论文题目：复式河槽水流阻力及泥沙输移特性研究
作者简介：杨克君，男，1973年7月出生，2003年9月师从于四川大学曹叔尤教授，于2006年6月获博士学位。
中文摘要
由冲积河流主槽和河漫滩组成的复式河槽在调节洪水、削减洪峰、贮存泥沙等方面不同于单一河槽。复式河槽随着流量增大、水位抬高，当水位超过滩地高程时洪水漫滩，产生水流结构、水流阻力、泥沙输移和河床演变的一系列复杂变化。研究复式河槽水流泥沙的复杂行为可以丰富和完善水力学、泥沙运动力学和河床演变学的研究内容，有重要的学术意义。其成果对防洪工程、航道整治、城市生态水利建设及河滩地合理开发利用等国民经济和社会发展中的重要问题，有着广阔的应用情景。本文系统地研究与探讨了复式河槽水流阻力、动量交换机理、过流能力、植被作用下的复式河槽水流特性以及全动床复式河槽水流阻力及泥沙输移特性。其主要研究内容为：
1．复式河槽水流阻力研究
(1) 复式河槽阻力系数研究。分析与讨论了曼宁系数和达西-韦斯巴赫阻力系数。通过分析英国科学工程研究协会洪水水槽设施 (SERC-FCF) 的大量的试验资料，建立了综合阻力系数、局部阻力系数和断面形态、床面粗糙度之间的关系，并指出单一河槽法与断面分割法不能准确估算复式河槽过流能力，其原因在于前者没有考虑阻力系数随水深的变化特性，而后者忽略了因滩槽动量交换而产生的附加阻力。分析表明，复式河槽阻力系数是雷诺数的复杂函数，但这种函数关系与单一河槽是不同的。
(2) 复式河槽综合糙率计算方法比较与分析。系统地总结了推求复式河槽综合糙率的各种典型方法。根据是否考虑动量交换及所需的断面几何参数，综合糙率计算方法可以简单地分为若干类。运用大量的复式河槽水槽试验资料和野外实测资料，验证了上述各类方法的有效性，并讨论了断面分割类型对综合糙率计算的影响。推求复式河槽综合糙率时，每类方法都将带来一定的误差。在各类方法中，Lotter方法类与断面分割类型密切相关，而 Einstein-Banks方法类与断面分割类型无关。通过对各类方法比较和分析，发现这些方法估算复式河槽综合糙率误差大，并对这些方法产生误差的原因进行了分析。
2．复式河槽动量交换机理研究
(1) 复式河槽动能损失强度分析。为了反映动能损失的强度，针对恒定、均匀、紊动水流，提出了横向动能校正系数和动能损失率的概念。通过分析复式河槽动能损失的机理发现，复式河槽横向动能校正系数大于1和动能损失率大于0。通过分析英国SERC-FCF系列试验成果发现，横向动能校正系数和动能损失率与复式断面形态有关。动能损失强度随着主槽边坡系数的增大而减弱，随着滩槽宽度比的增大而增强。对称复式河槽的动能损失强度大于非对称的复式河槽。对于所有断面形态，动能损失强度先随相对水深的增加而增强，后又随着的相对水深的增加而减弱，最终表现出单一河槽的特性。
(2) 复式河槽动量输运系数研究。从Boussinesq假定出发，并结合滩槽交互区流速分布特点，导出复式河槽动量输运系数的表达式；接着根据力的平衡，导出垂向表观剪切应力的表达式；运用SERC-FCF大量的水槽实验成果，分析了动量输运系数随相对水深及滩槽宽度比的变化关系；最后根据获得的动量输运系数关系，利用刘沛清方法计算复式河槽过流能力。计算结果表明：本文中的动量输送系数随相对水深及滩槽宽度比的变化关系是可行的。
3．复式河槽过流能力研究
(1) 复式河槽过流能力计算方法比较与分析。系统地总结了复式河槽流量计算的各种方法，运用这些方法分别计算整个复式断面的流量和滩槽流量分配，并利用SERC-FCF水槽实验成果加以比较。通过比较发现：计算流量时，断面垂直分割法、单一河槽法和等速剖分法误差都很大；而其它方法精度都比较高，精度由高到低依次为COHM方法、SKM方法、断面倾斜分割法类型 1 (IDMT1)、动量传递法 (MTM)、谢汉祥法 (XHXM)、断面叠加法 (SCSM)、童汉毅法 (THYM)。同时，运用流量计算精度比较高的方法计算流量分配。具体对某一断面形态而言，很难准确说哪种方法的精度最高，但综合所有系列总体而言，SKM方法最佳，其次是IDMT1和THYM，再次是COHM、MTM、XHXM，最后是SCSM。本文还分析了二次流对过流能力、流量分配及水流阻力的影响。二次流对过流能力的影响不大，但它对流量分配、垂线平均流速及床面剪切应力的横向分布、滩槽断面平均流速、滩槽床面平均剪切应力的影响却十分明显。通过对各种方法优劣地详细分析，建议：在计算天然复式河槽过流能力时，COHM方法是一种值得推荐的好方法。
(2) 复式河槽过流能力的系统动力学模型研究。通过分析SERC-FCF水槽实验成果，发现滩槽 Darcy-Weisbach阻力系数比值随着相对水深的增加而减小。主槽阻力系数随着相对水深的变化而变化，呈抛物线分布。根据获得的阻力系数关系，运用系统动力学方法，建立了洪水漫滩后，复式河槽过流能力的系统动力学模型。计算结果表明：该模型能确定水位流量关系，其模拟值与实测值吻合较好。
4．植被作用下的复式河槽水流特性研究
