# Beneath the Vendée dunes: former coastal systems, interface between marsh and foreshore revealed by ground-penetrating radar and core-penetration testing

Julie Billy, Vivien Baudouin, Manuel Garcin, Gildas Noury

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Julie Billy, Vivien Baudouin, Manuel Garcin, Gildas Noury. Beneath the Vendée dunes: former coastal systems, interface between marsh and foreshore revealed by ground-penetrating radar and core-penetration testing. 19e congrès français de sédimentologie (ASF), Association Française de Sédimentologie, Nov 2025, Lille, France. pp.133-137, 10.70665/IJUH4247. hal-05024165

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# BENEATH THE VENDÉE DUNES: FORMER COASTAL SYSTEMS, INTERFACE BETWEEN MARSH AND FORESHORE REVEALED BY GROUND-PENETRATING RADAR AND CORE-PENETRATION TESTING

Julie Billy¹, Vivien Baudouin², Manuel Garcin³ and Gildas Noury³

¹BRGM, Bureau de Recherche Géologique et Minières, F-33600 Pessac, France

² BRGM, F-44323 Nantes, France

³BRGM, F-45060 Orléans, France

Corresponding author: j.billy@brgm.fr

**Abstract:** The combined use of ground-penetrating radar and geotechnical testing by CPT in Pays-de-Monts (Vendée, France) effectively characterizes the coastal geosystem. The results reveal the internal structure and soil behavior type of coastal dunes and their underlying sedimentary units, highlighting distinct features beneath the dune system at two neighboring sites. At La Parée Grollier, the dunes migrated above former foreshore and marsh deposits, and the study reveals the sub-surface foreshore-marsh transition. At La Pège, a clay-rich paleo-channel was identified which seems connected to the modern beach/dune system. These findings emphasize the value of subsurface data for effective coastal management considering their specific features.

**Keywords:** Sandy coastal geosystem, Internal architecture, Geotechnical test, Soil behavior type, Paleo-environment.

## 1 INTRODUCTION

In the context of coastal evolution and territorial adaptation to climate change, understanding coastal geosystems has become increasingly essential. Generally, sandy coasts are primarily characterized by their geomorphology, focusing only on the surface features of the geosystem. Current tools such as LiDAR, satellite imagery, and cameras are commonly used to monitor shoreline variations and the morphological evolution of foreshore, beach, or dune systems. However, knowledge of coastal geosystems, particularly their internal architecture and intrinsic properties, remains limited or poorly understood. As a result, their specific characteristics and geological spatial variability are rarely integrated into site management, or even less, into the development of future adaptation strategies. The aim of this work is to improve understanding and investigation methods for sandy coastal geosystems in order to highlight their specific characteristics, especially beneath dunes systems.

The Pays-de-Monts coastal dune system is located on the Atlantic coast in the Vendée region, western France (Fig. 1A). This area is a 27-km-long and 0.5-2.3-km-wide Holocene sandy dune system (e.g., Ters, 1956) composed of four generations of vegetated dunes (with increasing elevation inland), separated by large slack areas and bordered to the east by low-lying marshes (<2 m NGF). From inland to the beach, the dune system comprises: (i) highly digitate overlapping parabolic dunes (18–25 m high; D₁); (ii) digitate parabolic dunes (10–13 m high; D₂); (iii) an irregularly-shaped series of dune (7–14 m high; D₃); and (iv) longitudinal dunes connected to the modern beach (4–10 m high; D₄). This work focuses on two specific sectors: La Parée Grollier (D₁-4 dune types; Fig. 1A) in the northern part of the area and La Pège (D₃-4 dune types; Figure 2A) in the southern part. The aim of this study was to investigate if these dunes located close to each other have similar internal structures and to determine the nature of the underlying material beneath the dunes. Even though these areas are close to each other, are they similar? And furthermore, what are these dunes based on?

