A frantic effort has begun deep in the mountains of northeastern Puerto Rico to repair a vital dam badly compromised by Hurricane Maria, bringing first glimmers of hope to local residents who are facing months without water or electricity in their battered and isolated homes.

The Independent was the first media organisation invited to witness the operation on the ground. A reporter was escorted to the site by the US Army Corps of Engineers in charge, Lieutenant Colonel Roberto Solorzarno.

Amid fears that any further heavy rains could cause a catastrophic rupture of the 100-year-old earth dam and unleash an epic 11-billion-gallon flood on towns and hamlets below, the US Army Corps of Engineers is leading a hazardous effort to halt dangerous erosion of its swollen spillway. Residents in the area were put under an evacuation order in the immediate wake of Maria but have been allowed back to their homes for the time being.

After days of preparation, including clearing narrow mountain roads of fallen trees, landslides and other debris left by Maria, the Army Corps has begun hurling concrete barriers from the air into the main spillway behind the Guajataca Dam. The channel has suffered serious erosion as much of its cement cladding was washed away. Each day brings evidence of further damage to its wall.

A risky choreography of an airborne V-22 Osprey tilt-rotor heavy-lift aircraft loaned by the US Marine Corps lifting the barriers from the dam itself and dropping them into the roaring waters of the spillway began this week.

Four additional heavy-lift helicopters were to join the effort on Wednesday. While a priority is to try to prevent any further gouging out of the spillway that could eventually weaken the dam itself, the Army Corps is trying also to restore severed pipelines. The reservoir supplies fresh water to 350,000 homes in the area. All are without water in the meantime.

“All that water is just being held by this thin line,” Lt Col Solorzano commented, gazing out across the water of the giant lake, rain clouds threatening overhead. The dam was built in 1923 and is mostly just soil and is topped by a roadway linking local villages on either side of the gorge. It is a race against time to fix the spillway before disaster might strike. “If we get maybe another 10 or 11 inches in a few days then we will really have an issue,” he added.

Local residents drawn to the dam by the strained clattering of the Osprey were clearly buoyed by seeing it. They watched in fascination, their faces lit up by evidence that the US military was coming to their aid. The scene was in stark contrast to the widespread expressions of frustration among many Puerto Ricans that the mainland seemed to have reacted slowly to their plight.

“I think it could have gone faster and I think they could have helped a little bit more,” said Yonaris Puig, 22, who rode out the hurricane in her grandmother's house less than a mile from the dam. “But they are here now and I hope they can fix it so we don’t have to be without water for too much longer. The point is they are trying and that makes us feel better. I have high hopes of them.”

The expressions of hope are gratifying to Lt Col Solorzano, who recognises that the perception of the relief effort has not always been positive. Part of the problem has been getting all the agencies involved in the effort, including the local government, to work together. “It’s never going to be fast especially when you have two or more agencies trying to work together. But we are getting there,” he said. As well as the Army Corps of Engineers and the Marines, the Puerto Rican energy utility is also involved in the dam’s rescue.

The badly damaged spillway that threatens integrity of the nearby dam (David Usborne)

Whether it will work – the main aim of the concrete barriers is to slow the flow of the water through the spillway – Lt Col Solorzano couldn’t say with any certainty. And even it does, more permanent work to rehabilitate the ageing dam will have to follow quickly. “It is urgent,” Lt Col Solorzano noted. “It’s really important that we deal with this. But this is an emergency bandaid so that they can do a proper fix for the long-term later on.”

“We have never tried this before,” he admitted, between pacing back and forth atop the dam trying to keep all the moving parts of the operation in sync. “Something is better than nothing, you know, little buckets are going to make a big difference in the end.” As each block was dropped into the spillway, the impression was of small bricks trying to stop the flow of a roaring torrent.

And on Tuesday afternoon there were problems. A hydraulic hook release used by the crew to drop the heavy barriers was not operating properly. That meant the crew on board attempting to pull the hook back by hand. To try to make it easier, the Osprey’s pilot attempted to make the aircraft hop up and down to give moments of slackness to the cables. When that was ineffective, they were forced to drop lower towards the water and set the blocks directly down into it.

US Army Corps of Engineers near the ruptured spillway are ready to guide the Osprey with its heavy load (David Usborne)

Lt Col Solorzano expressed the hope that in this lonely part of the island, residents now see that something is being done to help them. “We want people to understand that we are doing everything we can here to support the people of Puerto Rico. We want to make things better for them and we are working completely out of the box here.”

“We have never seen a plane like this before,” said a visibly excited Daniel Mendez, who had come from the town of Guajataca 15 miles away to witness the work. “And this is good because we have no electricity and we have no water. Life is very difficult for us.”