通过水槽试验，探讨了不同滩地植物 (乔木、灌木和野草) 对复式河槽流速分布的影响。试验时，选塑料吸管、鸭毛和塑料大草分别模拟乔木、灌木和野草。同时，考虑了流量、床面底坡对流速分布的影响。试验结果表明，滩地未种树的复式河槽在大的相对水深时，流速满足对数分布；滩地种树后，主槽流速增大，流速分布复杂，滩地流速减小，呈 S 形分布，不同植物的S 形分布是有差别的。这种S 型分布将水流划分为三区的复杂行为，每区的范围与水深、垂线位置和植物类型有关。床面坡度对流速分布的影响非常明显。纵向、横向和垂向三个方向的脉动流速基本上满足正态分布；时均流速与采样时间的长短有关；横向动量交换比垂向动量交换强，横向动量基本上是主槽向滩地传递；不同的滩地植物对水流紊动强度的影响是不同的。滩地种植植物后，水流的紊动强度增强，纵向和垂向的紊动强度相当，都服从S型分布。滩地种植植物前后雷诺应力的空间分布差异很大，不同的滩地植物对雷诺应力的空间分布十分明显。在主槽边坡区，雷诺应力变化复杂。由于受植物的影响，tyx 与tzx 基本上都为负，明显不同于滩地未种植植物的情况。
5．全动床复式河槽水流阻力及泥沙输移特性研究
通过全动床复式河槽水槽试验，探讨了河道完全粗化后能量损失、阻力系数的沿程变化，推移质输沙率的变化特性，床沙中值粒径的横向变化与沿程变化等，并探讨了运用仙农熵的概念对泥沙相关特性的模拟。试验结果表明，对于冲积河流，如果以水面坡度代替能坡，是会产生误差的。阻力系数与能坡的沿程变化趋势是相似的，而这种沿程变化趋势又与床沙代表粒径变化趋势总体相一致。河道急剧展宽发生在清水作用初期，而且作用时间很短。在特定的试验条件下，全动床复式河槽滩岸侵蚀速率在空间上变化趋势为，越往下游，同流量下的滩岸侵蚀速度越小。与初始河道相比，主槽床面沿程始终处于淤积状态，而主槽两侧均处于冲刷状态。冲刷或淤积面积均有沿程减小的趋势，这与流速总体沿程减小相一致。尽管初始断面是对称的，床沙初始级配是相同的，但主槽两侧断面冲刷面积并不相等。运用仙农熵的概念与理论，推导了清水作用下粗化过程中推移质输沙率公式。无论是输沙率单调递增段还是单调递减段，推移质输沙率公式都具有相同的表达形式。据此，导出了随机变量X表达式的一般形式，并从理论上分析了参数ki的取值与曲线 凹凸的关系。若曲线上凹，ki取负值；反之，若曲线上凸，ki取正值。无论是归槽水流还是漫滩水流，推移质中值粒径均有先随时间大幅度增大的趋势，后随着时间的增加中值粒径增加的幅度减小，最终将趋于某一恒定的值。这种变化特性可用X一般形式的表达式进行预测。全动床复式河槽推移质运动不仅存在纵向的沿程分选，还存在横向的沿断面分选。
关键词：复式河槽；水流阻力；动量交换；过流能力；植被；泥沙输移
A compound channel comprising the main channel and floodplains exists in natural rivers widely, especially in alluvial streams. It differs from a single channel in adjusting flood, cutting flood peak, transporting sediment etc. When water in the main channel flows in an out-of-bank condition and on to the adjoining floodplain, owing to abrupt change of the shape of cross section and heterogeneous boundary roughness, there are a bank of vertical vortices along the vertical interface between the main channel and its floodplain, which will result in complex variations in flow structure, flow resistance, sediment transport and fluvial processes. The study of the complex behavior of flow movement and sediment transport in compound channels is profoundly important for the basic theory of hydraulics, mechanics of sediment transport and river dynamics. The research results benefit to flood control, channel training, floodplain exploitation etc. The thesis will systematacially investigates flow resistance, mechanism of momentum transfer, conveyance capacity in non-vegetated compound channels, flow structure in a compound channels with vegetated floodplains, flow resistance and sediment transport in a self-formed one. The main contents of the thesis are as follows:
