泥沙输运问题是河口海岸地带面临的基础科学问题之一，也是发展海洋工程、港口建设过程中急需解决的实际问题。加强泥沙输运问题的基础研究，可为海洋的开发利用、海岸工程建设等提供理论基础和决策依据。波浪是近岸泥沙运动的主要动力之一，其流场很大程度上决定了泥沙的起动、悬浮和运移，准确描述波浪输沙过程对近岸工程建设具有十分重要的应用价值。波浪输沙是非定常的，近岸水波又往往具有很强的非线性，流场十分复杂，因此近岸波浪输沙是一个非常复杂的非定常过程。本文主要针对典型的浅水非线性波，通过理论方法和数值模型，研究近岸水波边界层的流动特征及其诱导的泥沙浓度和波挟沙力的变化规律。

首先，研究了典型浅水非线性波层流边界层特征。通过理论推导，提出了前倾波诱导的近底水质点轨道速度的一种新的级数表达式。基于此表达式，获得了前倾波层流边界层的理论解，并对速度、底部切应力、边界层厚度、底部切应力与自由外流速度之间的相位差随前倾度的变化规律进行了分析。同时，为了对非线性波边界层流场进行精细化模拟，采用大涡模拟方法，建立了振荡流数值模型。通过比较分析理论解与不同雷诺数下振荡流模型的速度和切应力及二者之间的定量误差，证明了理论解能够很好地预测层流和扰动层流的边界层特征，且可在一定程度上扩展到间歇湍流情形。

其次，研究了典型浅水非线性波湍流边界层的特征。通过引入新的变量变换和摩擦速度计算方法，推导出了前倾波湍流边界层理论解，经与实验数据比较，验证了理论解的准确性。同时，针对椭余波和前倾波，通过振荡边界层模型，研究了边界层底部切应力过程及其与外流速度的相位差随水波非线性参数的变化规律，获得了相位差与前倾度和非对称度的回归关系。基于此，对Nielsen提出的底部切应力模型进行修正，分别建立了前倾波和椭余波的底部切应力修正模型，得到实验数据的良好验证。

第三，对非线性波输沙过程进行了解析理论研究。针对非线性波输沙模型方程，提出了一个新的变量变换，因而可推导出常扩散系数和线性扩散系数及采用参考浓度和pickup函数确定底部边界条件的四种组合情形的理论解。通过与不同情形的实验数据比较发现，四种情形的理论解均能够很好描述近底附近周期平均泥沙浓度的垂向分布；随着离底床距离的增加，相比常扩散系数，基于线性扩散系数的泥沙浓度理论解在振幅和相位上更符合实验数据；相比参考浓度，采用pickup函数确定底部边界条件时，泥沙浓度周期变化的相位特征更符合实验结果。

最后，将数值波浪水槽与泥沙模型耦合，建立了非线性波悬沙输运模型，对非线性波输沙过程进行模拟，并将修正的底部切应力模型应用到悬沙输运模型中，建立了基于切应力修正的悬沙输运模型，通过与实验比较，证明此模型能够很好预测泥沙浓度的分布，且节省了大量计算时间和计算资源。基于此模型，研究了非线性波诱导的泥沙浓度和挟沙力随波参数的变化规律，发现波高相同时，不同高度处周期平均浓度随波高的增加而明显增加，随周期的增加亦增加，但增幅逐渐减小；挟沙力随波高和周期的增加而增加，但其增幅随周期增大而逐渐减小；一个波周期内不同阶段水体挟带的泥沙量随波高和周期的变化趋势与挟沙力相似，波峰阶段明显高于波谷阶段，且随水波非线性的增加，差异更显著。同时，建立了考虑底床形态的悬沙输运模型，比较沙纹底床和平床泥沙浓度的分布，发现沙纹能加速底部泥沙的掺混，但随离开床面距离的增加，沙纹的影响逐渐消失。

Sediment transportation is one of the basic issues in coastal areas. It is also a pragmatic task that should be urgently tackled for the development of coastal engineering, such as navigation channel, port and oversea bridge construction. The water wave is one of the main dynamic factors. Its flow field determines sediment incipience, suspension and transportation, so it is of great significance to describe the wave-induced sediment transport process accurately in order to serve for the coastal engineering construction well. Since the wave-induced sediment transport is unsteady and the nearshore wave is generally nonlinear with complex flow field, the nearshore sediment transport is very difficult to predict. Therefore, considering the typical nonlinear waves, the kinematic and dynamic characteristics of the boundary layer and the distribution of sediment concentration as well as sediment carrying capacity are studied through theoretical analysis and numerical models in this thesis.

