Inventory and Initiation Zone Characterization of Debris Flows on Mount St. Helens, Washington Initiated during a Major Storm Event in November, 2006

Abstract

The heavy precipitation event of November 3-8, 2006 dropped over 60 cm of rain onto the bare southern slopes of Mount St. Helens and generated debris flows in eight of the sixteen drainages outside the 1980 debris avalanche zone. Debris flows occurred on the upper catchments of the Muddy River, Shoestring Glacier, Pine Creek, June Lake, Butte Camp Dome, Blue Lake, Sheep Creek, and South Fork Toutle River. Debris flows were clustered on the west and southeast sides of the mountain. Of the eight debris flows, three were initiated by landslides, while five were initiated by headward or channel erosion. Six debris flows were initiated in deposits mapped as Holocene volcaniclastic deposits, while two were in 1980 pyroclastics on andesite flows. The largest (~975,000 m 2) and longest (~8,900 m) debris flow was initiated by landslides in the upper South Fork Toutle River Drainage. The average debris flow initiation zone elevation was 1,750 m, with clusters around 1,700 m and 2,000 m elevation. The lower cluster is associated with basins that host modern or historic glaciers, while the upper is possibly associated with recent pyroclastic deposits. Upper drainages with debris flows averaged 41% slopes steeper than 33 degrees, while those without debris flows averaged 34%. The geology of upper basins with debris flows averaged 6% snow and ice cover, 21% consolidated bedrock, and 74% unconsolidated deposits. Geology of basins without debris flows averaged 3% snow and ice cover, 27% bedrock, and 67% unconsolidated deposits. Drainages with debris flows averaged an 89% loss of glacier area between 1998 and 2009, while those without debris flows lost 68%. Further comparing glacier coverage during that period found that only five of ten glaciers still ii existed in 2009. On average, the glaciers had reduced in area by 67%, decreased in length by 36%, and retreated by an average of 471 m during that period. Basin attributes were measured or calculated in order to construct a predictive debris flow model based on that of Pirot (2010) using multiple logistic regression. The most significant factors were the percentage of slopes steeper than 33 degrees, unconsolidated deposits in the upper basin, and average annual rainfall. These factors predicted the 2006 debris flows with an accuracy of 94% in a debris flow susceptibility map for Mount St. Helens. iii ACKNOWLEDGEMENTS To begin, I would like to thank my family and friends for their continuing encouragement and support in my study of geology and of this thesis project. Special thanks to my brother, Craig Olson, and girlfriend, Melissa Carley, who both allowed themselves to be talked into spending very long days climbing on Mount St. Helens. My thesis advisor, Scott Burns, deserves much thanks for his encouragement and direction, as well as his infectious enthusiasm for this field. Also, my many field assistants deserve recognition for their vital support and efforts while in the field. My assistants for 2010 included

Debris Flow Susceptibility Map for Mount Rainier, Washington Based on Debris Flow Initiation Zone Characteristics from the November, 2006 Climate Event in the Cascade Mountains

“…Adams. Olson (2012) found percent steep slopes, percent unconsolidated material, and average annual precipitation to be the most significant factors on Mt. St. Helens.…”

“…My objective is to combine the methodologies from Pirot (2010), Williams 2011, and Olson (2012) and their findings to develop a susceptibility map for debris flows on Mt. Rainier, initiated by rainstorm events like the 2006 event.…”

“…Combine the attributes from this study and those from Pirot (2010), Williams (2011), and Olson (2012), perform a regression analysis on the data and develop a model for these four Cascade Range volcanoes.…”

“…Low snow pack will melt with warm rainfall and increase runoff. Increased precipitation and lack of snow cover to absorb the rain in the Cascades initiated debris flows on at least five volcanoes in 2006: Mount Jefferson (Sobieszczyk et al, 2009), Mount St. Helens (Olson, 2012), Mount Rainier (Copeland, 2009;Legg, 2013), Mount Adams (Williams, 2011), and Mount Hood (Pirot, 2010).Studies done by Pirot (2010), Williams (2011), and Olson (2012) developed methodologies to determine which drainages are most susceptible to debris flows and develop potential forecasting methods. Copeland (2009) and Legg (2013) looked specifically at recent debris flow history on Mt.…”

“…Hood, and Mt. Adams raised the predictive accuracy from 69% (Olson, 2012) to 77%. This model determined that percent glacier/ice and percent vegetation were the most significant.…”