Key Performance Indicators for the upgrade of existing coastal defense structures

Due to the effects of climate change on the sea levels and on the frequency and intensity of extreme weather events, many coastal defense structures are expected to be exposed to increased loads with respect to their design conditions in the near future and need to be upgraded. Often, several design alternatives are available, and the identification of the ideal solution is not straightforward. Solutions that are effective in reducing overtopping and ensuring hydraulic safety may reduce or compromise the structural performance. Solutions that are both hydraulically and structurally effective may be extremely expensive and/or environmentally harmful. Using consolidated techniques of Multi-criteria analysis, this contribution proposes a simple methodology to consistently compare and rank the performance of several alternative approaches to upgrade existing structures, and to individuate the best solution. The proposed methodology consists of Key Performance Indicators (KPIs) of alternative solutions of upgrades, considering the reduction in the wave overtopping discharge (q) and the wave forces (F) by limiting costs and environmental impact. The definition of the KPIs was developed on the basis of new experimental data of q and F at dikes with crown walls and parapets, but it can be applied to various structure types, sea levels, and wave conditions. The application of the KPIs to the new experiments prompts a few conclusions of practical utility concerning the effectiveness of berms, crown walls, and parapets as elements to upgrade existing dikes.