Location and probability of shoal margin collapses in a sandy estuary

Channel bank failure, and collapses of shoal margins and beaches due to flow slides, have been recorded in Dutch estuaries for the past 200 years but have hardly been recognized elsewhere. Current predictions lack forecasting capabilities, because they were validated and calibrated for historic data of cross‐sections in specific systems, allowing local hindcast rather than location and probability forecasting. The objectives of this study were to investigate where on shoal margins collapses typically occur and what shoal margin collapse geometries and volumes are, such that we can predict their occurrence. We identified shoal margin collapses, generally completely submerged, from bathymetry data by analyzing digital elevation models of difference of the Western Scheldt for the period 1959–2015. We used the bathymetry data to determine the conditions for occurrence, specifically to obtain slope height and angle, and applied these variables in a shoal margin collapse predictor. We found 299 collapses along 300 km of shoal margin boundaries over 56 years, meaning that more than five collapses occur on average per year. The average shoal margin collapse body is well approximated by a 1/3 ellipsoid shape, covers on average an area of 34 000 m² and has an average volume of 100 000 m³. Shoal margin collapses occur mainly at locations where shoals take up a proportionally larger area than average in the cross‐section of the entire estuary, and occur most frequently where lateral shoal margin displacement is low. A receiver operating characteristic curve shows that the forecasting method predicts the shoal margin collapse location well. We conclude that the locations of the shoal margin collapses are well predicted by the variation in conditions of the relative slope height and angle within the Western Scheldt, and likely locations are at laterally relatively stable shoal margins. This provides hypotheses aiding the recognition of these features in sandy estuaries worldwide. Copyright © 2018 John Wiley & Sons, Ltd.     

泥石流源地土体应力应变特性对降雨响应过程的分析——以蒋家沟泥石流为例

根据对蒋家沟泥石流源地的崩塌、滑坡、散流坡,在旱季与雨季不同时段,降雨入渗后的土壤含水状况观测,以及泉水季节性变化,详细分析了泥石流源地崩塌、滑坡、散流坡土体的应力应变特性对降雨的响应过程差异.在雨季滑坡临空面和滑坡体内的土体含水量分别为6.67%、4.8%,滑坡活动主要是通过前缘剪出口崩塌方式补给下方沟床.崩塌体由于土体结构极为松散,在雨季该土体含水量通常高达8.85%～16%,使其土体的抗剪强度(C、φ)处于极限应力状态,一旦遇到降雨浸润和冲刷,迅速触变液化,转化为高浓度泥石流的侵蚀产沙过程.     

Numerical Experiment on Two-Dimensional Line Thermal

The time evolution of a two-dimensional line thermal-a turbulent flow produced by an initial element with signifi-cant buoyancy released in a large water body, is numerically studied with the two-equation k - e model for turbulence closure. The numerical results show that the thermal is characterized by a vortex pair flow and a kidney shaped concentra-tion structure with double peak maxima; the computed flow details and scalar mixing characteristics can be described by self-similar relations beyond a dimensionless time around 10. There are two regions in the flow field of a line thermal: a mixing region where the concentration of tracer fluid is high and the flow is turbulent and rotational with a pair of vortex eyes, and an ambient region where the concentration is zero and the flow is potential and well-described by a model of doublet with strength very close to those given by early experimental and analytical studies. The added virtual mass coeffi-cient of the thermal motion is found to be approximat     

曝气对人工湿地氮去除效果的影响

选择不同的植物组合,分别在6组复合垂直流人工湿地的下行池、上行池种植金边石菖蒲(Acorus gramineus)、宽叶泽苔(Caldesia renif ormis);慈姑(Sagittaria sagittif olia)、水葱(Scirpus validus);灯芯草(Juncus effuses)、梭鱼草(Po...     

