J. L. M. P. de Lima

The influence of the pattern of moving rainstorms on overland flow

This study emphasizes the importance of spatial rainfall intensity patterns of moving rainstorms on overland flow. A simple numerical model, based on the non-linear kinematic wave, was used for comparing the results for hypothetical storms moving up and down an impervious plane surface. Simulations were undertaken by varying the storm pattern, length, speed and direction. No account was made for time varying losses, such as infiltration, evaporation, etc. The results indicate significant differences in peak discharges and hydrograph shapes for moving storms of various patterns. The sensitivity of runoff to storm patterns decreases as storm speed increases.

The influence of rainfall on transport of beach sand by wind.

This paper deals with the effect of rainfall on the process of wind erosion of beach sands and presents results from both field and wind tunnel experiments. Although sediment transport by splash is of secondary importance on coastal dunes, splash-saltation processes can move sediments in conditions where no motion is predicted by aeolian processes. The effect of raindrop impact on the movement of soil particles by wind was measured on a sand beach plain using an acoustic sediment sampler. In general, an increase of particle movement by wind at the sensor heights was observed during rainfall. Rainfall also affected the wind erosion process during and after rain by changing the cohesive conditions of the surface. The influence of the surface moisture content on the initiation of wind erosion and on the vertical distribution of transported sand particles was studied in a wind tunnel. Moisture significantly increased threshold wind velocities for the initiation of sediment transport and modified vertical sediment profiles.

Splash-saltation transport under wind-driven rain.

Abstract Soil detachment and transport by wind- driven rain differs from that by rainless wind and from that by windless rainfall. This study deals with field measurements of particle movement during periods in which rain and wind coincide. The effect of rainfall on the movement of soil particles by wind is analyzed using a sediment sensor called SALTIPHONE. Some results are presented of field experiments at two sites in the Netherlands: a cutover peat soil and a beach sand. Results show an increase of particle movement at the sensor height during rainfall. This can be attributed to the combined action of saltation and rain-induced uplift of soil particles and subsequent transport by wind.

The effect of oblique rain on inclined surfaces: a nomograph for the rain-gauge correction factor.

Inclined rainfall leads to errors in the assessment of effective precipitation in areas with rugged relief. This affects many hydrological studies such as hydrological forecasts, water erosion, determination of conditions for flash flood formation, determination of cropping conditions, etc. In this paper a nomograph is presented for the rain-gauge correction factor (Cgau), by which rainfall measured in a standard horizontal rain-gauge (Pgau) may be corrected to obtain the rainfall flux actually received on the inclined surface under study (Pact = Cgau × Pgau). The correction factor is assumed to be a function of wind (with constant wind velocity), type of rainfall, and inclination and orientation of the sloping surface with respect to the oblique rain.

Laboratory experiments on the influence of storm movement on overland flow

This study presents the results of laboratory experiments, conducted on an impermeable smooth plane surface with a movable sprinkling-type rainfall simulator, simulating a moving storm. In order to assess the effect of storm movement while eliminating variations in other factors that also influence the runoff response, the only parameters that were varied were storm velocity and direction. The results indicate considerable differences in runoff volumes and peaks and in overland flow hydrograph shapes, for storms moving upstream and downstream at differing velocities.

A mathematical model for evaluating the effect of wind on downward-spraying rainfall simulators

Abstract Rainfall simulators are used to study a variety of different processes (e.g., water erosion, infiltration, overland flow, irrigation, movement of agrochemicals, etc.). Wind affects field experiments that make use of rainfall simulators. Water-drop trajectories and velocities are altered, affecting water application and kinetic energy distributions. In this study, a three-dimensional numerical model was developed from the movement of individual drops after their release from the nozzle of a downward-spraying rainfall simulator. Drag forces, wind and gravity affect the original momentum of a single drop. Water application and kinetic energy were estimated from the coupling of a hydrodynamic model for drop movement, a drop generator representing a single full-cone spray nozzle, and an appropriate interception algorithm at the soil surface. The mathematical model should facilitate the selection of single full-cone spray nozzles and the size and configuration of the spray area for rainfall simulation in order to achieve high application uniformity values on the plot area. It can contribute to the adequate choice of nozzles as well as operating conditions necessary for laboratory and field purposes. Laboratory and field experiments were conducted to evaluate the adequacy of the proposed methodology.

Raindrop splash anisotropy: Slope, wind and overland flow velocity effects

Summary This study emphasises the importance of raindrop splash anisotropy as a factor affecting splash erosion. Slope, wind and overland flow velocity were found to be the factors contributing to the anisotropy of the splash. With the help of photography the asymmetry of the corona shape of the raindrop splash was recorded for the different factors. Only one equivalent drop diameter (3.5 mm) was studied.

Water depths at the upper boundary for overland flow on small gradients

In Hortonian overland flow caused by rainfall on a sloping plane, the usual presumption is that water depth is zero at the upper end of the slope. It is clear that this is impossible at zero slope or for very slight gradients. A numerical method was used to calculate this water depth in the final equilibrium (steady-state) situation. Depth appears to be finite but negligible at slopes of more than 0.5%. Laboratory experiments, conducted on an impermeable plane surface with a rainfall simulator, confirmed the numerical results.

An analytical kinematic model for the rising limb of overland flow on infiltrating parabolic shaped surfaces

Abstract Using the kinematic wave theory and Zarmis hypothesis, an analytical solution for overland flow over an infiltrating, parabolic shaped surface (concave or convex surfaces, that may be represented by a quadratic equation) is presented. The volocity of the water flow is assumed to be independent of time. The analytical solution developed is easier to utilize than numerical simulation and can be used to calibrate numerical methods devised for more complicated cases. An illustration is presented.

Incorporating the Effect of Moving Storms into Hillslope Hydrology: Results from a Multiple-Slope Soil Flume

The importance of storm movement on surface flow has long been acknowledged at varying scales ranging from headwater scales to drainage basins. Different studies have shown that moving rainstorms substantially affect surface flow hydrographs, although some of the results reported need further discussion. The main objective of this study is to quantify at the hillslope scale the hydrologic response to both non-moving and moving rainstorms in terms of discharge and soil loss. Controlled laboratory experiments were carried out using a multiple-slope soil flume subjected to a movable sprinkling-type rainfall simulator. To simulate moving rainstorms, the rainfall simulator was moved upstream and downstream over the soil surface at different speeds. Results show that the direction of storm movement, especially for very high intensity rainfall events, significantly affected runoff and water erosion with downstream-moving storms, causing higher peak runoff and erosion than did upstream-moving storms.