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东江是珠三角网河区入汇河流之一,属少沙河流,其上游浅滩-深潭序列密集分布,河床结构的演变特性尚不清晰。选取东江浅滩-深潭序列河段进行现场河床质与推移质测量,结合试验得到不同流量和坡降中水流与河床在无上游来沙补给中的互馈机理。试验结果表明:浅滩与深潭交替造成沿程平均流速不连续变化,浅滩中部在洪水流量的塑造下容易产生紊动能峰值。无上游来沙条件下,序列中粗化层的形成、破坏过程交替与循环发生,其中静态分选是主导因素且导致沿程级配不连续。流量与坡降的增大促进浅滩-深潭序列发育,同时使得深潭与浅滩的纵向长度缩短;有上游来沙条件下推移质输沙率增大,无来沙条件下输沙率减小。 相似文献
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长江泥沙输移特性分析 总被引：7，自引：0，他引：7
长江泥沙主要来源于上游，河流输沙特性与地理条件关系密切。宜昌以上属山区，泥沙来源于土壤侵蚀，其输沙量沿程递增，含沙量增减互见。宜昌以下属冲积河道，由于泥沙在湖泊（水库）中沉积、输沙量沿程减小。至于年内变化，则与径流量变化基本相应；年际间属随机变化，无系统性。悬沙级配有沿程细化趋势。卵石推移量，寸滩为２８．２万ｔ，沿程稍有增加；粗砂砾石推移量甚微；沙推移量与河床级配组成有密切关系，沙质河床的推移量大 相似文献
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对于山区河流低坝而言,平时淤积在坝前的推移质粗沙可能会在洪水期集中翻越坝顶,形成高强度输沙。本文开展水槽试验,研究推移质粗沙自上游起动、推进、再翻越坝顶后向下游输移的过程,分析了输沙参数的变化特性及数理规律,描述了翻坝输沙模式及运动特征,揭示了输沙规律与河床形态之间的自然联系。取得如下认识:①输沙量随时间大致以幂函数规律增长。②低坝附近区域河床形态终将趋于稳定,上游和下游均形成相对稳定的曲面斜坡淤积体。③在不同的水流强度下推移质翻坝输移模式存在差异。对于一般水流强度工况,上游淤积体曲面斜坡表面泥沙颗粒以滚动或滑动模式起动,推移至接近坝顶位置时再跃移翻坝,后向下游输移;对于更高水流强度工况,后期的翻坝输沙模式可能发生显著转变,周期性边壁漩涡成为翻坝输沙的主要动力来源。 相似文献
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对于山区河流低坝而言，平时淤积在坝前的推移质粗沙可能会在洪水期集中翻越坝顶，形成高强度输沙。本文开展水槽试验，研究推移质粗沙自上游起动、推进、再翻越坝顶后向下游输移的过程，分析了输沙参数的变化特性及数理规律，描述了翻坝输沙模式及运动特征，揭示了输沙规律与河床形态之间的自然联系。取得如下认识：①输沙量随时间大致以幂函数规律增长。②低坝附近区域河床形态终将趋于稳定，上游和下游均形成相对稳定的曲面斜坡淤积体。③在不同的水流强度下推移质翻坝输移模式存在差异。对于一般水流强度工况，上游淤积体曲面斜坡表面泥沙颗粒以滚动或滑动模式起动，推移至接近坝顶位置时再跃移翻坝，后向下游输移；对于更高水流强度工况，后期的翻坝输沙模式可能发生显著转变，周期性边壁漩涡成为翻坝输沙的主要动力来源。 相似文献
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提出了水量百分数这一非均匀沙不平衡输沙的一个重要概念和有效工具。给出了水量百分数定义及其表达式。分析了它的3个特性:它能严格地将均匀沙挟沙能力迭加为非均匀沙挟沙能力;在强平衡条件下,水量百分数恰等于床沙级配;水量百分数就是能量百分数。进而利用水量百分数证明了床沙质、冲泻质与全沙具有相同的挟沙能力;证明了用它可以导出非均匀沙不平衡输沙时的挟沙能力组成以及挟沙能力级配和有效床沙级配表达式。 相似文献
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有效流量是天然河流某一时段内悬移质输沙量最大所对应的流量,可反映中、短期造床作用。根据监利水文站1991—2016年逐月流量、输沙量及悬移质级配,分析三峡建库前后流量频率及不同粒径组悬移质泥沙输移特性;运用理论分析法与分组频率法计算下荆江分组悬移质输沙量对应有效流量的大小、重现期、历时。研究成果表明:受来水来沙、水流挟沙能力以及床沙补给等因素影响,有效流量随泥沙粒径增大而减小。建库后,因河床冲刷各粒径组间有效流量偏差增大,0. 062 mm0. 125 mm粒径组泥沙有效流量重现期减小;细颗粒泥沙含沙量严重不饱和河道输送粗颗粒泥沙的能力相对较大,悬移质级配粗化;累积50%的泥沙输移需43%～82%的累积流量以及62%～90%的累积历时,且累积流量和累积历时随着泥沙粒径的增大而减小和缩短。研究三峡建库前后有效流量变化对分析冲刷条件下下荆江河段河床演变具有重要意义。 相似文献
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三峡水库蓄水运用后,城汉河段悬移质输沙量大幅度减少,推移质输沙量所占比重增加,造床作用日益凸显,故开展其推移质输沙率计算公式研究具有理论与实际意义。利用螺山和汉口水文站实测资料建立了推移质输沙率与流量之间的指数关系式,并据此推算了两站逐日推移质输沙率,结果表明:螺山站、汉口站输沙率均与流量的0.912 78次方成正比,多年（2009—2015年）平均推移质输沙量分别为137万t、152万t,主要集中在汛期。利用推移质实测资料对Engelund、Einstein、Yalin 3个公式进行了检验与修正,结果表明:修正前各公式计算结果比较分散,且与实测值偏差很大;修正后各公式计算精度显著提高,综合比较发现,修正后的Yalin公式精度最高,Engelund公式、指数关系式次之,Einstein公式精度相对较低。因此,修正后的Yalin公式更适合于城汉河段推移质输沙率计算,可用于该河段的演变分析与数学模型计算。 相似文献
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采用图像识别与推移质动态监测技术,开展基于双峰型非均匀推移质的系列水槽试验.通过引入反映床面粗糙度、粘性底层特性与颗粒非均匀度η(粗细比)的综合水流强度函数Ψb、特征弗劳德数Frb,系统研究了不同水流强度与床沙组成条件下的推移质输移特性以及颗粒非均匀度对输沙率的影响.通过对关键因子的辨识与量纲分析,提出了双峰型非均匀推移质输移模式,建立了基于近壁特征因子的水流强度Ψb与非均匀推移质输移强度Φ'的函数关系.对双峰型底沙输移机理的分析表明,非均匀沙的组成特征使得η成为影响Φ'的重要参量;正是细粒对粗粒的解怙作用对粗沙运动产生重要影响,使推移质输移率与颗粒非均匀度间呈现驼峰关系,峰值对应的粗细比ηc约为3∶7. 相似文献
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根据Boltzmann方程得到了床面附近运动颗粒的速度分布函数,将其在速度空间上积分,同时考虑床面颗粒起动概率的影响得到了床面颗粒的冲刷率函数。在此基础上,将该函数代入Einstein提出的推移质输沙基本模式中得到了基于动理学理论的推移质输沙公式。研究结果表明:① 公式计算结果与实验数据吻合,相对误差为10%~50%,可用于高强度和低强度输沙,能够较好地反映出床面附近运动的推移质颗粒的统计属性;② 得到的推移质输沙率理论表达式与经典推移质输沙公式十分接近,同时大大改善了Einstein公式在高强度输沙情况下的计算结果,这也说明了本文理论推导的正确性。 相似文献
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以往对沙波的研究多针对河流中下游、河口海岸段的细沙(粒径D<1 mm),而长江上游等卵砾石输移河段(D>2 mm)是否会出现沙波以及卵砾石沙波的临界条件,尚需开展进一步研究。通过长江上游九堆子、筲箕背等卵石滩的现场踏勘,观察到明显的天然沙波形态。采用中值粒径D50=1.8 mm和5.3 mm的天然沙和D50=4.8 mm的轻质沙进行水槽试验,成功模拟出砾石沙波。根据恒定均匀流条件下,产生沙波的比降、水位、流量等水流条件及试验沙的粒径、比重,提出卵砾石沙波的临界条件公式。结果表明,卵砾石沙波的临界条件可以用量纲起动功率w* 、R／D及比降S表达,并据此提出判别系数GDcr。当研究河段的卵砾石输移带上的水流条件满足判别系数GDcr,则可以判断能够产生沙波现象。 相似文献
