Large values of hydraulic roughness in subglacial conduits during conduit enlargement: implications for modeling conduit evolution
Large values of hydraulic roughness in subglacial conduits during conduit enlargement: implications for modeling conduit evolution
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管道扩张过程中冰下管道水力粗糙度的大值：对管道演化建模的影响
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
G. Catania
中科院分区：
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作者：
Jason Gulley;P. Spellman;M. D. Covington;Jonathan B. Martin;Douglas I. Benn;G. Catania
水力粗糙度是热量消耗的原因，应该对冰下管道融化扩大的速度起到重要控制作用。然而，很少有研究量化冰下管道粗糙度如何随着时间的推移而演变，或者这种演变如何影响管道扩张模型。为了解决这一知识差距，我们计算了两个粗糙度参数，达西-魏斯巴赫摩擦系数(F)和曼宁粗糙度系数(N)，使用来自斯瓦尔巴特群岛Rieperbreen的绘制的冰下管道的染料跟踪数据。根据染料痕迹计算的f和n值与常用的表面粗糙度高度和管道水力直径之间的关系计算的f和n值进行了比较。染料示踪计算得到的粗糙度f为75-0.97，n为0.68-0.09 S m-1/3。与染料示踪研究结果相比，根据表面粗糙度高度计算粗糙度参数的方程对f的预测值低达326倍，对n的预测值低达17倍。我们认为，这些严重的低估是因为在导管扩张的早期阶段，冰下管道的相对粗糙度超过了5%的相对粗糙度范围，可以用来直接将f和n的值与水流深度和表面粗糙度高度联系起来。给出了简单的管道水文模型，展示了粗糙度的参数化对管道流量和扩大率模型的影响。我们使用从我们的染料跟踪研究中计算出的导管相对粗糙度与f和n值之间的关系来对导管放大模型进行参数化。假定水力坡度固定为0.01，不考虑蠕变闭合，管道直径从0.44m扩大到3m需要9.25d，比常用的粗糙度参数化法长6~7倍。版权所有©2013 John Wiley&Sons，Ltd.
Hydraulic roughness accounts for energy dissipated as heat and should exert an important control on rates of subglacial conduit enlargement by melting. Few studies, however, have quantified how subglacial conduit roughness evolves over time or how that evolution affects models of conduit enlargement. To address this knowledge gap, we calculated values for two roughness parameters, the Darcy–Weisbach friction factor (f) and the Manning roughness coefficient (n), using dye tracing data from a mapped subglacial conduit at Rieperbreen, Svalbard. Values of f and n calculated from dye traces were compared with values of f and n calculated from commonly used relationships between surface roughness heights and conduit hydraulic diameters. Roughness values calculated from dye tracing ranged from 75–0.97 for f and from 0.68–0.09 s m‐1/3 for n. Equations that calculate roughness parameters from surface roughness heights underpredicted values of f by as much as a factor of 326 and values of n by a factor of 17 relative to values obtained from the dye tracing study. We argue these large underpredictions occur because relative roughness in subglacial conduits during the early stages of conduit enlargement exceeds the 5% range of relative roughness that can be used to directly relate values of f and n to flow depth and surface roughness heights. Simple conduit hydrological models presented here show how parameterization of roughness impacts models of conduit discharge and enlargement rate. We used relationships between conduit relative roughness and values of f and n calculated from our dye tracing study to parameterize a model of conduit enlargement. Assuming a fixed hydraulic gradient of 0.01 and ignoring creep closure, it took conduits 9.25 days to enlarge from a diameter of 0.44 m to 3 m, which was 6–7‐fold longer than using common roughness parameterizations. Copyright © 2013 John Wiley & Sons, Ltd.