Firstly, the characteristics of laminar boundary layer of typical nonlinear waves in shallow water are studied. A novel series expression of the bottom orbital velocity beneath forward-leaning wave is worked out theoretically, based on which the theoretical solution of the laminar boundary layer beneath forward-leaning waves is obtained. Then using these theoretical results, the velocity, the bottom shear stress, the boundary layer thickness, and the phase lag between the free-stream velocity and bottom shear stress are analyzed and discussed. Meanwhile, the large eddy simulation approach is employed to establish the numerical model of oscillating flow in order to simulate the boundary layer flow field of nonlinear wave in detail. Comparisons with detailed quantitative error analysis of velocity and bottom shear stress obtained by the present theory and the numerical results with increasing Reynolds numbers demonstrate that the theoretical solution is capable of accurately predicting the properties of the boundary layer in laminar and disturbing laminar flow regimes, and can be roughly extended to cover part of the intermittent turbulent flow regimes.

Secondly, the characteristics of turbulent boundary layer of typical nonlinear waves in shallow water are analyzed. By introducing a new variable transfortmation and a new model of friction velocity, the theoretical solution of the turbulent boundary layer beneath forward-leaning waves is derived and the theoretical solution is tested by comparing with the experimental data. Additionally, the variation of the phase difference between the free-stream velocity and the bottom shear stress with different wave parameters is analyzed based on the oscillatory flow boundary layer model and the phase difference is then formulated as a function of the forward-leaning degree or asymmetric degree of nonlinear waves. Furthermore, the formulated phase difference is used to modify the bottom shear stress under the forward-leaning waves and the cnoidal waves, which is well validated by experiment data.

Thirdly, the analytical theory of sediment transport induced by nonlinear waves is developed. A new variable transformation is proposed to help solving the sediment transport equation in case of nonlinear waves. Consequently, theoretical solutions are obtained for four cases with different combination of the constant or linear varying sediment diffusion coefficients and the bottom conditions of sediment transport determined by the reference concentration or pickup function. Comparisons with experiment data reveal evidently that the theoretical results under the four different conditions are capable of describing the vertical profile of periodic averaged suspended sediment concentration near the bottom, and the theoretical solutions based on linear varying diffusion coefficient are more accurate to predict the profile of suspended sediment concentration away from the bottom compared with those obtained by constant diffusion coefficient, as well as the amplitude and phase of the periodic variation of sediment concentration. In addition, the solutions with the bottom condition determined by pickup function rather than the reference concentration can provide more precise results which show a good agreement with experiment in phase of periodic variation of sediment concentration.

Finally, by coupling the numerical wave flume with the sediment transport model, a suspended sediment transport model under nonlinear waves is established to simulate the nonlinear wave-induced sediment transport process, in which the modified shear stress model obtained above is introduced. Comparisons with experiment data demonstrate that the distribution of seiment concentration can be accurately predicted by this model with merits of low computational cost and less computing resources required. Based on this model, the variation of sediment concentration and sediment carrying capacity of nonlinear waves with different wave parameters is studied. It is found that the periodic average sediment concentration at different heights increases with the increase of wave height and period when the water depth is unchanged, but the growth rate with wave period decreases gradually. Besides, the sediment carrying capacity of waves increases while the wave height and wave period are increased, but the growth rate with wave period gradually decreases. Moreover, the sediment carried by water at different stages with wave height and period show a similar changing trend to that of sediment carrying capacity of waves. In particular, the sediment carried by water at peak stage is higher than that at trough stage, and the difference is more notable with the increase of wave nonlinearity. Furthermore, the suspended sediment model over sand ripple bed is established to research the effect of sand ripple on the variation of sediment concentration. It is found that sand ripple can accelerate the mixing of bottom sediment with upper water, albeit the influence of the sand ripple gradually diminishes with the increase of the distance from the bed