Soil Flume Research Articles

Numerical modeling of surface runoff and erosion due to moving rainstorms at the drainage basin scale

Summary A physically-based distributed erosion model (MEFIDIS) was applied to evaluate the consequences of storm movement on runoff and erosion from the Alenquer basin in Portugal. Controlled soil flume laboratory experiments were also used to test the model. Nine synthetic circular storms were used, combining three storm diameters (0.5, 1 and 2 times the Alenquer basin’s axial length) with three speeds of storm movement (0.5, 1 and 2 m/s); storm intensities were synthesized in order to maintain a constant rainfall depth of 50 mm. The model was applied to storms moving downstream as well as upstream along the basin’s axis. In all tests, downstream-moving storms caused significantly higher peak runoff (56.5%) and net erosion (9.1%) than did upstream-moving storms. The consequences for peak runoff were amplified as the storm intensity increased. The hydrograph shapes were also different: for downstream-moving storms, runoff started later and the rising limb was steeper, whereas for upstream moving storms, runoff started early and the rising limb was less steep. Both laboratory and model simulations on the Alenquer basin showed that the direction of storm movement, especially in case of extreme rainfall events, significantly affected runoff and soil loss.

A fractional dispersion model for overland solute transport

Using the kinematic‐wave overland flow equation and a fractional dispersion‐advection equation, a process‐oriented, physically‐based model is developed for overland solute transport. Two scenarios, one consisting of downslope and the other of upslope rainstorm movements, are considered for numerical computations. Under these conditions, the hydrograph displays a long‐tailed distribution due to the variation in flow velocity in both time and distance. The solute transport exhibits a complex behavior. Pollutographs are characterized by a steep rising limb, with a peak, and a long, stretched receding limb; whereas the solute concentration distributions feature a rapid receding limb followed by a long stretched rising limb. Downslope moving storms cause much higher peak in both hydrographs and pollutographs than do upslope moving storms. Both hydrographs and the pollutographs predicted by the fractional dispersion model are in good agreement with the data measured experimentally using a soil flume and a moving rainfall simulator.

Effects of cattle manure on erosion rates and runoff water pollution by faecal coliforms

The large quantities of slurry and manure that are produced annually in many areas in which cattle are raised could be an important source of organic matter and nutrients for agriculture. However, the benefits of waste recycling may be partially offset by the risk of water pollution associated with runoff from the fields to which slurry or manure has been applied. In this paper, the effects of cattle manure application on soil erosion rates and runoff and on surface water pollution by faecal coliforms are analysed. Rainfall simulations at a rate of 70 mm h −1 were conducted in a sandy loam soil packed into soil flumes (2.5 m long×1 m wide) at a bulk density of 1400 kg m −3, with and without cattle slurry manure applied on the surface. For each simulation, sediment and runoff rates were analysed and in those simulations with applied slurry, presumptive faecal coliform (PFC) concentrations in the runoff were evaluated. The application of slurry on the soil surface appeared to have a protective effect on the soils, reducing soil detachment by up to 70% but increasing runoff volume by up to 30%. This practice implies an important source of pollution for surface waters especially if rainfall takes place within a short period after application. The concentrations of micro-organisms (presumptive faecal coliforms (PFCs)) found in water runoff ranged from 1.9×10 4 to 1.1×10 6 PFC 100 mL −1, depending on the initial concentration in the slurry, and they were particularly high during the first phases of the rainfall event. The result indicates a strong relationship between the faecal coliforms transported by runoff and the organic matter in the sediment.

Transport rate-based model for overland flow and solute transport: Parameter estimation and process simulation

A one-dimensional mathematical model, termed a transport rate-based model, is developed for solute transport over infiltrating soil slopes under constant rainfall and declining sources. The model is comprised of (1) the kinematic-wave equation for overland flow, (2) a transport rate-based advection equation for overland solute transport, (3) a moment-based method for estimation of the parameters involved in the flow and solute transport equations, and (4) a semi-Lagrangian algorithm for numerical solution of the solute transport equation. Data from a single soil flume experiment under constant rainfall established the proof-of-concept for this new model. Sodium chloride was applied to the soil surface to simulate the presence of a declining diffuse pollutant. The parameters involved in the flow equation were found to take on the values that correspond to turbulent flow. With these turbulent flow parameters the simulated hydrograph displayed an initial rising limb, followed by a constant flow discharge. The profile of solute concentration exhibits a steep receding limb transitioning into an elongated tail. The solute transport rate follows a non-Gaussian distribution that does not appear to have been derived before. These theoretical hydrographs and pollutographs are in good agreement with those measured in laboratory, demonstrating the laboratory proof-of-concept for the transport rate-based model on soil and pavement blocks.

The effect of incorporating slurries on the transport of faecal coliforms in overland flow

Abstract. Rainfall simulation experiments on a laboratory soil flume were conducted to test the hypothesis that the incorporation of slurry into the soil would reduce bacterial transport in overland flow. Presumptive faecal coliform (PFCs) concentrations were higher in the runoff from the surface applied treatment than from the incorporated treatments. The transport of PFCs and organic sediment were strongly correlated, with values of r ranging from 0.72 to 0.91.

