A laboratory on the beach: quantifying aeolian transport of nourished sediment using SfM in a mobile flume

Abstract

A key feature for the safety of our coasts is the long-term growth and maintenance of dunes. These processes depend strongly on the presence of aeolian transport, where wind mobilizes sand particles on the beach, causing grains to blow landward, thereby feeding the dunes and enabling them to grow and adapt to sea-level rise and storm impacts. In this study, we present a novel ‘laboratory on the beach’ approach to quantify aeolian transport under controlled yet realistic field conditions. Along Dutch sandy beaches, large-scale nourishments are widely used to maintain the coastline, introducing sediment that often differs in grain size distribution and shell content. Numerical dune development models, key tools in coastal planning, typically rely on simplified uniform sediment properties. Further model calibration is therefore essential. Relying solely on large-scale field measurements limits the ability to resolve fine-scale processes. Natural variability in wind and sediment heterogeneity introduces substantial uncertainty in transport observations; however, traditional laboratory experiments approximate unknown variables such as grain size, vegetation species, age, roots, and density, shell fragments, beach slope, and armouring effects. Consequently, there is a need for experimental approaches that investigate small-scale aeolian processes without simplifying complex beach conditions. We address this knowledge gap by constructing a field-deployable mobile wind flume and applying it in a series of experiments to quantify small-scale aeolian sediment transport and to evaluate the performance of co-registered Structure from Motion (SfM) photogrammetry. Our approach proved effective for investigating small-scale aeolian processes. Using SfM data acquisition, we resolved surface changes within the mobile flume with a vertical resolution of 0.35 mm and a volumetric accuracy of 0.056 dm3. This allowed a spatial interpretation of subtle redistribution patterns and an accurate quantification of volumetric sediment transport. We further demonstrate the value of mobile flume experiments for improving process understanding. Theoretical transport equations overestimated transport by 58%, increasing to 127% with an upwind poorly sorted sediment patch. We were able to link this difference to the formation of surface armouring layers. Results show a surface roughness increase of 26.3%, which suppresses downwind transport, even beyond the initial coarse patches. This process resulted in an exposed roughness value of k_s = 4.28 mm, a value that closely correlates with existing field observations of nourished beaches. Our findings show that roughness, induced by shell surface cover, leads to a strong nonlinear reduction in transport with a statistically significant exponential trend (R2 ≈ 0.94). This quantified relation demonstrates how our mobile flume can parameterize sediment supply limitations. To bridge the remaining knowledge gap, research should focus on integrating these specific armouring and shell-reduction functions into long-term coastal models to better predict how nourished beaches and dunes will evolve. Concretely, this means that future research would benefit from improving the flume design, but more importantly, extending experiments to actual field sites along the Dutch coast. Understanding these processes is critical for predicting dune resilience under rising sea levels and overseeing the consequences of coastal nourishments. Our approach presents a humble step in further uncovering the hidden processes that shape beaches and dunes.