ICE FRACTURING DURING JO ¨ KULHLAUPS: IMPLICATIONS FOR ENGLACIAL FLOODWATER ROUTING AND OUTLET DEVELOPMENT MATTHEW J. ROBERTS 1,2 *, ANDREW J. RUSSELL 2 , FIONA S. TWEED 1 AND O ´ SKAR KNUDSEN 3 1 Division of Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire ST4 2DE, UK 2 School of Earth Sciences and Geography, Keele University, Keele, Staffordshire ST5 5BG, UK 3 Klettur Consulting Engineers, Bı ´ldsho ¨fa 12, IS 112 Reykjavı´k, Iceland Received 19 January 2000; Revised 16 June 2000; Accepted 10 July 2000 ABSTRACT Theoretical studies of glacial outburst floods (jo ¨kulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jo ¨kulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jo ¨kulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jo ¨kulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðara ´rjo ¨kull and So ´lheimajo ¨kull. Video footage and aerial photographs, taken before, during and after the Skeiðara ´rjo ¨kull jo ¨kulhlaup and immediately after the So ´lheimajo ¨kull jo ¨kulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jo ¨kulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jo ¨kulhlaup outlets provides a new mechanism for rapid englacial debris entrainment. Copyright # 2000 John Wiley & Sons, Ltd. KEY WORDS: jo ¨kulhlaup; flood routing; hydrofracturing; subglacial hydrology; Iceland BACKGROUND, RATIONALE AND AIMS Glacial outburst floods (jo ¨kulhlaups) are a recurrent hazard in glaciated terrain, with floodwaters inflicting widespread damage to infrastructure and causing rapid geomorphic change. Jo ¨kulhlaup research to date has focused mainly on: (i) mechanisms of drainage initiation (e.g. Fowler, 1999); (ii) predicting the timing and magnitude of jo ¨kulhlaups (e.g. Bjo ¨rnsson, 1992); and (iii) interpreting proglacial flood deposits (e.g. Maizels, 1991). Issues of intraglacial floodwater routing and outlet location have rarely been addressed due to the theoretical assumption that jo ¨kulhlaups drain from pre-existing outlets, which remain stationary during flooding (Spring and Hutter, 1981; Szilder and Lozowski, 1997; Fowler, 1999). However, field observations confirm that jo ¨kulhlaups take multiple routes through glaciers and are not always restricted to channelized Earth Surface Processes and Landforms Earth Surf. Process. Landforms 25, 1429–1446 (2000) Copyright # 2000 John Wiley & Sons, Ltd. * Correspondence to: M.J. Roberts, Division of Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire ST4 2DE, UK. E-mail: m.j.roberts@staffs.ac.uk Contract/grant sponsor: UK NERC; contract/grant number: GR3/10960; contract/grant number: GR3/12969 Contract/grant sponsor: Earthwatch Contract/grant sponsor: Icelandic Research Council Contract/grant sponsor: Icelandic Public Roads Administration Contract/grant sponsor: Icelandic Power Company Contract/grant sponsor: Division of Geography, Staffordshire University