深水截流的堤头坍塌机理与稳定性研究及工程应用

以往，人们对水利水电工程截流难度的衡量，通常采用流速、落差等指标。其后， 有关专家学者也试图进行截流难度的综合指标研究，并提出了最大龙口水流能量或 最大龙口水流单宽能量的概念，导出了有关公式。再后，由于没有大型截流工程特别 是深水截流工程验证，使该截流难度公式一直沿用至今，直到三峡工程大江深水截流 特例的出现，使得对截流难度的认识有了新的突破。究竟深水截流工程有着怎样的特 征？深水截流最关键的难点表现在哪些方面？如何建立理论模型并得到问题的求解？ 采取怎样的工程措施和对策攻克上述难点等已成为该领域亟待解决的重大课题。 深水截流的特征主要表现在：由于堤高坡长，抛填料不能一次滚落到坡底；新的 稳定坡度需要经过多次坍塌才能形成；堰体沉陷引发堤头失稳等。 围绕截流龙口处的水力参数获得以便为截堤堤头坍塌数学模型计算提供必要的 已知条件，本文建立了河道截流水流数学模型，并给出其求解算法及工程计算实例。 建立截流龙口附近的水流数学模型，多采用平面二维模型。由于有限体积法简单、灵 活、物理意义明确，且大多采用守恒型离散格式，求解时不需要进行线性化，容易合理 处理非线性作用较强的问题，故本文采用有限体积法。对于紊动粘性系数的计算方 法，本文采用零方程模型。 平面二维数学模型的基本方程为连续方程、X方向（顺流向）、Y方向（垂直流向） 动量守恒方程及相应的湍流封闭方程。运用其通用微分方程对于不同的φ值，只要 重复调用该程序，并赋以Г和S适当的表达式以及适当的初始条件和边界条件便可 以求解。在通用微方程的求解中，采用Simpler算法进行编程和计算。 工程计算实例选取了长江三峡工程大江截流项目。所给的计算条件包括计算范 围及地形资料、围堰■堤进占各阶段情况、龙口位置及水流条件等。计算得出了龙口 各项水力特征参数包括：龙口上、下游水位、■堤轴线水位、落差、导流明渠分流量、分 流比及龙口中线平均流速等。数学模型计算结果与水工模型试验结果相比基本一致， 其中导流明渠分流量除个别情况外，相差小于5％。在三峡工程大江截流胜利合龙 后，又将数学模型计算结果与现场实测资料进行了比较，表明两者结果基本一致，实 测明渠分流比与计算分流比的误差在6％以内。 本研究着重进行了深水截流堤头坍塌机理与稳定性研究，并给出了堤头坍塌计 算实例和预报实例，在此基础上，进行了堤头坍塌规律分析。对于■堤稳定的理论分 析，进行了无水流冲刷和有水流冲刷条件下堤头坍塌研究。在无水流冲刷条件下，导 摘 要出了完全坍塌状况下的堤顶坍塌长度、高度和体积计算式，随之进一步导出了坍塌体积变化率关系和临界水深，揭示了坍塌规模决定于水深的重要规律。同时，还得出了不完全坍塌和有水流冲刷条件的坍塌长度、高度、体积随临界坡度、凸体高度以及水深的变化关系，并将这些关系点给成图表，以便工程人员实施中直观查用。 作为堤头坍塌计算及预报的实际例子，选取了长江三峡工程大江截流项目。通过计算，给出了不同口门宽度时的冲刷深度、堤顶坍塌长度、坍塌体积等系列结果，并将这些结果绘成图表。计算结果表明，堤头最大坍塌发生在口门宽80～100m的范围，该范围对应于从最深处向浅处转化的龙口水深部位。 根据以上分析研究，本文提出了以“坍塌强度”作为衡量截流难度的新指标，并提出深水截流最突出的难点是由水深引起的大规模堤头坍塌问题以及由于大规模提头坍塌而引起的堤头进占安全问题。 围绕防止堤头坍塌工程措施的研究，本文比较系统地提出了如预平抛垫底等四个方面的重要措施。预平抛垫底即是在战堤进占前，向战堤基础深槽部位从水面抛投砂卵石料或块石等物料，以使战堤底部河床高程整体抬高的工程措施，它是一项防止堤头坍塌的关键措施。作为防止堤头坍塌重要工程措施的防护性进占，其主要有浮桥超前抛投、堤头挑流和堰体尾随抛投、小粒径材料抛投、高强度进占抛投以及变换堤头抛投方法抛投等。防止堤头坍塌的另一有效工程措施是诱导坍塌。它是在堤头坡度陡于填料的自然稳定坡并接近于临界坡度时，所实施的人为改变提头坡度的措施。诱导坍塌的方法主要包括机械扰动、流体扰动和水下爆破三种。最后是开展坍塌的预报与预防，通过预测坍塌的可能发生时刻，提前进人防坍塌状态，以避兔在坍塌过程中蒙受损失。坍塌的预报可采用理论模型计算测报、实际观测资料分析测报和经验方法判断测报等方法。坍塌的预防主要是指一系列的组织和保障措施，包括组织机构
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Difficulty of river closure used to be measured by velocity of flow, water drop and the like. Later, some experts and scholars concerned tried to perform study on comprehensive index to measure difficulty of river closure,, and advanced concepts of maximum flow energy in closure gap or maximum flow energy per unit width in closure gap and the relating formula was derived, which has been used to up now due to lack of examples of large-scale closure works, especially deep-water closure works, to verify it. Knowledge of closure difficulty has not been broken through until appearance of special example of deep-water closure of the main channel for the Three Gorges Project. What are characteristics of deep-water closure works? What are key difficulties in deep-water closure? How to establish theoretical model and find out resolution? What are the engineering measures and countermeasures to overcome the above difficulties? All those above-mentiond problems demand prompt solution in this field.Characteristics of deep-water: dumped stone materials are unable to roll down to the