Responses of river bed evolution to flow-sediment process changes after Three Gorges Project in middle Yangtze River:A case study of Yaojian reach

The Three Gorges Project(TGP)has changed the flow-sediment process in the middle Yangtze River.For navigation purposes,there is an urgent need to study the changes of the river regime over a long-term period and the shoal-channel evolution over different seasons since the completion of the TGP.Based on analysis of the measured data and the results of a two-dimensional mathematical model,the changes of the river regime and river bed evolution in the Yaojian reach downstream of the TGP were studied.Results show that a high sediment transport flux helps to keep the main flow in the North Branch,while a low sediment transport flux helps to keep the main flow in the South Branch.Thus,the main branch will not change in the near future because of the low sediment transport load.In this study,the flow-sediment process adjusted by the TGP was restored to the conditions before the TGP,and the river bed evolution under the adjusted and non-adjusted flow-sediment conditions was calculated.After the completion of the TGP,the reservoir storage accelerated the flood recession process and decreased the erosion by 11.9%under the flow-sediment conditions in 2010,and the deposition in the flood season decreased by 56.4%.

Stability of Geothermal Convection in Anisotropic River Beds

The onset of thermal convection, due to heating from below in a system consisting of a fluid layer overlying a porous layer with anisotropic permeability and thermal diffusivity, is investigated analytically. The porous medium is both anisotropic in permeability whose principal axes are oriented in a direction that is oblique to the gravity vector and in thermal conductivity with principal directions coincident with the coordinate axes. The Beavers-Joseph condition is applied at the interface between the two layers. Based on parallel flow approximation theory, a linear stability analysis is conducted to study the geothermal river beds system and documented the effects of the physical parameters describing the problem. The critical Rayleigh numbers for both the fluid and porous layers corresponding, to the onset of convection arising from sudden heating and cooling at the boundaries are also predicted. The results obtained are in agreement with those found in the past for particular isotropic and anisotropic cases and for limiting cases concerning pure porous media and for pure fluid layer. It has demonstrated that the effects of anisotropic parameters are highly significant.

Analysis on Scour Characteristics under River Bed Variation in Weir Downstream

Various river projects are underway in small rivers in Korea that typically have natural flows. However, recent findings have shown that damages could be aggravated by structures such as weirs and drop structures during flood incidents. Experimental studies for securing the stability of flood control for these artificial structures have been insufficient, and designs applying the existing domestic design standards would not be suitable for the steep flow sections such as the actual small rivers, possibly aggravating the damages. The present study involved hydraulic model experiments conducted in a laboratory to investigate the surrounding flow patterns according to the river bed slope at the downstream part of the weir model. Further, the scour characteristics in the apron section during the overflow of the structure were analyzed to determine the appropriateness of the apron length. Thus, as the upstream river bed slope gradually increased, the experimental scour length deviated more from the design criteria formula. The results suggest that both the formula suggested by the National Construction Research Institute and Bligh’s formula presented in the River Design Criteria are not suitable for steep-slope rivers, such as small rivers in Korea, because both formulas were proposed based on the seepage line distance and river bed materials without considering the slope of the river bed. Thus, in designing the apron and bed pitching of weirs and drop structures, the river bed slope, scour characteristics of weir overflow, and existing design factors should be comprehensively considered to devise a design formula appropriate for environment of the small rivers in Korea.

Analysis on River Bed Changes in Weir Downstream by Real-Scale Experiment

Flood damage has aggravated recently owing to artificial structures in high flow rare areas such as small rivers, which can lead to secondary damage. In this regard, studies are required to examine the conventional design criteria formulas to secure the stability of structures such as weirs and drop structures. Although studies on the stability of these structures have been conducted through small-scale experiments, few empirical studies have investigated the hydraulic phenomena occurring near actual artificial structures. In this study, we fabricated real-size models of weir and drop structure at the Andong River Experiment Center and investigated the flow patterns around the structures by applying the particle image velocimetry analysis technique with a flow tracker. We also measured the scour length in the waterspout section when the structures are overflowing, and compared it with the values calculated using the formula. Consequently, as the supply flow increases, the result is different from the value calculated using the formula given in the existing design standard, and it is judged to be inappropriate for a small stream area with high flow rate. Thus, it is necessary to consider the design factors such as energy gradient and the flow amount per unit width into weir and drop structure as well as the existing design factors in designing an apron section for a weir and drop structure.

