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Yarra River Users
A major component of the Melbourne Main Sewer Replacement is the crossing of the Yarra River, upstream of the Charles Grimes Bridge.
What is a cofferdam?
A cofferdam uses steel sheets to build an enclosed area in which to work. The cofferdam for this project will be a wet cofferdam, which means water will remain inside the work area.
The river crossing will be completed using a staged wet cofferdam construction method. The cofferdam will be constructed in three stages working from the south bank to the north bank, ensuring two thirds of the Yarra River will remain open to traffic at all times. One third of the Yarra River will be closed during each of the three stages of work. Each stage will take approximately eight months to complete. Construction methods for the river crossing have been designed to withstand and cause minimal impact in the event of a one-in-100 year flood.
The Yarra River crossing is expected to take two years to complete.
River traffic changes
A river traffic management plan for each stage of the partial river closure has been developed in consultation with commercial and recreational river users and waterway manager, Parks Victoria. Marine Safety Victoria is responsible for reviewing and approving all closures of the Yarra River.
Traffic changes will be communicated in advance via signage on the river, in bulletins and in Notices to Mariners prior to each new stage of works.
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Yarra River closure - Stage 1
Enlarge Stage 1 river crossing side view
Enlarge Stage 1 river crossing traffic management plan
The southern arch of both the Charles Grimes Bridge and Webb Bridge will be closed to all river traffic during Stage 1 of the river crossing work.
During this time, the middle arch of the bridge will be used for both upstream and downstream traffic. Buoys and signage will be prominently displayed to ensure the smooth flow of traffic. This traffic management plan has been approved in principle by Marine Safety Victoria and Parks Victoria. Once Stage 1 has been completed, the southern arch will be re-opened before Stage 2 works begin.
Yarra River closure - Stage 2 and 3
Stage 2 bridge side-view Stage 2 traffic view Stage 3 bridge side-view Stage 3 traffic view
It is proposed during Stage 2 of the Yarra River crossing, the middle arch of the Charles Grimes Bridge and Webb Bridge will be closed. River traffic will be restricted to travel under the southern arch of both bridges.
In Stage 3 of construction, it is proposed that the northern arches of the Charles Grimes Bridge and Webb Bridge will be closed, with river traffic using the southern and middle arches.
Traffic management plans for stage 2 and 3 of the river crossing have been approved in principle by Marine Safety Victoria and Parks Victoria.
Constructing the river crossing
Each cofferdam will be constructed using steel sheets to create an enclosed rectangular box in which to work. For each stage of construction, the cofferdam will extend across the river approximately the same length as the corresponding arch of the Charles Grimes Bridge and will be approximately six metres in width. A floating barge with a crane will sit alongside the cofferdam to service the works.
Current sediment levels mean the bottom of the Yarra River is higher than the original riverbed. To make sure the new sewer has a firm and supported base, it must be laid on the original riverbed. Inside the cofferdam, the riverbed will be excavated to allow the sewer to be laid and then backfilled to the original riverbed level. Permanent piles will also be installed into the riverbed to support the new sewer.
Each cofferdam will be split into two halves or cells. Construction activities will take place in one cell while the second cell is used to temporarily store the excavated material from the first cell.
Most of the excavated material will be replaced back over the new sewer. Any excess material will be transported to shore and then trucked to an approved onshore disposal site.
To service the cofferdam and assist with the works, a 55 metre long, winched barge with a 180 tonne crawler crane will sit alongside the cofferdam. Specialist divers will work underwater within the wet cofferdam. This work will include the installation of pipe supports, steel pipe, sealing and protection works.
River health
Construction methods for the river crossing have been selected following consultation with EPA Victoria, to ensure that the potential for impacts on river health are minimised.
Fish movements are not likely to be affected as two thirds of the river will remain open at all times.
Strict environmental controls will be implemented to protect the health of waterways and aquatic life during construction works. These controls are in accordance with EPA Victoria, Melbourne Water and Parks Victoria requirements.
Further information
