Sediment sources and sediment transport by rill flow and interrill flow on a semi-arid piedmont slope, southern Arizona

Abstract

Summary In this study, sediment transport by rill flow and interrill flow on a piedmont hillslope were compared. Four experiments ranging in duration from 10 to 30 minutes were conducted under simulated rainfall delivered at 80 mm h−1 on a 18×35m runoff plot located on a gentle piedmont slope near Tombstone, southern Arizona. We observe that while the sediment concentration is similar, size of sediment transported is considerably coarser in interrill flow than in rill flow. Sediment transport by interrill flow on thee piedmont slopes is influenced by the nature of splash-wash interactions. As flow concentration increases downslope, the proportion of splash-detached particles which reach competent threads of overland flow is reduced. Because raindrop detachment is aselective for size ranges up to several mm and the competence of sediment transport by interrill flow is enhanced by raindrop impact, rather coarse material is transported. Sediment transport in rill flow is strongly influenced by overland flow input from upslope, and above all, the scour and fill processes which occur in the rills. Because of local deposition of sediment derived from upslope in the rills, the reduced surface transport of splash-detached particles from the adjacent contributing area to the rills, and the selective nature of flow detachment, relatively finer particles are transported in the rills. On the basis of a clastic sediment budget evaluated for the bifurcation rill system, we estimate that on a per storm basis, 0% to 56% of the total sediment exported by the rill system is derived from sources in the rills. However, this result is strongly influenced by the sequence of events. Overall, apart from some well-established factors, we suggest that sediment availability, the degree of flow concentration, and the propensity of scour and fill processes in the rills, profoundly influence sediment transport on the experimental plot.