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气候变化的第四次评估报告中省略了从冰盖到全球海平面的潜在贡献。这反映了在格陵兰岛和南极洲现存的冰盖的冰动力学（运动）和质量平衡（雪和冰的质量平衡与冰雪损失）方面的不确定性水平。对冰动力学的一个潜在的关键控制是冰川曲面，它可以促进表面熔体水与冰盖床的路由，从而导致出口冰川上的滑动速度增加。此外，曲面控制了海洋终止边缘的冰山的生产，格陵兰冰盖通过该冰山的生产约50％，南极冰盖几乎所有的年度冰损失。冰山产量（产犊）可能是通过自下而上和自上而下的曲面和大气变暖的结合，通过增加熔体水以填充表面裂缝的可用性，至少在短期内可能是变化的主要驱动力。直到最近，基于物理学的裂缝/产犊关系与预测性数值模型的发展发展。这些进步对于改善我们对大冰块对未来变暖的反应的预测至关重要。但是，迄今为止，只有一项研究测试了这些基于物理的曲面关系，然后仅用于无水裂缝。鉴于当前的研究重点是冰川曲面，因此迫切需要测试曲面模型。然而，由于在冰川的裂隙区域工作的困难，在冰川区域工作，测量了最终在深度的发际线裂缝中结束的深度，并将裂缝与瞬时应力/应变场相关联，因此在田野上做到这一点。项目合作伙伴DB有一个项目，准备在Svalbard的Kronebreen上部署仪器进行连续的水位监测。地球物理成像目前是有问题的，例如，“温暖”温带冰川，信号干扰裂缝磁场的信号衰减，并获得分辨率以对裂缝（裂纹）尖端进行成像。同样，控制水的深度迫使裂缝穿透将提出重大挑战，例如，填充裂缝所需的水量或与沟渠排水系统的连接所需的水量，导致水损失等。因此，建模方法代表了一种理想的前进方式。但是，实验室模型是无用的，因为与裂缝传播相关的应力增加了自重应力（重力x冰密度x冰厚度）和裂纹长度的函数，即裂缝深度。岩土工程离心机是一种独特的建模工具，可以通过缩减模型中的尺寸，但“增强”重力来缩小原型（现实世界）和模型之间的应力等效性，从而允许重现幅度的自重应力。这是通过“飞行”（旋转）在离心机中的模型来实现的，以使在N时重力在N TIMES中飞行的N量表模型与原型产生相同的自重应力。已经针对与本研究相关的所有参数建立了扩展关系，因此没有预期的扩展问题，但是将采用模型离心技术的标准建模来确认这一点。然后将运行一系列模型，重复冰川冰川部分〜50x80x50 m中所经历的应力水平。预铸前的裂缝将充满水，以促进逐步的全深处透明渗透，从而可以测试基于物理的最新模型的当前状态。该项目将提供概念证明，该证明将有助于进一步的赠款应用程序，其中可以构建和使用更复杂的模型（例如自下而上和自上而下）来测试和开发物理模型。