Wrack as a Roughness Element: Nonlinear Controls on Wind Reduction and Sediment Deposition in a Coastal Dune System

Phillip Schmutz, Samantha Seals, Joseph Agin, Peter Tereszkiewicz

Coastal dunes are shaped by interactions among wind, sediment supply, and surface roughness elements. While vegetation is widely recognized as a driver of aeolian sediment dynamics, beach wrack remains understudied despite its frequent occurrence on sandy coastlines. This study investigated how wrack modifies airflow and influences sediment deposition. Paired 2D anemometers positioned upwind and downwind of a natural wrack pile and a single-point terrestrial LiDAR were used to quantify wind reduction and bed-level change. Generalized linear models (GLMs) and generalized additive models (GAMs) were used to evaluate relationships among wind speed, wind angle, wind reduction, and sediment deposition. Wind speed and wind angle significantly influenced wind reduction (p < 0.001), with the GAM providing a significantly better fit than the GLM (ΔAIC = 612.7; p < 0.001). Wind reduction increased with wind speed and varied according to wind orientation, indicating that wrack functions as a dynamic aerodynamic roughness element. Wind speed and wind angle also significantly influenced sediment deposition dynamics, although depositional responses exhibited complex nonlinear patterns and substantial variation. For both airflow reduction and sediment deposition, the GAM identified relationships that were oversimplified or undetected under linear assumptions. Overall, these findings demonstrate that wrack significantly modifies local airflow and creates conditions favorable for sediment retention. More broadly, the results suggest that wrack functions as a condition-dependent geomorphic agent whose influence on aeolian transport varies across environmental scales. The study highlights the importance of nonlinear approaches for characterizing aeolian processes and identifying threshold-dependent responses within coastal sediment transport systems.