Cause-Effect Time-Series Mapping of Urban Floods Using GIS for Patna, India

Patna, the capital city of Bihar, has witnessed rapid urbanization at the rate of 44.3% in the last decade, leading to considerable disturbances in the city's original topography. With the increasing population of Patna, with a decadal growth of 23.25% in 2011, the demand for land has risen, leading to land encroachment in Patna city, majorly in the core of the city for residential purposes and public services. Along with this, the rainfall pattern has also changed in recent decades. According to the India Meteorological Department, the Coefficient of Variation (CV) values for rainfall for the state of Bihar vary between 15 and 70%. The prominent flooding in Patna has primarily occurred in September, which falls under the southwest monsoon season (June–September), showing a CV range of 10–85%. Due to this fluctuating rainfall intensity and insufficiency of natural and engineered drainage in Patna, chocking and encroachment of the drainage system have led to significant urban flooding in the last two decades, resulting in several life losses and substantial economic losses. The urban poor were the most affected sections of the society, mainly because of their settlement pattern, primarily in low-lying areas. The primary factor contributing to the urban flooding in Patna is the increase in surface runoff. The prime aim of this study was to investigate the cause behind the frequent urban flooding in Patna during the monsoon season, among natural or man-made reasons. The study found that the average number of rainy days and the maximum rainfall in the study period did not change drastically. The seasonal average rainy days have been around 77 days throughout the study period (2000–2019), and the change in rainfall has shown a rising trend and was found to increase by 12.69% between the two major floods of 2007 and 2019, based on the CHIRPS satellite data. The study found a sharp increase of about 23.44% in the built-up area in the study period. The change in the built-up percentage increased the surface runoff by 3.87% for Patna between the 2007 and 2019 flood events, which could be traced as the significant reason behind the two urban flooding events in Patna in the last two decades. The study began with a null hypothesis (H0), considering the natural event responsible for the urban flooding in Patna, which was rejected based on the rise in the built-up area triggering a higher surface runoff in the city. The study concludes that the main reason behind the frequent urban flooding in Patna is because of the higher surface runoff, which is directly related to the increase in the built-up area in the city.