A dam-breach erosion model: II. Application

In Part I, the earthen dam breach erosion was analyzed. To this end, the BEED (breach erosion of earthfill dams) model was modified and extended. In this paper, Part II of the series, the extended BEED model is applied to three historical cases: South Fork Dam, Buffalo Creek Dam, and Teton Dam. The testing before it can be used for forecast purposes. A sensitivity analysis of the model to its parameters shows that the model is very sensitive to friction factors; Chenzy's for breach erosion, and Manning's for flood routing.     

坝体溃决过程与溃坝洪水演进耦合数值模拟

研究坝体的溃决过程与溃坝洪水演进对于处置由溃坝引起的洪水灾害、提升水利安全具有重要的意义。鉴于目前大多模型均将坝体溃决过程与溃坝洪水演进分别进行模拟,不能反映土体与水流相互耦合的特点,模拟结果精度有限。基于对土体有限抗冲能力的考虑,选取双曲线型冲蚀速率表达式描述坝体冲蚀、采用简化Bishop法搜索临界滑裂面描述溃口边坡坍塌和具有总变差不增特性的MacCormack有限体积法离散控制方程,建立了坝体溃决过程与溃坝洪水演进耦合的平面二维数值模型。实际算例表明模型合理地模拟了溃口的发展过程与洪水演进过程,在溃口急缓流转换区展现了较强稳定性,守恒性良好,可作为溃坝洪水风险评估与洪灾预报的有力工具。     

土石坝漫顶溃决过程数值模拟研究

针对水动力条件变化复杂、水土耦合作用强烈的土石坝溃决过程,结合水库调洪演算、清水冲刷以及溃口冲刷侵蚀机理,在Breach模型基础上建立了土石坝漫顶溃口流量过程计算物理模型。结果表明:模型对JP水库大坝溃决过程的模拟,很好地再现了溃决洪水流量过程线、溃口展宽和下切过程,验证了模型的合理性和应用潜力。     

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.     

Not all breaks are equal: Variable hydrologic and geomorphic responses to intentional levee breaches along the lower Cosumnes River,California

The transport of water and sediment from rivers to adjacent floodplains helps generate complex floodplain, wetland, and riparian ecosystems. However, riverside levees restrict lateral connectivity of water and sediment during flood pulses, making the re‐introduction of floodplain hydrogeomorphic processes through intentional levee breaching and removal an emerging floodplain restoration practice. Repeated topographic observations from levee breach sites along the lower Cosumnes River (USA) indicated that breach architecture influences floodplain and channel hydrogeomorphic processes. Where narrow breaches (<75 m) open onto graded floodplains, archetypal crevasse splays developed along a single dominant flowpath, with floodplain erosion in near‐bank areas and lobate splay deposition in distal floodplain regions. Narrow breaches opening into excavated floodplain channels promoted both transverse advection and turbulent diffusion of sediment into the floodplain channel, facilitating near‐bank deposition and potential breach closure. Wide breaches (>250 m) enabled multiple modes of water and sediment transport onto graded floodplains. Advective sediment transport along multiple flow paths generated overlapping crevasse splays, while turbulent diffusion promoted the formation of lateral levees through large wood and sediment accumulation in near‐bank areas. Channel incision (>2 m) upstream from a wide levee breach suggests that large flow diversions through such breaches can generate water surface drawdown during flooding, resulting in localized flow acceleration and upstream channel incision. Understanding variable hydrogeomorphic responses to levee breach architecture will help restoration managers design breaches that maximize desired floodplain topographic change while also minimizing potential undesirable consequences such as levee breach closure or channel incision.     

