Dune Initiation in a Bimodal Wind Regime

Pauline Delorme,Giles F S Wiggs,Philippe Claudin,Andrew Valdez,Matthew C Baddock,Joanna M Nield

doi:10.1029/2020jf005757

Abstract

AbstractEarly‐stage bedforms develop into mature dunes through complex interactions between wind, sand transport, and surface topography. Depending on varying environmental and wind conditions, the mechanisms driving dune formation and, ultimately, the shape of nascent dunes may differ markedly. In cases where sand availability is plentiful, the emergence and growth of dunes can be studied with a linear stability analysis of coupled transport and hydrodynamic equations. Until now, this analysis has only been applied using field evidence in unidirectional winds. However, in many areas of the world and on other planets, wind regimes are more often bimodal or multimodal. Here, we investigate field evidence of protodune formation under a bimodal wind regime by applying linear stability analysis to a developing protodune field. Employing recent development of the linear stability theory and experimental research, combined with in situ wind, sediment transport, and topographic measurements during a monthlong field campaign at Great Sand Dunes National Park, Colorado, USA, we predict the spatial characteristics (orientation and wavelength) and temporal evolution (growth rate and migration velocity) of a protodune field. We find that the theoretical predictions compare well with measured dunefield attributes as characterized by high‐resolution Digital Elevation Models measured using repeat terrestrial laser scanning. Our findings suggest that linear stability analysis is a quantitative predictor of protodune development on sandy surfaces with a bimodal wind regime. This result is significant as it offers critical validation of the linear stability analysis for explaining the initiation and development of dunes toward maturity in a complex natural environment.

Highlights

The variability in sand dune patterns is, in part, due to complex interactions between fluid flow, bed morphology, and sediment transport
This figure shows that sediment flux originated almost exclusively from two directions, (221 ± 15° and 303 ± 10°) corresponding with the bimodality evident in the wind directional data. These flux determinations allowed the characterization of the wind regime using drift potential (DP) and resultant drift potential (RDP) (Equations 3 and 4)
We found RDP=DP 1⁄4 0:72 with a resultant drift direction (RDD) = 65°

Summary

Introduction

The variability in sand dune patterns is, in part, due to complex interactions between fluid flow, bed morphology, and sediment transport. The coupling between flow and sediment transport controls dune morphology, and concurrently, there is a strong modification of airflow dynamics driven by dune topography (Walker & Nickling, 2002; Wiggs et al, 1996; Wiggs & Weaver, 2012). These interactions are well studied on mature dunes, we lack field observations of early‐stage (proto)dunes because the small scale of relevant morphological features and associated flow processes are extremely difficult to measure (Kocurek et al, 2010). Requisite process measurements are challenging on small bedforms, the quantification of these fundamental relationships on protodunes is crucial because of the importance this bedform type has as a precursor to fully developed dunes (Baddock et al, 2018; Claudin et al, 2013; Kocurek et al, 2010; Phillips et al, 2019)

Methods

Results

Discussion

Conclusion