三维正交非结构网格数值模型中物理流场的定义方法与改进

使用Casulli等(2000)的数值方法建立了基于正交非结构网格的U型渠道的三维近岸正压水动力数值模型,检验了模型使用ELM(Eulerian-Lagrangian Method)方法处理平流项时,物理流场的定义对模拟结果的作用。数值试验和结果分析说明,使用ELM方法计算平流项所需的物理流场对计算结果影响明显,而目前常用的两种物理流场定义方法都存在一定不足——水位计算值对流速变化的反映不灵敏或者会产生小扰动等。针对常用物理流场定义方法的不足之处,作者在Casulli方法的基础上利用对多边形切向流速分量进行平均的方法改进了物理流场定义,抑制了小扰动。     

Efficient Methods to Simulate Planar Free Surface in the 3D 4th-Order Staggered-Grid Finite-Difference Schemes

We numerically tested accuracy of two formulations of Levander's (1988) stress-imaging technique for simulating a planar free surface in the 4^th-order staggered-grid finite-difference schemes. We have found that both formulations (one with normal stress-tensor components at the surface, the other with shear stress-tensor components at the surface) require at least 10 grid spacings per minimum wavelength ( _min÷h = 10) if Rayleigh waves are to be propagated without significant grid dispersion in the range of epicentral distances up to 15 _dom ^S.Because interior 4^th-order staggered-grid schemes usually do not require more than 6 grid spacings per minimum wavelength, in the considered range of epicentral distances, it was desirable to find alternative techniques to simulate a planar free surface, which would not require denser spatial sampling than _min÷h = 6. Therefore, we have developed and tested new techniques: 1. Combination of the stress imaging (with the shear stress-tensor components at the surface) with Rodrigues' (1993) vertically refined grid near the free surface. 2. Application of the adjusted finite-difference approximations to the z-derivatives at the grid points at and below the surface that uses no virtual values above the surface and no stress imaging. The normal stress-tensor components are at the surface in one formulation, while the shear stress-tensor components are at the surface in the other formulation.The three developed formulations give for the spatial sampling _min÷h = 6 results very close to those obtained by the discrete-wavenumber method. Because, however, the technique with the vertically refined grid near the free surface requires 3 times smaller time step (due to the refined grid), the technique with adjusted finite-difference approximations is the most accurate and efficient technique from the examined formulations in the homogeneous halfspace.     

The September 8–9, 1998 Rain-Triggered Flood Events at Motozintla, Chiapas, Mexico

In September 1998 tropical storm “Earl” swept southern Mexico, producing intense rainfall in the states of Oaxaca and Chiapas. Among the most devastated cities was Motozintla, located in the drainage basin of the Allende, La Mina and Xelajú Grande Rivers. The rainfall from the tropical storm totaled 175 mm on September 8 and 130 mm on September 9, duplicating in two days the average monthly precipitation in the region. Numerous landslides occurred in the vicinity of Motozintla, depositing large volumes of material into the Xelajú Grande stream. Much of this sediment was subsequently remobilized, yielding debris flows, hyperconcentrated flows, and sediment-laden flows that inundated most sections of Motozintla city. The flows covered an approximate area of 3.15 km² with a minimum volume of 4.4 × 10⁶ m³ of sediment. Communication of Motozintla with the rest of the Chiapas State was interrupted for about a month, as was the supply of potable water, food, electricity, and fuel. The geologic record around Motozintla indicates that the Xelajú Grande River has been a pathway for similar large floods during the last 6000 years. The oldest deposit yielded a radiocarbon age of 5320 ± 100 ¹⁴C years. B.P. At least two historic floods have occurred during the last 100 years, a time period defined by a stratigraphically distinct tephra of 1902. Frequency analysis of the historical record of daily rainfall in the Motozintla area suggests that events like that of September, 1998, have a recurrence interval of about 25 years. After the catastrophic flows of 1998, the mitigation measures by Municipal Authorities were made without regard to geological and environmental factors, or to taking into consideration the flow magnitude and appropriate hazard-mitigation techniques, with the result that Motozintla remains at serious risk for future floods. Unfortunately, prior to the publication of this study, in early October 2005, Motozintla was seriously damaged again by intense rain provoked by Hurricane Stan.     

