Numerical Simulation of Bed Load and Suspended Load Sediment Transport Using Well-Balanced Numerical Schemes

Sediment transport can be modelled using hydrodynamic models based on shallow water equations coupled with the sediment concentration conservation equation and the bed con-servation equation.The complete system of equations is made up of the energy balance law and the Exner equations.The numerical solution for this complete system is done in a seg-regated manner.First,the hyperbolic part of the system of balance laws is solved using a finite volume scheme.Three ways to compute the numerical flux have been considered,the Q-scheme of van Leer,the HLLCS approximate Riemann solver,and the last one takes into account the presence of non-conservative products in the model.The discretisation of the source terms is carried out according to the numerical flux chosen.In the second stage,the bed conservation equation is solved by using the approximation computed for the system of balance laws.The numerical schemes have been validated making comparisons between the obtained numerical results and the experimental data for some physical experiments.The numerical results show a good agreement with the experimental data.

Bed Forms and Sediment Characteristics along the Thalweg on the Tanana River near Nenana, Alaska, USA

Sediment sampling and longitudinal river-bottom surveys were conducted along the thalweg on the Tanana River near the city of Nenana, Alaska, USA, to provide basic information for the engineering design requirements of hydrokinetic devices to be deployed in the area. The study reach was located at approximately 64°33'50'N and 149°04'W. The Tanana is a large glacier-fed river, with open-water flow conditions from May to October. The river presents a single channel in the study area. Granulometric analyses of sediment moving near the riverbed reveals the coexistence of three distinctive types of sediment along the study reach: 1) nearly uniform fine sand;2) bimodal distributions containing fine sand and medium gravel;and 3) medium gravel. Preliminary relationships between sediment loads and discharge were developed. Dunes with small superimposed dunes were found along the reach. The basic geometric parameters (i.e., wavelength and height) of dunes were measured, and steepness was calculated. In general, dune wavelength increased with increasing discharge. Dune wavelengths ranged from 41 to 67 m, while small-dune wavelengths ranged from 13 to 16 m. Steepness increased slightly with increasing discharge.

Equilibrium bed—concentration of nonuniform sediment

Knowledge of the equilibrium bed-concentration is vital to mathematic a l modeling of the river-bed deformation associated with suspended load but prev i ous investigations only dealt with the reference concentration of uniform sedime nt because of difficulties in observation of the bed-concentration. This work i s a first attempt to develop a theoretical formula for the equilibrium bed-conce n tration of any fraction of nonuniform sediment defined at the bed-surface. The f ormula is based on a stochastic-mechanistic model for the exchange of nonunifor m sediment near the bed, and described as a function of incipient motion probabil ity, non-ceasing probability, pick-up probability, and the ratio of the averag e single-step continuous motion time to static time. Comparison of bed-concentra ti on calculated from the proposed formula with the measured data showed satisfacto ry agreement, indicating the present formula can be used for solving the differe ntial equation governing the motion of suspended load.

Numerical simulation of sediment transport in coastal waves and wave-induced currents

Prediction of coastal sediment transport is of particularly importance for analyzing coast erosion accurately and solving the corresponding coast protection engineering problems.The present study provided a numerical scheme for sediment transport in coastal waves and wave-induced currents.In the scheme,the sand transport model was implemented with wave refraction-diffraction model and near-shore current model.Coastal water wave was simulated by using the parabolic mild-slope equation in which wave refraction,diffraction and breaking effects are considered.Wave-induced current was simulated by using the nonlinear shallow water equations in which wave provides radiation stresses for driving current.Then,sediment transport in waves and wave-induced currents was simulated by using the two-dimensional suspended sediment transport equations for suspended sediment and the bed-load transport equation for bed load.The numerical scheme was validated by experiment results from the Large-scale Sediment Transport Facility at the US Army Corps of Engineer Research and Development Center in Vicksburg.The numerical results showed that the present scheme is an effective tool for modeling coastal sediment transport in waves and near-shore currents.

