Head above water: São Tomé and Príncipe's path to flood resilience

• São Tomé and Príncipe is a  Small Island Developing State (SIDS)  with a fragile economy, enduring double-insularity and high susceptibility to external shocks. It is especially vulnerable to the effects of climate change in the form of increased precipitation and sea-level rise, with most assets along the coast and near rivers.
• The expected yearly losses due to floods, on average, are projected to be 3.06% of its GDP in 2020, 4.16% in 2050, and 6.17% in 2080. This means that the overall flooding risk more than doubles in 60 years. The number of people affected by floods each year is expected to increase from around 30,000 today to 37,000 in 2080.
• The  WACA Resilience Investment Project in West Africa Coastal Areas (WACA ResIP)  is carried out with the financial and technical backing of the World Bank. It aims to benefit six countries in West and Central Africa. In São Tomé and Príncipe, the project focuses on enhancing the resilience of coastal communities. This involves building adaptive capacities in 12 communities that are highly susceptible to climate impacts. The actions include establishing a more robust early warning system, implementing safety measures at sea for fishers, and introducing interventions to protect the coastal areas.

São Tomé and Príncipe today

Situated in the Gulf of Guinea, São Tomé and Príncipe is the smallest economy in Africa and is faced with the challenges that characterize its  Small Island Developing State (SIDS)  and lower middle-income status.

Islanders currently face challenges from changing climate conditions, the socio-economic impact of COVID-19, and escalating fuel and food costs. Moreover, one-third of the population lives on less than US$ 1.9 per day, and more than two-thirds fall below the poverty line of US$ 3.2 per day.

This small two-island nation is characterized by a fragile economy and geographic insularity, and is highly susceptible to external shocks. It is especially vulnerable to the effects of climate change in the form of increased precipitation and sea-level rise, given that most of its population and infrastructure are situated along the coast and near rivers.

A rapid, unplanned urbanization

In towns and cities, amid global urbanization trends, the urban population proportion in São Tomé and Príncipe has already reached 73% (as of 2023). Urban residents prefer to settle in flat areas in coastal regions and near waterways, but ideal conditions are not always available.

As of 2023, approximately 85,000 inhabitants live in São Tomé, the capital city, while the smaller Príncipe Island houses a compact population of around 8,500 inhabitants.

Informal settlements in at-risk areas

Given the rapid urbanization and significant rural-to-urban migration flows, the growth of informal settlements with inadequate water and sanitation and energy infrastructure is observed, often in zones exposed to coastal flood hazards. Estimates from 2020 indicate that more than 85,000 residents (53% of the total urban population) currently live in informal settlements across the country.

Typical of local construction methods are houses crafted from wooden planks, elevated above the ground, alongside numerous concrete structures. Furthermore, there are dispersed settlements locally referred to as lucháns, where many people reside in urban areas.

Waterways define Sao Tomé and Príncipe

The rivers and streams in São Tomé and Príncipe form upland and flow down to the coast; many of these streams are very steep and pristine waterfalls are scattered across both islands. The main rivers are:

• Ió Grande: it is the largest river of São Tomé, and its main tributaries are Rio Ana Chaves, Rio João and Rio Umbugo;
• Do Ouro: part of its course is diverted for irrigation and community water supply; its volume decreases significantly during the dry season.
• Manuel Jorge and Abade: the former has some tributaries on the right bank on its upper and middle course, but only one on the left bank on its lower course.
• Quija and Xufexufe: the two rivers, which intersect at 400m from the mouth, tend to form flood plains;
• Papagaio: it is the largest river on the island of Príncipe; it has several tributaries on the left bank.

Complex challenges

São Tomé and Príncipe is confronted by a trifecta of threats—climate change, COVID-19, and soaring fuel and food prices. Its unique challenges, compounded by insularity and a rugged landscape, result in complex challenges.

Frequent floods, aggravated by rising sea levels and extreme weather, currently put at risk the health, food security, and water resources of São Tomé and Príncipe, impacting livelihoods and key sectors that include agriculture and fisheries, affecting almost 20% of the nation’s workforce.

