Rain Splash Transport Research Articles

Estimating the Effect of Rain Splash on Soil Particle Transport by Using a Modified Model: Study on Short Hillslopes in Northern China

Splash erosion is an important soil erosion process in sloping lands. This study aims to improve the model of rain splash transport based on the results of previous studies and field experiments involving rainfall simulation. A field study was conducted to examine the effects of rainfall properties, herbaceous cover and surface flow on splash processes on hillslopes in northern China. On the basis of the experimental results, a comprehensive model of rain splash was established through the quantitative analysis of the interactive effects of rainfall kinetic energy, vegetation coverage and overland runoff depth on splash erosion rate and the probability density of splashed particles and maximum splash distance. The results showed that the estimated and observed values of splash transport exhibit high consistency and adaptability. However, several discrepancies were observed between the estimated and observed values for events with high vegetation coverage. These differences can be ascribed to the variation in overland runoff connectivity and the differences in soil surface cohesion at various wetness degrees. The proposed model provides insights into splash erosion characteristics and suggestions for erosion control practices on hillslopes.

Modeling aggregate size distribution of eroded sediment resulting from rain-splash and raindrop impacted flow processes

Soil susceptibility to detachment and transport sub-processes of erosion is generally controled by the aggregate breakdown mechanism. Measuring particle size and aggregation to the estimate erodibility potential of soils is important under erosive rainfall conditions. The Aggregate Size Distribution (ASD) is one of the most important determinants of soil structure along with soil organic matter content for describing the efficiency of applied, sustainable management strategies. This study aimed to compare the performances of three different aggregate size distribution models to predict the characteristic aggregate size parameter (median diameter, D50) for eroded sediment from interrill erosion processes of Rain-Splash Transport (RST) and Raindrop Impacted Flow Transport (RIFT). The ASDs of 1143 collected sediment samples from the RST and RIFT processes were measured and modeled by the Log-normal, Fractal, and Weibull approaches. The D50 value, as a characteristic parameter for aggregate size distributions, derived from the cumulative ASD curve was compared for soils from different land use types and different slope and rainfall intensity conditions. The performance of each model was evaluated using the Mean Square Error (MSE) and Coefficient of Determination (R2). The Weibull approach was the most accurate model showing the best fit with the lowest MSE values (0.0002 ≤ MSE ≤ 0.0048) and having the greatest R2 values (0.936 ≤ R2 ≤ 0.998) when compared with the Log-normal and Fractal models. Herewith, for semi-arid land use and soil, specific shape and scale parameters for the Weibull distribution, the respective ASDs were successfully re-generated for modeling the eroded sediment of the simulated RST and RIFT interill processes.

The elements and richness of particle diffusion during sediment transport at small timescales

AbstractThe ideas of advection and diffusion of sediment particles are probabilistic constructs that emerge when the Master equation, a precise, probabilistic description of particle conservation, is approximated as a Fokker–Planck equation. The diffusive term approximates nonlocal transport. It ‘looks’ upstream and downstream for variations in particle activity and velocities, whose effects modify the advective term. High‐resolution measurements of bedload particle motions indicate that the mean squared displacement of tracer particles, when treated as a virtual plume, primarily reflects a nonlinear increase in the variance in hop distances with increasing travel time, manifest as apparent anomalous diffusion. In contrast, an ensemble calculation of the mean squared displacement involving paired coordinate positions independent of starting time indicates a transition from correlated random walks to normal (Fickian) diffusion. This normal behavior also is reflected in the particle velocity autocorrelation function. Spatial variations in particle entrainment produce a flux from sites of high entrainment toward sites of low entrainment. In the case of rain splash transport, this leads to topographic roughening, where differential rain splash beneath the canopy of a desert shrub contributes to the growth of a soil mound beneath the shrub. With uniform entrainment, rain splash transport, often described as a diffusive process, actually represents an advective particle flux that is proportional to the land surface slope. Particle diffusion during both bedload and rain splash transport involves motions that mostly are patchy, intermittent and rarefied. The probabilistic framework of the Master equation reveals that continuous formulations of the flux and its divergence (the Exner equation) represent statistically expected behavior, analogous to Reynolds‐averaged conditions. Key topics meriting clarification include the mechanical basis of particle diffusion, effects of rarefied conditions involving patchy, intermittent motions, and effects of rest times on diffusion of tracer particles and particle‐borne substances. Copyright © 2016 John Wiley & Sons, Ltd.

