The water stored above the flood control level of the reservoir was regulated and released from the Xiaolangdi Reservoir as a man-made flood peak to enhance the erosion of the Lower Yellow River. Based on the field data of the Xiaolangdi Reservoir and the Lower Yellow River from 2002 to 2015, this paper analyzes impacts of man-made flood peaks of the Xiaolangdi Reservoir. The result shows that from 2002 to 2015, the nineteen man-made flood peaks were carried out, the total water for these peaks amounted to 1.24×109m3～6.04×109m3 and the total sediment load were 0～0.08×109t. The amount of erosion was increased with the increasing of incoming water of flood, the amount of erosion and the efficiency of channel erosion was gradually declined from 2006.The erosion appeared along the whole river, the total erosion for the nineteen man-made flood peaks amounted to 0.427×109t, 1 m3 of incoming water could erode averagely about 6 kg of bed sediment. Moreover, all the erosion happened in the main channel, which was beneficial to the enlargement and comparatively narrower of cross-sections, dropping the water level and increases of flood conveying and sediment transporting capacities. The incoming runoff for the nineteen man-made flood peaks accounted for 19.1% of the total runoff in 2002-2015, the erosion was 17.5% of the total correspondingly. All of these imply that the man-made flood peak resulted from the utilization of abandoned water of the reservoir is much affective to strengthen the channel erosion although it is just a part of the total erosion.

ObjectiveA spur dike is an in-stream structure that is widely employed for the protection of riverbank erosion. In river engineering, traditional regulating structures exhibit certain limitations, such as occupying large areas, requiring significant quantities of stone, being difficult to maintain, and involving high construction costs, which make them unsuitable for use in some regions. Therefore, scholars propose the application of floating dike structures in river regulation engineering. These structures exhibit advantages such as minimal impact on sediment transport, improved adaptability to upstream water and sediment discharge, and reduced impact on the riverbed and ecological environment. However, research on the effects of floating dikes on flow structure and the riverbed remains insufficient, and the understanding of their practical application is limited. Considering practical application, ships are used as substitutes for floating dikes; therefore, this structure is also referred to as a ship-shaped floating dike. This study focuses on the effects of the submergence ratio (D is submergence depth/water depth), length ratio (K is ship length/river width), arrangement angle (θ > 0 indicates the dike oriented downstream), and transverse bed slope at the location of the ship-shaped floating dike on the surrounding flow field and turbulence characteristics. The findings provide a hydrodynamic basis for further understanding the influence of ship-shaped floating dikes on sediment transport and riverbed evolution, as well as technical support and reference for their application in practical river engineering projects.MethodsThis study investigated the influence of geometric characteristics, including submergence depth and ship length, as well as arrangement parameters, including ship angle and transverse bed slope, of ship-shaped floating dikes on the surrounding flow field and turbulence structure, with the objective of identifying the optimal arrangement configuration. The same flume dimensions (length is 1.4 m, width is 0.3 m, and height is 0.05 m) and simulation conditions as those used in the physical model experiments were adopted to enhance the reliability of the numerical model. The RNG k‒ε turbulence model was employed to simulate the flow structure around the ship-shaped floating dike. Considering the negligible variation in the water surface, the rigid-lid assumption was applied, with a constant inlet velocity and free outflow conditions at the outlet. The velocity-pressure coupling equations were solved using the semi-implicit SIMPLE algorithm.Results and DiscussionsThe numerical simulation results for velocity distribution and Reynolds shear stress across different longitudinal sections were consistent with those obtained from physical experiments, confirming the reliability of the numerical model. The three-dimensional flow structure and turbulence kinetic energy distribution were analyzed by considering variations in the submergence ratio (D), length ratio (K), arrangement angle (θ), and transverse bed slope (SL) of the ship-shaped floating dike. The numerical results were verified against experimental observations. The results demonstrated that: 1) As D increased from 0.2 to 0.8, the maximum relative streamwise velocity below the ship-shaped floating-dike (SSFD), defined as the ratio of local maximum flow velocity/inlet average flow velocity (U/U0max), gradually increased, from 1.52 to 1.69. The length of the back-flow area and U/U0max increased from 1.0L to 3.5L (L is the length of SSFD) and from ‒0.35 to ‒0.51. Increasing D also intensified near-bed flow velocity and enhanced turbulence intensity downstream of the SSFD. 2) As K increased, the effective flow-passed cross-section decreased, resulting in increased flow velocity below and outside the SSFD. The area of the back-flow and low-velocity zones expanded with increasing K, whereas the length and intensity of the back-flow region remained nearly unchanged. Turbulence kinetic energy gradually increased and formed a concentrated core region near the bottom and downstream of the SSFD. 3) When the SSFD was tilted upward with θ = ‒30º, a strong blocking effect was observed, with U/U0max upstream of the SSFD reduced to 0.4, while near the shore and bottom of the SSFD, U/U0max = 1.53, which was less than θ = 30º with U/U0max = 1.63, but both were less than θ=0 with U/U0max = 1.72. The distribution of the slow-flow zone behind the SSFD was larger when θ = ‒30º. When θ = 30º, the flow remained relatively stable, characterized by weak turbulence kinetic energy, and localized acceleration zones and circulation structures developed downstream of the SSFD. When the SSFD was positioned on the deeper side of the channel, the flow concentrated beneath the structure, resulting in an intensified back-flow region. The transverse slope of the riverbed enhanced downstream circulation. The results indicated that the SSFD achieved optimal slow-flow and sedimentation promotion effects when it was arranged on the deeper side, tilted downward, with a submergence ratio ≤ 0.4 and a length ratio ≤ 0.5.ConclusionsThe geometric dimensions and spatial arrangement of the ship-shaped floating dike, along with the transverse bed slope, play a significant role in influencing the surrounding hydrodynamic characteristics and turbulence structure. As an innovative river regulation structure, the ship-shaped floating dike exhibits advantages including the formation of a stable slow-flow zone downstream of the dike, well-developed secondary circulation, and minimal disturbance to the bottom flow field, which collectively contribute to favorable ecological compatibility. These findings provide valuable technical support and practical reference for the application of ship-shaped floating dikes in river regulation engineering projects.

