Improvement Works at Alajo Drain

Scope of Work

- The Ministry of Works, Housing and Water Resources invites sealed bids from eligible contractors for the improvement works at Alajo drain. The works involve two main interventions: - The construction of a covered reinforced concrete culvert parallel to the Eastern Railway Line (Railway drain), aiming to improve flood management in the Alajo urban area. The new infrastructure consists of a 1,596-meter-long rectangular box culvert with internal dimensions of 5.00m by 2.00m, designed with a constant invert slope of 0.3% to ensure optimal hydraulic performance and prevent water stagnation. The proposed drain alignment passes through densely urbanized areas, cultivated plots, and near existing structures, requiring the relocation or demolition of some buildings, careful excavation work including vertical sheet piling and retaining walls where needed, as well as connections with secondary drainage. Special attention is given to the crossing under the Eastern Railway Line, where the culvert will be placed at a depth of 1.4 meters to prevent interference with the rail infrastructure. The culvert discharges into the Odaw River with an outfall designed at a 45-degree angle to the river flow. Secondary drainage connections are included, with inspection manholes spaced every 50 meters and fitted with sediment traps and grates to manage solid waste. Additional protective works include, especially in proximity to agricultural land. - The refurbishment/reconstruction of primary/secondary channels of the Alajo Urban Drainage Network. The Alajo urban area has a surface of around 270 hectares and it is enclosed by the existing railway at the North side, by the Onyasia drain at the East side and by the Nsawam road at the West side. The main Odaw river flows within that area, bounding the Alajo area on the West side. The confluence between the Odaw and Onyasia is located at the southern limit of this area, at Caprice. The existing drains consist of open/covered rectangular concrete sections, varying in size from 0.4m (width) x 0.4m (height) for the smallest drains to 1.5m (width) x 1.5m (height) for the largest ones. The slope of these drains ranges from 0.2% to 1.5%, except in a few cases where the slope approaches zero. The current drainage network is insufficient, as many existing drains are in poor condition or completely absent in some areas.

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