The Seaford technical report (available at http://levelcrossings.vic.gov.au/media-library/publications/frankston-publications/reports/seaford-technical-report) states:
LXRA Myth… When considering the best way to remove the Seaford Road level crossing, environmental factors are particularly challenging in terms of groundwater flows, flooding and potential impacts to the nearby Edithvale-Seaford wetlands and Kananook Creek.
These environmental problems ruled out the building of a trench at this site. This image shows the underground conditions at Seaford. In Seaford, there are clay layers that are close to the surface.
A trench structure, combined with the clay, would change groundwater flow, as water would need to divert around the trench. This means that water levels on the inland side could rise, causing potential impacts to the wetlands. Water levels on the coastal side could fall, which would in turn affect the environmental values of Kananook Creek.
Image from LXRA technical report
We have 3 points to consider here:
1. Groundwater issues have been successfully tackled on many sites on Mornington Peninsula, and everywhere else in the world.
Groundwater obstacles can be overcome at the Seaford Rd site just like any other site. A number of train stations have ground water pumps. A more challenging example is of Martha Cove road tunnel under the water channel, close to the shore.
2. Seaford Wetlands has own engineering controls for water levels
Seaford Wetlands are now largely Artificial and Actively managed with current engineering controls that regulate the hydrological operations controlling the water coming in and out from the various sources. Water levels are regulated through the various cells and then in turn the water is released to storm water drains or the Kananook creek via further engineering controls.
There would be no change to water levels at the Seaford Wetlands with the Rail under solution, due to the current engineering controls designed to regulate the hydro-logical operation of the area.
In an effort to fast track the Seaford Rd level crossing removal the LXRA have skipped detailed technical investigations at the Seaford Rd site. They have not looked into the current engineering controls at the Seaford wetlands, which could manage the water levels of the Seaford wetlands with a rail under road solution