均质土坝漫顶溃坝过程数学模型研究及应用

国内外大量模型试验表明,"陡坎"式冲蚀是均质土坝漫顶溃决的重要机理。近年来,各国学者开发了一系列的考虑"陡坎"式冲蚀的溃坝过程数学模型,但模型均采用了"陡坎"出现在下游坡脚的假设。通过大比尺均质土坝漫顶溃决模型试验发现,对于坝高较大的均质土坝,"陡坎"出现的位置与漫顶水头和下游坝坡坡比存在内在联系,且"陡坎"的移动速率与坝料的物理力学指标相关,因此初始冲坑的位置和"陡坎"移动参数的选取对于溃坝过程模拟结果的合理性具有重要意义。本文借鉴国内外的漫顶溃坝过程数学模型,提出一个可考虑均质土坝漫顶溃决过程中"陡坎"移动的数学模型。该模型通过漫顶水流特征和坝体形状参数确定下游坡初始冲坑的位置,采用能量分析方法模拟"陡坎"移动,并通过室内与现场模型试验提出可考虑坝料黏粒含量、含水率、干密度等指标的"陡坎"移动参数;利用基于水流剪应力原理的冲蚀速率公式模拟溃口纵向下切与横向扩展;采用宽顶堰流量公式计算溃口流量,通过极限平衡法分析溃坝过程中溃口边坡的稳定性,采用迭代的数值计算方法模拟整个溃坝过程。选择国内外典型的大比尺均质土坝漫顶溃坝试验和有实测资料的溃坝案例对模型进行验证,并研究了是否考虑"陡坎"冲蚀对溃坝模拟结果的影响;通过模型计算分析可以得出,本文提出的数学模型可合理模拟均质土坝的漫顶溃坝过程。     

尾矿库漫顶溃决动态演化及溃口展宽模型试验

为研究尾矿库漫顶溃决动态演化过程与溃口发展机理及规律,通过3组相似模型试验,对溃口展宽过程和演变机理进行了系统分析,提出了尾矿坝漫顶模式下溃口展宽速率的幂函数公式,并讨论了库水位上升阶段浸润线的变化过程及溃决阶段不同浸润线埋深条件下流速、流量、溃口宽度的变化过程。研究结果表明：尾矿坝的溃决过程可分为坝面漫流与冲沟形成、坝体崩塌与溃口快速扩张及稳定泄流3个阶段;浸润线埋深对流量、流速及溃口最终宽度有较大影响,排渗失效时峰值流量及溃口最终宽度接近正常排渗时的1.5倍,峰值流速可达1.2～1.3倍;降低浸润线高度可以延缓溃决泥石流到达下游的时间,紧急撤离时间也可延长3 min左右;提出的幂函数公式计算值与实测值吻合较好。     

堰塞坝漫顶溃决试验及相关数学模型研究

针对当前堰塞坝溃决试验粒径取值偏低和粒径相差不大的现状,采用两组粒径差别明显的砂样进行了堰塞坝垭口漫顶溃决试验。试验表明,同条件下粗、细两种颗粒坝体的溃决现象有着较明显的不同。垭口挡板提起后,细颗粒坝体以下切侵蚀为主,冲刷强度比较剧烈,坝体较容易发生溃决;而粗颗粒坝体则是以渗流出流形成的溯源冲刷为主,冲刷强度较低,溯源面逐渐向上发展,只有当其发展到垭口下端附近时坝体才有可能迅速发生溃决。试验还发现,下游坝坡对溃决过程的影响比较显著,坝坡越陡,坝体越易溃决,溃口的平均展宽速率也越大。此外以deVries输沙率公式为基础建立了具有物理意义的概念性溃口出流计算模型,并采用试验实测数据对该模型进行了验证,结果表明该模型具有良好的适用性。     

考虑水沙相互作用的河堤溃决二维数值模型

该文建立了洪水漫堤导致河堤溃决的平面二维水沙运动数值模型,以模拟溃口扩展和堤身冲刷的过程。该模型基于有限体积法、采用显式格式求解平面二维水流运动和非平衡全沙泥沙输移方程,并考虑了边坡崩塌、底床急剧变形以及高浓度泥沙输移对水流运动的影响。溃口处的混合流场采用HLL approxim ate Riemann solver处理,非黏性土溃口边坡的崩塌采用不同干、湿静止角处理。通过对英国HR Wallingford实验室一组实验的计算,结果表明该模型能较好地模拟非黏性土堤坝溃决过程中的溃口展宽、坝体冲刷、流场分布以及溃口流量过程线。对模型参数的敏感性分析显示了水沙耦合溃决模型的非线性特征。     

