LABORATORY EXPERIMENTS ON SURFACE SEAL DEVELOPMENT AND ITS EFFECT ON INTERRILL EROSION PROCESSES

Despite extensive research, the processes involved in seal and crust formation are not yet fully understood. This paper provides detailed information on the dynamics of surface sealing and its effect on the erosion process. Seal development of an artificial soil mixture was investigated by measuring changes in splash detachment, soil bulk density and near-surface soil strength, and by detailed observation and analysis of thin-section photomicrographs taken at different sealing stages. Raindrop impact was found to be the principal force governing the formation of the surface seal. The sequence of seal development was as follows: (i) rapid disruption of surface aggregates by direct raindrop impact; (ii) rearrangement of disrupted fragments and textural separates; and (iii) compaction of surface material by continued drop impact. Eventually, an 'equilibrium' complete seal of the surface was established. Other micromorphological characteristics included (i) a mean seal thickness of 0.3-0.8 mm, (ii) a complex void structure dominated by compound packing voids and vughs, (iii) an absence of any 'washing-in' of fine material, and (iv) development of miniature soil pedestals (micro-hoodoos), micro-depressions and micro-knolls formed by drop impact. Measured changes in splash detachment and soil bulk density were consistent with the observed fabric changes. Splash detachment increased during the early seal stages but thereafter decreased rapidly with incorporation of material into the developing seal. The decrease was matched by an overall increase in soil bulk density. Contrary to expectations, soil shear increased only during the initial wetting phase, and subsequently decreased to give minimum values coincident with maximum seal development. After the initial wetting phase shear strength had a negative linear relationship with soil moisture content. It is concluded that soil shear strength may not be a reliable indicator of seal development.