HORIZONTAL 2-D BASIC EQUATIONS AND SIMULATION OF INTERCHANGE BETWEEN SUSPENDED SEDIMENT AND BED MATERIALS IN ALLUVIAL RIVER

A set of new 2-D equations of interchange between suspended sediment and bed materials was serived by theoretical deduction based on the systematic summarization and assessment the previous studies of simulating the interchange between suspended sediment and bed materials in the Lower Yellow River. This model was used to simulate the erosion and deposition processes caused by the interchange between suspended sediment and bed materials in a sketch channel. The results show that these equations are well consistent with the laws of interchange between suspended sediment and bed materials. Furthermore, compared with previous models, it has important practical value not only because of strong theoretical foundation, but also for smaller amount of calculating work and convenient application in practice.

七一冰川融水径流中悬移质与推移质动态输沙特征

[目的]冰川融水输沙不仅可以反映冰川侵蚀速率,而且是下游河流泥沙的重要来源之一,因此,认识青藏高原地区冰川融水径流中悬移质和推移质动态输沙特征对于冰川地貌演化预测和下游防灾减灾工作具有十分重要的意义。[方法]基于2023年8月11-24日七一冰川下游河道的水文泥沙实测数据,分别构建流量-水位、悬移质含沙量-流量和单宽推移质输沙率-水流剪切应力之间的相关关系,进而重建整个研究时段的流量、悬移质和推移质动态输沙过程。[结果]七一冰川融水径流中悬移质含沙量和推移质输沙率受径流变化控制,日变化十分剧烈。悬移质含沙量每日最小值一般出现在10:00,最大值出现在16:00,中值粒径约为0.3 mm。推移质输沙过程与悬移质含沙量的日变化趋势基本相同,但推移质中值粒径远大于悬移质,约12 mm,而且在每日05:00-09:00时段,由于水流搬运能力的限制,没有推移质输沙。就输沙量而言,七一冰川融水输沙以悬移质泥沙为主,推移质日输沙量远小于悬移质日输沙量,约占悬移质日输沙量的1.73%~2.81%。冰面气温是影响冰川融水径流量、悬移质含沙量和推移质输沙率的关键因素,随着冰面气温的增加,径流量和悬移质含沙量均呈线性增加趋势而推移质输沙率呈指数函数增加趋势。[结论]在未来气候变化情景下冰川融水径流增加时,七一冰川融水径流中悬移质和推移质输沙均将变得更加显著,值得更加重视。

The suspended sediment transport equation and its near-bed sediment flux

The suspended sediment transport equation and its near-bed sediment flux are one of the key problems of sediment transport research under nonequilibrium condition. Based on the three-dimensional primitive suspended transport equation, the two-dimensional suspended sediment transport equation is deduced. The derived process indicates that the physical essence of the near-bed sediment flux is right the bottom boundary condition for the suspended sediment transport equation. This paper analyzes the internal relations between the two methods of sediment carrying capacity and shear stress in common use, points out the consistency of these two methods in terms of form and physical meaning, and unifies these two methods theoretically. Furthermore, based on the analysis and comparison of the expressions of the near-bed sediment flux, this paper summarizes some problems to which attention should be paid, thus offering a novel approach to the study and the solution of the problems of suspended sediment transport and exchange flux of near-bed water sediment.

