Overland Flow Experiments Research Articles

Estimation of erosion model erodibility parameters from media properties

The aim of this research was to enable erodibility values for hillslope-scale erosion prediction models to be determined from easily measured media properties. Simulated rainfall and overland flow experiments were carried out on 34 soils and overburdens from 15 Queensland open-cut coal mines at The University of Queensland Erosion Processes Laboratory. Properties of the 34 media determined included aggregate stability, Atterberg limits, bulk density, cation exchange capacity, dispersion ratios, electrical conductivity, exchangeable sodium percentage, organic carbon content, pH, texture, and water content at field capacity and wilting point. Correlation and stepwise multiple regression procedures were used to determine those media properties that could best be used to predict rill and interill erodibility. Correlations between media properties and sediment delivery at each of 5, 10, 15, 20, and 30% slope revealed that different media properties were correlated with erosion rates at different slopes. A media property could show a strong correlation with erodibility at 30% slope, and a low correlation at 5% slope. Splitting the data set into soils only, and overburdens only, showed that properties that were positively correlated with erosion rates for one group could be negatively correlated for the other group. Therefore, in this study, erodibility could not be explicitly linked to one set of media properties for all medium types and erosive conditions. It was concluded that a single regression equation could not be used to predict erodibility under all conditions. Instead, 4 equations were developed to predict rill and interill erodibility, for soils and overburdens separately. The need for separate regression equations was attributed to the presence of different erosive sub-processes for specific combinations of medium type and slope gradient.

Resistance to Overland Flow on Desert Pavement and Its Implications for Sediment Transport Modeling

Seventy‐three overland flow experiments were performed on eight runoff plots located on a gentle piedmont hillslope in Walnut Gulch Experimental Watershed, southern Arizona. These plots were sited on gravel‐covered desert pavement surfaces where overland flow occurs during natural rainstorms. Multiple regression analyses indicate that the Darcy‐Weisbach friction factor ƒƒ can be well predicted by the Reynolds number and gravel concentration and size, with gravel concentration being by far the single best predictor. Grain resistance ƒƒ', calculated using Savat's algorithm, is found to be typically less than 10% of ƒƒ. Given that sediment transport capacity is controlled by grain shear stress τ' rather than total shear stress τ and that the ratio of τ' to τ is the same as that of ƒƒ to ƒƒ, sediment transport modeling on the gravel‐covered hillslope under study must be based on τ' rather than τ to avoid gross overestimation of transport capacity. This conclusion is relevant to all gravel‐covered desert hillslopes and probably also to most humid hillslopes and agricultural lands where plant stems, litter, and tilled surfaces replace gravel as sources of form resistance and ensure that grain resistance remains relatively small.