Hydrometeorological Extremes in South-Eastern Africa: characterization of the Idai event in Beira (Mozambique) and flooding projections in the Sofala Province

The cyclone Idai made landfall on the city of Beira, in the Sofala Provinceof Mozambique, on the 14th March 2019 causing a huge number of fatalities and extensive damage to infrastructures. Most of the damages were caused by extended flooding in the area of Beira due to the concurrence of different meteorological conditions: torrential precipitation over the city of Beira, flooding of the Buzi and Pungwe rivers, and huge storm surges caused by the strong winds and low central pressure characterizing the cyclone. For its location, layout, and socio-economic context, Beira is among the most threatened city by Climate Change all over the world. The knowledge of climatic conditions that are likely to affect the area in the next decades is fundamental for designing adaptation strategies and actions. In this context, the interdisciplinary master thesis laboratory of Politecnico di Torino has been launched with the objective of supporting the “Comunità di Sant’Egidio” in developing a vision for the future of the city. The aim of this study is twofold: 1) the quantification of the rarity of the Idai cyclone event in terms of rainfall amount, wind intensity, and generation of storm surges, and 2) the analysis of the temporal evolution of the meteorological conditions leading to extended flooding in the Sofala Province, in order to understand how these conditions are changing and how they are likely to change in the future according to climate projections. ERA5 reanalysis hourly data on single levels from 1979 to 2020 are used to obtain the values of precipitation, wind, and significant height of combined wind and swell waves. Based on these data, the “Multi-driver Flood Stress” (MFS) indicator is proposed, which assesses the hydrological stress due to flooding over the city of Beira accounting for the intensity of precipitation and storm surges over critical locations, and their concurrency. The most extreme events over the past 42 years (on average 3 events per year), according to the MFS indicator, have been identified through a “Peak over Threshold” approach and analysed to assess the trend in time of frequency of “MFS”. Our results suggest that an increase of one extreme event per year is expected to occur over a period of 80 years. In order to connect the results of our work to the literature on climate projections, the MFS3 index has been defined as the number of events per year exceeding the above mentioned threshold of the MFS indicator, and its correlation with climatic extreme indices (i.e., ETCCDI) calculated for the central Mozambique area is analysed to produce a multi regressive model. This model is used to estimate the MFS3 index in the future using the climatic extreme indices modelled by an ensemble of 18 climatic models from CMIP6. No significant trend in the mean of the MFS3 index is found, but 95th percentile values increase in the second half of the 21th century, i.e., the variance of MFS3 increases, suggesting a larger probability of having years characterized by an unusually high number of joint extreme hydrometeorological events over the city of Beira at the end of the 21th century. The results obtained in this thesis demonstrate the need for the sustainable development of the city including a more resilient design of drainage infrastructures, in order to cope with extreme events such as the Idai cyclone in view of the projected increase in the population of Beira.