Limits on the morphogenetic role of rain splash transport in hillslope evolution

AbstractAn experimentally parameterized equation for rain splash transport was generalized in order to compute annual sediment transport rates for a range of climates. The resulting equation was then used to examine the conditions under which rain splash transport can play a significant morphogenetic role in water‐eroded landscapes. Rain splash can generate convex hillslopes on short, steep hillslopes such as the margins of gullies in unconsolidated sediments. However, comparison of predicted profiles with long, convex hillslopes in postorogenic landscapes indicates that rain splash cannot transport sufficient quantities of sediment to produce the observed convex hillslope profiles. Rates of biogenic transport estimated from the literature also appear to be insufficient. Sheetwash transport appears to be the only process capable of molding these extensive convex hillslope profiles despite the common assumption that it generates exclusively concave forms.

Effects of vegetation cover on sediment particle size distribution and transport processes in natural rainfall conditions on post-fire hillslope plots in South Korea

Sediments were collected from four slow vegetation recovery plots, six fast vegetation recovery plots and five unburned plots at a post-fire site on a rainfall event basis and sorted for size distribution. The aim was to evaluate the effects of vegetation cover, soil aggregate stability, slope and rainfall intensity on sediment size distribution, transport selectivity and erosion processes between the burned and unburned treatment plots. Sediment detachment and transport mechanisms and the particle size transport selectivity of the eroded sediment were assessed based on enrichment ratios (ER) and mean weighted diameter (MWD) methods. The most eroded particle size class in all treatment plots was the 125–250μm class and, generally, the percentage of eroded particle sizes did not increase with slope and rainfall intensity. Higher MWD of the eroded sediment was related to a higher percentage of bare soil exposed and gravel content associated with high soil burn severity and soil disaggregation in the slow vegetation recovery plots. The enrichment of finer clay silt particle sizes increased with varying maximum 30-min rainfall intensity (I30) in the slow vegetation recovery plots, and reflected increased aggregate breakdown and transport selectivity, whereas no good relationship was found in the fast vegetation recovery and unburned plots with varying I30. A minimum I30 of <3.56mmh–1 and a maximum of 10.9mmh–1 were found to be the threshold rainfall intensity values necessary for aggregate breakdown and transport of finer particles by both rainsplash and rainflow in the slow vegetation recovery plots, whereas the response was weak in the fast vegetation recovery and unburned plots following varying I30 dominated only by rainsplash transport closer to the plot sediment collector. The results show that higher vegetation cover in the fast vegetation recovery and unburned plots reduces erosive rainfall energy by 5.6- and 17.7-fold respectively, and runoff energy by 6.3- and 21.3-fold respectively, limiting aggregate breakdown and transport selectivity of finer particles compared with the slow vegetation recovery plots.

Water flux and sediment transport within a forested landscape: the role of connectivity, subsurface flow, and slope length scale on transport mechanism

AbstractThe connectivity and upscaling of overland runoff and sediment transport are important issues in hillslope hydrology to identify water flux and sediment transport within landscape. These processes are highly variable in time and space with regard to their interactions with vegetation and soil surface conditions. The generation of overland runoff and its spatial connectivity were examined along a slope to determine the variations in the transport mechanism of runoff and soil particles by rain splash and overland runoff. Field experiments were conducted by erosion plots on a steep hillslope at lengths of 5, 10, and 15 m. The overland runoff connectivity and flow transport distance decreased with the slope length, while spatial variability of infiltration increased significantly with the slope length. Observation of subsurface flow revealed that surface soil and litter layer could have important role in water transport. However, the surface soil water content and water flux transport along the slope was highly variable for different storm events; the variability was related to the complexity of the system, mainly by way of the initial wetness conditions and infiltration characteristics. Only net rain‐splashed soil was measurable, but examination of the water flux, overland runoff and sediment transport connectivity, characteristics of sheetwash, and the variability in spatial infiltration indicated an increase in the contribution of the rain splash transport mechanism along the slope. Copyright © 2013 John Wiley & Sons, Ltd.

