Tolosa Dam decommissioning

Client: TasWater
Location: Tasmania / Australia
Date: 2016 - 2024

Supporting the safe decommissioning of a 140-year-old dam

Background

Tolosa Dam is a water supply dam located at the top of Tolosa Street in Glenorchy, Hobart. The dam entered the final phase of its life cycle, decommissioning, in mid-September 2023.

Tolosa Dam was originally constructed in 1880 and was raised in 1915 and 1922. In its final configuration, it was a zoned earthfill dam with a puddle clay core, a maximum height of 20 m and a crest length of 283 m. The dam eventually fell into disuse as a water supply reservoir and a risk assessment found that the dam no longer complied with modern safety requirements. This offered the dam’s owner the opportunity to repurpose the asset, eliminate potential safety risks, protect the environment and community, and convert the area to a new use in line with the priorities of the local community.

The dam owner and operator, TasWater, decided to decommission the dam and hand the land back to the Glenorchy City Council.

Solution

Entura was engaged to support the application for decommissioning the dam, which included undertaking environmental and heritage impact assessments, sediment and erosion control planning, engineering design, rehabilitation planning and a dam safety emergency plan. Our multidisciplinary team undertook these studies and prepared a decommissioning design report for the decommissioning permit application.

We later updated this 2017 decommissioning plan to take account of a revised final masterplan with an updated erosion and sediment control plan, environmental and heritage assessment, and hydrological and hydraulic modelling. Once works began on site, our team provided construction supervision to ensure the works were completed in accordance with the decommissioning permit.

With Tolosa Dam being located adjacent to a school, public park, and residential buildings, it was crucial for the dam owner to engage with the local community and council. A masterplan was developed which involved converting the dam and reservoir area into a park with ponds and expansive flat areas that could be used for recreation and outdoor public events. Retaining some heritage aspects of the dam was important to the community and was a major focus of the decommissioning plan.

During decommissioning, most of the embankment was removed; however, the ends of the left and right abutment were retained and integrated into the surrounding park. This made it possible to visualise the footprint of the dam embankment, enabling the community to remember the past use of the land. Furthermore, five ornamental ponds were constructed throughout the park, where the existing reservoir was located, to incorporate a water theme echoing the area’s previous use.

Our team conducted safety checks on the gradients of the retained abutments to ensure rainfall runoff wouldn’t erode the downstream area and that the remaining abutments would not retain any water. The designs of the ornamental ponds were reviewed to ensure their volume was less than 1 ML as per regulatory requirements.

To manage sediment and erosion during the decommissioning process, we recommended continuously shaping the site’s landform to direct rainfall and inflows into a temporary sediment basin using small bunds and hessian silt rolls. The lowest point of the reservoir served as the temporary sediment basin until the majority of the earthworks were completed. This approach allowed turbid water to settle or undergo treatment before being pumped out.

Outcome

Decommissioning dams takes a wide range of skills to deliver a successful outcome – and it can be a lengthy process from early decision-making through planning, design, construction and remediation. The Tolosa Dam decommissioning project is a great example of how decommissioning can reduce safety risks to the community, protect the environment during the works, and ultimately create a new asset which enhances the amenity of the area for the benefit of the community.

The decommissioning project is now complete and the new park has been handed back to the council and is ready for the community to enjoy.