# Characterization of barchanoid dune systems on the Cap Ferret spit (Aquitaine)

Clélia Billières, Julie Billy, Raphaël Bourillot, Hugues Feniès

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Clélia Billières, Julie Billy, Raphaël Bourillot, Hugues Feniès. Characterization of barchanoid dune systems on the Cap Ferret spit (Aquitaine). 19è congrès de sédimentologie, Association Française de Sédimentologie, Nov 2025, Lille, France. pp.1-4, 10.70665/AEMK1901. hal-05004261

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# CHARACTERIZATION OF BARCHANOID DUNE SYSTEMS ON THE CAP FERRET SPIT (AQUITAINE)

Clélia BILLIÈRES¹, Julie BILLY², Raphaël BOURILLOT³ and Hugues FÉNIÈS⁴

¹ BRGM, Bureau de Recherche Géologique et Minières, F-33600 Pessac, France;
Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France

² BRGM, F-33600 Pessac, France

³ Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France

⁴ CV Associated Engineering

Corresponding author: c.billieres@brgm.fr

**Abstract:** The Cap Ferret spit (Atlantic coast, France) is defined by barchanoid dune systems formed by a large input of sand remobilized by aeolian dynamics. GPR investigations have revealed distinct internal architectures within the barchanoid dunes, mainly characterized by eastward-dipping reflectors and complex structures illustrating sand remobilization. These structures highlight a progressive evolution in dune morphology, from west to east, i) low topographic and interconnected dunes without specific morphology (proto-barchans), ii) interconnected dunes with crescent-shaped morphology (barchanoid ridge) and iii) isolated dunes with a well-defined crescent-shaped morphology (isolated barchans). This study investigates these distinct stages of development, linking internal stratigraphy and geomorphological evolution, and provides insights on the dynamics of coastal barchanoid dune formation and migration.

**Keywords:** Barchanoid dunes, Spit, GPR, Internal architecture, Dune migration, France.

## 1 INTRODUCTION

The coastal zone is the interface between marine and terrestrial environments and can extend several kilometers inland. It represents a complex system with interactions between land and marine dynamics processes. In the literature, the beach/dune interface, coastline or geomorphological changes are the most documented topics (e.g. Brossard and Nicolae Lerma., 2020), but fewer studies include the retro-littoral domain. Indeed, holistic study of the coastal zone is crucial, especially in the context of coastal evolution and territorial adaptation to climate change, and requires consideration of the marine part, the land/sea interface, and the inland domain. For example, sedimentary spits and coastal barriers need to be considered on their entire geosystem, i.e., along an ocean/barrier/lagoon continuum.

Barchan dunes are one of the most iconic dune categories in the literature and can be located in desert areas as well as along the coast (e.g. Bristow, 2019; Fu et al., 2019; Gomez-Ortis et al., 2008; Oliveira, et al., 2008). They are mainly defined as crescent-shaped, with the convex side facing into the wind and horns pointing downwind, and literature suggests that they are formed on dry substrates. Unlike desert barchanoids, those on the coast remain understudied, with limited documentation on their internal structures. Studies referring to barchan migration or geomorphological evolution often consider high dome-shaped sand deposits at starting point (e.g., numerical models or theoretical evolutionary models based on distinct wind directions, etc., Hersen, 2004). However, these idealized cases are not representative of the barchan dune initiation, especially in coastal areas.

