The coastal cliffs of Saint Anne's Point, near Hendaye, formed from pinkish Damian limestone dating from the Cretaceous-Tertiary period (Hendaye, SW France, 2006). © BRGM - François Michel

Characterisation of sedimentary stocks and dynamics at Bidart and Guéthary beaches for operational management purposes

09.20.2020
The storms of winter 2013/2014 had a major impact along the Basque coast. Scientific observation of the flooding and erosion processes affecting the Basque coast needs to be maintained and reinforced prior to the implementation of coastal risk management and prevention policies adapted to the specific characteristics of the rocky coastline of the Basque Country. Local authorities and scientists have thus been working together, via the Basque Coast Scientific Interest Group, to set up the MAREA research project (under the POCTEFA programme).

Aerial view of the Centre beach, including rocky outcrop, sand stock and structures, at Bidart (June 2018). © Observatoire de la Côte Aquitaine / Com' by AVM

The need

Besides numerous emergency coastline restoration works, the local authorities and the Basque Country Urban Community (CAPB) expressed a need for new knowledge on how coastal systems work, in order to be better prepared for managing coastal risks. Various tools were therefore developed as part of an interdisciplinary project, to predict erosion and flooding processes at the beach scale. In particular, BRGM produced new knowledge on the beach systems of the municipalities of Guéthary and Bidart, to gain a better understanding of the particularities of the changes to this complex site (with alternating areas of cliffs, rocky outcrops, pocket beaches and raised beaches) and to assess the effectiveness of existing management measures.

The results

The functioning of the coastal zone in question is highly complex. Due to the presence of rocky outcrops and reefs, hydro-sedimentary dynamics change on the time scale of the tidal cycle. Over the period and the entire area in question, sedimentary stocks including submarine areas and beaches were found to be relatively stable, with seasonal and interannual variability of less than 10%. However, there were sometimes major and rapid changes to the volumes of sediment stored on the beaches of the different areas studied (with stock fluctuations of +30% to -40% locally during one winter).

This part of the coastline is mainly a transport zone where sediments circulate parallel to the coast (from south to north) at depths of between 2 and 10 m. Generally speaking, the quantities of sedimentary stocks accumulated on the beaches are relatively limited compared to those found in the shallower depths, with a ratio of around 25% on the beaches and 75% in the underwater part.

Using the results

A general diagram of the hydro-sedimentary functioning of the area under study was drawn up, along with thematic fact sheets describing each beach. They summarise existing knowledge on the area and form the necessary basis for analysing the future trajectories of changes to the beaches. This information provides managers with a framework for developing plans for managing sediments or prior to shoreline restoration projects.

Geological sampling campaign (Bidart Beach, November 2017). © Basque Country Urban Community

THE PARTNERS

  • Basque Country Urban Community
  • Casagec Ingénierie consultancy
  • University of Pau and the Pays de l’Adour
  • Rivages Pro Tech - Suez Eau France's Coastal R&D centre
  • AZTI Foundation
  • Euskalmet, Basque Weather Agency

PUBLIC REPORT

Caractérisation des stocks et des dynamiques sédimentaires des plages de Bidart et Guéthary en vue d’une gestion opérationnelle (Projet MAREA). Rapport final. (in French)

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