Geotechnical centrifuge modelling of crevassing in glaciers and ice sheets

冰川和冰盖裂缝的岩土离心机模拟

• 批准号: NE/J014419/1
• 负责人: Brice Rea
• 金额: $ 6.57万
• 依托单位: University of Aberdeen
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ014419%2F1
• 关键词: Geotechnical; centrifuge; modelling; crevassing; glaciers

Omitted from the 2007 IPCC Fourth Assessment Report on Climate Change was the potential contribution from ice sheets to global sea level. This reflected the level of uncertainty with respect to the ice dynamics (motion) and mass balance (snow and ice accumulation vs. snow and ice loss) of the extant ice sheets in Greenland and Antarctica. One potential key control on ice dynamics is glacier crevassing which can facilitate the routing of surface melt water to the ice sheet bed leading to increased sliding velocities on outlet glaciers. Additionally, crevassing controls the production of icebergs at marine terminating margins, through which the Greenland Ice Sheet disposes of ~50% and the Antarctic Ice Sheet almost all of their respective annual ice loss. Iceberg production (calving) may be through a combination of both bottom-up and top-down crevassing but atmospheric warming, by increasing the availability of melt water to fill surface crevasses, is likely to be the main driver of change, in the short term at least. Only recently have advances been made in the development of physics-based crevassing/calving relationships with incorporation into predictive numerical models. These advances are vital for improving our predictions for the response of the big ice sheets to future warming. However, only one study to date has tested these physics-based crevassing relationships and then only for shallow water-free crevasses. Given the current research focus on glacier crevassing, there is an urgent need to test crevassing models. To do this in the field is however challenging, due to difficulties of working in crevasse zones of glaciers, measuring the depth of what ultimately ends in a hairline crack at depth and associating the crevasse with the instantaneous stress/strain field. Project Partner DB has a project in preparation to deploy instrumentation for continuous water level monitoring in crevasses on Kronebreen, Svalbard. Geophysical imaging is currently problematic for example signal attenuation on 'warm' temperate glaciers, signal interference from adjacent crevasses in crevasse fields and obtaining the resolution to image the crevasse (crack) tip. Likewise controlling water-depth to force crevasse penetration would present significant challenges for example, the volume of water needed for filling a crevasse or connection with the englacial drainage system leading to water loss etc. Field monitoring of glacier crevassing is thus in its infancy. A modelling approach therefore represents an ideal way forward. However, lab-floor models are useless because the stresses relevant to crevasse propagation increase as a function of both the self-weight stress (gravity x ice density x ice thickness) and crack length i.e. the crevasse depth. The geotechnical centrifuge is a unique modelling tool which allows the magnitude self weight stresses to be reproduced, with stress equivalence between the prototype (real world) and the model by scaling down the dimensions in the model but 'enhancing' gravity. This is achieved by 'flying' (spinning) the model in the centrifuge such that an Nth scale model flown at N times gravity generates the same self-weight stress as the prototype. Scaling relationships are already established for all the parameters relevant to this study so no scaling issues are anticipated, but the standard modelling of models centrifuge technique will be employed to confirm this. Then a series of models will be run, replicating the stress levels experienced in a prototype glacier section ~50x80x50 m. Pre-cast crevasses will be filled with water to facilitate step-wise full-depth crevasse penetration allowing the current state of the art physics-based models to be tested. This project will provide a proof of concept which will facilitate further grant applications where more complex models (e.g. bottom-up and top-down) can be built and used to test and develop physical models.

2007年IPCC第四次气候变化评估报告中省略了冰盖对全球海平面的潜在贡献。这反映了格陵兰岛和南极洲现有冰盖的冰动力学（运动）和质量平衡（冰雪积累与冰雪损失）的不确定性程度。对冰动力学的一个潜在的关键控制是冰川裂隙，它可以促进表面融水流向冰盖床，从而导致出口冰川的滑动速度增加。此外，裂隙控制了海洋终端边缘冰山的产生，格陵兰冰盖通过其处理了约 50% 的冰山，而南极冰盖则几乎消耗了各自每年的全部冰损失。冰山的产生（崩解）可能是通过自下而上和自上而下的裂缝相结合的方式进行的，但通过增加融水填充表面裂缝的可用性，大气变暖可能是短期内变化的主要驱动力至少。直到最近，基于物理的裂隙/崩解关系的发展并结合到预测数值模型中才取得了进展。这些进展对于改善我们对大冰盖对未来变暖反应的预测至关重要。然而，迄今为止，只有一项研究测试了这些基于物理的裂缝关系，而且仅针对浅层无水裂缝。鉴于目前的研究重点是冰川裂缝，迫切需要测试裂缝模型。然而，在野外做到这一点具有挑战性，因为在冰川裂缝区域工作、测量最终在深度处形成细裂缝的深度以及将裂缝与瞬时应力/应变场相关联是困难的。项目合作伙伴 DB 有一个项目正在准备部署仪器，用于斯瓦尔巴群岛 Kronebreen 裂缝中的连续水位监测。地球物理成像目前存在问题，例如“暖”温带冰川上的信号衰减、裂缝场中相邻裂缝的信号干扰以及获取裂缝（裂缝）尖端成像的分辨率。同样，控制水深以迫使裂缝渗透也会带来重大挑战，例如填充裂缝所需的水量或与冰川排水系统连接导致水损失等。因此，冰川裂缝的现场监测还处于起步阶段。因此，建模方法代表了一种理想的前进方向。然而，实验室地板模型是无用的，因为与裂缝扩展相关的应力随着自重应力（重力 x 冰密度 x 冰厚度）和裂缝长度（即裂缝深度）的函数而增加。岩土离心机是一种独特的建模工具，可以通过缩小模型尺寸但“增强”重力来重现自重应力的大小，并在原型（现实世界）和模型之间实现应力等效。这是通过在离心机中“飞行”（旋转）模型来实现的，这样在 N 倍重力下飞行的第 N 比例模型会产生与原型相同的自重应力。与本研究相关的所有参数都已经建立了比例关系，因此预计不会出现比例问题，但将采用模型离心机技术的标准建模来证实这一点。然后将运行一系列模型，复制原型冰川部分 ~50x80x50 米所经历的应力水平。预制裂缝将充满水，以促进逐步全深度裂缝渗透，从而允许测试当前最先进的基于物理的模型。该项目将提供概念验证，这将有助于进一步的资助申请，可以构建更复杂的模型（例如自下而上和自上而下）并用于测试和开发物理模型。