Adelaide's largest reservoir has been drained for the first time in 26 years after being drained for inspection and maintenance works ahead of a major upgrade proposed by SA Water.
Catchment from the 46.4-gigalitre capacity Mt Bold dam has been sent to Happy Valley Reservoir, or released into the Onkaparinga River for scheduled environmental flows, enabling access to areas normally covered by the city's drinking water supply.
SA Water Asset Operations general manager Mark Gobbie said they wanted to take advantage of lower levels following a drier than average 2019 and the supply of desalinated water into Adelaide's mains.
The city's desalination plant was recently cranked up for the first time to supplement the city's potable supply with 40 gigalitres under the Federal Government's Water for Fodder program.
"The key point here is we haven't wasted any water from Mount Bold," Mr Gobbie said.
"It's only a three-week exercise and then we'll start to fill the dam again from there.
"Otherwise we would have had to do that work underwater with divers, so this is a much safer — and probably more accurate way — of doing the work."
Mt Bold was last drained in 1994 for three days of inspection works, but no upgrade works were undertaken.
Upgrade in the works
SA Water is replacing guides for valve gates on the inside face of the dam wall as well as undertaking a detailed structural assessment ahead of a major safety upgrade expected to start in 2022-'23.
Mr Gobbie said those more significant works would improve the structure's ability to withstand earthquakes and large flooding events.
Similar works have recently been completed at Kangaroo Creek Reservoir where SA Water raised and strengthened the dam wall and widened the spillway for a combined cost of $94 million.
The upgrade is part of a $91m, four-year investment proposal from SA Water, which will also include early works towards the upgrades at Baroota and Hindmarsh Valley reservoirs.
Mr Gobbie said the utility did not have any concerns about missing out on catchments while Mt Bold Reservoir was being inspected.
"We get the majority of our rainfall in winter and through spring," he said.
"Because over summer the catchment soil profile tends to dry out quite a bit, it takes quite a bit of rain in our Adelaide Hills catchment before we start to get run-off into the dam.
"So this is actually the ideal time of the year to be doing this work."
