This paper shows the first results of measured spatial variability of beach erosion due to aeolian processes during the recently conducted SEDEX2 field experiment at Long Beach, Washington, U.S.A.. Beach erosion and sedimentation were derived using series of detailed terrestrial LIDAR measurements of beach morphology during three low tide periods. Results show significant measured sedimentation and erosion up to 10-20 mm/hour during moderate wind conditions. Spatial variability in bed level changes were found which appeared to be related to the wind orientation and varying bed level characteristics. Around the high waterline, erosion is found during onshore winds whereas sedimentation is observed on the upper beach. The terrestrial lidar data also resolves the migration of bed forms migrating on the upper beach demonstrating its utility of for a range of aeolian sediment transport applications.@en

Seasonal variability in wave conditions drive corresponding cycles of erosion and accretion along sandy beaches. Despite the fact that these oscillations are well documented at numerous sites throughout the world, the physical processes driving beach recovery remain poorly understood. Using field data from a low sloping, dissipative beach in the U.S. Pacific Northwest we show that the onshore migration of intertidal sandbars contributes to beach growth in a rapidly prograding system. Over a six week period two intertidal sandbars are shown to migrate onshore resulting in the generation of a low relief berm and local beach width increases of up to 20 m. Although significant alongshore variability of intertidal morphological change was observed, a 2.5 km stretch of coast is shown to experience beach growth as a result of intertidal bar welding.@en

Dunes provide the first line of defense from elevated water levels in low-lying coastal systems, limiting potentially major flooding, economic damages, and loss of livelihood. Despite the well documented importance of healthy dunes, our predictive ability of dune growth, particularly following erosive storm events, remains poor – resulting in part from traditionally studying the wet and dry beach as separate entities. In fact, however, dune recovery and growth is closely tied to the subtidal morphology and the nearshore hydrodynamic conditions, necessitating treating the entire coastal zone from the shoreface to the backshore as an integrated system. In this context, to further improve our understanding of the physical processes allowing for beach and dune growth during fair weather conditions, a large field experiment, the Sandbar-aEolian Dune EXchange EXperiment, was performed in summer 2016 in southwestern Washington, USA. Measurements of nearshore and atmospheric hydrodynamics, in-situ sediment transport, and morphology change provide insight into the time and space scales of nearshore-beach-dune exchanges along a rapidly prograding stretch of coast over a 6 week period. As part of this experiment, the hypothesis that dune growth is limited by the welding of intertidal sandbars to the shoreline (Houser, 2009) was tested. Using laser particle counters, bed elevation sensors (sonar altimeters and Microsoft Kinect), continuously logging sediment traps, RGB and IR cameras, and repeat morphology surveys (terrestrial lidar, kite based structure from motion, and RTK GPS), spatial and temporal trends in aeolian sediment transport were assessed in relation to the synoptic onshore migration and welding of intertidal sandbars. Observations from this experiment demonstrate that (1) the intertidal zone is the primary source of sediment to the dunes during non-storm conditions, (2) rates of saltation increase during later stages of bar welding but equivalent wind conditions, and (3) alongshore variability in rates of backshore fluxes appear to be related to alongshore variability in intertidal morphology. These observations quantitatively support the Houser (2009) bar welding hypothesis and provide valuable new insights on nearshore-beach-dune sediment exchanges@en

Due to the wide range of complex processes in the active coastal zone, individual studies have tended to focus on specific time scales (e.g., event-scale erosion) and/or specific morphological units, (e.g., the nearshore bar zone). As a result, the wet and dry portions of the beach have typically been studied independently. In nature, however, the nearshore and the backshore are highly interdependent and understanding the linkages between these units is critical to characterizing coastal evolution. For example, during periods of intense storm conditions (e.g., major El Niños on the U.S. West Coast), elevated water levels and large waves commonly lead to the scarping, or even destruction, of wind formed dunes. Given that dunes act as a form of green infrastructure and are a major asset to the coastal zone, it is critical to be able to forecast backshore evolution. Existing models for backshore recovery, however, are typically based on local historical trends rather than a mechanistic understanding including onshore sediment transport, dune growth, and the role of ecomorphodynamic feedbacks. Therefore, most likely as a result of the historical academic separation of wave and wind driven processes, geomorphology and ecology, and short- and long-term processes, our understanding of beach and dune building is still in its infancy. Here we describe SEDEX2, the Sandbar-aEolian-Dune EXchange Experiment, a comprehensive summer 2016 field campaign in which measurements of waves, currents, wind, dune ecology, subaqueous and aeolian sediment transport, and subsequent morphological changes were collected along the Long Beach Peninsula, WA. The data collected during the six-week experiment are contextualized by nearly two decades of focused research on the seasonal-centennial scale evolution of this rapidly prograding system. The findings of this study, actively bridging across disciplines, morphometric units, and temporal scales are informing conceptual and numerical models of beach-dune interaction and helping to improve management of vital backshore resources.@en

