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<br />124"
<br />
<br />122"
<br />
<br />42'
<br />
<br />
<br />
<br />WASHINGTON
<br />
<br />Seattle
<br />
<br />46' ~
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<br />44' "-
<br /> OREGON
<br />
<br />o 100 MILES
<br />I I I 1 I r
<br />11111
<br />o 100 KILOMETERS
<br />
<br />Fig_ I. Location map of Centralia. Washington, and Porter
<br />Hill, Oregon, dam failures.
<br />
<br />cracked and deteriorated concrete panel seams into the
<br />fractured bedrock foundation; (b) stress patterns caused by
<br />the quarterly draining and refilling of the reservoir; or (c)
<br />a recent increase of 0.6 m in the water level in the
<br />reservoir. Sedimentological characteristics of deposits,
<br />high-water mark distribution, transport of unbroken beer
<br />bottles, and landforms preserved on the valley floor
<br />indicated the dam-failure flood consisted initially of a
<br />debris flow that deposited coarse gravel and boulders along
<br />the channel and floodplain. The debris flow had an
<br />estimated volume of 1,800 m', and was immediately
<br />followed by a water flood that achieved a stage about 0.3-
<br />0.5 m higher than the debris flow.
<br />A four-section slope-area indirect discharge estimate was
<br />made on Oct. 10, 1991, five days after the dam failure, at
<br />a site 275 m below the emptied reservoir (Figure 3)_ Scour
<br />and deposition, a steep channel slope of 0.09, and
<br />uncertain roughness coefficients all contribute to some
<br />uncertainty in the final peak-discharge estimate of 71 m'/s.
<br />An official for the city of Centralia, responsible for the
<br />operation of the reservoir, reported that the reservoir
<br />drained in three to five minutes. At a constant discharge
<br />rate of 71 m'ls, it would take 3.1 minutes to drain the
<br />
<br />COSTA AND O'CONNOR 47
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<br />reservoir.
<br />Several pieces of data about the dam-failure and
<br />resulting flood, such as reservoir volume, reports of
<br />drainage time, peak discharge calculations, and average
<br />velocity of the flood, allow construction of a flood
<br />hYdrograph. Using the average peak-flow velocity of 4.2
<br />m/s calculated for the slope area reach, it would take 1.1
<br />minutes for the flood to travel 275 meters from the
<br />reservoir to the measurement site. If a triangular-shaped
<br />hydrograph is assumed, considering the 13,250 m'
<br />reservoir volume and the 71 m'ls peak discharge, the
<br />duration of the flood past the slope-area site would be
<br />about 6.2 minutes. Consequently, after about 7.3 minutes
<br />from the time of the reservoir failure, the flood had passed
<br />the indirect-discharge measurement site, and moved into
<br />the city (Figure 4).
<br />
<br />2.2 Failure of Porter Hill dam near Roseburg, Oregon
<br />
<br />
<br />A private landowner constructed several small earthen
<br />dams to collect spring discharge on the flanks of Porter
<br />Hill in southwestern Oregon. The dams blocked an
<br />unnamed tributary into Olalla Creek, which flows into
<br />Lookingglass Creek and eventually into the South Umpqua
<br />River. The Porter Hill dam is the largest of these dams,
<br />and is located in the NWI/4, SEI/4, sec_ 32, T28S, R7W
<br />(Tenmile Quadrangle, Oregon). Porter Hill is underlain by
<br />rhythmically bedded sandstone and siltstone that has been
<br />folded, faulted, and weathered [Baldwin, 1974] (Figure 1).
<br />The Porter Hill dam was 5.8 m high, about 20 m wide,
<br />and stored an estimated 15,000 m' of water at the time of
<br />failure. The earthen dam was constructed of local clayey
<br />residuum. The exact date of the dam failure is unknown,
<br />but it is believed to have failed on or about February 27,
<br />1993 (John FaIk, Oregon State Dam Safety Coordinator,
<br />personal communication, April 15, 1993). The dam
<br />apparently failed during a rainstorm when a large slump on
<br />the downstream face of the dam opened a breach with a top
<br />width of about 20 m (Figure 5). The large slump led to a
<br />near instantaneous failure of the dam, and the release of
<br />about 15,000 m' of water down a steep upland valley.
<br />Peak discharge from the dam failure was estimated to
<br />have been about 30 m'ls at a location about 150 m
<br />downstream from the dam, using the slope-conveyance
<br />method (Figure 6). If a triangular-shaped hydrograph is
<br />assumed, considering the 15,000 m' reservoir volume and
<br />the 30 m'ls peak discharge, the duration of the flood past
<br />the slope-conveyance site would be about 16.6 minutes. A
<br />reconstructed hydro graph for the flood is shown in Figure
<br />4. Data for the two dams are summarized in Table I.
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