USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
The 1980 Polallie Creek Debris Flow and Subsequent Dam-Break Flood, East Fork
Hood River Basin, Oregon
--
Gary L. Gallino and Thomas C. Pierson, 1984,
The 1980 Polallie Creek Debris Flow and Subsequent Dam-Break
Flood, East Fork Hood River Basin, Oregon:
U.S. Geological Survey Open-File Report 84-578, 37p.
Abstract
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.
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03/03/06, Lyn Topinka