Limited land surface and a scarce workforce constrain economic diversification, leaving the country vulnerable to trade shocks. Additionally, inadequate infrastructure and heavy reliance on food imports lead to price fluctuations, which disproportionately impact the most vulnerable households.

Drivers of climate vulnerability

São Tomé and Príncipe ranks among the most vulnerable countries to climate change due to its unique geography and economy, characterized by fragile ecosystems and low socioeconomic development.

One of the dominant reasons is that both islands feature coastal zones with slopes from extinct volcanoes - a factor that tends to aggravate soil degradation and coastal erosion.

Several of São Tomé and Príncipe's coastal communities witness flooding up to 10 times a year and yearly landslides, causing significant damages to homes and livelihood assets.

Rising temperatures, rainfall patterns and sea level rise

The impacts of climate change are already being felt in São Tomé and Príncipe in the form of higher climate variability and rising temperatures:

• From 1950 to 2010, São Tomé and Príncipe experienced a 1.5 °C increase in average annual temperature.
• São Tomé and Príncipe also witnessed a slight increase in precipitation since the 1950s, with more intense, harder to forecast rainfall events in the last 30 years.

With projections indicating that the average sea level will increase by approximately 30 cm and rainfall events will become 20% more intense by 2050, coastal communities are likely to face severe consequences.

Flood impacts: present and future

Linking risk knowledge with risk reduction

Despite recent efforts in data collection and flood risk awareness in São Tomé and Príncipe, a critical gap existed: there were no detailed, quantitative spatial datasets focusing on national-scale flood hazard that are publicly available. This deficiency hampers the effective integration of flood knowledge and risk information into sectoral planning and poses a significant obstacle for engaging local communities in risk communication initiatives about impending climate risks.

The World Bank is addressing this gap, empowering São Tomé and Príncipe with tools to integrate precise flood hazard information into strategic planning. This initiative strengthens the country's capacity to confront climate challenges and facilitates public outreach to engage and inform local communities about increasing flood risks.

The importance of combined flooding in São Tomé and Príncipe

The flood hazard and risk assessments goes beyond traditional approaches by examining simultaneously coastal flooding (caused by tide, surge, and wave setup) and rainfall-related flooding (pluvial/fluvial). These assessments cover the 'present climate' (2020) and also provide insights into future climate projections: 2050 and 2080, assuming the emissions scenario 'SSP3-7.0' (medium- to high-end future forcing pathways).

In São Tomé and Príncipe, studying the combined effects of these floods is crucial for a comprehensive assessment as they often occur simultaneously during extreme weather events. Interaction between coastal and pluvial/fluvial flooding can amplify the overall impact, such as heavy rainfall coinciding with high tides and surges, worsening flooding conditions in a given location.

In fact, the most vulnerable communities are poor fishing villages, located between the mouth of a river (for drinking water and washing purposes) and the beach (access to fishing), thus exactly where the two flooding mechanisms combine at their worst.

In this context, the concept of 50-year return period flood events becomes useful

In the semi-probabilistic method applied several representative events were simulated, in order to capture flood hazards (and risk) between a common "once in 1 year" (100% annual likelihood) and a rarer "once in 100 years" (1% annual likelihood) events.  The 50-year "return period" flood maps strike a balance between planning for rare yet significant floods without over-engineering infrastructure.

Standard practice involves designing infrastructure to withstand such events, ensuring resilience against moderate to severe floods cost-effectively, taking also in account the expected situation in 2050. This enhances the Government's ability to anticipate and mitigate the compounding effects of different flood mechanisms.

Decision-makers can use this flood risk information to strike an optimal balance between preparing for rare yet impactful events and ensuring cost-effective and resilient infrastructure and flood risk reduction measures that can withstand the challenges of present and future conditions.

How to interpret flood hazard maps under different climate change scenarios?

The flooding maps below (2020 climate) indicate areas prone to coastal and rainfall-related flooding, by examining a 50-year "return period" event. Results under present-day conditions can be compared with future events climates (dates of 2050 or 2080). A 50-year return period flood event expresses the likelihood of a flood of a certain magnitude occurring in any given year, with a 2% chance annually (rather than exactly once every 50 years).

