Sediment transport capacity of overland flow during rainfall in a silty clay soil

Abstract

The process of sediment transport over eroding surfaces during rainfall involves several additional considerations than fluvial and channelized sediment transport due to its small spatial scale, including the impact of raindrops on flow properties, variations in boundary geometry, surface roughness effects, and differences in particle and aggregate sizes and densities. Although soil transport via splash occurs during rainfall, it's considered less significant compared to transport via flow. But when the surface flow comes to shallow, neglecting the effect of rainfall impact can lead to a significant underestimation of sediment transport and in particular the size of the transported particles. However, understanding these overland flow processes remains limited, leading erosion modelers to utilize various fluvial transport equations to estimate overland flow transport capacity. Previous studies examining the suitability of fluvial sediment transport equations for overland flow revealed that most equations were inadequate for this purpose. Equations used or recommended for soil erosion modeling performed poorly when applied to overland flow, particularly under simulated rainfall. Issues included underestimation of transport rates, considerable variability in results, and inadequate representation of transport thresholds. Studies also indicated that sediment delivery during rainfall was significantly higher than during uniform baseflow, with a substantial portion attributed to changes in flow characteristics caused by rainfall impact. However, these models were not specifically developed or validated for overland flow conditions, leading to perplexity about their applicability, especially for scenarios involving steep slopes, shallow flow, rainfall impact, material non-uniformity in very fine soils, and light-weight material under rainfall. While numerous transport equations exist, their efficacy in estimating sediment transport during overland flow remains uncertain due to limitations in their development and validation for these specific conditions. Studies consistently demonstrate the challenges in accurately predicting sediment transport rates and thresholds during rainfall-induced overland shallow flow particularly in agricultural fields where a reliable estimate of sediment transport capacity of surface flows is a key component of any deterministic model designed to route sediment. Although extensive literature exists on the transport of sand by streamflow, very little information is available on the transport of aggregates and very fine particles by overland flow. The authors have evaluated several transport formulas against laboratory data collected in simulated rainfall experiments performed in replicated plots in silty clay loam soil at SERLAB station (central Italy). The paper presents a summary of the formulas and the data used in the study, and the results of the evaluation. The research was financially supported  by PNRR “Finanziato dall'Unione europea – Next Generation EU”.