Erosion and Sediment Transport by Water on Hillslopes

Much of the terminology and many of the concepts within the field of erosion and sediment transport by water on hillslopes, derive from the literature of agricultural engineering. Of particular importance has been the distinction between rills and interrill areas. One definition of the former is that they are channels small enough to be removed by ploughing; gullies, by contrast, are not. The distinction of gullies, rills, and interrill areas is artificial. None the less, it provides a convenient framework for an examination of the processes of hillslope erosion and sediment transport. In interrill areas, the dominant mechanism of sediment detachment is that of raindrop impact. Sediment detached by raindrops may be split into that which is transported away from the location of detachment in splash droplets (rainsplash), and that which is simply dislodged by the impact of raindrops but which either remains at, or falls back to, the site of detachment. The former is relatively easy to measure; the latter is not, but may be quantitatively much more important. The rate of detachment is a function of the rainfall energy at the soil surface, so that where vegetation intercepts some of the energy of the rainfall, or a layer of surface water exists, some of the energy of the falling rain will be dissipated. The rate of detachment is also affected by surface gradient. Whereas detachment in interrill areas is due to the energy of falling raindrops, sediment transport is mainly controlled by flow energy. Several authors have attempted to apply transport-capacity equations developed for alluvial rivers even though the hydraulic conditions in shallow overland flow are very different from those in rivers. However, sediment transport by interrill flow needs to consider not only the capacity of the flow, but also its competence. As threads of interrill flow become deeper and faster, a threshold is reached beyond which significant flow detachment begins to take place. Once this occurs the flow begins to erode definable channels, or rills. As with interrill overland flow, the transport capacity of rill flow has typically been estimated using equations taken from the literature developed for alluvial rivers. However, sediment transport by rill flow is equally determined by the competence of the flow. This is particularly the case for stony soils. Gullies have been relatively neglected in the agricultural literature, so that, whereas there is considerable qualitative literature on gully growth and development, quantitative information is limited. However, gullies may account for between 10% and 94% of total soil loss on hillslopes. Modelling of hillslope erosion and sediment transport has been undertaken by both geomorphologists and agricultural engineers. There is a shared history, inasmuch as, through time, both demonstrate increasingly explicit representation of processes in their modelling as understanding of processes has increased and increased computing power has become available. Keywords: interrill erosion; rill erosion; gulley erosion; sediment detachment; sediment transport; transport capacity; transport competence; modelling hillslope erosion