Numerical Analysis of Adaptation-to-capacity Length for Fluvial Sediment Transport

Over the last several decades,various sediment transport capacity formulations have been used by geomorphologists and engineers to calculate fluvial morphological changes.However,it remains poorly understood if the adaptation to capacity could be fulfilled instantly in response to differing inflow discharges and sediment supplies,and thus if the calculation of morphological changes in rivers based on the assumed capacity status is fully justified.Here we present a numerical investigation on this issue.The distance required for sediment transport to adapt to capacity(i.e.,adaptation-to-capacity length) of both bed load and suspended sediment transport is computationally studied using a coupled shallow water hydrodynamic model,in line with varied inlet sediment concentrations.It is found that the adaptation-to-capacity length generally decreases as the Rouse number increases,irrespective of whether the inlet sediment concentration increases or reduces.For cases with vanishing inlet sediment concentration a unified relationship is found between the adaptation-to-capacity length and the Rouse number.Quantitatively,the adaptation-to-capacity length of bed load sediment is limited to tens of times of the flow depth,whilst that of suspended sediment increases substantially with decreasing Rouse number and can be up to hundreds of times of the flow depth.The present finding concurs that bed load sediment transport can adapt to capacity much more rapidly than suspended sediment transport,and it facilitates a quantitative criterion on which the applicability of bed load or suspended sediment transport capacity for natural rivers can be readily assessed.

Sedimentation Processes at the Navigation Channel of the Liquefied Natural Gas (LNG) Port, Nile Delta, Egypt

Liquefied Natural Gas (LNG) port is located at Abu Qir Bay on the northwestern coast of the Nile delta, Egypt. The port was constructed in 2004 to export liquefied natural gas worldwide. The offshore basins of this port including the turning and berthing areas (15-m depth) are connected to the deep water by a 15-m depth dredged channel that extends 4 km offshore. However, the navigation channel and its contiguous basins have experienced problematic shoaling that might affect the navigation activities of gas tankers. Sedimentation processes have been investigated by analyses of waves, currents, bathymetry, grain size of seabed and channel dimensions. Sedimentation rates are estimated using a developed numerical model. Sedimentation rate fluctuates between 0.048 × 106 m3/month and 0.388 × 106 m3/month, with an annual sedimentation rate of 1.977 × 106 m3/yr. The variance in the sedimentation rates between winter and summer resulted in increasing of current speed and direction flowing towards offshore. The sedimentation process is influenced by the temporal variability in the direction and intensity of the predominant waves, currents, orientation of navigation channel, basin breakwaters, seafloor morphology and sediment sources. Due to the geographic location of LNG port it lays within a sediment sink for sediments supplied from different alternating directions by several pathways, flowing towards the N-W, S-W, N-E, and S-E quadrants. Most of these currents components are substantially effective in transporting fine-grained sediment towards the navigation channel axis and contiguous basins. Together with these currents, the predominant NW and SE waves acting to agitate and stirrup sediments in the vicinity of the port, and thereby accelerating sedimentation rates.

Application of 2-D sediment model to fluctuating backwater area of Yangtze River

Based on the characteristics of backflow, a two-dimensional mathematical model of sediment movement was established. The complexity of the watercourse boundary at the confluence of the main stream and the tributary was dealt with using a boundary-fitting orthogonal coordinate system. The basic equation of the two-dimensional total sediment load model, the numerical calculation format, and key problems associated with using the orthogonal curvilinear coordinate system were discussed. Water and sediment flow in the Chongqing reach of the Yangtze River were simulated. The calculated water level, flow velocity distribution, amount of silting and scouring, and alluvial distribution are found to be in agreement with the measured data, which indicates that the numerical model and calculation method are reasonable. The model can be used for calculation of flow in a relatively complicated river network.

江乌航道整治工程水文泥沙原型观测方法与实践

为解决长江下游航道整治工程原型观测的问题,以长江下游江心洲至乌江河段航道整治二期工程原型观测为例,结合现场工作实践,采用多种设备和技术方法,包括水位及比降观测、水文断面测量、表面流速流向测量、悬移质和河床质取样及分析等,针对长江下游感潮河段和部分江段流速大、水深深的特点,采取在合理的时间段进行相关原型观测工作,并利用深水底质采样器解决在深水及流速大的江段采样困难问题。实践结果表明:在江乌航道整治工程中采用的水文泥沙原型观测方法,能获取完整详实的原型观测数据,避免成果失真。采用文中的水文泥沙原型观测方法,解决了长江下游感潮河段原型观测问题,为航道整治工程设计及相关专题研究提供了可靠的基础资料。