凌汛期堤防漫顶溃口破坏规律断面模型试验Ⅱ：非黏性土堤

为了研究凌汛期堤防漫顶破坏溃口发展规律,分别进行了洪水和冰凌条件下的均质非黏性土堤溃决模型试验。结果表明：两种条件下的溃堤过程均可划分为4个阶段：渗透过流、堤后冲刷、溃口展宽和冲淤平衡,各阶段发展速度和形式有所不同;通过分析溃堤过程、溃口宽度以及流速和水位变化过程的异同点可知,两种条件下流速和水位变化趋势基本一致,冰凌条件下峰值流速明显增大且出现时间提前,峰值流速最大可增加81.5%;同一流量下,增大堤顶宽度对流速和水位的影响不大,而减缓堤身断面的边坡能显著降低堤后水位的上升速度和溃口处峰值流速。试验结果有助于认识凌汛期堤防溃决的过程和特点,可为凌汛期非黏性土堤险情防控和抢险措施的制定提供科学的参考。     

沿海城市溃堤洪水模拟技术研究进展

综述了国内外漫顶溃堤过程和海岸洪水在城市内部演进的数值模拟技术的研究进展,从溃口底部的冲刷、溃口侧壁的展宽和下游坡面的溯源冲刷三方面评述了溃堤机理模型,分析了各类洪水演进模型和建筑物概化方法的优缺点和适用性。针对当前沿海城市溃堤洪水模拟技术研究中存在的不足,指出未来需要通过物理模型试验完善海堤溃决洪水模型,加强溃口泥沙冲蚀输运研究,优化城市洪水孔隙度模型,并充分考虑城市排水系统、汽车等对洪水传播的影响。     

均质土坝漫顶溃坝模型相似准则研究

由于土石坝溃坝过程具有强非线性,溃坝机理非常复杂,土石坝溃坝模型试验相似理论研究有较大的难度。基于近年来南京水利科学研究院开展的5次国内外最高黏性土均质坝漫顶溃坝实体试验及20多组室内漫顶溃坝试验,"陡坎"式侵蚀被认为是土石坝漫顶溃坝的重要机理之一。从这一机理出发,该文推导获得了均质土坝漫顶溃坝过程模型试验的两个重要相似准则:"陡坎"移动速度相似比尺λR及溃口流量过程时间比尺λt。两次现场大尺度溃坝试验数据验证表明,该文获得的溃坝模型相似率能够较好的反演均质土坝漫顶溃坝过程,有较好的参考价值。     

Using PTV through an embankment breach channel

Particle tracking velocimetry was used to determine for the first time the surface velocities upstream, downstream, and through experimental embankment dam breach channels at all stages of breach development. A series of experimental tests was performed, which used varying initial breach geometry. The measurements were made using Styrofoam surface tracking particles and three different video camera angles to obtain 3-dimensional velocity measurements, which were calculated using ArcGIS. Space-time contour maps of velocity were then interpolated for each of the breach scenarios. A maximum surface flow velocity of 2.1 m/s, corresponding to 21 m/s at prototype scale, was measured at the downstream end of the breach channel near the time to peak for the breach outflow. The surface velocities were also converted to average velocities and used in conjunction with breach flow measurements to determine the depth of flow through the breach channel and to make estimates of the sub-aqueous breach geometry. Estimates of flow depth through the breach were used to compare measured outflow results with those determined analytically using the broad-crested weir equation.     

凌汛期堤防漫顶溃口破坏规律断面模型试验Ⅰ:黏性土堤

凌汛期河道存在大量冰凌,导致河流壅塞、水位上涨,对堤防安全造成了严重隐患。为了探究凌汛期黏性土堤的漫顶溃口破坏规律,建立了室内水槽物理模型,模拟了冰凌条件下的堤防漫顶破坏,通过改变堤身含水率和河道流量,分析了冰凌条件下溃堤时的溃口破坏形式、溃口发展过程和水位变化过程。结果表明:溃堤过程中冰凌挤压对堤体的破坏作用明显,堤身含水率和河道流量对溃堤过程有显著的影响;与洪水条件相比,冰凌条件下溃堤时溃口前期发展较为缓慢,而中、后期较为强烈;冰凌条件下的堤前水位峰值要高于洪水条件。     

无黏性土坝漫顶溃决过程及机理研究

以室内试验为技术手段,以无黏性非连续宽级配沙砾料土坝为研究对象,首次尝试采用"埋入式轻型冲蚀捕捉器"动态记录溃决过程。在上游恒定来流量30 L/s条件下,研究了洪水漫顶条件下坝体(1 m高)的冲蚀过程和溃口水力要素变化过程。明确提出了坝体溃决过程的4个阶段:即侵蚀沟形成发育阶段(下游坡面侵蚀缓慢渐进)→"多级跌坎"冲刷阶段(下游坡面冲蚀能力逐渐加大)→"瀑布状跌落水流"冲蚀阶段(坝体快速溃塌)→坝体冲蚀趋于稳定阶段。初步分析了坝体溃决的机理,揭示了冲蚀过程的非均衡性和溃口水力要素的强非恒定流特性。     

