Tsunami evacuation mapping co-design through science-informed and participatory decision making

Tsunami preparedness strategies are essential in tsunami risk governance and management due to three main factors that characterize these phenomena: they may have great devastating potential, they are unpredictable until an earthquake occurs, and they move extremely fast. Preparedness strategies are prospective measures that should be planned based on risk understanding in a pre-event phase, to best identify the emerging needs. Among them, tsunami evacuation planning is one of the most relevant strategies, especially in terms of protecting lives.

Tsunami evacuation mapping constitutes the basis of an evacuation planning process. Maps must be useful for both emergency managers and population. For that reason, they must be scientifically robust, detailed, and comprehensive, but also easy to understand and attractive. Beyond their main purpose, they are also a powerful tool for public awareness and communication activities.

The methodology developed in this study to elaborate tsunami evacuation mapping is based on the proposed concept of “science-informed participatory decision-making” that has been applied in Chipiona (Spain), and Larnaca (Cyprus). “Science-informed” means that the scientific community provides the hazard and evacuation approaches, models, and their implementation within the study area. “Participatory decision-making" refers to the active involvement of all relevant actors in the discussions, analysis, problem-solving, and decision-making to collaboratively develop the final tsunami evacuation maps. Involved actors include all public, private, academic, and civil association personnel that may be related to tsunami risk management and planning. Decision-makers and stakeholders were extensively involved in the process of understanding and translating the information from science to risk management.

Tsunami hazard assessment and the analysis of the elements of the evacuation strategy constitute two main steps for the elaboration of tsunami evacuation maps. In the case of Larnaca, a Seismic Probabilistic Tsunami Hazard Assessment (S-PTHA) approach was applied. As a result, a series of hazard zones were obtained, corresponding to different return periods and percentiles of uncertainty. The first decision asked to be made by the competent decision-makers was to select the tsunami hazard zone to be used for evacuation planning and mapping. This process was facilitated through a dedicated participatory workshop in which the concepts and methods applied (such as PTHA, average return period, and uncertainty) were explained. Additionally, the implications of selecting different tsunami hazard zones for emergency and evacuation planning (including exposed areas, population, and buildings critical for evacuation) were discussed, and stakeholder's perception and concerns were analysed and addressed.

Then, preliminary tsunami evacuation maps were developed based on a least-cost distance model to determine the optimal evacuation routes leading from any point inside the designated tsunami hazard zone to a series of assembly areas, previously identified through a multi-criteria approach. Subsequently, an additional participatory workshop and field visits were carried out with key stakeholders to identify well-known places and landmarks and potential evacuation barriers to finally validate the preliminary evacuation routes and assembly areas. Local knowledge provided by stakeholders effectively contributed to ensure the understanding and usefulness of the mapping end products for local emergency/risk managers and the community.