The influence of storm movement on water erosion: storm direction and velocity effects

Although the problem of storm movement affecting flows (shape of the hydrograph and peak discharges) has been recognised for a long time, most overland flow and water erosion studies do not take into account the effect on the runoff response caused by the movement of the storm across the catchment. Ignoring of the storm movement can result in considerable over- and underestimation of runoff volumes and peaks, and associated soil loss by sheet erosion. This work shows the results of laboratory experiments that were undertaken to study the effect of moving storms on the water erosion process. The experiments were carried out using a soil flume adjustable to different slopes and a movable sprinkling-type rainfall simulator. Both the effects of storm velocity and direction, and surface slope were studied. To simulate moving rainstorms, the rainfall simulator was moved upstream and downstream over the soil surface. The results show that the storm direction and velocity strongly affect the water erosion process. The soil loss caused by the downstream moving rainstorms is higher than that caused by the identical upstream moving rainfall storms.

Numerical simulation of NAPL flow in the subsurface

A three-dimensional, three-phase numerical model is presented for simulating the movement of immiscible fluids, including nonaqueous-phase liquids (NAPLs), through porous media. The model is designed to simulate soil flume experiments and for practical application to a wide variety of contamination scenarios involving light or dense NAPLs in heterogeneous subsurface systems. The model is derived for the three-phase flow of water, NAPL, and air in porous media. The basic governing equations are based upon the mass conservation of the constitutents within the phases. The descretization chosen to transform the governing equations into the approximating equations, although logically regular, is very general. The approximating equations are a set of simultaneous coupled nonlinear equations which are solved by the Newton-Raphson method. The linear system solutions needed for the Newton-Raphson method are obtained using a matrix of preconditioner/accelerator iterative methods. Because of the special way the governing equations are implemented, the model is capable of simulating many of the phenomena considered necessary for the sucessful simulation of field problems including entry pressure phenomena, entrapment, and preferential flow paths. The model is verified by comparing it with several exact analytic test solutions and three soil flume experiments involving the introduction and movement of light nonaqueous-phase liquid (LNAPL) or dense nonaqueous-phase liquid (DNAPL) in heterogeneous sand containing a watertable.

Surface Sealing and Hydraulic Conductances under Varying‐Intensity Rains

AbstractIn the past, investigations on surface seals developing under simulated rains usually were performed with uniform rainfall intensities. Recent studies, however, showed that varying‐intensity rains affect erosion and volumes of runoff. We conducted a study on surface sealing under varying‐intensity rains using physical and morphological methods. Four 1‐h‐duration rainfall patterns were used: S1, S2, S3 and S4, with mean rainfall intensities of 83, 69, 65, and 51 mm h‐1, respectively, and highest intensities of 148, 120, 111, and 100 mm h‐1, respectively. Two experiments were performed in triplicate: (i) in packed soil columns, pressure heads at 1‐cm depth were measured, and runoff (Rt) and wash sediment (St) collected at short time intervals during S1, S3, and S4 patterns; and (ii) packed shallow soil flumes were successively exposed to 10, 15, 30, 40, and 60 min of S2 and S4 patterns, and the surface was then characterized by micromorphological analysis of thin sections (S2) and top‐view photographs (S2 and S4). The largest differences between rainfall patterns occurred during the intensity peaks (10–30 min): at the end of these periods, measured surface seal hydraulic conductances (β) attained values of 0.80, 0.25, and 0.51 h‐1 for S1, S3, and S4, respectively. These relative β values are thought to be due to partially open surfaces as a result of microrill erosion (S1), complete sealing without erosion (S3), and incomplete sealing (S4). Presented Rt, St, and morphological data support these conclusions. After 5 min of the intensity peaks of S2, surface seals attained maximum observed thickness and minimum macroporosity (>30 µm). Planar voids (>500 µm) were eliminated at this point. Under continuing rain, erosion reduced the seal thickness and increased macroporosity, mainly as planar voids.

Overland Flow on an Infiltrating Surface

The partial differential equation for vertical, one‐phase, unsaturated moisture flow in soils is employed as a mathematical model for infiltration rate. Solution of this equation for the rainfall‐ponding upper boundary condition is proposed as a sensitive means to describe infiltration rate as a dependent upper boundary condition. A nonlinear Crank‐Nicholson implicit finite difference scheme is used to develop a solution to this equation that predicts infiltration under realistic upper boundary and soil matrix conditions. The kinematic wave approximation to the equations of unsteady overland flow on cascaded planes is solved by a second order explicit difference scheme. The difference equations of infiltration and overland flow are then combined into a model for a simple watershed that employs computational logic so that boundary conditions match at the soil surface. The mathematical model is tested by comparison with data from a 40‐foot laboratory soil flume fitted with a rainfall simulator and with data from the USDA Agricultural Research Service experimental watershed at Hastings, Nebraska. Good agreement is obtained between measured and predicted hydrographs, although there are some differences in recession lengths. The results indicate that a theoretically based model can be used to describe simple watershed response when appropriate physical parameters can be obtained.