Nebkha alignments and their implications for shadow dune elongation under unimodal wind regime

Abstract

Compared with longer-crested linear dunes formed in the bimodal wind regime, little is known about the driving forces of linear dunes that elongate in the lee side of obstacles under a unimodal wind regime. Here, we present the flow dynamics and sand deposition patterns around the aligned nebkhas with different spacing distances (∆) based on computational fluid dynamics (CFD) simulation, wind tunnel experiment and field measurement. In terms of the relationships between the width of horseshoe vortex around two laterally aligned nebkhas (T1 and T2, respectively) and ∆, four wake flow patterns were identified: distinctly merged wake flow when ∆ → 0, merging flow when 0 < ∆ < T1 + T2, two isolated and weakly interfered flows when 0 ≪ ∆ < T1 + T2, and two independent wake flows when ∆ ≫ T1 + T2. Considered two aligned nebkhas with the same dimension of diameter D, ∆ < 1.5D commonly contributes to the merging and elongation of shadow dunes. We suggest that wake flow interference has been reported to produce a region of enhanced deposition with the minimum wind speed and shear stress, i.e., secondary deposition zone, which is determined by sediment supply and ∆ of nebkha alignments. Spatial distance ∆ between tandem aligned nebkhas strongly controls the morphology of bead-like shadow dunes. It is found that the elongation of shadow dunes in the unimodal winds can be well explained by the deflection flow around sand ridges, thus demonstrating the driving forces for the elongation of unimodal-wind-subjected shadow dunes.