Abstract:

Aeolian sand ripple attracts people's attention because of its regular wavy geometric pattern， but its formation process is still lack of in-depth understanding. The lack of high-precision and high-resolution measurement method of sand ripple profile is one reason. The laser sheet is inclined at some angle to the sand bed in the traditional measurement method. It will cause the inaccurate measurement of height and the deformation of sand ripple profile. In this study， we correct the problem by irradiating the sand bed vertically and analyze the influence of fetch length on the evolution of geometric parameters with time. The results show that： （1） The durations of the initial formation stage， the growth stage and the stable stage of the sand ripple decrease with the increase of the fetch length， and the shorter the fetch length， the slower the development process of the sand ripple， forming the "backward phenomenon". （2） The height and wavelength of sand ripple increase rapidly with time at first and then are gradually become stable in the end. The growth rate of ripple height increases with the increase of fetch length. The mean ripple height and wavelength increase with fetch length. （3） The ripple indexes， windward indexes and leeward indexes decrease with time following an exponential law. At the stable stage of the sand ripples， the mean ripple index decreases with fetch length. （4） The propagation velocity of sand ripple shows a decreasing trend with ripple height. The mean propagation velocity decreases according to a power law with respect to time， and it decreases with fetch length when saturated sand flux is not reached. Therefore， the fetch length is a key factor in the study of sand ripple.

Key words: sand ripple profile, wavelength, fetch effect, wind tunnel