The resulting flood hazard maps assume no additional defensive measures beyond existing ones, with 2020 building locations and population distribution remaining consistent. Future hazard maps include the expected effect of climate change on precipitation patterns, river discharge and sea levels which influence flood hazard intensity and distribution.

What type of insights can be derived from Flood Risk Maps?

1. Vulnerability Zones: Areas prone to coastal flooding, considering tide, surge, and wave setup, can be identified. Additionally, zones at risk of pluvial/fluvial flooding from rainfall-runoff can also be mapped, thus providing a holistic view of flood vulnerability zones.
2. Hotspots for Combined Effects: The maps can highlight areas where the combined effects of coastal and inland flooding are particularly pronounced. This information is critical for prioritizing current and upcoming intervention measures and allocating resources effectively. For example, a precise understanding of vulnerability zones ensures that infrastructure projects in the future, such as roads and buildings, can withstand a spectrum of flooding scenarios.
3. Emergency Response Planning: Flood risk maps can also facilitate emergency response planning by clearly delineating high-risk areas from safer zones. This allows for the development of targeted evacuation plans and the establishment of emergency shelters in strategic locations.

Built Environment and Population in São Tomé and Príncipe

For example, on a rainy day, it would be difficult to determine which neighborhoods are likely to be the most flooded. The only information readily available in São Tomé, the capital city, is that it is home to approximately 85,000 inhabitants, with more than 21,000 buildings identified by remote sensing, thus constituting 37% of the country's population and 35% of the country's total building footprint.

2020 Climate Conditions

The map shows predicted flooding patterns for a 50-year Return Period flood event (annual average probability of 2%) under current climate conditions.

For instance, in São Tomé city, a flood event with a 20cm threshold would impact about ~ 3,100 buildings, accounting for 14% of the city's existing total buildings

2050 Climate Conditions

The map shows predicted flooding patterns for a 50-year Return Period flood event (annual average probability of 2%) under 2050 climate conditions.

For instance, in São Tomé city, a flood event with a 20cm threshold would impact about ~3,500 buildings, accounting for 16% of the city's existing total buildings.

2080 Climate Conditions

The map shows predicted flooding patterns for a 50-year Return Period flood event (annual average probability of 2%) under 2080 climate conditions.

For instance, in São Tomé city, a flood event with a 20cm threshold would impact about ~3,800 buildings, accounting for 18% of the city's existing total buildings.

A national flood Risk study was performed, by combining the above high-resolution flood hazard maps (integrating flood extents and intensities of all Return Periods) with maps representing exposed assets and with vulnerability / damage curves, so that a dollar value can be attributed to Average Annual Losses (AAL) both in today's Climate and for the expected conditions in 2050 and in 2080, if no adaptation measures are taken. 

AAL's were estimated at 3.06 % of GDP for 2020, 4.16 % for 2050 and 6.17 % for 2080. This is using 2020's exposure data and assuming 2022's GDP (of US$ 547 million). It is shown that the integrated national flood risk more than doubles in only 60 years. Numbers for population affected annually were also derived based on current population figures, ranging from approximately 30,000 today (13%), to 32,000 in 2050 (14%), and up to 37,000 in 2080 (16%).

The study considered every sub-district in São Tomé and Príncipe, examining the count of buildings and population affected by flooding, defined as flood depths of 20cm or more. This analysis was conducted for all return periods and under three climate scenarios. The charts below provides a summary of (i) the number of buildings affected by a flood event with a 50-year return period in 2020, 2050, and 2080, and, (ii) the population affected by a flood event with a 50-year return period in 2020, 2050, and 2080.

In response to the growing risks faced by coastal communities in São Tomé and Príncipe, the World Bank is supporting the  West Africa Coastal Areas (WACA) Management Program  to safeguard lives, livelihoods and key assets from the escalating challenges of water-related hazards intensified by climate change.

São Tomé and Príncipe has experienced substantial loss of life and assets due to these hazards. With anticipated increasing impacts, the World Bank is intensifying efforts to bolster technical assessments of adaptation options and accelerate the execution of resilience-building interventions. This includes efforts to enhance communication strategies to convey urgent needs, determining technical requirements, and laying out strategic options for climate change adaptation.