Movement regimes and mechanisms of soil aggregates destruction in the process of soil erosion

An integrated model of slope soil erosion is hard to elaborate because of the up to billion-fold difference in scales of the processes involved. Therefore, the elements of the general theory of soil erosion should be developed with due consideration of their scale. The aim of the work was to apply the hydraulic approximation to obtain an analytical solution of the problem of soil aggregate size reduction as a result of attrition during transportation by a steady slope flow. To achieve it, we studied separation, transport, transformation and re-sedimentation of an individual soil particle (aggregate), initially resting on the bottom of a temporary water flow. Unlike empirical approaches and those based on arbitrarily introduced equations and statistical distributions, the study, as in the previous case of wind erosion, was carried out using the methods of multiphase media mechanics at mesoscale. Particle transformation is defined as its attrition due to repeated impact interaction with the bed of a temporary slope water flow. As a result, a novel analytical solution of the problem of soil aggregate size reduction due to attrition during transport by a steady slope flow has been found (in equations format) under the hydraulic approximation. It has been determined, that degree of abrasion, defined as a ratio of an actual soil particle radius to the initial one, depends on its mechanical properties and the relation between the hydraulic parameters of the flow and those of the aggregates. At this soil aggregate mechanical properties are represented by two empirical coefficients of the abrasion equation, while the hydraulic parameters of the flow and the aggregates are represented by the erosion number. As a result of theoretical solution, the physical meaning of the erosion number has been expanded; it appeared to be both a ratio of the vortex lift to the aggregate weight, governing the mode of aggregate motion in the flow, and a coefficient of its maximum abrasion during transportation by a steady water flow. The coefficients representing mechanical properties of aggregates abraded by water flow can be found empirically by experiments in hydraulic flumes, such as we used in our study.

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