Downslope soil detachment–transport on steep slopes via rain splash

AbstractThis study developed a one‐dimensional model of downslope rain splash transport based on field experiments and previous studies. The developed model considers soil detachment processes, ground cover, probability densities, and the effect of overland run‐off in preventing detachment. Field monitoring was conducted to observe precipitation run‐off, ground cover, and sediment production on steep hillslopes. Field‐observed data were used to develop the splash detachment rate equation, probability densities for splash transport, and the maximum splash transport distance. Observed and estimated splash transport showed overall agreement, with some differences for small storm events or events with relatively low intensity, probably caused by variation of overland run‐off depth and connectivity as well as differences in soil surface cohesion at various degrees of wetness. Our model can provide insights on the interactions among rainfall intensity, soil surface condition, soil wetness, and splash transport on forested hillslopes. Copyright © 2011 John Wiley & Sons, Ltd.

A rain splash transport equation assimilating field and laboratory measurements

Process‐based models of hillslope evolution require transport equations relating sediment flux to its major controls. An equation for rain splash transport in the absence of overland flow was constructed by modifying an approach developed by Reeve (1982) and parameterizing it with measurements from single‐drop laboratory experiments and simulated rainfall on a grassland in East Africa. The equation relates rain splash to hillslope gradient, the median raindrop diameter of a storm, and ground cover density; the effect of soil texture on detachability can be incorporated from other published results. The spatial and temporal applicability of such an equation for rain splash transport in the absence of overland flow on uncultivated hillslopes can be estimated from hydrological calculations. The predicted transport is lower than landscape‐averaged geologic erosion rates from Kenya but is large enough to modify short, slowly eroding natural hillslopes as well as microtopographic interrill surfaces between which overland flow transports the mobilized sediment.

Rain splash of dry sand revealed by high‐speed imaging and sticky paper splash targets

Rain splash transport of sediment on a sloping surface arises from a downslope drift of grains displaced ballistically by raindrop impacts. We use high‐speed imaging of drop impacts on dry sand to describe the drop‐to‐grain momentum transfer as this varies with drop size and grain size and to clarify ingredients of downslope grain drift. The “splash” of many grains involves ejection of surface grains accelerated by grain‐to‐grain collisions ahead of the radially spreading front of a drop as it deforms into a saucer shape during impact. For a given sand size, splash distances are similar for different drop sizes, but the number of displaced grains increases with drop size in proportion to the momentum of the drop not infiltrated within the first millisecond of impact. We present a theoretical formulation for grain ejection which assumes that the proportion of ejected grains within any small azimuthal angular interval dθ about the center of impact is proportional to the momentum density of the spreading drop within dθ and that the momentum of ejected grains at angle θ is, on average, proportional to the momentum of the spreading drop at θ. This formulation, consistent with observed splash distances, suggests that downslope grain transport involves an asymmetry in both quantity and distance: more grains move downslope than upslope with increasing surface slope, and, on average, grains move farther downslope. This latter effect is primarily due to the radial variation in the surface‐parallel momentum of the spreading drop. Surface‐parallel transport increases approximately linearly with slope.

Sediment transport from interrill areas under wind-driven rain

In nature, erosive rainstorms are usually associated with high winds. Therefore, a quantification of wind and rain interactions and the effects of wind on detachment and transport processes provides a great opportunity for a given prediction technology to improve the estimation of erosion. This paper presents experimental data obtained on interrill erosion processes under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, windless rains and the rains driven by horizontal wind velocities of 6, 10, and 14 m s −1 were applied to three agricultural soils packed into 20 by 55 cm soil pan at both windward and leeward slopes of 7, 15, and 20%. Wind-driven rainsplash transport was measured by trapping the splashed particles at distances on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. The process was adequately described by relating the rainsplash transport rate to the rainfall parameter, fluxes of rain energy or momentum, and wind shear velocity by log-linear regression technique. Sediment transport by rain-impacted thin flow was measured by collecting sediment and runoff samples at 5-min intervals after runoff onset. This process was also adequately described by rainfall and flow parameters. Comparing the process contribution to the total sediment transport, it is concluded that rainsplash transport is a significant process that should not be neglected in accurately predicting interrill water erosion under wind-driven rain. Mean values for the ratio of rainsplash erosion to the corresponding total sediment transport were 14.6±6.4, 23.9±7.8, and 38.1±8.7% for the rains driven by 6, 10, and 14 m s −1 wind, respectively.

