Project context and challenge
The Nairobi Commuter Rail connects fast growing residential areas such as Ruiru, Syokimau, Embakasi and Kikuyu with the city centre, offering an affordable alternative to Nairobi’s congested roads. Under the Kenya Urban Mobility Improvement Project, the Government of Kenya and the World Bank are modernising the network and developing the areas around selected stations.
Flooding is a serious threat to that ambition. Large parts of the railway run through cuttings and low-lying land, often below the surrounding roads, so stormwater collects on the tracks. Bridges and culverts at river crossings are too small, causing water to back up upstream. The floods of April and May 2024 damaged bridge abutments and disrupted services. With rapid urbanisation and heavier rainfall, these problems will only grow: by 2050, a storm that now occurs once every 50 years may occur once every ten.
The World Bank asked for a clear evidence base of flood risk along the network, a plan to make the railway flood resilient, and guidance on how station areas can be developed safely.
CDR’s approach and contribution
CDR led the study together with REBEL East Africa, urban planners VE-R and mobility specialists Move-Mobility, working closely with Kenya Railways and county authorities through meetings, field visits and workshops.
The team analysed rainfall records and satellite data to define design storms, including the effects of climate change. A hydrologic model calculated river discharges, and detailed SFINCS flood models of the city centre and the lines towards Thika and Lukenya simulated both river floods and urban flooding from local rainfall, including the effect of rail and road bridges. In total, 18 scenarios were modelled for today, 2050 and 2080. The results were checked against observed floods and validated with stakeholders.
The flood maps were combined with data on rail assets, buildings and passenger use to assess damage to the railway, loss of service, and impacts on surrounding communities and the wider economy. This showed where flooding hurts the network most.
Building on these insights, CDR developed a Flood Risk Management Plan that combines engineering, nature based solutions, operational measures and spatial planning. Measures were prioritised in a multi-criteria workshop with stakeholders. For three priority station areas along the Thika line, Githurai, Mwiki and Kariobangi South, the team carried out detailed risk assessments, prepared conceptual spatial plans and drafted guidelines to bring flood risk into transit oriented development.
Result
Kenya Railways and the World Bank now have a network wide picture of flood risk and a prioritised plan to tackle it. In the short term, the plan focuses on upgrading undersized railway bridges and improving drainage along some 80 km of track. In the longer term, it sets out river improvements, room for the river and upstream retention to address flooding at its source.
The station area plans and guidelines show how new development can avoid flood-prone land and add value to the city, providing a solid basis for the feasibility studies and designs that follow.
