The Northside Storage Tunnel helps protect public health, recreational activity and aquatic ecosystems in the Sydney Harbour catchment. It does this by storing wastewater and stormwater that would otherwise have overflowed into the Sydney Harbour during heavy rain.
The Northside Storage Tunnel forms a complete underground system from North Head, near Manly, to the Lane Cove overflow site on the western side of the Lane Cove River. There's a junction at Tunks Park, 9.5 kilometres west of North Head. From this junction, a 3.7-kilometre branch tunnel runs to Scotts Creek near Castle Cove. There are also large underground caverns at North Head and Tunks Park. They were originally built to park tunnel boring machines, store equipment and be used as underground workshops. They're now used for ventilation.
The wastewater system is built with overflow points that act as relief valves when pipes reach their capacity. The largest of these overflow points are at:
The tunnel can store almost 500 million litres of water collected from these 4 overflow sites. It's then transported to North Head Wastewater Treatment Plant.
We have important tunnel ventilation equipment at Lane Cover River West and Scotts Creek. If wastewater levels rise above a certain point at these sites, control gates or penstocks open and channel the overflow into the tunnel.
If the tunnel capacity nears 70 million litres in wet weather, the flow of the air is reversed. The ventilation fan at North Head is automatically turned off and ventilation fans at Lane Cove and Scotts Creek start up. As the tunnel fills, air is extracted by the Lane Cove and Scotts Creek fans.
Air discharged from the Lane Cove and Scotts Creek sites passes through 3 filters before being released into the atmosphere. The filters include a pre-filter, granular activated carbon filter and final filter.
We manage overflows through a fixed-level weir at Tunks Park. If an overflow occurs, a siphon is activated to channel the diluted wastewater into an underground pipeline and finally into the tunnel.
All equipment at the Quakers Hat Bay site is underground. It's connected to the Northern Suburbs Ocean Outfall System (NSOOS).
If the flow of diluted wastewater rises in the NSOOS, it's channelled into the Quakers Hat Bay site and into the tunnel. If the tunnel is full, an isolation control gate or penstock closes. This channels the overflow into a purpose-built weir and discharges it into Quakers Hat Bay.
At Shelly Beach, a weir collects any overflow from the Manly Ocean Outfall System and channels it into a borehole that flows into the Northside Storage Tunnel.
If the tunnel is full, the overflow is discharged into the ocean. The Marine Parade outfall, near Shelly Beach, is no longer needed.
The North Head Wastewater Treatment Plant is on the coast near Manly. The cliff-face bypass weir and siphon at North Head Wastewater Treatment Plant's inlet channel prevents underground flooding if there's power or major equipment failure.
The tunnel reduces the need for the cliff-face discharge by allowing wastewater to divert into the tunnel during power or major equipment failure. The tunnel pumping station is monitored and controlled by the plant’s SCADA (Supervisory Control and Data Acquisition) system.
We monitor all sites 24 hours a day, 7 days a week. All data is recorded and sent via telemetry to our Systems Operations Centre. We track, monitor and report all critical data and assess and respond to any alarms. Although the Systems Operations Centre coordinates communication for all sites, each site is designed to operate safely and independently if there's a communications failure.