A methodology to study rain splash and wash processes under natural rainfall

AbstractThe complex interactions between rainfall‐driven erosion processes and rainfall characteristics, slope gradient, soil treatment and soil surface processes are not very well understood. A combination of experiments under natural rainfall and a consistent physical theory for their interpretation is needed to shed more light on the underlying processes. The present study demonstrates such a methodology. An experimental device employed earlier in laboratory studies was used to measure downslope rain splash and ‘splash‐creep’, lateral splash, upslope splash and rainfall‐driven runoff transport (wash) from a highly aggregated clay‐rich oxisol exposed to natural rainfall in West Java, Indonesia. Two series of measurements were made: the first with the soil surface at angles of 0°, 5°, 15° and 40°; and the second all at an angle of 5° but with different tillage and mulching treatments. A number of rainfall erosivity indices were calculated from rainfall intensity measurements and compared with measured transport components. Overall storm kinetic energy correlated reasonably well with sediment transport, but much better agreement was obtained when a threshold rainfall intensity (20 mm h−1) was introduced. Rain splash transport measurements were interpreted using a recently developed theory relating detachment to sediment transport. Furthermore, a conceptually sound yet simple wash transport model is advanced that satisfactorily predicted observed washed sediment concentrations. The lack of replication precluded rigorous assessment of the effect of slope and soil treatment on erosion processes, but some general conclusions could still be drawn. The results stress the importance of experiments under conditions of natural rainfall. Copyright © 2002 John Wiley & Sons, Ltd.

Raindrop-induced and wind-driven soil particle transport

A wind tunnel study under wind-driven rains was conducted to determine the combined effect of rain and wind on the rainsplash transport process. The rains driven by horizontal wind velocities of 6, 10 and 12 m s−1 were applied to three agricultural soils packed into a 20×55-cm soil pan placed at both windward and leeward slopes of 4.0°, 8.5° and 11.3°. Transport rates were measured by trapping the splashed particles at set distances in the upslope and downslope directions, respectively, for windward and leeward slopes. Rainsplash transport under wind-driven rains was adequately described (R2=0.93) by relating the transport rate to the rain impact pressure and wind shear velocity by log–linear regression technique. Average trajectory of a raindrop-induced and wind-driven particle was also adequately predicted by the momentum loss per unit time per unit length of travel (u*2/g). The travel distance is found to be three times greater than the path of a typical saltating sand grain.

Gravel dispersion on a granite pediment (East Mojave Desert, California): a short-term look at erosional processes

Hypotheses about the influence of surface shape, landscape unit and vegetation cover on gravel dispersion were tested on a shallowly dissected portion of a low-sloping granite pediment in the East Mojave Desert of California. Painted gravels (2 to 20 mm diameter) were placed at 117 nodes on a 6m × 3m grid. Gravel movements were recorded after 9.7 cm of precipitation over a four-month period. Vectors indicating the magnitude and direction of gravel movement were longest for summits (24 cm, 34 nodes observed) and shortest for backslopes (14 cm, 27 nodes observed). Gravels beneath shrub canopies were protected significantly from rainsplash transport. To describe dispersion symmetry, eccentricity values were calculated using a ratio of variances of major and minor axes of an ellipse. Mean eccentricity values ranged from about 3 to 250 with dispersion on summits being the most symmetrical and dispersion in washes being the most elongated. Erosion is the most important soil- and pediment-modifying process at upper elevations of the Granite Cove Pediment which is cut off from sediment additions because of washes incised at the base of the mountain front. Copyright © 1999 John Wiley & Sons, Ltd.

The significance of rainsplash in the surficial debris cascade of the colorado front range foothills

AbstractThe surficial characteristics and precipitation regime of sparsely vegetated hillslopes in the montane zone of the Colorado Front Range suggest that rainsplash may be an important component of the surficial debris cascade. Site sediment flux data from two study periods reveal marked spatial and temporal variability. Comparison of these date with sediment movement data from splashcups suggests the following conclusions: (1) Detachment rates of the rainsplash process appear great enough to account for the sediment flux in the open Gerlach‐type traps; (2) Areal extrapolation of rainsplash transport suggest that 88 per cent of the fine sediment flux in 1982 can be attributed to rainsplash; (3) Estimates of rainfall energy and changes in the potential energy of hillslopes by mass transport suggests a process efficiency of 0.05 per cent for rainsplash. If this procedure is applied to sediment flux values from open traps, the low precipitation‐energy cascade of 1982 appears to be largely rainsplash‐transported sediment. Extrapolation with the 1981 data suggests more aggressive overland flow erosion and transport.