Sensitivity and uncertainty analysis of a sediment transport model: a global approach

Computerized sediment transport models are frequently employed to quantitatively simulate the movement of sediment materials in rivers. In spite of the deterministic nature of the models, the outputs are subject to uncertainty due to the inherent variability of many input parameters in time and in space, along with the lack of complete understanding of the involved processes. The commonly used first-order method for sensitivity and uncertainty analyses is to approximate a model by linear expansion at a selected point. Conclusions from the first-order method could be of limited use if the model responses drastically vary at different points in parameter space. To obtain the global sensitivity and uncertainty features of a sediment transport model over a larger input parameter space, the Latin hypercubic sampling technique along with regression procedures were employed. For the purpose of illustrating the methodologies, the computer model HEC2-SR was selected in this study. Through an example application, the results about the parameters sensitivity and uncertainty of water surface, bed elevation and sediment discharge were discussed.     

Evaluation of safety of hydraulic structures affected by migrating pits

Because it can be carried by flowing water, a sand/gravel pit on the river bed could migrate downstream. Consequently, the presence of pits on river beds could pose a safety threat to in-stream hydraulic structures such as bridge piers. A pit migration model can be used to predict progressive changes of pit geometry as it migrates downstream. However, due to the existence of many uncertainties, the maximum pit depth cannot be predicted with certainty. This paper adopted a simple pit migration model and evaluated the uncertainty associated with the calculated maximum pit depth. Such information is essential for evaluating the probability that a migrating pit could pose a safety threat to a downstream hydraulic structure. Three reliability analysis techniques were applied and their performances were compared.     

Theory and implementation of switch‐based hybrid simulation technology for earthquake engineering applications

Hybrid simulation (HS) is a novel technique to combine analytical and experimental sub‐assemblies to examine the dynamic responses of a structure during an earthquake shaking. Traditionally, HS uses displacement‐based control where the finite element program calculates trial displacements and applies them to both the analytical and experimental sub‐assemblies. Displacement‐based HS (DHS) has been proven to work well for most structural sub‐assemblies. However, for specimens with high stiffness, traditional DHS does not work because it is difficult to precisely control hydraulic actuators in small displacement. A small control error in displacement will result in large force response fluctuations for stiff specimens. This paper resolves this challenge by proposing a force‐based HS (FHS) algorithm that directly calculates trial forces instead of trial displacements. The proposed FHS is finite element based and applicable to both linear and nonlinear systems. For specimens with drastic changes in stiffness, such as yielding, a switch‐based HS (SHS) algorithm is proposed. A stiffness‐based switching criterion between the DHS and FHS algorithms is presented in this paper. All the developed algorithms are applied to a simple one‐story one‐bay concentrically braced moment frame. The result shows that SHS outperforms DHS and FHS. SHS is then utilized to validate the seismic performance of an innovative earthquake resilient fused structure. The result shows that SHS works in switching between the DHS and FHS modes for a highly nonlinear and highly indeterminate structural system. Copyright © 2017 John Wiley & Sons, Ltd.     

Discriminant analysis of the geomorphic characteristics and stability of landslide dams

Landslides can cause the formation of dams, but these dams often fail soon after lake formation. Thus, rapidly evaluating the stability of a landslide dam is crucial for effective hazard mitigation. This study utilizes discriminant analysis based on a Japanese dataset consisting of 43 well documented landslide dams to determine the significant variables, including log-transformed peak flow (or catchment area), and log-transformed dam height, width and length in hierarchical order, which affect the stability of a landslide dam. The high overall prediction power (88.4% of the 43 training cases are correctly classified) and the high cross-validation accuracy (86%) demonstrate the robustness of the proposed discriminant models PHWL (with variables including log-transformed peak flow, and log-transformed dam height, width and length) and AHWL (with variables including log-transformed catchment area, and log-transformed dam height, width and length). Compared to a previously proposed “DBI” index-based graphic approach, the discriminant model AHV – which uses the log-transformed catchment area, dam height, and dam volume as relevant variables – shows better ability to evaluate the stability of landslide dams. Although these discriminant models are established using a Japanese dataset only, the present multivariate statistical approach can be applied for an expanded dataset without any difficulty when more completely documented worldwide landslide-dam data are available.     