The Pays-de-Monts coastal geosystem is an emblematic study site for the BRGM's coastal research team. An innovative, multi-geophysical approach was developed and deployed in this area during 2016–2017 (SIBLES project). This approach combines continuous, multiscale, and non-destructive methods, including terrestrial seismic surveys (MASW), ground-penetrating radar (GPR), and electrical resistivity tomography. One of the key findings of previous work in La Parée Grollier area (Billy et al., 2020) is that the depth of Eocene limestone bedrock (fractured to highly fractured) reach -10 to -15 m NGF. GPR (Billy et al., 2014; Robin et al., 2023) and geotechnical testing (cone penetration test; e.g. Campo et al., 2023; Zhang et al., 2018) both demonstrate significant potential for characterizing the coastal geosystem in detail. Here we present a coupled approach GPR-CPT that has been experimented (ongoing SIBLES2 project) to characterize the coastal geosystem, and specifically the dunes as well as the underlying units down to the limestone bedrock.
## 2 MATERIALS AND METHODS

Ground-penetrating radar is based on the propagation and reflection of transmitted electromagnetic pulses to image sedimentary structures (Neal, 2004). The system used for the project is a Mala ProEx with two antennae centered at a frequency of 250 and 500 MHz, respectively. Data were processed using REFLEXW V10.5 (i.e. time-zero drift, background removal, band-pass filtering, gain, topographic correction) and interpreted following principles of radar stratigraphy (Neal, 2004) using DELPH RoadMap (V5). A constant radar velocity is estimated at 0.122 m/ns (Billy et al., 2020) and used for the time-depth conversion.

The cone penetration testing (CPT) is a standardized geotechnical test (ISO 22476-1, TE1 class). It measures soil resistance to the penetration of an instrumented cone that advances at a constant speed of 2 cm/s. Sensors measure two pressures (in MPa) every 2 cm: 1) the cone tip resistance, Qc, and 2) the lateral friction exerted by the soil on the sides of the cone, fs. Based on these data, parameters can be calculated to assess the properties of the soil traversed, such as the friction ratio, Rf (=100 * fs/qc, %), which provides an indication of soil granulometry, and relative density, Dr (%), representing grain packing (similar to porosity). Based on established charts (e.g., Robertson, 1990), these values can be used to classify soils based on their behavior (Soil Behavior Type, SBT 1-9, e.g. dense sands, soft clays, etc.). Existing cores and Log (from the French national BSS database) are also used to support the interpretation of soil classes identified through CPT. Only part of the data set is presented here: 4 and 8 CPTs for La Parée Grollier and La Pège, respectively. Each of these 12 sounding was performed up to the refusal (50 MPa).

## 3 RESULTS AND DISCUSSION

The Pays-de-Monts geosystem is basically defined by three main units: 1) fractured limestone (U Basal), overlain by 2) fluvio-marine deposits and 3) dune systems. The coupled GPR-CPT approach is suitable to characterize these two sedimentary cover units and to provide additional information on them.

### 3.1 Foreshore and marsh interface at La Parée Grollier

The internal structures of dunes at La Parée Grollier are well imaged using GPR. The parabolic dunes (UD2 dune type, in Fig. 1B) are primarily characterized by eastward-dipping reflectors (yellow dotted-line: UD2a, UD2b). This internal architecture reveals the former shape of the parabolic sandy dune and its past migration pattern towards the east (inland) before its fixation by vegetation (maritime pine forest) during the 19th century. CPT analyses, based on established charts, indicate that parabolic dunes are defined by soil behavior domain SBT 7: "dense sand to gravel" (yellow dots in Fig. 1D), and measurements are sparsely scattered.