Pyroclastic flow deposits of the 1991 eruption of Volcán de Colima,Mexico

The April 16, 1991, eruption of Volcán de Colima represents a classical example of partial dome collapse with the generation of progressively longer-runout, Merapi-type pyroclastic flows that traveled up to 4 km along the El Cordoban gullies (East, Central and West). The flows filled the gullies with block-and-ash flow deposits up to 10 m thick, of which, after 7 years of erosion, only remnants remained in the El Cordoban West and East gullies. The El Cordoban Central gully, however, provided a well-preserved and incised longitudinal section of the 1991 deposits. The deposits were emplaced as proximal and distal facies, separated by a change in slope angle from >30° to <20°. The proximal facies consists of massive, clast-supported flow units (up to 1 m thick) with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading. The distal facies consists of a massive, matrix-supported deposit up to 8 m thick, which contains dispersed andesite blocks in a fine ash matrix. In the distal facies, a train of blocks marks flow-unit upper boundaries and, although sorting is poor, some grading is present. Thin, finely stratified, or dune-bedded layers of fine ash material are locally present above or below units of both facies. Sedimentologic parameters show that the size or fraction of large pyroclasts (larger than –1 ) decreases from proximal to distal facies, as the percentage of matrix (0 to 4 ) increases, especially immediately beyond the break in slope. We propose that the propagation of the Colima pyroclastic flows is critically dependent on local slope angle, the presence of erodible slope debris, and the decrease in grain size with distance from the vent. The progressive fining is probably caused by some combination of erosion, clast breakup and deposition of larger pyroclasts, and is itself influenced by the slope angle. In the proximal region, the flows moved as granular avalanches, in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud. The maximum runout distance of the avalanches was controlled by the angle of repose of the material, and the volume and grain size of source and eroded material. Because the slope angle is close to the repose angle for this debris, granular avalanches were not able to propagate far beyond the change in slope. If, however, an avalanche had enough mass in finer grain size fractions, at least part of the flow continued beyond the break in slope and across the volcano apron, propagating in a turbulent state and depositing surge layers, or in an otherwise settling-modified state and depositing block-and-ash flow layers.Editorial responsibility: T Druitt     

Disaster diaspora and the consequences of economic displacement and climate disruption,including Hurricanes Matthew (October 8, 2016) and Florence (September 14, 2018) in Robeson County,North Carolina

Natural Hazards - This paper presents a case study of Robeson County’s challenges in addressing the double-barrel disasters of Hurricane Matthew in 2016 (Category 5) and Hurricane Florence in...     

Detrital provenance of the Grenvillian Oaxacan Complex,southern Mexico: a zircon perspective

The Oaxacan Complex is the largest exposure of Grenvillian-age rocks in Mexico, constituting the backbone of the Oaxaquia microcontinent. Whereas the main rock-forming events were previously established at 1,150–1,200 Ma (charnockite–syenite–gabbros), 1,020 Ma (AMCG suite), 990 Ma (granulite-facies metamorphism), and ca. 970 Ma post-tectonic pegmatites, no data are yet available to establish provenance links with other Grenville-age terranes. In this work, we studied detrital zircons belonging to 12 samples, all metamorphosed under granulite facies but variably affected by retrogression. Laser ablation inductively coupled plasma mass spectrometry U–Pb geochronology was employed on selected zircons to determine their crystallization age and geochemistry. The results of the analysis of about 100 crystals per sample show that the studied zircons range between ca. 940 and 1,400 Ma, with only three samples having zircons between 1,400 and 1,600 Ma, and only one showing older zircons up to ca. 1,775 Ma. Whereas some of the slightly discordant (1–5 %) zircons in several samples show ages younger than the granulite metamorphism (probably as a result of Pb loss), and thus a disturbed geochemical pattern (abnormal enrichment in LREE, decreasing HREE), a few metamorphic zircons show flat and depleted HREE patterns, contrasting with the igneous pattern of older zircons (positive Ce anomaly, negative Eu anomaly, enriched HREE pattern). The main distributions observed using the kernel density estimator diagrams fall in the range 975–995 Ma (six samples), 1,100 Ma (four samples) and 1,120–1,170 Ma (six samples). Only the southernmost sample shows a marked peak at ca. 1,400 Ma. The application of the Kolmogorov–Smirnov (K–S) statistical test to the studied samples and particularly the comparison of obtained P values yield interesting similarities. Overall, two sample groups show internal similarities, i.e., they may belong to the same source area, whereas only one sample is dissimilar, failing to pass the K–S test. Comparison of these data with the timing of comparable events in the Sveconorwegian orogens, the Sunsas and Rondonia-San Ignacio belts of Amazonia, and some of the Precambrian massifs cropping out in the Andes help to constrain possible Mesoproterozoic conjugate margins of Oaxaquia.