SYNERGETICS – Engineering assessment of West Gate Tunnel ventilation exhaust stacks

Executive summary

Maribyrnong Truck Action Group (MTAG) members are increasingly concerned by the prospect of the West Gate Tunnel (WGT) beginning operation without tunnel exhaust filters. MTAG engaged Synergetics Consulting Engineers in May 2025 to address the following questions.

- Does the WGT exhaust stack design represent best practice?

- Are exhaust filters necessary to protect community health?

The as-built WGT ventilation exhaust design was found to have an unusual and complex geometry that does not comply with good engineering practice (GEP) guidelines for exhaust design in a number of respects resulting the downwash of the plume.

• The exhaust stack does not have sufficient vertical clearance above the surrounding architectural façade and does not satisfy Good Engineering Practice (GEP) guideline of > 2.5 x the height of nearby structures (USEPA, 1985) to prevent downwash.

 • The very low discharge velocity of 6 m/s from the WGT exhausts will result in the exhaust plume being downwashed for most wind speeds based on the 1.5 x local wind speed threshold for road tunnel exhausts and other neutrally buoyant plumes (Snyder & Lawson, 1991).

Previous assessments by the WGT proponent’s consultants: Golders, GHD, and AECOM[1] failed to properly consider and account for these adverse downwash effects on the dispersion of the discharge plume. The most recent community update “Westgate Tunnel Project Community update April 2025”, published by Victoria’s Big Build in partnership with Transurban included as Appendix E, misrepresents the benefits of the WGT ventilation exhaust design.

Computational fluid dynamics (CFD) was used to characterise the wind field and dispersion and short term ground level concentration (GLC) distribution of the down washed plume for two representative wind directions at an average wind speed of 18 km/h (5 m/s). The highest GLC were observed for wind directions oblique to the long axis of the architectural façade surrounding the WGT discharge stacks.

When these GLCs are assessed over an entire year, and including the background levels of fine particulate, exceedances of the EPA Victoria standards are likely. As a consequence, the impacted residential community is likely to experience adverse health outcomes including increased levels of cardiovascular and respiratory disease, hospital admissions, emergency department visits and mortality.

Installation of 90% efficient particulate filtration such as ESPs to the two as-built stacks is technically feasible and is the only engineering control considered that would reduce the ground level concentration to levels comparable to a simple best practice cylindrical stack.

It is recommended that this CFD study be extended to identify and optimise all feasible engineering control options so that the most cost effective option/s for both the northern and southern WGT ventilation exhaust discharge stacks can be identified, designed, fabricated and installed prior to WGT operation.

[1] (Aurecon Jacobs Joint Venture, 2017), (Golder Associates, 2017) and (Aurecon Jacobs Joint Venture, 2019)