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Pantano de Puentes Dam Failure
The Pantano de Puentes Dam Failure was a major catastrophe in Lorca, Spain in 1802 which left more than 600 people dead. It is widely believed that the reason for the failure was internal erosion, as the dam was built on sandy soil.

History
On February 11, 1785, King Carlos III of Spain gave permission for the construction of a dam in Lorca to create a reservoir to be used for irrigation. The project was to be headed by Jerome Martinez-Lara and Antonio Roblez-Vives and paid for by the royal treasury. The dam was constructed between 1785 and 1791, and at its completion it was the longest dam in Spain, at 286 meters across. The dam was designed by Jerónimo Ortiz de Lara and built on the Guadalentin riverbed. This location was chosen since a dam had already been built there in December of 1647 but had collapsed less than a year later in August of 1648.

Collapse
At around 2:30 p.m. on Sunday April 30th, 1802, a dam laborer noticed that water had begun to spurt out and stream into the woods near the base of the dam. According to eye-witness accounts “’water of an exceedingly red color was boiling up in great quantities.’” At this point a messenger was sent to warn Antonio Roblez-Vives, the works commissioner, on his ranch in Palomares, located between the dam and Lorca, the city downstream. There are reports that many employees and neighbors of the dam had come to Roblez-Vives that morning after hearing loud crunching sounds coming from the wall of the dam during the night; he had offered to inspect the dam later that evening to calm their anxieties.

Unfortunately, at around 3:00 p.m., loud explosions were heard and a flood of water carrying the wooden beams of the structure’s foundation was released. The torrent of water and dam materials rushed downstream, picking up stones and trees and catching up to Roblez-Vives who was making his way to Lorca after receiving the initial news. The hole in the structure was so large that it was less than an hour later that the reservoir had been emptied completely.

A report sent to the Municipality of Murcia describes the losses and damages resulting from the breakage of the Pantano de Puentes. In addition to 1800 houses, many factories were destroyed, including those for cloth, soap, and saltpeter. Around 900 acres were flooded, 41,000 trees were uprooted, and local irrigation channels were demolished. The total estimated cost in damages was 21,718,185 reales.

Reason for Collapse
The Pantano de Puentes Dam was constructed on 22 foot long piles driven into a bed of sand and gravel. The dam was originally planned to be anchored in solid rock, but it was later found that this would be too deep. Due to being built upon an unstable foundation, the dam was damaged by internal erosion as a result of piping and percolation. The pile foundation held up for 11 years when the water level never reached more than 25 meters above the apron, but on April 30, 1802, strong rainfall led to a water level that rose to 46 meters. The soft soil material was carried away rapidly by this extra load, causing a break in the middle part of the concrete dam. This eventually grew to a large hole 17 meters wide and 33 meters meters across.