Project context and challenge
São Tomé and Príncipe is a small island state where most communities live along the coast. Since 2011, the World Bank has supported the country in adapting to climate change, first through a pilot project and later through the West Africa Coastal Areas (WACA) programme. In its latest phase, two more communities were added: Ribeira Afonso on São Tomé and Chimalô on Príncipe.
Ribeira Afonso, a fishing town of some 8,000 people at the mouth of a river, floods regularly. During high seas, water levels in the river rise until it overflows its banks, while rainwater in the town cannot drain away. An undersized and poorly maintained culvert under the main road and waste in the drains make matters worse.
In Chimalô, the coastal road along the bay is being eroded by waves during storms from the northeast and by runoff from the steep slopes behind it. The narrow road is the only access to the fishing facilities, and landslides threaten the hillside. The challenge was to find practical, durable solutions for both communities that could be built and maintained locally.
CDR’s approach and contribution
CDR led the assignment with local partner AP Studio. Shore Monitoring carried out the topographic survey and installed a water level gauge to measure local sea levels.
For each community, the team carried out a hazard assessment of flooding and erosion, using hydrological and hydraulic models for present conditions and for 2050. In Ribeira Afonso, this revealed that flooding is caused by the combined effect of rainfall, river discharge and high sea levels. In Chimalô, the analysis showed that erosion results from both wave attack and uncontrolled runoff.
Possible measures were compared in an options study and discussed with the client, the communities and other stakeholders in a multi-criteria workshop and a national workshop. The preferred strategies were then worked out into detailed designs with drawings, bills of quantities and technical specifications. In parallel, CDR prepared an environmental and social impact assessment and management plan for each location.
Result
In Ribeira Afonso, raising the lowest land, a larger culvert under the main road and upgraded drainage channels with a stable outlet to the river lower flood depths by up to 1 metre in the lowest areas. Flood duration in the town centre drops by 30 to 40% for a typical storm in 2050, and the main road overtops less often.
In Chimalô, a raised and widened road with rock protection, a drainage channel and an upgraded culvert protect 265 metres of road and the homes behind it against erosion, while improving year round access to the fishing facilities. Both designs are ready for tendering and construction.
