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1.
动床阻力在冲积河道洪水演进与河床冲淤计算中具有十分重要的作用。黄河下游不同水沙条件下的床面形态变化较大,动床阻力变化规律十分复杂,因此需要研究动床阻力的计算方法。利用黄河下游花园口、高村、利津等7个水文站1958—1990年的686组实测数据,确定了影响动床阻力变化的关键水沙因子——水流弗劳德数（Fr）与相对水深（h/D50）,前者表示水流强度,后者表示床面相对粗糙度;建立了基于水流能态分区的动床阻力计算公式,并采用这些实测数据率定了公式中的相关参数;利用黄河下游各水文站1991—2016年的2 288组实测资料,进一步验证了公式的计算精度。计算结果表明:动床阻力的大小随弗劳德数或相对水深的增加而减小;基于水流能态分区动床阻力公式的计算精度明显高于未分区的公式及其他4个动床阻力公式,且决定系数（R2）总体接近0.80,说明水流强度与床面相对粗糙度对动床阻力影响十分显著。 相似文献
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植物的存在改变了河流水动力特性,造成独特的床面冲淤态势。利用实验室水槽模拟含淹没植物的河道,对床面形态和紊流统计特性参数进行测量,研究不同类型紊流作用下的床面冲淤特征以及床面起伏对流动的影响。结果表明:床面剪切紊流条件下,床面形态为马蹄坑-沙沟/沙脊与沙波复合分布,床面变形加剧了流速沿水深不均匀分布并促进水流动量交换;在自由剪切混合层紊流条件下,床面形态为植物根部马蹄形冲坑及其后方沙沟、沙脊交错分布,床面变形对流动的影响并不显著;“类二重紊流”条件下,床面形态同样表现为马蹄坑-沙沟/沙脊-沙波复合,床面变形促进植物层内部的水流动量交换、抑制紊动清扫,抑制植物层外部的动量交换、促进紊动喷射。 相似文献
3.
小浪底水库运用后，黄河下游河床冲刷粗化严重、水流阻力变化十分明显。为定量描述小浪底水库运用对水流阻力的影响，基于黄河下游水文站流速、河宽、床沙粒径、曼宁系数等实测数据分析，结合床面形态控制数理论及实测床面形态资料，建立了包含床面形态因子的动床阻力计算公式，计算精度得到了黄河下游水文站1 508组实测数据的验证。分析计算表明：黄河下游床沙粒径上段粗、下段细的特征更加突出，高村以上游荡型河段沙垄发育、动床阻力增加明显；高村以下河段，河床冲刷粗化程度明显减少，动床阻力变化相对较小，但河槽断面趋于窄深，洪水期岸壁阻力增大明显。小浪底水库运用促进了黄河下游床面形态的发育，增大了各河段的水流阻力。 相似文献
4.
明渠非恒定流流速分布及非恒定流作用下推移质运动分别是水力学和泥沙运动力学研究中的重要问题之一。近几十年来,国内外许多学者在这些领域做过很多探索,取得了一定进展,也存在较多问题。在总结相关研究成果的基础上,简述了明渠非恒定流试验方法、流速分布、床面切应力、非恒定流输沙与水流运动不同步现象、产生原因、以及非恒定流推移质输沙公式等方面的成果,指出了现有研究中存在的不足,提出了需要进一步研究的关键问题:应用先进的量测仪器,研究非恒定流作用下非均匀沙运动输移规律。 相似文献
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基于床面形态控制数理论,结合已有水槽试验研究成果,分析了床面形态控制数与不同床面形态的对应关系,提出了包括沙粒弗劳德数及相对水深(水深与泥沙中值粒径之比, 下同)的床面形态参数, 论述了简化床面形态控制数的计算方法及适用范围,研究了床面形态控制数与床面形态参数之间的相互关系及适用性;结合黄河下游床面形态运动特征,建立了床面形态判别方法。研究认为床面形态控制数能够反映高、低能态区的水流阻力特征;床面形态控制数与床面形态参数之间的关系,既适用于室内水槽试验也适用于天然河流;建立的黄河下游床面形态判别方法,得到了野外实测资料的初步检验,能够用于黄河下游不同能态区床面形态的判别。 相似文献
8.
