汶川地震诱发山地斜坡震动的地形放大效应

汶川地震诱发大量山地斜坡次生灾害具有明显的地形分布特征,对其地形放大效应研究具有重要的理论及实际意义.斜坡震害调查表明,宏观地貌单元对次生地质灾害的发育类型具有控制作用,中高山区以大中型滑坡、崩塌为主,低山及丘陵区以山体震裂、小型崩塌为主;峡谷形态对次生地质灾害的发育密度具有明显影响,其中“V”型峡谷发育密度高于“U”型峡谷;单薄山脊、条形山体、坡型转折部位、凸出地形等微地貌地形放大效应明显.监测显示,不同方向水平分量地震波峰值(PHA)相差可达10余倍,研究表明:地形尺寸与地震波波长的耦合作用使得地震动水平分量放大显著.     

A simplicial complex‐based DBMS approach to 3D topographic data modelling

This paper introduces a new compact topological 3D data structure. The proposed method models the real world as a complete decomposition of space and this subdivision is represented by a constrained tetrahedral network (TEN). Operators and definitions from the mathematical field of simplicial homology are used to define and handle this TEN structure. Only tetrahedrons need to be stored explicitly in a (single column) database table, while all simplexes of lower dimensions, constraints and topological relationships can be derived in views. As a result the data structure is relatively compact and easy to update, while it still offers favourable characteristics from a computational point of view as well as presence of topological relationships.     

Effects of hillslope topography on runoff response in a small catchment in the Fudoji Experimental Watershed,central Japan

We investigated the role of different hillslope units with different topographic characteristics on runoff generation processes based on field observations at two types of hillslopes (0·1 ha): a valley‐head (a convergent hillslope) and a side slope (a planar hillslope), as well as at three small catchments having two types of slopes with different drainage areas ranging from 1·9 to 49·7 ha in the Tanakami Mountains, central Japan. We found that the contribution of the hillslope unit type to small catchment runoff varied with the magnitude of rainfall. When the total amount of rainfall for a single storm event was < 35 mm, runoff in the small catchment was predominantly generated from the side slope. As the amount of rainfall increased (>35 mm), the valley‐head also began to contribute to the catchment runoff, adding to runoff from the side slope. Although the direct runoff from the valley‐head was greater than that from the side slope, the contribution from the side slope was quantitatively greater than that from the valley‐head due to the proportionally larger area occupied by the side slope in the small catchment. The storm runoff responses of the small catchments reflected the change in the runoff components of each hillslope unit as the amount of rainfall increased and rainfall patterns changed. However, similar runoff responses were found for the small catchments with different areas. The similarity of the runoff responses is attributable to overlay effects of different hillslope units and the similar composition ratios of the valley‐head and side slope in the catchments. This study suggests that the relative roles of the valley‐head and side slope are important in runoff generation and solute transport as the catchment size increases from a hillslope/headwater to a small catchment. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of groundwater and topography on stream drying in semi-arid headwater streams

Non-perennial streams comprise over half of the global stream network and impact downstream water quality. Although aridity is a primary driver of stream drying globally, surface flow permanence varies spatially and temporally within many headwater streams, suggesting that these complex drying patterns may be driven by topographic and subsurface factors. Indeed, these factors affect shallow groundwater flows in perennial systems, but there has been only limited characterisation of shallow groundwater residence times and groundwater contributions to intermittent streams. Here, we asked how groundwater residence times, shallow groundwater contributions to streamflow, and topography interact to control stream drying in headwater streams. We evaluated this overarching question in eight semi-arid headwater catchments based on surface flow observations during the low-flow period, coupled with tracer-based groundwater residence times. For one headwater catchment, we analysed stream drying during the seasonal flow recession and rewetting period using a sensor network that was interspersed between groundwater monitoring locations, and linked drying patterns to groundwater inputs and topography. We found a poor relationship between groundwater residence times and flowing network extent (R² < 0.24). Although groundwater residence times indicated that old groundwater was present in all headwater streams, surface drying also occurred in each of them, suggesting old, deep flowpaths are insufficient to sustain surface flows. Indeed, the timing of stream drying at any given point typically coincided with a decrease in the contribution from near-surface sources and an increased relative contribution of groundwater to streamflow at that location, whereas the spatial pattern of drying within the stream network typically correlated with locations where groundwater inputs were most seasonally variable. Topographic metrics only explained ~30% of the variability in seasonal flow permanence, and surprisingly, we found no correlation with seasonal drying and down-valley subsurface storage area. Because we found complex spatial patterns, future studies should pair dense spatial observations of subsurface properties, such as hydraulic conductivity and transmissivity, to observations of seasonal flow permanence.     

Quantifying ice storage in upper Indus river basin using ground-penetrating radar measurements and glacier bed topography model version 2

Ice reserve estimates is a fundamental prerequisite for water resources management. We selected the UIB (upper Indus river basin) as study area because it contains the most abundant mid-latitude glaciers outside the polar region, however, the ice reserve estimates remain unclear due to the harsh topography. In this study, we validated the parameters of the GlabTop2 model (Glacier Bed Topography version 2) using the ice thickness measured by GPR (ground-penetrating radar) and compared the “GPR-measured ice thickness and ice bed elevation” versus “the estimated results obtained from GlabTop2.” Integrated with IDW (inverse distance weighted) interpolated results of glaciers of various sizes, a reasonable parametric scheme (τ = 120 kPa and f = 0.8) of GlabTop2 was applied on vast amounts of glaciers in the UIB region. The GlabTop2 estimates indicated that the ice thickness of the UIB varied from 0 to 736.0 ± 110.0 m, with an average value of 74.5 ± 11.2 m. A significant spatial heterogeneity exists in the sub-basins. The Shyok, Shigar and Hunza that contain the most abundant ice reserve. Lesser quantities are stored in the Western Himalaya and Hindu Kush ranges, which account for 11.3 % and 6.7 % of the total ice reserve in the UIB, respectively. A total volume of 1162.4 ± 175.1 km³ of glacier can be converted to 1046.2 ± 157.6 Gt ice reserve; this is 13.6 times the annual average discharge obtained from the outlet of the Besham hydrological station. We aim to present estimates that can provide the baseline information for glaciology study of the Indus river.     

