TAKE THE A-FRAME: DEBRIS FLOW DURING 1996 RAIN-ON-SNOW EVENT, BLUE MOUNTAINS, WASHINGTON

At 3:30 A.M. on 9 February 1996, four Kooskooskie residents of an A-frame on an alluvial fan heard what sounded like a train. Seconds later the building was surrounded by a debris flow, and its first floor was filled with mud and rocks. Nearby Mill Creek flowed at about 100 m³/s as a result of warm rain on deep snow over frozen ground in the foothills of the Blue Mountains. Thousands of small slumps of loess and colluvium liquefied to become long thin mudflows. Many of the failures were initiated at breaks-in-slope such as agricultural berms and road fills. The A-frame debris flow was fed by about 20 small slumps, and had a total volume of about 17 x 10³ m³. With an area of a little less than 1 km², the mean denudation for this drainage basin was about 18 mm during the one storm. The four largest slumps occurred on logging road fill above a deforested zero-order drainage basin, and evidently formed a debris dam below the road. The high runoff burst the dam causing a debris flow, which ricocheted down a first-order stream, ripped out large stumps, surrounded the A-frame, and buried the main road along Mill Creek. Using the Manning equation with a resistance factor of 0.046, the peak discharge of the debris flow is estimated at 62 m³/s. This equation is designed for water, not a debris flow with higher viscosity. Using velocity-area discharge relations and assuming a velocity range of 3 to 6 m/s, the estimated peak discharge of the debris flow was 28.5 to 57 m³/s.