基于八阶NAD算子的保辛分部Runge-Kutta方法及其波场模拟(英文)

基于声波方程扩充的哈密尔顿系统,本文给出了空间精度为八阶的近似解析离散化(NAD)保辛分部Runge-Kutta方法,简称八阶NSPRK方法。该方法采用八阶精度的近似解析离散算子近似空间高阶偏微分算子,并使用二阶精度的辛分部Runge-Kutta方法进行时间离散。我们从理论和数值计算两个方面研究了八阶NSPRK方法的稳定性条件和数值频散关系,并同四阶NSPRK方法、八阶Lax-Wendroff(LWC)方法和八阶交错网格(SG)方法进行了比较。结果表明八阶NSPRK方法压制数值频散的能力显著优于传统数值计算方法。与四阶NSPRK方法和传统四阶辛格式(SPRK)方法相比,八阶NSPRK方法具有最小的数值误差和最高的计算效率:在达到同样消除数值频散的前提下,八阶NSPRK方法的计算速度约为四阶NSPRK方法的2.5倍、为四阶SPRK方法的3.4倍;八阶NSPRK方法的存储量仅为四阶NSPRK方法的47.17%、为四阶SPRK方法的49.41%。在双层介质、非均匀介质和Marmousi等复杂速度模型中,八阶NSPRK方法模拟得到的波场快照非常清晰,无可见数值频散。这些结果表明,八阶NSPRK方法在粗网格条件下能有效地压制数值频散,从而能够极大地节省计算内存,提高计算速度。总体而言,八阶NSPRK方法是一种在地震探测领域和地震学研究中有着巨大应用潜力的数值计算方法。     

用经验格林函数法和随机方法模拟卢龙地震的比较

在介绍用经验格林函数方法和随机方法模拟地震动的基础上,分别用这两种方法模拟了卢龙5．7级地震。模拟结果表明,用经验格林函数方法模拟地震动的结果与地震记录符合得很好。用随机方法模拟地震动在高频部分与地震动记录符合得比较好,而在中、低频部分符合得不太好。两种方法比较,经验格林函数方法模拟的结果更符合实际记录。但随机方法具有计算简单、无需小震记录的优点,因此发展随机的方法是有意义的。随机方法中中、低频部分不太符合的原因是随机方法中的很多经验关系是在美国大陆得到的。因此建议加强对中国大陆经验关系的研究,并把中国大陆的经验关系加入到随机方法中,使随机的方法成为中国大陆普适的模拟地震动的方法。     

地震模式识别“PR”程序包的几种方法在北京及其邻区易发震地点识别中的应用

本文使用地震模式识别应用程序包—简称“PR” 程序包(the packageof program-to apple algorithms of pattern recognition)提供的CORA-3修改方法和Hamming方法,对北京及其邻近地区的地质、地貌和地球物理资料进行了新的研究。得到的结果如下:北京及其邻近地区M≥6.0级地震的震中主要分布在自新生代以来活动断裂(特别是主活动断裂)的交汇带内,新生代断陷盆地和新生代断陷盆地与平原或山区的接触地带。 本文对同一组资料用CORA-3方法、CORA-3修改方法、Hamming方法和加权Hamming方法的识别分类结果进行了比较。其中加权Hamming方法识别分类效果最好,CORA-3修改方法次之,Hamming方法和CORA-3方法更次之。 CORA-3方法可以通过识别的示性特征给出震中地区的地质背景及其相应的图象。Hamming方法计算简便、节约机时和内存,而且识别分类效果就本文而言,加权Hamming方法还略好于CORA-3修改方法。     

解析法图像重建中的理想斜变滤波器的进一步研究(英文)

理想斜变滤波器是一种被傅里叶逆变换定义的广义函数。设计理想斜变滤波器的实用数字滤波器是最重要的任务之一。推导了理想斜变滤波器的单位冲激响应的公式。提出了两种实用数字滤波器的设计模式:连续模式和离散模式。分析了3种已经存在的实用滤波器,设计了2种新的实用滤波器。通过一个仿真实验,比较了这5种滤波器。总的来说,以离散模式设计的离散方法优于以连续模式设计的连续方法。对于2种离散方法来说,无限长离散方法优于自适应长度离散方法,它们均可以得到较好的重建质量。对于3种连续方法而言,线性方法是最好的,可以重建出工程可用的CT图像,而常数方法和抛物线方法不能重建出可用的CT图像。     

