Authors: C.M. Bhatt, Pankaj R Dhote, Nishant Rana & Abhay Masiwal

The Indo-Gangetic Plains are among the most chronically flood-affected regions in India. The area receives a substantial amount of rainfall during the monsoon season (June–September), which leads to increased river discharge and frequent flooding. Flooding in the Ganga floodplains is a recurrent phenomenon (Bhatt et al.,2016).

In July 2025, continuous rainfall in the upstream catchment areas resulted in the swelling of the Ganga River and its tributaries, particularly the Yamuna River. This caused flood-like conditions, especially around the confluence of the Ganga and Yamuna rivers near Prayagraj, Uttar Pradesh, during the second and third weeks of July. The situation worsened further due to the release of water from several barrages, including the Haridwar Barrage, Narora Barrage, and Kanpur Barrage.

This study focuses on a segment of the Ganga River flowing through Prayagraj, which experienced significant flooding in July 2025. To assess the spatial extent of the flood and observe changes in water surface elevation (WSE), the study utilized data from two satellite missions: Sentinel-1 Synthetic Aperture Radar (SAR) data for mapping flood extent (Figure-1 & 2) using a change detection method (Bhatt et al., 2021, Dhote et al., 2025), and Surface Water and Ocean Topography (SWOT) data for WSE analysis (Figure-3).

Figure 1 depicts the spatial extent of floodwaters along the Ganga River between Prayagraj and Varanasi based on Sentinel-1 satellite imagery.  The difference in extents reveals the substantial spread of floodwaters, especially in the meandering zones upstream and downstream of Prayagraj. Three Central Water Commission (CWC) gauging stations—Naini, Allahabad, and Phaphamau—are marked and played a key role in monitoring the event. Major inundation is visible along the concave banks of river meanders. Also, Urban and peri-urban zones near Allahabad experienced flooding. The river widened significantly during the flood event, inundating agricultural fields and low-lying settlements.

Figure 2 presents a detailed flood mapping of the upstream region of the Sangam in Prayagraj, using Sentinel-1 imagery captured before and after the flood event (07 July and 19 July 2025, respectively). The comparison highlights a significant expansion in the flood extent, particularly on the northern floodplain, where the inundation spread laterally up to approximately 3.5 kilometers. Other stretches show considerable lateral flooding of around 1.7 and 1.9 kilometers, demonstrating the severity of the floodplain encroachment.

One of the most impacted areas is the newly developed district situated along the banks of the Ganga River. This zone, which was prepared to host the Mahakumbh Mela 2025, appears to be substantially submerged during the flood event. The analysis also reveals that floodwaters entered large tracts of barren and reclaimed land along the riverbanks, much of which have hosted temporary infrastructure. The inundation of these low-lying, unprotected areas highlights their vulnerability to flood pulses.

Figure-1 shows the changes in the flood situation pre-flood (July 07, 2025) and swelling of River Ganga due to monsoon rains (July 19, 2025) along Ganga River.

The SWOT “L2_HR_PIX D” data product from the descending pass number 342, right swath tile 110R, and scene 055F was utilized to extract water level information (Figure 3). The data was accessed from the open-source NASA portal https://search.earthdata.nasa.gov/ on July 20, 2025.

Figure-2 shows the spatial extent of flood mapped using Sentinel-1 data of July 19, 2025. Flood water accumulation in the floodplains and adjacent low lying areas along the River Ganga and Prayagraj is clearly visible.

The “L2_HR_PIXD” product (released on May 06, 2025) is the fundamental water detection product derived from Single-Look-Complex (SLC) radar imagery. It provides pixel cloud data that contain detailed height and backscatter information for water surfaces. Standard geophysical corrections were applied, including the conversion of height measurements from WGS84 ellipsoidal heights to orthometric heights, using the EGM2008 geoid model.

Following the corrections, noise removal procedures were conducted to eliminate spurious water pixels (Bhatt, 2025). This was achieved by applying data quality flags and denoising algorithms, resulting in the generation of a clean Water Surface Elevation (WSE) map (Figure 3).

The Water Surface Elevation (WSE) map derived from SWOT data (Figure 3) shows elevation values ranging from 77–80 meters in the upstream of the confluence of Ganga and Yamuna and 72–75 meters in the downstream stretch of the Ganga River on July 11, 2025. At the Phaphamau site, the in-situ gauge observation recorded a water level of around 79.20 meters on July 11, 2025 and from SWOT derived WSE it was observed to be approximately 79.31 meters (Figure 3). The close agreement between the SWOT-derived and in-situ measurements demonstrates the accuracy and reliability of SWOT data for surface water elevation monitoring in flood-prone river systems.

Figure-3 shows the spatial water surface elevation (WSE) derived from SWOT data of July 11, 2025.

The spatially continuous nature of Water Surface Elevation (WSE) derived from SWOT offers a significant advantage in understanding flood dynamics along the 130 km stretch of the Ganga River. Unlike in-situ gauge measurements, which provide only point-based observations, SWOT data captures spatial variations in WSE, enabling a more comprehensive assessment of flood behavior across the river reach.

Observations from SWOT and Sentinel-1 datasets (July 11 and 19, 2025), supplemented by CWC data (July 10–22, 2025), indicate an increase in water extent and surface elevation. However, no significant flooding was observed, as water levels remained below the warning threshold and began receding from July 22, 2025 onward.

This study demonstrates that, when carefully processed, SWOT data can effectively provide spatially continuous water surface elevation (WSE) information with high accuracy, along with the spatial extent of flooding. This capability makes SWOT a valuable complement to existing Earth Observation (EO) systems and in-situ monitoring networks especially for flood hazard dynamics.

References:

Bhatt, C. M., Gupta, A., Roy, A., Dalal, P., & Chauhan, P. (2021). Geospatial analysis of September, 2019 floods in the lower gangetic plains of Bihar using multi-temporal satellites and river gauge data. Geomatics, Natural Hazards and Risk, 12(1), 84-102.

Bhatt, C. M., & Rao, G. S. (2016). Ganga floods of 2010 in Uttar Pradesh, north India: a perspective analysis using satellite remote sensing data. Geomatics, Natural Hazards and Risk, 7(2), 747-763.

Bhatt (2025). SWOT Two Dimensional Water Surface Elevation (WSE) A New Era in Flood Risk Management. https://science.iirs.gov.in/swot-two-dimesional-water-surface-elevation-wse-a-new-era-in-flood-risk-management/

Dhote, P.R., Thakur, P.K., Gaur, K. et al. SWOT Mission with Wide-Swath Altimetry: Observations and Insights into India’s Inland Waterbodies. J Indian Soc Remote Sens 53, 2367–2376 (2025). https://doi.org/10.1007/s12524-025-02234-8