Morphology and dynamics of aeolian mega-ripples in Nahal Kasuy, southern Israel

Aeolian sand ripples are a common feature on sandy deserts and beaches. Aeolian ripples often have wavelengths of 10-15 cm and amplitudes of a few millimeters. Mega-ripples are bigger than regular ripples and have a mean wavelength of about 70 cm. They are characterized by a bimodal distribution of coarse and fine particle sizes, which is necessary for their formation. We present here the results of a 1 1/2-year field study at the Nahal Kasuy mega-ripple field, located in the southern Negev Desert. The regular sand ripples superposed on the mega-ripples were formed by weaker winds blowing from different directions. The time evolution of mega-ripples developing from a flat surface was monitored. They grow due to a sand coarsening mechanism. Initially, regular ripples form, which subsequently undergo coarsening by winnowing of the finer particles, thereby producing a coalescence of the regular ripples. The smaller, faster-moving ripples overtake the larger, slower-moving ripples, resulting in increased size and spacing. This state was analyzed by a new technique we developed, using a digital elevation model (DEM) constructed from stereo digital photographs. Data on the wind power (drift potential) during the fieldwork and grain size of samples taken from the mega-ripple crest and trough are presented. The grain-size characteristics demonstrate that only fine particles saltate, while coarse grains creep due to the low wind power at Nahal Kasuy.