The application of terrestrial laser scanning to measure small scale changes in aeolian bedforms

Abstract

Traditional methods used to measure aeolian sediment transport rely on point based sampling, such as sand traps or saltation impact sensors, which ignore the spatial heterogeneity displayed in the transport system. Obtaining an accurate transport rate is important to parameterise predictive models, which currently show large deviations between measured and predicted rates.

Terrestrial laser scanning (TLS) is a tool that is rapidly emerging in the field of geomorphology. It provides the ability to capture surface elevation data of in-situ bedforms at the spatial and temporal scale necessary to link change, such as ripple migration, to the processes that drive them. Repeat scans provide digital elevation models which can be differenced to provide volumetric rates of change, in a process known as the morphologic method. However, utilising data at such high resolutions requires an accurate estimation of error in order to provide meaningful results.

Typically the morphologic method documents surface change between geomorphic events. However, due to the high temporal variability displayed by the aeolian transport system, measuring topographic change during a transport event would be beneficial. Using TLS during active transport removes the ability to take multiple convergent scans. Therefore current methods of approximating error in TLS derived surfaces by using convergent scans from multiple scan locations cannot be applied.

The influence of scanning geometry and survey set up is explored in order to quantify and reduce errors when scanning small scale bedforms from a single location. This is then applied to an active transport event to measure wind ripple migration, and derive a sediment transport rate using the morphologic method. The results suggest TLS is a viable tool for capturing in-situ aeolian ripples. Scan incidence angle is shown to significantly affect point density and therefore point cloud accuracy. The influence of incidence angle is different depending on the extent of the bedform studied. Ripples were measured during an active transport event in the Great Sand Dunes National Park, Colorado. Ripple morphologies and migration rates were within previously observed ranges. Applying the morphologic method highlighted ripple migration patterns, surface change and enabled an overall sediment budget to be calculated.

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Identifiers

ORCID for Joanna M. Nield: orcid.org/0000-0002-2657-0525

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