Driven by continuous channel degradation，the water level-discharge relationship has undergone significant changes in the Lower Yellow River (LYR). Based on the hydrological and topographic data from six hydrometric stations in the LYR from 1986 to 2023，this study investigated the characteristics and mechanisms of flood-level changes in the LYR，and quantified the effects of channel morphological adjustment and channel resistance variation on flood levels at a given discharge. Results show that: ①Before the operation of the Xiaolangdi (XLD) Reservoir (1986-1999)，flood levels at all six stations showed an overall upward trend，with the values at a discharge of 4000 m3/s increasing by 1.43-1.78 m. After the XLD Reservoir operation (1999-2023)，flood levels at 4000 m3/s decreased by 1.51-3.40 m. ②The decline in flood levels at identical discharges was positively correlated with channel scour volumes. For each 100 million m of scour in the braided，transitional and meandering reaches，flood levels at 4000 m3/s dropped by 0.17-0.85 m at the corresponding stations. ③Numerical experiments were used to quantify the contributions of different factors on flood levels at 4000 m3/s between 1986 and 2020. It revealed that increased channel resistance raised flood levels of 0.45-0.91 m，whereas channel morphological changes lowered them by 0.59-1.58 m. Downstream of the Sunkou Station，these two effects largely offset each other，resulting in little net change. While in the upstream reach，the flood-level rise attributable to increased channel resistance was less pronounced than the decline caused by riverbed incision，but it still counteracted 24.3% of the total flood-level decrease.

