Wind tunnel experiments on the formation of mega-ripples according to the amount of coarse particles
Authors: Kodama Yoshinori 
Published: June 2022

&lt;p&gt;In Japan's most famous Tottori Sand Dunes, mega-ripples were observed from 2013 to 2015 around the exposed volcanic ash layer in sand dunes. Gully erosion progressed during rainfall in exposed volcanic ash, and aggregated coarse particles of volcanic ash were supplied to the sand surface of the surrounding dunes. In other words, it is suggested that mega-ripples may have appeared and disappeared depending on the amount of coarse particles supplied to the sand surface. This hypothesis was first confirmed in a wind tunnel experiment.&lt;/p&gt;&lt;p&gt;I made acrylic rectangular cross-section wind tunnel experimental device with a width of 9 cm, a depth of 60 cm, and a length of 7.28 m. The maximum wind speed was 16-17 m / s. Fine sand was spread with a thickness of 10 cm, and polypropylene particles (diameter 4 mm, density 0.9) were evenly sprayed on the surface. Seven preliminary experiments were conducted in which the amount of spray was doubled. The experiment was conducted without sand supply. As a result, when the amount of the polypropylene particles sprayed was small, the polypropylene particles were buried in the sand. On the contrary, if the amount was too large, polypropylene particles covered the sand surface and became stable on a desert pavement-like smooth bed. When the amount of polypropylene particles sprayed was moderate, in a 30-minute experiment, five convex downward mega-ripples were formed at a wavelength of 50-80 cm.&lt;/p&gt;&lt;p&gt;Finally, fine sand was spread with a thickness of 19 cm, the amount of polypropylene particles sprayed was 45 g / m, and this experiment was carried out for 40 minutes under a wind speed of 17 m / s. From the side of the wind tunnel, a cross-section was recorded every minute with five digital cameras. As a result, after one minute elapsed, an accumulation zone of polypropylene particles was formed at a wavelength of about 10 cm, and they repeatedly coalesced, and after 10 minutes, they changed to an accumulation zone with a wavelength of about 25 cm. Gradually, the accumulation zone of polypropylene particles became peak-shaped, and in the polypropylene particles passage area, fine sand was wind-eroded and changed to a convex downward cross-sectional shape. After 20 minutes, mega-ripples with a wavelength of 60 cm were formed, and after 40 minutes, six mega-ripples with an average wavelength of 75 cm and an average wave height of 4.3 cm were formed. Mega-ripples moved downstream at a rate of 4 cm / min.&lt;/p&gt;&lt;p&gt;From the wind tunnel experiment, it became clear that mega-ripple was formed in the sand surface lowering area. This is because the development of mega-ripple became unclear when the sand feeding experiment was conducted. In the field, mega-ripple is observed in a limited range unlike the wind ripples. One of the reasons for this is that the area where the sand surface is lowered is limited in sand dunes. I will introduce the results of mega-ripple investigations at the Great Sand Dunes of the United States at this point.&lt;/p&gt;