Megaripple flattening due to strong winds

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 property being a pre-requisite for their formation. In our three-year project, megaripple development was monitored using a digital elevation model (DEM) constructed from above-ground stereo digital photographs. Temporal dynamics of wind power (drift potential, DP) were measured, and grain-size analyses were performed on samples taken from different parts of the megaripple. The coarse grains that protect the crest enable the ripple to grow, but when ripple height becomes too high, the bed shear stress increases, thus allowing the wind to move the armoring layer. When this happens (during strong wind storms) the megaripple will flatten and even disappear, as was observed in our field study. We present measurements that for the first time directly show the megaripple to normal ripple transition, and we suggest two possible physical processes as potential causes of this phenomenon. Megaripple flattening can occur either when the wind exceeds the fluid threshold for a sufficient length of time or after a sequence of storms with winds blowing from the same direction.