slope foot because of high dike and long slope; new stable slope can form only through repeatedly collapse; dike caving leads to unsteady of the dike nose.In order to attain hydraulic parameters in closure gap and so that present the needed condition for mathematical model solution of dyke nose slump, a mathematical model of current in river channel is established and its solution and engineering computation example are given. In establishing current mathematical model near the closure gap, two-dimensional model is usually adopted. Finite volume method is adopted in this paper thanks to its simplicity, flexibility, conservation type discrete pattern and non-necessity of linearization. For turbulent viscosity coefficient, zero equation model is adopted in this paper.Basic equations for two-dimensional mathematical model are consisted of continuous equation, momentum conservation equations in direction X (with the stream) and direction Y (against the stream) and corresponding closed turbulent flow equation.In finding out solution of the general microdifferential equation, finite volumemethod is used to discrete and establish a two-dimensional mathematical model of turbulence. Later, Simpler method is used to doing programming and calculation. Closure works of the main channel for the Three Gorges Project is selected as example for engineering computation. The given computation conditions consist of calculation scope and topography data, situation in stages of closure dike advancement, closure gap location and current condition. Various hydraulic parameters for closure gap after computation include upstream and downstream water levels in closure gap, water level along axis of closure dike, drop, diversion volume and ratio for the open diversion channel and mean velocity in central line of the closure gap. Results computerized according to mathematical model and hydraulic model test are basically identical. Difference between diversion volumes of the open diversion channel is less than 5 %, except for individual cases. Upon successful closure of the main channel for the Three Gorges Project, results computerized according to mathematical model and field real measured data are compared, showing that they are basically identical. Difference between real measured and computerized diversion ratios is within 6%.Study on deep-water closure dike nose slump mechanism and its stability is carried out specially in this paper, examples of dike nose slump for computation and forecast are put forward, and slump law is analyzed based on the above study.As a theoretical analysis for closure dike stability, study on slump of dike nose is carried out under the conditions of erosion by current and without current erosion. In condition of no current erosion, the formulas for slump length, height and volume are derived in case of complete slump and the relationship of slump volume varying rate and critical water depth have been further derived, a i