This study focuses on the inland domain of an emblematic coastal system on the Atlantic coast of Gironde, the Cap Ferret spit, a 23-km-long and 5-km-wide sedimentary spit (Fig. 1-A). This coastal spit has evolved over the last 3000 years through southward progradation (Féniès et al., 2005), shifting the Leyre estuary and shaping the geomorphology of the Arcachon Basin. The spit displays several dune systems and interdune areas, in equal proportion. This spit is particularly well suited for examining the coastal zone in its entirety, nevertheless only the lagoonal part and ocean domains were studied previously (e.g. Allard, et al., 2009; Féniès, et al., 1998; Nahon, 2018). The terrestrial domain (spit itself) has been given little consideration, apart from the coastline and the modern foredune (e.g. Brossard and Nicolae Lerma., 2020). However, the spit configuration provides an excellent opportunity to explore the complexity and interconnection of the coastal environment as well as the study of barchanoid dune systems in coastal environments. The study focuses specifically on the different dune systems present on this spit. Analysis of 2016 LIDAR data (1 m resolution) enabled the characterization of four dune typologies on the Cap Ferret spit (Billières, 2022). From west to east (Fig. 1-B), these include a longitudinal active foredune (modern coastal dune, not studied here) followed by barchanoid dune systems fixed since the mid-19th century (maritime pine forestation), and defined as: (i) proto-barchan (PB) dunes, referred to as undetermined dune type in Froidefond et al. (1985); (ii) barchanoid ridges (BR); and (iii) isolated barchans.
Here, each barchanoid dune displays distinct morphological characteristics, notably in height or shape. What insights can be obtained regarding their internal architecture? In this study, the internal structure of coastal barchans is analyzed using ground-penetrating radar (GPR) profiles, which enable the identification of sedimentary structures.

<figure><img src="image_2.png" /><figcaption>Figure 1. A. Location map of the studied areas; B. Barchanoid dune typologies identified on the Cap Ferret Spit (Atlantic coast, France): Proto-barchan, Barchanoid ridge and isolated barchan.</figcaption></figure>

## 2 MATERIALS AND METHODS

Several GPR surveys have been carried out between 2022 and 2024 to provide insights into the dunes' internal structures and their underlying substrate. GPR is a non-intrusive geophysical method, based on the transmission and reflection of electromagnetic pulses to visualize sedimentary structures. Two instruments were used: a Mala ProEx system with three antennas operating at center frequencies of 100, 250 and 500 MHz and a dual frequency 200-900 MHz IDS-CHASER, which enables deep penetration while maintaining high resolution even with a high frequency. A total of 65 km of profiles were recorded along the spit. REFLEXW V10.5 software was used to process data, applying corrections such as background removal, gain adjustment and topographic correction, etc. The interpretation of these processed profiles was conducted using the DELPH Road Map software (V5), which also allows 3D visualization of the data. A constant radar velocity of 0.14 m/ns was estimated, with CMP method (Billy et al. 2020), for the time to depth conversion. Profile analysis allowed (i) visualization of internal architecture via reflectors, (ii) identification of various dune facies, (iii) calculation of internal structure dip angles, and (iv) dune development phases identification. Proto-barchan and barchanoid ridge systems were the most extensively surveyed due to their accessibility (via dune paths).

## 3 RESULTS AND DISCUSSION

### 3.1 Internal architecture of barchanoid dunes

The internal architecture of the three families of barchanoids dunes analyzed in this study is primarily characterized by eastward-dipping oblique reflectors (Fig. 2-B,C), downlapping onto the top of a basal unit. In their western part, these reflectors are truncated at the top, illustrating the effects of dune migration by aeolian processes. This erosion surface represents the current dune topography and is concordant with the 19th century dune topography (at the period of dune fixation by vegetation). GPR images also reveal distinct sequences within dunes, which correspond to phases of sediment mobilization influenced by wind dynamics. From west to east:

- Smaller PB dunes are defined as low altitude (5-15 m) interconnected dunes (Fig. 1-B.2). They are featured by homogeneous internal structures, with reflectors displaying gentle dip angles, from 5° to 15°, becoming steeper toward the eastern part of the dunes. Some PB dunes reach twice the altitude of smaller PB dunes (15-30 m) suggesting dune overlap. These are characterized by reflectors dipping up to 25° and draping stratification at the dune crest.

- BR dunes are taller than PB (25-40 m) and the crescent-shaped dune is well defined (Fig. 1-B.3) but dunes are still interconnected. GPR images (Fig. 2-B) demonstrate greater complexity of their
internal architecture. Their western parts share similarities with small PB in terms of facies, but with steeper dip angles, ranging from 25° to 30°. Their eastern parts show distinct overlapping units. It is interpreted here as evidence of eastward dune migration, with dunes growing and overlying existing sand accumulations (former dunes or part of nearest dunes).

