Transition from Riverine to Pluvial Flooding under Rapid Urbanization: Evidence from Varanasi, India

Urban flooding in rapidly growing cities is undergoing significant changes in both its driving mechanisms and spatial patterns. In many historic cities of South Asia, flooding was traditionally associated with river overflow during the monsoon season. In recent years, however, frequent flood events have been reported in urban areas located far from river channels, indicating an increasing role of rainfall-driven pluvial flooding. This shift is closely linked to rapid urban expansion, increased impervious surface coverage, and the degradation of natural drainage systems. Despite its growing relevance, the transition from riverine to pluvial flooding remains insufficiently documented, particularly in historic cities. This study investigates the evolution of urban flooding under rapid urbanization in Varanasi, a historic city located in the Middle Ganga Plain, India. Urban growth was analyzed using multi-temporal satellite imagery from 2000 to 2025, including Landsat and Sentinel data, to quantify changes in built-up areas and land-surface characteristics. Information on past flood events was compiled from historical records, satellite-derived flood observations, and reported urban flooding locations. Flood events were classified as riverine or pluvial based on their proximity to river channels and local drainage conditions. The spatial distribution of flood events was analyzed over time in relation to urban expansion patterns and distance from the river network. Event-based rainfall data were examined to assess the role of short-duration intense rainfall in recent flooding episodes, while changes in drainage density and land-surface conditions were considered to support the interpretation of flooding mechanisms. The results indicate a clear temporal shift in flooding patterns. Earlier flood events were predominantly concentrated near river corridors, reflecting riverine flooding, whereas recent flood events increasingly occur within newly urbanized areas away from rivers. This shift highlights the growing dominance of pluvial flooding associated with rapid urban expansion, increased impervious surfaces, and reduced drainage efficiency. The findings emphasize the need to move beyond river-focused flood management approaches and to strengthen urban drainage planning, land-use regulation, and climate-resilient urban design. The approach presented here is transferable to other rapidly urbanizing historic cities facing similar flood challenges.