Predicting monthly to multi-annual foredune growth at a narrow beach

An open-source quantitative model for predicting coastal foredune growth at monthly to multi-annual (meso)temporal scales is developed. The model builds on the established fetch framework as a surrogate for the complex micro-scale aeolian processes on the beach, to which rain and groundwater-induced spatiotemporal surface moisture dynamics are added as factors limiting aeolian sand supply to foredunes. The model shows great skill in an application at Egmond aan Zee, The Netherlands, with a predicted growth of 16.5 m³/m/yr comparing favourably to the observed growth of 17.3 m³/m/yr. Rain, surface moisture dynamics as well as beach width reduction by storm-induced elevated sea levels are shown to be important factors that jointly reduce meso-scale sand supply below the potential (i.e., unlimited) maximum, in our case study by almost 5 m³/m/yr. These factors are most relevant for strong (here, above 15.5 m/s) onshore winds. Consistent with expectations from the literature, meso-scale foredune growth results primarily from moderately strong (9.5–12.5 m/s) shore-oblique winds, which are frequent and do not result in supply-limited conditions. At the study site these winds are most common in winter and hence foredune growth is predicted to vary seasonally, consistent with the observations. Because of the promising results we believe that our model has potential for quantifying how quickly a foredune can recover after an episodic erosion event because of storm waves.