Anthropogenic drivers and impacts of urban floding- A case study in Lower Kelani River Basin, Colombo Sri Lanka

Abstract

Flooding is the most frequent natural disaster among the top ranking disasters causing destructive effects on people and property of Sri Lanka. Colombo, the commercial capital of Sri Lanka and surrounding suburban areas were experiencing a significant flood events during the last couple of decades. The intensified anthropogenic activities have accelerated severity and frequency of recent floods. Therefore this study was attempted to investigate the pressure generated by the population on the land by human activities causing frequent and severe flooding in Kolonnawa DS Division, Sri Lanka. A questionnaire survey and focus group discussions were carried out to collect primary data whilst collecting secondary data such as long term annual flood peak records, flood inundation data from different government agencies. The gridded population data was obtained from Oak Ridge National Laboratory (LandSacn) from 2000 to 2018 to examine the population pressure generated from the commercial capital to the study area. GIS and Remote Sensing based spatial analysis such as Change Detection, Normalized Difference Vegetation Index (NDVI), Normalized Difference Built-up Index (NDBI), Zonal Statistics and Concentric Circles were used interactively in data analysis. The results reveled that there is an influence of anthropogenic activities of area on recent flood events, severity and damages. Results of NDVI and NDBI analysis depicted a significant decline in green cover. Marshy lands have decreased by 42% from 2005 to 2017. The study further indicated a conversion of paddy and wetlands in to home gardens from 1972 to 2018 due to suburbanization. The decreasing trend of marshy lands, home gardens and paddy lands occurred due to the population pressure generated from the commercial city. Hence, the results of the study would be useful for policy makers and urban planners in developing future flood mitigation plans in study area.