·1–0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra‐grid‐scale roughness also allows a reduction in the height of the near‐bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near‐bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel‐bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh‐independent solutions. Copyright © 2001 John Wiley & Sons, Ltd.     

Representing the bed roughness of coarse‐grained streams in computational fluid dynamics

Computational fluid dynamics (CFD) applications are increasingly utilized for modelling complex flow patterns in natural streams and rivers. Although CFD has been successfully implemented to model many complex flow situations in natural stream settings, adequately characterizing the effects of gravel and cobble beds on flow hydraulics in CFD is a difficult challenge due to the scale of roughness lengths and the inadequacy of traditional roughness representations to characterize flow profiles in situations with large roughness elements. An alternative method of representing gravel and cobble beds is presented. Appropriate drag forces associated with different grain sizes are computed and included in the momentum equations to account for the influence of a hydraulically rough bed. Comparisons with field measurements reveal reasonable agreement between measured and modelled profiles of spatially averaged velocity and turbulent kinetic energy, and model fidelity to the non‐logarithmic behaviour of the velocity profiles. The novel method of representing coarse beds expands the utility of CFD for investigating physical processes in natural channels with large bed roughness. Copyright © 2005 John Wiley & Sons, Ltd.     

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

The morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.     

Determining hydraulic resistance in gravel‐bed rivers from the dynamics of their water surfaces

Traditionally, approaches to account for the effect of the boundary roughness of a gravel‐bed river have used a grain‐size index of the bed surface as a surrogate for hydraulic resistance. The use of a single grain‐size does not take into account the spatial heterogeneity in the bed surface and how this heterogeneity imparts resistance on the flow, nor the way in which this relationship changes with variables such as flow stage. A new technique to remotely quantify hydraulic resistance is proposed. It is based on measuring the dynamics of a river's water surface and relating this to the actual hydraulic resistance created by a rough sediment boundary. The water surface dynamics are measured using a new acoustic technique, grazing angle sound propagation (GRASP). This proposed method to measure hydraulic resistance is based on a greater degree of physical reasoning, and this is discussed in the letter. By measuring acoustically the temporal dynamics of turbulent water surfaces over a water‐worked gravel bed in a laboratory flume, a dependency is demonstrated between the temporal variation in the reflected acoustic pressure and measured hydraulic resistance. It is shown that the standard deviation in acoustic pressure decreases with increasing hydraulic resistance. This is shown to apply for a range of relative submergences and bed slopes that are typical of gravel‐bed rivers. This remote sensing technique is both rapid and inexpensive, and has the potential to be applied to natural river channels and to other environmental turbulent flows, such as overland flows. A whole new class of low‐cost, remote and non‐intrusive instruments could be developed as a result and used in a wide range of hydraulic and hydrological applications. Copyright © 2006 John Wiley & Sons, Ltd.     

Evolution of grain size distributions and bed mobility during hydrographs in gravel-bed braided rivers

Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel-bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally-stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel-bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high-resolution (~1 mm precision) digital elevation models generated from close-range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel-forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near-equal mobility at peak channel-forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D₉₀ and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel-forming discharge, the active depth had a local maximum of 9D₉₀ while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd.     

Comparison of hydraulic geometry between sand‐ and gravel‐bed rivers in relation to channel pattern discrimination

A comparison has been made between the hydraulic geometry of sand‐ and gravel‐bed rivers, based on data from alluvial rivers around the world. The results indicate a signi?cant difference in hydraulic geometry among sand‐ and gravel‐bed rivers with different channel patterns. On this basis, some diagrams for discrimination of meandering and braided channel patterns have been established. The relationships between channel width and water discharge, between channel depth and water discharge, between width–depth ratio and water discharge and between channel slope and water discharge can all be used for channel pattern discrimination. The relationship between channel width and channel depth can also be used for channel pattern discrimination. However, the accuracy of these relationships for channel pattern discrimination varies, and the depth–discharge relationship is a better discriminator of pattern type than the classic slope–discharge function. The cause for this difference has been explained qualitatively. To predict the development of channel patterns under different natural conditions, the pattern discriminator should be searched on the basis of independent or at least semi‐independent variables. The relationship between stream power and bed material grain size can be used to discriminate channel patterns, which shows a better result than the discriminator using the slope–discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.     

