Urban sprawl and its stress on the risk of extreme hydrological events (EHEs) in the Kelani River basin, Sri Lanka

Abstract

Land modification and urban sprawl cause incremental changes in hydrological processes due to the results of novel or hybrid ecosystems. This study mainly investigated the unforeseen urban expansion in the lower Kelani River basin, Sri Lanka, and its impact on the increment of hydrological extremes. Remote sensing data, including night-time light images (NOAA/AVHRR) and Landsat (TM/ETM+/OLI) data of different wavelengths, were analysed in this study. Land use and land cover data of the river basin were obtained from 1995. Shannon's entropy was used to demarcate urban sprawl in the river basin over nearly two decades. A spatial regression model was built to identify the correlation between increments of hydrological extremes and urban sprawl. This study revealed that the Kelani River basin has experienced a high urban sprawl rate over the past 23 years and that the total urban land area has increased by 130%. The flood risk analysis revealed that the flood frequency has also dramatically increased due to urban sprawl, and nearly 20 minor flood events have been recorded over the last two decades. Most of the urban areas situated in the lower river basin have invaded into the area with a higher flood risk, and the total flood damage has increased over the study period. The built-up land extent increased from 24.07 km2 to 56.39 km2. In 2018, the flood plain and the lower basin were mostly occupied by human settlements. Therefore, it is essential to improve current policies and mitigation plans to minimize the negative impact of rapid urban sprawl in the study area.