【作者】 胡勇；

【导师】 李义天； 邓金运；

【作者基本信息】 武汉大学 ， 水力学及河流动力学， 2023， 博士

【摘要】 三峡大坝建成后较为深刻地改变了水库下游河道的水沙输移特点,使得长江中游河道长期处于非平衡态的调整过程中,河道阻力特性也随之出现了响应性调整,进而引起水位的变化。然而,冲积河流滩槽格局复杂、河势宽浅,阻力的影响因素较多,基于室内试验提出的传统阻力公式适用性受到限制,难以解释阻力在不同地貌单元的变化规律。同时,天然洲滩植被实勘资料难以获取,植被演替特征和阻力变化特性不易确定,利用水沙数学模型研究水位变化规律尚存在较多的不确定性。因此,系统分析三峡水库蓄水后长江中游滩槽阻力响应特性并确定各地貌单元阻力的计算方法可更好的揭示地貌演化下水位调整的驱动机制,具有重要的理论意义与实践价值。本文基于水力学、泥沙运动力学、河床演变学的基本理论,结合大量实测水沙、地形、遥感数据,采用理论分析、实测资料分析、机器学习、植物辐射传输模型、一维和二维水沙数学模型等方法,在系统分析三峡工程影响下长江中游滩槽阻力变化特性及其对河道水位的影响规律的基础上,改进了大型冲积河流中枯水河槽阻力计算方法,改进了植物叶面积指数反演模型并基于此构建了洲滩植被阻力公式,研究了洲滩植被演替和阻力变化规律及其对水文泥沙过程响应特性,最后依据阻力叠加原理构建了考虑大型冲积河流不同地貌单元的阻力计算方法,并通过多种阻力计算模式的对比分析,采用水沙数学模型讨论了其对水位模拟的影响。主要结论如下:(1)揭示了三峡水库运用后长江中游同流量下“枯水位下降,洪水位不降”现象的主要原因是地形下切和河道阻力增大。不饱和水流对河道的剧烈冲刷是地形下切的驱动因子,导致水位下降;河底纵向起伏程度加剧、沙波发育、床沙粗化促使河床阻力增大,洲滩植被茂密生长使得洲滩植被阻力增大,导致水位抬升。但河床阻力增大的影响难以抵消掉地形下切的影响,使得同流量下枯水位下降;河床阻力和洲滩阻力的共同增大,能够与地形下切的影响大致相当,使得同流量下洪水位不降。(2)考虑中枯水河槽地貌特征,改进了大型冲积河流河槽阻力计算公式,研究了三峡水库蓄水后长江中游河槽阻力变化规律及其成因。综合考虑初级沙波、次级沙波、沙粒与水流间的相互作用,采用Van Rijn公式反映次级沙波和床沙对阻力贡献的基础上,引入随机波分析方法评估河道内初级沙波特征,改进了河床阻力计算公式。考虑水流从枯水河槽漫至中水低滩时由于过水断面突然增大而引起的流速损失,通过引入附加阻力,构建了低滩阻力计算公式。三峡水库蓄水后,长江中游河槽阻力呈增大趋势。河床阻力的增大主要来源于非饱和水流对河道沿程的不均匀冲刷而引起的初级沙波和次级沙波的发育伴以床沙颗粒的粗化,同时,逐渐变小的水流能态也一定程度上促使了次级沙波发育;低滩阻力的增大原因一方面是滩区床沙的粗化和沙波的发育增大了滩面阻力,另一方面是河道深宽比的增大提升了附加阻力。(3)采用改进后的基于机器学习和辐射传输的植物叶面积指数反演模型,提出了洲滩植被阻力计算公式,揭示了长江中游河道洲滩植被演替和阻力变化规律及其成因。三峡水库蓄水后,长江中游洲滩面积略有减小,植被面积明显增加,植物平均叶面积指数明显增大,多数洲滩植物群落呈现由滩尾向滩头、由滩缘向滩心的扩张趋势;河段尺度洲滩植被阻力增幅约为24%,典型洲滩植被阻力增幅约为17%。洲滩植被阻力增大源于河道内植被面积扩张,同时伴随着植被密度增大。洲滩冲淤幅度不大给植被扩张提供了基本的空间,而水流漫滩频率降低则是植被茂密生长的主要原因。(4)构建了考虑大型冲积河流滩槽地貌的综合阻力公式,应用于长河段一维水沙数学模型,为水位趋势预测提供技术支持。与已有阻力公式相比,综合考虑滩槽地貌的阻力公式对水位的模拟效果更好。该模式可在计算过程中根据河床冲淤调整、水流条件变化、洲滩植被演替对河道的糙率取值进行自动调整,能较为真实的反映河道阻力在较长时期的变化。更多还原