Based on the field data of the Xiaolangdi Reservoir and Lower Yellow River from 1999 to 2018, this paper analyzes reservoir sedimentation and its Influence on channel in the Lower Yellow River during the nearly 20 years operation of the Xiaolangdi Reservoir. The result shows that from 1999 to 2018, the total deposition of Xiaolangdi Reservoir reached 3.488×109m3, only 26.7% of the incoming sediment load could be sluiced out of the reservoir. Under the new conditions of incoming runoff and sediment, the Lower Yellow River unavoidably suffers erosion. From 1999 to 2018, the whole erosion of the river amounted to 2.92×109 t. The erosion in flood season is 61.1% of the annual erosion, about 83.6% of the erosion occurs in upper reach of the river covering a length of 495 km. The efficiency of channel erosion becomes weaker and weaker. The smallest bankfull discharge of the Lower Yellow River in the post-flood season were increased from 1800 m3/s in 2002 to 4300 m3/s in 2018. The water levels corresponding to the discharge of 3000 m3/s in the pre-flood season of 2018 obviously dropped down by 1.67m-3.20m in the Lower Yellow River, as compared with those in the pre-flood season of 1999. The surface bed materials in 2018 were nearly two times as coarse as those in 1999. Further research shows that, the increased flow energy coursed by 1m3 sedimentation in the Xiaolangdi Reservoir are capable to scour 0.84t sediment in the downstream channel, therefore, the sediment detention by reservoirs are still one of the most effective measures for sedimentation reduction in the Lower Yellow River.

The Xiaolangdi (XLD) Reservoir stands as a pivotal reservoir for the water-sediment regulation and power generation in the Yellow River Basin. The coexistent process of water-sediment-energy during the pre-flood period causes the competition among multi-objectives: power generation, water supply, deposition reduction in the reservoir area and the downstream channel. To balance the trade-off between reservoir sustainability and utility value, a model framework is proposed in this study, following the route of "scheme design-model simulation-scheme evaluation". The framework integrates a reservoir operation module, a hydrodynamic-based water-sediment-energy simulation module, and a benefit evaluation method based on the Fuzzy Neural Network (FNN). Firstly, the water-sediment-energy simulation module was validated against the 2013 and 2014 water-sediment regulation events, showing good agreement with the field measurements. Subsequently, by altering two key scheduling parameters of CWL (connecting water level) and RWL (refilled water level) based on the 2014 water-sediment regulation event, different operation schemes were evaluated based on the comprehensive performance of VS (vented sediment amount from reservoir), TV (total volume of discharge greater than 2600 m3/s), PG (power generation amount), and SC (final storage capacity) with the trained FNN. The proposed optimal values of CWL and RWL were 217 m and 211 m, with the relative membership degrees of 0.750 and 0.754, respectively. This finding suggests that maintaining a continuous low pool level in the XLD Reservoir is optimal for securing higher comprehensive benefits in case of insufficient water inflow, with the effect of deposition reduction being a notable advantage.

Abstract Confluences are marked by converging streamlines, complex hydrodynamics features and mixing processes. Understanding such intricate flow patterns and their effects on the mixing at natural river junctions is difficult, particularly for large confluences with a compound channel tributary, due to the scarcity of field data. This research aims to assess the effects of density differences on hydrodynamics and mixing at the large river confluence between the Yangtze River and its tributary, the Poyang Lake, whose outflow channel has a large inner-side floodplain. Field data were collected during three surveys under various flow conditions to analyse how the alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel influences the confluence mixing process. In high flow and relatively high flow conditions, the Poyang Lake outflow channel functioned as a compound channel and switched to a single channel in low flow. Acoustic Doppler current profiling (ADCP) was used in all three surveys to explore the features of hydrodynamics and the mixing process at the confluence, supplemented by water quality sampling to characterise the mixing patterns. Secondary flows observed during the field surveys were found to be affected by alterations in the flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel and the density effects, which were characterised using the densimetric Froude number Frd. Ultimately, the findings obtained in these field surveys confirm the role of density differences between the tributaries in significantly affecting hydrodynamics features and the mixing process at river confluences.

