An assessment of conditions before and after the 1998 Tam Pokhari outburst in the Nepal Himalaya and an evaluation of the future outburst hazard

On 3 September 1998, a glacial lake outburst flood (GLOF) that originated from Tam Pokhari occurred in the Hinku valley of the eastern Nepal Himalaya. This study analyses the lake's geomorphic and hydrologic conditions prior to the outburst, and evaluates the conditions that could contribute to a future flood through photogrammetric techniques. We processed high‐resolution Corona KH‐4A (2.7 m) and ALOS PRISM (2.5 m) stereo‐images taken before and after the GLOF event, and produced detailed topographic maps (2‐m contour interval) and DEMs (5 m × 5 m). We (re‐) constructed lake water surfaces before (4410 ± 5 m) and after (4356 ± 5 m) the outburst, and reliably estimated the lake water surface lowering (54 ± 5 m) and the water volume released (19.5 ± 2.2 × 10⁶ m³) from the lake, showing good agreement with the results obtained from ground‐based measurements. The most relevant conditions that may have influenced the catastrophic drainage of Tam Pokhari in 1998 include the presence of: (i) a narrow (75 ± 6 m), steep (up to 50°) and high (120 ± 5 m) moraine dam; (ii) high lake level (8 ± 5 m of freeboard) and (iii) a steep overhanging glacier (>40°). The lake outburst substantially altered the immediate area, creating a low and wide (>500 m) outwash plain below the lake, a wide lake outlet channel (~50 m) and a gentle channel slope (~3–5°). Our new data suggest that the likelihood of a future lake outburst is low. Our results demonstrate that the datasets produced by photogrammetric techniques provide an excellent representation of micro‐landform features on moraine dams, lake water surfaces and the changes in both over time, thereby allowing highly accurate pre‐ and post‐GLOF (volumetric) change analysis of glacial lakes. Furthermore, it enables precise measurement of several predictive variables of GLOFs that can be useful for identifying potentially dangerous glacial lakes or prioritizing them for detailed field investigations. Copyright © 2015 John Wiley & Sons, Ltd.     

Tidal estuary width convergence: Theory and form in North Australian estuaries

In order to better understand the relations between tidal estuary shape and geomorphic processes, the width profiles of 79 tidal channels from within 30 estuaries in northern Australia have been extracted from LANDSAT 5 imagery using GIS. Statistics describing the shape and width convergence of individual channels and entire estuaries (which can contain several channels) are analysed along with proxies for the tidal range and fluvial inputs of the estuaries in question. The width profiles of most individual channels can be reasonably approximated with an exponential curve, and this is also true of the width profiles of estuaries. However, the shape of this exponential width profile is strongly related to the mouth width of the system. Channels and estuaries with larger mouths generally exhibit a more pronounced ‘funnel shape’ than those with narrower mouths, reflecting the hydrodynamic importance of the distance over which the channel or estuarine width converges. At the estuarine scale, this ‘convergence length’ also tends to be higher in estuaries which have larger catchments relative to their size. No clear relation between the estuarine width convergence length and tidal range could be discerned within the Northern Australian estuaries although, when these data are combined with data from other studies, a weak relationship emerges. Copyright © 2010 John Wiley & Sons, Ltd.     

Abiotic,residual and functional components of landforms

The concept of functional ecogeomorphology developed originally from the study of fluvial systems represents an insightful framework for describing landforms as structures adjusting to external physical forces, and also as functional components contributing to the complex play of interactions and reciprocal adjustments between biotic and abiotic elements of ecosystems. In this commentary we propose to extend the model of functional ecogeomorphology by classifying landforms in three fundamental components: abiotic, residual and functional. The various possible combinations of these three components are presented and discussed in the scope of evolutionary geomorphology. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydraulic parameter estimations of a 2D model validated with sedimentological findings in the point bar environment

There have been a number of flume tests of flow round bends with idealized geometry and recently hydraulic simulations of such experiment. However, studies of hydraulic models in natural river bend are rather limited because of greater complexity of the flow characteristics and lack of detailed data. In this article, we study how 2D hydraulic model and raster‐based hydraulic parameter calculations predict flow characteristics on the natural point bar environment. We will compare calculations of various hydraulic parameters (velocity, bed shear stress and stream power) by the 2D model and the associated sedimentology of the point bars. As a result of comparison, the usability of the 2D model for flow‐form‐product relationship predictions will be evaluated in natural river bend environment. The study shows that the 2D model can be generally utilized to predict the flood‐generated flow‐form‐product relationship in coarse‐grained and structurally complex point bar environments with sand‐dominated bedload. For example, point bar sections submerged in water depths greater than 50 cm showed a relatively good match laterally between the model and sedimentological estimations. Furthermore, this approach allows us to estimate flood processes on a local scale in similar point bar environments with width–length ratio. The flow direction estimates of the 2D model coincided relatively well with the sedimentological estimations on the bar head. However, flow directions on the downstream section could not be modelled because the 2D model cannot handle the helicoidal flow of the river bend. Copyright © 2010 John Wiley & Sons, Ltd.     

A comparative study of boron isotopes and trace elements of the marine foraminifers during the last glacial maximum and Holocene

Boron isotopic compositions in marine planktonic foraminifers can record changes in seawater pH and hence provide a new tool to reconstruct the changes of paleo-atmospheric PCO2.Here a comparative study was done on boron isotopes and trace element abundances of planktonic foraminifers,Globigerinoides sacculifer and Globigerinoides ruber from three tropical ODP sites,Site 806B,Site 664C and Site 999A.It is demonstrated here that G.ruber is a better species to be used for downcore boron isotope analyses on gl...     