耦合溃口演变的二维洪水演进数值模型研究

为更好地模拟溃坝洪水过程,本文采用源项法耦合了溃口演变模型DB-IWHR与基于GPU加速技术的二维水动力模型,建立了一个包含上游库区二维水动力过程、溃口演变和下游淹没区二维洪水演进的高性能耦合模型。模型中所用源项法为在同一时间步长内,通过二维浅水方程的源项将溃口演变模型计算的流量转换为二维水动力模型溃口上下游各标记网格水深的变化值,以此来实现溃口上下游之间的水量交互。该模型的优势在于所用源项法简单易实现,充分考虑溃口的冲刷过程及上下游水动力过程,同时引入GPU技术加速计算。最后,将耦合模型应用于一个土石坝和两个堰塞坝溃决算例,所得结果与实测吻合较好,模型运行快速高效,这表明基于源项法的耦合模型可实现对土石坝、堰塞坝溃坝等灾害事故的合理高效预测,为应急抢险工作提供有力支撑。     

堰塞坝漫顶溃口流量变化过程的数值模拟

根据一般滑坡堰塞坝特点和实际观测到的堰塞坝溃口发展规律建立了一个溃口扩展模式,并将溃口扩展过程归纳为溃口垂直下切、横向扩展和坝坡溯源冲刷3种主要表现形式,采用通过试验资料建立的高强度泥沙冲刷计算公式将这3种表现方式联系在一起,建立了堰塞坝逐渐溃决数学模型,并利用实测溃坝资料验证了模型的可靠性.考虑到溃坝洪水计算的极大不确定性,本文对计算模型中的关键参数给定一定变幅范围进行计算,研究了其对计算结果的影响.研究结果表明,堰塞坝残留坝体高度和坝体物质抗冲性是影响溃坝流量的最重要因素,库容特性的影响相对较小.     

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。     

土石坝溃口流量过程分析

基于土石坝漫顶溃决物理过程基础上,综合水库调洪演算、溃口泥沙冲蚀计算和溃口边坡稳定性分析,构建溃口流量过程的数值计算模型。利用该模型计算分析某土石坝水库发生漫顶溃决的溃口流量过程,计算分析结果表明,模型合理,计算精度较好,能反映土石坝溃口流量过程变化特点。     

溃口近区二维数值模拟与溃坝洪水演进耦合

基于黏土心墙砂石坝的溃决过程,以及溃坝洪水传播和运动的特性,建立黑河金盆水库大坝溃口近区二维数值模型和下游地区溃坝洪水演进耦合数学模型。使用DAMBRK法计算逐渐溃坝,并应用其结果进行后续模拟。采用Abbott-Ionescu六点隐式有限差分格式求解一维模型,采用单元中心的有限体积法求解二维模型方程。采用侧向连接方式,将黑河两岸计算水位点与二维网格单元相连,实现一、二维模型的耦合。采用所建立的二维模型对溃口近区进行计算与模拟,得到计算区域某一时刻的水深及流速分布。应用所建耦合模型对黑河金盆水库万年一遇入库洪水漫顶致溃坝洪水进行数值模拟,得到一维河道内各断面的水位和流量变化过程,以及二维计算区域内不同时刻的水深分布图、流速矢量图和淹没范围变化过程。溃口的形成过程不仅包括漫顶水流的直接作用,同时包括溃口形成过程中两侧漩涡状水流的反冲刷作用。耦合模型可以同时兼顾河道内的水流变化以及河道外计算区域内的洪水演进过程,从而减少由于计算结果偏大或偏小所带来的防洪资源浪费和防洪措施不利等不良影响。     

考虑材料冲蚀性沿深度变化的堰塞体漫顶溃决模拟

合理预测堰塞体的溃决过程对于致灾后果评价和防灾减灾工作的开展具有至关重要的意义,但由于堰塞体结构和材料的复杂性,给预测工作带来了挑战。基于堰塞体的地质勘察资料和溃决机理,建立了一个可考虑材料冲蚀特性随深度变化的堰塞体漫顶溃决过程数学模型。模型主要包括水动力模块、材料冲蚀模块和溃口发展模块,并采用按时间步长迭代的数值计算方法模拟堰塞体溃决时的水土耦合过程。选择拥有实测资料的白格“11·03”堰塞体溃决案例对模型进行验证,模拟结果验证了模型的合理性。参数敏感性分析结果表明,堰塞体材料冲蚀系数对溃口流量过程具有重要影响,堰塞体材料临界剪应力对溃决过程影响较小;另外,开挖泄流槽可大幅降低库容较大堰塞湖溃决时的溃口峰值流量,是一种行之有效的减灾手段。     

