Nicolas Robin

Blowouts are geomorphic features formed through the interplay of aeolian processes, vegetation cover and anthropogenic disturbance. Although the basic dynamics of airflow over blowouts under unidirectional winds are relatively well understood, fine‐scale airflow patterns under bi‐directional wind regimes remain poorly characterised. This study investigates airflow dynamics within a small trough blowout located in the coastal dune system of Canet‐en‐Roussillon (SE France), where foot traffic has modified the original morphology. Wind data were collected using 26 anemometers deployed across and within the blowout, capturing spatial and vertical variations during both offshore and onshore wind events. The blowout exhibits a bifurcated morphology that alters airflow asymmetrically: offshore winds are steered oblique to the shoreline and sustain acceleration along the main axis (pattern [c]), whereas onshore winds generate more parallel flow, resulting in deceleration and turbulence (pattern [a]). This contrast gives rise to distinct internal flow structures, flow bifurcation under offshore winds and widespread near‐surface disturbance under onshore winds. A shallow, disturbed near‐surface flow develops within the blowout, constrained vertically by blowout dimensions, and decoupled from the more coherent higher and above‐blowout flows aligned with the incident wind. Peak velocities for both wind directions are observed in the constricted throat region (pattern [b]), though classic jet flow does not develop, likely due to bifurcation and throat morphological constraints. These findings highlight the sensitive coupling between blowout morphology, wind regime and anthropogenic alteration in shaping airflow behaviour within a small trough blowout.

This study presents the first high‐resolution spatial and temporal analysis of wind flow, sediment transport and topographic evolution under simultaneous storm conditions across two morphologically contrasting beach‐dune systems, characterized by a gently sloping dune face (11°) and a steep, scarped dune face (36°). Results demonstrate that the dune slope strongly controls near‐surface wind acceleration, the development of secondary airflow structures (amplitude, spatial positions), and the continuity of sediment transport pathways. Over the gentle slope, airflow accelerates progressively up the stoss face, promoting sustained, landward‐directed sediment fluxes across the entire beach–dune system and enabling efficient sediment recycling. In this configuration, beach‐derived contributions account for only 12%–15% of the total sediment flux. In contrast, the steep scarp induces flow deceleration and separation at the dune toe, limiting sediment transfer from the beach and favoring seaward‐directed transport associated with secondary vortices at the crest. These contrasting airflow organizations result in fundamentally different storm responses. The gently sloping dune undergoes landward translation with minimal net volume change, whereas the scarped dune experiences dominant marine erosion, leading to a 4 m retreat of the dune front and a sediment loss of ∼30 m ³ m ⁻¹ . A new conceptual model of storm‐driven airflow over contrasting dune morphologies is proposed, illustrating how inherited dune slope governs airflow structure and circulation patterns. Overall, these results identify inherited dune morphology as a primary control on airflow organization, sediment pathways, and dune resilience during extreme events.

Dunes are key geomorphological features controlling airflow and sediment transport. While these processes are well documented under onshore conditions, this study provides the first high-resolution spatial analysis of dune-beach dynamics under offshore winds, integrating wind flow, sediment transport, and topographic data. The investigated site is a low-elevation (<1 m) dune typical of Mediterranean coasts, characterized by a mixed sand–gravel patch and a distinct beach slope break. Results show that dune height strongly controls the magnitude of airflow adjustment. Directional deflections and accelerations remain limited (<15° and 50%), sediment transport initiates immediately beyond the crest, feeding offshore-directed fluxes. Under weaker winds (gusts < 20%), enhanced surface roughness from the mixed sand–gravel patch and flow stagnation at the slope break shift the active transport zone toward the lower beach, where the most pronounced morphological changes occur. These findings demonstrate that small dunes provide limited aerodynamic shelter and fail to prevent sediment export under offshore winds. They highlight the need to incorporate additional factors (e.g., microtopography, surface properties) when assessing sediment budgets and the long-term evolution of low-relief coastal systems.

The European coasts are among the most densely populated of the world, with natural sand dune barriers urbanised and replaced by traditional hard coastal protection structures. Without the needed measures to adapt, the number of people exposed to floods is anticipated to increase 187 million worldwide by the end of the 21st century. In Europe, the coasts of The North Sea, the Baltic Sea, and the Atlantic are anticipated to experience substantial flood risks from sea-level rise, and climate extremes are also expected to impact southern Europe Mediterranean coasts (Vousdoukas et al., 2017). Future coastal management surpasses the current fixed and non-adaptive flood coastal protection setup: Hybrid Nature-based-Solutions (abbreviated as NbS) that can efficiently integrate static hard infrastructure with dynamic aeolian, and vegetated sediments are currently developed along urbanized areas of most of the European sandy coasts, yet still at small scales.

Across the coastal dunes, wind velocity and consequently sand transport is strongly controlled by the interaction between regional wind flow, vegetation cover and surface topography (topographic steering, Schwarz et al., 2020). Recently, knowledge has been improved on how winds erode, transport and deposit sand across coastal dunes, including the specific effects of vegetation and moisture as exemple (e.g Bauer et al., 2012; Hesp et al., 2015; Davidson et al., 2022). However, most of the existing studies are based on measurements collected along a single profile, thus limiting the understanding of alongshore variability and of the influence of the beach-dune profile on windblown sand pathways. The aim of this study is to address the differences on the wind flow pattern and resulting sediment transport during the same storm events at two beach-dune systems separated by only 7 km. The two field sites, which are exposed to the same ambient wind conditions, have contrasting dune profile morphologies primarily due to different human action (fixation/remobilization), allowing to discuss the impact of coastal dune management on aeolian processes.

