AN EXPERIMENTAL STUDY OF GLACIER SLIP OVER HARD AND SOFT BEDS

随着气候变暖，冰川和冰盖的移动会影响它们的稳定性，并导致海平面上升。此外，更新世期间的冰盖运动在景观上留下了壮观的印记。移动最快、对景观影响最大的冰川主要是通过在岩石（硬）或沉积物（软）床上滑动而移动的。在过去的半个世纪里，已经提出了越来越复杂的理论，但是这些理论在很大程度上未经检验，因为冰川床难以接近，而且那里的条件具有空间和时间的可变性。本项目的目的是利用新建成的实验装置，研究冰川滑动速度、基底阻力（滑阻）和有效压力（冰压减去河床水压）之间的关系。该装置拖着一个融化的冰环（外径0.9米，宽0.2米，厚0.15米）穿过坚硬或柔软的床。该装置的特点包括无限滑动位移，温度控制到百分之几度，并在床上连续观察冰滑动和流动分离。一组实验将检验这样一个理论：随着滑移速度的增加，粗糙坚硬的床层上的稳定阻力达到最大值，然后根据床层的几何形状保持不变或减小。当冰在阶梯式和正弦床面上的融化温度下滑动时，基底阻力、滑动速度和有效压力将分别发生变化。实验几何约束下冰流的数值模拟将使理论结果与实验数据直接进行比较。第二组实验将提供滑移速度、阻力和软地层有效压力之间的关系。这些实验还将揭示冰通过融化和再冻结侵入软床孔隙空间的程度，以及软床变形以适应滑动的程度。评价冰川泥沙输送的核心问题。为了预测冰川和冰盖的速度，需要强有力的冰川滑动定量模型。已经观测到冰川以异常高的速度向前移动，并以不同的速度后退，这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从实地观测中确定的方式检查冰川运动。对滑动过程进行更全面的分析，将使冰川滑动在冰川和冰盖运动的计算机模型中得到更精确的处理，而不那么随意性。这将有助于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.