Work has begun on cavernous sewers to prevent sewage overflows at one of the North Shore's most popular swimming beaches.
A tunnelling machine has been lowered into a pit at Browns Bay, from where it will carve out a pair of 2.1m-diameter sewers 5m beneath the beach reserve and AnzacRd.
Most days, the sewers will serve the rapidly growing housing areas of the East Coast Bays.
But the pipes will come into their own after fierce cloudbursts and days of steady rain in the bay suburbs, becoming storage for the huge increase in sewage flow when stormwater enters sewers during heavy rain.
Presently, sewer pumping stations cannot cope and they overflow.
As a result, Browns Bay and Mairangi Bay beaches and streams are fouled by more than 12 wet-weather sewage overflows a year.
The $12 million project should cut that number to two a year, said North Shore City Council water services project engineer Thien Yet Liew.
Repairs were also being made to leaky old public and private sewers in the Browns Bay area so stormwater would not get into the sewer system.
Improving the reliability of pumping stations was also part of the solution and extensions to the Browns Bay pumping station were under way.
The council's $210 million pollution works plan, Project Care, includes expanding the capacity of the city's far-flung sewer network by building or upgrading storage tanks and sewer tunnels in several suburbs.
Two major storage projects have been completed - a $4.5 million one at Beachhaven and a $6.2 million scheme at Silverfield in the Wairau Valley.
The Silverfield project aims to reduce overflows into the Wairau Stream and onto Takapuna and Milford beaches.
