ABSTRACT
Through wind tunnel experiments, we measured the structure of surface drifting sand flux and sand transport rate on a bed surface that contained widely but uniformly spaced and non-erodible ridges. We found that under the condition of no ridges, the sand transport rate within the height of 0~70 cm on the bed surface decreases in a power function law with the increase of height, increases with the increase of friction velocity, and the proportion of sand transport rate at different high layers increases with the increase of height. The variation of sand transport rate with height can be divided into two cases for all the ridge heights and spacings: one shows that sand transport rate decreases exponentially with height, while the other shows that sand transport rate increases with height under a certain height, and above the certain height decreases exponentially with the increase of height, known as ”elephant nose” effect which seems similar to the structure of drifting sand flux in Gobi desert. For all the ridge heights and spacings, the total sand transport rate in the height of 0~70 cm increases with the increase of friction velocity in a power function law, and increases with the increase of ridge spacing. When the friction velocity and ridge spacing are both large, the total sand transport rate of some ridge structures are larger than that with no ridges. Our results will contribute to the study on recognize the process and mechanism of soil wind erosion in ridge farmland.
Key words :Wind tunnel experiments; ridge microtopography; drifting sand flux structure; sand transport rate; friction velocity
INTRODUCTION
Sand transport rate is a measure of the capacity of transporting sand particles by wind. It is the amount of sand carried by the air flow through the unit width in unit time, also known as the solid flow of drifting sand flux (Lancaster et al., 1996; Zhao et al., 2021). There are many factors affecting the sand transport rate, including wind force, density, particle size, specific gravity and shape of sand, as well as moisture rate of sand, earth surface features and atmosphere stability (Kok et al., 2012; Miao et al., 2012; Avecilla et al., 2017; Favaro et al., 2020).
The study of drifting sand flux originated with Bagnold’s research on the physics of blown sand and desert dunes (Bagnold, 1941). Exner obtained the distribution of sand transport rate with elevation according to the diffusion theory (Wu, 2003). Horikawa (1982) regarded the phenomenon of wind-blown sand as a group movement of sand particles, made statistical treatment, and put forward the theory of sand density distribution and sand transport rate distribution with height in the sand transport layer. Znamenski (Ding, 2010) studied the relationship between the structural characteristics of drifting sand flux and sand wind erosion and accumulation through wind tunnel experiments and field observations, and he used the structure number of drifting sand flux (Q max/Q ) to describe the structure of drifting sand flux, which was used as the basis for judging the process of wind erosion. Ma et al. (1987) put forward the three laws of the structure of drifting sand flux according to the research of domestic and international scholars. Wu Zheng (2003) proposed using the characteristic value λ of drifting sand flux structure to measure the variation characteristics of surface erosion and deposition. Fryear and Saleh (1993) put forward the distribution function of drifting sand flux with height according to the long-term observation and research of soil wind erosion research station in Big Spring, Texas, USA. They believe that the variation of saltation sand transport with height follows the distribution law of power function, while the suspended sediment follows the distribution law of exponential function. The study on the structure of drifting sand flux on different underlying surfaces shows that the distribution of sand transport rate on the sand surface with vertical height basically satisfies the exponential function (Ha, 2004; Zhang et al., 2022). There is a turning height of Gobi drifting sand flux on the sand bed. Below this height, the sand transport rate increases with the increase of height, and beyond this height, the sand transport rate decreases with the increase of height. The distribution characteristics of this drifting sand flux structure are vividly called ”elephant nose” effect (Qu et al., 2005; Zhang et al., 2007).
The type of underlying surface affects the turbulence of near surface airflow. Therefore, for a specific type of underlying surface, there is a unique structure model of drifting sand flux. Ridge microtopography increases the surface fluctuation and enhances the turbulence of near surface airflow, which changes the spatial distribution of sand-carrying airflow energy, and then changes the structure of near surface drifting sand flux. This paper intends to explore the variation law of drifting sand flux structure and sand transport rate under the condition of ridge microtopography by studying the drifting sand flux structure and sand transport rate under different ridge microtopography conditions.
RESEARCH METHODS