黄河下游游荡型河段洪水演进与河床变形过程的数值模拟

黄河下游游荡型河段的河床变形过程相当复杂，横向变形尤为突出，现有水沙数学模型一般仅能模拟河床纵向冲淤，不能模拟滩岸横向冲刷与崩塌，本文将平面二维水沙数学模型与粘性河岸冲刷的力学模型相结合，建立河床变形的平面二维混合模型。然后以黄河下游花园口至夹河滩的游荡型河段为例，采用该混合模型，首次较为全面地模拟了1982年汛期的洪水演进与河床变形过程。沿程最高水位、出口断面流量过程等计算值与实测值符合较好。计算结果还表现为大水时主槽冲刷，滩地淤积；小水时主槽淤积，滩地崩塌。该计算结果与游荡型河段的年内冲淤变化规律一致。     

基于FREAD溃坝体系的堰塞湖溃决过程反演分析

为了提出适用于堰塞湖溃决模拟仿真的方法,在系统梳理FREAD溃坝洪水分析体系DWOPER、DAMBRK、BREACH和FLDWAV模型的基础上,对各模型的基本原理、适用条件及优缺点进行了汇总。基于各模型的功能特点,联合使用BREACH溃坝计算模型及FLDWAV洪水演进模型反演了尼泊尔逊克西(Sunkoshi)堰塞坝的溃决过程。结果表明:逊克西堰塞坝溃决过程历时68 min达到溃决洪峰流量1 794 m3/s,考虑到支流入流的情况,溃决洪峰历时154 min演进至下游37.9 km处的库帕瓦加特(Pachuwarghat)水文站,计算流量结果与该水文站实测数据较为一致,从而验证了联合使用BREACH和FLDWAV模型进行堰塞湖溃决计算的合理性和可行性。研究成果可以为制定类似堰塞湖溃决的应急处置方案提供参考。     

金沙江“10·10”白格堰塞湖溃坝洪水反演分析

准确预测堰塞湖溃坝洪水流量过程在堰塞湖应急抢险过程中极其重要。以白格堰塞湖下游水文站实测的洪水过程为依据,通过DB-IWHR溃坝洪水分析程序和GST洪水演进模型,分别采用不同冲刷侵蚀参数对"10·10"白格堰塞湖漫顶自然泄流过程进行了反演分析。结果发现:冲刷参数a=1.100 0、b=0.000 6时,叶巴滩、拉哇水文站模拟结果与实测流量结果最为接近。由此判断"10·10"白格堰塞湖溃决洪峰流量为10 882.78 m~3/s,溃决历时6.2 h到达洪峰流量,最终溃口水面宽度为99.66 m。运用DB-IWHR溃坝洪水分析程序结合基于GPU加速技术的GST洪水演进模型,计算效率得以大大提高,可以在应急抢险工作中实现快速、精准的预测。     

土石坝漫顶溃决模式与溃坝参数预测

利用黏土和黄砂混合配制试验筑坝材料,开展水槽试验,模拟均质土石坝漫顶破坏过程,研究溃坝模式对溃坝参数的影响。试验中观测到3种漫顶破坏模式:陡坎蚀退冲刷溃决模式(M1)、剪切蚀退坍塌溃决模式(M2)和浸泡剥蚀破坏模式(M3)。相同库容和坝高条件下,不同模式最大溃决流量差异较大:Q_p(M2)>Q_p(M1,M3);定义水流开始漫顶至水库内库存水流构成漫坝破坏的主要动力源的时间为漫顶临界时间T_C,则T_C(M2)<T_C(M1)<T_C(M3)。溃坝模式反映了漫顶流量、初始溃口、坝高和筑坝材料等因素对漫顶溃决的综合影响,是除坝高、库容外溃坝参数预测的重要影响因子。     

