Flash floods in high-relief Himalayan River corridors present significant and escalating geomorphic hazards, yet the precise local mechanisms governing their destructive power remain inadequately constrained. This study presents a scenario-based analysis of a catastrophic flash flood in the Harshil-Dharali corridor of the Bhagirathi River, Uttarakhand, India, a representative high-risk mountain environment. We introduce an integrated methodology that couples daily discharge records with hydraulic modeling across 15 high-resolution, DEM-derived river transects, validated against satellite-derived (NDWI) erosion mapping. Our results reveal critical hydrodynamic amplification at specific geomorphic nodes. On August 5, 2025, Unit Stream Power (USP) abruptly increased to over 1000 W/m2, with corresponding cross-sectional velocities of 3–5 m/s, at four key transects (XS5, XS6, XS9, and XS14). These modeled high-energy zones exhibit a strong spatial correlation with observed erosion footprints. Notably, USP at transect XS14 exceeded 1700 W/m2 (and XS9 exceeded 3000 W/m2) despite only moderate discharge, a phenomenon attributed to extreme channel slope and severe morphological constriction. At XS9 and XS14, an amplifying effect reveals fundamental nonlinearities in mountain river hydraulics. At these constrictions, channel width (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(w\)</EquationSource> </InlineEquation>) narrows by at least 40% relative to upstream reaches. By the continuity equation (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(Q=V\cdot A\)</EquationSource> </InlineEquation>), these geometric narrowing forces an abrupt increase in velocity (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(V\)</EquationSource> </InlineEquation>) to maintain the same discharge. As flow accelerates, the regime often shifts from subcritical to supercritical (Froude number > 1). Energy is then dissipated through hydraulic jumps and intense turbulence. It explains the exponential spike of Unit Stream Power (USP⩰ƮV), which exceeded the theoretical threshold for bedrock incision (<InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(1000\, W/{m}^{2}\)</EquationSource> </InlineEquation>). Thus, hazards were not solely driven by hydrological input (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(Q\)</EquationSource> </InlineEquation>), but were amplified mechanically via constriction ratio—acting like a hydraulic nozzle to focus kinetic energy onto the banks. The findings confirm that localized geomorphic controls, such as channel narrowing and steepening, rather than discharge magnitude alone, were the dominant drivers of the event’s destructive potential. Concurrently, flood-driven geomorphic instability triggered episodic pulses of carbon mobilization, with peak annual losses exceeding 5000 tC. This represents a substantial climate-relevant flux, of nearly 20,000 tCO2e directly linking catastrophic erosion events to disruptions in the terrestrial carbon cycle. The proposed methodology provides a robust and rapid framework for identifying hydro-geomorphic hotspots. This approach enhances flood risk assessment and mitigation planning in structurally controlled mountain catchments worldwide, offering a transferable tool for proactive disaster management in sensitive and developing regions.

Hydro-geomorphic amplification of flash flood hazards: transect-based analysis of the 2025 Dharali flood, Himalayan Bhagirathi river, India

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发表日期：

2026年6月29日

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Shravankumar S. Masalvad,
Manish Pandey,
N. V. Umamahesh,
Anil Kumar Gupta

摘要

Flash floods in high-relief Himalayan River corridors present significant and escalating geomorphic hazards, yet the precise local mechanisms governing their destructive power remain inadequately constrained. This study presents a scenario-based analysis of a catastrophic flash flood in the Harshil-Dharali corridor of the Bhagirathi River, Uttarakhand, India, a representative high-risk mountain environment. We introduce an integrated methodology that couples daily discharge records with hydraulic modeling across 15 high-resolution, DEM-derived river transects, validated against satellite-derived (NDWI) erosion mapping. Our results reveal critical hydrodynamic amplification at specific geomorphic nodes. On August 5, 2025, Unit Stream Power (USP) abruptly increased to over 1000 W/m², with corresponding cross-sectional velocities of 3–5 m/s, at four key transects (XS5, XS6, XS9, and XS14). These modeled high-energy zones exhibit a strong spatial correlation with observed erosion footprints. Notably, USP at transect XS14 exceeded 1700 W/m² (and XS9 exceeded 3000 W/m²) despite only moderate discharge, a phenomenon attributed to extreme channel slope and severe morphological constriction. At XS9 and XS14, an amplifying effect reveals fundamental nonlinearities in mountain river hydraulics. At these constrictions, channel width ( \(w\) ) narrows by at least 40% relative to upstream reaches. By the continuity equation ( \(Q=V\cdot A\) ), these geometric narrowing forces an abrupt increase in velocity ( \(V\) ) to maintain the same discharge. As flow accelerates, the regime often shifts from subcritical to supercritical (Froude number > 1). Energy is then dissipated through hydraulic jumps and intense turbulence. It explains the exponential spike of Unit Stream Power (USP⩰ƮV), which exceeded the theoretical threshold for bedrock incision ( \(1000\, W/{m}^{2}\) ). Thus, hazards were not solely driven by hydrological input ( \(Q\) ), but were amplified mechanically via constriction ratio—acting like a hydraulic nozzle to focus kinetic energy onto the banks. The findings confirm that localized geomorphic controls, such as channel narrowing and steepening, rather than discharge magnitude alone, were the dominant drivers of the event’s destructive potential. Concurrently, flood-driven geomorphic instability triggered episodic pulses of carbon mobilization, with peak annual losses exceeding 5000 tC. This represents a substantial climate-relevant flux, of nearly 20,000 tCO₂e directly linking catastrophic erosion events to disruptions in the terrestrial carbon cycle. The proposed methodology provides a robust and rapid framework for identifying hydro-geomorphic hotspots. This approach enhances flood risk assessment and mitigation planning in structurally controlled mountain catchments worldwide, offering a transferable tool for proactive disaster management in sensitive and developing regions.