Duran and Moore (2013) recently extended the model of Hermann et al. (2008) to create an aeolian eco-morphodynamic model (the Coastal Dune Model, CDM) to simulate the formation of coastal foredunes. De Vries et al. (2014) initiated development of a model to simulate the influence of supply-limiting factors on aeolian transport (now the AeoLiS model). Roelvink et al. (2009) developed a modelling approach to dune erosion, overwashing and breaching (XBeach), which connects the upper shoreface with dune systems during storms. Despite the importance of interactions that occur across the spatial domains and range of conditions represented by these process-based models, CDM and AeoLiS treat storm erosion in a schematized way and XBeach does not address inter-storm development of topography. Thus, we are collaborating to develop WindSurf, consisting of: (1) XBeach (2) CDM and (3) AeoLiS. In addressing both subaqueous and subaerial sediment transport and erosion during storms as well as inter-storm evolution of subaerial topography, the resulting coupled model will allow, for the first time, process-based simulation of event- and decadal-scale co-evolution of nearshore, beach and dune systems. @en

The interface between the land and sea is complex. During high energy conditions large waves and elevated water levels can cause severe beach and dune erosion, whereas recovery takes place during extended periods of calm. Recent advances in numerical modeling have improved our ability to accurately model both storm-induced coastal hazards (e.g., XBeach; Roelvink et al., 2009), aeolian sediment transport in supply limited conditions (de Vries et. al. 2014), and dune eco-morphodynamics (e.g., Coastal Dune Model; Duran and Moore, 2013). However, process based numerical models incorporating the co-evolution of the coastal zone due to both subaqueous and subaerial processes to our knowledge, do not exist; hindering accurate forecasts of seasonal- to decadal-scale coastal evolution. Addressing this community need, a recent international collaboration has initiated the development of the open-source coupled numerical model Windsurf. Under the Windsurf framework, the coupled system resolves the most important subtidal and supratidal physical and ecological processes during both calm accretive conditions and high energy erosive periods. Here we present the coupled model’s ability to simulate short-term beach and dune building processes on daily to seasonal time scales. The welding of intertidal sandbars to the shoreline has long been recognized as an important mechanism for beach and dune building (e.g., Houser, 2009) as beaches are often supply limited. Field experiments on the Oregon coast indicate that discrete sandbar welding events can deliver as much as 20 m3/m of sand from the nearshore to the backshore via the intertidal, resulting in shoreline progradation, backshore aggradation, and dune growth. Here we compare these field observations to model simulations demonstrating Windsurf’s ability to simulate beach-dune exchanges in supply limited scenarios. @en

Coastal dunes are generally dynamic due to a combination of marine and aeolian sediment transport processes. The growth of coastal dunes is generally governed by wind driven sediment transport. Quantifying and predicting aeolian sediment transport processes is a scientific and practical challenge. This is caused by the uncertainties in the relative importance of the transport capacity of the wind and the availability of sediment (or sediment supply). Especially sediment supply has recently been hypothesized to be of governing importance but no quantitative knowledge is available yet. The intertidal zone adds further complexity since sediment supply is likely influenced by alternating marine and aeolian processes on the tide timescale. However, while sediment availability is likely to vary along the coastal zone, no measurements of sediment supply and availability has been successful in the past. In this study we use detailed measurements of wind driven erosion and sedimentation in the intertidal and supra-tidal coastal zone to quantify sediment supply for aeolian sediment transport and associated dune growth. During the 6 weeks SEDEX2 field campaign, a RIEGL 200VZ laser scanner is used to collect high resolution topographic data with 15-minute intervals during several tidal cycles. The data reveals the small but significant erosion in the intertidal zone due to wind driven processes within a tidal cycle for the first time. At the same time the small but significant sedimentation at the dry beach and dunes is measured on the tidal timescale. These data are essential in understanding sediment exchange between marine and aeolian zones and the growth of coastal dunes.@en

The Sand Motor is an artificial sandy peninsula extruding from the Dutch coast about 1 kilometer into the North Sea (Stive et al., 2013). It is virtually permanently exposed to tides, waves and wind and is consequently highly dynamic. In order to understand the complex morphological behavior of the Sand Motor, it is vital to take both subtidal and subaerial processes into account. About 70% of the Sand Motor area is located above 2m+MSL and is therefore uniquely shaped by subaerial processes. These dry areas are hardly eroded due to the presence of a coarse sand armor layer that was naturally established over time. However, significant aeolian transport is observed originating from the intertidal beaches surrounding the Sand Motor. Due to periodic flooding no armor layer can be established in the intertidal zone. Consequently, subtidal processes significantly influence the subaerial morphology. An international collaboration initiated the development of the open-source Windsurf modeling framework that enables us to simulate multi-fraction sediment transport due to subtidal and subaerial processes simultaneously. The Windsurf framework couples separate model cores for subtidal (XBeach; Roelvink, 2006) and subaerial morphodynamics (Coastal Dune Model; Duran and Moore, 2013) and multi-fraction aeolian sediment supply (AeoLiS; based on work by de Vries, 2015). Preliminary model results from a one-year one-dimensional simulation show a concentration of aeolian sediment supply from the intertidal beach area during calm conditions and an elevated aeolian activity shortly after a strong wind or surge event. The period of elevated transport may last for over a week resulting in the immediate initiation of recovery after a surge. Here we will present an application of the Windsurf modeling framework on the Sand Motor and a detailed description on how the interaction between subtidal and subaerial processes explain its complex morphological development. @en