土坝溃决跌坎水流水动力特性数值模拟

黏性土坝漫顶溃决涉及多学科交叉,过程极其复杂,尽管国内外大量物理模型试验成果表明其溃决多以“跌坎式”溯源冲蚀为主要特征,然而对该冲蚀发展形式下的水流-坝体微观作用机制尚不清晰。水流作为漫顶溃决的冲刷主动力,对坝体溃决发展起着主导性作用,采用RNG k-ε紊流模型和VOF自由液面捕捉技术针对黏性土坝漫顶溃决代表性水流结构——溃决跌坎水流开展了三维数值模拟研究,对跌坎水流的水流结构、流态、水力特性指标等进行了细致分析,揭示了不同工况下坝体跌坎上的剪切应力、流速分布规律,进而从水动力学的角度对坝面进行受力分析,初步推断了黏性土坝漫顶溃决过程中各级跌坎的主要合并方式为“台阶水平面刷深下切”。研究成果为进一步掌握黏性土坝漫顶溃决发展演变机理提供了理论基础。     

加筋无黏性土石坝漫顶溃坝试验研究

研制了封闭式循环供水土石坝溃坝模型试验装置,并应用该装置开展了室内模型试验,对加筋无黏性均质土坝的溃坝过程进行了系统研究,探讨了坝体加筋对溃坝流量过程线的影响规律。研究结果表明:在水泵出水口设置消能弯管和挡水隔板可保证模型上游供水的稳定性,在试验装置侧面设置连接上下游的U型测流弯管,把明渠流转化为满管管流,在U型弯管下侧平直段安装电磁式流量计量测溃口流量,可有效提高流量过程线的测量精度;加筋无黏性均质土坝的漫顶溃坝过程可分为坝体下游未加筋区域、加筋嵌固区和上游未加筋区域破坏3个阶段;随着加筋体埋深的增加,峰值流量呈现先增大、后减小,再增大的变化规律,在坝高的35%~50%范围内埋设加筋体,能够降低溃坝峰值流量;减小加筋体竖向间距能有效延滞溃坝峰现时间。     

Riprap Effect on Hydraulic Fracturing Process of Cohesive and Non-cohesive Protective Levees

A comprehensive experimental study was conducted to evaluate the effects of soil properties and the rip-rap as well as cohesive and non-cohesive soil material and the riprap particle size on the breaching process and the failure mechanism of the levee. The results showed the crucial role of the riprap coverage and the soil properties in the breaching of the protective levees. In this regard, breaching was developed in both vertical and transverse directions. In the levee without riprap coverage, breach development was observed in the transverse direction while the levee was eroded in the streamwise direction. In contrast, in the levee with riprap coverage, breach development mainly occurred in the vertical direction. Furthermore, the flow rate across the breach was higher in the tests with no riprap compared to those with the riprap coverage. Comparisons suggested that the shorter the failure time, the longer the equilibrium time. The present research has some major implications for coastal and hydraulic engineering designs since the construction of levees is of great importance. Furthermore, the findings can be used to predict flooding and erosion induced by the embankment failure.

     

A simplified physically-based breach model for a high concrete-faced rockfill dam: A case study

A simplified physically-based model was developed to simulate the breaching process of the Gouhou concrete-faced rockfill dam (CFRD), which is the only breach case of a high CFRD in the world. Considering the dam height, a hydraulic method was chosen to simulate the initial scour position on the downstream slope, with the steepening of the downstream slope taken into account; a headcut erosion formula was adopted to simulate the backward erosion as well. The moment equilibrium method was utilized to calculate the ultimate length of a concrete slab under its self-weight and water loads. The calculated results of the Gouhou CFRD breach case show that the proposed model provides reasonable peak breach flow, final breach width, and failure time, with relative errors less than 15% as compared with the measured data. Sensitivity studies show that the outputs of the proposed model are more or less sensitive to different parameters. Three typical parametric models were compared with the proposed model, and the comparison demonstrates that the proposed physically-based breach model performs better and provides more detailed results than the parametric models.     

堰塞坝溃决模拟研究综述与展望

堰塞坝几何形态、粒径级配和库容决定了其溃决机理的复杂性,而溃决过程的精细模拟和峰值流量的准确预测是应急处置的基础和关键.堰塞坝溃决过程与模拟技术是面向国家防灾减灾重大需求的前沿热点问题.在系统梳理国内外试验和数值模拟研究进展的基础上,指出以往试验研究坝体尺度小,足够大的库容基本未模拟,难以显示最终溃口形态;数学模型假设...     

