Roughness of a subglacial conduit under Hansbreen, Svalbard

• Published in: Journal of Glaciology
• Main Authors: MANKOFF, KENNETH D., GULLEY, JASON D., TULACZYK, SLAWEK M., COVINGTON, MATTHEW D., LIU, XIAOFENG, CHEN, YUNXIANG, BENN, DOUGLAS I., GŁOWACKI, PIOTR S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2017
• Subjects: Hansbreen; Svalbard; Journal of Glaciology
• Online Access: http://dx.doi.org/10.1017/jog.2016.134; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143016001349

ABSTRACT Hydraulic roughness exerts an important but poorly understood control on water pressure in subglacial conduits. Where relative roughness values are <5%, hydraulic roughness can be related to relative roughness using empirically-derived equations such as the Colebrook–White equation. General relationships between hydraulic roughness and relative roughness do not exist for relative roughness >5%. Here we report the first quantitative assessment of roughness heights and hydraulic diameters in a subglacial conduit. We measured roughness heights in a 125 m long section of a subglacial conduit using structure-from-motion to produce a digital surface model, and hand-measurements of the b -axis of rocks. We found roughness heights from 0.07 to 0.22 m and cross-sectional areas of 1–2 m 2 , resulting in relative roughness of 3–12% and >5% for most locations. A simple geometric model of varying conduit diameter shows that when the conduit is small relative roughness is >30% and has large variability. Our results suggest that parameterizations of conduit hydraulic roughness in subglacial hydrological models will remain challenging until hydraulic diameters exceed roughness heights by a factor of 20, or the conduit radius is >1 m for the roughness elements observed here.