At approximately 9 p.m. on December 25, 1980, intense rainfall and extremely wet antecedent conditions combined to trigger a landslide of approximately 5,000 cubic yards at the head of Polallie Creek Canyon on the northeast flank of Mount Hood. The landslide was transformed rapidly into a debris flow, which surged down the channel at velocities between about 40 and 50 ft/s, eroding and incorporating large volumes of channel fill and uprooted vegetation. When it reached the debris fan at the confluence with the East Fork Hood River, the debris flow deposited approximately 100,000 cubic yards of saturated, poorly sorted debris to a maximum thickness of 35 ft, forming a 750-ft-long temporary dam across the channel. Within approximately 12 minutes, a lake of 85 acre-feet formed behind the blockage, breached the dam, and sent a flood wave down the East Fork Hood River. The combined debris flow and flood resulted in one fatality and over $13 million in damage to a highway, bridges, parks, and a water-supply pipeline.
Application of simple momentum- and energy-balance equations, and uniform flow equations resulted in debris flow peak discharges ranging from 50,000 ft3/s to 300,000 ft3/s at different locations in the Polallie Creek Canyon. This wide range is attributed to temporary damming at the boulder- and log-rich flow front in narrow, curving reaches of the channel. When the volume of the solid debris was subtracted out, assuming a minimum peak debris-flow discharge of 100,000 ft3/s at the canyon mouth, a minimum peak-water discharge of 40,000 ft3/s was obtained.
A computer dam-break model simulated peak flow for the outbreak flood on the East Fork Hood River in the range of 20,000 to 30,000 ft3/s using various breach shapes and durations of breach between 5 and 15 minutes. A slope conveyance computation 0.25 mi downstream from the dam gave a peak water discharge (solids subtracted out) for the debris-laden flood of 12,000 to 20,000 ft3/s, depending on the channel roughness coefficient selected.