Climate and land use change impact on urban flooding

Urban floods can cause damage to properties, disruption to business and traffic, discomfort to community and in some worse cases human loss. Floods occur due to meteorological, hydrological and human factors, which can be categorized as climate change, the most significant challenges facing human beings in 21st century, and land use change (urbanization). The specific objectives of this study were to develop future climate scenarios on extremes temperature and rainfall based on downscaling the output of Global Climate Models (GCMs) and to estimate flood volumes from climate and land use change scenarios at Nyabugogo catchment in Rwanda. GCMs have been used to project climate change in next 100 years based on different development trends and global greenhouse gas emission scenarios. As the outputs of GCMs are at low resolutions and cannot be used for any local area directly, in this study, we have adpted Statistical Downscaling Model (SDSM), a hybrid of stochastic weather generator and regression based methods to generate climate variable at high resolution. The meteorological variables downscaled from SDSM and land use projection data were used as input to the hydrological model (HEC-HMS) to estimate the flood discharge peak and volume in order to analyze the impact of climate and land use change in Nyabugogo watershed. The results of SDSM predict an increase trend in extreme temperature for the future periods for both A2 and B2 SRES emissions which are respectively comparable to high emissions RCP 8.5, and intermediate emission RCP 6 used in 5th Assessment report of IPCC. The average annual minimum temperature will be increased by 1.4 °C and 1.2 °C under A2, B2 scenario respectively and the average maximum temperature will be increased by 2.84 °C under A2 and 2.4 °C under B2 towards the end of century. The results of downscaled rainfall do not manifest a systematic increase or decrease in all future time horizons for both A2 and B2 scenarios. However the total annual rainfall decrease of 0.2% and 5.9 % are expectable for 2020s under A2 and B2 respectively. A decrease of 4.4% and increase of 1.08% are likely to happen for 2050s under A2 and B2 respectively. In 2080s the Annual rainfall will be increased by 4.51% and 7% underA2 and B2. The hydrological modeling(HEC-HMS) results show that the climate change has a considerable impact in decreasing the expected peak discharge whereas the land use change pronounce an increasing of discharge volume for different return periods of storm.