Geomorphological imprints of episodic surface runoff in deserts

Flash floods in desert, however they are rare, have significant geomorphic effects. The present paper reports the 4th November 2015 flash flood that occurred in the Western Desert of Egypt. Satellite images processing, field survey, close range photogrammetry and sediment analyses allow the quantification of flood geomorphologic impacts at different size scales. At Wadi An-Natrun depression, lake size doubled, hundreds of new ephemeral ponds appear. All ponds were dry by the next summer and lake margins were transformed into salt pans. At a local scale, four detailed case studies show how much geomorphic impacts of the same flash flood event vary in terms of gully morphology, sediment failure or stability, sediment size transport from clays to blocks and fan extension. This variability depends on local soil erodibility, including bedrock heterogeneity, soil cohesion and permeability, vegetation cover and inherited topography. Organic muds formed into ephemeral ponds and playa may provide a relevant sedimentary proxy for past flood events in the Western Desert of Egypt.