Geotechnical centrifuge modelling of crevassing in glaciers and ice sheets

2007年IPCC第四次气候变化评估报告中忽略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态（运动）和质量平衡（冰雪积累与冰雪损失）的不确定性。冰动力学的一个潜在的关键控制是冰川裂缝，它可以促进表面融水的路线冰盖床，导致出口冰川的滑动速度增加。此外，冰缝控制着海洋终端边缘冰山的产生，格陵兰冰盖和南极冰盖每年的冰损失量分别约为50%和几乎全部。冰山的产生（产犊）可能是通过自下而上和自上而下的裂缝的组合，但大气变暖，通过增加融化水的可用性来填充表面裂缝，可能是变化的主要驱动力，至少在短期内。直到最近才取得了进展，在发展的物理学为基础的裂缝/产犊关系纳入预测数值模型。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，迄今为止只有一项研究测试了这些基于物理学的裂缝关系，然后只为浅水无裂缝。鉴于目前的研究重点是冰川裂缝，有一个迫切需要测试裂缝模型。然而，由于在冰川的裂缝区工作的困难，在该领域中这样做是具有挑战性的，测量最终以深度处的发丝状裂缝结束的深度，并将裂缝与瞬时应力/应变场相关联。项目合作伙伴DB有一个准备在斯瓦尔巴群岛Kronebreen的裂缝中部署连续水位监测仪器的项目。地球物理成像是目前的问题，例如信号衰减的“温暖”温带冰川，信号干扰，从邻近的裂缝在裂缝领域和获得分辨率成像的裂缝（裂缝）尖端。同样，控制水深，迫使裂缝渗透将提出重大的挑战，例如，所需的水的体积填充裂缝或连接与冰川内排水系统导致水的损失等，因此，冰川裂缝的实地监测处于起步阶段。因此，建模方法是一种理想的前进方式。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作为自重应力（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。土工离心机是一种独特的建模工具，它允许复制自重应力的大小，原型（真实的世界）和模型之间的应力等效，通过缩小模型中的尺寸，但“增强”重力。这是通过在离心机中“飞行”（旋转）模型来实现的，使得在N倍重力下飞行的第N比例模型产生与原型相同的自重应力。已经为与本研究相关的所有参数建立了比例关系，因此预计不会出现比例问题，但将采用模型离心技术的标准建模来确认这一点。然后，将运行一系列模型，复制原型冰川段~ 50 x80 x50 m中经历的应力水平。预制裂缝将充满水，以促进逐步全深度裂缝渗透，允许测试当前最先进的物理模型。该项目将提供一个概念证明，这将促进进一步的赠款申请，其中更复杂的模型（如自下而上和自上而下）可以建立和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.