Measuring ignitability for in situ burning of oil spills weathered under Arctic conditions: from laboratory studies to large-scale field experiments

This paper compares the ignitability of Troll B crude oil weathered under simulated Arctic conditions (0%, 50% and 90% ice cover). The experiments were performed in different scales at SINTEF’s laboratories in Trondheim, field research station on Svalbard and in broken ice (70-90% ice cover) in the Barents Sea. Samples from the weathering experiments were tested for ignitability using the same laboratory burning cell. The measured ignitability from the experiments in these different scales showed a good agreement for samples with similar weathering. The ice conditions clearly affected the weathering process, and 70% ice or more reduces the weathering and allows a longer time window for in situ burning. The results from the Barents Sea revealed that weathering and ignitability can vary within an oil slick. This field use of the burning cell demonstrated that it can be used as an operational tool to monitor the ignitability of oil spills.     

Influence of irregular topography and random soil properties on coherency loss of spatial seismic ground motions

Coherency functions are used to describe the spatial variation of seismic ground motions at multiple supports of long span structures. Many coherency function models have been proposed based on theoretical derivation or measured spatial ground motion time histories at dense seismographic arrays. Most of them are suitable for modelling spatial ground motions on flat‐lying alluvial sites. It has been found that these coherency functions are not appropriate for modelling spatial variations of ground motions at sites with irregular topography (Struct. Saf. 1991; 10 (1):1–13). This paper investigates the influence of layered irregular sites and random soil properties on coherency functions of spatial ground motions on ground surface. Ground motion time histories at different locations on ground surface of the irregular site are generated based on the combined spectral representation method and one‐dimensional wave propagation theory. Random soil properties, including shear modulus, density and damping ratio of each layer, are assumed to follow normal distributions, and are modelled by the independent one‐dimensional random fields in the vertical direction. Monte‐Carlo simulations are employed to model the effect of random variations of soil properties on the simulated surface ground motion time histories. The coherency function is estimated from the simulated ground motion time histories. Numerical examples are presented to illustrate the proposed method. Numerical results show that coherency function directly relates to the spectral ratio of two local sites, and the influence of randomly varying soil properties at a canyon site on coherency functions of spatial surface ground motions cannot be neglected. Copyright © 2010 John Wiley & Sons, Ltd.     

汶川地震崩滑灾害影响因素分析

为研究汶川地震崩滑灾害主要影响因素,在掌握汶川地震灾区公路沿线地震崩滑灾害资料基础上,选取典型段进行灾害统计分析,研究表明动力条件下斜坡失稳主要受斜坡岩体结构特征、地震动峰值加速度和斜坡动力响应特征三方面因素影响。地震动峰值加速度越高,地震崩滑灾害越发育。斜坡动力响应特征主要取决于地形地貌和地层岩性,陡坡硬岩段为地震崩滑灾害高发区。斜坡岩体结构是控制斜坡变形破坏的主要因素,从研究斜坡动力失稳角度,提出了斜坡岩体结构类型的划分,分为土层及强风化层——基岩二元结构、块状结构、层状及似层状结构、碎裂结构、土层等5个大类12个亚类。     

电磁法测量的地形影响及校正方法

针对地形对电磁测量结果的影响,设计了一组带地形模型,采用有限差分法对其进行了正演模拟计算。对模拟结果进行了分析,总结了地形对电磁测量视电阻率的影响规律,即TM模式比TE模式易受地形影响,山顶比山谷易受地形影响。在此基础上,分析了常用地形校正方法的优缺点;理论模型和实测资料反演结果表明带地形的二维联合反演可有效消除地形影响。     

Numerical Study on the Initiation of the Severe Convective Weather in Chongqing on 6 May 2010

As a follow-up of a previously published article on the synoptic background of the development of the severe convective weather that happened in Chongqing on 6 May 2010, this study further examines the initiation of the severe convective weather via a better high-resolution simulation with the Weather Research and Forecasting (WRF) model. It is found that the cold front approaching Chongqing from the northwest played a critical role in the initiation of the severe convective weather. As the cold front approached Chongqing, the low-to-mid level updrafts ahead of the front acted to increase the atmospheric lapse rate via the stretching effect, which in combination with the low-level diabatic heating induced by the sensible heat fluxes and infrared radiation emitted from the ground surface led to the continuous decrease of the low-level static stability and the increase of the convective available potential energy (CAPE) in Chongqing area. This provided necessary unstable energy for the development of deep moist convection. Furthermore, along with the reaching of a nearly east-west-oriented mesoscale convergence line from the southeast of Chongqing, the outflow right above the cold front began to interact with that above the mesoscale convergence line and induced distinct convergence at the altitude of approximately 1-2 km in the triangular area sandwiched by the cold front and the mesoscale convergence line. It is found that the updrafts associated with this convergence provided lifting necessary for the initiation of the severe convection. The sensitivity experiment without the terrain west of Chongqing indicates that the local topography did not play an important role in the initiation of this severe convective weather.