THE EFFECT OF PREVIOUSLT DEATACED PARTICLES ON DETACHMENT

Erosion involves the detachment of particles held within the soil surface by cohesion and interparticle friction, and the subsequent transport of those particles away from the site of detachment. The zone that extends a distance x_pDd of the downstream boundary of an area eroding by Raindrop Detachment-Raindrop Induced Saltation (RD-RIS) is called the active zone. Once particles are lifted into the flow by a drop impact affecting the active zone, the particles pass across the downstream boundary without the need of subsequent drop impacts. Drop impacts within a distance x_pDd of upstream of the active zone cause particles to move into the active zone and settle on the surface. In terms of the drop impacts affecting the active zone, the particles that settle on the surface in the active zone are previously detached particles while particles held within the soil surface by cohesion and interparticle friction are not. Since the previously detached particles are above the particles held within the soil surface by cohesion and interparticle friction, they are lifted first. While only held to the surface by gravity, energy is absorbed in lifting previously detached particles and the thicker the layer of previously detached material, the greater the amount of energy absorbed. This reduces the energy available to detach the particles held within the soil surface by cohesion and interparticle friction. Consequently, these previously detached particles provide a degree of protection (H_R) against detachment. If the layer is sufficiently thick, it will provide absolute protection (H_R=1) and only particles detached upstream of the active zone will be discharged across the downstream boundary. When there are no previously detached particles in the active zone (ie, H_R=0), M_pDd is controlled by cohesion and interparticle friction within the soil surface. When H_R=1, then M_pDd is controlled by the characteristics of the layer of previously detached particles. The effect can be expressed by

M_pDd = H_R M_pDd.PD + (1 - H_R) M_pDd.M

where M_pDd.PD is the value of M_pDd when H_R=1 and M_pDd.M is the value of M_pDd when H_R=0. The layer of previously detached particles is dynamic when particles of various sizes and densities are being transported. The dynamic nature of the layer will be illustrated later. Experiments with close sorted sand minimise variations in M_pDd.PD and x_pDd.

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