Thousands of Puerto Ricans evacuated as dam threatens to breach

Danger: full to the brim

Reuters/Alvin Baez

Stay away or be swept away.” This grim advice was issued to the people of Puerto Rico by the US National Weather Service on Monday, after a dam overwhelmed by rainfall from Hurricane Maria developed a large fissure.

The Guajataca Dam is located on the Rio Guajataca in the north-west corner of Puerto Rico. The 90-year-old earthen dam houses a reservoir, 4 square kilometres in area and 4 kilometres long, which stores almost 40 billion litres of water. The reservoir is full to bursting, after Maria dumped 40 centimetres of rain on the surrounding mountains.

If the dam fails, it could cause life-threatening flash flooding downstream.

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Since Friday, when the crack appeared, local officials have been warning of the danger and managing an evacuation. “We don’t know how long it’s going to hold,” Governor Ricardo Rosselló told the media on Sunday. “The integrity of the structure has been compromised in a significant way.”

Up to 70,000 people are being evacuated from the area. Dam experts contacted by New Scientist were full of praise for the evacuation plan.

Dam bottom eroding

“Photographs of the incident show there’s structural damage at the base of the dam’s spillway, where overflows appear to be eroding the bottom of the dam,” says Tracey Williamson, chairman of the British Dam Society. Similar incidents and near misses have happened before. “Depending on whether the overflow continues or increases for a significant amount of time, there’s a possibility that erosion could progress to the point where the dam could breach.”

There are several ways to stop the dam bursting. One is to reduce the reservoir level by draining water through all other available outlets: this would relieve the water pressure on the dam and slow the flow of water over the spillway. Another approach is to deploy high-volume pumps to divert the overflow around the zone of erosion. Finally, relief workers could jam additional material into the existing crack to prevent further erosion.

The crisis has also further depleted Puerto Rico’s power supply, as the Guajataca lake supplies hydroelectric power as well as drinking water.

Communications have been severely hampered by damage from Hurricane Maria. 1360 of the country’s 1600 cellphone towers have been downed, and 85 per cent of above-ground phone and internet cables knocked out. At least 16 people are confirmed dead, with 15,000 residents housed in shelters.

As of Sunday, the US Federal Emergency Management Agency had supplied Puerto Rico and the US Virgin Islands with 1.5 million meals, 1.1 million litres of water and 12,000 emergency roofing kits.

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