Laboratory Experiments for Sediment Deposition and Detachment in a Rill Under Different Vertical Hydraulic Gradient Conditions.

Soil erosion has led to agricultural and environmental problems throughout the world. And soil erosion is a fundamental phenomenon of the material movements in the nature. In this study, laboratory experiments were conducted to estimate the sediment deposition and detachment in a rill under different vertical hydraulic gradient conditions. Experiments were also focused about sediment redetachment subsequent to deposition.

Miami clay loam was used for this experiments. Sediment was supplied using sediment box (1.8m length, 1.2m width, and 5% slope) with rainfall simulator. The amount of supplied sediment was controlled by changing the soil surface cover of the box. Experimental rill was 5m length, 0.25m width, 2.3% slope and it can be controllable soil hydraulic gradient, drainage condition or seepage condition. Flow rate at the rill was about 0.10 L/s and seepage rate was 0.005 L/s. Sediment deposition was estimated as the difference of sediment supply and sediment runoff in the rill. Sediment detachment was divided into erosion of original soil and erosion of deposited soil.

Deposition was increased with increasing of sediment supply under the both hydraulic gradient conditions. Deposition under the drainage condition was larger comparing seepage condition. On the other hand, detachment under the seepage condition was larger comparing drainage condition. Sediment transport capacity, Tc was 0.66 g s^-1 m^-1 under drainage condition and 3.95 g s^-1 m^-1 under seepage condition. Detachment rate was increased with increasing previous deposition rate under the drainage condition. Detached sediment was included redetached sediment from deposited layer under this condition. The ratios, redetachment/deposition were from 18% to 50%. On the other hand, redetachment under the seepage condition was not shown obviously. It was because the erosion of original soil was much larger than the erosion of deposited soil. From the sediment particle size distribution measurements, sediment, up to 1000μm diameter was deposited in the rill under drainage condition. After the deposition, particles from 0.5μm to 100μm diameter were redetached under this condition.