The Origin of Aeolian Dunes (TOAD)

TOAD is a jointly funded NERC-NSF Standard Grant that aims to understand the genesis and subsequent evolution of aeolian early stage bedforms by quantifying for the very first time the role and importance of flow, transport and surface feedbacks in the initiation and emergence of dunes through a combination of fieldwork, flume measurements and modelling. The project is an innovative and interdisciplinary collaboration between Dr Jo Nield (Southampton), Prof Giles Wiggs (Oxford), Dr Matthew Baddock (Loughborough), Prof Jim Best (Illinois, US) and Prof Ken Christensen (Notre Dame, US). The project is supported by two post doctoral researchers Dr Pauline Delorme (Southampton) and Dr Nate Bristow (Notre Dame, US). The project also benefits from a host of international project partners.

Background
Aeolian (wind-blown) sand dunes occupy 10% of the Earth’s surface, both in vast desert sand seas and as important natural defences against flooding along coasts. While the environmental conditions that influence the shape, movement and patterns of fully grown dunes have been extensively studied, arguably the most enduring deficiency in our understanding of these landforms is also the most profound: how do wind-blown dunes initiate?

Initiation is central to understanding dunes as major geological units, including the response of these landscapes to climatic drivers, environmental change and societal impact. The significance of dune initiation for the wider understanding of wind-blown sandy systems and their contexts, for which the discovery of extra-terrestrial dune fields has added a recent impetus, ensures that the question of initiation has remained prominent throughout the history of desert research. Despite this, existing ideas proposed to explain processes of dune origin have remained largely descriptive and uncorroborated. The persistence of the question regarding dune initiation is not due to an absence of appreciation of its importance but, rather, a lack of the means to tackle this fundamental issue.

The critical obstacle to a fully developed understanding of dune initiation is that, until now, measurement of the necessary variables, at the ultra-high spatial and temporal resolutions required to detect small-scale variations in surface conditions and wind-blown sand transport, has been impossible. Recent technological advances in the geosciences both inspire and underpin this proposal, as they now provide the opportunity to meet the demanding requirements of process measurement.

Methods
Surmounting the abiding problem of dune initiation requires novel approaches in research design and our project tackles the issues of measurement at small scales by forging complementary links between fieldwork and physical modelling, as well as an ability to widen the application of detailed process findings through computer modelling. Specifically, this research will for the first time examine the key inter-relationships between airflow, surface properties, changes in sand transport and bedform shape that lie behind a meaningful understanding of how nascent dunes emerge. Full measurement of controlling processes and bedform development will be achieved through field monitoring of surface properties and bedform change at extremely high resolution. We will use multiple terrestrial laser scanners (TLS) to measure surface change, while 3D sonic and hotwire anemometers will measure the changing wind conditions as the bedforms grow. Transport directly measured by TLS will be complemented by Wenglor and sensit sediment transport measurements. A key novelty of the fieldwork is that it will be carried out at carefully chosen locations of known dune development, with each location representing the ‘type site’ for different drivers of dune initiation; surface roughness, surface moisture and sand bed instability. Field locations include the UNESCO World Heritage listed Namib Sand Sea, Namibia and the Great Sand Dunes National Park, Colorado, USA.

The fieldwork will inform experiments undertaken in a bespoke refractive index matching (RIM) laboratory flume that is designed to enable accurate characterisation of flow very close to the 3D surface of modelled dunes using state-of-the-art imaging techniques. The use of an iodine solution, means that the perspex models in the flume literary vanish and flow patterns on and through them can be measured using PIV. Our field and laboratory dataset will be used to drive a hybrid cellular automaton (CA) and computational fluid dynamics (CFD) computer model that we will then run to test the sensitivity of dune initiation and growth to different controls in a range of environmental conditions in deserts, coasts and on other planets.

Outcomes
This research project utilises a new capability to make field observations at the requisite exceptional levels of detail, augmented by closely coupled state-of-the-art laboratory flow simulations, plus the development and application of evidence-based modelling to examine drivers of dune initiation. In concert, this approach represents an unprecedented opportunity to overcome a truly enduring plateau for understanding the origins of one of the major terrestrial landform systems.

Project Partners
The project benefits from 10 international project partners: Prof Gary Kocurek (UT-Austin), Prof Nick Lancaster (Desert Research Institute USA), Dr Serina Diniega (JPL, CalTech), Dr William Anderson (UT-Dallas), Prof Philippe Claudin (CRNS, Paris), Andrew Valdez (Great Sand Dunes National Park, Colorado), Dr Gillian Maggs-Kölling (Gobabeb Research and Training Centre, Namibia), Dr Martin Hipondoka (University of Namibia), Caroline Hern (Shell International) and the Royal Geographical Society (RGS).