MATHEMATICAL MODEL OF RIVER BED CHANGE DOWNSTREAM OF XIAOLANGDI RESERVOIR

A mathematical model of river be d change downstream of the Xiaolangdi Reservoir was developed based on the most recent achievement of sediment theory in the Yellow River. The model was verified by the comparison of computed results and measured data from 1986 to 1996. Num erical prediction of the erosion and deposition downstream of the Xiaolangdi Res ervoir in its first operation year was carried out, and a series of suggestions were given for reservoir operation mode in its early operation period.

Roughness coefficient and its uncertainty in gravel-bed river

Manning＇s roughness coefficient was estimated for a gravel-bed river reach using field measurements of water level and discharge, and the applicability of various methods used for estimation of the roughness coefficient was evaluated. Results show that the roughness coefficient tends to decrease with increasing discharge and water depth, and over a certain range it appears to remain constant. Comparison of roughness coefficients calculated by field measurement data with those estimated by other methods shows that, although the field-measured values provide approximate roughness coefficients for relatively large discharge, there seems to be rather high uncertainty due to the difference in resultant values. For this reason, uncertainty related to the roughness coefficient was analyzed in terms of change in computed variables. On average, a 20% increase of the roughness coefficient causes a 7% increase in the water depth and an 8% decrease in velocity, but there may be about a 15% increase in the water depth and an equivalent decrease in velocity for certain cross-sections in the study reach. Finally, the validity of estimated roughness coefficient based on field measurements was examined. A 10% error in discharge measurement may lead to more than 10% uncertainty in roughness coefficient estimation, but corresponding uncertainty in computed water depth and velocity is reduced to approximately 5%. Conversely, the necessity for roughness coefficient estimation by field measurement is confirmed.

Distribution and number change of the Littorinidae (Gastropoda) on intertidal hard bed in the Jiulong River Estuary

There are 4 species of Littorinidae on the intertidal hard bed in the Jiulong River Estu-ary. They are Littorina scabra, L. brevicula, Nodilittorina pyramidalis pyramidalis and N.millegrana. Of these species, L. scabra is a dominant species. According to the results of in-vestigation, we find distribution patterns of the Littorinidae on intertidal hard bed in theestuary as follows: (1)There is no Littorinid species at Haicheng Station. (2)There is only Lit-

Theoretical analyses on bed topography responses in large depth-to-width ratio river bends with constant curvatures

Bed morphology is the result of a dynamic response to a complex meandering river system. It is an important factor for the further development of river. Based on the meandering river characterized by a large depth-to-width ratio, a theoretical model is established with the coupling of Navier-Stokes （N-S）~ sediment transport, and bed deformation equations. The flow characteristics and bed response of river are obtained with the perturbation method. The research results show that, under the effect of two- dimensional flow disturbance, the bars and pools present the regular response. For a given sinuousness, the amplitude of the bed response can be used as a criterion to judge the bedform stability. The effects of the Reynolds number, disturbance wavenumber, sinuousness, and bed morphology gradient on the bed response development are described.

An Investigation of Stochastic Nature of Bed Load Motion in Chuanjiang River

Bed Load Motion and its transport rate is one of the basic issues in river dynamics.In this paper, the authors discussed the stochastic nature of bed load motion in Chuanjiang River in details.Chungjiang lies in the upstream reach of Yangtze River.Its stochastic nature is shown in the following four aspects.Firstly, even though all the conditions are the same,due to the fluctuation of the flow,the bed load discharge and the location and width of sediment transport belts are different.Secondly,during the ...

Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler （PC-ADP） and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods： log profile （LP）, eddy correlation （EC）, turbulent kinetic energy （TKE）, and inertial dissipation （ID）. The results show that （a） all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; （b） the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and （c） in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

冲积性弯曲河道造床过程及演变机制

冲积性弯曲河道独特的几何边界特征,使得弯曲河道的水沙运动特性和河床演变特性与其他类型河道显著不同。为模拟自然条件下的冲积性弯曲河道河床冲淤演变特性,采用自然模型法的理念在试验水槽中塑造出近似的弯曲河道,探究不同水沙条件对弯曲河道形成和发育过程的影响。试验表明:进口水沙循环周期长、来水来沙强度较小时,弯曲河道达到动态平衡状态所需的时间相对较短,河道形成稳定河宽也较小。弯曲河道在形成和发育过程中,河道的形态要素主要由水沙作用条件决定,在一定变化范围内,当水沙作用循环变换周期较短、来流流量较小、含沙量较大时,有利于形成弯曲度较大的河道;而当来流量较大、水位漫滩冲刷浅滩时,所塑造的弯曲河道会偏向于宽浅形断面形态。在河道弯曲程度和河道横断面形态的调整中,水沙作用变化对横断面形态塑造的作用相对较强。