Assessment of the fraction of bed load concentration towards the sediment transport of a monsoon-dominated river basin of Eastern India

Given the challenges of re-creating complex bed load(BL)transport processes in rivers,models are preferred over gathering and examining field data.The highlight of the present research is to develop an approach to determine the ungauged bed load concentration(BLC_(u))utilizing the measured suspended sediment concentration(SSC)and hydraulic variables of the last four decades for the Mahanadi River Basin.This technique employs shear stress and SSC equations for turbulent open channel flow.Besides,the predicted BLC_(u)is correlated with SSC using a power relation to estimate BLC_(u)on the river and tributaries.Eventually,different BL functions(BLF)efficiency is assessed across stations.The model predicted BLC_(u)is comparable with the published data for sandy rivers and falls within±20%.Outliers in hydraulic and sedimentological statistics significantly influence estimating the BL fraction apart from higher relative ratios and catchment geology.The constants of power functions are physically linked to sediment transport configuration,mechanism,and inflow to the stream.The stream power-based BLF best predicts the BL transport,followed by shear stress and unit discharge approaches.The disparity in the estimation of BLC_(u)results from station-specific physical factors,sampling data dispersion,and associated uncertainties.

漏斗式全沙排沙技术及其应用

主要介绍了一种新型泥沙处理技术 ,即“漏斗式全沙排沙术”。该项技术既可以排除粗颗粒的推移质泥沙 ,也可以排除细颗粒的悬移质泥沙。对 0 .5mm以上的粗颗粒泥沙的截沙率为10 0 % ,对 0 .5mm以下细颗粒泥沙的截沙率为 90 % ,排沙耗水量仅占渠道引水量的 2 %～ 5%。

长江河口波-流共同作用下的全沙数值模拟

针对长江河口地形、水文、泥沙运动等复杂的特点,建立了波-流共同作用下的二维全沙及河床演变模型.在合理计算研究区域流场等的基础上,利用切应力概念确定悬沙扩散方程中的源函数;通过系列数值试验和实测资料的统计分析,在经典的泥沙临界起动速度中引入反映河床底质结构及固结程度的局地系数;选用由流速、盐度、含沙量浓度确定的泥沙颗粒絮凝沉降速度,从而提高长江口悬沙场数值模拟精度.在底沙输运计算中,提出一种较为合理确定有关参数的方法.通过洪、枯季大、中、小潮水文、泥沙资料和典型台风引起航槽冲淤变化的实测资料验证,表明该文提出的模型能较合理地反映长江河口流场、泥沙场及地形的演变.

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间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 。

黄河下游河道悬移质泥沙与床沙交换计算研究

在前人研究成果的基础上,根据挟沙水流任一粒径组泥沙在输移过程中质量守恒原理,建立了一维非恒定挟沙水流悬移质泥沙和床沙交换基本方程;通过引入平衡冲淤物粒径的概念,建立了河床处于淤积与冲刷时冲淤物粒径的计算公式,并提出了一套完整的一维非恒定挟沙水流悬移质泥沙和床沙交换计算方法。将该成果引入黄河下游一维扩展泥沙数学模型中,采用黄河下游1977年高含沙洪水与1999年汛后至2002年汛前冲刷系列进行了验证。结果表明,该方法能较好地模拟悬沙与床沙的交换过程,克服分组挟沙力方法的缺陷,使得非均匀沙计算理论上更加完善,应用上更加方便。

