Controls on geomorphological stage of trough blowout evolution: Evidence from computational fluid dynamics (CFD) airflow modelling

Trough blowouts are an important component of coastal dune systems since they facilitate sand transport from the beach to the backdune which can increase foredune width and resilience to erosion. The contemporary conceptual model of trough blowout evolution supposes that airflow acceleration over the deflation basin declines when the blowout reaches a critical cross-section. To date, however, there is little evidence for this model. The current study examines how trough blowout morphology - including the slope of the bed surface (from 10° to 25°), the slope of lateral walls (from 60° to 90°) and trough width (from 10 to 50 m), controls airflow dynamics and shear velocity during various incident wind speeds and directions. Computational Fluid Dynamics (CFD) is used to model flow where the incident wind angle varies from 0° to 80° relative to the long axis of the blowout. The results support the contemporary conceptual model. Increasing bed slope, decreasing wall slope and increasing blowout width result in a decrease in average shear velocity on the lateral walls and bed surface of the blowout. We also found that measurements of wind speed at the middle of the blowout might not be sufficient to predict trough blowout evolution in oblique incident wind conditions because of the occurrence of helicoidal flow inside the blowout, which results in a complex erosion pattern on the lateral walls and bed surface. We also found a strong linear relationship between the shear velocity of incident winds and the average shear velocity on the lateral walls and bed surface of the blowout. This relationship could assist coastal managers to select locations where the incident wind speed is strong enough to maintain artificial trough blowouts to address coastal problems.