After recent weather  events, the ever-tranquil Torrens Lake is in danger of getting a new reputation: Actually flowing with water.

The Torrens Lake sits in the heart of Adelaide on the River Torrens, and is supported by a weir at the downstream end. It is a typically used for rowing, paddle boating, ‘Pop-eye’ cruises and postcard photographs. It is also known for its cute ducks, swans, algal blooms, accidental drainages and pollution.

A much rarer sight is to see it flowing, so I am taking this opportunity to share the moment with you and offer some thoughts on stormy weather.

Flooding in the Torrens

Given the anomalous weather of the past 48 hours, a colleague and I decided to take a walk along the ‘mighty’ Torrens this morning. The video below shows the Torrens flowing at bank-full height, with:

  • minor flooding along some footpaths;
  • eddying waters around bridge piers; and
  • significant discharge through the sluice gates at the Torrens Lake Weir.

Flow in the Torrens from 30 September 2016.

Statistics on Rainfall in Adelaide

At 143.6 mm, this September has experienced more than 2.5 times its median rainfall. With 630 mm to date, 2016 is already well above its annual average of 550 mm.

An event two weeks ago caused significant flooding, including along Brownhill Creek.  In the past two days there have been two more storms back-to-back.

The first storm notoriously caused a state-wide power outage, but there was also a significant deluge. The second has brought flooding and more sleep-deprivation to state emergency response services, but thankfully no fatalities.

023090_136_13_3967701711033897713Graph of 2016 Rainfall in Adelaide. It shows falls higher than average from May to September.

2016 Adelaide Rainfall Statistics

Adelaide rainfall is winter-dominated. The majority of rainfall lands between the months of May and September. Coming at the end of the wetter months a catchment will be highly saturated. Therefore rainfall in September can produce disproportionately larger flows than earlier months.

For example, compare the two radar images below. One is from January 2016 (top) and the other from September 2016 (bottom).

The January event had much higher instantaneous intensities (see black and red colouring in centre of storm cells). This led to flash flooding and localised flooding (see footage of Goodwood underpass further below). But it did not cause much flow in larger catchments such as the Torrens.

The recent September events have had lower peak intensities in Adelaide (e.g. mostly blue/yellow intensities, the Goodwood underpass during my drive home was not blocked). However, they resulted in more widespread flooding.

Adelaide rain radar showing high intensity falls moving across the city.

22 January 2016Rain radar from September 2016 depicting high rainfall.

28-29 Sep 2016

Localised flooding of Goodwood underpass from 22 January 2016.

Thoughts on engineering design

Thinking about these events, it is clear that impacts we experience and their relationship to weather hazards are complex and multi-causal.

  • Whereas engineering design prefers to consider idealised  scenarios, a single event event can have multiple attributes. For example, strong winds, storm surge and heavy rainfall. The one hazard can also lead to multiple impacts, such as an electricity outages,  floods and storm damages.
  • Engineering designs also prefer to consider events in isolation. But the circumstances leading up to an event can be equally as relevant. For example, the September events have come at the end of a wet period in an above average wet year. Had the same storms come earlier in the year, or a drier year, the impacts would be lessened.

To better understand the reliability of our engineered systems requires less reliance on idealised and isolated design methods and stronger appreciation for the compound behaviour of real events.  For those interested, read more on compound events here.