continue like an avalanche. They are composed of at least 50% silt and clay-sized materials and up to 30% water. Because mudflows mobilize a significant amount of sediment, mudflows have higher flow heights than a clear water flood for the same water discharge. Also, sediment within the mudflow increases granular friction within the flow structure of the flow relative to clear water floods, which raises the flow depth for the same water discharge. Difficulty predicting the amount and type of sediment that will be included in a mudflow makes it much more challenging to forecast and engineer structures to protect against mudflow hazards compared to clear water flood hazards. Mudflows are common even in the hills around Los Angeles, California, where they have destroyed many homes built on hillsides without sufficient support after fires destroy vegetation holding the land. On 14 December 1999 in Vargas, Venezuela, a mudflow known as The Vargas tragedy significantly altered more than 60 kilometers (37 mi) of the coastline. It was triggered by heavy rainfall and caused estimated damages of US$1.79 to US$3.5 billion, killed between 10,000 and 30,000 people, forced 85,000 people to evacuate, and led to the complete collapse of the state's infrastructure. Mudflows and landslides Landslide is a more general term than mudflow. It refers to the gravity-driven failure and subsequent movement downslope of any types of surface movement of soil, rock, or other debris. The term incorporates earth slides, rock falls, flows, and mudslides, amongst other categories of hillslope mass movements. They do not have to be as fluid as a mudflow. Mudflows can be caused by unusually heavy rains or a sudden thaw. They consist mainly of mud and water plus fragments of rock and other debris, so they often behave like floods. They can move houses off their foundations or bury a place within minutes because of incredibly strong currents. Mudflow geography When a mudflow occurs it is given four named areas, the 'main scarp', in bigger mudflows the 'upper and lower shelves' and the 'toe'. The main scarp will be the original area of incidence, the toe is the last affected area(s). The upper and lower shelves are located wherever there is a large dip (due to mountain or natural drop) in the mudflow's path. A mudflow can have many shelves. Largest recorded mudflow The world's largest historic subareal (on land) landslide occurred during the 1980 eruption of