三峡工程明渠截流水流数学模型研究及其应用 I:模型建立与率定

三峡工程三期截流具有流量大、落差大、龙口流速大等特点。为保证截流顺利实施,将一维和二维水流数学模型紧密结合,先用一维模型推求明渠的过流量及上下游戗堤承担的水位落差,再用二维模型模拟戗堤进占时明渠内的水力条件。然后利用已有的水工模型试验资料,对建立的数学模型进行了率定。率定结果表明:一维模型计算的明渠过流量、截流总落差、上下游戗堤承担的水位落差等结果与试验值相当接近;二维模型的计算结果不仅与试验值基本符合,同时又符合弯道水力学的基本特点。     

坝体土料黏粒质量分数对均质土坝漫顶溃决过程的影响

为了研究坝体土料黏粒质量分数对均质土坝漫顶溃决过程的影响,建立了描述均质土坝溃坝溃口发展规律的溃坝数值模型,对实体溃坝案例进行了反馈分析,验证了模型的合理性,并利用该模型重点研究了坝体土料黏粒质量分数对均质土坝溃口发展规律和洪水流量过程的影响。结果表明:坝体土料的黏粒质量分数对均质土坝的溃口发展规律、最终溃口形状以及溃口洪水流量过程具有明显影响,土体黏粒质量分数越高,其临界起动流速越大,冲蚀率越小,均质土坝溃口的发展速率越慢,溃口边坡的失稳坍塌临界深度越大,从而导致最终溃口形状也越小,相应地溃口洪峰流量及最大下泄水量也越小,溃口洪峰流量出现的时间越迟。     

冻土条件下均质土堤漫顶溃决试验研究

为了研究冻土条件下的溃堤机理,开展了弯道水槽概化模型试验。基于结构光的三维成像原理,提出了一种无干扰全方位的实时溃口三维参数测量方法,并结合温度传感器监测堤内温度变化,研究了冻土条件下均质土堤漫顶溃决过程中溃口形态的演变规律和溃决区域的水位、流速、流量变化特性。依据试验中溃口冲蚀特点,将溃堤过程划分为渗透过流、堤后冲蚀、横向展宽、冲淤平衡4个阶段,黏性土堤和非黏性土堤第2阶段、第3阶段分别以陡坎冲蚀和失稳坍塌为主要特征,且黏性土堤会出现明显的陡坎合并现象。试验结果还表明:冻结后土体间隙的冰晶体与土颗粒相互胶结,土体的黏聚力增大,溃口扩展速率及溃决区域的水位、流速变化率均减小,且黏性土堤受冻土深度的影响更加明显;冻土条件下堤防背水侧与堤顶的连接处薄弱易溃,应加强维护,且一旦溃堤应优先对临水侧进行封堵。     

Numerical modeling of earthen dam breach due to piping failure

Based on model tests of earthen dam breach due to piping failure,a numerical model was developed.A key difference from previous research is the assumption that the cross-section of the pipe channel is an arch,with a rectangle at the bottom and a semicircle at the top before the collapse of the pipe roof,rather than a rectangular or circular cross-section.A shear stress-based erosion rate formula was utilized,and the arched pipe tunnel was assumed to enlarge along its length and width until the overlying soil could no longer maintain stability.Orifice flow and open channel flow were adopted to calculate the breach flow discharge for pressure and free surface flows,respectively.The collapse of the pipe roof was determined by comparing the weight of the overlying soil and the cohesion of the soil on the two sidewalls of the pipe.After the collapse,overtopping failure dominated,and the limit equilibrium method was adopted to estimate the stability of the breach slope when the water flow overtopped.In addition,incomplete and base erosion,as well as one-and two-sided breaches were taken into account.The USDAARS-HERU model test P1,with detailed measured data,was used as a case study,and two artificially filled earthen dam failure cases were studied to verify the model.Feedback analysis demonstrates that the proposed model can provide satisfactory results for modeling the breach flow discharge and breach development process.Sensitivity analysis shows that the soil erodibility and initial piping position significantly affect the prediction of the breach flow discharge.Furthermore,a comparison with a well-known numerical model shows that the proposed model performs better than the NWS BREACH model.     

土石坝漫顶过程水力特性分析

土石坝漫顶过程极其复杂,溃决过程研究成果存在诸多争议。采用标准k~ε紊流模型和VOF方法对土石坝水流漫顶过程进行了水气两相流数值模拟,对漫顶过程水流流态、流速分布等进行了详细分析,揭示了坝面切应力分布规律。该规律的发现可以用于解释笔者先前提出的"落点冲蚀"猜想及"剪剥式"冲蚀理论,即漫顶条件下最先发生冲蚀的部位可能在坝坡中上部,漫顶冲蚀以台阶式冲蚀为溃决的主要特征。