Spatial distribution and abundance of Unionidae mussels in a eutrophic floodplain lake     

《Limnologica》2016

Although Unionidae mussels produce large biomass and reach high density in freshwater habitats, little is known about their ecology. We studied the distribution of 5 species of freshwater unionids in a eutrophic floodplain lake, on transects, along the lake shore and across the depth gradient. The clam distribution within the water body was not random: all species form a crowded zone along the lake shore, showing the highest density at ca. 0.5 m depth. The distribution of the most numerous species changed along the shore in Anodonta anatina and Unio pictorum but not in A. cygnea, whose numbers remained constant. The population numbers of the most numerous species showed a positive correlation with silt layer thickness. The generalized model of all the analyzed factors influencing the unionids’ distribution confirmed this relation and indicated a trade-off between water depth and distance from bank, which might be responsible for the occurrence of the zone at some optimum depth. Unionids have an important influence on freshwater ecosystem functions, thus their zonation implies that their functions are also spatially structured.  相似文献   

Impact of hazard‐consistent ground motion duration in structural collapse risk assessment          下载免费PDF全文

Reagan Chandramohan  Jack W. Baker  Gregory G. Deierlein 《地震工程与结构动力学》2016,45(8):1357-1379

This study evaluates the effect of considering ground motion duration when selecting hazard‐consistent ground motions for structural collapse risk assessment. A procedure to compute source‐specific probability distributions of the durations of ground motions anticipated at a site, based on the generalized conditional intensity measure framework, is developed. Targets are computed for three sites in Western USA, located in distinct tectonic settings: Seattle, Eugene, and San Francisco. The effect of considering duration when estimating the collapse risk of a ductile reinforced concrete moment frame building, designed for a site in Seattle, is quantified by conducting multiple stripe analyses using groups of ground motions selected using different procedures. The mean annual frequency of collapse (λ_collapse) in Seattle is found to be underestimated by 29% when using typical‐duration ground motions from the PEER NGA‐West2 database. The effect of duration is even more important in sites like Eugene (λ_collapse underestimated by 59%), where the seismic hazard is dominated by large magnitude interface earthquakes, and less important in sites like San Francisco (λ_collapse underestimated by 7%), where the seismic hazard is dominated by crustal earthquakes. Ground motion selection procedures that employ causal parameters like magnitude, distance, and Vs₃₀ as surrogates for ground motion duration are also evaluated. These procedures are found to produce poor fits to the duration and response spectrum targets because of the limited number of records that satisfy typical constraints imposed on the ranges of the causal parameters. As a consequence, ground motions selected based on causal parameters are found to overestimate λ_collapse by 53%. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Ductility‐dependent intensity measure that accounts for ground‐motion spectral shape and duration          下载免费PDF全文

Nasser A. Marafi  Jeffrey W. Berman  Marc O. Eberhard 《地震工程与结构动力学》2016,45(4):653-672

The calculated nonlinear structural responses of a building can vary greatly, even if recorded ground motions are scaled to the same spectral acceleration at a building's fundamental period. To reduce the variation in structural response at a particular ground‐motion intensity, this paper proposes an intensity measure (IM_comb) that accounts for the combined effects of spectral acceleration, ground‐motion duration, and response spectrum shape. The intensity measure includes a new measure of spectral shape that integrates the spectrum over a period range that depends on the structure's ductility. The new IM is efficient, sufficient, scalable, transparent, and versatile. These features make it suitable for evaluating the intensities of measured and simulated ground motions. The efficiency and sufficiency of the new IM is demonstrated for the following: (i) elastic‐perfectly plastic single‐degree‐of‐freedom (SDOF) oscillators with a variety of ductility demands and periods; (ii) ductile and brittle deteriorating SDOF systems with a variety of periods; and (iii) collapse analysis for 30 previously designed frames. The efficiency is attributable to the inclusion of duration and to the ductility dependence of the spectral shape measure. For each of these systems, the transparency of the intensity measure made it possible to identify the sensitivity of structural response to the various characteristics of the ground motion. Spectral shape affected all structures, but in particular, ductile structures. Duration only affected structures with cyclic deterioration. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Evaluating nonlinear effective stress site response analyses using records from the Canterbury earthquake sequence     