川藏公路二郎山隧道和平沟泥石流特征研究与治理设计

泥石流作为地质环境较差的山区的主要自然灾害之一,对于泥石流的研究思路的进一步发展和完善,在很大程度上将对山区的工程建设有很重要的指导意义。从工程地质研究思路的角度出发,对位于川藏公路二郎山隧道西口和平沟泥石流进行了研究。采用工程地质分析的方法对和平沟泥石流的形成环境(地层岩性条件、构造条件、地貌条件、气象条件、植被发育及人类活动)、基本特征(泥石流沟的基本特征、泥石流的堆积特征)进行了系统的研究,并在此基础上对泥石流沟沿岸的岸坡稳定性(可能泥石流的物源)等进行了评价分析。对泥石流的活动趋势以及可能泥石流的方量做出了较为科学的评价预测。经过这样的系统分析对工程建设中的泥石流防治提出了治理设计方案。     

脐带缆在潜器下放过程中的运动建模与仿真

潜器在下放布置过程中需要考虑变长度的脐带缆对其运动的影响,在考虑流的情况下,对脐带缆的有限差分模型进行了改进,并采用最小二乘方法求解由时间和空间上的中心差分格式离散后的非线性方程组。为了验证模型改进的有效性,将潜器在均匀定常流中水平匀速直航时的数值解与该特殊情况下的解析解进行比较,两组解的吻合不仅证实了模型改进的有效性,而且表明上述数值计算方法是可靠且有效的。在潜器下放布置运动给定的情况下,脐带缆的运动仿真结果表明,潜器姿态角的调整会对拖曳点处的缆绳张力大小和变化趋势产生显著的影响,流在改变缆绳空间形状的同时引起了缆绳对潜器作用力和力矩的非线性和时变性,脐带缆总是阻止潜器自身动力对运动速度和姿态角的改变,且对纵荡运动的影响最大。     

Concurrent Use of OSCAT and AltiKa to Characterize Antarctic Ice Surface Features

Radar altimetry provides an important geophysical parameter, backscatter coefficient (σ₀), which is useful in studying target surface characteristics. Ku-band (Oceansat-2 scatterometer- OSCAT) and Ka-band (SARAL-AltiKa altimeter) data are concurrently used to characterize polar surface features over the Antarctic region. Maximum-likelihood classification has been employed to classify combined data set (AltiKa and OSCAT) for discrimination among sea ice, open water, and ice sheet (interior and exterior). The sea ice region obtained using the current approach has been compared with sea ice boundary derived from passive microwave data.     

基于LAHARZ的泥石流堆积范围预测模型的建立及应用

为了探究泥石流的堆积范围,利用LAHARZ软件,对北京市密云县泥石流沟喇嘛栅子南沟进行了数值模拟。结合泥石流沟小流域1：10 000数字高程模型图,模拟了泥石流的堆积范围。首先利用中国部分地区泥石流体积和堆积范围的数据资料,获得了泥石流体体积与其堆积范围的新的统计模型B=11.42V^0.7156;然后通过模拟沟道与实际沟道的对比,确定了最佳沟道阈值为15 000;再结合现场调查统计和降雨历史资料,确定了10年、20年、50年和100年一遇暴雨条件的泥石流体积值,分别为56 500、72 900、94 200和113 100 m³;最后在此基础上对该条泥石流沟的堆积范围进行了预测。结果表明,100年一遇的暴雨条件下泥石流堆积面积为48 729 m²,到达最远距离约为490 m,已影响下游村庄。     

Bench development along the regulated,lower River Dee,UK

Channel change to regulated flows along large lowland rivers with cohesive bank materials has been investigated on the lower Welsh Dee, including the tidally influenced reach. Reduction of channel width has involved the formation of a 5–40m wide discontinuous bench, often linking ‘point’ and ‘concave’ locations. Map evidence shows that wide benches occur where historically the channel had migrated laterally; narrow benches were found at stable channel locations. Auger cores of the bench deposits clearly differentiated the two contrasting depositional environments within meandering rivers: ‘point bench’ and ‘concave bench’. Around an individual bend a morphologically continuous bench showed a gradient in sediment characteristics from coarser sediments (point locations) to finer organic deposits (concave locations); it also showed a topographic gradient, gaining 0.5m in elevation around the bend suggesting that bench accretion at concave locations is faster than at point locations in fluvially dominated reaches. Such patterns are suggested to have important implications for riparian ecosystems. Copyright © 1999 John Wiley & Sons, Ltd.