Splash Distance Research Articles

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.

Splash distance and size distributions for various soils

Splash is an important process in interrill erosion because it produces movement of soil fragments. However, this process is technically difficult to measure and little is known about its size selectivity. In this study, a splash ring device was used to characterise the spatial variation of the quantity and the aggregate size distribution of splashed soil fragments. Soil aggregates were placed at the centre of an experimental device subjected to a 29 mm h −1 simulated rainfall with a kinetic energy of 17 J mm −1. Splashed soil fragments were collected in concentric rings and analysed for masses and fragment size distributions. Four different soils, with various textures, were tested. Soils fragments were splashed across the whole splash device up to 45 cm from the source, and the quantity of splashed fragments decreased exponentially with the distance. For the four tested soils, the splash parameters were significantly correlated to the results of aggregate stability measurements with r=−0.96 and r=0.95, respectively, for the total splashed mass and the mean weight diameter (MWD) of the whole splashed fragments. The measurement of the splashed fragment size distributions showed that fragments up to 2000 μm were transported by raindrop impacts. The mass percentage of the coarsest fractions of splashed soil fragments exponentially decreased with the distance from the source. The extent of this decrease depends on the soil type. The size distributions of splashed soil fragments were compared with those of soil fragments produced by breakdown. Comparison of splash data to aggregate breakdown data showed an enrichment of the 200–1000 μm size fraction in the splashed fragments.

Measurements of rain splash on bench terraces in a humid tropical steepland environment

AbstractSoil loss continues to threaten Java's predominantly bench‐terraced volcanic uplands. Sediment transport processes on back‐sloping terraces with well‐aggregated clay‐rich oxisols in West Java were studied using two different techniques. Splash on bare, cropped, or mulched sub‐horizontal (2–3°) terrace beds was studied using splash cups of different sizes, whereas transport of sediment on the predominantly bare and steep (30–40/deg ) terrace risers was measured using a novel device combining a Gerlach‐type trough with a splash box to enable the separate measurement of transport by wash and splash processes. Measurements were made during two consecutive rainy seasons. The results were interpreted using a recently developed splash distribution theory and related to effective rainfall erosive energy. Splash transportability (i.e. transport per unit contour length and unit erosive energy) on the terrace risers was more than an order of magnitude greater than on bare terrace beds (0·39–0·57 versus 0·013–0·016 g m J−1). This was caused primarily by a greater average splash distance on the short, steep risers (>11 cm versus c. 1 cm on the beds). Splashed amounts were reduced by the gradual formation of a protective ‘pavement’ of coarser aggregates, in particular on the terrace beds. Soil aggregate size exhibited an inverse relationship with detachability (i.e. detachment per unit area and unit erosive energy) and average splash length, and therefore also with transportability, as did the degree of canopy and mulch cover. On the terrace risers, splash‐creep and gravitational processes transported an additional 6–50% of measured rain splash, whereas transport by wash played a marginal role. Copyright © 2002 John Wiley & Sons, Ltd.

Laboratory Study of Wave‐Breaking over Bars and Artificial Reefs

One‐hundred eight regular‐wave tests and 12 random‐wave tests were performed in a tank to investigate properties of waves breaking on irregular beach profiles. In the tests, solid objects representing natural barred profiles, terraced profiles, and artificial reefs were installed on a concrete slope of 1/30. Wave conditions covered deep‐water steepnesses from 0.0085 to 0.09. A large data base of previous measurements of wave breaking on plane slopes was also compiled and analyzed for comparison to the barred profile measurements. Significant differences were found in such properties as breaker type, breaker depth and height indices, plunge distance, and splash distance for plane and barred slopes. The ratio of wave height and water depth of random waves transforming over a bar was not constant, as previously reported from field measurements, but showed a bimodal distribution with maxima of approximately unity at the bar crest and foreshore and minima of approximately 0.45 in the offshore and bar trough.

A physically‐based model of the dispersion of splash droplets ejected from a water drop impact

AbstractA model has been developed which predicts the dispersion of splash droplets produced by the impact of a water drop on a sloping soil surface.Experimental results of the ejection velocities and ejection angles of the splash droplets are generalized to a planar slope and the resulting splash distances are calculated taking into account the effects of air resistance. The predictions are presented in terms of the numbers of splash droplets from the impact point to surrounding squares arranged in a grid on the slope.The model explains many experimentally observed features of raindrop splash in terms of the mechanics of the processes involved and can make predictions of the effects of slope, wind, raindrop size, and soil properties on droplet dispersion. The component of the raindrop velocity parallel to the surface of the slope is identified as the main factor determining the degree and the direction of the asymmetry in the splash droplet dispersion.By combining the model with a theory of the entrapment of soil in the splash droplets it is possible to extend it to predict the dispersion of soil particles by raindrop impact, which is the basis of a model of soil erosion by rainsplash.

Work done by Splash: Laboratory experiments

AbstractLaboratory experiments, based on the theory proposed by van Heerden (1967) that net splash discharge equals the difference between yMiVi and miyi, where M and Y denote downslope masses and projected distances and m upslope projected distances and masses, demonstrate that the work done by splash is a function of the slope angle to (sin a)1.9.Net discharge of sediment (q) therefore increases to (sin a)0.9, It is equally a function of the susceptibility to splash erosion K, which is a complex function of the moisture content of the sands and their grain size distribution.The technique proposed for the assessment of the mean projected splash distances is rapid and gives very accurate results. When carefully applied, it can also be used for fine material like, for example, loess.The splash board technique which is currently used for many field experiments, provides none of the basic information necessary for establishing a discharge‐slope relationship.