Managing Flood Risk in Coastal Areas Considering Climate Change Uncertainties

Abstract:

Coastal areas are particularly vulnerable to flooding from heavy rainfall, sea storm surge or the combination of the two. Recent studies indicate higher intensity and frequency of heavy rains, and progressive sea level rise continuing for the next decades. Pre-emptive and optimal flood defence policies that adaptively address climate change are needed. However, future climate projections are have significant uncertainty due to multiple factors: a) future CO2 emission scenarios; b) uncertainties in climate modelling; c) discount factor changes due to market fluctuations; d) uncertain migration and population growth dynamics. In this study a methodology is proposed to identify the optimal design and timing of flood defence structures in order to minimize both the intervention costs and the probabilistic damages associated with flooding. An optimization model is developed to minimize both the cost of the flood defence infrastructure system and the flooding hydraulic risk expressed by Expected Annual Damage (EAD). The latter accounts the joint probability density functions of extreme rainfall, storm surge and sea level rise, as well as the damages, which are determined by the defence system state considering the probability and consequences of system failure, using a water depth – damage curve related to the land use (CORINE Land Cover). Uncertainties in climate projections are explicitly considered probabilistically. The decision variables of the optimization problem are the size of defence construction and their timing schedule. The input variables are the coupled heavy rainfall and storm-surge events as well as the average sea level rise as indicated by climatic projections. A hydraulic model for the assessment of damage is integrated within the optimization algorithm (non-dominated sorting genetic algorithm 2, NSGA2). A set of optimal policies to mitigate the risk of flooding to aid decision makers is thus identified with reference to a case study which is focused on Pontina Plain (Lazio Italy), a coastal reclamation region particularly vulnerable to hydrological extremes and sea level rise.