The spatial and temporal patterns of aggradation in a temperate,upland, gravel‐bed river

Intensive field monitoring of a reach of upland gravel‐bed river illustrates the temporal and spatial variability of in‐channel sedimentation. Over the six‐year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land‐use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

While clay and silt matrices of gravel‐bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms, their impact on sediment entrainment and transport has been explored less. A recent increase of such a matrix in the bed of Nahal Eshtemoa, an ephemeral river of the northern Negev, has more than doubled the boundary shear stress needed to initiate bedload, from 7 N m^‐2 (τ^* = 0.027) during the flash floods of 1991–2001 to 15 N m^‐2 (τ^* = 0.059) during those of 2008–2009. The relation between bedload flux and boundary shear stress continues to be well‐defined, but it is displaced. The matrix now contains a significant amount of silt and clay size material. The reasons for the increased entrainment threshold of bedload are explored. Large‐scale laser scanning of the dry bed reveals a reduction in grain‐scale morphological roughness, while artificial in situ tests of matrix integrity indicate considerable cohesion. The implications for adopting bed material sampling strategies that account for matrix development are assessed. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of sediment size,relative grain size and channel slope on initiation of sediment motion in boulder bed rivers. A lichenometric study

Sediment transport of four boulder bed rivers is studied using lichenometry. The presence of lichens on boulders in the river channel is used to date the last mobilization of the blocks. Using size frequency diagrams and regional growth curves calibrated with dated reference points it is possible to determine the flood event responsible for the last mobilization of each boulder with lichens present. The specific stream power of flood events over the last 60 years is then calculated, and thresholds of sediment transport based on the sediment size are calculated. The results from the four studied rivers are compared to similar relationships in the literature. Sediment motion thresholds appear to be very variable within the same type of river (mountainous boulder bed rivers). The critical specific stream power necessary to mobilize a particle of a given diameter may vary by up to 10 times from one river to the next. Bed sediment size and river slope may explain this large range of stream powers. Calculation of the relative size of the transported particles (D_i/D₅₀) also shows that both hiding and protrusion effects, as well as channels slope, are important factors in sediment transport. Copyright © 2010 John Wiley & Sons, Ltd.     

转换波在储层预测中的研究分析

岩石的各向异性与油气的运移和储集有着密切关系.在油气勘探和油气田开发中,横波对各向异性的敏感性具有重要价值,它可提供一些其他方法无法获取的新信息.纵波在地下传播时,当通过路径的岩石存在各向异性时,会在波阻抗界面产生纵波和横波.这相当于纵波震源同时激发出纵波和横波,利用这些信息就可以对地下岩石的物性进行研究.本文以有限差分为基础,以地下油气田的储藏地质特性为对象,利用弹性波波动方程的传播特性, 研究P_SV转换波,分析其传播特征,用来指导其在储层预测中的应用.通过研究分析可以看出,P_SV转换波通过含油气介质时受影响比较少,能够得到比纵波好的多的成像资料.     

Influence of external loading and halocline on phosphorus release from sediment in an artificial tidal lake

Many of the dams built in estuaries in the last century have difficulty with water quality management.Numerous factors have affected the estuary lake water,most importantly external loadings,tidal currents,and increases in the phosphorus(P) release from sediment,so that most water quality characteristics in the estuary are highly interactive and dynamic.In the current study,water quality measurements were made in the laboratory and field,and a series of phosphorus release experiments was done to understand the behavior of P in an estuary lake.The concentrations of chemical oxygen demand(COD),chlorophyll-a(Chl-a),and total P(TP) showed an increasing trend when the pollutant loading of the influent stream water was high.The measurements showed increasing trends,which indicates the constituents are produced in the internal environment of the lake.When a large amount of freshwater flowed in from the upper watershed,density stratification was observed,which forms strongly because of the salinity of seawater.During the period of stratification,a hypoxic layer formed,which can accelerate P release.Comparing the open and dosed conditions of the release experiments,the P release rate was much higher under the closed condition than under the open condition.The maximum P release rates from the sediment collected from the five main sites of the lake were more than 2.5 times the P loading from the inflowing streams in April.Spatially,the release rate was higher mid-reservoir than down-reservoir where a halocline was evident The pollutant load discharged from the tributary watershed was deposited on the bottom mid-reservoir,whereas it was washed out downreservoir because of the density stratification and strong tide in that area.To sustainably manage water quality and decrease lake eutrophication in brackish environments formed by freshwater from streams mixing with seawater entering through sluice dikes,different measures than those applied in strictly freshwater environments are required.Considering the spatial characteristics of an estuary lake,these measures include 1) blocking settleable particles discharged from the rivers upstream,2) controlling hypoxia to avoid P release from the sediment and inhibiting algae growth mid-reservoir,and 3)decreasing stratification caused by the halocline down-reservoir.     

