Flood management challenges in transitional environments: Assessing the effects of sea-level rise on compound flooding in the 21st century

Abstract

One of the main challenges facing decision-makers, particularly in highly modified transitional systems (i.e., coastal and inland waters, estuaries, river mouths, deltas, and coastal lagoons), is to determine whether current flood defenses will be sufficient throughout the 21st century. In case they are not, address the questions of where and when flood defenses will fail and how much. The methodology presented in this paper addresses the effects of the mean sea-level rise on flooding failures caused by the occurrence of multiple concomitants (compatible and simultaneous) forcing agents in channelized transitional systems throughout the 21st century. It analyzes the spatial and temporal evolution of overflowing failures and compares the crest height of flood defenses with the extreme total water level events expressed as a function of the return periods. The presented procedure then characterizes the mechanism responsible for flooding and identifies the channelized transitional system areas that should be protected at different temporal horizons. The results are converted into management maps, where the spatio-temporal variability and uncertainty of the results are analyzed to facilitate knowledge-based decision making and long-term adaptation planning. The application of the proposed method to the Guadalete estuary, a highly modified estuary representative of the coasts of southern Europe, indicates that: (1) the middle part of the estuary is the most sensitive to flooding due to the sea-level rise; (2) if no actions for mitigation are taken, the number of overflowing failures by the end-century will increase by a 2000% in the middle estuary as a consequence of the combination of the extremes of the astronomical tide and river discharge with the mean sea-level rise; (3) the extreme total water level events of 50-year return period in 2050 will be 1.8 m higher than the current crest height of the levees with a 95% confidence. Therefore, it is recommended that reliable methodologies such as the one proposed here be incorporated into the recommendations for the cost-optimization of the construction and maintenance of protection structures against sea-level rise, such as the Spanish Recommendations of Maritime Works (ROM Program).