Mass‐Wasting Controls

A variety of conditions affect the development of mass wasting in a particular area. Steep slopes, widely varying altitude ranges (relief), the thickness of the loose earth material, planes of weakness parallel to the slopes, frequent freezing and thawing, high water content in the earth material, dry conditions with occasional heavy rainfall, and sparse vegetation are the factors that contribute to the unstable conditions that result in mass wasting. Movements can be triggered by the motion of earthquakes or too much weight added to the upper part of a slope, such as snowpack.

Angle of repose. The angle of repose is the steepest angle at which loose material will remain in place. It is largely dependent on the size, shape, and roughness of the particles. The angle varies from about 25 degrees to about 40 degrees. If the angle is exceeded by additional sedimentation or tilting, a slide or disturbance will result.

Gravity and friction. Rock particles and soil move downslope because of the forces of gravity. The gravity that acts on an object is a combination of the normal force and the shear force. The normal force is perpendicular to the slope the object rests on, and the shear force is parallel to the surface of the slope (Figure 1). Steep slopes have high shear forces; the steeper the slope, the greater the chance an object will slide. Friction, such as that from a rough bedrock surface, counteracts shear force. Rough, angular particles maintain steeper slopes than smooth rounded particles do. Water acts as a lubricant and reduces the force of friction, increasing the tendency to slide.

Figure 1

Gravitational Forces That Affect Mass Wasting

The shear strength is an object's resistance to movement that needs to be overcome in order to make it move. Shear strength is proportional to how solid the mass is, the density of plant roots, the amount of water present, and the roughness of the particles in the mass.

The effects of water. In addition to acting as a lubricant, water increases the weight of a mass of earth material. Water reduces the shear strength by forcing sediment particles apart through pore pressure, which reduces the friction between the particles. Alternatively, smaller amounts of water that don't completely fill the pores are distributed as thin films around the sediment particles, which are attracted to each other through surface tension, increasing the cohesiveness of the mass. Thus, saturated materials are much more likely to flow than a mass that is only a little wet.