Hydrological Observations of a Two-Stage Flood Dynamics and Implications for Ecological River Regulation in the Cai Nha Trang River Basin, Vietnam

ABSTRACT: Flooding in coastal river basins is shaped by the temporal interaction of upstream hydrological forcing and downstream hydraulic conditions, yet such processes remain difficult to characterize in data-limited regions. This study investigates the hydrological evolution of the November 2025 flood in the Cai Nha Trang River Basin, Vietnam, using water-level observations from the Dong Trang and Dien Phu hydrological stations, discharge observations at Dong Trang, rainfall records, tidal-level data, and post-flood field observations. Hydrograph-derived indicators characterized flood magnitude, evolution, and persistence, with the event examined as two successive flood phases. The first phase reached a water level of 12.24 m and a maximum discharge of 3180 m³·s⁻¹ at Dong Trang, while the second phase reached 13.14 m and 3720 m³·s⁻¹. The second phase followed substantially greater antecedent rainfall and produced longer exceedance of the official flood-warning levels. The observations indicate that the November 2025 flood was characterized by successive hydrometeorological forcing and contrasting upstream–downstream responses. However, the Dien Phu water-level record was interrupted by sensor inundation during the second phase; therefore, the 6.73 m value represents the maximum observed downstream level before the sensor was lost rather than the absolute downstream peak. Downstream field observations of floodplain encroachment, channel narrowing, and hydraulic infrastructure provide evidence of potential constraints on flood conveyance, but do not establish their causal contribution to the event. The findings support ecological river regulation as a basin-scale management perspective emphasizing functional connectivity among rivers, floodplains, estuarine pathways, and coastal waters.