洪水对洪泛区建筑会造成较大的危害,洪水荷载研究是洪泛区建筑脆弱性研究的基础。通过模型试验,研究不同水深流速的洪水对建筑表面的水流压力及其分布规律。试验在波流水槽中设置可调整出水口来控制作用于模型迎流面水流的深度和流速,模型分为2层,1层有洞口,2层无洞口,固定于出水口下游。结果表明:模型迎流面水流流速分布不均匀,水流压力从下至上逐渐减小;基于试验数据发现动水压力随着流速的增加而增加,且呈非线性关系,并据此提出了动水压力修正系数,改进了水流压力计算公式;水流压力和水深近似呈线性关系;洞口和边界会在一定程度上减小迎流面水流流速与水流压力。研究结果为洪泛区建筑的抗洪设计和抗洪能力评价提供依据。 相似文献
9.
冰湖溃决洪水的二维水动力学数值模拟 总被引：1，自引：0，他引：1
为分析冰湖溃决洪水过程,建立了一个二维非恒定流数值模型,可以定量的开展冰湖溃决洪水预测.数值模型的基本控制方程采用二维浅水水流方程,数值方法采用可以捕捉激波的WAF TVD二阶精度格式,并用非界面追踪的方法处理干湿边界.通过理想情况下一维解析解的验证和二维复杂地形情况的数值试验,证明数值模型具有较好的精度和可靠性,并且可以适应复杂地形.将数值模型应用于一个典型的冰湖溃决事件,定量地预估了该冰湖溃决洪水过程,结果表明:水流流速很高,床面剪切应力很大,可能诱发活跃的泥沙运动,形成泥石流灾害. 相似文献
10.
为探明降雨条件下沙黄土坡面水流水动力学特性,以流体力学和泥沙运动力学理论为依据,通过5个坡度和5个雨强组合条件下室内模拟降雨试验,系统研究了坡面水流水力参数的变化规律。结果表明,降雨条件下坡面薄层水流雷诺数均小于580,处于层流失稳区;水流流型随雨强和坡度的变化而发生转捩,当坡度较缓、雨强较小时,床面形态处于低能态区和过渡区,床面出现沙纹现象,水流宏观上呈缓流,反之,坡度较陡、雨强较大时,床面由沙纹和沙垄向动平床过渡,宏观上多呈急流;并根据薄层水流阻力组成特点,推导出沙黄土坡面薄层水流阻力计算公式。验证结果表明,该式误差较小,可为坡面侵蚀预报模型的构建提供参考。 相似文献
11.
Superimposed large-scale transverse bed forms are currently assigned to classes in addition to dunes. In modern rivers superimposition of the forms occurs only where the hydrological variables change rapidly with time over a large range. An analysis of the time-series of large-scale transverse forms in the Fraser River, Canada, indicates that the small forms, which here abruptly arise on the backs of large ones on the recession, are able during the flood to evolve back into the large forms. Hence superimposed large-scale transverse forms may all be classified hydrodynamically as dunes, though only the members of one order (the smallest in scale) may be expected to be active at any instant. The concept of the bed-form hierarchy remains valid but, in the context of unsteady natural flows, it should be recognized that the members of some classes of form may experience a discontinuous cyclical evolution. 相似文献
12.
结构面力学性质的定量鉴定 总被引：8，自引：0，他引：8
变形带力学性质的鉴定是地质力学研究中的先行基础步骤.近来出现一些新的概念和方法,可用以定量表征变形带的力学性质.天然变形带通常是一般剪切作用的产物,是纯剪切(共轴缩短或伸展)和简单剪切的组合.为了定量说明两者间的相对贡献,提出了运动学涡度(Wk)这一物理量,并简单地定义为cos υ.υ是主变形面内两特征方向间的夹角.纯剪切的υ=90°,Wk=0;简单剪切的υ=0°,Wk=1.一般剪切的υ介于0°和90°之间,Wk为0到1.运动学涡度符号的正负分别代表变形带的减薄和增厚.υ可通过极摩尔圆法求出.主压应力(σ1)方向与Wk的关系为Wk=sin2ξ.ξ是σ1与变形带法线间的夹角.因此,可用以确定变形带的Wk和力学性质.根据最大有效力矩准则,韧性变形带与主压应力(σ1)方向间的夹角为55°,可用以确定古应力轴的方向,并可能确定变形时差应力的大小. 相似文献
13.
Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA 总被引：3，自引：0，他引：3
SHARON L. GABEL 《Sedimentology》1993,40(2):237-269
The geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day-long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three-dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted. Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate. Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant. 相似文献
14.
J. R. L. ALLEN 《Sedimentology》1973,20(2):323-329
In nature where unsteady flows prevail, the characters of bed configurations lag, or differ in phase from, the flow conditions. This is demonstrated from quantitative data for dune beds in the Fraser River (Canada) and the Gironde Estuary (France), where the flow conditions change ahead of the bed form by as much as 1/4 period. Qualitative evidence for lag abounds, for instance, substantially unmodified dunes exposed during low water. The existence of lag appears to confuse attempts to establish from field data the relationships between bed-form properties and flow conditions and, at the same time, makes difficult the confident application to field situations of formulae based on steady-state experiments or theories. Unfamiliar types of field study and experiments are needed to increase understanding of the degree and precise significance of lag. 相似文献
15.
地面径流单位线在水文计算和水文预报工作中应用十分普遍,但当净雨时段较多时,传统的单位线推求方法存在工作量大、效率不高、过分依赖专业经验、计算过程复杂、模拟结果可能出现负值等问题。为此,本文引入源于动物群体迁移行为的自由搜索(Free Search,FS)算法,将FS单个动物每步探查行走的位置向量作为一条初始单位线,在FS算法迭代过程中再将所有的初始单位线转化为符合定义的单位线,从而利用动物群体的迁移行为推求出最优单位线。该方法的优点是概念清晰,操作简便,能够直接给出单位线的各时段流量值。三个典型实例模拟结果表明:利用FS推求单位线时,寻优一次即可得到满意结果,所得单位线不需要修正,并且能够有效克服锯齿现象,对地面径流量的模拟精度也较高。 相似文献
16.
The mathematical properties of the normalized diffusive hydrograph allow for easy determination of intrinsic basin characteristics.
These include lag times between storm events and peak flow, recession rate, and the total, temporally integrated flow volume,
all in terms of a single parameter, the basin time constant “b”. This simple function displays surprising fidelity to measured hydrographs of springs and hundreds of streams and small
rivers. We explain this fidelity by showing that the curvature of the theoretical hydrograph matches that of the natural hydrographs
better than several alternate models, and by demonstrating that the simple hydrograph function can be integrated over a range
of time constants (0 to b
max ) to represent the hierarchy of flow paths of varying lengths that exist in real watersheds. Surprisingly, the unwieldy analytical
results from this integration are almost numerically indistinguishable from a simple hydrograph using a single, suitably-weighted
average for the time constant. The peak flow times are shifted slightly. The accuracy with which the simple hydrograph approximates
the integrated results for hierarchies of hydrographs representing individual flow paths explains why the former can realistically
describe the discharge behaviors of complex natural watersheds. 相似文献
17.
