Application of the Central Composite Design for Predicting the Effects of Surface Rock Fragments on Soil Loss and Surface Flow Velocity

The effect of surface rock fragments on soil erosion processes has been an important challenge in erosion studies during the last two decades. Rock fragment characteristics including size, position and coverage may affect soil erosion and cause a complication in predicting their effects on soil loss. The aim of this study was to model the effects of rock fragment coverage, size and the flow rate on soil loss and surface flow velocity employing response surface method and central composite design. Two sets of run-on simulation experiments were carried out in a laboratory flume (6×0.5m). The range of the independent variables were 0-45 percent for rock fragment coverage, 3-9cm for rock fragments diameter (size) and 1.67-5 cm3cm-1s-1 for flow rate. The second set of experiments used to develop the predictive model based on the central composite design and the results of the first set of experiments were applied to validate the predictive model. Results indicated that the central composite design models have high performance in predicting flow velocity (R2=0.993) and soil loss (R2=0.994). Models validation with the first data set also indicated a good agreement between the predictive values of flow velocity (R2=0.887) and soil loss (R2=0.851) with the experimental values of these two variables. Rock fragments coverage, flow rate and the size of the rock fragments have the highest influence on soil loss and flow velocity, respectively. There was a significant interaction between the flow rate and rock fragment coverage, which should be considered in modeling of their effects. A linear relationship was also observed between the flow velocity and soil loss.