Estimating the impact of rainwater harvesting system installation on floods using hydrological models Case study: Nyabugogo valley, Kigali

Abstract

Rwanda, a landlocked country located in East Africa, is under the temperate climate due to the high altitude though near to the equator. 5.3% of the territory is covered by water bodies, lakes, and rivers. Both rainy and dry season come twice a year with frequent inundations and water shortage. Kigali, the capital city, and the most populated city in the country, is affected by floods during rainy season. Nyabugogo valley, the downtown of the capital is the typical affected area with devastating damage. The mitigation measures such as widening Mpazi drainage channel were taken by the government based on a number of researches. However, the inundation still occurs every year. This research aims to propose rainwater harvesting (RWH) that can supply water to each household, as a mitigation measure to floods. In order to assess the effect of RWH installation on the flood control, Runoff modeling and simulation of Flood inundation was carried out.

Nyabugogo river catchment area, delineated using DEM, is 1663.95 km². The dominant land cover is forest (65%) and crop land (22%). Run-off from the upper catchment area was simulated using SWAT model with input data of elevation, soil, and land use data. The simulation was done for 3 years 2011-2013 and calibrated using SWAT CUP.

IRIC Nays2Dflood Model was used to simulate and visualize the flooded area with the input data of the discharge simulated by the above modeling and water level at downstream of the flooded area for the flood event reported on March 2^nd and 3^rd 2020. The boundary conditions were set along the river, with two inflow points: Nyabugogo river and Mpazi outlet. The maximum water depth and the maximum velocity were observed at the confluence of the two streams; and flood extended throughout the valley. The accuracy of the simulation was checked by comparison with the sentinel2 satellite images.

The RWH system installation will be simulated and its hydrological impact on flood will be assessed as the final outcome of this research. Rainwater harvesting system installation, additionally to reducing flood mitigation, can contribute to improvement of the population’s health and sanitation, which aligns with the SDGs. Knowing the extent at which RWH can reduce flood, will open to further studies on design and cost analysis of the system.