After the devastating mudslide in Washington last month I decided to find out a little more about what causes mudslides. Mudslides occur when porous soil becomes over saturated with water and becomes to heavy for the soil beneath to support it. The mass of the soil increases as it fills with water causing a greater force to be applied to the lower layers causing the bottom of a hill to explode outward. The extra water in the soil also decreases friction causing it to slide more easily down a slope. Mudslides are most common in areas with porous soil which is usually caused by powerful storms that deposit sediment followed by dry periods in which the sediment dries. This creates a structure that is stable but is severely weakened by future rainfall. The problem is compounded if the trees in an area are cut down as seems to be the case in the Washington incident.