解析法图像重建中的理想斜变滤波器的进一步研究(英文)

理想斜变滤波器是一种被傅里叶逆变换定义的广义函数。设计理想斜变滤波器的实用数字滤波器是最重要的任务之一。推导了理想斜变滤波器的单位冲激响应的公式。提出了两种实用数字滤波器的设计模式:连续模式和离散模式。分析了3种已经存在的实用滤波器,设计了2种新的实用滤波器。通过一个仿真实验,比较了这5种滤波器。总的来说,以离散模式设计的离散方法优于以连续模式设计的连续方法。对于2种离散方法来说,无限长离散方法优于自适应长度离散方法,它们均可以得到较好的重建质量。对于3种连续方法而言,线性方法是最好的,可以重建出工程可用的CT图像,而常数方法和抛物线方法不能重建出可用的CT图像。     

地下结构抗震分析的振动法与波动法对比研究

针对地下结构抗震分析中应用广泛的波动方法与振动方法,从物理方程和有限元格式出发,明确了两种方法的区别与联系。通过标准数值试验,证明了振动方法是波动方法采用下卧刚性基岩假设和地层水平无限延伸假设,仅考虑SV波垂直向上入射的一种特例。采用波动方法模拟弹性地基,对比振动方法结果,表明弹性地基和刚性地基上地表位移响应和隧道结构内力响应均有显著差异。建议应根据具体场地条件,选择振动或波动方法进行地下结构抗震设计研究。     

标贯击数液化判别方法的比较

依据标贯击数进行液化判别的方法,国外以NCEER推荐方法(改进Seed法)为代表,国内以《水利水电工程地质勘察规范》(GB50487-2008)和《建筑抗震设计规范》(GB50011-2010)为代表。NCEER方法与国内规范方法所依据的地震液化现场调查资料不同,采用的液化判据、反映震级影响的方法和考虑黏粒含量影响的方法也不同。将NCEER方法以液化临界标贯击数与深度的变化曲线表示,并将其与国内规范方法确定的液化临界标贯击数随深度的变化曲线进行比较。结果表明,在相同烈度下:近震时,国内规范方法偏于安全;远震时,对于7.5级以下地震,国内规范方法偏于安全;对于7.5~8.5级地震,在一定加速度(烈度)下,NCEER方法与国内规范方法计算液化临界标贯击数接近,某些加速度(烈度)下NCEER方法偏于安全,某些加速度(烈度)下国内规范方法偏于安全。研究成果可为《水工建筑物抗震设计规范》的修订提供参考。     

用经验格林函数方法研究台站的场地响应

本研究利用成都遥测数字地震台网数字地震波形数据资料,采用经验格林函数方法研究台站的场地响应,并与计算台站场地响应的常规方法-Moya方法得出的结果进行了比较。结果表明,两种方法得到的结果是比较相似的。本研究所采用的经验格林函数方法简单、有效,得到的台站场地响应结果真实可靠。     

土-结构相互作用分析中几类地震输入方式的比较

地震作用下土-结构动力相互作用的整体有限元分析需要在人工边界处输入地震动。目前可能采用的地震输入方法包括黏弹性边界自由场输入方法、自由场应力方法、自由场位移方法以及侧边界自由方法。由于采用近似人工边界条件或者未完全考虑地震自由场,上述地震输入方法均为近似方法。本文以大开地铁车站二维有限元分析为例,根据规范建议的边界位置,研究了上述地震输入方法的精度,研究成果可为土-结构相互作用分析的合理地震输入提供一定参考。     

动力反应分析的显式积分方法及其稳定性

Newm ark-更新精细积分法是动力方程求解的隐式的时域逐步积分法,其稳定性条件非常容易满足。与隐式方法相比较,显式积分方法不需要求解耦联的方程组,可以有效地减少内存占用和机时耗费。因此,根据显式积分方法的特点和优点,基于Newm ark-更新精细积分法的基本思想,提出其显式积分格式。对显式积分方法的精度与稳定性进行了初步的分析,指出该显式积分方法具有极好的稳定性,其精度比隐式积分方法的精度稍低。随着时间步长的增加,其精度优于传统的方法。     

