How can a reservoir be more than 100%?

How can a reservoir be more than 100%?

Our reservoirs supply homes and businesses across South Australia. At full capacity, they can collectively hold just under a year’s supply of water for Adelaide. In recent weeks, the heavy rainfall has resulted in a large amount of inflow into our reservoirs. For the first time in over 10 years, our combined reservoir level has exceeded 100% and multiple reservoirs are still full.

You may be wondering – how can a reservoir be above 100% capacity?

It is possible for a reservoir to measure more than 100%. A reservoir can hold slightly more water than the set capacity levels we report against, and spills once it’s above full capacity. Essentially, any increases above 100% mean the reservoir is spilling. However, we perform controlled releases of water at the Mount Bold and Myponga reservoirs before they reach 100% capacity. Our Mount Bold and Myponga reservoirs have adjustable gates to manage outflows, and all other reservoirs apart from Barossa have spillways to discharge water collected above capacity.

For a reservoir to spill, the water level must be above the spillway, which is positioned at full capacity. As the water level in the reservoir rises above the base of the spillway, the volume increases beyond 100% until inflows begin to drop and the reservoir is no longer spilling.

Click diagram below to enlarge
How can a reservoir be more than 100%?

  • Major faults

  • Underway

  • Mcarthur Av
  • Plympton
  • 18/01/2019
  • Water Supply On
  • 19/01/2019 01:42 AM - We are attending to an incident in Plympton with no interruption to the water supply. The safety of our crews and customers comes first, and we always aim to minimise inconvenience by restoring services as quickly as we can. Reference Number WO: 06755851.
  • See all major faults

  • Scheduled works

  • Underway
  • Hog Bay Rd
  • Cygnet River
  • 21/01/2019
  • Temporary Supply Interruption
  • Estimated start time and water supply off: 22/01/2019 09:00 AM
    Estimated restore time and water supply back on: 22/01/2019 04:00 PM

    We’re improving your services and undertaking maintenance work in Cygnet River. Sometimes our crews need to temporarily interrupt the water supply to our customers and/or manage traffic while they are working. Temporary traffic management may remain in place until reinstatement of the impacted road is complete. We always aim to minimise inconvenience by restoring services as safely and quickly as we can.


  • See all scheduled works

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