1. Flow resistance in compound channels
(1) To study resistance coefficients in compound channels. This thesis analyzes and discusses the effect of cross-sectional shape on Manning’s and Darcy-Weisbach resistance coefficients. By analyzing the experimental data from Science and Engineering Research Council Flood Channel Facility (SERC-FCF), the relationships between overall, zonal, local resistance coefficients and a wide range of geometries and different roughness between the main channels and its associated floodplains have been established. Moreover, the reason why the conventional methods can not assess the conveyance capacity of compound channels is analyzed. The reason is that single channel method doesn’t consider the fact the composite roughness varies with flow depth and cross-sectional division method ignores the extra resistance produced due to the momentum transfer between the main channel and floodplains, in assessing the conveyance capacity in compound channels. According to the experimental results of SERC-FCF, it is shown that the overall Darcy-Weisbach coefficient for a compound channel is the function of Reynolds number, but the function relationship is different from that for a single channel.
(2) To compare and analyze the method for predicting composite roughness in compound channels. This thesis systematically sums up all kinds of different representative methods for predicting composite roughness. According to the hydraulic parameter required and whether the momentum transfer is considered or not, they can be simply classified into several groups. A vast number of experimental data and field data for compound channels are applied to check the validity of the mentioned methods. Meanwhile, the effect of the division type of cross section on the computation of composite roughness is analyzed. Any method for predicting composite roughness will result in error. Among them, Lotter method is closely related to the division type of cross section, while Einstein-Banks method is not related to that. By comparing and analyzing the above methods, it is pointed out the methods are not fit to assess the composite roughness in compound channels. The reasons why the methods result in errors are analyzed.
