Flood hazards at ford stream crossings on ephemeral channels (south‐east coast of Spain)

Most road‐stream crossings over ephemeral channels are vulnerable to extreme hydrologic events. Ford stream crossings (FSCs) are usually dangerous for the road traffic during periods of high flow, in particular under flash flood conditions. The present paper analyzes the flood hazards on the Mediterranean coast in the Region of Murcia (south‐east Spain), affecting this type of road‐stream crossing over dry channels, according to hydraulic variables and bedload transport rates estimated for discharges at bankfull and flood‐prone stages. Under such conditions, the safety of people and vehicles was obtained using numerical models, developed by previous researchers; in particular, water levels and flow velocities across ford reaches were compared with different trend curves between water depths and corresponding critical velocities for children and adults, and for various prototype vehicles. Specifically, two approaches to assess this type of hazards were proposed: a specific Hydraulic Hazard Index and an algorithm for estimating the flood hazard from criteria of bed stability and bedload transport capacity (Flood Hazard at Fords, FHF). In addition, different exposure levels were established, using a Flood Vulnerability Index, based on the FHF, the road category, and the annual average daily traffic. The FHF model gave the best results with regard to the magnitude of the damage observed in recent flash floods for flow stages similar to those simulated. According to the danger thresholds established for this index, half‐bankfull flows represent here a high risk: 27.3% of FSCs for mini‐cars and 18.2% for large cars. At bankfull, the FHF exhibits very high values for mini‐cars (77.3% of FSCs) and for large passenger vehicles (50% of FSCs), while at the floodprone stage, extreme FHF values are reached for all kinds of vehicles at most of the ford crossings.     

洪水再生稻高产栽培技术研究

2007年7月8—9目,四川省泸县发生了特大洪涝灾害,嘉明和福集2镇洪水淹没面积多达8000hm^2,淹没时间为60-92h,处于不同生育期的水稻受到了不同程度的生长抑制。通过野外追踪调查和示范区比较试验,提出在洪灾后结合水稻生育期、淹没时间、淹水深度、植株长势等指标判断水稻受害程度,并以各指标为依据果断采取相应的抗灾、减灾对策。初步分析认为,蓄留洪水再生稻是行之有效的灾后生产自救措施,同时揭示了早割低留桩、晚割高留桩、早割多施肥、晚割少施肥、保持浅水层和防治病虫害是蓄留洪水再生稻的关键技术。     

Shallow water SPH for flooding with dynamic particle coalescing and splitting

In this paper an adaptive algorithm for Smoothed Particle Hydrodynamics (SPH) for the Shallow Water Equations (SWEs) is presented. The area of a particle is inversely proportional to depth giving poor resolution in small depths without particle refinement. This is a particular limitation for flooding problems of interest here. Higher resolution is created by splitting the particles, while particle coalescing (or merging) improves efficiency by reducing the number of the particles when acceptable. The new particle coalescing procedure merges two particles together if their area becomes less than a predefined threshold value. Both particle splitting and coalescing procedures conserve mass and momentum and the smoothing length of new particles is calculated by minimizing the density error of the SPH summation. The new dynamic particle refinement procedure is assessed by testing the numerical scheme against analytical, experimental and benchmark test cases. The analytical cases show that with particle splitting and coalescing typical convergence rates remain faster than linear. For the practical test case, in comparison to using particle splitting alone, the particle coalescing procedure leads to a significant reduction of computational time, by a factor of 15. This makes the computational time of the same order as mesh-based methods with the advantage of not having to specify a mesh over a flood domain of unknown extent a priori.     

