Determination of the North Rukuru river flooding extent in Karonga District, Malawi

Abstract

Karonga district has experienced devastating floods since 1946, and it was the worst-hit district in Malawi in 2017. The district faces an increasing flood risk due to the downstream overflow of the North Rukuru River (NRR) and dyke failure. A reliable mitigation strategy involves using flood inundation maps to assess flood extent and dyke effectiveness. This study aimed to determine North Rukuru River flooding extent. Flood frequency analysis using streamflow data (1979–2022) from Mwakimeme gauge station applied Gumbel’s and Log-Pearson Type III (LP3) distributions to estimate peak flows for various return periods from NRR. Chi-square, Anderson-Darling, and Kolmogorov-Smirnov tests identified LP3 as the best-fit distribution. Peak flows for 2, 5, 10, 25, 50, and 100-year return periods were 1028 m³/s, 1306 m³/s, 1474 m³/s, 1675 m³/s, 1815 m³/s, and 1950 m³/s, respectively. These were input into a HEC-RAS model to simulate flood depths along river reaches. Maximum channel flood depths at Mwakimeme gauge station were 5.15 m, 6.00 m, 6.47 m, 6.99 m, 7.33 m, and 7.65 m for the respective return periods. ArcGIS was used with HEC-RAS outputs to generate flood inundation maps. Flooded areas under the same return periods were 254,209.5 m², 289,555.8 m², 309,817.3 m², 332,673.4 m², 348,384.7 m², and 362,771.7 m². Inundation maps developed showed Peter Mwangalaba, Mwanyesha, Mwamatope, Mwanganda, Mwanyongo, Mwahimba, Mweniyumba, Mwanjabala, and Katolora as consistently affected by all flood magnitudes. The 1.5 m height dyke was breached across all flood magnitudes. Household survey was conducted to further evaluate effectiveness of the dyke. Notably, 96.70% (381 out of 394) supported reinforcement and extension of the dyke, confirming persistent vulnerability despite its presence. Therefore, this study recommends the use of flood inundation maps developed as a flood control measure to demarcate safe zones, and to reinforce, and extend the dyke height to 2-4.6 m according to a 100 year flood depth.