Harnessing nature to advance sponge city in Rwanda: Empirical analysis of the impacts of restoration of Nyandungu urban wetland on flood risk reduction

Abstract

In Rwanda, Kigali's urban wetlands have been degraded by agriculture, settlements, and commercial activities, which have been exacerbated by increased flooding. This degradation has diminished the wetlands' ability to mitigate floods and provide other ecosystem services. To address this issue, Kigali initiated a citywide wetlands restoration plan, converting major urban wetlands into parks. In this line, Nyandungu Eco-Park, was the first wetland to be restored as a pilot project using Sustainable Urban Design (SUDS) and ecosystem-based adaptation interventions where those approaches were expected to be replicated in the restoration of Gikondo, Rwampara, Rugenge-Rwintare, Kibumba, and Nyabugogo wetlands. This study evaluates the flood control benefits of the restored Nyandungu wetland and recommends further interventions based on the Sponge City approach. An intensive literature review, spatial data analysis, and field observations were conducted to assess the restoration's impact on flood risk reduction and to identify suitable Sponge City infrastructures for the Nyandungu catchment. Results show a significant reduction in flood susceptibility in the area where interventions were established. Area which was highly susceptible to flooding reduced from 26 hectares in 2010 to 2 hectares in 2023 and areas less susceptible to flooding increased from 30 hectares to 119.7 hectares. The Normalized Difference Vegetation Index (NDVI) analysis showed an increase in vegetation health in Nyandungu Wetland, with values rising from 0.351 in 2010 to 0.495 in 2023. This indicates that previously degraded vegetation areas have significantly recovered, with higher vegetation density observed in 2023. The Normalized Difference Water Index (NDWI) analysis of Nyandungu from 2014 to 2023 shows a slight increase in areas with higher water content, which is equal to 1.23 ha. Area with low water content decreased by 0.76 ha. These shifts indicate improved water content and vegetation moisture in areas where interventions were established. Regarding the analysis conducted on the whole catchment, the analysis of Land use Land cover (LULC) revealed that built area increased from 149 Ha to 239 ha in a total of 3149 Ha for the whole catchment. Normalized Difference Built-up Index (NDBI) of 2023 was found to be high with where 239 Ha were found to be in area with high NDBI, Sediment Transportation Index (STI) analyzed for the whole catchment indicated that this index ranges from 0 to 204.368 which is very high. Results from flood susceptibility analysis conducted for the whole catchment was found to be high where 60 % of the catchment were found to be highly susceptible, 21 % moderate, 10% low and 9% very low. In this line, the study recommends implementing additional Sponge City infrastructures in the whole catchment, such as green roofs, vegetated swales, rain gardens, rainwater harvesting, detention basins, and porous pavements. In the whole catchment. Other sponge city infrastructures such Bioswales, grass swales, and infiltration ponds/basins should be established at runoff entry points to further mitigate flooding impacts on the restored wetland. These measures can contribute to reduce future flood risks by addressing runoff from the catchment area