Wind Enhanced Raindrop Splash Sand Transport

Raindrop splash is widely accepted as an important mechanism of soil erosion (e.g. Van Dijk, Bruijnzeel, and Eisma 2003) as it can cause both particle detachment and transport. Since 1950, a number of papers have been published to identify and quantify the factors that influence splash transport (e.g. Ekern 1944; Rose 1960; Hudson 1963; Morgan 1981). More recently there has been attention focused on the combined processes of splash detachment and grain transport enhanced by the presence of wind (e.g. Erpul et al., 2009, 2008; Foulds and Warbuton 2007a, b; Cornelis et al. 2004a, 2004b; Erpul, Norton, and Gabriels 2002). However, in terms of splash transport rate estimation, these models are based on empiricism, which limits their real world applications. Therefore, the purpose of this paper is to propose a more physically based model to estimate the wind-enhanced splash transport rate. For simplicity, this model assumes the splashing occurs on a flat, sandy surface, the wind condition is steady, and the wind-raindrop- sand system has reached an equilibrium status, i.e. splash transport rate becomes constant. Raindrops become oblique to the surface due to the effect of wind. Therefore, the terminal velocity can be divided into horizontal and vertical components. When an incident raindrop hits the sandy surface, it will generate a splash crater formed by thousands of ejecting droplets. This model will not simulate the trajectory of each individual ejecting droplet; instead, it will only focus on the bulk transport rate for all the ejecting droplets within the crater. If there is no wind, the crater will be symmetrical and the bulk (net) transport is zero. Raindrop terminal horizontal and vertical velocities were obtained by applying the law of motion on a raindrop falling in a given wind field. Detachment rate was estimated from an empirical model. Average splash transport distance was obtained using the “Stokes Law”. The bulk sediment transport rate was calculated using the product of splash distance and detachment rate. Simulation results indicate that 1) raindrop terminal vertical velocity is the function of raindrop size, which is related to rainfall intensity; 2) raindrop horizontal velocity is the function of wind profile, especially the free stream velocity; 3) detachment rate is a function of soil (sand here) type and incident raindrop terminal vertical velocity; 4) average splash distance is mainly related to incident raindrop terminal horizontal velocity; 5) bulk transport rate is a function of wind velocity and rainfall intensity if ignoring the effect of soil type. Finally, scenario analysis on different wind speeds and rainfall intensities indicates that: for slow wind, synergy between wind and raindrops is more effective than wind alone; for fast wind, wind alone is more effective.