The Influence of Immersion Duration on Erosion Differentiation Along Riverbanks in the Middle Yangtze River

The middle reaches of the Yangtze River, especially the Jiujiang region with unconsolidated soil, suffer frequent erosion‐induced collapses, yet the mechanism by which immersion duration regulates soil properties and drives differential erosion remains unclear. To address this gap, field investigations and sampling were focused on Jiujiang's riverbanks, laser diffraction analysis, potassium dichromate oxidation‐spectrophotometry, and dry‐wet sieving methods were utilized to examine soil property evolution under four immersion durations (severe, moderate, mild, slight). Results indicate: (1) As immersion duration shortens, soil particles transition from fine‐dominated (clay, silt) to coarse‐dominated (sand), with the lowest sand content in the severe immersion zone (43.1%–76.3%) and highest in the moderate immersion zone (69.08%–98.98%); (2) Organic carbon content first decreased, then increased with shorter immersion duration, reaching a trough in the moderate immersion zone (minimum < 0.2%) and a peak in the mild immersion zone (maximum > 1.0%); (3) Aggregate stability was poorest in the moderate immersion zone ( = 50.12%–67.35%, highest PAD) and optimal in the slight immersion zone ( = 78.39%–82.20%, lowest PAD); (4)Erosion phenomena progressively transition from Category IV (collapse, scouring) to Category I (minor topsoil loss) as immersion duration shortens. This phenomenon originates from immersion duration modifying soil particle size and aggregate stability, thereby changing anti‐erosion capacity. These findings provide a scientific basis for projecting erosion‐prone zones and offering targeted bank protection strategies, thereby serving as a valuable reference for similar studies on large alluvial rivers with regulated flow regimes and comparable dual‐layer bank structures.