Lahars

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:

Movie 1

Movie 2

Movie 3

(Movies courtesy of M.T. Dolan)

Lahars usually travel down valleys. They have a wide range of velocities varying from 1 m/s to 40 m/s. The velocity of a lahar depends on the channel width, channel slope, volume of the flow, and grain size composition (Scott, 1989). Lahars can travel long distances. Some lahars have traveled hundreds of kilometers from their source (Scott, 1989). The deposits of a lahar that traveled 60 km from its source at Mount Rainier can be found near the large city of Seattle, Washington (Pierson et al., 1992). The lahar's origin at Mount Rainier helped make that volcano a decade volcano.

Lahars have been known to transport very large boulders. At Mount Pinatubo, boulders measuring 1.5 m long were not uncommon in lahar deposits (Pierson et al., 1992). The lahars from Nevado del Ruiz transported a boulder with a volume of 208 cubic meters, 300 m downstream (Mileti, 1991).

When a lahar travels down valley, the high point of the lahar is usually marked by the mudline it leaves on trees, valley walls, and buildings. This mudline marks the upper limit of how high a lahar will go. This upper limit is important because it defines how high people must go to be out of danger from the lahar. The small eruption of Nevado del Ruiz in 1987, produced large lahars that destroyed the city of Armero. Unfortunately, the 30,000 people who lost their lives might have been saved had they established an appropriate line of communication and evacuated to higher ground (Francis, 1993).

In 1991, Mount Pinatubo erupted. Some of the pyroclastic flows initiating from this eruption were transformed into lahars as they moved downslope through river valleys. Secondary lahars were formed when rain mixing with ash from the eruption became unstable. The formation of these lahars often occured within 30 minutes of as little as 10-15 mm of precipitation falling on the loose ash near the summit of Pinatubo (Primer, 1992). Secondary lahars are still forming today from the unconsolidated ash.

Lahars are extremely dangerous especially to those living in valley areas near a volcano. Lahars can undercut banks and cause houses on those banks to be destroyed. Lahars can bury and destroy manmade structures including roads and bridges. At Nevado del Ruiz, lahars destroyed an entire city; filling the first floor of a hospital with mud, breaking windows, floating cars, and leaving debris in the tops of trees (Mileti, 1991).

Many methods have been used to stop or at least detour a lahar. These methods include building retention basins, alternate channels, tunnels, and concrete structures. Some of these methods have met with success and others have not. The best preventative measure is to establish a warning system. Warning systems that have been used include seismometers that pick up the signal of a lahar as it moves down valley and rain gauges that collect water and warn when lahar formation is possible. But, as was shown at Nevado del Ruiz, warning systems are not enough. Evacuation plans and lines of communication must also be established.

Text by C.M. Riley, Photos by M.T. Dolan