Comparative analysis of rating curve and ADP estimates of instantaneous water discharge through estuaries in two contrasting Brazilian rivers

This work quantifies, using ADP and rating curve techniques, the instantaneous outflows at estuarine interfaces: higher to middle estuary and middle to lower estuary, in two medium‐sized watersheds (72 000 and 66 000 km² of area, respectively), the Jaguaribe and Contas Rivers located in the northeastern (semi‐arid) and eastern (tropical humid) Brazilian coasts, respectively. Results from ADP showed that the net water balances show the Contas River as a net water exporter, whereas the Jaguaribe River Estuary is a net water importer. At the Jaguaribe Estuary, water retention during flood tide contributes to 58% of the total volume transferred during the ebb tide from the middle to lower estuary. However, 42% of the total water volume (452 m³ s^?1) that entered during flood tide is retained in the middle estuary. In the Contas River, 90% of the total water is retained during the flood tide contributing to the volume transported in the ebb tide from the middle to the lower estuary. Outflows obtained with the rating curve method for the Contas and Jaguaribe Rivers were uniform through time due to river flow normalization by dams in both basins. Estimated outflows with this method are about 65% (Contas) and 95% (Jaguaribe) lower compared to outflows obtained with ADP. This suggests that the outflows obtained with the rating curve method underestimate the net water balance in both systems, particularly in the Jaguaribe River under a semi‐arid climate. This underestimation is somewhat decreased due to wetter conditions in the Contas River basin. Copyright © 2011 John Wiley & Sons, Ltd.     

The bed morphology of upland single‐thread channels in semi‐arid environments: evidence of repeating bedforms and their wider implications for gravel‐bed rivers

Single‐thread, gravel‐bed streams of moderate slope in the northern Negev are characterized by three channel units: bars exhibit steeper than average slopes and poorly sorted mixtures of small–medium cobbles and coarse–very coarse pebbles; flats are associated with more gentle slopes and well‐sorted medium–fine pebbles and granules; and transitional units have intermediate slopes and grain size. In general, all three units are planar, span the full channel width and have well‐defined boundaries. Bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres, forming sequences that are reminiscent of the riffle–pool structure commonly observed in humid‐temperate gravel‐bed rivers. A notable contrast is the absence of significant bed relief: bars lack crests and flats lack depressions. The relative lack of bed relief in bar–flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport. This reduction of bed relief lowers channel roughness, which in turn increases flow velocity and, therefore, the ability of the channel to transmit the large sediment loads it receives. Although our analyses pertain to a semi‐arid river system, the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments. Copyright © 2012 John Wiley & Sons, Ltd.     

Assessment of erosion and settling properties of fine sediments stored in cobble bed rivers: the Arc and Isère alpine rivers before and after reservoir flushing

Cohesive sediment dynamics in mountainous rivers is poorly understood even though these rivers are the main providers of fine particles to the oceans. Complex interactions exist between the coarse matrix of cobble bed rivers and fine sediments. Given that fine sediment load in such environments can be very high due to intense natural rainfall or snowmelt events and to man‐induced reservoir or dam flushing, a better understanding of the deposition and sedimentation processes is needed in order to reduce ecohydrological downstream impacts. We tested a field‐based approach on the Arc and Isère alpine rivers combining measurements of erosion and settling properties of river bed deposits before and after a dam flushing, with the U‐GEMS (Gust Erosion Microcosm System) and SCAF (System Characterizing Aggregates and Flocs), respectively. These measurements highlight that critical shears, rates of erosion, settling velocities and propensity of particles to flocculate are highly variable in time and space. This is reflective of the heterogeneity of the hydrodynamic conditions during particle settling, local bed roughness, and nature and size of particles. Generally the deposits were found to be stable relative to what is measured in lowland rivers. It was, however, not possible to make a conclusive assessment of the extent to which the dynamics of deposits after reservoir flushing were different from those settled after natural events. The absence of any relationships between erosion and deposition variables, making it impossible to predict one from another, underlined the need to measure all of them to have a full assessment of the fine sediment dynamics and to obtain representative input variables for numerical models. While the SCAF was found to be effective, an alternative to the U‐GEMS device will have to be found for the erodibility assessment in cobble bed rivers, in order to make more rapid measurements at higher shears. Copyright © 2017 John Wiley & Sons, Ltd.     

黄河尾闾段一维耦合水沙数学模型研究及其应用

小浪底水库于1999年运用以后,该河道经历了长时间持续冲刷过程.为掌握小浪底水库运用后黄河尾闾段洪水演进特点及河床冲淤规律,采用一维水沙数学模型研究是一条重要的途径.本研究首先采用浑水控制方程,建立了一维耦合水沙数学模型,并利用2003年利津-西河口段汛期实测水沙及汛前断面地形资料对该模型进行率定,计算的流量、水位及含沙量等过程与实测值吻合较好;然后采用2015年利津—汊3段汛期实测资料对该模型进行验证,结果显示水位与冲淤量计算值与实测值较为符合;最后基于2015年实测洪水过程,计算了若干组不同断面间距下的洪水演进及冲淤过程,分析了不同断面间距对沿程水位及河段冲淤量等计算结果的影响,结果表明:采用不同断面间距对水位计算结果影响较小,而对冲淤量计算结果会产生一定影响;在河段水沙及冲淤特性复杂的情况下,采用一维数学水沙模型计算时应考虑断面间距的选择.