Roughness of a subglacial conduit under Hansbreen, Svalbard

• Published in: Journal of Glaciology
• Main Authors: KENNETH D. MANKOFF, JASON D. GULLEY, SLAWEK M. TULACZYK, MATTHEW D. COVINGTON, XIAOFENG LIU, YUNXIANG CHEN, DOUGLAS I. BENN, PIOTR S. GŁOWACKI
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press 2017
• Subjects: glacier hydrology; roughness; subglacial conduits; Environmental sciences; GE1-350; Meteorology. Climatology; QC851-999; Svalbard; Hansbreen; glacier; Journal of Glaciology
• Online Access: https://doi.org/10.1017/jog.2016.134; https://doaj.org/article/23cf24a6f69b419f81975bc091351163

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 m2, 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.