Geotechnical centrifuge modelling of crevassing in glaciers and ice sheets

2007年气专委第四次气候变化评估报告遗漏了冰盖对全球海平面的潜在贡献。这反映了格陵兰和南极洲现存冰盖的冰动力学(运动)和物质平衡(雪和冰的积累与雪和冰的损失)方面的不确定性程度。冰川破裂是冰川动力学的一个潜在的关键控制因素，它可以促进地表融化水向冰盖床输送，从而增加出口冰川的滑动速度。此外，裂隙作用控制了海洋末端边缘冰山的产生，格陵兰冰盖和南极冰盖通过冰山处理了大约50%的冰山，它们各自每年的冰损失几乎都是通过这些冰山来进行的。冰山的产生(崩解)可能是自下而上和自上而下两种方式的组合，但至少在短期内，大气变暖可能是变化的主要驱动力，因为它增加了融化水的可获得性来填充表面裂缝。直到最近，才在发展以物理为基础的破裂/崩解关系方面取得进展，并将其纳入预测数值模式。这些进展对于改善我们对大冰盖对未来变暖的反应的预测至关重要。然而，到目前为止，只有一项研究测试了这些基于物理的裂缝关系，而且只针对浅水无裂缝。鉴于目前对冰川破裂的研究重点，迫切需要测试冰川破裂模型。然而，在实地做到这一点是具有挑战性的，因为在冰川裂隙带工作、测量最终在深处形成细微裂缝的深度以及将裂隙与瞬时应力/应变场联系起来是困难的。项目合作伙伴DB有一个项目正在准备部署仪器，用于在斯瓦尔巴特群岛Kronebreen的裂缝中持续监测水位。地球物理成像目前存在问题，例如“温暖的”温带冰川上的信号衰减、来自决口区邻近裂隙的信号干扰以及获取对裂隙(裂缝)尖端成像的分辨率。同样，控制水深以迫使冰川穿透将带来巨大的挑战，例如，填补冰川裂缝所需的水量或与冰川排水系统的连接导致水流失等。因此，建模方法代表了一种理想的前进方式。然而，实验室地板模型是没有用的，因为与裂缝扩展有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)的增加而增加。土工离心机是一种独特的建模工具，它允许复制大小自重的应力，通过缩小模型中的尺寸但“增强”重力，使原型(现实世界)和模型之间的应力相等。这是通过在离心机中飞行(旋转)模型来实现的，这样N比例的模型在N倍重力下飞行时产生与原型相同的自重应力。已经建立了与本研究相关的所有参数的比例关系，因此预计不会出现比例问题，但将使用离心机技术对模型进行标准建模来确认这一点。然后将运行一系列模型，复制在约50x80x50米的原型冰川段中经历的应力水平。预制的裂缝将被充满水，以促进逐步全深度裂缝渗透，从而允许测试当前基于物理的模型的最新状态。这一项目将提供概念验证，这将促进进一步的赠款申请，在那里可以建立更复杂的模型(例如自下而上和自上而下)，并用于测试和开发物理模型。

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.