Question

Circumstances conducive to future lahars (i.e. substantial volumes of rock, substantial topographic relief, great volumes of ice, and the potential of renewed volcanism) are all present at Mount Rainier. Thus, lahars are a greater threat to communities down-valley from Mt. Rainier than any other volcanic phenomenon. For purposes of lahar hazard assessment, past lahars were studied and classified into 4 categories (Cases): M, I, II, and III (in order of decreasing size and increasing frequency). The largest lahar to occur in the past 10,000 years (5,700 years ago) was 10 times larger than any other and would belong to the 'Case M' category. It flowed down stream 50 miles and its deposits covered an area of about 212 sq. miles in the Puget Sound lowlands, extending north beyond Auburn (see hazards map). It crossed the divide separating the White and Green Rivers then flowed down the Duwamish River. Flows of this magnitude are too infrequent to estimate an annual probability. The smallest lahars (Case III flows) are the most frequent. Moderate to large lahars (Cases II and I) occur less frequently, but can affect larger areas. These flows can extend all the way to Puget Sound (see hazards map).

Answer 1

Answers:

11. Yes, the area where Case M lahar that occured about 5700 years ago, is highly populated today. From the perspective of this map, the area where this lahar occured is the entire northern part i.e. from around King and Pierce counties up to Puget Sound. If it would have happened today, many cities like Enumclaw, Buckley, Auburn, Kent, Orting would have been buried under the mudflow.

12. Sources of water that contribute to such flows include snow and ice present on the top of the volcano, which get melted by hot pyroclastic flows, and also rainwater which occurs during eruption.