To improve the comprehensive benefits of the Xiaolangdi Reservoir in gaming among multi-objectives during the flow and sediment regulation processes before flood seasons,an integrated model has been proposed,which can simulate the flow-sediment transport in the reservoir area and quantitatively evaluate the comprehensive benefits of reservoir operation.The integrated model consists of three sub-modules,including:a flow-sediment transport module,which considers the process of turbidity currents and flow exchanges between the mainstream and tributaries;a reservoir regulation module,which serves as a connection between the flow-sediment simulation in the reservoir area and the calculation of channel deformation volume in the Lower Yellow River;a ben-efit evaluation module,which adopts an economic indicator to measure the comprehensive benefits.Flow and sediment regulation e-vents in the Xiaolangdi Reservoir in 2013 and 2014 were used for model calibration and verification,showing that the whole proces-ses of plunging,transport and venting of turbidity current,as well as the outflow and power generation processes,were in good a-greement with the measured data.Key operation parameters were selected based on the practical experience of flow-sediment regula-tion events over the years.Based on the 2014 actual operation process,5 new operation schemes were designed by reducing the connecting water level or decreasing the degree of backwater,and comprehensive benefits of these five operation schemes were as-sessed.Calculation results showed that the changes of connecting water levels and backwater conditions in a reservoir regulation process would have a conflict between sedimentation reduction and water conservancy.By comparing the economic indicators of de-signed operation schemes,the operation scheme with the connecting water level of 222.57 m and less backwater has been suggested for gaining higher comprehensive benefits in the Xiaolangdi Reservoir.

AbstractThere is independence and integration between water supply and sediment reduction in sediment-laden river reservoirs. How to maintain effective storage capacity and meet water supply requirements for a long time is one of the problems to be solved in the efficient operation of sediment-laden river reservoirs. In this paper, Dongzhuang Reservoir of Jinghe River is taken as the research object. Through the analysis of measured data and mathematical model calculation, the measured hydrological sediment and cross-section erosion and deposition in the lower reaches of Jinghe River and Weihe River are analyzed. The reservoir sediment discharge flow index which is beneficial to reduce the sediment deposition in the lower reaches of Weihe River and maintain the effective reservoir capacity for a long time is studied, and the joint regulation mode of reservoir runoff and sediment is put forward. The results show that the sediment regulation of Dongzhuang Reservoir during the sediment retention period is mainly to reduce the deposition of the lower reaches of the Weihe River, and the sediment regulation during the normal operation period of the reservoir is mainly to maintain the effective storage capacity of the reservoir for a long time. The joint regulation of the Dongzhuang Reservoir and the surrounding four storage reservoirs, such as the Ganhe Reservoir, the Xijiao Reservoir, the Longtan Reservoir, and the Helan Reservoir, can reduce the deposition of the lower reaches of the Weihe River by 11 million tons per year, increase the guarantee rate of agricultural irrigation from 30 to 50%, and increase the guarantee rate of industrial water supply from 57 to 95%. And the outflow of the reservoir meets the requirements of the ecological base flow of the river. The research results provide technical support for the optimal operation of reservoirs in sediment-laden rivers.

新时期黄河水沙情势巨变,水库及河道边界条件也发生重大调整,小浪底水库调水调沙需要进一步创新和优化,以便持续发挥其效益.在黄河中游潼关站实测百年水沙系列分析的基础上,提出来水量较多年平均值偏多30％、偏少30％作为丰水年、枯水年的判别标准.对小浪底水库排沙和下游河道输沙规律的分析表明:丰水年具有冲刷能力强和输沙潜力大的优势,水库可多排沙;枯水年具有冲刷能力弱和输沙潜力小的劣势,水库可少排沙.进一步提出考虑黄河来水来沙丰、平、枯变化的小浪底水库调水调沙优化运用方式,采用2000年以来实测水沙系列,分析优化运用方式在当前水沙情势下的适用性.结果表明:优化运用方式可在不淤积下游河道的基础上延长水库拦沙年限8 a,减少水库淤积量8.18亿m3,提高水库多年平均日蓄水量3.99亿m3.

