Civil Engineering June 2022 | Vol 30 No 5

40 June 2022 Civil Engineering behind the use of what we have termed the “direct measurement method” is shown in Figure 6. PEAK FLOW OF THE GAHIRA BRIDGE Once the 1:50-year peak flow was known, the 1:100-year design peak for the Gahira Bridge could be calculated. This in turn was verified with the Regional Maximum Flood, an empirical method based on the Francou-Rodier ‘K’ statistic value. The K is taken from a series of envelope curves and describes the distribution of observed global maximum peak flood discharges for different climatic regions. When the two results (1:100-year design peak and RMF) are compared the calculated design peak falls in a similar order of magnitude to the RMF method. CONCLUSION When working on a project with complex hydrology, lack of available data and lim- ited flood studies, one realises how fortu- nate we are in South Africa, where there is significantly more available data and where a large number of engineers and specialists have been actively contributing to the refining of hydrology methods over many years. Since the inception of the Gahira Bridge Project in September 2021, the project has required a flexible and pioneering design approach in which mul- tiple avenues have had to be investigated to simplify seemingly complex problems. Among other things, this has included having to process and obtain useable de- sign information from large databases of raw data, identifying alternative credible methods to derive reliable design flood peaks and having to research available historical records. If it had not been for the information shared via social media (i.e. the Twitter video), the specialists that assisted the Ovum team, and the ongoing refinement of the design approach, an appropriate level of confidence in the adopted design flood peaks could not have been obtained. Ultimately, the “direct measurement” flood peak for the May 2020 flood event, along with the documented streamflow data, was used to obtain the adopted 1:100-year design peak for the Gahira Bridge of 1 450 m 3 /s. The available deterministic, empirical and statistical methods yielded results that were within 5% and 20% of the direct measurement method. ACKNOWLEDGEMENTS We would like to express our gratitude towards DNMZ Consulting Engineers who gave us the opportunity to be part of the Rwanda Project Design Team and the great exposure given within this project.  REFERENCES Leurquin, P.P. 1963. Agricultural Change in Ruanda-Urundi: 1945–1960. Rwanda, Food Research Institute, Stanford University, pp. 39–93. Alexander, W. 2006. Climate change and its consequences – an African perspective. Pretoria. Figure 6 Direct measurement method flow diagram Age of the bridge? What floods has the bridge experienced during this time? Flow of the May 2020 flood? Empirical Method Peak Flow Verification Francou-Rodier, K-factor K Tropical Africa = 2-3 K Gahira Bridge = 3 Gahira Bridge ≈ Q 1:100 Twitter Video Topographic Survey Bathymetric Survey Google Earth Site Reconnaissance Return period of May 2020 flood, exceeded Q > 1:50 year Rwanda Atlas and World Catalogue of Maximum Observed Floods: Q Q Floods greater than 1:25-year occured in: 1963, 1979, 1990, 1998 Q Q 1990 flood amounts to a return period of greater than 50-years Q Q Baily type bridge – designed and used throughout the world during 1945–1960 Q Q Agricultural change period in Ruanda–Urundi in 1945–1960 and transportation network was rapidly expanded Q Q By 1960, 11 000 km of roads and permanent bridges were constructed Q Q Therefore, built during 1945–1960, around 60–75 years old 1 2 3 Calibrated River Model (HEC-RAS) Calculated flow