Building a 3-D model of a flood basalt: an example from the Etendeka, NW Namibia

Flood basalts represent large outpourings of lavas which often cover, and interact with, sedimentary basins. For this reason areas with significant flood basalt cover are often targets for hydrocarbon exploration. Problems exist, however, when trying to image sediments and structures in offshore regions covered by basalts. Here we present preliminary 3-D models of the Etendeka flood basalt province from NW Namibia, which can act as an aid in understanding the internal and external architecture of the flood basalt cover. Satellite images, digital elevation models, measured geological logs, sections and maps, are used to create the geological model. Models are presented in 2 parts; 1) models created using topography with images such as Land Sat and geological maps draped over them, and 2) a 3-D model of key lava and sediment surfaces in the basin as defined by measured geological sections. Initial results show a palaeo-volcanic feature early in the flood basalt history which is onlapped by later Iavas. The modelling also allows a simple correction for post emplacement subsidence by assuming an original sub horizontal position for the first basin wide silicic flow unit.     

流域洪涝指数方法研究

应用降水产生流量过程线的变化原理 ,研究了一个考虑前期影响雨量的设计洪涝面雨量计算方法。然后复核水位与设计洪涝面雨量的关系，确定出各级洪涝指数的设计洪涝面雨量的大小。     

Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial valley floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channels that are requisite habitat for native fish. Rapid Neogene uplift and Quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine–riparian ecosystems. Uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharge during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and channel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and much greater flood plain impact. Nonetheless, 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of Soda Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 flood plain abandoned as a terrace. Gravelly overbank deposits raise flood-plain surfaces above levels of frequent inundation and are well drained, therefore flood-plain soils are drier. Noncohesive gravels also reduce bank stability and may have persistent effects on channel form. Overall, floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley-floor ecosystems in northeastern Yellowstone.     

Appraisal of regional and index flood quantile estimators

Our results illustrate the performance of at-site and regional GEV/PWM flood quantile estimators in regions with different coefficients of variation, degrees of regional heterogeneity, record lengths, and number of sites. Analytic approximations of bias and variance are employed. For realistic GEV distributions and short records, the index-flood quantile estimator performs better than a 2-parameter GEV/PWM quantile estimator with a regional shape parameter, or a 3-parameter at-site GEV/PWM quantile estimator, in both humid and especially in arid regions, as long as the degree of regional heterogeneity is moderate. As regional heterogeneity or record lengths increases, 2-parameter estimators quickly dominate. Flood frequency models that assign probabilities larger than 2% to negative flows are unrealistic; experiments employing such distributions provide questionable results. This appraisal generally demonstrates the value of regionalizing estimators of the shape of a flood distribution, and sometimes the coefficient of variation.     

Environmental mitigation and the Upper Yazoo Projects

The US Army Engineer District, Vicksburg (CELMK), evaluated an array of flood control alternatives, which included up to 167 water control structures, 52 confined disposal facilities, and 47 borrow pits as part of a major flood control effort known as the Upper Yazoo Projects (UYP). Many of these project features are capable of ponding water and thus can be managed to mitigate for aquatic, terrestrial, waterfowl, and wetland resource losses expected to occur as a result of the UYP. The benefits to be derived will depend upon the land uses and management of the ponded areas. The US Army Engineer Waterways Experiment Station developed procedures to quantify the cost and habitat benefits of the many management options for these sites. The mitigation strategy was derived by optimizing various combinations of land acquisition, reforestation, land-use change, and site hydrology so that the least-cost mitigation plan could be selected.     

武义县暴雨洪涝的时空变化特征

为了揭示武义县暴雨洪涝的时空变化特征，基于1970-2018年洪涝及暴雨实测数据，统计分析反映时空变化特征的一系列指标，应用多元线性回归模型，建立水位与径流、降雨的多元相关关系。研究结果表明：洪涝在20世纪70至80年代中期次数偏少，其后开始逐渐增多，且具有3a左右的年际周期变化，12a左右和25a左右的年代际周期变化；洪涝主要发生在主汛期6-8月，其中，6月中旬至7月中旬以系统性暴雨为主，洪涝占全年60%，7月下旬至8月短时暴雨明显增多，山洪占全年50%；洪涝与西部型南亚高压、西太平洋副热带高压的变化特征正相关，同时中低层冷暖气流的汇合强度亦有利于产生洪涝；构建武义江流域水位与径流、降雨多元逐步回归模型，提出一种以水位预报为目标的洪水趋势预测方法。