Project context and challenge
Morogoro Road is one of the main arteries of Dar es Salaam, carrying both regular traffic and the city’s Bus Rapid Transit system. Where it crosses the Msimbazi River in the Jangwani valley, the existing bridge floods frequently, bringing traffic in large parts of the city to a standstill. A new, higher Jangwani Bridge was therefore designed in 2021 as part of the Jangwani Flood Protection Project for TANROADS.
After the bridge design was completed, the Lower Msimbazi Upgrading Project developed a much larger plan for the valley: the river and floodplain are reshaped, and the dredged material is reused to build flood safe terraces for urban development and a city park around the bridge.
Severe floods in November 2023 and April 2024, together with the high rates of sedimentation in the river, raised a critical question. Would the new bridge remain safe under the most extreme future conditions, and does its design still fit with the new plans for the valley? TANROADS and PO-RALG asked CDR to find out.
CDR’s approach and contribution
Working in a joint venture with CDR East Africa and supported by Deltares, CDR started with a kick-off meeting at the World Bank office in Dar es Salaam with the World Bank, PO-RALG and TANROADS to agree on the approach and design criteria.
The team reviewed all earlier models and data from both projects and updated the boundary conditions for climate change over the lifetime of the bridge. A morphodynamic Delft3D model simulated how the riverbed would develop over ten years without maintenance dredging, for present conditions and for 2050 and 2100. The resulting bed levels were then fed into a detailed 1D-2D Delft3D FM flood model to test the bridge against a 1 in 1,000 year flood under a severe climate change scenario, both with and without sedimentation.
CDR also reviewed the sedimentation safety margin used in the original design and determined which flood a bridge at the level of the nearby Morogoro Road and United Nations Road junction could withstand.
Result
The study confirmed that the bridge as designed is safe. Even during a 1 in 1,000 year flood in 2100, combined with ten years of sedimentation, there would still be around 4.4 metres of clearance between the water and the underside of the bridge. No raising or widening is needed, and the original sedimentation margin proved to be conservative.
To align the bridge with the new valley design, CDR recommended lowering the pile caps to the new riverbed level and replacing retaining walls with sloped embankments that connect to the urban terraces. The World Bank described the study as a good re-assessment of the bridge’s safety.
