Temporal and spatial variability in event scale aeolian transport on Ameland, The Netherlands

Abstract

a b s t r a c tCoastal dunes are the primary defence protecting the coastline from the destructive forces of the sea in The Netherlands. Aeolian processes are important in this context as they contribute to dune accretion and thus the safety of the coastal hinterland. In this study, we analyze horizontal and vertical variability of event scale aeolian sand transport on a wide beach on the island of Ameland, The Netherlands. Data were obtained from a meteorological station, groundwater monitoring wells and a camera installed on the beach. Fifteen aeolian transport events (two involving onshore winds, seven longshore and six offshore) were measured using a comprehensive grid of 37 customized MWAC traps. The highest sand transport rates and largest variability was found for alongshore events. Surface moisture, governed by groundwater, was found to be an important controlling parameter for aeolian transport rates and vertical flux profiles. Groundwater levels were largely dominated by beach inundation, influencing the groundwater table for a two week period. Variations in vertical flux profiles between traps were larger for wet sand transport events than dry ones. In general, sand transport rates were highest at the foreshore and lowest at the dune toe. Sand transport dynamics are dependent on local conditions such as beach dimensions, beach orientation and also meteorological and surface characteristics. Moderate (high frequency, low magnitude) events are also capable of transporting large amounts of sand. Future studies should include spatially explicit measurements of elevation and surface moisture to obtain a more complete understanding of the complex sand transport dynamics.

Terrestrial LiDAR monitoring of coastal foredune evolution in managed and unmanaged systems

“…Generally, winds exceeding the sediment entrainment velocity threshold mobilize sand from the sub‐aerial portion of the beach and deposit it when wind speeds are reduced around vegetation and topographic features, typically within the upper backshore and incipient dune (Hesp, ). Transport magnitudes are controlled by wind speed, duration and fetch (Davidson‐Arnott et al ., ; Anthony et al ., ; Delgado‐Fernandez, ; Bauer et al ., ; Keijsers et al ., ), vegetation (Feagin, ; Bitton and Hesp, ; Charbonneau et al ., ), topographic slope breaks (Walker et al ., ), as well as supply‐limiting factors including moisture content (Davidson‐Arnott et al ., , ), beach width, sediment supply and sediment characteristics (Bauer and Davidson‐Arnott, ; Poortinga et al ., ; Crapoulet et al ., ). Dune growth, however, may be interrupted by hydrodynamic processes, especially during stormy periods when elevated water levels and runup may reach and erode the dune (Sallenger, ).…”

The influence of groundwater depth on coastal dune development at sand flats close to inlets

“…Sand supply, surface moisture, vegetation characteristics, and wind velocity are, among others, characteristics that can drive spatial variations in coastal dune development, although just a few studies have been published on sand flat environments (Hesp 2002;van Heteren et al 2006;Poortinga et al 2015;Engelstad et al 2017;van Puijenbroek et al 2017). Zarnetske et al (2015) argue that, for a straight beach, vegetation and sand supply are important factors that control foredune morphology, depending on the time-scale considered and coastline position variability.…”

“…Zarnetske et al (2015) argue that, for a straight beach, vegetation and sand supply are important factors that control foredune morphology, depending on the time-scale considered and coastline position variability. Regarding sediment supply, some authors have suggested that water table depth and consequently surface moisture are important determinants for sediment supply and, consequently, for dune development (Hesp 2002;Bauer et al 2009;Poortinga et al 2015).…”

“…Surface moisture is known to affect aeolian sediment transport, which on natural beaches can be related to groundwater levels (Arens 1996;Yang and Davidson-Arnott 2005;Oblinger and Anthony 2008;Bauer et al 2009;Houser and Ellis 2013). Water table fluctuations, which are related to several atmospheric (e.g., pressure, precipitation) and oceanographic (e.g., tidal cycle, wave run-up) variables, influence the variability of surface moisture and hence the potential for aeolian transport (Yang and Davidson-Arnott 2005;Darke and Neuman 2008;Houser and Ellis 2013;Poortinga et al 2015). In a study on the Dutch island of Ameland, Poortinga et al (2015) suggest that high groundwater levels can be supply limiting in an event scale (i.e., length of days), and therefore limit the amount of sand available for aeolian transport.…”

“…Water table fluctuations, which are related to several atmospheric (e.g., pressure, precipitation) and oceanographic (e.g., tidal cycle, wave run-up) variables, influence the variability of surface moisture and hence the potential for aeolian transport (Yang and Davidson-Arnott 2005;Darke and Neuman 2008;Houser and Ellis 2013;Poortinga et al 2015). In a study on the Dutch island of Ameland, Poortinga et al (2015) suggest that high groundwater levels can be supply limiting in an event scale (i.e., length of days), and therefore limit the amount of sand available for aeolian transport. Water table levels are often higher than the tidal elevation, and this effect (termed overheight) behaves in inverse proportion to beach face slope and sediment size but is directly proportional to tidal range and wave infiltration (Turner et al 1997;Horn 2006).…”