Breaking the seal at Grimsvotn, Iceland
Breaking the seal at Grimsvotn, Iceland
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DOI：
10.3189/s0022143000001362
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发表时间：
1999-01-01
影响因子：
3.4
通讯作者：
Fowler, AC
中科院分区：
文献类型：
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作者：
Fowler, AC
在与冰岛东南部瓦特纳约库尔冰盖出口冰川Skeioarjokull出现的约库拉普的理论解释相关的几个问题中，洪水引发的机制是迄今为止无法解释的一个问题。在对经典的Nye-Rothlisberger模型进行仔细分析的基础上，我们提出了这样的解释：在冰下湖泊Grimsvotn附近，水力势梯度朝向湖泊，因此在冰下存在集水边界，其位置取决于冰下融水的排水特征。随着洪水条件的接近，我们发现分水岭向湖泊迁移，同时湖泊压力增加。当向湖的水力势梯度较低且补给速度较慢时，当集水边界到达湖面时，封堵将会“破裂”，此时湖面仍低于浮选压力；而如果补给速度较快，则可在洪水开始前实现浮选。因此，这一理论可以解释为什么当格里姆斯沃顿的湖水位仍低于漂浮水位约60米时，封口通常会被打破。此外，我们能够解释为什么1996年喷发后的约库勒劳普直到达到浮选水平才发生，我们还说明了如何在这个理论中解释约库勒劳普斯的旋回性和强度。
Of several problems associated with theoretical explanations of the jokulhlaups which emerge from the outlet glacier Skeioararjokull of the ice cap Vatnajokull in southeast Iceland, the mechanism of flood initiation is one that has hitherto defied explanation. We provide, such an explanation based on a careful analysis of the classical Nye-Rothlisberger model; near the subglacial lake Grimsvotn, the hydraulic potential gradient is towards the lake, and there is therefore a catchment boundary under the ice, whose location depends on the subglacial meltwater drainage characteristics. As the conditions for a flood approach, we show that the water divide migrates towards the lake, while at the same time the lake pressure increases. When the hydraulic potential gradient towards the lake is low and the refilling rate is slow, the seal will "break" when the catchment boundary reaches the lake, while the lake level is still below flotation pressure, whereas if refilling is rapid, flotation can be achieved before a flood is initiated. This theory can thus explain why the seal is normally broken when the lake level at Grimsvotn is still some 60 m below flotation level. In addition, we are able to explain why the jokulhlaup following the 1996 eruption did not occur until flotation level was achieved, and we show how the cyclicity and magnitude of jokulhlaups can be explained within this theory.