Evaluation of storm water in Kigali City. Case study of Nyarurama Cell catchment, Gatenga Sector, Kicukiro District

Abstract

Globally, the rapid expansion of impermeable surfaces in urban areas has exacerbated stormwater management challenges, necessitating more effective drainage systems, and this is the case in Rwanda. This study aims to evaluate the current drainage infrastructure in Kigali City, Gatenga Sector, Nyarurama sub-catchment areas and assess the impact of land use changes on runoff patterns. Using GIS tools (QField, SAGA Next Gen, QGIS), the study applied the Rational Method and Manning’s Equation to estimate runoff and evaluate drainage effectiveness. Results reveal that 91.3% of roads are unpaved, leading to substantial stormwater overflow. Rectangular (U-shaped) drainages cover 89.6% of paved areas but only 4.9% of unpaved zones, leaving 83.1% of the region lacking adequate drainage. Land use changes forecasted from 2024 to 2050 show a significant increase in built-up areas (from 19.9% to 74.4%) and a decline in grassland and cropland (from 61.8% to 7.0%). The projected changes in runoff from 2024 to 2050 for various return periods and rainfall durations were analyzed. For the short durations (0.5 hr to 6 hr), the peak discharge in 2050 shows both increases and decreases compared to 2024, with changes ranging from a +60.55% increase in peak discharge during a 2-year return period for 24-hour rainfall events to a -35.44% decrease during a 50-year return period for a 6-hour rainfall event. Notably, higher return periods (50 and 100 years) demonstrate more significant reductions in peak discharge by 2050, reaching up to -33.69% for a 6-hour event and -23.59% for a 24-hour event under a 100-year return period. The results suggest that climate change will have varying impacts on flood risks depending on the rainfall duration and return period. The study highlights that without immediate infrastructure improvements, the increasing urbanization and changing rainfall patterns will exacerbate erosion and flood risks. Enhanced drainage systems and adaptive planning are essential to manage future stormwater runoff effectively. The substantial upgrades in drainage systems were recommended including the addition of water reservoirs, to mitigate the impacts of urbanization and climate change on stormwater management in Nyarurama cell