三峡水库运用后长江中游河漫滩演变趋势分析

长江中游历来是防洪重点,河漫滩的演变速率虽然相对缓慢,但新水沙条件下河漫滩的长期累积性变化值得关注。以三峡水库175 m运行期长江中游多处典型河漫滩淹没特征流量的统计分析作为基础,量化分析了三峡水库蓄水前后径流过程在河漫滩淹没特性方面的变化,结合滩面植物对季节性水文波动淹没耐受能力的分析,指出三峡水库蓄水前后径流过程变化造成的季节性淹没不足以阻断滩面植物的生长发育。由于一系列涉水工程的控制作用,长江中游河漫滩的横向变形受到了极大的抑制,但在滩面植被缓流滞沙效应下,长江中游河漫滩总体仍呈现缓慢淤高的长期趋势。

导流兼泄洪冲沙洞消力池体型优化及河床冲刷研究

导流兼泄洪冲沙洞中消力池经常存在水跃波动范围不稳定、水流跃出边墙等问题。为获得良好的消能防冲效果,通过数值模拟与物理模型试验相结合的方法,对导流兼泄洪冲沙洞泄洪过程的流体运动进行研究,获得流场水力性能参数以及下游河床冲刷情况,分析不同方案对消力池水力特性的影响并进行物理试验验证。结果显示:消力池末端在增设矩形尾坎后水流流态得到改善;同时池内进出口流速降幅最大为7.66 m/s,而压强分布均匀、无负压产生,且消能率达到80.60%。在设计和校核工况下,增设矩形尾坎后河床冲坑范围相较于原方案分别减少12.20 m和13.30 m,其最大冲坑深度分别较原方案减少0.48 m和1.00 m,对下游防护效果明显。研究表明:所设的体型优化方案均能有效的提高消力池的消能效果,在增设矩形尾坎后流场分布均匀且减轻下游河床冲刷的效果最佳。该研究成果可为导流兼泄洪冲沙工程提供参考。

黄河下游平滩流量的模拟方法

在黄河流域水沙变异背景下,准确计算黄河下游平滩流量对于河道冲淤趋势预测与悬河治理十分重要。本文在比较分析已有多个黄河下游平滩流量计算方法基础上,首先建立了考虑非汛期水沙条件影响的平滩流量滞后响应模型,再提出了基于滞后响应模型和能量法的平滩流量模拟方法,结果表明:(1)小浪底运行后非汛期水沙条件发生了重要改变,已有研究提出的三种黄河下游平滩流量计算方法中,滑动平均法、滞后响应模型、能量法均无法有效模拟2000年后的黄河下游河道平滩流量。(2)建立了考虑非汛期水沙条件影响的滞后响应模型,以反映小浪底水库运行以来非汛期来水来沙条件的影响,虽然提高了平滩流量的模拟精度,但平滩流量平衡值的计算公式中部分参数差异较大,结构形式不稳定。(3)借鉴能量法的优点,改进了平滩流量平衡值计算公式,改进后的平滩流量计算方法不仅可反映全年来水来沙的影响,而且可同时反映以床沙中值粒径为代表的床面阻力的影响,新方法不仅具有一定的理论基础,而且提高了平滩流量的模拟精度。

近40年思贤滘河床演变规律

通过分析不同年代的地形数据和遥感影像,揭示了思贤滘岸线、等深线和河道形态特征参数的变化规律,结果显示,河道整体呈现窄深化的趋势,深水区面积不断增加;此外,上游来沙量的减少和人类活动,如水利水电工程和采砂等,对思贤滘河床演变产生了显著影响。研究结果可为珠江河口治理提供科学支撑,有助于明确新时期思贤滘控导工程的定位和功能。

川西龙门山前陆盆地南段前缘砾石层成因及青衣江演化过程:基于宇宙成因核素^(26)Al/^(10)Be埋藏年龄

四川西部(川西)龙门山前陆盆地自中新世以来发生强烈隆升,形成了陡峻的地形梯度带。在紧邻龙门山断裂带的四川盆地堆积一系列砾石层,其中包括名邛(名山—邛崃)砾石层、丹思(丹棱—思蒙)砾石层。名邛砾石层、丹思砾石层为古青衣江堆积砾石层,是古青衣江改道的产物,也是探究青衣江演化及该地区古环境变化的良好对象。基于龙门山前缘地区野外调查和名邛砾石层、丹思砾石层的宇宙成因核素^(26)Al/^(10)Be定年测试,发现名邛砾石层埋藏年龄为(0.6089±0.0187)Ma,丹思砾石层埋藏年龄为(0.2180±0.0941)Ma,由此推断名邛砾石层形成时间为早更新世晚期—中更新世早期(Q_(1)^(3)—Q_(2)^(1)),丹思砾石层形成时间为中更新世中期(Q_(2)^(2))。结合前人研究结果分析,两套砾石层分别对应于深海冰心δ^(18)O曲线和中国黄土序列曲线,都形成于温暖的间冰期。结合前人对该地区的砾石成分、砾径、砾向统计分析和古流向分布等研究,认为青衣江水系经历三次主要的改道事件,早期的青衣江由雅安往北北东方向流向建山,随着熊坡背斜的隆起和名邛台地的持续抬升,由南西-北东流向的青衣江被迫在草坝东南方向切断熊坡背斜从而改道南东方向。