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Bedform geometry is widely recognized to be a function of transport stage. Bedform aspect ratio (height/length) increases with transport stage, reaches a maximum, then decreases as bedforms washout to a plane bed. Bedform migration rates are also linked to bedform geometry, in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment. However, how bedform morphology (height, length and shape) and kinematics (translation and deformation) change with transport stage and suspension have not been examined. A series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated, mixed‐load dominated and suspended‐load dominated conditions. The results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage, against a continuously increasing bedform length. Bedform size variability increased with transport stage, leading to less uniform bedform fields at higher transport stage. Total translation‐related and deformation‐related sediment fluxes all increased with transport stage. However, the relative contribution to the total flux changed. At the bedload dominated stage, translation‐related and deformation‐related flux contributed equally to the total flux. As the transport stage increased, the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster. At the suspended‐load dominated transport stage, the deformation flux increased and the translation flux decreased as a fraction of the total load, approaching one and zero, respectively, as bedforms washed out to a plane bed. 相似文献
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The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data 总被引：1，自引：0，他引：1
The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and, at many sites, only suspended load data are available. Often the bedload fraction is estimated with ‘rule of thumb’ methods such as Maddock’s Table, which are inadequately field‐tested. Here, the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short‐term and long‐term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year‐to‐year averages. Using published simultaneous short‐term field measurements of bedload and suspended load transport rates, Maddock’s Table is reviewed and updated. Long‐term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel‐bed rivers, whereby in large catchments the bedload fraction is insignificant at ca 1%. For sand‐bed rivers, the bedload fraction may be substantial (30% to 50%) even for large catchments. However, available data are scarce and of varying quality. Long‐term partitioning varies widely among catchments and the available data are currently not sufficient to discriminate control parameters effectively. 相似文献
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Joanna C. Curran 《Sedimentary Geology》2007,202(3):572-580
It is generally accepted that a gravel-bed river will aggrade if the supply of sediment to the river is increased. In a series of flume experiments using constant discharge and gravel feed rate, sand feed rates were increased to 6.1 times that of gravel. The slope of the bed decreased with increasing sand supply, indicating that the increased sediment load could be transported at the same rate due to a decrease in shear stresses. These results extend previous experiments to a wider range of boundary conditions. A recent surface transport model is used to predict the changes in bed composition and transport using the same sediment supply composition and feed rates as in the laboratory experiments. This model reasonably predicts a decrease in the reference shear stresses of the sand and gravel fractions as the sand supply is increased. An increase in sand supply can increase the mobility of gravel fractions in the stream bed, which can lead to bed degradation and preferential evacuation of these sediments from the river. 相似文献