2. Mechanism of momentum transfer in compound channels
(1) To undertake the analysis of kinetic energy loss intensity in compound channels. The intense momentum transfer on the vertical interface between the main channel and floodplains in a compound channel, makes its conveyance capacity decrease. To reflect the kinetic energy loss intensity in a compound channel, two new concepts, transverse kinetic energy correction coefficient (TKECC) and kinetic energy loss rate (KELR) are put forward for steady, uniform and turbulent flow in the thesis. By the analysis of the mechanism of kinetic energy loss in compound channels, a conclusion is drawn that TKECC is larger than 1 and KELR is larger than 0 in compound channels. By analyzing the experimental data from SERC-FCF, it is found that TKECC and KELR are both related to shapes of cross section. Kinetic energy loss becomes weaker with main channel side slope factor increasing and becomes stronger with the ratio of main channel and floodplain widths increasing. Kinetic energy loss in a symmetric compound channel is stronger than that in an asymmetric one. For all the shapes of cross section, kinetic energy loss increases with the relative depth increasing. After it reaches the largest value, it decreases with the relative depth increasing, and the compound channel ultimately shows a characteristic of single channels.
(2) To investigate momentum transfer coefficient in compound channels. Momentum transfer coefficient plays an important role in computing conveyance, apparent shear stress on the vertical interface between the main channel and floodplain, mean boundary shear stress on floodplains or in the main channel respectively. In this thesis, beginning with Boussinesq’s assumption and combining the characteristics of velocity distribution in the interacting region, the expression of momentum transfer coefficient is derived theoretically; on the basis of force balance, the expression of vertical apparent shear stress is obtained; applying experimental data from SERC-FCF, the variation of momentum transfer coefficient with relative depth and the ratio of floodplain and main channel width, is analyzed; basing on the momentum transfer coefficient relationship obtained and applying Liu Peiqing method, the conveyance capacity in compound channel is calculated. The computed results show the momentum transfer coefficient relationship obtained is viable.
3. Conveyance capacity in compound channels
(1) To compare and analyze the method for predicting the conveyance capacity in compound channels. For a compound channel, when water in a main channel flows in an overbank manner and inundates its floodplains, if the conveyance capacity is directly calculated by Manning equation, it results in a deal of error. All kind of methods for predicting discharge are systemically summarized in this thesis and are applied to compute the cross-sectional discharge and the distribution of discharge in the main channel and floodplains, respectively. By comparing with different series of experimental data from SERC-FCF, it is found that the discharge error is large by cross-sectional vertical division method (VDM), single channel method (SCM) and equivalent velocity division method (EVDM). Otherwise, the calculation accuracy is high by the other methods. According to the order from high accuracy to low, the methods are，in turn， channel coherence method (COHM), Shiono and Knight method (SKM), inclined division method type 1 (IDMT1), momentum transfer method (MTM), Xie Hanxiang method (XHXM), superposing cross section method (SCSM), Tong Hanyi Method. In the meantime, the methods with high-calculation accuracy are used to calculate the discharge distribution. It is found that for certain given cross section, it is difficult to determine which method has the highest discharge distribution’s accuracy, but for all the series, as a whole, the highest is SKM, the next is IDMT1 and THYM, and the rest is COHM，MTM，XHXM and SCSM. In addition, the thesis analyses the effect of secondary flow on conveyance capacity, discharge distribution and flow resistance. The results show that it has a very small influence on conveyance capacity, but it does distinct influences on discharge distribution, lateral distributions of depth mean velocity and boundary shear stress, the mean velocities and boundary shear stresses of main channel and floodplains, respectively. By analyzing the merit and demerit of the above mentioned methods, it is pointed out that COHM is a good method in calculating discharge.