风暴过程中潮滩悬沙浓度和悬沙输运的变化及其动力机制——以长江三角洲南汇潮滩为例

悬沙浓度是淤泥质海岸重要的环境指标。为探讨潮滩悬沙浓度和悬沙输运对风暴事件的响应过程及其动力机制,于2014年9月"凤凰"台风过境前、中、后在长江三角洲南汇潮滩进行了现场观测,获得同步高分辨率的水深、波高、近底流速和浊度剖面时间序列(9个潮周期)。结果表明,风暴中平均和最大波高、波-流联合底床剪切应力、悬沙浓度和悬沙输运率可比平静天气高数倍;风暴期间高潮位低流速阶段悬沙沉降导致近底发育数十厘米厚的浮泥层(悬沙浓度大于10g/L)。研究认为风暴事件中淤泥质海岸悬沙浓度和悬沙输运的剧烈变化其根本动力机制是风暴把巨大能量传递给近岸水体,进而显著增大波-流联合底床剪切应力,导致细颗粒泥沙再悬浮。

长江流域沿江镉异常源追踪与定量评估的方法技术研究:以长江流域安徽段为例

长江全流域性的Cd异常是被中国正在进行的多目标地球化学调查发现的重大生态环境问题。以长江流域安徽段为研究对象,对沿江镉异常源追踪与定量评估的方法技术进行了系统研究。通过对安徽段长江干流及其主要支流悬浮物中元素含量的测量查明:悬浮物是流域内重金属元素大跨度迁移的主要载体;Cd在悬浮物中的富集程度远远大于其他重金属元素,这或许正是Cd可以形成沿江流域性异常的主要原因;长江干流悬浮物中重金属元素含量的变化明显受到沿江支流的影响。安徽段长江干流及其主要支流重金属元素输出通量定量计算结果表明:秋蒲河是安徽段重金属元素输出通量最大的支流,每年在安徽段长江两岸土壤中沉积下约4.45t的Cd。利用悬浮物加密测量、1∶5万土壤测量和岩石测量对顺安河流域Cd异常源追踪结果显示:内生金属矿床特别是铅锌矿是悬浮物中Cd的最大的供应源。采用河漫滩沉积物分层采样技术和同位素测年技术,初步恢复了研究区Cd等重金属元素沉积和污染的地球化学历史,对研究区Cd等重金属元素异常的未来演变趋势进行了预警预测。

破碎波作用下沙质海床床面形态变化试验

波浪在斜坡沙质海床上破碎会加剧泥沙输移导致海床形态变化,研究破碎波作用下沙质海床形态变化机制对于岸滩演变分析极为重要。在波浪水槽中采用中值粒径0.47 mm原型沙铺设1∶20坡度的底床模型进行试验研究,测量不同波浪条件下床面形态和沙坝顶端悬浮泥沙浓度变化。通过测量和计算破碎带输沙率、沙坝尺度和沙坝移动速度,分析破碎波作用下沙质斜坡海床上床面形态变化规律。试验结果表明,破碎带沙坝顶端的悬浮泥沙浓度与水深和底部床面密切相关,在形成沙坝和沙坝水平方向移动时,悬浮泥沙浓度较大;斜坡上沙坝前后来回运动的周期大小具有随机性,沙坝既有向岸又有离岸移动;在多组波长时间作用后沙坝尺度趋于稳定,底床净输沙量趋于0。

长江河口北港河道泥沙特性及河床沙再悬浮研究

基于北港上段和下段以及南支河道三个站点的7天连续同步水沙观测数据(包括潮流、悬沙含量和表层沉积物样品),通过对不同时刻悬沙浓度、悬沙和沉积物粒径组成分析,以及基于沙量平衡的粒度谱计算,对长江口北港河道泥沙特性和河床沉积物再悬浮作用进行了研究。结果表明:北港上段河道落潮时水动力强于涨潮,单宽输沙量表现为净向海输沙;河床沉积物颗粒较粗,细砂和粉砂含量可达90%以上;近底层悬沙浓度在一个涨或落潮周期内可存在两个峰值;河床沉积物再悬浮作用落潮强于涨潮。