Thresholds for incipient rilling and particle entrainment

Abstract

The most cited definition of a rill is "a microchannel (...) small enough to be removed by normal tillage operations" (FAO 1965, op. cit. Bryan 1987, op. cit. Selby 1993) although, according to Bryan (1987), a more precise definition of the term ‘rill’ would be useful. Its width and depth is normally measured in centimeters (Summerfield 1991; Carson & Kirkby 1972) or even in millimeters (Van Asch 1997) and its maximum depth is often arbitrarily chosen as half a meter. When a rill cannot be removed anymore by agricultural practice it has become a gully. The formation of gullies is quite different from the formation of rills, and will therefore not be treated here. Rill initiation by overland flow can be viewed in two ways (Kirkby 1980; Rauws 1987). The first view is that rill generation is caused by the lack of balance between the infilling of juvenile rills by inter-rill processes - such as splash and wash erosion - at one side, and widening and deepening of young rills by rill processes - such as small mass movements and headcutting - at the other side. This view was first proposed by Smith & Bretherton (1972) in a conceptual model.