Safety questions raised over ageing Viseu dam – Portugal Resident

Questions are being raised over how long the ageing Fagilde dam can safely remain in operation after the government acknowledged that its spillway cannot handle the flood conditions required under current regulations.

The dam, built on the River Dão in 1984, supplies water to approximately 150,000 people across Viseu, Mangualde, Nelas, Penalva do Castelo and Sátão.

It has suffered progressive deterioration of its concrete and limitations affecting its hydraulic safety systems, but must remain operational until a replacement is completed — currently expected at the end of 2029.

Viseu mayor João Azevedo is now calling on the government to disclose the safety limits governing its continued use.

“The question that remains without a clear answer is simple: until when is the existing Fagilde dam considered safe?” Azevedo tells Expresso.

The Ministry of the Environment, responding after the newspaper first published its report, said the dam is under continuous surveillance and that no evidence has emerged to change the National Laboratory for Civil Engineering’s previous assessment of its structural safety.

However, the ministry confirms that a study completed in 2022 found that the existing dam “does not have a flood spillway with sufficient capacity to discharge the regulatory flood conditions”.

That limitation remains unresolved – and was one of the principal reasons for deciding to build a replacement approximately 100 metres downstream.

Concrete deterioration documented

The National Laboratory for Civil Engineering (LNEC) has studied Fagilde’s condition for more than two decades, monitoring deformation, concrete expansion, cracking and the operation of its discharge systems.

A 2021 study found that internal sulphate reactions were driving structural changes and identified extensive areas of severe cracking.

Internal sulphate reactions cause concrete to expand, potentially producing deformation and progressive cracking. The presence of such deterioration does not in itself mean that a dam is unsafe, but requires close monitoring to establish whether its behaviour remains within defined limits.

Despite the defects identified, LNEC concluded in 2021 that Fagilde remained functional and safe.

By 2023, however, the laboratory had produced an assessment of possible rehabilitation measures. When presenting plans for its replacement in 2025, the government referred to “serious structural anomalies”, including internal sulphate reactions in the concrete and limitations affecting the hydraulic safety equipment.

Azevedo argues that the government must explain how those findings are reconciled with plans to operate the existing dam for at least another three years.

“If the current dam presents anomalies that the government classifies as serious, and if its replacement is necessary, what exactly is the level of safety that allows it to remain in operation over the coming years?” he asked.

Structural engineer João Alves told Expresso that the presence of defects do not automatically make a dam unsafe.

Safety is determined through inspections, instruments and regular assessments of structural behaviour, he explained. A dam can continue operating for decades provided its readings remain within established limits.

Miguel Fernandes, an engineer with the Norvia Group who has experience of dams and hydraulic infrastructure, also stressed that evidence of deterioration should not be confused with an imminent risk of collapse.

Portuguese dams are closely monitored, he said, allowing developing risks to be identified before a structural failure occurs.

History of discharge problems

Concerns surrounding Fagilde extend beyond deterioration of its concrete.

The government has acknowledged that the dam was overtopped (the water inside it overflowed) in 1995 and again in 2000. During heavy rainfall in December 2000, a failure in the gate-opening system was associated with water passing over the dam. An automatic opening system was subsequently installed.

In 2007, a technical failure caused one gate to open by approximately two metres. It had to be closed manually after releasing an estimated 1.3 million cubic metres of water.

The Environment Ministry said the 2022 assessment considered rehabilitating the existing structure but concluded that building a new dam was the most appropriate response to its limitations and to present and future water demand.

Construction of the replacement is expected to begin in 2028 and finish by the end of 2029. Until then, the region will remain dependent on the existing structure.

The ministry has promised continued monitoring of concrete expansion and cracking to detect any deterioration promptly.

Azevedo nevertheless wants the government to disclose the thresholds that would require reduced operation or the dam’s removal from service, as well as the contingency plan should its condition worsen faster than expected.

“The question is what limit values determine that the dam can continue operating, what alert levels require its use to be reduced, and what criteria determine its withdrawal from service,” he said.

Source: Expresso