《Soil Dynamics and Earthquake Engineering》2016

A widely used one-dimensional nonlinear effective stress site response analysis program is used to model the response of potentially liquefiable soils during strong shaking. Ground motion records from six events of the 2010–2011 Canterbury earthquake sequence and the extensive site investigation data that have been obtained for the Christchurch area provide the basis for the analyses. The results of the analyses depend significantly on the input motions and soil profile characterization, so these important aspects are examined. Deconvolved Riccarton Gravel input motions were generated, because recorded rock or firm layer motions were not available. Nonlinear effective stress seismic site response analyses are shown to capture key aspects of the observed soil response through the comparison of acceleration response spectra of calculated surface motions to those of recorded surface motions; however, equivalent-linear and total stress nonlinear analyses capture these aspects as well. Biases in the computed motions compared to recorded motions were realized for some cases but they can be attributed primarily to the uncertainty in the development of the input motions used in the analyses.  相似文献   

基于地震动参数的RC框架结构易损性分析          下载免费PDF全文

徐超  温增平 《地震工程学报》2016,38(2):201-211

以某典型的12层钢筋混凝土框架结构作为研究对象,研究基于非线性动力时程分析和地震动参数的RC框架结构易损性分析方法。首先采用静力pushover分析判定结构薄弱层,并确定结构性能(capacity)参数;然后应用非线性动力时程分析估计结构地震反应,研究以峰值加速度和基本周期加速度反应谱作为地震动参数结构反应的不确定性,并进一步分析结构地震需求(demand)参数与地震动参数的关系;在此基础上,分别建立该结构基于峰值加速度和加速度反应谱的易损性曲线,通过考虑场地条件对地震动特性的影响,研究场地条件对结构易损性的影响,结果表明不同场地条件下的结构易损性曲线有一定差异。应用本文方法,根据新一代地震区划图或地震安全性评价确定的地震动参数,可以直接估计结构在未来地震中出现不同破坏的概率,这在结构的抗震性能评估和地震损失预测中有一定意义。  相似文献   

土层结构对油气管线工程地震动峰值加速度区划图的影响研究          下载免费PDF全文

孙译  石玉成  卢育霞  马林伟  任栋 《地震工程学报》2016,38(2):219-225

油气管线工程是生命线工程的重要组成部分,工程跨度通常超越几十甚至几千公里,从而导致横穿地区覆盖层中土层结构存在明显差异,对地震动峰值加速度(PGA)产生较大影响,进而影响区划结果。本文采用分区拟合放大系数的方法,对华北平原地区某大型管线进行研究,给出研究区不同土层结构条件下场地放大系数KS-基岩PGA拟合函数结果,得到沿线附近10km范围内的PGA区划图结果,并与第四代和第五代中国地震动参数区划图提出的场地系数公式的计算结果进行比较。三种计算方法的结果表明,研究区内50年超越概率10%条件下实际场地放大系数为1.30~1.45,50年超越概率5%条件下实际场地放大系数为1.15~1.30,均高于我国第四代和第五代区划图对场地系数的建议值。50年超越概率10%下的PGA区划图结果显示,局部区域在第四代和第五代地震动参数区划图场地系数的结果中位于0.15g或0.20g区,由于KS的提高,其实际计算结果会提升为0.20g或0.25g分区,这说明场地系数对峰值加速度区划图结果具有较大影响。  相似文献   

黄土丘陵河谷场地地震动特征研究          下载免费PDF全文

马林伟  卢育霞  王良  孙译 《地震工程学报》2016,38(3):373-381,390

研究黄土丘陵河谷场地在地震作用下强地面运动特征的变化情况,可以揭示强震对该类场地上震害的触发机理。结合黄土高原的地貌特征,建立具有代表性的动力数值分析模型,通过输入不同幅值、频谱特性和持续时间的地震波,对起伏地形和覆盖黄土层共同影响下的黄土河谷场地进行地震反应分析。结果表明:黄土层和地形耦合作用控制了地表的PGA变化,使其趋于复杂,在同一输入波不同振幅作用下,与基岩河谷各测点相比,黄土覆盖河谷场地的地震动频谱幅值均有所增加,并且频谱主峰均向高频移动。在不同地震波输入下,场地不同部位的固有频率受地形高程和土层影响;而地震动大小和频谱幅值不仅与场地的基本频谱和地形起伏有关,也与输入地震波的频谱成分相关。输入波PGA与地震频谱特征都不变时,同一场地输出的地震频谱形状具有相似的特征,随着地震持时增长,能量向场地基本频率附近集中,从而可能导致场地上相应频率建筑物震动幅值增加,造成累积破坏。  相似文献   