汉江兴隆水利枢纽下游近坝河段枯水位下降及成因分析

汉江兴隆水利枢纽运行后,坝下游近坝段枯水位较建坝前下降明显,500~800 m^(3)/s流量下,2021年较运行前坝下水位累计下降2.47~2.55 m,对工程安全及效益发挥带来不利影响。基于水文、断面、水下地形等观测资料,对枯水位下降的原因进行深入剖析。结果表明,兴隆水利枢纽的拦沙作用有限,皇庄以上沙量减少是兴隆以下河床冲刷加剧的重要原因,高水调平导致的年内枯水上滩几率下降和航道整治护滩工程是强冲刷过程中“滩淤槽冲”的核心因素。坝下游河段枯水河槽冲刷及汉口最低水位下降是造成兴隆近坝段枯水位下降的直接原因。随机森林算法分析显示,对兴隆坝下水位变化影响最大的因素为兴隆站输沙率的锐减。此外,航道整治工程、河床边界条件对枯水位下降也有十分重要的驱动作用。兴隆坝址及下游河段的河床组成偏细,2012-2022年虽然河床剧烈冲刷,但床沙组成没有出现粗化的现象,预计河床仍将继续冲刷下切,枯水位尚未达到稳定状态。

三门峡水库原型试验以来渭河下游河床稳定性变化及其影响因素

为探究渭河下游河道形态演变特征及影响机理,基于多年水文观测资料分析渭河下游河床稳定性演变规律,探讨三门峡水库原型试验以来(2003~2020年),来水来沙条件、潼关高程变化、河道边界条件与河床稳定性的响应关系。结果表明,原型试验期内,渭河下游河道年均径流量呈缓慢增加趋势,年均含沙量均呈缓慢减小趋势,河段河槽呈扩张趋势,潼关高程呈逐年下降趋势。原型试验期前后,渭河下游河段河床横向、纵向和综合稳定性系数均增加,原型试验期内,河床稳定性呈逐年上升趋势。前期3年平均水流冲刷强度和潼关高程变化是影响渭河下游河床综合稳定性的重要因素,利用多元回归分析方法建立了渭河下游河床综合稳定性的经验计算公式,验证了所提经验公式的准确性和可靠性。研究成果可为渭河下游及类似河流管理提供参考。

金沙江下游三堆子河段卵石推移质输沙率公式研究

金沙江上中游梯级水库运用后,进入下游的悬移质输沙量大幅减少,水沙条件显著变化;同时,卵石推移质输沙量在总输沙量中的比重明显增加,对水库淤积进程影响显著,开展变化水沙条件下金沙江下游卵石推移质输移规律研究具有重要意义。基于下游入口三堆子水文站2008—2021年卵石推移质实测资料,对经典起动流速公式和推移质输沙率公式进行了检验与修正。结果表明:与经典起动流速公式计算结果相比,通过引入以卵石粒径为变量的系数,所得到的不同粒径范围的卵石推移质起动流速修正公式,计算精度显著提高;经典推移质输沙公式的计算结果在输沙强度较低时与实测值偏差很大,通过建立输沙强度与水流强度的关系,并引入隐暴参数对公式进行修正,明显提升了其在低输沙率情况下的计算精度。修正后的起动流速公式和推移质输沙率公式能更有效地反映三堆子河段卵石推移质的输移规律。

组合生态修复技术对河道水质净化与保持的应用研究

为探究生态修复技术对河道水质净化和保持的实际应用效果,对上海市某河道采用“生物床+微曝气+沉水植物”技术进行生态修复治理。结果表明,河道水质得到明显改善,对氨氮、总磷的去除率分别达到了54.5%~64%和48.4%~52.8%,水质可满足V类水要求,多数时段可达到地表水Ⅱ、Ⅲ类标准。同时水生态结构和功能得到了较好的恢复,生物多样性增加,水生态系统稳定。经过连续5年的水质跟踪检测,河道水质基本能稳定达到V类水质要求,证明本工程技术切实可行,稳定有效,值得在类似城市河道治理修复项目中推广。