Aeolian Sediment Availability and Transport

This thesis explores the nature of aeolian sediment availability and its influence on aeolian sediment transport. The aim is to improve large scale and long term aeolian sediment transport estimates in (nourished) coastal environments. The generally poor performance of aeolian sediment transport models with respect to measurements in coastal environments is often accredited to limitations in sediment availability. Sediment availability can be limited by particular properties of the bed surface. For example, if the beach is moist or covered with non-erodible elements, like shells. If sediment availability is limited, the aeolian sediment transport rate is governed by the sediment availability rather than the wind transport capacity. Aeolian sediment availability is rather intangible as sediment availability is not only affected by aeolian processes, but also by marine and meteorological processes that act on a variety of spatial and temporal scales. The Sand Motor 21 Mm3 mega nourishment is used to quantify the spatiotemporal variations in aeolian sediment availability and its effect on aeolian sediment transport. The Sand Motor was constructed in 2011 along the Dutch coast. Aeolian sediment accumulation in the Sand Motor region is low compared to the wind transport capacity, while the Sand Motor itself is virtually permanently exposed to wind and accommodates large fetches. Aeolian sediment availability is therefore likely to dominate aeolian sediment accumulation. Multi-annual bi-monthly measurements of the Sand Motor's topography are used for a large scale aeolian sediment budget analysis. The analysis revealed that aeolian sediment supply from the dry beach area, that is almost permanently exposed to wind, diminished a half year after construction of the Sand Motor. The reduction in aeolian sediment supply is likely due to the development of a beach armor layer. In the subsequent years, two-third of the aeolian sediment deposits originate from the low-lying beaches that are frequently flooded and therefore often moist. The importance of the low-lying beaches in the Sand Motor region is tested during a six-week field campaign. Gradients in aeolian sediment transport are measured during the field campaign as to localize aeolian sediment source and sink areas. A consistent supply from the intertidal beach area was measured that was temporarily deposited at the higher dry beach. The temporary deposits were transported further during high water, when sediment supply from the intertidal beach ceased, resulting in a continuous sediment supply to the dunes. The temporary deposition of sediment at the dry beach was likely promoted by the presence of a berm that affects the local wind shear. Moreover, the berm edge coincided with the onset of the beach armor layer that might have further promoted deposition of sediment. The measurements on spatiotemporal variations in aeolian sediment availability and supply inspired an attempt to capture the characteristics of aeolian sediment availability in coastal environments in a comprehensive model approach. The resulting model simulates spatiotemporal variations in bed surface properties and their combined influence on aeolian sediment availability and transport. The implementation of multi-fraction aeolian sediment transport in the model introduces the recurrence relation between aeolian sediment availability and transport through self-grading of sediment. The model was applied in a four-year hindcast of the Sand Motor mega nourishment as first field validation. The model reproduces the multi-annual aeolian sediment erosion and deposition volumes, and the relative importance of the intertidal beach area as source of aeolian sediment well. Seasonal variations in aeolian sediment transport are incidentally missed by the model. The model accuracy is reflected in a R2 value of 0.93 when comparing time series of measured and modeled total aeolian sediment transport volumes in the four years since construction of the Sand Motor. The results suggest that indeed significant limitations in sediment availability, due to soil moisture content and beach armoring, govern aeolian sediment transport in the Sand Motor region. A comparison with a simulation without limitation in sediment availability suggests that aeolian sediment availability in the Sand Motor region is limited to about 25% of the wind transport capacity. Moreover, both spatial and temporal variations in aeolian sediment availability as well as the recurrence relation between aeolian sediment availability and transport are essential to accurate long term and large scale aeolian sediment transport estimates.