Constrained scaling approach for design rainfall estimation

Rainfall depth (or intensity) of the same frequency should follow a non-decreasing relationship with rainfall duration. However, due to the use of finite samples and sampling error, rainfall frequency analysis could yield rainfall intensity (depth)–frequency (IDF, DDF) curves of different durations that might intersect among them. Results of this kind violate physical reality and it is more likely to occur when rainfall record length gets shorter. To ensure the compliance of the physical reality, this paper applied the scale-invariant approach, in conjunction with constrained regression analysis, to circumvent intersections in rainfall IDF or DDF curves. Rainfall data of various durations at rain gauge in Hong Kong are used to demonstrate the procedure. Numerical investigation indicates that the proposed procedure yields more reasonable results than those based on the conventional frequency analysis, especially when only a small sample of data are available.     

散落核素7Be和137Cs在洱海和红枫湖沉积物中蓄积对比

沉积物柱芯分别采自滇西地区的洱海和黔中地区的红枫(及百花)湖,散落核素7Be和137Cs在沉积物中的蓄积特征对比分析表明,7Be在洱海及红枫湖沉积物中的累计值分别为(237±73)Bq/m2和(783±44)Bq/m2;按校正到沉降年代的数值,1986年以前137Cs的累计值分别为(519±26)Bq/m2及(3704±56)Bq/m2.由模式计算获知,(1)7Be和137Cs在红枫湖沉积物中的蓄积以侵蚀影响为主;在洱海的蓄积受直接散落控制.(2)在洱海和红枫湖地区7Be大气散落累计值分别为(0.07±0.02)Bq/cm2及(0.08±0.01)Bq/cm2;1986年以前137Cs大气散落累计值分别为(0.11±0.01)Bq/cm2及(0.37±0.01)Bq/cm2,显示出滇西与黔中地区之间137Cs散落的地区差异.这一现象可能反映出青藏高原对滇西地区存在着全球性大气扩散污染物散落的屏蔽效应.     

柴达木盆地南缘金水口群的早古生代构造热事件:锆石U-Pb SHRIMP年龄证据

通过柴达木盆地南缘(东昆仑北缘)原定为金水口群麻粒岩相片麻岩和花岗质岩石中锆石的SHRIMP测定,确定其麻粒岩相变质时代为460Ma±8Ma,而具有深熔成因特征的花岗质岩石深熔作用时间为402Ma±6Ma。这些年龄数据表明,金水口群经历了早古生代与麻粒岩相变质和深熔作用有关的构造热事件,原认为是柴达木地块的前寒武纪变质基底明显在早古生代造山过程中发生了活化作用。花岗质岩石的继承锆石给出了少量太古宙和大量1600～1800Ma之间的年龄,代表了其锆石的主要源区物质年龄,这与祁连—柴达木地区以及扬子地块的地壳形成年龄基本一致,反映柴南缘(东昆仑北缘)的变质基底与扬子克拉通具有明显的亲缘性。     

Evaluation of dam overtopping probability induced by flood and wind

This study develops a probability-based methodology to evaluate dam overtopping probability that accounts for the uncertainties arising from wind speed and peak flood. A wind speed frequency model and flood frequency analysis, including various distribution types and uncertainties in their parameters, are presented. Furthermore, dam overtopping probabilities based on monthly maximum (MMax) series models are compared with those of the annual maximum (AMax) series models. An efficient sampling scheme, which is a combination of importance sampling (IS) and Latin Hypercube sampling (LHS) methods, is proposed to generate samples of peak flow rate and wind speed especially for rare events. Reservoir routing, which incorporates operation rules, wind setup, and run-up, is used to evaluate dam overtopping probability.     

Determination of unit hydrographs by multiple storm analysis

This paper discusses the derivation of a unit hydrograph by multiple storm analysis using least squares methods. Variations of least squares method was generalized using the framework of weighted ridge analysis. The paper also shows two theorems to support using multi-storm analysis to derive UH. In addition, an issue was addressed on the scaling effect in the conventional multi-storm analysis which could create potential bias toward large storms in deriving a multi-storm UH. For that, a simple scaling procedure was proposed to reduce such potential bias problem. Numerical investigations were conducted to examine the performance of the scaling procedure by comparing with the conventional multi-storm analysis (without scaling) and the HEC-1 weighing procedures. Based on various performance criteria using a total of 39 storms from three watersheds, the proposed scaling procedure was found to produce a quite desirable UH.