<figure><img src="image_2.png" /><figcaption>Figure 1. A. Location map of the studied areas; B. Example of an interpreted GPR profile and C. Qc curves (CPT) at La Parée Grollier, and D. interpretation of depositional environments based on the soil behavior type chart, illustrating parabolic dunes, fluvio-marine and marsh deposits.</figcaption></figure>
Beneath this parabolic dune (U<sub>D2</sub>), GPR reveals a non-uniform underlying deposit that can be split in three domains, from beach (0 m) to land (~2 km distance from the shoreline):

i. From the beach to 850 m inland, the deposits are characterized by westward-dipping oblique reflectors (Fig. 1B, U<sub>FM</sub>). They are interpreted as a continuum of shoreface, foreshore and beach deposits (from -10 m to +2 m NGF). U<sub>FM</sub> illustrates the progradation pattern of this coast and the successive seaward sediment accumulation.

ii. From 850 m to 1 km, a mound-shape structure is observed on GPR profiles (Fig. 1B, U<sub>T</sub>). This unit is spatially restricted, but it also observed on profiles further north. U<sub>FM</sub> is adjoined to U<sub>T</sub>. It is interpreted as the boundary between fluvio-marine (U<sub>FM</sub>) and marsh deposits.

iii. From 1 to 2 km, a unit is identified on GPR profile based on its basal discontinuity. It consists of a 2 m thick unit characterized by a transparent facies (U<sub>M</sub>).

CPT analyses indicate that units under the dunes can be grouped into two populations. U<sub>T</sub> and U<sub>FM</sub> have a similar soil behavior domain, SBT 6, defined as "clean sands to silty sands" interpreted as foreshore to beach deposits (blue dots in Figure 1D). Like for parabolic dune SBT, measurements are sparsely scattered, and friction ratio (median: 0.6%) and spread are similar for both populations. However, foreshore to beach deposits have lower average normalized peak resistances than dune (median ~200 vs ~350). CPT Measurements at the depth of the unit U<sub>M</sub> (1 to -1 m NGF, green curve with low Qc values; Figure 1C) reveal a third population corresponding to the domain SBT 4, defined as "silt mixtures: clays silt and silty clay". These sediments are clearly distinguishable from the other layers, with Qtl and Fr medians at around 27 and 3.5% respectively. U<sub>M</sub> is hence interpreted as marsh deposits. Finally, CPT sounding refusals show the boundary between sediment and limestone (U<sub>Basal</sub>, Figure 1B), which is consistent with the depth and spatial variability of the limit determined by geophysics (Billy et al., 2020).

At La Parée Grollier, the parabolic dunes had migrated above foreshore and marsh deposits. Consequently, the current location of the marsh boundary identified using maps, photographs or LiDAR data does not show the real foreshore/marsh transition, contrary to the GPR-CPT approach.

## 3.2 Filled channel under the dune at La Pège

To the south, at La Pège (Fig. 1A), the dune system is less developed inland and can be narrowest (~40 m, Fig. 2A). Here, dunes are a natural barrier between the sea and touristic areas (parking area, campsite, etc.), as well as critical infrastructures such as water treatment plant. CPT soundings were realized on this narrow part of the dune along a north-west to south-east transect (pink dots; Fig. 2A).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** A. Location map of La Pège; B. Qc curves of each CPT, along a NW-SE longitudinal profile and C. Analysis of CPT data following the soil behavior type chart.</figcaption></figure>
Analyses highlight that the sand dunes have similar soil behavior than those described at La Parée Grollier. The CPT behavior of the fluvio-marine deposits is quite similar to that of the dunes, with both deposits corresponding to the SBT 7 domain: “dense sand to gravel”, but the fluvio-marine deposits appear to be coarser, i.e. with a lower Fr (0.3%; Figure 2C). Intercalated between the dune (U<sub>D4</sub>) and fluvio-marine deposits, a 1-2 m thick layer is identified on curves with low Qc values (CPT 234, 235, 202, 204 and 338; Fig. 2B). This unit corresponds to the SBT 3 domain: “clay to silty clay”. This clayed layer is observed along a 400 m-long transect (from CPT 234 to 338; Fig. 2A) and is absent on CPT 236 and 339, at the north and the south, respectively. These observations suggest that the clay layer could extend over a distance of 500 to 600 m (NW-SE orientation). This unit is also recorded on CPT and in cores more distant from the longitudinal dune (white dots, Fig. 2A), and seems to be related to the low topography inland corresponding to a paleo-channel. Thus, this clay layer is interpreted as a part of a filled former channel (e.g. former meander). Whereas the existence and the location of the paleo-channel beneath the dune system was previously unknown, our study shows a connection between the paleo-channel and the beach/dune system. In a context of coastal erosion and shoreline retreat, a better knowledge of the sub-surface geology is important because this layer is highly susceptible to appear on the beach in case of coastal retreat due to its altitude (-1 to 1.5 m NGF).