This paper examines the transport of calcareous sand in unidirectional flow and its prediction through existing sediment transport models. A flume experiment of four sand samples collected on Oahu, Hawaii, provides 29 sets of sediment transport data in the bed-form and suspended transport stages. The measured transport data are compared with direct predictions from four energy-based transport models developed for siliceous particles. Corrections for the grain-size, fall velocity, and critical velocity of calcareous sand based on recent research are applied to the models and the results are compared with the direct calculations and measured data. The comparison illustrates the important role particle shape plays in the transport of calcareous sand. All four sediment transport models give consistent predictions and good agreement with the majority of the measured data. Two of the models respond positively to the corrections in both the bed-form and suspended transport stages indicating that such an approach may provide an interim solution for the transport of calcareous sand. 相似文献
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不同流量级下,分汊河段进口主流摆动导致江心洲头部的平面冲淤分布呈现出复杂变化特征。为探究江心洲头部冲淤与来流过程之间的响应关系,借助长江中下游多个分汊河段内水沙、地形资料,对江心洲头部水动力特征实施了数值模拟,并对不同流量级下的河床变形强度进行综合比较。研究表明:江心洲头部低滩的冲淤动力随着流量涨落呈现出明显的临界特性,冲淤过程中存在大、小两个临界流量Q1与Q2,在介于两个临界流量之间的流量级持续作用下,洲头低滩发生冲刷;流量大于Q1或小于Q2时,洲头低滩表现为淤积。由于不同流量级造成的冲淤效果不同,因而水文过程中各级流量的持续时间,是影响洲头低滩年内、年际变形的主要因素。 相似文献
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三峡工程运行后,长江中游荆江河段含沙量大幅度降低,次饱和水流冲刷河床使含沙量在沿程逐渐恢复,但其恢复特点由于沿程床沙组成差异而有所不同。本文基于Markov随机过程及泥沙起动理论,推导非均匀悬沙的泥沙落距表达式;结合悬移质扩散理论,修正非均匀悬沙恢复饱和系数公式。在此基础上,基于荆江河段实测输沙过程,提出考虑床沙组成影响的分组悬沙恢复饱和系数计算方法;该方法主要与泥沙粒径、悬浮指标、床沙组成及止滚概率、止悬概率等因素有关,无需考虑非饱和调整系数的影响。计算结果表明:①不考虑床沙组成时,沙市、监利站恢复饱和系数计算值均在区间0.12~0.27内变化,而考虑床沙组成时分别为0.000 3~0.171 8和0.003 5~0.157 9。②不考虑床沙组成时,分组悬沙恢复饱和系数计算值细沙>中沙>粗沙;考虑床沙组成时细沙 < 中沙 < 粗沙;除落水期外,沙市站粗沙的恢复速度均大于监利站。③沙市、监利站恢复饱和系数在各计算时段的变化过程对悬沙分组具有敏感性,其中洪水期恢复饱和系数大于枯水期、涨水期和落水期。采用河床变形方程反算分组悬沙恢复饱和系数,并与本文计算结果进行对比;通过比较分析,本文公式能够用于描述荆江河段分组悬沙恢复特性,可为三峡工程下游非均匀悬沙沿程恢复过程研究提供参考。 相似文献
20.
A series of wave‐flume experiments was conducted to closely look at characteristics of geometry and migration of wave‐generated ripples, with particular reference to the effect of velocity ‘hiatuses’ during which the near‐bed flow velocity becomes much smaller than the threshold of sediment movement. Three types of wave patterns were generated: two types for simulating waves with intervening velocity hiatuses; and regular waves for comparison purposes. In the former two types, two different wavelengths of water waves were generated alternately in the course of a wave test: the wave with a longer wavelength was set large enough to mobilize the bottom sediment, whereas the wave with a shorter wavelength was set too small to mobilize the sediment. The former two types were designed to be different in sequence of convexity and concavity of wave patterns. The sequence with the convex–concave longer wave and successive convex–concave shorter wave was described as a ‘zero‐up‐crossing’ wave pattern, and the inverse sequence was described as a ‘zero‐down‐crossing’ wave pattern. The ripples developed under oscillatory flow with intervening hiatuses manifested the following characteristics in geometry and migration. (i) The morphological characteristics of ripples, namely wavelength, height and the ripple steepness, are unaffected by the intervening hiatuses of velocity. (ii) The directions of ripple migration under the zero‐up‐crossing and zero‐down‐crossing wave patterns corresponded well with the directions of the flow immediately before onset of the hiatuses. (iii) The observation of sand particle movement on the ripple surface indicated that, under the zero‐up‐crossing waves, the velocity hiatus prevents the entrained sediment cloud from being thrown onshore, and thus the sediment grains thrown onshore are fewer than those thrown offshore. As a result of the sediment movement over one wave‐cycle, the net sediment transport is directed offshore under the zero‐up‐crossing wave pattern. (iv) The velocity of ripple migration was highly correlated with acceleration skewness. Under most of the zero‐up‐crossing (zero‐down‐crossing) wave patterns, flow acceleration skewed negative (positive) and ripples migrated offshore (onshore). 相似文献