Civil Engineering June 2022 | Vol 30 No 5

Civil Engineering June 2022 37 T he location of the Gahira Bridge and a photo of the surviving bridge abut- ment structures are shown in Figure 1. OvumCorporation was subsequently appointed to carry out the hydrological and hydraulics study for the Gahira Bridge. The project requirements included the following: Q Q Full detailed design with tender documentation for the Gahira Bridge (including inception, preliminary and detailed design phases) in a timeframe of six months from the appointment date Q Q Specifically, in relation to the hy- drological and hydraulics study, the Gahira Bridge is to be designed for a 1:100-year flood with a freeboard of 600 mm. HYDROLOGICAL ANALYSIS The Nyabarongo River is the largest river in Rwanda and forms part of the Nile basin. The hydrology of this catchment can be described as complex due to the varia- bility of the catchment characteristics. The river consists of five tributaries originating from three different sub-catchments. HYDROLOGICAL COMPLEXITY OF THE CATCHMENT The Gahira Bridge catchment can be defined as being ‘large’ with an area of 5 840 km 2 . Due to the multiple streams that flow into the Nyabarongo River, varied runoff times are expected. To illustrate the complexity described above, an analogy can be drawn from a hypothetical rainfall event. When looking at Figure 2 and imagining that, in the instance when a rainfall event occurs simultaneously over the catchments of the Mukungwa and Rukarara Rivers, the respective flood peaks will reach the Gahira Bridge at different times, while the intensity and distribution of rainfall will also differ, depending on the Hendrik Peens Flood and stormwater specialist Independent Consultant hendrik.peens@gmail.com Susan Cloete Technical Director: Water Ovum Corporation susang@ovumcorporation.com Social media saves the day: The Gahira Bridge Project Balarka Robinson Flood and stormwater specialist Independent Consultant balarka@zoho.com Nicholas Featherston Managing Director Ovum Corporation nicholasf@ovumcorporation.com The old Gahira Bridge, situated on the Nyabarongo River in Rwanda, was swept away during a major flood event in May 2020. The Rwanda Transport Development Agency (RTDA) appointed DNMZ Consulting Engineers to design a new Gahira Bridge to connect the Gakenke and Muhanga Districts. Figure 1 Map of Rwanda showing the location of the Gahira Bridge on the Nyabarongo River Gahira Bridge  Direction of flow