Sorting of a sand-gravel mixture in a Gilbert-type delta

Abstract

This paper presents the results of a Gilbert-type delta progradation experiment within an impoundment created by a dam. The delta was composed of a poorly sorted sand-gravel mixture in a bedload-dominated environment. The main goal of the paper is to analyse the sorting process of material within the deposit as the delta progrades towards the dam. Bed profile evolution has been documented and the entire delta has been extensively sampled in order to study sorting processes. Longitudinal and vertical sorting mechanisms are illustrated. What is novel in this investigation is the complete record, within an entire deltaic deposit, of the vertical distribution of streamwise sorting in the absence of suspended load. The data presented herein provide a detailed description of sorting processes in a Gilbert-type delta. The experimental set-up, the water flow and the sediment feed rate chosen determine the evolution of the delta: it initially progrades with little topset aggradation and degrades afterwards. Experimental results fit well with a previously presented empirical sorting model, despite the fact that the experimental conditions used here were well outside the range of those used to derive that model. The relative coarsening of the upper layers of the delta is found to be related to the slow speed at which the delta progrades, the formation of a mobile armour layer and the erosion of the topset towards the end of the run. Furthermore, a strong correlation between the coarsening of the bottom layer of the delta and its front height has been documented and explained: as the delta gets higher, as there is more space to sort sediment, it is more likely that coarse particles failing near the top of the foreset reach the bottom of the foreset. These findings provide new and useful data documenting sediment sorting in granular, bedload-dominated deltas.