Linear analysis of concrete arch dams including dam–water–foundation rock interaction considering spatially varying ground motions

The available substructure method and computer program for earthquake response analysis of arch dams, including the effects of dam–water–foundation rock interaction and recognizing the semi‐unbounded size of the foundation rock and fluid domains, are extended to consider spatial variations in ground motions around the canyon. The response of Mauvoisin Dam in Switzerland to spatially varying ground motion recorded during a small earthquake is analyzed to illustrate the results from this analysis procedure. Copyright © 2009 John Wiley & Sons, Ltd.     

Modelling the impact of land-use change and farm dam construction on hillslope sediment delivery to rivers at the regional scale

Landscapes in southeastern Australia have changed dramatically since the spread of European colonisation in the 19th century. Due to widespread forest clearance for cultivation and grazing, erosion and sediment yields have increased by a factor of more than 150. In the 20th century, erosion and sediment yield were reduced again due to an increasing vegetative cover. Furthermore, during the last decades, thousands of small farm dams were constructed to provide drinking water for cattle. These dams trap a lot of sediment, thereby further reducing sediment delivery from hillslopes to river channels. Changes in sediment delivery since European colonisation are documented in sediment archives. Within this study, these changing rates in hillslope erosion and sediment delivery were modelled using a spatially distributed erosion and sediment delivery model (WATEM/SEDEM) that was calibrated for Australian ecosystems using sediment yield data derived from sedimentation rates in 26 small farm dams. The model was applied to the Murrumbidgee river basin (30,000 km²) under different land-use scenarios. First, the erosion and sediment yield under pre-European land-use was modelled. Secondly, recent land-use patterns were used in the model. Finally, recent land-use including the impact of farm dams and large reservoirs was simulated. The results show that the WATEM/SEDEM model is capable of predicting the intensity of the geomorphic response to changes in land-use through time. Changes in hillslope erosion and hillslope sediment delivery rates are not equal, illustrating the non-linear response of the catchment. Current hillslope sediment supply to the river channel network is predicted to be 370% higher compared to the pre-European settlement period, yet farm dams have reduced this back to 2.5 times the pre-19th century values. The role of larger reservoirs is even more important as they have reduced the current sediment supply downstream to their pre-European values, thus completely masking the increased hillslope erosion rates from land-use change. However, the model does so far not include valley widening and sediment storage in river systems. Therefore, modelled rates of sediment delivery are lower than observed values.     

Seismic assessment of concrete gravity dams using capacity estimation and damage indexes

A new concept to determine state of the damage in concrete gravity dams is introduced. The Pine Flat concrete gravity dam has been selected for the purpose of the analysis and its structural capacity, assuming no sliding plane and rigid foundation, has been estimated using the two well‐known methods: nonlinear static pushover (SPO) and incremental dynamic analysis (IDA). With the use of these two methods, performance and various limit states of the dam have been determined, and three damage indexes have been proposed on the basis of the comparison of seismic demands and the dam's capacity. It is concluded that the SPO and IDA can be effectively used to develop indexes for seismic performance evaluation and damage assessment of concrete gravity dams. Copyright © 2012 John Wiley & Sons, Ltd.     

Distinguishing the effects of climate on discharge in a tropical river highly impacted by large dams

Abstract

The hydrological regime of a river is defined by variables or representative curves that in turn have characteristics related to fluctuations in flow rates resulting from climate variability. Distinguishing between the causes of streamflow variations, i.e. those resulting from human intervention in the watershed and those due to climate variability, is not trivial. To discriminate the alterations resulting from climate variation from those due to regulation by dams, a reference hydrological regime was established using the classification of events based on mean annual streamflow anomalies and inferred climatic conditions. The applicability of this approach was demonstrated by analysis of the streamflow duration curves. An assessment of the hydrological regime in the lower reaches of the São Francisco River, Brazil, after the implementation of hydropower plants showed that the operation of the dams has been responsible for 59% of the hydrological changes, while the climate (in driest conditions) has contributed to 41% of the total changes.

Editor Z.W. Kundzewicz

Citation Genz, F. and Luz, L.D., 2012. Distinguishing the effects of climate on discharge in a tropical river highly impacted by large dams. Hydrological Sciences Journal, 57 (5), 1020–1034.     

Effects of near-fault and far-fault ground motions on nonlinear dynamic response and seismic damage of concrete gravity dams

As the forward directivity and fling effect characteristics of the near-fault ground motions, seismic response of structures in the near field of a rupturing fault can be significantly different from those observed in the far field. The unique characteristics of the near-fault ground motions can cause considerable damage during an earthquake. This paper presents results of a study aimed at evaluating the near-fault and far-fault ground motion effects on nonlinear dynamic response and seismic damage of concrete gravity dams including dam-reservoir-foundation interaction. For this purpose, 10 as-recorded earthquake records which display ground motions with an apparent velocity pulse are selected to represent the near-fault ground motion characteristics. The earthquake ground motions recorded at the same site from other events that the epicenter far away from the site are employed as the far-fault ground motions. The Koyna gravity dam, which is selected as a numerical application, is subjected to a set of as-recorded near-fault and far-fault strong ground motion records. The Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is employed in nonlinear analysis. Nonlinear dynamic response and seismic damage analyses of the selected concrete dam subjected to both near-fault and far-fault ground motions are performed. Both local and global damage indices are established as the response parameters. The results obtained from the analyses of the dam subjected to each fault effect are compared with each other. It is seen from the analysis results that the near-fault ground motions, which have significant influence on the dynamic response of dam–reservoir–foundation systems, have the potential to cause more severe damage to the dam body than far-fault ground motions.     