- Barchans are individual dunes (Fig. 1B.4,5), with a clearly defined crescent shape. Its internal architecture is close to BR dunes in terms of prograding and overlapping facies, the dip angles (10 to 25°) and heights (25-40 m) but barchans can be wider (up to 500 m).

The internal architecture described below is coherent with findings by Bristow (2019) and Oliveira et al. (2008) for desert and coastal barchans, respectively. Indeed, in both studies, key features include oblique and parallel reflectors indicative of sedimentary layering formed by wind and sediment transport. Prograding facies align with sediment migration direction, showing stratification that flattens near the crest of the lee slope. Dip angles range from 24° to 31° (Bristow, 2019) decreasing near the slip face. Differential slope inclinations are also observed, with the windward slope shows a higher dip angle compared to the leeward slope, reflecting variations in sediment deposition and transport processes.

<figure><img src="image_3.png" /><figcaption>Figure 2. A. Location map of the GPR profile studied; B. Example of a GPR profile of a barchanoid ridge; C. Representative 2D model of a barchanoid ridge (Vertical Exaggeration = 9)</figcaption></figure>

## 3.2 Barchanoid development model: initiation to individual barchan

Geomorphological observations complete these internal architecture analyses, revealing a main west to east gradient across the 5-km-wide spit in terms of dune heights and shape. This west to east evolution pattern reflects the gradual transformation of the dunes, which is also identified in their internal architecture (e.g. oblique reflectors, slopes, overlying units). The evolution of dune shapes begins with sand accumulation (embryonic dune) lacking specific forms (PB), progressing to interconnected dunes showing the initiation of crescent shape (BR) and to individual dune (barchans). This pattern highlights distinct phases of barchanoid dune development that are rarely described in the literature.

The progression from PB dunes to BR dunes marks a critical phase in the evolutionary sequence, where dunes evolve from embryonic, poorly organized sand masses (PB) into larger, more structured forms with increased internal complexity (BR). The final stage, represented by mature barchans, reflects the isolation and individualization of dunes, completing the maturity sequence. These individual barchans continued their eastward migration with this mature shape.

# 4 CONCLUSION

By integrating GPR-based internal architecture data with geomorphological observations, the processes underlying the transition from PB dunes to BR dunes, and then to mature barchans have been highlighted. Our study site provides a rare opportunity to observe the earliest phase of this evolutionary sequence, with the observation of PB dunes. These embryonic dunes are scarcely documented in the literature. Additionally, the poorly defined morphology of PB dunes often excludes them from studies in contrary of well-defined
shape such as BR dunes or isolated barchans. Their identification at the Cap Ferret Spit enriches the understanding of coastal sedimentary dynamics. Without the intensive vegetation of the spit in the 19th century, this information, particularly on PB dunes, might have been partly masked by the natural mobility of the dunes.

Moreover, the evolutionary sequence observed in this study offers a broader framework for interpreting coastal barchanoid systems in other regions. The evolutionary pattern in dune height, width, and internal architecture reflects a maturity gradient that is likely influenced by sediment availability, wind regime, and local topographic constraints, need to be further investigated. The ability to document and preserve early dune stages provides insights into the initial conditions that precede more developed forms, contributing to a comprehensive understanding of coastal barchanoid dune dynamics.

## 5 ACKNOWLEDGMENT

The work is part of the DUNES research program, co-funded by the BRGM, The Nouvelle-Aquitaine Region, the ONF, the municipality of Lège Cap Ferret, the Landes Department, Vermilion REP SAS, CCGL, AEAG, and the EC2CO HYBIGE CNRS program. The authors would like to thank our ONF collaborators, Eric Lenain and Aurelien Brossard, and our BRGM colleagues Nicolas Bernon, Pierre Bourbon and Marie Branellec for their field assistance. This paper benefitted from the reviews of Marie-Hélène Ruz and Anne-Lise Montreuil.

## 6 REFERENCES

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