One of the greatest volcanic hazards is lahars. Lahars are similar to pyroclastic flows but contain more water. Lahars form 1) from debris avalanches that contain water from snow and ice which, when released, mixes with loose debris to form a lahar, 2) from pyroclastic flows and surges which release water that mixes with debris, 3) from pyroclastic flows which dilute themselves with river water as they travel downslope, 4) from natural dam failure (i.e. a lava flow dam or crater lake), and 5) from rainfall on loose material such as ash. Lahars that contain 20 to 60 percent sediment are usually very turbulent. Lahars that contain greater than 80 percent sediment usually flow more smoothly (laminar flow). These smooth flowing lahars usually travel much faster than their turbulent counterparts and can float boulders, cars, buildings, and bridges (Philippines, 1992).
To see some movies of lahars click on the following: