What phenomenon occurs when soil movement on a slope leads to a decreased angle of repose?

The phenomenon of decreased angle of repose due to soil movement on a slope is primarily referred to as a slide. This process occurs when the forces attempting to maintain the soil on the slope are overcome by the gravitational forces acting on the soil mass. As soil or rock material shifts, it often leads to an increase in water saturation or destabilization of the material, which decreases the angle at which these materials can naturally rest without sliding.

In a slide, the movement often takes place along a predetermined plane, which differentiates it from other forms of movement such as landslides, where a larger volume of material may be involved, or erosion, which typically describes the gradual wearing away of soil and rock surfaces rather than a more abrupt movement. A slip may also occur in conjunction with a slide but generally refers to more localized material movement rather than the whole mass shifting along a distinct plane.

Overall, within the context of this question, recognizing that a slide represents the specific type of downward movement of soil that results in a change to the angle at which it can remain stable underscores its impact on slope stability and the dynamics of grading and drainage management.