澧阳平原晚冰期短尺度气候变化的风成沉积物稀土元素和粒度记录

对澧阳平原SL剖面的系统采样和室内稀土元素分析、粒度分析和AMS14C年代测定,揭示了澧阳平原岗地黄土剖面成因特征,探讨了澧阳平原晚冰期短尺度的气候变化过程。结果表明,澧阳平原岗地黄土在稀土元素配分模式上与黄土高原黄土-古土壤、镇江下蜀黄土具有高度的一致性,而且岗地剖面沉积物无任何水流作用痕迹,因此表明,澧阳平原岗地黄土剖面为风成成因。粒度结果表明,SL剖面较好地记录了晚冰期短尺度的气候变化过程,SL遗址剖面自然沉积物记录了晚冰期千年尺度的气候事件,如BL事件、OlderYD事件、AL事件、YD事件。SL剖面全新世早期表现为升温阶段,9.0～8.00kaBP的彭头山文化在此区兴起,表明此时气候相当适宜,适合人类生存。     

祁连山老虎沟12号冰川雷达测厚和冰下地形特征研究

冰川地形是构建冰川流动模型的基础,对于认识冰川响应气候变化的动力机制具有重要意义.在2009年和2014年消融季,使用探地雷达对祁连山老虎沟12号冰川进行了厚度测量和冰下地形观测,获得了沿冰川中流线和多条横剖面的厚度资料,并对中流线上的厚度分布特征和槽谷形态进行了研究.研究结果表明,东、西支冰川的平均厚度分别为190m和150m,东支冰川冰下地形起伏大于西支,支冰川的表面坡度都较缓和.东、西支冰川进入汇合区时厚度分别为122m和157m,由于支冰川的横向挤压和汇流,汇合区中部冰川厚度增加到162m.冰川槽谷形态具有空间差异,东、西支冰川槽谷形态近似于对称的V型,但是在冰川汇合区,槽谷底部变宽,边坡变缓,发育有不对称槽谷.     

ESR测年技术在第四纪冰川年代学中的研究综述

近些年,宇宙成因核素暴露年代、光释光和放射性碳埋藏年代等方法在第四纪冰碛物的年代测定中已经取得显著进展.但对于第四纪冰川沉积物10万年甚至百万年以上的冰碛物样品的埋藏年代和暴露年代,由于方法自身问题或沉积物后期地质地貌过程的改造,使上述方法存在定年方面的挑战.而电子自旋共振（ESR）广泛的测年范围,使其在第四纪样品的测年中发挥着不可替代的作用.目前,石英ESR信号的衰退特征已有许多研究进展,但是对于冰碛物中的ESR信号的研究还很少.ESR测年方法由于缺乏测年机理的研究,西方学者和国内一些测年学者对这种方法的定年存有疑虑.因此,我们在典型冰川区开展对冰碛物中石英砂ESR信号变化的机理研究,得到人工研磨可以使ESR信号强度下降到原来值的53%~69%,但并不能使ESR信号完全归零.如果在以后的研究中能够找出这些残留信号值大小,并予以扣除,将会大大提高ESR冰碛物测年精度.在今后的研究中应探求其测年机理,提高测试样品的石英纯度,增强ESR测年信号的精度,使这种方法成为冰碛物交叉测年中独立可信的测年方法之一,为第四纪冰川研究中的老冰碛物的定年提供准确的年代依据.     

西藏喜马拉雅山地区冰湖溃决的预测模型及其应用研究

以西藏喜马拉雅山地区的冰湖为研究对象,基于现有的冰湖溃决预测方法,提出了建立冰湖溃决预测方法的关键点,即选取的指标必须能够体现冰湖的动态变化特征.在定量分析的建模过程中应该采用不确定性的数学理论,对于冰湖溃决可能性的等级划分需要进行合理性及实用性验证.选取坝顶宽度、湖水面距坝顶高度与坝高之比、冰湖面积和补给冰川面积为预测指标,通过对西藏喜马拉雅山地区29个冰湖样本进行逻辑回归分析,建立了冰湖溃决的预测模型,并用所有样本进行了交叉验证.结果表明：该模型能够在分类应用中取得较好效果,根据溃决冰湖累积百分数随冰湖溃决可能性大小的变化曲线,将冰湖溃决的可能性划分为四个等级.以黄湖为例,把湖水面距坝顶高度与坝高之比作为冰湖溃决的诱变指标,分析了冰湖溃决可能性大小的变化规律.结合现有的冰湖溃决预测的定性方法,讨论了所建立的冰湖溃决预测模型的优点和缺点.     

中国第四纪冰川研究的现状与争议——兼记首届“中国第四纪冰川与环境变化”研讨会

冰川是塑造地表形态最积极的外营力之一, 对第四纪冰期与间冰期旋回中冰川进退留下的丰富且形态独特的冰川地形的研究, 不仅能重建古冰川时空演化的规律, 在构造活跃的山区还可为山体抬升提供重要的理论参考. 2012年8月, 中国从事冰川地貌与环境变化研究的学者相聚兰州, 总结了以技术定年为主要特征的第四纪冰川研究新阶段取得的成绩与突破, 并探讨了现阶段工作的重点及未来的发展方向. 与会代表认为, 可对冰川地形进行直接定年的测年技术的发展与应用, 促进了我国第四纪冰川研究的发展, 现阶段及今后应着重加强与开展如下几个方面工作: 1)精确年代学框架的建立是现阶段我国第四纪冰川研究的重点; 2)青藏高原及周边山地最老冰碛的追溯及其年代测定是回答我国第四纪冰川开始发育的关键, 也是理解青藏高原构造抬升与冰期气候耦合的内在要求; 3)我国东部(105° E以东)第四纪冰川与环境问题需作进一步的研究与澄清, 科普工作亟待加强.