Urbanisation and Geomorphic Transformation of the Drainage Network in the Ram Nadi Watershed, Pune, India

Urban expansion modifies watershed hydrology and geomorphology through growth of impervious cover, slope modification, channel encroachment, and disruption of low-order drainage pathways. This study evaluates land-use/land-cover (LULC) change and drainage-network transformation in the 50.95 km² Ram Nadi watershed, Pune, India. The quantitative historical analysis integrates Survey of India topographic sheets, multi-temporal Landsat imagery for 1991, 2000, 2010 and 2015, GIS-based drainage mapping, cross-profile analysis, and field reconnaissance. The legacy LULC workflow used supervised pixel-based classification in ERDAS IMAGINE; because the archived project files do not retain the classifier specification, training-sample inventory, or an independent confusion matrix, numerical accuracy statistics cannot be reconstructed and the class percentages are treated as descriptive mapped estimates. Drainage loss was assessed using an operational definition that required historical map evidence of a channel, loss or physical interruption of the mapped channel trace in 2015 spatial data, and field evidence of conversion, filling, construction, or persistent obstruction; seasonal absence of surface water alone was not considered sufficient evidence of channel loss. Settlement cover increased from 22.19% in 1991 to 47.55% in 2015, whereas agricultural land declined from 10.65% to 2.01%. Of 416 historically mapped drainage segments, 106 (25.48%) were classified as obliterated or converted by 2015, representing 47.42 km of mapped channel length; most losses occurred in first-order channels. Corrected regional calculations show that total built-up area increased from 1,165.49 ha to 2,074.81 ha, an increase of 909.32 ha (78.02%). Post-2015 institutional records from 2023-2024 indicate that wastewater discharge and channel-management pressures remain relevant within the watershed. The results demonstrate a strong spatial association between urban expansion and drainage-network fragmentation; however, event-specific flood causation cannot be established without rainfall-runoff and hydraulic modelling. The study therefore provides a geomorphic and spatial basis for drainage protection, restoration prioritisation, and future high-resolution monitoring of the Ram Nadi watershed.