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: 2017
• Subjects: Glacier hydrology; Roughness; Subglacial conduits; Hansbreen; Svalbard; glacier; Journal of Glaciology
• Online Access: https://research-portal.st-andrews.ac.uk/en/publications/90c333fa-3c42-4c60-9952-8889b48a12b0; https://doi.org/10.1017/jog.2016.134; https://research-repository.st-andrews.ac.uk/bitstream/10023/10934/1/Benn_2017_JG_Roughness_CC.pdf

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.