充分挖掘黄河中游水库群调控作用潜力,提高黄河下游河道的输沙能力,对黄河下游河道与滩区治理具有重要指导意义.通过实测资料分析、数学模型计算等方法,计算不同水沙条件下优化现状水库群调控方式对黄河下游河道输沙能力的影响,进一步分析古贤、东庄水库投入运行后黄河下游河道输沙能力的变化.结果表明:优化现状水库群调控方式,水库增加泄放较大含沙量的水流过程,可提高下游河道输沙能力10％～12％;古贤、东庄水库投入运行后拦沙期内进入下游河道的沙量和输沙入海量减少,正常运用期可进一步提高输沙能力11％～12％.

黄河泥沙主要来源于中游,而水沙灾害在下游最为严重,导致黄河下游河道治理与滩区发展矛盾十分突出.为此,构建了黄河下游水沙演进与洪水风险评估的耦合模型,包括长河段水沙演进的一维模型与局部河段二维水沙演进及洪水风险评估模型.对一维模型,嵌入改进后的动床阻力与水流挟沙力计算公式;对二维模型,采用无结构三角网格,能精确反映主槽及滩区的不规则边界,同时采用有限体积法求解水沙耦合控制方程,并嵌入滩区洪水风险评估模块.采用2017年低含沙洪水过程与2004年高含沙洪水过程分别对不同模块进行了验证.最后采用该稱合模型计算了 1958型极端洪水过程中长河段及重点河段的水沙输移过程,并开展了不同治理模式下的滩区洪水风险评估.计算结果表明:1)在当前黄河下游河床边界条件下,洪峰沿程衰减较快.其中夹河滩至高村河段几乎全部漫滩,漫滩历时较长,平均为140 h,漫溢系数平均为1.5,是下游漫滩最严重的河段;2)在现状边界条件下,群众及农作物等受淹对象处于高风险区(风险等级大于0.85)的面积均最大.在防护堤模式下,群众处于高风险区的面积很小,占比仅为23％,但农作物处于高风险的面积较大.在三滩分治模式下,房屋及农作物处于高风险区面积均最小,高滩上仅7.3 km2区域中的人群将处于高洪水风险中.

黄河调水调沙连续开展二十余年,是协调黄河水沙关系行之有效的措施,在减淤、塑槽、生态等方面发挥了显著效益.但新时期黄河水沙情势异变,水库及河道边界条件也发生调整,调水调沙面临冲刷效率下降和后续动力不足两个突出问题.为适应新时期新形势变化,有必要对调水调沙进行优化创新,以更好地发挥其长期综合效益.采用实测资料分析的方法,结合工程实际调度经验,提出基于现状水沙调控工程体系优化调水调沙的措施,包括小浪底水库多排沙提供沙源以及挖掘干流水库群潜力为调水调沙补充后续水流动力.从保障黄河长久安澜的角度,提出完善调水调沙的建议:一是加快推动骨干工程建设,完善水沙调控体系;二是积极创新调水调沙运用模式,动态优化调控指标.上述工程措施和非工程措施的结合,可有效解决黄河泥沙治理难题.

Optimizing the flood limit water level (FLWL) of reservoirs in sediment-laden rivers under changing water and sediment conditions is an important research topic that could improve comprehensive utilization benefits. Because reservoir operation has multiple objectives in sediment-laden rivers, this study established a water–sediment mathematical model, a comprehensive benefit evaluation model, and an evaluation index system. Taking the Xiaolangdi Reservoir of the Yellow River as an example, the operation mode of the FLWL under changing water and sediment conditions was studied. Under the scenarios of incoming sediment amounts of 300–800 million tons, when using the operation mode of gradually raising the FLWL, the sediment retention period was 4–13 years longer; the lower average annual siltation of the downstream channel and minimum bank-full discharge of the downstream channel after 50 years was larger by 150–260 m3/s than the operation mode of raising the FLWL at one time. However, with enhanced benefits of sediment blocking and siltation reduction, other benefits such as water resources supply, hydropower generation, and ecological improvement are reduced. The average annual number of days that do not meet the downstream water resources supply requirements, irrigation, and ecological improvement was increased by 0.64–2.16 days, and 91–197 million kW·h reduced average annual hydropower generation. The critical amount of incoming sediment was 350 million for conversion between the two FLWL operation modes, and it will increase to 450 million tons if the incoming runoff of the Yellow River increases by 20%. After constructing the Guxian Reservoir in the middle of the Yellow River, the critical amount of incoming sediment will increase to 600 million tons. This study is of great significance for improving the utilization efficiency of water resources and promoting the socio-economic development of river basins.