TerEx Toolbox for semi‐automated selection of fluvial terrace and floodplain features from lidar          下载免费PDF全文

Justin C. Stout  Patrick Belmont 《地球表面变化过程与地形》2014,39(5):569-580

Terraces and floodplains are important indicators of near‐channel sediment dynamics, serving as potential sediment sources and sinks. Increasing availability of high resolution topography data over large areas calls for development of semi‐automated techniques for identification and measurement of these features. In this study we introduce a novel tool that accommodates user‐defined parameters including, a local‐relief threshold selected by a variable‐size moving window, minimum area threshold, and maximum distance from the channel to identify and map discrete terrace and floodplain surfaces. Each of the parameters can easily be calibrated for a given watershed or reach. Subsequently, the tool automatically measures planform area, absolute elevation, and height relative to the local river channel for each terrace polygon. We validate the tool in two locations where terrace maps were previously developed via manual digitization from lidar and extensive field mapping campaigns. The tool is also tested on six different types of rivers to provide examples of starting selection parameters, and to test effectiveness of the tool across a wide range of landscapes. Generally, the tool provides a high quality draft map of terrace and floodplain surfaces across the wide range of environmental conditions for which it has been tested. We find that the tool functions best in catchments where the terraces are spatially extensive, with distinct differences between the terrace and floodplain. The most challenging environments for semi‐automated terrace and floodplain mapping include steep catchments with dense riparian vegetation, and very small terraces (~10 m² in areal extent). We then apply the tool to map terraces and floodplains in the Root River watershed, southeastern Minnesota and generate exceedance plots for terrace heights. These plots provide a first pass analysis to indicate the tributaries and reaches of the river where terraces constitute a significant source of sediment. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Ecomorphodynamics of rivers with converging boundaries          下载免费PDF全文

Paolo Perona  Benoît Crouzy  Stuart McLelland  Peter Molnar  Carlo Camporeale 《地球表面变化过程与地形》2014,39(12):1651-1662

Many rivers worldwide show converging sections where a characteristic limiting front for vegetation establishment on gravel bars is observed. An important conceptual model was advanced in 2006 by Gurnell and Petts, who demonstrated that for the convergent section of the Tagliamento River the downstream front of vegetation establishment can be explained by unit stream power. We introduce a theoretical framework based on 1D ecomorphodynamic equations modified to account for the biological dynamics of vegetation. We obtain the first analytical result explaining the position and river width where vegetation density is expected to vanish in relation to a characteristic streamflow magnitude and both hydraulic and biological parameters. We apply our model to a controlled experiment within a convergent flume channel with growing seedlings perturbed by periodic floods. For a range of timescales where hydrological and biological processes interact, we observe the formation of a front in the convergent section beyond which vegetation cannot survive, the location of which is explained by flow magnitude. This experiment confirms that the timescales of the involved processes and the unit stream power determine the existence and the position of the front within convergent river reaches, respectively. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Parameter-independent model reduction of transient groundwater flow models: Application to inverse problems     

《Advances in water resources》2014

A new methodology is proposed for the development of parameter-independent reduced models for transient groundwater flow models. The model reduction technique is based on Galerkin projection of a highly discretized model onto a subspace spanned by a small number of optimally chosen basis functions. We propose two greedy algorithms that iteratively select optimal parameter sets and snapshot times between the parameter space and the time domain in order to generate snapshots. The snapshots are used to build the Galerkin projection matrix, which covers the entire parameter space in the full model. We then apply the reduced subspace model to solve two inverse problems: a deterministic inverse problem and a Bayesian inverse problem with a Markov Chain Monte Carlo (MCMC) method. The proposed methodology is validated with a conceptual one-dimensional groundwater flow model. We then apply the methodology to a basin-scale, conceptual aquifer in the Oristano plain of Sardinia, Italy. Using the methodology, the full model governed by 29,197 ordinary differential equations is reduced by two to three orders of magnitude, resulting in a drastic reduction in computational requirements.  相似文献