Role of geomorphic processes in the morphology of semi-arid nebkhas

Nebkha is a sand dune formed in many arid and semi-arid regions by the accumulation of sediments around vegetation. The appearance and development of the sand dunes can be a sign of the expansion of desertification. The interaction between wind performance and the geomorphic characteristics of sand dune leads to heterogeneity in the distribution of sediment particles within sand dune. The aim of this study was to investigate the effects of hydrological and aeolian processes on the morphology of nebkha. Overall, 9 sand dunes were randomly selected. For each nebkha, sediment sampling was carried out from the different positions, including upwind and leeward slopes, dune top, edge, and the interdune spaces. Data on the parameters of particle size distribution, hydraulic conductivity and moisture were extracted. The measured particles were classified to 5 groups, including clay (> 2 µm), fine silt (2-5µm), medium silt (5-20µm), coarse silt (20-64 µm), and sand (65≤ 2000 µm). The accumulation of aggregates with different sizes along the interdune-dune continuum could exhibit the contribution of hydro-aeolian processes in sediment transport and deposition. The findings showed that average aggregate size reduced from the interdune areas to the dunes, and the finest aggregates were observed on the dune edge, illustrating the functioning of hydrological processes on the evolution of the nebkha structure. By contrast, predominant sediments within the nebkha structure mostly were coarse-grained (sand) fractions, illustrating the functioning of aeolian processes. Based on the findings, the processes involved in the evolution of dune include the saltation and creep of sand-sized sediments, transportation of sediments through runoff in the interdune space, as well as sediment-laden runoff that flows from the top of coppice dune into the edge positions.