完善水沙调控体系是应对黄河水少沙多、水沙关系不协调的关键措施.结合实测资料,计算了河道输沙能力,分析了水沙调控体系运行机制,计算了水沙调控体系对黄河水沙关系的调节作用,结果表明:河道输沙效率随着水流流量的增大而增大,且在平滩流量附近存在明显的拐点,骨干水库群联合调控可泄放接近下游河道最小平滩流量的大流量过程,提高河道输沙能力,减轻河道淤积.当黄河来沙8亿t,水库群拦沙期内通过拦沙和调水调沙,进入下游的水沙关系协调度小于1,水沙关系是协调的;正常运用期通过调水调沙可改善黄河水沙关系的不协调程度.与现状工程条件相比,水沙调控体系完善后的正常运用期可年平均减少下游河道淤积量1.28亿t,南水北调西线一期工程调水20亿m3前提下可进一步减少下游河道淤积量1.53亿t.

Sediment accumulation is severe in the Lower Yellow River. Improving sediment transport efficiency is an important way to save water resources. In this study, we analyzed the sediment transport efficiency at different sediment concentrations and discharge levels using data from 306 non-overbank floods in the lower reaches of the Yellow River from 1960 to 2016. When the sediment concentrations were below 20 kg/m(3), 60 kg/m(3), and greater than 100 kg/m(3), the clear water volumes for transporting sediment were above 50 m(3)/t, approximately 16 m(3)/t, and less than 10 m(3)/t, respectively. Given current scheduling, the Xiaolangdi Reservoir can lower its water level and increase non-overbank large water flows with an outflow discharge of > 3,000 m(3)/s when the incoming water flow has a sediment concentration of > 60 kg/m(3). Mathematical modeling indicates that, for an incoming sediment load of 600 million tons in the Middle Yellow River, the average sediment-transport water volume at the downstream Huayuankou station decreased by 27%, Optimized scheduling at the Xiaolangdi Reservoir could reduce the sedimentation in the downstream channel by 1.1 billion tons more than current scheduling, and extend the reservoir's sediment-trapping period by 10 years. These findings can be applied widely in sandy rivers.

水沙调控是解决黄河上游宁夏至内蒙古河道淤积萎缩、凌汛威胁的重要措施.采用实测资料分析和数学模型计算等方法,考虑水沙情势和流域生态保护、经济社会发展需求变化,研究了黄河上游不同水沙调控模式及其调控效果.现状调控模式,未来宁蒙河段处于淤积态势,年平均淤积量0.59×108t,最小平滩流量减小到1 000m3/s.调整龙羊峡、刘家峡水库调控模式,对减少宁蒙河段淤积、恢复中水河槽有一定作用,但淤积态势未发生调整,龙刘水库汛期少蓄水25×108～40×108m3,宁蒙河段年均淤积量为0.36×108～0.44×108t,同时影响流域供水、发电.在黄河上游黑山峡河段建设水库,与现状工程联合拦沙和调水调沙运用,协调水沙关系,一级开发方案可在100a内使宁蒙河道基本不淤积,最小平滩流量恢复并维持在2 500m3/s;二级开发方案可在水库运用50a内使宁蒙河段不淤积,最小平滩流量恢复并维持在2 500m3/s.黄河上游适宜的水沙调控模式为黑山峡水库与现状水库群联合运用的调控模式.

