BEGINNING OF MOTION FOR SELECTED UNANCHORED RESIDUE MATERIALS

Conservation tillage systems help to maintain residue materials from the previous crop on the soil surface. The potential for serious erosion may exist if crop residues are removed by overland flow. This study is conducted to identify the hydraulic conditions required to initiate residue movement by overland flow. Corn, cotton, peanut, pine needles, sorghum, sunflower, and wheat residue are placed in a flume on smooth and sand surfaces, and flow is then introduced in progressive increments. The discharge rate and flow velocity required to initiate residue movement are identified. Hydraulic measurements are used to calculate the ratio of critical flow depth to residue diameter, critical Reynolds number, critical shear stress, dimensionless shear stress, and boundary Reynolds number. Regression equations are developed to relate dimensionless shear stress to boundary Reynolds number. Close agreement is found between predicted and actual dimensionless shear stress. If residue diameter is known, the regression equations can be used to estimate the beginning of motion for other residue materials. Information obtained in this study can be used to help identify proper residue management practices for conservation tillage systems.