(2) To establish the system dynamics model of conveyance capacity in compound channels. By analyzing a vast number of experimental data from SERC-FCF, it is found that the ratio of the Darcy-Weisbach resistance coefficients between the floodplain and main channel decreases with the relative depth increasing. That in the main channel varies with the relative depth and follows the paraboloidal distribution. On the basis of the resistance coefficient relationships obtained, applying system dynamics method, the thesis establishes the system dynamics model of conveyance capacity when water flows in an out-of-bank manner and onto the adjoining floodplain. The model gives the relationship between stage and discharge. There is a good agreement with modelled and experimental values. The absolute values of the relative errors are very little.
4. Flow patterns in compound channels with vegetated floodplains
The thesis experimentally studies the velocity distribution of flows over different types of vegetations such as arbors, shrubs and grass. For vegetations on the floodplain, the thesis chooses plastic grass, duck feathers and plastic straws as model grass, model shrubs and model arbors, respectively. ADV is used to measure the local flow velocities for the cases of different types of vegetation on the floodplain, discharges and flume slopes. All measured streamwise velocities follow the logarithmic distribution for the case of non-vegetated floodplain, and obey S-shaped distribution for vegetated floodplain. The S-shaped distribution divides the flow into three regions. The range of every region is related to flow depth, lateral location and vegetation type. For different vegetation, the S-shaped distribution is different. In the meantime, it is found that the velocity in the main channel increases and that on floodplain decreases after the floodplain is vegetated. The increasing or decreasing degree is related to vegetation type. Furthermore, the influences of flume slope on local velocity distribution for different types of vegetation are distinct. For all cases, the fluctuating velocity follows approximately a normal distribution. The time-averaged velocity is related to the sampling duration. After the floodplain is vegetated, the lateral exchange of momentum becomes intensive. On the whole, the momentum transferrs from the main channel to its associated floodplain. In addition, the effects of different types of vegetation on turbulence intensity are distinct. The turbulence intensity increases after the floodplain is vegetated. The lateral and vertical ones are proximately equal and follow an S-shaped distribution. After the floodplain is vegetated, the distribution of Reynolds stress distinctly differs from that for non-vegetated case. Meanwhile, for different types of vegetation, the distributions are largely different. In the main channel side-slope zone, the Reynolds stresses become particularly complex. The effect of vegetation makes both tyx and tzx negative as a whole, differing from that of the non-vegetated-floodplain compound case.
5. Flow resistance and sediment transport in a self-formed compound channels
The thesis experimentally studies flow resistance and sediment transport for inbank and overbank flows in a self-formed channel with complex cross section, including the variation of energy loss and resistance coefficients along the channel, the streamwise and lateral variations of median diameter of bed material after armoring is completed, the variation of bed load discharge with time during the armoring process. By analyzing the experimental data, it is found that water surface slope is largely different from energy slope for alluvial channels. Energy loss varies along the channel. The variation trends of resistance coefficient and energy slope are similar, as a whole, corresponding to the variation of representative diameter of bed material. Under the action of clear water, the river becomes wide quickly at the begging of the experiment. Then, the rate of river width adjustment becomes smaller. For the given experiment, retreat rate of river bank becomes small along the channel for the same discharge. Comparing with the original cross section, the main channel deposits while its two sides erode. The erosion or deposition areas decrease, corresponding to the decrease of mean velocity of cross section along the channel. It is surprised that the erosion areas of the two sides of the main channel are not equal although the original cross section is symmetric and the original size distribution is the same. Applying the concept of Shannon’s entropy, the equation of bed load rate is derived during the armoring process. Whether the bed laod rate increases or decreases with time, the structure and form of the equations are both the same. According to the characteristics, the general form of stochastic variable, X, is obtained. By theoretic analysis, the relationship between the parameter, ki, and the shape of Curve is established. If the curve is concave, ki is less than zero; otherwise, ki larger than zero. For inbank and overbank flows, the median diameter of bed load increases with time. Finally, it will approach to certain constant. The variation trend may be modeled by the expression of X. For the type of channel, during the course of armoring process, not only sorting along the channel but also across the channel exists.
Key words: Compound channels; Flow resistance; Momentum transfer; Conveyance capacity; Vegetation
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