Evolution of megaripples from a flat bed

Megaripples in Nahal Kasuy in the southern Negev desert of Israel are characterized by a mean wavelength of about 70 cm and by a bimodal distribution of coarse and fine particle sizes, the latter of which is necessary for megaripple formation. The goal of the following work was to explore long-term megaripple evolution and its dependence on wind power and directionality and to study the grain-size distribution by advanced grain-size analysis as a process indicator. Temporal dynamics of wind power such as drift potential (DP) were measured, and samples taken from megaripple crests were analyzed for grain size distribution (GSD) as the megaripple evolved from a flat bed. The GSD was initially unimodal, but it became more bimodal as the ripples grew. At the ripple crest, GSD is sensitive to storms that blow from directions perpendicular to the prevailing winds. These storms can slow the process of ripple recovery as the layer of coarse particles at the crest become finer. After two years the megaripples recovered almost fully, but a series of storms destroyed them and created small ripples in their place. Such complete ripple destruction can occur only when the megaripples are high enough (∼5 cm in Nahal Kasuy), i.e., the crest must be above the saltation layer. Thus, periods of megaripple construction are ended by destructive episodes due to strong storms, and the process repeats itself. At the larger scale, the ripple dimensions of different plots may cause them to respond differently to the same storms.

► Evolution of megaripples from a flat bed entailed alternate phases of building and destruction.
► Grain size distribution at the crest becomes bimodal during the building phase.
► Series of strong uni-directional storms can destroy the megaripples.
► Coarse grain removal and flattening are more pronounced for small megaripples.