High-energy megafloods usually occur in relatively narrow and deep, confined reaches supplied by large volumes of water. Examples of planetary settings include modern and ancient glacial outburst floods (jökulhlaups) of Iceland, glacial lake spillways south of the Pleistocene Laurentide Ice Sheet, the Channeled Scabland of the north-western USA, mountain lake bursts from central Asia, spillways connecting Pleistocene lake basins in Asia, and the immense outflow channels of Mars. The palaeohydraulic analyses of all these floods indicate that they generate values of stream power per unit area (> 103 W m-2) and bed shear stress (> 103 N m-2) that are two orders of magnitude larger than are typical for floods on large alluvial rivers such as the Amazon and Mississippi. Flood discharges can be comparable to flows in ocean currents, indicating important short-term roles in planetary water and sediment fluxes. Significant sedimentary processes in the confined reaches for megafloods include streamlining (and related bar formation), scour around obstacles and giant current ripple (dune) formation. Sediment transport involves the entrainment of large boulders and phenomenally high loading of the flow with extremely course suspended and wash load. The outflux of high-energy, sediment-charged megafloods from confined continental settings to ocean basins results in hyperpycnal flows, and unusually powerful turbidity currents. These have been documented recently for the Pleistocene Missoula floods that formed the Channeled Scabland. They also were probably very important for the Martian outflow channel floods, which may have exerted the primary trigger for climatic change during Mars' geological history.
|Original language||English (US)|
|Number of pages||13|
|Journal||International Association of Sedimentologists Special Publications|
|State||Published - 2002|
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