Particle comminution defines megaflood and superflood energetics

Abstract

The concept of ‘megafloods’ and ‘superfloods’ was introduced at the end of the last century to define exceptionally large-discharge floods, primarily those associated with the failure of Quaternary ice-dams. These floods exceeded, by one or two magnitudes, historically recorded floods, megafloods having peak discharges equalling, or exceeding, 1 M m ³ s ⁻¹. Herein, the sediment associations of exceptional floods are found to be distinctive, being dominated by comminuted (smashed) grain-size distributions, which contrast to the sediment deposits of more moderate floods. A consideration of the energetics of moderate floods, superfloods and megafloods with respect to the entrainment, dis-entrainment, suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits. A threshold of energy expenditure - 20 kJ s ⁻¹ m ⁻² - is proposed to separate moderate floods from superfloods; the latter by this definition can be ~ 0.1 M m ³ s ⁻¹. This threshold separates floods competent to entrain cobbles and boulders (with limited comminution of the load) from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension. The granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods. The energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux. Significance: Exceptional floods occurred in prehistoric times; the largest are referred to as ‘megafloods’ or ‘superfloods’. To distinguish such exceptional floods from other large floods, megafloods were defined as having a peak discharge exceeding 1 M m ³ s ⁻¹. However, many reports of ‘megafloods’ are flows with a peak discharge smaller than 1 M m ³ s ⁻¹. Recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel, which is not the case for smaller floods. The gravel is smashed by exceptionally high-energy expenditures. Consideration of the energetics of floods to transport gravel allows a robust quantitative definition of ‘superflood’ based on the energy needed to smash a range of gravel sizes.

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