## 4 CONCLUSION

This study highlights the effectiveness of combining ground-penetrating radar and cone penetration testing for a detailed characterization of sandy coastal geosystems, particularly in complex dune environments. First, this study shows that sandy coasts can present notable singularities, even if they are spatially close and characterized by comparable dune morphologies. Our results reveal the internal structure of dunes and those of the underlying sedimentary units, providing valuable insights into stratigraphic transitions, and previously unknown features of the geosystem such as paleo-channel.

At La Parée Grollier, the coupling of GPR and CPT shows that i) the dune system, fluvio-marine and marsh deposits have distinct soil behavior types; ii) the eastward dune migration has covered foreshore/beach and marsh deposits, and iii) that the effective underlying location of the marsh-foreshore transition is beneath the dune system.

At La Pège, the study revealed the existence of a filled paleo-channel beneath the dune system, establishing a previously unknown link between former fluvial deposits and modern coastal landforms. This finding is particularly significant in the context of shoreline retreat and coastal erosion, as the clay-rich paleo-channel may become exposed, and thus have an impact on coastal sediment dynamics.

The integration of GPR and CPT provides a robust methodological framework for distinguishing sedimentary units and understanding their properties. While GPR offers high-resolution imaging of internal structures, CPT complements it by quantifying geotechnical properties and identifying sedimentary domains. Our results highlight the importance of incorporating subsurface geological data into coastal management strategies, particularly for adapting to global changes such as rising sea levels or coastal erosion.

## 5 ACKNOWLEDGMENT

This paper is a contribution of SIBLES2 research project co-funded by the BRGM, French Geological survey, and the Pays de La Loire Regional direction of the environment, DREAL. The authors would like to thank Bastien Ruaud and Emeric Gautier for their field assistance.

## 6 REFERENCES

Billy, J., Baudouin, V., Portal, A., Deparis, J., Bitri, A., Garcin, M., 2020. An Innovative Approach for a Comprehensive Characterization of Coastal Dune Systems through Internal Architecture and the Associated Intrinsic Geophysical Properties. Journal of Coastal Research, 95, 387–391.

Billy, J., Robin, N., Hein, C.J., Certain, R., FitzGerald, D.M., 2014. Internal architecture of mixed sand-and-gravel beach ridges: Miquelon-Langlade Barrier, NW Atlantic. Marine Geology, 357.

Campo, B., Bruno, L., Amorosi, A., 2023. Sedimentary facies characterization through CPTU profiles: An effective tool for subsurface investigation of modern alluvial and coastal plains. Sedimentology, 70, 1302–

Neal, A., 2004. Ground-penetrating radar and its use in sedimentology: Principles, problems and progress. Earth-Science Reviews, 66, 261–330.

Robertson, P.K., 1990. Soil classification using the cone penetration test. Canadian Geotechnical Journal, 27, 151–158.
Robin, N., Billy, J., Nicolae Lerma, A., Castelle, B., Hesp, P.A., Rosebery, D., Fauny, C., Deparis, J., Marieu, V., Bouchet, C., Miot da Silva, G., 2023. Natural remobilization and historical evolution of a modern coastal transgressive dunefield. Earth Surface Processes and Landforms, 48, 1064–1083.

Ters, M., 1956. La Vendée littorale : étude de géomorphologie. Oberthur, Paris, 578 p.

Zhang, X., Lin, C.-M., Dalrymple, R.W., Gao, S., Canas, D.T., 2018. Use of the Cone Penetration Testing (CPT) method to interpret late Quaternary tide-dominated successions: A case study from the eastern China coastal plain. Continental Shelf Research, 161, 49–57.