Jökulhlaups, or glacier outburst floods, have occurred during the Holocene from the northern margin of the Vatnajökull ice cap, Iceland. Relatively little is known about the origin, magnitude and frequency of these jökulhlaups. The volcanic rifting zone of northern Iceland provides a new environment in which to examine jökulhlaups. Jökulhlaup reconstructions have to date omitted 2D hydrodynamic modelling techniques. This research therefore reconstructs jökulhlaups from Kverkfjöll volcano, a discrete source of meltwater from northern Vatnajökull. This research describes a suite of erosional and depositional landforms that distinguish Kverkfjöll jökulhlaup routeways. Some of these; clinker-scoured lava, gorges with walls of pillow and subaerial lava, lava steps, cataract-fill mounds and imbricated boulder clusters and run-ups, are previously undocumented jökulhlaup impacts. These landforms may be diagnostic of volcanic and/or rifting landscape jökulhlaups. Cross-cutting relationships and sedimentary stratigraphy suggest at least three Holocene jökulhlaups from Kverkfjöll. Kverkfjöll jökulhlaups were reconstructed using palaeocompetence, slope-area and 2D hydrodynamic modelling. Jökulhlaups were volcanically triggered, had linearly-rising hydrographs and peak discharges of 50,000-100,000 m3s-1, which attenuated by ~75% within 25km. Flows were highly varied spatially and temporally, and strongly controlled by topography, geology and sediment supply. Frontal flow velocities were ~2ms-1 but as stage increased, mean velocities reached 5-15ms-1. Shear stress and stream power reached 1x104 Nm-2 and 1x105 Wm-2 respectively. Flows were initially hyperconcentrated and subsequently more fluidal, supercritical and highly turbulent. Kverkfjöll jökulhlaups achieved geomorphic work comparable to that generated by the largest known terrestrial floods. Landscaping resulted from topographic confinement, steep channel gradients, high hydraulic roughness and an initially abundant but rapidly depleted supply of volcaniclastic sediment. These controls on, and impacts of, jökulhlaups are important for distinguishing high-magnitude water-sediment inputs to the North Atlantic, for recognising jökulhlaups in the rock record and for flood hazard mitigation in similar landscapes and upon glaciated volcanoes.

Hydrograph shape as a control of the sedimentary impact of jökulhlaups (glacial outburst floods) Existing studies have associated distinctive vertical sedimentary successions with different jökulhlaup hydrograph shapes, however this remains to be tested. This study investigates the role of hydrograph shape as a control on the sedimentary impact of jökulhlaups in a modern jökulhlaup channel where the hydrograph shapes were known, and by isolating the control of hydrograph shape in a flume. Two modern jökulhlaups in a well-confined channel at Kverkfjöll, northern Iceland, provided an opportunity to determine the role of hydrograph shape as a control on the sedimentary impact of jökulhlaups. Contrasting jökulhlaup hydrograph shapes were studied; an exponentially-rising jökulhlaup with a prolonged rising and rapid falling stage, and a linearly-rising jökulhlaup with a rapid rising and prolonged falling stage. This study indicates that hydrograph shape exerts controls on sedimentary processes, which influence the resultant sedimentary impact of jökulhlaups. Rapid discharge acceleration and deceleration rates cause rapid deposition, providing little time for sediment sorting or grading to occur, producing massive, ungraded, matrix-supported and poorly-sorted deposits. Rapid rising stage discharge acceleration facilitates high bedload transport rates, producing large-scale gravel bars and bedload sheets. More time is available for bedforms to develop during prolonged rising and falling stages. Gradual deposition during prolonged falling stages produces normal-grading. During prolonged falling stages, time is allowed for winnowing, armouring, channel incision and erosion to occur, producing armoured layers, coarsening-upward units, terrace surfaces and the exhumation of cobbles, ice-blocks and rip-up clasts. Armoured layers are absent during rapid falling stages, as discharge wanes too quickly for armouring to occur. A revised model is presented that outlines the role of hydrograph shape as a control on the sedimentary impact of jökulhlaups. This model can be used to interpret hydrograph shape from ancient and modern flood deposits, and to predict the sedimentary impact of floods in glacial and non-glacial environments.

The role of ice blocks in the creation of distinctive proglacial landscapes during and following glacier outburst floods (jökulhlaups) In recent years, it has been recognised that ice blocks form a major component of jökulhlaups. There are, however, very few published hypotheses of ice-block impact during and following jökulhlaups. The November 1996 jökulhlaup in southern Iceland, which transported ice blocks as large as 55 metres in diameter on to Skeioarärsandur, provided an opportunity to study ice-block impact produced during a high-magnitude flood. This thesis aims to (i) determine the impact of ice blocks on the morphology and sedimentology of proglacial river channels during and following a jökuihlaup, and (ii) provide a model which links distinctive landscapes created by ice blocks with specific controls on ice-block impact. A range of ice-block related features are produced during and following a jökulhlaup reflecting glacial and topographical constraints, ice-block characteristics and jökulhlaup hydraulics. In locations where sediment flux remains high throughout a flood, large ice blocks form kettle-scours. Rapid sediment deposition around ice blocks results in the formation and preservation of antidune stoss sides, entirely aggradational ice block obstacle shadows and hummocky topography. The grounding of ice blocks in flows of low sediment concentration or total exhumation of buried ice blocks results in the formation of classic U-shaped obstacle marks. Where channel geometry abruptly expands ice-block berms form. On outwash fans kettle holes and obstacle marks occur in distinct clusters. 11 Kettle holes form post-flood by the in situ melt of (1) progressively buried ice blocks and (2) small ice blocks incorporated into flow deposits. Ice block debris is superimposed onto obstacle marks and kettle holes and deposited on the post-flood streambed to form rimmed kettle holes and obstacle marks and ice-block till mounds respectively. Knowledge of associations between ice blocks and the bedforms and facies produced during and following a jökulhlaup will aid jökulhlaup identification and reconstruction in modern and ancient proglacial environments.