Estimation of recharge from floods in disconnected stream-aquifer systems

Stream-aquifer interaction has been the subject of much research for cases of good hydraulic connection (continuous saturated zone) between a river and an aquifer. Under these conditions, floods do not represent a very large net input to the aquifer because most of the water that enters the aquifer during the flood returns to the river when its stage recedes. The situation is different in disconnected stream-aquifer systems, where the streambed lies above the water level in the aquifer, thus preventing return flow from the aquifer. Under these conditions, floods may represent large, but hard to quantify, water inputs. Here, we present a methodology to estimate recharge from floods for disconnected stream-aquifer systems. Recharge is estimated as the product of a flood time function (dependent on the streamflow) and an unknown factor, which is obtained from calibrating a ground water flow model to aquifer heads. The approach can also benefit from concentration data, which can be very informative when river water concentrations vary over time. This methodology is applied to a field situation where recharge from river flooding is found to amount to nearly 15 million m(3)/year on the average, which represents 40% of the total aquifer inputs. Recharge from flooding helps explain major head recoveries, suggesting that basin water management programs should allow some floods to occur.     

The response of braided planform configuration to flow variations,bed reworking and vegetation: the case of the Tagliamento River,Italy

Morphological features of braided rivers (bars, channels and pools) experience major changes in area, shape and spatial distribution as a response to (i) the pulsation of discharge during a flood and (ii) the bed evolution induced by floods. In this work, at‐a‐station relationships between water level and planform configuration were investigated on the Tagliamento River, a large gravel‐bed braided river in northeast Italy, over a 2‐year study period comprising three bankfull events and several small‐to‐medium floods. The analysis was performed on two 1‐km‐long reaches, characterized by different riparian vegetation cover. Ground‐based images with an hourly temporal resolution were acquired using software‐controlled, digital cameras. Bars, channels, pools and vegetated patches were manually digitized on more than 100 rectified images. Sequences of constant‐level images spanning the study period were used to quantify the impact of floods on the stability of at‐a‐station relationships and on the turnover rate of water bodies. The analysis shows that wetted area increased almost linearly with water level in both reaches. The average number of branches per cross‐section peaked at intermediate flow levels, increasing from 2 at low flow up to 6–7. The number of branches displayed the largest fluctuations over time, with significant changes produced also by moderate floods. Turnover rates were high in both reaches, with more than 30% of wetted areas at low flow converting into bare gravel in less than 2 months. Vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer, deeper anabranches. The number of channels per cross‐section was 30–40% less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3% to 14%. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantifying peakflow attenuation/amplification in a karst river using the diffusive wave model with lateral flow

The aim of this paper is to quantify peakflow attenuation and/or amplification in a river, investigating lateral flow from the intermediate catchment during floods. This is a challenge for the study of the hydrological response of permeable/intermittent streams, and our contribution refers to a modelling framework based on the inverse problem for the diffusive wave model applied in a karst catchment. Knowing the upstream and downstream hydrographs on a reach between two stations, we can model the lateral one, given information on the hydrological processes involved in the intermediate catchment. The model is applied to 33 flood events in the karst reach of the Iton River in French Normandy where peakflow attenuation is observed. The monitored zone consists of a succession of losing and gaining reaches controlled by strong surface‐water/groundwater (SW/GW) interactions. Our results show that despite a high baseflow increase in the reach, peakflow is attenuated. Model application shows that the intensity of lateral outflow for the flood component is linked to upstream discharge. A combination of river loss and overbank flow for highest floods is proposed for explaining the relationships. Our approach differentiates the role of outflow (river loss and overbank flow) and that of wave diffusion on peakflow attenuation. Based on several sets of model parameterization, diffusion is the main attenuation process for most cases, despite high river losses of up to several m³/s (half of peakflow for some parameterization strategies). Finally, this framework gives new insight into the SW/GW interactions during floods in karst basins, and more globally in basins characterized by disconnected river‐aquifer systems.     

水位波动对长江中下游湖泊湖滨带底质环境的影响

以长江中下游湖泊为对象,研究了水位波动对湖滨带底质环境的影响.结果表明,底质环境参数沿高程梯度变化明显.底质含水率沿高程梯度增加而减小,pH值沿高程梯度变化不大,有机质、总氮和总磷沿高程梯度呈先增大后减小的趋势.底质环境参数的季节变化也较大.分析表明,水位波动幅度、淹没深度、高水位持续时间等对湖滨带底质环境有较大影响.在中等波动幅度下,底质参数沿高程的变异系数最大,表明此时湖滨带底质异质性较高.夏季湖滨带淹没时间越长、淹没深度越大,底质养分流失越快.高水位持续时间越长,底质pH值变化就越大、营养盐流失越快.本研究结果可为湖滨带生态修复及管理提供依据.     

