Effect of Slope Length on Soil Loss in Reclaimed Fields of Pliocene Sandy Soil

Abstract

The effect of slope length on soil loss was investigated by field test in a new reclaimed fi eld of Pliocene sandy soil, which was rather erosive, similar to decomposed granite soil in Japan.
The test field was selected to have a gentle graded temporary levee at the lower side, and had various slope lengths from 0 to 80 m in the shape of a trapezoid. The slope gradient was 4 degrees. After smoothing the configurations, the process of soil erosion was observed periodically after rainfall over a 2-month-period. As a result, a shallow gully (rill) occurred on the whole slope. Along the temporary levee, eroded sand accumulated downstream to make the longitudinal gradient uniform gradually. The sedimentation along the temporary levee, which was 15 cm of the maximum depth and 2 m of the maximum width, became smaller in scale because of its secondary movement. It was considered that the gully (rill) development on the slope was not so notably influenced by the sedimentation that the effect of slope length on soil loss was able to be analyzed every each of gully (rill) network.
In test field, rill and interrill erosion was observed predominantly. Such is the form of erosion, soil loss per unit area has been considered to increase as the slope becomes longer. However, there were no obvious relations between soil loss per unit area and slope length. In this paper, the relation between soil loss per unit slope width and slope length was proposed from the viewpoint of planning a graded temporary levee along which much eroded sand accumulates.