小浪底水库运用以来,为实现下游河道减淤,水库运用方式以蓄水拦沙和调水调沙运用为主,下游河道输沙能力较低.当前,黄河下游河道适宜的中水河槽规模(泄流能力为4000 m3/s)已形成,长期维持该河槽规模,充分发挥黄河下游河道输沙能力输沙入海,成为当前及今后一个时期水库减淤调度的新要求.根据黄河下游1960—2016年306场非漫滩洪水实测资料,分析不同含沙量级、流量级黄河下游河道的冲淤效率和输沙效率,提出含沙量在60 kg/m3以上的大流量洪水过程在下游河道具有较高的输沙效率,但也容易造成河道淤积.进一步提出基于下游河道中水河槽维持的河道输沙对水库运用的要求,即当下游河道中水河槽泄流能力维持在4000 m3/s左右时,为提高河道输沙能力,小浪底水库可适时增加泄放含沙量不超过100 kg/m3的大流量过程;当下游河道中水河槽泄流能力扩大至4500 m3/s以上时,小浪底水库可适时增加泄放含沙量不超过200 kg/m3的大流量过程;若未来下游河道中水河槽泄流能力缩减至4000 m3/s以下时,小浪底水库调度仍以蓄水拦沙和调水调沙运用为主,以重塑河槽规模.数学模型计算结果表明,小浪底水库运用方式调整可提高下游河道输沙能力,同时减缓小浪底水库和下游河道的淤积,且在更长的时间内维持下游河道泄流能力为4000 m3/s左右的中水河槽.

The inverse relationship between water and sediment is hindering the regulation of sediment in silt-laden rivers. Under natural conditions and insufficient water flow, sediment deposits along river channels, resulting in the accumulation of suspended river sediments above the ground level on both sides of the river. Then, during flooding, dam collapse and overflow can easily occur, threatening the life of residents in floodplain. The operation of upstream reservoirs to maximise the sediment transport capacity of downstream rivers into the sea is deemed the best strategy for the management of silt-laden rivers. However, as incoming water and sediment form a dynamic system, optimising the operation mode of reservoirs has become the most severe challenge in the field. This study proposes a method to remodel the water-sediment relationship of natural rivers by optimising the reservoir operation. First, data were collected and analysed to investigate sediment transport in the downstream river channel and the response relationship between erosion-deposition. Then, an empirical equation for calculating the desired incoming sediment was derived, and the threshold for sedimentary transport balance in the lower reach of silt-laden rivers was determined through mathematical modelling. Finally, the reservoir operation mode was optimised within the threshold range to study the discharge process of the reservoir with the highest sediment transport efficiency downstream. The Yellow River in China exhibits the highest sediment concentration worldwide and was used as a case study to verify the practicability of the proposed method. The critical threshold for sediment transport balance in the lower Yellow River was determined to be 250 million tons. The results demonstrate that the proposed method improved the sediment transport capacity of the lower reaches of the Yellow River. Moreover, it reduced the siltation of the XiaoLangDi Reservoir and the lower reaches of the Yellow River. The XiaoLangDi Reservoir can be jointly operated with the Guxian Reservoir, which is under construction and is expected to maintain the bankfull discharge of the lower reaches of the Yellow River at 4000 m(3) for a long time. The optimisation process described in this study can be used to improve the management of silt-laden rivers.

In order to optimize the operation mode of reservoir groups in the Middle and Lower of the Yellow River, and to give full play to the advantages and potential of the water-sediment joint regulation of reservoir groups and river channel, taking the reservoir groups and river channel in the Middle and Lower of the Yellow River as the research objects, the method of joint dynamic regulation of reservoir groups and river channel was studied, and the mutual feedback index, mutual feedback model, regulation principle and application of joint dynamic regulation of reservoir groups and river channel were constructed. In this way, we developed a water-sediment joint regulation simulation model of reservoir groups and river channel, and analyzed regulation effects of the current project in the Middle and Lower of the Yellow River. The results show that under the scenario of 800 million to 300 million tons of incoming sediment of the Yellow River in the future, the water-sediment joint dynamic regulation of reservoir groups and river channel comprehensively takes into account the reduction of siltation in the reservoir and the river course, and can extend the sediment retention period of the reservoir by 4 to 9 years. In a relatively long period of time, the coordination degree of the water-sediment relationship entering the downstream has been improved. The total siltation of the downstream river channel is less, and the minimum bank-full discharge of the river channel has increased by 200—250 m3/s.