黄土高原沟道坝系模型设计方法

在回顾前人研究成果及分析土壤侵蚀模型试验特点的基础上，提出了黄土高原小流域沟道坝系模型设计的方法。认为在水力侵蚀最活跃的黄土高原地区，降雨强度是影响产流产沙的关键因子，从而需要抓住降雨与侵蚀产沙这对主要矛盾，根据降雨侵蚀空间或时问的集积效果来实现模型流域与原型流域产沙特征的相似。为保证沟道坝系地貌演变相似，除降雨历时遵循重力相似条件外，模型降雨产沙关系还应通过借助天然资料率定产沙量比尺的途径与原型相对应。     

平面二维溃坝水沙输移动床数学模型研究

为数值模拟溃坝洪水作用下泥沙输移及河床变形,建立了无结构三角形网格的平面二维水沙耦合数学模型。水动力学模型采用浑水流动方程,考虑了泥沙输移及床面冲淤变化对水流动量的影响;泥沙输移采用不平衡泥沙输移模式。模型数值计算格式采用守恒性较好的有限体积法,单元界面的对流通量采用基于近似黎曼解的Roe格式计算,扩散通量采用中心格式,不平衡泥沙输移方程采用坡度限制的二阶空间精度格式离散。模型经经典算例和小尺度瞬间全溃的动床实验资料检验,水面、床面及流速等计算结果与实验资料或解析解等比较吻合,分析了动床与定床水面及洪水传播速度的差异。在上述检验的基础上,采用动床模型对某河道型水库溃坝诱发的水沙运动进行了数值模拟,结果表明溃坝洪水下水流、泥沙及河床变形发生强烈的相互作用,采用水沙及河床变形耦合的数学模型是必要的。     

Two-Dimensional Composite Mathematical Alluvial Model for the Braided Reach in the Lower Yellow River

Abstract

The fluvial processes are considerably complicated in the braided reach of the Lower Yellow River, and the lateral or transverse channel deformation process is especially pronounced. The state-of-the-art of simulation techniques for the channel deformation in the Lower Yellow River was analyzed in this paper, and such a conclusion was drawn that existing mathematical alluvial models only can simulate the longitudinal channel deformation and cannot simulate such the lateral deformation as lateral erosion and failure of bank. Then a new two-dimensional (2D) composite model was developed herein, consisting of a 2D flow and sediment transport submodel and a riverbank-erosion submodel. This model not only can simulate the longitudinal and lateral channel deformations, but also can be used to simulate the fluvial processes of natural alluvial rivers with complex bed morphology. Through using the proposed composite model, the flood routing and channel deformation processes during the August 1982 flood were simulated. The maximum stage along the reach and the discharge hydrograph at the outlet so obtained were in good agreement with the observed data. Simulated results of the channel deformation processes showed that during the flood season, the main channel scours and the flood plain deposits; during the dry season, the main channel deposits and banks of the floodplain retreat due to lateral erosion by the flow and failure under gravity. Such results accord with the annual varying law of bed scouring and deposition in the braided reach.     

HEC-RAS模型在二维溃坝洪水研究中的应用

为准确模拟大坝失事后溃坝洪水的下游演进,运用HEC-RAS二维水动力学模型,修正面板坝溃口发展曲线,设计两种闸门开度的小井沟面板坝漫顶溃坝工况,模拟水库泄洪影响下溃坝洪水的下游演进并生成相应的洪水风险图、最大流速分布图、滞留时间图。研究结果展现了溃坝洪水在中下游平原丘陵地区的泛滥情况、洪水风险的分布差异以及水库泄洪对溃坝洪水的影响。分析得出不同闸门开度下溃坝洪水在中下游平原丘陵地区的淹没水深和范围差异明显,最大流速和洪水滞留时间区别不大, 说明水库全力泄洪能有效降低溃坝洪水对下游人员聚居的平原地区的危害。研究成果对后续的人员疏散和损失估计具有重要参考意义。     

堤防溃决的流动分析及冲刷坑计算

以平面二维浅水方程和不平衡输沙理论作为建模基础 ,采用高分辨率有限元格式捕捉溃坝涌波的传播 ,建立了堤坝瞬间溃决后水流运动和河床冲刷的二维数学模型。计算了不同溃口宽度、不同流量和不同粒径土体组成条件下的堤坝瞬时溃决的水流演进和冲坑发展 ,为钢木土石组合坝的结构受力及稳定性分析提供水动力学条件。