Inventory and initiation zone characterization of debris flows on Mount St. Helens, Washington initiated during a major storm event in November, 2006 Keith V. Olson ([email protected]) Department of Geology, Portland State University, Portland, OR 97201 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 south-east 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 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. References Burns, S., Pirot, R., Sobieszczyk, S., and Williams, K., 2009, Massive debris flow event on Pacific Northwest volcanoes, November 2006: More to come as the climate changes: Geological Society of America annual meeting Portland, Abstracts With Programs, vol. 41, No. 7, 716 p. Copeland, E. A., 2009, Recent periglacial debris flows from Mount Rainier, Washington: Oregon State University, Master’s thesis. Pirot, R., 2010, Initiation zone characterization of debris flows in November, 2006 on Mount Hood, Oregon: Portland State University, Master’s thesis. Williams, K. J., 2011, Analysis and characterization of debris flows in November, 2006, Mount Adams, Washington: Portland State University, Master’s thesis. Acknowledgements • Advisor: Scott Burns, Portland State University Geology Department • Thesis Committee: Thomas C. Pierson (Cascade Volcano Observatory), Christina L. Hulbe (Portland State University Geology Department) • Field assistants: Jon Weatherford, Harold Shields, Kent Stallings, Matt Gibson, Lina Ma, Rachel Secrest, Craig Olson, Melissa Carley Lahar plain, 1986 8/2005 8/2011 Courtesy of David Anderson Muddy River/Shoestring Glacier Debris flow inventory map showing defined drainage outlines and 2006 event flow paths. The largest and longest debris flow occurred in the South Fork Toutle Drainage in the western quadrant. The depositional area comprised about 975,000 m 2 and a total length of 8,900 m. The next largest debris flow deposit, at 420,000 m 2 , was from the June Lake drainage. The smallest of 300,000 m 2 occurred in the Butte Camp Dome drainage. South Toutle Sheep Creek June Lake Butte Camp Dome Blue Lake Debris flows • Channelized flow path • Initiated by a glacial outburst, landslide, erosion, or dam failure • Transports debris at ~25 mph • Bulks up during transport with addition of soil, rock, organic debris • Creates a lobate debris fan Location • Southwestern WA state • 50 miles (80 km) northeast of Portland Background Geology • Composite volcano • ~300,000 years old • Largely constructed during the past 3,000 years • Eruptive phases: fallout tephra and pyroclastic events then a long hiatus • Three primary river systems drain the volcano: Lewis River, North and South Toutle Rivers, Kalama River Initiation event • Extreme rain event (pineapple express) on Nov. 6-9, 2006 • Over 3x normal daily rainfall within a 24 hour period • Initiated debris flows on all central Cascade volcanoes: Rainier (northern-most) to Jefferson (southern-most) 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 3 4 5 6 7 8 Precipitation, cm Days, November 2006 June Lake Sheep Canyon Lone Pine Spencer Meadow Spirit Lake Swift Creek 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 June Lake Sheep Canyon Lone Pine Spencer Meadow Spirit Lake Swift Creek Total Storm Precipitation, cm INITIATION DEPOSITION TRANSPORTATION Map showing the gridded PRISM average annual precipitation data for 2006 in hundredths of millimeters and as contours. Locations of nearby SNOTEL sites are also shown. The highest average rainfall values for this area are 430 cm. Crater is denoted by black circle. Graph of rainfall measurements during the period of November 3-8 at the SNOTEL sites June Lake (1,049 m), Sheep Canyon (1,216 m), Lone Pine (1,198 m), Spenser Meadow (1,036 m), Spirit Lake (1,072 m), and Swift Creek (1,353 m). June Lake and Swift Creek (see map), experienced the greatest rainfall accumulation (USDA-NRCS, 2007). Initiation Type Landslide Elevation (m) 1,695 Length (m) 8,900 Deposit Area (m 2 ) 975,000 Geology Qvc Glacier No Direct Connection NA Initiation Type Channel Erosion Elevation (m) 1,706 Length (m) 4,800 Deposit Area (m 2 ) NA Geology Qva/Qvc Glacier No Direct Connection NA Initiation Type Channel Erosion Elevation (m) 1,986 Length (m) 6,700 Deposit Area (m 2 ) 300,000 Geology Qvc Glacier No Direct Connection NA Initiation Type Landslide Elevation (m) 1,706 Length (m) 5,300 Deposit Area (m 2 ) - Geology Qva/Qvc Glacier Yes Direct Connection Yes Pine Creek Initiation Type Channel Erosion Elevation (m) 1,403 Length (m) 3,760 Deposit Area (m 2) NA Geology Qvc Glacier No Direct Connection NA Shoestring Glacier Initiation Type Channel Erosion Elevation (m) 1,706 Length (m) 6,400 Deposit Area (m 2 ) 460,000 Geology Qvc Glacier Yes Direct Connection No Muddy River Initiation Type Channel Erosion Elevation (m) 1,706 Length (m) 6,600 Deposit Area (m 2 ) 340,000 Geology Qvc Glacier No Direct Connection NA Basin Comparisons 0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000 6,000,000 7,000,000 8,000,000 9,000,000 10,000,000 Basin Area (m 2 ) Upper Basin Total Basin 0 500 1,000 1,500 2,000 2,500 Basin Elevations (m) Total Basin Low Upper Basin Low Total Basin High 0 500 1,000 1,500 2,000 2,500 Basin Height (m) Upper Basin Total Basin 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 Basin Length (m) Upper Basin Total Basin 0.00 0.10 0.20 0.30 0.40 0.50 0.60 Basin Gradient Upper Basin Total Basin 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 Upper Basin Melton's Ruggedness Number Initiation Type Landslide Elevation (m) 2,124 Length (m) 5,900 Deposit Area (m 2 ) 420,000 Geology Qvc Glacier No Direct Connection NA View of the Blue Lake Debris Flow deposit covering about 350 meters of Road 8123 and all of the Blue Lake trailhead parking area by deposits of up to three meters thick (USDA-FS, 2007). 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 Upper Basin WaDNR Geology (%) Ice Bedrock Unconsolidated 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 Upper Basin Measured Geology (%) Snowpack & Ice Consolidated Bedrock Unconsolidated 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Vegetation (%) 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 Percent Steep Slopes (%) 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 Upper Basin Average Annual Precip. (m) The 8 of 16 drainages that experienced a debris flow in November 2006 are described in detail in the maps and tables below and to the right. Further comparisons are made below and in the basin comparison charts in the central panel. Debris Flows 0 200,000 400,000 600,000 800,000 1,000,000 Deposit (m 2 ) 0 2,000 4,000 6,000 8,000 10,000 Length (m) Glacial Change Mount St. Helens edifice with post-May 18, 1980 eruption glacier distribution and extents (left map). Redrawn from Brugman and Post (1981). On The right are glacier extents and distributions as they existed as of 2009. The glaciers remaining are Nelson, Ape, Shoestring, June (#1), and Swift. The diagrams show the decrease in glacier area and length between 1982 and 2009. 0 100,000 200,000 300,000 Toutle Glacier Talus Glacier Dryer Glacier Snowfield Swift Glacier June Lake Shoestring Glacier Ape Glacier Nelson Glacier Glacier Area (m 2 ) 2009 1983 0 20 40 60 80 100 Toutle Glacier Talus Glacier Dryer Glacier Snowfield Swift Glacier June Lake Shoestring … Ape Glacier Nelson Glacier Glacier Percent Change 1982-2009 (-%) Length Area