【Abstract】 The construction of the Three Gorges Dam(TGD)has profoundly changed the flow and sediment transport characteristics of the downstream channel of the reservoir,which makes the Middle Yangtze River(MYR)in the unbalanced adjustment process for a long time.Concurrently,the flow resistance characteristics of the channel undergo a responsive adaptation,culminating in the fluctuation of water levels.However,the intricate nature of the alluvial river morphology,coupled with the expansive and shallow river regime,introduces a multitude of factors influencing flow resistance.The applicability of conventional flow resistance equations,derived from laboratory experiments,is considerably constrained in such contexts.Consequently,elucidating the variation patterns of flow resistance across distinct geomorphological units proves to be a formidable challenge.Simultaneously,acquiring accurate field data pertaining to natural bar vegetation presents a considerable challenge.Determining the characteristics of vegetation succession and resistance alterations remains a complex endeavor.Consequently,substantial uncertainties persist when employing mathematical models of hydrological and sedimentary processes to investigate the governing principles of water level fluctuations.Hence,it is of paramount theoretical and practical importance to systematically examine the flow resistance response properties of the channel and bars within the MYR.Identifying the computational methodologies for the flow resistance of individual geomorphological units will facilitate a more comprehensive understanding of the driving mechanisms governing water level adjustments under the geomorphic evolution.Drawing upon the fundamental principles of hydraulics,sediment transport mechanics,and fluvial processes,this thesis integrates an extensive array of measured hydrological,topographical,and remote sensing data.Employing a diverse range of methodologies,including theoretical analysis,in situ data examination,machine learning,leaf area index(LAI)inversion model,as well as one-and two-dimensional hydro-sedimentary mathematical models,this thesis systematically scrutinizes the channel and bar resistance variation characteristics within the MYR,attributable to the influence of the TGD.Meanwhile,it explores the TGD’s subsequent impact on riverine water levels.This thesis advances the flow resistance calculation methodology for expansive alluvial river mid-low-flow channels,refines the LAI inversion model,and,based on these developments,constructs a flow resistance formula for high bar vegetation.It further investigates the variation patterns of bar vegetation and its response characteristics relative to flow and sediment processes.The efficacy of various flow resistance methods on water level simulations is assessed through comparative analysis,utilizing hydro-sedimentary mathematical models.The principal findings of this thesis can be summarized as follows:(1)It is revealed that the primary cause for the observed“decline in low water levels and slightly changed high water levels”in the MYR,despite maintaining equivalent discharge following the TGD operation,can be attributed to topographical alterations and augmented flow resistance.The intense scouring effect of unsaturated flow on the channel serves as the driving force behind topographical cutting,subsequently resulting in water level reductions.The augmented degree of longitudinal undulation in the riverbed,the development of dunes,and the coarsening of riverbed grains collectively contribute to an increased channel resistance.Concurrently,the dense proliferation of bar vegetation elevates the bar resistance,ultimately resulting in a rise of water levels.Nonetheless,the impact of the increased channel resistance is hard to offset the influence of topographical cutting,leading to a decline in low-flow water levels.The joint escalation of channel and bar resistances can be approximately commensurate with the effects of topographical cutting,thereby maintaining consistent high-flow water levels.(2)Given the distinct geomorphological features inherent to the mid-flow channel,we have enhanced the formulae used in determining the channel resistance within the MYR.This improved understanding forms the foundation upon which we investigate the shifts in channel resistance and the associated drivers in response to the operation of the TGD.Base on the comprehensive consideration of the interactions between primary dunes,secondary dunes,riverbed grains,and water flow,the Van Rijn formula is employed to quantify the contributions of secondary dunes and riverbed grains to the flow resistance.Furthermore,random wave analysis is introduced to evaluate the primary dune signatures within the channel,thereby enhancing the calculation formula for channel resistance.Considering the loss of flow velocity due to the abrupt expansion of the cross-section from channels to low bar regions,the low bar resistance calculation equation is established by incorporating additional resistance elements.Subsequent to the TGD operation,the channel resistance within the MYR has exhibited an upward trajectory.The escalation of channel resistance can be primarily attributed to the development of primary and secondary dunes,as well as the arming of riverbed grains,resulting from non-uniform scouring along the channel by unsaturated flow.Concurrently,the diminishing flow energy state also fosters the development of secondary dunes to a certain extent.The ascent in low bar resistance can be ascribed to two primary factors:firstly,the enlargement of riverbed grains coupled with the development of secondary dunes contribute to an increase in riverbed resistance;secondly,growth in the channel depth-to-width ratio contributes to the heightened additional resistance.(3)The enhanced LAI inversion model,utilizing machine learning and radiative transfer,is employed to propose the bar vegetation resistance formulae,hence unveiling the vegetation succession patterns and resistance variations and their underlying causes within the MYR.Following the TGD operation,the bar area within the MYR has undergone a modest decrease,while the vegetation area and average LAI of these bars experienced a significant rise.Moreover,the plant community on most river bars demonstrated a tendency to expand from the bar tail to the bar head and from the edge to the center.The bar vegetation resistance has experienced a notable surge,with an approximate 24%rise at the reach scale,and a typical bar vegetation resistance growth of about 17%.The escalation in bar vegetation resistance can be attributed to the expansion of the vegetation area within the channel,paired with an increase in vegetation density.The minimal degree of bar erosion provides fundamental space for vegetation proliferation,while the decrease in the bar submerged frequency predominantly drives the dense growth of vegetation.(4)A comprehensive flow resistance equation that accounts for the substantial river channel and bar topographies.This equation has been integrated into the one-dimensional hydro-sedimentary mathematical model within the extended river system,thereby offering indispensable technical assistance for future water-level predictions.In comparison to existing channel resistance formulas,the enhanced resistance formula that takes into consideration the channel and bar landform offers a more accurate simulation of water levels.This enhanced model is designed to automatically recalibrate the flow resistance based on adjustments in channel erosion and deposition,alterations in flow conditions,and the succession of bar vegetation throughout the calculation process.This approach provides a more authentic reflection of long-term changes in flow resistance.更多还原