Civil Engineering June 2022 | Vol 30 No 5

position and elevation profiles of the respec- tive catchments. It is therefore preferable to relate the calculated flood peaks to an independent evaluation based on statistics and/or measured data from site. It was accord- ingly decided to consider deterministic, empirical and statistical methods for the selection of a design flood peak for the Gahira Bridge catchment. RAINFALL The catchment experiences large-scale pre- cipitation, predominantly from the south- eastern and eastern Indian Ocean monsoon flow, which brings significant moisture into Eastern Africa, including Rwanda. Elevation ranges from 1 450 m to 4 450 m within this catchment and, as orographic precipitation is dominant in this region, this change in elevation greatly influences the rainfall intensity. A typical example of the resulting tropical environment, with its dense vegeta- tion, is shown in Figure 3. Consecutive days of rainfall are prevalent in this humid region. To capture this trend for design purposes, rainfall data provided by the Rwanda Meteorological Agency was used to carry out a statistical analysis. Given the short project duration and some delays in obtaining this rainfall data, the first hurdle that the team had to address was conducting a statistical analysis on the large amounts of raw rainfall data received. To solve this problem, a Python programming script was written to directly capture consecutive days of rainfall (rainfall depths) for durations of 24, 48 and 72 hours. DETERMINISTIC METHODS AND RESULTS Once the required design rainfall depths were calculated, the deterministic methods as recommended by the RTDA Drainage Manual could be explored. Unfortunately, the team had justified concerns about the resulting flood peaks. The main reason for this was that the modified rational method, as described in the RTDA Drainage Manual, does not comply with similar modifications of the rational method from other countries, 1 450 m 1 750 m 2 050 m 2 350 m 2 650 m 2 950 m 3 250 m 3 550 m 3 850 m 4 150 m 4 450 m Elevation (msl) Mukungwe catchment Upper Nyabarongo catchment Lower Nyabarongo catchment 0 20 40 60 80 100 km Figure 2 Sub-catchments of the Gahira Bridge and the locations of the Rukarara and Mukungwa Rivers 38 June 2022  Civil Engineering Figure 3 Dense vegetation on the western side of the Nyabarongo River