Safety concerns for 100-year-old dam situated above Hobart's Southern Outlet

Updated

TasWater has been warned the safety of Hobart's 100-year-old Ridgeway Dam is of "serious concern", after structural analysis conducted last year by technical services firm GHD.

Key points: TasWater has been told that the 100-year-old Ridgeway Dam is of "serious concern," and that work is needed to address the safety risk

TasWater is planning significant upgrades by 2024

Immediate risk mitigation measures have been recommended, including lowering the dam level at the major Hobart water supply

The ABC has obtained a copy of advice provided by GHD to TasWater in July 2019, and an internal TasWater briefing note from the same period.

The Ridgeway Dam is situated above Hobart's Southern Outlet.

The documents reveal GHD told TasWater its investigations showed the safety of the arch portion of the dam was of "serious concern".

The documents indicate that areas along the Southern Outlet and Channel Highway could be impacted if there was an incident.

"The stress and stability conditions of the dam, including the arch section, are more adverse than previously assessed in the period between 2007 and 2012," the documents said.

"GHD has undertaken an initial assessment of the risk position for Ridgeway Dam which indicates that the current level of risk is higher than previously assessed and more than an order of magnitude above the limit of tolerability for existing dams, as set out by [the Australian National Committee on Large Dams]."

The limit of tolerability refers to an assessment of individual and societal risks posed by large dams.

According to Victorian dam safety principles, owners of dams found to be more than an order of magnitude above the limit of tolerability should take interim measures to reduce the level of risk.

Upgrades to be completed by 2024

GHD advised TasWater to immediately lower the Ridgeway Dam level to 4 metres below full to reduce pressure on the wall, implement daily visual inspections, install new monitoring equipment, and clear vegetation around the dam.

TasWater said it had lowered the dam level and increased visual inspections, was in the process of improving monitoring equipment and had not yet removed vegetation.

GHD said its initial recommendations were made on the expectation that major medium and long-term safety improvements would be made within two years.

"If these major works are deferred beyond 2021 then further interim improvements would be required to be implemented by TasWater to justify satisfactory progress in safety improvement," the documents say.

TasWater said the upgrades were being prioritised and under its current program of works the Ridgeway Dam would be upgraded by 2024, at an expected cost in excess of $30 million.

The dam would then meet modern dam engineering requirements.

"Ridgeway Dam is one of a number of dams (13) where we are actively lowering their dam safety risk," a TasWater spokeswoman said.

"We would like to be further progressed at Ridgeway however the investigations and analysis that we are currently undertaking are complex and have taken longer than expected.

"Due to the significance of the project the process cannot be rushed."

Ridgeway among state's highest risk dams

The documents suggested TasWater engage in "enhanced emergency planning" with the Department of State Growth and Kingborough Council to "explore means of limiting the exposure of downstream road users [Southern Outlet and Channel Highway] in the event of a dam safety incident".

However, that planning was ultimately not recommended as an immediate risk mitigation measure, because TasWater and Tasmania Police conducted a training exercise in regards to the Ridgeway Dam in 2018.

Modelling available on Tasmania's Listmap site shows the potential impact if Ridgeway Dam were to overflow during a severe wet weather event.

TasWater said most dams had inundation maps that were used to "determine a dam's consequence category, dam risk and emergency planning".

It said TasWater was currently reviewing the inundation mapping and revised mapping would be included as part of the upgrade design, and that inundation from a dam collapse would be less severe than it would in a wet weather event.

According to the documents, the saftey issues at Ridgeway Dam were first identified 12 years ago.

Ridgeway Dam is one of Hobart's main water supplies, with water restrictions introduced in the Greater Hobart area late last year.

The documents said the dam was one of the two highest risk dams in TasWater's portfolio — alongside Mikany near Smithton — because of the probability and consequences of failure.

Ridgeway Dam is classified in the "extreme" consequence category, due to the size of the dam and amount of downstream development.

TasWater said it intended to reduce the safety risk at 13 of its dams by 2024, including Mikany and Ridgeway.

It also said its board regularly reviewed the safety risks and key dam safety performance indicators for its more than 300 dams.

A Government spokesman said matters relating to assets owned and managed by TasWater were entirely for the company's consideration.

Topics: dams-and-reservoirs, rivers, environment, government-and-politics, industry, disasters-and-accidents, politics-and-government, tas, hobart-7000, launceston-7250, kingston-7050

First posted