Obstacle marks as palaeohydraulic indicators of Pleistocene megafloods

Pleistocene megafloods generated several large-scale obstacle marks that could not be interpreted hydraulically with the present knowledge of submerged obstacles. Thus, flume and field data of classical obstacle marks, characterised by a frontal scour hole and an adjacent depositional ridge, are analysed to estimate flow velocities from obstacle mark geometry, especially scour depths, length, width and ridge width. These data reveal a consistency of correlations between obstacle mark morphometries across a wide spatial scale. Two existing analytical models, basically integrating obstacle size, flow velocity as well as sediment size and grading, are transformed so that the magnitude of individual geometric parameters can be used as variables for the estimation of mean and tip flow velocities. These reconstructed velocities have to be regarded as minimum velocities during the rising limb of the hydrograph, as peak discharge might not last long enough to significantly influence the obstacle mark dimensions. A universally applicable practical outline is developed for palaeohydraulic reconstruction. This framework is applied on three examples of obstacle marks generated by Pleistocene megafloods. The reliability and scale-invariance of these reconstructions is confirmed by similar results of velocity estimations by other independent approaches at the same locations.