超标准洪水条件下土石坝安全性应急判别分析

当前极端气候频发,溃坝事件时有发生,提出一种超标准洪水条件下快速判别土石坝安全性的方法对于提升中国防洪决策水平、提高抗洪抢险能力具有重要的意义。然而,由于坝体结构复杂,不确定因素众多,超标准条件下快速判断土石坝安全性具有非常大的难度。借鉴堰塞湖天然坝安全性判别的研究方法,将判别分析法应用于超标准洪水条件下土石坝的安全性快速判别分析,结果证明,该套思路是可行的。从各坝体安全性判别分析方法的预测结果来看,判别分析法和I_DB指标判别方法具有较高的判别正确率,两种判别方法正确率均达到90.0%,优于其它简单指标判别方法。     

中国喜马拉雅山地区滑坡堵江编目及空间特征分析

滑坡堵江数据获取与编目是其区域研究开展的基础。喜马拉雅山脉地处中国西南边陲,新构造运动强烈,滑坡堵江事件频发,在产生巨大经济损失的同时也造成了不良国际影响。鉴于该区区域滑坡堵江现场调查难以开展的问题,本文利用遥感技术、地理信息技术,结合野外验证获取了区内136处滑坡堵江事件的空间位置、基本属性和几何形态,建立了中国喜马拉雅山地区滑坡堵江编目。区内滑坡堵江集中分布在米林、札达、加查、错那、隆子、郎县等县,成因类型以滑坡、崩塌、泥石流为主。基于环境要素信息量计算得出该区滑坡堵江的易发程度随高程、坡度、地震加速度的增大先增大后减小,随地震点密度增大先减小后增大,随构造线密度增大逐渐增大,随与水系距离增大逐渐减小。不同坡向中,西向斜坡更容易诱发滑坡堵江,东南坡向最不容易诱发滑坡堵江。高位高山地貌类型、地层条件中的朗县构造混杂岩组和坚硬岩组,构造分区中的高喜马拉雅分区和雅鲁藏布江分区是滑坡堵江形成的有利条件。对比各环境要素不同类别的信息量取值认为影响该区滑坡堵江事件形成的主要背景因素是高程、地貌类型、地层岩组、构造分区和地震点密度。这些滑坡堵江事件几何参数的研究结果表明坝体长度-坝体面积与滑坡面积-坝体面积之间具有拟合程度较高的乘幂函数关系,而其他参数间的相关性并不突出。     

Beaver dams attenuate flow: A multi-site study

Beavers can profoundly alter riparian environments, most conspicuously by creating dams and wetlands. Eurasian beaver (Castor fiber) populations are increasing and it has been suggested they could play a role in the provision of multiple ecosystem services, including natural flood management. Research at different scales, in contrasting ecosystems is required to establish to what extent beavers can impact on flood regimes. Therefore, this study determines whether flow regimes and flow responses to storm events were altered following the building of beaver dams and whether a flow attenuation effect could be significantly attributed to beaver activity. Four sites were monitored where beavers have been reintroduced in England. Continuous monitoring of hydrology, before and after beaver impacts, was undertaken on streams where beavers built sequences of dams. Stream orders ranged from 2nd to 4th, in both agricultural and forest-dominated catchments. Analysis of >1000 storm events, across four sites showed an overall trend of reduced total stormflow, increased peak rainfall to peak flow lag times and reduced peak flows, all suggesting flow attenuation, following beaver impacts. Additionally, reduced high flow to low flow ratios indicated that flow regimes were overall becoming less “flashy” following beaver reintroduction. Statistical analysis, showed the effect of beaver to be statistically significant in reducing peak flows with estimated overall reductions in peak flows from −0.359 to −0.065 m³ s⁻¹ across sites. Analysis showed spatial and temporal variability in the hydrological response to beaver between sites, depending on the level of impact and seasonality. Critically, the effect of beavers in reducing peak flows persists for the largest storms monitored, showing that even in wet conditions, beaver dams can attenuate average flood flows by up to ca. 60%. This research indicates that beavers could play a role in delivering natural flood management.     

Hydrological functioning of a beaver dam sequence and regional dam persistence during an extreme rainstorm

It is becoming increasingly popular to reintroduce beaver to streams with the hopes of restoring riparian ecosystem function or reducing some of the hydrological impacts of climate change. One of the risks of relying on beaver to enhance ecosystem water storage is that their dams are reportedly more apt to fail during floods which can exacerbate flood severity. Missing are observations of beaver dam persistence and water storage capacity during floods, information needed to evaluate the risk of relying on beaver as a nature-based flood solution. A June rainstorm in 2013 triggered the largest recorded flood in the Canadian Rocky Mountains west of Calgary, Alberta. We opportunistically recorded hydrometric data during the rainfall event at a beaver-occupied peatland that has been studied for more than a decade. We supplemented these observations with a post-event regional analysis of beaver dam persistence. Results do not support two long-held hypotheses—that beaver ponds have limited flood attenuation capacity and commonly fail during large flood events. Instead we found that 68% of the beaver dam cascade systems across the region were intact or partially intact after the event. Pond fullness, in addition to the magnitude of the water-sediment surge, emerged as important factors in determining the structural fate of dam cascade sequences. Beaver ponds at the instrumented site quickly filled in the first few hours of the rain event and levels were dynamic during the event. Water storage offered by the beaver ponds, even ones that failed, delayed downstream floodwater transmission. Study findings have important implications for reintroducing beaver as part of nature-based restoration and climate change adaptation strategies.