Compound flood hazard assessment of atoll islands based on representative scenarios for typhoons and non-typhoon conditions: A Majuro case study

Abstract

Small Island Developing States, including many low-lying atoll islands, are among the most vulnerable countries to natural hazards and climate change disproportionately amplifies this vulnerability. Hence, there is a strong need for disaster risk reduction and risk management. Further development and implementation of methodologies for flood hazard assessment of atoll islands contributes to this. The methodology proposed in this thesis was applied to Majuro, an atoll island and capital of the Republic of the Marshall Islands. More specifically, the flood hazard related to different flood drivers, and including compound events (i.e. the combination of coastal flooding and precipitation) was assessed for the densely populated Delap, Uliga, and Djarrit region, in the east of Majuro Atoll. Main flood drivers are waves during typhoon events and distantly generated (swell) waves, but precipitation and high water levels (mainly tide) are important as well. To include all possible combinations of these flood drivers, and to include the spatial variation in events, 1000 years of synthetic events was generated based on data for historical events. Accurate simulation of inundation depths was computationally unfeasible for all synthetic events. Hence, a method was developed to reduce the number of model simulations, without losing information on the probability of occurrence of each event. The main steps of this method seem applicable to many other study areas where many scenarios are needed to include all (combinations of) drivers. Main steps are: (1) Selection of representative events – by Maximum Dissimilarity Algorithm, based on parameters that characterize the events. Hereby, the most extreme events are included as well. (2) Simulation of inundation depths for the representative events – by use of Delft3D, SWAN, and XBeach models. The XBeach model included a module for rainfall (first application). (3) Weighted interpolation to obtain the inundation depths for the synthetic events – based on the same parameters as in step 1. Based on the inundation depths for 1000 years of synthetic events, flood maps for different return periods and flood drivers were derived. These provide insight in the flood hazard for the DUD region due to the different flood drivers and for different return periods. The importance of different flood drivers varies significantly per area. Generally speaking, flooding related to (swell) waves in combination with high water levels is more frequent, while infrequent typhoons lead to the most severe flooding. Precipitation is an important flood driver as well, and exclusion would lead to underestimation of the flood hazard. Analysis of inundation depths suggests that for many areas these are limited to a maximum, where after excess water drains to the ocean – mainly into the lagoon. The derived flood maps could be used as a base for assessment of flood risk, climate change impacts, and closely related freshwater availability. In relation to the latter, more in-depth understanding of the contributions of precipitation and coastal flooding to the total flood hazard is needed, as on the long term infiltration of precipitation seems favourable, while that of oceanic water is not.