EES assessment of natural environment impacts
Although Metro Tunnel will be built through heavily populated areas, the project may still impact the natural environment. We are committed to minimising impacts on these community and environmental assets.
Specialist investigations have identified that important natural assets, native plants and animals will be protected across the project's proposed project area.
Trees and open space
See the Managing trees and the natural environment page for the most recent information.
Plants and animals
With approximately 180 years of growth and development, much of the city's original biodiversity values have changed. The construction and operation of the Metro Tunnel is not expected to impact any threatened plant species.
With the exception of a small number of planted trees, impacts on native vegetation across the alignment will be minimal. Only one threatened plant species, small burr-grass (Tragus australianus), has been recorded in the past 30 years within one kilometre of the project's proposed area however was not located during the surveys undertaken for the EES.
The Domain and eastern portal tunnel entrance precincts are the only areas where threatened animal species are known to exist. Trees in these areas provide temporary habitat for the Grey Goshawk, the Powerful Owl and the Grey Headed Flying Fox.
These animals are listed as vulnerable species in Victoria, while the Grey Headed Flying Fox is also protected under Commonwealth law. As these species are highly mobile, the potential for the Metro Tunnel to have any impact on them is very low.
A range of listed and other important migratory species were also assessed and it was determined that no significant migratory species habitat will be impacted by the construction of the project.
The Metro Tunnel alignment will interface with three of the city's major waterways – the Yarra River, Moonee Ponds Creek and the Maribyrnong River. Protecting these waterways is an important part of the project's design.
The use of tunnel boring machines (TBMs) crossing approximately 10 metres beneath the Yarra River and Moonee Ponds Creek riverbeds will ensure these waterways are protected from direct impact. The Maribyrnong River will also remain protected from direct impact, as there will be no construction activity near the riverbank.
Although these waterways are located next to a large urban environment, the estuary and freshwater reaches are home to a number of migrating fish species. Of these fish, the Australian Grayling, found in the Yarra River, is a threatened species listed under Commonwealth legislation.
During construction of Metro Tunnel, it may be necessary to stabilise riverbed sediments in certain parts of the Yarra River. This will involve injecting grout into the riverbed using either a floating barge on top of the water or via injection from the TBM as it passes underneath.
The project will adopt construction techniques to minimise the risk of any impact on the habit of the Australian Grayling in the Yarra River.
Surface water and flooding
There will be exposed surface areas at construction work sites which are typical of any major works site. During high volume rainfall events, runoff from these surfaces could affect water quality in nearby waterways, either directly or via stormwater drainage systems.
Commonly used site management techniques, including minimising the area of exposed ground and isolating site runoff from the existing drainage system, will minimise the risk of contaminated runoff entering the stormwater drainage system and receiving waterways during construction.
As is typical of major construction projects, work will also have the potential to block or reduce drainage flows, increasing the risk of overland flooding. Overland flooding is caused by water that runs across the land after rain, either before it enters a waterway, after a waterway breaks its banks.
Flood modelling and field assessments have been carried out to determine flows and levels for a range of flood events, including allowances for the impacts of climate change. Design solutions will be adopted to minimise the impact of flooding on the project area and surrounding land.
The tunnels, tunnel entrances and stations will be designed to provide protection against heavy rainfall and rare flood events during construction and operation.
Proposed protection measures may include installing barriers and retaining walls to intercept and divert surface water flows away from entrances, raising station entrances well above flood levels, and installing flood gates between the City Loop and the new tunnels. Emergency management measures, including flood warning systems and emergency evacuation procedures will also be developed.
Once the Metro Tunnel is operational, drainage runoff from above ground structures such as station and tunnel entrances will be discharged into existing local drainage systems. Adopting water sensitive design principles and meeting Environment Protection Authority Victoria requirements in the design of the project's stormwater treatment system will ensure Melbourne Metro's operations do not have any adverse impacts on water quality.
Opportunities to capture and reuse runoff for the irrigation of parks, sports fields or gardens will also be explored as part of delivering the project.
More information on biodiversity and waterways can be found in Chapter 17 Surface Water and Chapter 21 Biodiversity of the EES.
A number of mitigation measures were identified in the project's Environment Effects Statement (EES) to avoid, reduce or offset environmental impacts. These measures form the basis of the recommended Environmental Performance Requirements for the Metro Tunnel and have been recommended by specialists through the EES process.
Some of the recommended Environmental Performance Requirements to manage biodiversity and surface water impacts include:
- During detailed design, review potential tree impacts and provide for maximum tree retention where possible.
- Re-establish trees to replace loss of canopy cover and achieve canopy size equal to (or greater than) healthy, mature examples.
- Replacement of trees to offset the loss of canopy cover and to restore amenity and landscape values in accordance with relevant council policies and master plans.
- Preparation and implementation of Tree Protection Plans for each precinct.
- Where ground treatment works are required in waterways, design and implement methods that prevent discharge of sediments into the water column.
- Design permanent and temporary works and, if necessary, develop and implement emergency flood management measures for the tunnels, tunnel portals, access shafts and station entrances to provide appropriate protection against floodwaters and overland stormwater flows.