| Summary: | 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.
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