10.3.3 Mudflows are a highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus, after a heavy rain, streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous. Mudflows involve the downslope movement of soil or unconsolidated, clay-rich sediment in a fluid motion. Mudflows occur when the material within the sloped surface are saturated or nearly saturated with water. The slopes are stable when dry, but become unstable when saturated with water. Mudflows occur on steep slopes where vegetation is not sufficient to prevent rapid erosion but can occur on gentle slopes if other conditions are met. Other factors are heavy precipitation in short periods and an easily erodible source material. The speeds of mud flow at speeds as great as 100 km per hour and can cause great damage to life and property (figure 11). Boulders as large as houses have been moved by mudflows.
Figure 11: Example shows the damage of house and other construciton due to mud flow