Longitudinal revetments to mitigate overtopping and flooding: Effectiveness, costs and benefits

Several anthropogenic actions have been weakening sediment supplies to coastal areas, reducing the amount of sediments available on beaches and leading to a general and recurrent retreat of the shoreline position. As a consequence, different coastal defense interventions, oftentimes longitudinal revetments, have been established over the past decades in areas with high erosion rates. However, the lack of monitoring of these coastal defense structures combined with the high exposure of the urban areas to erosion, have led to an increased frequency of damage events in several locations, damaging infrastructures, commercial establishments and housing. This study purposes to assess the effectiveness, costs and benefits of different types of longitudinal revetments that aim to protect urban waterfronts from erosion and overtopping. A statistical analysis was developed and applied in order to assess the overtopping frequency of longitudinal revetments, depending on the seabed depth at the toe of the structure and on the structure's freeboard. Given that wave climate and beach profile are intrinsically associated with overtopping frequency over time, causes for the deepening of the beach profile were identified and various formulations were analyzed in order to estimate the maximum scour depth, to anticipate the frequency of damage events and to assess the severity of overtopping and flooding events. The effectiveness of different interventions to reduce wave overtopping at longitudinal revetments was assessed, including crest elevation, artificial beach profile nourishment and construction of an intermediate berm. A cost-benefit analysis was performed to assess the economic viability of these interventions, based on their additional investment, operation and maintenance costs and corresponding avoided flood damage costs over time. Results for the case of Furadouro beach (Portugal) show that reinforcement of longitudinal revetments is usually economically viable in the mid-term, especially when an increase in overtopping frequency is anticipated. The avoided costs associated with the negative impacts of overtopping tend to break-even with the interventions' costs within a period of around 10 years.