How should storm surge barrier maintenance strategies be changed in light of sea-level rise? A case study

Sea-level rise and changes in storminess, together with population growth and coastward migration, are increasing the risks of coastal flooding. The impacts are amplified in coastal cities due to the high concentration of inhabitants, infrastructure and services in low lying areas. Many coastal cities are located in estuaries, and storm surge barriers are often constructed to provide flood protection in these areas with long exposed coastlines. For these complex and unique structures, maintenance is vital to ensure they remain reliable and comply with legal protection standards. To ensure safe conditions for workers, storm surge barriers typically define water level thresholds, at which maintenance work must stop when these water levels are reached or exceeded. This paper evaluates the changes in past and future water levels exceeding the maintenance threshold to inform management, maintenance and operation strategies, and design, of storm surge barriers. The Maeslant barrier in Rotterdam, Netherlands is used as a case study to test this analysis. Water levels from measurements taken at the Hoek van Holland tide gauge are compared to the maintenance threshold level. The number of past threshold exceedances is determined and the sensitivity of exceedances to threshold level are assessed. Results show that the maintenance threshold has been exceeded 991 times of which 13% occurred during the maintenance season. Conversely, there were periods in the storm season when water levels were below the maintenance threshold and work could have been carried out safely. The effect of sea-level rise and natural inter-annual tidal cycles on future threshold exceedances is also assessed. Findings reveal that the maintenance window will shift earlier in the year and narrow until exceedances of the maintenance threshold occur regularly all year around. With 1 m sea-level rise maintenance threshold exceedances are likely to occur regularly all year around by 2048. This analysis highlights that maintenance strategies at the Maeslant barrier will need to be adapted for the barrier to remain operational until its design life of 2 100. This is due to the increase in maintenance threshold exceedances resulting from natural interannual tidal cycles combined with sea-level rise. This analysis framework is applicable to existing barriers worldwide to assess future intervention points and for barriers in the design phase to verify the implications of design decisions on planned maint