Contrasting Infiltration Dynamics of Alluvial and Non-Alluvial Urban Soils: A Laboratory Study in Padang, Indonesia

Variations in soil physical properties fundamentally dictate water absorption capacity in urban environments. The urgency of this factor was clearly evident in the flooding of Padang City in late 2025, where soil structure degradation was shown to have exacerbated the disaster's intensity. This study evaluates the infiltration behavior of alluvial and non-alluvial soils through controlled laboratory experiments, utilizing the Horton infiltration model as a framework for analysis. Representative soil samples from Padang, Indonesia, were subjected to simulated heavy rainfall conditions to determine their hydraulic response. The results reveal a significant disparity: alluvial soils exhibited an infiltration capacity of 29.8 mm/h, while non-alluvial soils reached 64.9 mm/h. Over a 60-minute duration, cumulative infiltration in non-alluvial soil (139 mm) nearly doubled that of alluvial soil (85 mm). Furthermore, the critical infiltration point was reached significantly faster in alluvial samples (17 minutes) than in non-alluvial samples (33 minutes). This rapid decline suggests a limited saturation threshold in alluvial soils, driven by higher silt and clay content, which facilitates a denser, stratified structure with restricted pore connectivity. Conversely, the porous and coarse-grained nature of non-alluvial soils allows for more sustained and efficient water transmission.