El Guapo Dam, which impounded a reservoir volume of over 32,000 acre-feet, was constructed from 1975 to 1980 on the Rio Guapo in the state of Miranda in northern Venezuela. Over about a two-week period in December 1999, heavy rainfall in the mountains of northern Venezuela were relentless. On December 14, 1999, the reservoir at El Guapo Dam rose to about 3 feet above normal pool, but still had an additional 15 feet of freeboard . However, due to additional heavy rainfall overnight and the extremely saturated ground conditions in the watershed, the reservoir levels quickly rose to within about 2 feet of the dam crest by the morning of December 15. By December 16, the reservoir rose to within a 1/2 foot of the dam crest. Due to the high-water surface elevation and increased runoff, the spillway chute sidewalls began to overtop from the record amount of discharge. Erosion of the fill behind the walls began to occur almost immediately. Within a matter of hours and with no means to further manage the enormous inflows, the areas below the dam were evacuated. The reservoir levels began to decrease as the morning progressed; however, additional rainfall in the afternoon caused the reservoir to rise again sharply. The spillway chute sidewalls continued to overtop, and the erosion quickly accelerated beneath and up the chute. As the chute failed into the scoured areas, the head cutting progressed and ultimately led to a breach of the spillway control section at about 5 PM on December 16. The failure of El Guapo Dam has many striking similarities to the recent spillway incident in Oroville Dam. This presentation will explore those similarities, reflect on emergency preparedness and response, and re-examine many of the potential failure modes and critical design and construction considerations for these structures.
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