Potential Size and Flow Velocity of Lahars at Crater Lake

We can make an educated guess of the size and properties of potential lahars at Crater Lake by analogy with documented events at other volcanoes. The analysis presented in table 1 is based on a 1982 lahar at Mount St. Helens (Scott, 1988) and several lahars from Ruapehu Volcano, New Zealand (Nairn and others, 1979; Ruapehu Surveillance Group, 1996; S.J. Cronin, written commun., 1996).

Ruapehu is an andesite composite volcano with a crater 0.5 km in diameter. A 9x106 m3 (cubic meters) lake (also called Crater Lake) occupied the active vent at 2,540 m altitude prior to 1995. Numerous past eruptions have catastrophically displaced lake water, which probably transformed to debris flows and their hyperconcentrated runouts. At least 1.6x106 m3 of water and lake deposits were ejected by hydromagmatic explosions in a 1975 eruption, resulting in an 8 m fall in lake level (Nairn and others, 1979).

Recorded discharges in downstream drainages reached 5,000 m3/s and flow velocities as high as 12 m/s were estimated (Nairn and others, 1979). Estimated flow volume indicated substantial bulking of the flow. From data for the 1953, 1975, and 1995 flows at Ruapehu Volcano and the 1982 flow at Mount St. Helens, we can synthesize the dynamics and behavior of a flow appropriate for planning and design purposes at Crater Lake (table 1). The greater size of Crater Lake, Oregon, (~9 km, Oregon, vs. 0.5 km diameter, New Zealand) may represent a greater hazard, but more probably the greater volume and depth would absorb a significant part of the energy of an explosive event. There should be ample warning to prepare for noncohesive lahars because any plausible scenario for their generation at Crater Lake is linked to renewed volcanic activity that should be preceded by detectable seismicity.