Mud flows occur very often on steep slopes covered by poorly permeable quaternary layers (clay-rich moraines or loam deposits). Concave areas in a slope with the concentration of surface discharge or the occurrence of surface seepage are particularly prone. Mud flows are normally triggered by intense rainfall. The volume of the moving mixture is rather small. However, the high water content results in high velocities (more than 30 km/h). Therefore, a mud flow can be very destructive. The development of the mud flow into a debris flow is possible if the process reaches a steep ravine or creek.
Terrain movements with large erosional scars, debris deposition and water cause damage to or destruction of buildings. Human beings may be injured or even killed. Very frequently roads and railway lines are interrupted. The determining factors for the extent of damage are the velocity, the flow depth, and the volume of the slope failure.