Bathing waters: Saint-Palais-sur-Mer “predicts” possible preventive closures

The City has developed with the University of Bordeaux a so-called "predictive" model based on several statistical data, to refine its decisions on swimming bans.

Bathing water quality (BWQ) is a hot topic in Saint-Palais-sur-Mer, not because of its poor quality, quite the opposite, but rather because of preventive closures following adverse weather conditions. Summer storms and the resulting rainfall are among the most common.

Having been awarded the "Bathing Water Quality Approach" label since 2013, the municipality must undergo an annual monitoring audit. On July 10, 2025, the city received the renewal of this certification. As a reminder, this certification, created by the Ministry of the Environment, attests to a rigorous monitoring methodology to protect the health of bathers. This certification applies to the beaches of Bureau, Platin, Grande Côte, Combots, and Nauzan, shared with the municipality of Vaux-sur-Mer.

Faced with the unpopularity of preventive closures, which are often misunderstood by locals and tourists alike, the Saint-Palais municipality has adopted a predictive model based on statistics. Stéphane Magrenon, deputy mayor in charge of bathing water quality, contacted professional statisticians from the University of Bordeaux in 2023 to create this tool specifically adapted to Saint-Palais-sur-Mer, and more specifically to Bureau Beach, the beach most affected by these closures.

“Our wish is to find a balance between taking risks and not taking them.”

The label's regulations state that if it rains 5 mm in one hour or 8 mm in twenty-four hours, the beach must be closed as a precaution to ensure health safety. Thus, between five and eight preventive closures are ordered each summer. However, the municipality has realized that it might be possible to better calibrate these closures, as some analyses have not concluded that the results are inconsistent with health safety.

Fed by a database created since 2017 including various parameters such as tide times, their coefficients, their heights, the amounts of rainfall on D-1, D-2, and beach attendance, the predictive tool offers two possibilities: 0, no risk; 1, risk. The 2024 summer season served as a test and allowed "us to run in," says Stéphane Magrenon. "It's a decision-making tool. If it rains 9 mm in twenty-four hours, the regulations advise preventive closure. Thanks to the predictive tool and its statistical data, the displayed information confirms our decision either for a preventive closure if the risk is proven, or to open the beach if the risk is not significant. Reliability is estimated at 95%. Our desire is obviously to reduce the number of preventive closures and to find a balance between taking risks or not taking them while ensuring the safety of bathers." »

While July did not experience a precautionary closure, with the July 21 closure being due to a poor result, statistics show that August could be more likely to experience one. A full-scale test is planned.