Moisture-dependent erodibility and flooding potential of alluvial clay and peat soils in Selangor, Malaysia

Study region This study focuses on the Quaternary alluvial plains of Selangor, Malaysia. Flood-driven erosion in these urbanizing tropical monsoon catchments remains poorly understood despite high vulnerability to interacting hydraulic forces, rainfall, and varying soil moisture. Study focus Controlled flume experiments evaluated cohesive clay and peat soils under three rainfall intensities (16, 42, 105 mm/h), three slopes (0°, 15°, 30°), and five moisture levels (0%–40%). Measured detachment rates and excess-shear metrics, examined by principal component analysis, identified a primary hydraulic forcing axis and a secondary moisture-influenced soil-state axis for precise determination of sediment transport control. New hydrological insights for the region The soils display distinct behaviours with important hydrological implications. Dry clay is highly erodible with a critical shear stress of 8.02 Pa and detaches 1.3 times faster than dry peat. Above 30% moisture, mineral swelling and matrix sealing increase clay thresholds to 17.50 Pa, reducing erodibility by two orders of magnitude and detachment by 11.5-fold. This response alters runoff dynamics during intense rainfall. Conversely, peat thresholds increase from 19.43 Pa under dry conditions to 44.11 Pa under wet conditions, though erodibility declines only modestly. This trend promotes continuous fibrous sloughing and greater surface water retention than clay, potentially contributing to prolonged inundation and elevated sediment transport. These empirical parameters support process based geomorphic models and enhance the understanding of flooding potential in tropical climates.