# Exploring Future Shoreline and Nearshore Evolution Under Sea-Level Rise Projections: Application of the LX-ST Model to Lacanau

Mohammad Traboulsi, Déborah Idier, Bruno Castelle, Arthur Robinet, Vincent Marieu, Rémi Thiéblemont, Alexandre Nicolae Lerma

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Mohammad Traboulsi, Déborah Idier, Bruno Castelle, Arthur Robinet, Vincent Marieu, et al. Exploring Future Shoreline and Nearshore Evolution Under Sea-Level Rise Projections: Application of the LX-ST Model to Lacanau. 14th Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2025), Sep 2025, Barcelone, Spain. hal-05056317

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# Exploring Future Shoreline and Nearshore Evolution Under Sea-Level Rise Projections: Application of the LX-ST Model to Lacanau

Mohammad Traboulsi¹,², Déborah Idier¹, Bruno Castelle², Arthur Robinet³, Vincent Marieu², Rémi Thieblemont¹, Alexandre Nicolae Lerma³

¹BRGM, 3 Av. Claude Guillemin, Orléans, 45100, France

²Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, Allée Geoffroy Saint-Hilaire, Pessac, 33600, France

³BRGM Nouvelle Aquitaine, 24 av. Léonard de Vinci, Pessac, 33600, France

e-mail corresponding author: m.traboulsi@brgm.fr

**Keywords:** shoreline change, shoreface translation, sea-level rise, nearshore morphology, climate projections

## 1 Introduction

In the context of rising sea levels and increasing coastal vulnerability, understanding the long-term evolution of shorelines under future climate scenarios is critical for effective coastal management. While process-based models can provide detailed insights, their computational demands make them impractical for decadal-to-centennial scales. Instead, reduced-complexity models offer an efficient alternative by simulating dominant processes over large spatial and temporal scales [2]. This study applies LX-ST to explore future shoreline position and associated nearshore-beach-dune morphology at a wave-dominated partially urbanized sea front. Simulations are performed under IPCC sea-level rise projections through 2100, integrating local geomorphological features, and multiple climate scenarios.

## 2 LX-ST

LX-ST is a reduced-complexity numerical model developed to simulate shoreline and nearshore evolution over medium to long-term timescales. By integrating the one-line shoreline evolution model LX-Shore [2] with the shoreface profile translation tool ShoreTrans [1], LX-ST captures both shoreline and 3D nearshore morphology changes. The model accounts for processes such as sea-level rise (SLR), longshore sediment transport, and the presence of natural and artificial coastal features.

## 3 Study site

Lacanau-Océan, a seaside resort on the central sandy coast of Gironde, southwest France, has experienced chronic erosion over the last century, with rates reaching up to 1.5 m/year in recent decades. A 1.2-km seawall was built to protect the urbanized coastline; however, it may disrupt sediment transport dynamics under future sea-level rise scenarios, particularly by interfering with longshore drift if erosion persists. This makes Lacanau an ideal test case for assessing the impacts of sea-level rise and sediment dynamics under varying climate scenarios. LX-ST was applied on a 5-km section of the beach, incorporating local topobathymetry from LiDAR and bathymetric data, realistic wave conditions derived from ERA5 data (assuming no significant changes in future wave climate),

and considerations of sediment bypassing to assess shoreline and nearshore evolution.

## 4 Results

Simulations predict a shoreline retreat ranging from O(10 m) to O(100 m) by 2100, depending on the considered IPCC scenario and SLR decile. The most severe erosion occurs downdrift of the seawall due to reduced sediment supply. In the second half of the century, the retreat of the beach-dune system contrasts with the relative stability of the shoreline position along the seawall sector, resulting in partial retention of longshore sediment transport. As a result, updrift erosion is smaller compared to downdrift where the dune can be fully eroded in the most extreme scenarios. The beach in front of the seawall experiences shoreface lowering.

## 5 Conclusion

This study illustrates the use of LX-ST in projecting long-term shoreline and nearshore evolution under varying climate scenarios, providing valuable insights into the future trajectories of vulnerable urbanized coasts. The findings underscore the combined influence of rising sea levels and artificial structures on coastal morphology.

## Acknowledgements

This work was conducted within the framework of the ANR SHORMOSAT project (ANR-21-CE01-0015). OCNA for the LiDAR data.

## References

[1] R. Jak McCarroll, Gerd Masselink, Nieves G. Valiente, Timothy Scott, Mark Wiggins, Josie-Alice Kirby, and Mark Davidson. A rules-based shoreface translation and sediment budgeting tool for estimating coastal change: Shoretrans. *Marine Geology*, 435:106466, May 2021. ISSN 0025-3227. doi: 10.1016/j.margeo.2021.106466.

[2] Arthur Robinet, Déborah Idier, Bruno Castelle, and Vincent Marieu. A reduced-complexity shoreline change model combining longshore and cross-shore processes: The lx-shore model. *Environmental Modelling & Software*, 109:1–16, November 2018. ISSN 1364-8152. doi: 10.1016/j.envsoft.2018.08.010.