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Corbett, I. 2018. The Influence of the Benguela Low-Level Coastal Jet on the Architecture and Dynamics of Aeolian Transport Corridors in the Sperrgebiet, Namibia. Communications of the Geological Survey of Namibia, 20: 9-58


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The Influence of the Benguela Low-Level Coastal Jet on the Architecture and Dynamics of Aeolian Transport Corridors in the Sperrgebiet, Namibia.

Summary / Notes:

Exploration for aeolian diamond placers within the southern Sperrgebiet requires a thorough understanding of aeolian transport across a broad range of scales from a systemic regional level to a micro-topographic-scale at the individual particle level. Within this arid zone the transport of coarse-grained aeolian bedload, including diamonds, is driven by the impact of saltating sandflow, which produces a uniquely characteristic diamond dispersal pattern as well as a variety of coarse-grained bedload features, textures and fabrics. For the first time the Namibian Aeolian System (NAS : see Annex 1 for abbreviations) which runs along the entire continental margin from the Orange River in the south to southern Angola in the north can be studied within the context of the recently discovered Benguela Low-Level Coastal Jet (BLLCJ). The structures produced in response to the hydraulic behaviour of the BLLCJ flow are shown to influence strongly the location of zones of high-energy erosion and aeolian sand accumulation throughout the system. The boundary conditions of the Namib Aeolian Erosion Basin are redefined to include the influence of the BLLCJ on the architecture and dynamics of sandflow pathways through this high-energy aeolian erosion landscape. Empirical sandflow measurements were previously used to identify narrow, linearly extensive Aeolian Transport Corridors characterised by high sandflow conditions. The corridors are commonly marked by the development of mono-trains of large barchan dunes along their length. The advent of Google Earth Engine time-lapse video provides an observational platform enabling spatial and temporal changes in sandflow and bedforms to be examined over a 32 year period. It thus provides many new insights into sandflow-dune and dune-dune interaction throughout this large-scale system down to the resolution of individual protobarchan genesis. The influence of the hydraulic behaviour of the BLLCJ flow together with localised effects of topography on surface wind flow structure is examined through analysis of a unique 10 km wide regional Airborne Laser Scanner (ALS) dataset stretching 170 km from Chameis Bay in the south to Schmidtfeld, to the north of Lüderitz. Aeolian bedforms and erosional features and patterns of coarse-grained aeolian bedload textural features and fabrics are mapped in detail throughout the Sperrgebiet. Changes in bedform interaction are shown to reflect variation in the pattern of surface flow created by localised topographic blocking and steering effects. The new system-wide perspective on the pattern of sandflow that feeds into, and maintains, the present-day Namib Sand Sea shows clear evidence of the influence of the BLLCJ as well as a variety of different types of vortices, which is a newly recognised element in the boundary conditions of this high-energy system. It is concluded that favourable conditions exist for the formation of horizontal rolls and vortex structures of varying length-scales within the Marine Boundary Layer (MBL) associated with the BLLCJ. It is proposed that these strongly influence both the development of the erosional aeolian landscape and the pattern of bedforms that develop within Aeolian Transport Corridors in response to surface wind flow over complex topography. Keywords: Namib Aeolian System, Low-Level Coastal Jet, Sandflow, Barchan Dune, Bedload Dynamics, Turbulent Flow.

Year: 2019

Type: Journal Article

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