Inundation extent as a key parameter for assessing the magnitude and return period of flooding events in southern Iceland

Abstract

River flow conditions in many watersheds of Iceland are particularly disturbed during winter by the formation, drifting and accumulation of river ice, whose impact on water encroachment and extent of inundations is not reflected in the discharge records. It is therefore necessary to use river discharge with great caution when assessing the magnitude of past inundations in Iceland, and to give attention to other flood magnitude parameters. A GIS-based methodology is presented that focuses on inundation extent as an alternative parameter for the assessment and ranking of the magnitude of past flooding events in the Ölfusá-Hvítá basin, known as one of the most dangerous flood-prone river complexes in Iceland. Relying ultimately on a macro-scale grid, the method enabled the reconstruction of the extent of inundations, the delineation of the flood plain, and, finally, some estimation of the likelihood of flooding of exposed areas that include marine submergences and river floods for both open water and ice conditions.

Citation Pagneux, E., Gísladóttir, G. & Snorrason, Á. (2010) Inundation extent as a key parameter for assessing the magnitude and return period of flooding events in southern Iceland. Hydrol. Sci. J. 55(5), 704–716.     

Influence of changes in river system structure on hydrological processes in Taihu Basin,China

ABSTRACT

With the rapid economic development and urbanization in Taihu Basin (eastern China), the river system has decayed and the connectivity of rivers and lakes has weakened, resulting in frequent floods. The impact of changes in river system structure on hydrological processes in the plain river network area was analysed against a background of urbanization. An indicator system was built to describe the quantitative, morphological and spatial structure of the river system. Analysis of the change in annual average and extreme water levels revealed the influencing mechanism of the changes in river system structure on the hydrological processes. The results indicate a decreasing tendency in the density of the river system in the study area, with a reduction in water surface ratio by about 20% in the past 50 years. Since the 1960s, the maximum and annual average water levels have increased. The degree of change in the mean monthly, annual lowest and annual highest water levels was great, with that in the non-flood season being higher than in the flood season. The decrease in the number of rivers directly reduced the storage and adjustable capacity of the basin.     

调水引流工程对平原河网水动力调控的效果

平原河网多存在河道流动性不强,河道生态流速难以保障的问题,以典型平原河网地区——宁波市海曙区为例,开展野外原型同步监测,以河道分流比、换水率及槽蓄量为研究对象,系统分析引调水工程对河网水动力的调控效果.结果表明:平原河网水动力改善效果与引排水口分布、引水流量密切相关,为保证河网区水体交换良好,引排水口的连线应尽可能贯通...     

A comparative flood frequency study of Ganga and Brahmaputra river systems of north India — a brief appraisal

An attempt has been made to study the occurrence of floods in the two important river systems of north India, viz. Brahmaputra and Ganga. Both these river systems are located north of Lat. 22°N and lie in the longitudinal belt of north India from Long. 73°E to 97°E. Both these river systems are affected by floods during monsoon months of June–September. It has been seen after examining rainfall and floods of the period 1986–1999 that although variation of monsoon rainfall magnitudes received by these two basins differ considerably in each monsoon season, but by and large, the frequency of floods at their terminal gauge/discharge (G/D) sites at Dhubri and Farakka do not differ very much from each other.     

Channel form and processes of the flood-dominated Narmada River,India

The 1300 km long Narmada River flows along a structural lineament, alternating between constricting rocky gorges and rapids, and meandering wide alluvial reaches. Channel forms and processes were studied in a 120 km long section of an alluvial reach. Channel size, shape and bedforms in the Narmada River are related to very large floods which have occurred three times in this century. During such floods the entire 400 m wide channel is utilized and 10–15 m high cliffs on both sides operate as riverbanks. Normally, even the high flows of the south-western monsoon are insufficient to fill the whole channel, and hence their effects are limited to building of discontinuous floodplains between the cliffs and modifying bedforms and bars. A channel-in-channel topography is thus created. The very large floods are also responsible for erosion of the rocky stretches and building of point bars. The river meanders, but its movement is restricted because of (1) rocky gorges and scablands operating as anchor points at intervals, and (2) the presence of high alluvial cliffs which are topped on extremely rare occasions. In spite of being located in a tectonically active zone in a monsoon setting, it is the exceptional high-magnitude floods at irregular intervals which control the form and behaviour of the Narmada River.