Many coastal dunes are experiencing human stabilization by fencing, vegetation planting or reshaping the dune profile, resulting in a modification of air flow and sediment transport patterns. For example, a taller and steeper dune may result in an increased zone of wind flow separation and deceleration, resulting in the reversal of flow and sediment transport to the dune toe from the backshore during offshore winds. (Walker and Nickling, 2002; Hesp and Smyth, 2019). Globally, the pattern of reversed or deflected flow plays a key role in dune evolution, particularly when an offshore wind regime is dominant (Bauer et al., 2012; Nordstrom and Jackson, 2018). Recently, Computational Fluid Dynamics (CFD) has given new insights to complex flow recirculation on the lee-side of heavily managed coastal dunes on developed coasts during onshore winds (Smyth and Hesp, 2015; Nguyen et al., 2022). This research is the first to investigate wind flow and sediment transport dynamics on a anthropogenically constructed dune in offshore wind conditions. CFD simulations were completed for three offshore wind directions over a computational domain constructed from a 3D topographic LiDAR surface. Simulations were validated by a comparison with wind

In the context of sea levels rising, monitoring spatial and temporal topographic changes along coastal dunes is crucial to understand their dynamics since they represent natural barriers against coastal flooding and large sources of sediment that can mitigate coastal erosion. Different technologies are currently used to monitor coastal dune topographic changes (GNSS, UAV, airborne LiDAR, etc.). Satellites recently emerged as a new source of topographic data by providing high-resolution images with a rather short revisit time at the global scale. Stereoscopic or tri-stereoscopic acquisition of some of these images enables the creation of 3D models using stereophotogrammetry methods. Here, the Ames Stereo Pipeline was used to produce digital elevation models (DEMs) from tri-stereo panchromatic and high-resolution Pleiades images along three 19 km long stretches of coastal dunes in SW France. The vertical errors of the Pleiades-derived DEMs were assessed by comparing them with DEMs produced from airborne LiDAR data collected a few months apart from the Pleiades images in 2017 and 2021 at the same three study sites. Results showed that the Pleiades-derived DEMs could reproduce the overall dune topography well, with averaged root mean square errors that ranged from 0.5 to 1.1 m for the six sets of tri-stereo images. The differences between DEMs also showed that Pleiades images can be used to monitor multi-annual coastal dune morphological changes. Strong erosion and accretion patterns over spatial scales ranging from hundreds of meters (e.g., blowouts) to tens of kilometers (e.g., dune retreat) were captured well, and allowed to quantify changes with reasonable errors (30%). Furthermore, relatively small averaged root mean square errors (0.63 m) can be obtained with a limited number of field-collected elevation points (five ground control points) to perform a simple vertical correction on the generated Pleiades DEMs. Among different potential sources of errors, shadow areas due to the steepness of the dune stoss slope and crest, along with planimetric errors that can also occur due to the steepness of the terrain, remain the major causes of errors still limiting accurate enough volumetric change assessment. However, ongoing improvements on the stereo matching algorithms and spatial resolution of the satellite sensors (e.g., Pleiades Neo) highlight the growing potential of Pleiades images as a cost-effective alternative to other mapping techniques of coastal dune topography.

A 24-channel seismic display recording on a DaQLink is used to carry out tomographies of P and S waves down to 40m deep in the Holocene deposits of the coastal Flemish plain west of Dunkirk and the Slack estuary in the Boulonnais. The flat topography along the profiles collected along a road, on the beach, or in farmed fields, makes it possible to avoid complex processing of topographic correction. The results, validated by a number of cores, provide details on the morphology of the incision at the bottom of Holocene transgressive sediments, either infilling estuarine valleys or onlapping bedrock fault scarps. Vertical and horizontal heterogeneities in shear-wave velocity, well expressed in the upper layers, may reflect lateral changes in lithology or water content.

The morphodynamics of the Leucate double-crescent bar system was studied over twenty years using bathymetric data supplemented by satellite images and video monitoring. Eleven different bar typologies were identified, mostly based on existing beach state classifications (Low-Tide Terrace, Transverse Bar and Rip, Rhythmic Bar and Beach), also including new heterogeneous typologies (TBR/LTT, RBB HP/RBB, TBR/RBB). The inner bar shows greater variability, with 10 different typologies observed, while the outer bar shows only three different typologies. Summer low-energy periods are dominated by TBR/LTT and TBR typologies, while RBB, although common throughout the year, dominates winter periods along with disrupted bar configurations. The return to less energetic periods in spring is associated with the establishment of heterogeneous typologies. The outer bar has a fairly stable position, although breaches at the embayments and slight movements of its horns can occur following particularly energetic episodes. The inner bar, on the other hand, is much more dynamic, with more common breaches at the embayments and significant cross-shore movement of the horns. Seasonal changes in bar typology do not lead to bar renewal through destruction/reconstruction. Overall, the morphological and typological characteristics of the bar system described here seem somewhat unique compared to the existing literature.