Urban Expansion and Riverbank Encroachment in the Sangam Confluence Zone, Prayagraj: A Geospatial Assessment

Abstract

Rapid urban expansion is placing growing pressure on river corridors and floodplains across fast-developing cities. The Sangam confluence at Prayagraj is a particularly sensitive case: an ecologically fragile and culturally significant landscape where construction restrictions have been imposed by the Allahabad High Court with reference to a 500 m distance from the highest flood level. This study applies an operational 500 m river buffer for geospatial assessment. This study maps and quantifies permanent built-up encroachment within that analytical boundary. The workflow integrates Google Earth Engine and ArcGIS 10.8 to process Sentinel-2 Level-2A imagery from the May–June 2025 dry season, when water extent and sandbar exposure are most stable. Three spectral indices — NDVI, NDWI, and NDBI — served as inputs to a Random Forest classifier, which separated built-up surfaces from water and exposed sand. A post-classification refinement step then filtered out vegetation anomalies and temporary Kumbh Mela structures, retaining only permanent construction. The results reveal substantial permanent development within the study buffer. Of the 22.48 km² buffer zone, permanent structures cover 1.69 km², or 7.53% of the study area, corresponding to an estimated 16,936 individual units. This encroachment is spatially uneven: it clusters along the western and north-western edges adjoining Prayagraj's urban core, while the central confluence remains largely free of permanent construction. These findings indicate that development pressure persists within the assessed river corridor. The method offers planners and environmental authorities a repeatable, spatially precise basis for monitoring and enforcing riverfront protection.