Towards an Operational coastal flooding Early Warning System in a confined bay: The case of Pasaia Bay

The Basque Country currently operates a regional coastal flooding Early Warning System (EWS) designed for exposed open-coast environments, with a primary focus on urban coastal areas backed by beaches and coastal protection structures. This system serves as a short-term coastal management tool to forecast potential storm hazards and assist coastal authorities in deploying appropriate mitigation solutions. However, the Basque Coast also contains several confined coastal environments, such as enclosed bays, where hydrodynamic behaviour differs markedly from that of the open coast, requiring tailored approaches to forecast accurately the level of hazard.

Pasaia Bay, hosts several urban centres (Pasai San Juan, Pasai San Pedro and Pasai Antxo) located within a confined estuarine port environment. Pasai San Juan, in particular, experiences recurrent passive flooding events, which are expected to increase due to sea level rise, requiring the development and implementation of a specific Early Warning System to anticipate the hazard.

The first step of this work was to review the existing historical data and relate it to the prevailing hydrodynamic conditions. Next, the work proceeded with the design, calibration, and validation of a coastal flooding Early Warning System specifically developed for the Pasaia Bay. The system is based on the Total Water Level indicator that combines: (i) sea level (astronomical tide and meteorological contribution), and (ii) port agitation or longwave contribution. Using water level time series collected during local measurement campaigns within the bay, functional relationships are established between offshore wave forcing and the bay’s internal hydrodynamic response. This enables the anticipation of the Total Water Level indicator in the critical zone (Pasai San Juan) and the definition of operational activation thresholds.

The system is designed to provide alerts to the Pasai San Juan City Council more than 72 hours in advance, supporting preventive management, the protection of urban areas, and informed decision-making during flooding events. This development represents an initial step towards extending early warning capabilities tailored to confined coastal environments to other localities in the Basque Country.