AN EXPERIMENTAL STUDY OF GLACIER SLIP OVER HARD AND SOFT BEDS

随着气候变暖，冰川和冰盖的运动会影响它们的稳定性和随之而来的海平面上升。此外，更新世期间的冰盖运动在风景上留下了壮观的印记。移动最快和改变地形最严重的冰川主要是通过滑过岩石(硬)或沉积物(软)床而移动的。在过去的半个世纪里，关于这种滑动的越来越复杂的理论已经被提出，但这些理论在很大程度上是未经检验的，因为那里无法进入冰川床，而且那里的条件在空间和时间上都是可变的。该项目的目标是使用新建造的实验室设备来研究冰川滑动速度、基面阻力(滑动阻力)和有效压力(冰压减去床面水压力)之间的关系。该装置拖着一圈融化的冰(外径0.9米，宽0.2米，厚0.15米)穿过硬床或软床。该装置的特点包括无限滑动位移，温度控制在几百分之一度，并在床上连续观察冰的滑动和流动分离。一组实验将测试这一理论，即随着滑移速度的增加，粗糙、坚硬的床面上的稳定阻力达到最大值，然后根据床面的几何形状保持不变或减小。在阶梯型和正弦型床面上，当冰在融化温度下滑移时，基本阻力、滑移速度和有效压力将分别改变。在实验的几何约束下对冰流进行数值模拟，可以将理论结果与实验数据直接进行比较。第二组实验将提供软床滑动速度、阻力和有效压力之间的关系。这些实验还将揭示冰通过融化和重新冻结侵入软床孔隙空间的程度，以及软床为适应滑动而变形的程度？这是评估冰川泥沙输送的核心问题。需要可靠的冰川滑动定量模型来预测冰川和冰盖的速度。观察到冰川的涌动，即以异常高的速度向前移动，并以不同的速度后退，显然是对气候变化的反应。该项目的实验研究将以一种无法通过实地观察确定的方式来检查冰川的运动。对滑动过程进行更完整的分析将使冰川滑动在冰川和冰盖运动的计算机模型中得到更准确和更少的武断处理。这将使我们能够更好地描述近期的环境变化和遥远过去的景观演变。

Movement of glaciers and ice sheets affects their stability and attendant sea-level rise as the climate warms. In addition, ice-sheet movement during the Pleistocene left a spectacular imprint on landscapes. Glaciers that move fastest and modify landscapes most severely move primarily by slip over their rock (hard) or sediment (soft) beds. Increasingly sophisticated theories of this slip have been advanced over the last half-century, but these theories are largely untested, owing to the inaccessibility of glacier beds and the spatial and temporal variability of conditions there. The objective of this project is to use a newly constructed laboratory device to study relationships among glacier slip velocity, basal drag (slip resistance) and effective pressure (ice pressure minus water pressure at the bed). The device drags a ring of melting ice (0.9 m outside diameter, 0.2 m wide, 0.15 m thick) across a hard or soft bed. Features of the device include unlimited slip displacement, temperature control to a few hundredths of degree, and continuous observation of ice sliding and flow separation at the bed. One set of experiments will test the theory that as slip velocity increases, steady drag on a rough, hard bed attains a maximum value and then either remains constant or decreases, depending upon the bed geometry. Basal drag, slip velocity, and effective pressure will be individually varied during slip of ice at its melting temperature over stepped and sinusoidal bed surfaces. Numerical modeling of ice flow under the geometric constraints of the experiment will allow theoretical results to be compared directly with experimental data. A second set of experiments will provide relationships among slip velocity, drag, and effective pressure for soft beds. These experiments will also reveal both the extent to which ice invades the pore spaces of soft beds by melting and refreezing and the degree to which soft beds deform to accommodate slip?problems central to evaluating sediment transport by glaciers. Robust quantitative models of glacier slip are needed to forecast the speeds of glaciers and ice sheets. Glaciers have been observed surging, i.e. moving forward at unusually high velocities, and retreating at various rates, apparently in response to changing climate. The experimental studies of this project will examine glacier movement in a a way that can not be determined from field observations. A more complete analysis of the sliding process will allow glacier slip to be treated more accurately and less arbitrarily in computer models of glacier and ice-sheet motion. This will permit better characterizations of environmental change in the near future and of landscape evolution in the distant past.