Field-Based Analysis of Saltwater Intrusion Distance, Arrival Time, and Stratification Index Following a Controlled Barrage Opening in the Nakdong River Estuary, South Korea

Saltwater intrusion in regulated estuaries is commonly assessed using intrusion distance; however, its response to barrage operation may also depend on arrival timing, vertical stratification, and bathymetrically influenced near-bed transport. This study investigated these processes during a controlled saltwater intrusion experiment conducted on 17–18 September 2019 in the freshwater reach upstream of the Nakdong River Estuary Barrage, South Korea. One barrage gate was opened for 51 min, with an officially reported seawater inflow of approximately 1.01 × 106 t. Repeated vertical profiles of water temperature and salinity were collected at Vessels A and B (3.0, 7.0 km upstream). The salinity stratification index (SI) was defined as the difference between near-bottom and surface salinities. Before intrusion, salinity at both stations was approximately 0.1 psu. At Vessel A, near-bottom salinity increased abruptly at approximately 12:40 on 17 September, and the maximum recorded salinity later reached 7.83 psu. At Vessel B, near-bottom salinity began increasing progressively at approximately 20:00 on 17 September, and the maximum recorded salinity reached 4.69 psu at approximately 10:00 on 18 September. Saline water appeared earlier at Vessel A (3.0 km) and showed a substantially delayed response at Vessel B (7.0 km), indicating spatially heterogeneous adjustment of the saline bottom layer rather than uniform propagation. Mean SI values were 3.34±2.60 psu at Vessel A and 2.49±1.98 psu at Vessel B, confirming predominantly near-bed salt-wedge propagation beneath persistent freshwater surface flow. The observed bottom-intensified salinity structure was consistent with bathymetry-driven transport through deeper sections of the S-shaped channel, producing an abrupt intrusion at the 3.0 km station and delayed but sustained stratification at the 7.0 km station. Subsequent freshwater discharge is consistent with freshwater flushing, with an inferred retreat pathway involving seaward retreat and flushing of the saline layer, and the observed retreat pattern suggesting an influence of bathymetry. Surface salinity observations and single-distance criteria may underestimate saltwater penetration in regulated estuaries. Assessments should therefore consider intrusion distance, arrival time, vertical stratification, and near-bottom transport pathways.