Influence of rock fragment coverage on soil erosion and hydrological response: Laboratory flume experiments and modeling

Abstract

[1] Two laboratory flume experiments on the effect of surface rock fragments on precipitation-driven soil erosion yields were carried out. The total sediment concentration, the concentration of seven individual size classes, and the flow discharge were measured. Digital terrain models (DTMs) were generated before and after one of the experiments. The results revealed that the rock fragments protected the soils from raindrop detachment and retarded the overland flow, therefore decreasing its sediment transport capacity. Rock fragments were found to affect selectively the different size classes in a manner that changed according to the time scale. For short times, the rock fragment coverage reduced erosion of the finer particles (<20 mm). For the midsize classes the protection decreased, while erosion of the larger size classes (>100 mm) was unaffected. At long times the rock fragment cover decreased the concentration of the individual size classes in proportion to effective rainfall intensity and the area exposed to raindrops. An area-based modification of the Hairsine and Rose (H-R) soil erosion model was employed to analyze the experimental data. The H-R model predictions agreed well with the measured sediment concentrations when high rainfall intensity and low rock fragment cover were used. Predictions were instead less accurate with low rainfall intensity and high rock fragment cover. The DTM results showed that the presence of rock fragments on the soil surface led to increased soil compaction, perhaps due to higher soil moisture content (from greater infiltration) within the rock fragment-covered flumes.

Linear scaling of precipitation-driven soil erosion in laboratory flumes

“…The initial moisture content affects the short time hydrological response through the time-to-ponding and runoff, which in turn influences the overland flow depth development and consequently soil erosion detachment (H6 versus H7-E2 and H7-E3). For example, the times-to-runoff for experiment H7-E1, where the compaction effects were significant (Jomaa et al, 2012b), were 14.3 and 27.1 min for Flumes 1 and 2, respectively (Table 1). However, for the other experiments the time-to-runoff was less than 3 min independent of the precipitation rate (H7-E2-E4).…”

“…However, for the other experiments the time-to-runoff was less than 3 min independent of the precipitation rate (H7-E2-E4). Jomaa et al (2012b) reported that the presence of rock fragments on the topsoil affects the surface sealing development and infiltration rate compared with the bare flume (Table 1).…”

“…Four sequential experiments, denoted H7-E1, H7-E2, H7-E3, and H7-E4 are taken from Jomaa et al (2013;2012b), and experiment H6 from Jomaa et al (2012b). Experiment H6 used two flumes, H6-F1 (bare soil) and H6-F2 (20% rock fragment coverage), each subjected to 3 h precipitation at a rate of 74 mm h -1 .…”

“…For all experiments, the rock fragments were placed on the top surface (not embedded in the soil). The design of experiments, the rainfall simulator characteristics and the soil property as well as its preparation procedure were described previously (Jomaa et al, 2010;Jomaa et al, 2012b;Tromp-van Meerveld et al, 2008), so only key features are discussed here. The flume was filled to a depth of 0.32 m with an agricultural loamy soil from Sullens, Switzerland, and underlain by 0.10 m of coarse gravel facilitating the drainage.…”

“…For experiments H6, H7-E1-E4, and H8, Flume 1 was bare soil while Flume 2 was covered by surface rock fragments (Table 1). Jomaa et al (2012bJomaa et al ( , 2013 While noting that there are exceptions for some size classes, this proportionality appears to hold also under unsteady conditions, i.e., for the entire erosion event. In particular, Figs 2, S1…”