Strong low (972mb) with flooding!, 28-30 September 2016

After the enormous storm event that was dubbed ‘the worst in 50 years’ to begin with, and ‘the most significant weather event we’ve ever had’ in the end, a large amount of damage was seen across South Australia after the low had passed. And whilst it certainly was a very big event, it is also a very big call to say it was the most significant ever, especially when you go back through the history books. Given the highly connective nature of the modern world and electricity/telecommunications being such an integral part of our lives, it is reasonable to say we are more vulnerable than ever to such storm events. Suggestions of stronger tornadoes, much bigger rain events and wilder winds have all been recorded in the past, but that isn’t to downplay this event, just a reminder that whilst events like this are certainly rare, they have happened before, and will happen again. But I will say this was up there with the best, a big part of which was due to its broad impact, and the particularly severe storms on the Wednesday.

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The deep low near its maximum intensity, and had even maintained a small ‘eye’ for many hours. The first heavy rainbands associated with it can be seen impacting Eyre Peninsula, while the decaying thunderstorms can be seen on its long tail, the moisture being wrapped back around into our state. 

A breakdown of some of the main events. This certainly isn’t to say this was all that happened, but these are events that stand out for me.

  • 5 confirmed tornadoes, definitely more, potentially many more. I’d hedge my bets that we had a dozen touchdowns at least during the Wednesday simply due to many of these cells going unobserved. There is every possibility that this was one of the biggest tornado outbreak in Australia’s history. In South Australia we often see atmospheric shear profiles supportive of supercells and tornadoes given our latitude, our biggest issue is and always will be available moisture due to the huge dry continental air to our north, which is why we often see our best storms in the wettest years.
  • Hail larger than tennisballs recorded in Blyth and apparently Snowtown, golf balls at Cleve, which caused huge crop losses. What makes this so significant, is it is RARE to see hail larger than golfballs in South Australia, even in November when we still have cold uppers and much higher instability potential. The fact that hail of this INCREDIBLE SIZE occurred in late September, with sub 20 degree temperatures is astounding. Snowtown obs prior to the supercell peaked out at 19.2/15.6. It is clear the relatively cold weather allowed a lower freezing level, combined with the strong upper divergence, high dewpoints and near dry adiabatic lapse rates for a short distance (in height) through the lower-mids, all contributed to this. And of course properly rotating updrafts to allow the recycling of downdrafts/updrafts in the storm. In all honestly, this stands out more for me than almost any other aspect of this system. About the maximum size you will ever see in South Australia.
  • Widespread strong to gale force winds, locally storm force on coasts in association with the western flank of the low.
  • A central pressure in the low of 972mb just south of the gulfs. Very very rare to have such a deep low, hence the severe weather.
  • Widespread heavy rainfall in two lots, the first from the aforementioned thunderstorms on the Wednesday, and the majority on the Thursday from the low and its associated rainbands running ashore.
  • Flooding associated with this rainfall. I’m not aware (yet) of any rivers reaching historical levels, however the broad scale of the flooding is quite rare. Most of the Mid North and the Adelaide Hills suffered some form of flooding with many rivers bursting their banks.
  • A significant storm surge which severely damaged a number of jetties in South Australia, including long standing ones such as Port Germein, which has been in existence since 1879. The impact however has likely been increased as a result of sea level rises since its inception, as sea levels have risen 10-20cm in this time. Some locals quoted as saying the worst damage in 50 years.
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    As the sun finally rose we could get a clear picture of the incredible structure now of this low. A cold front and dryline surging across eastern Australia, a warm front south of Tasmania wrapping back around , the occlusion to the sw of South Australia, and the heavy convective rim of the low being wrapped directly into the mid north ranges. In Clare we heard several thunderstorms move through in 9 degree conditions, and had a couple of quite heavy hail showers with it as well! The first burst of wind had already brought many trees down.

Given the broad scope of the flooding, the statewide outages which lasted two days where I live and the telecommunication blackout which lasted over 5 days in Clare, I spent the entirety of the event around the Clare Valley, simply due to the dangers of leaving (and getting stuck), and who doesn’t want to be there when their own town floods! Doesn’t happen everyday.

The big part of why the flooding was so widespread, is simply due once again to the negative IOD influencing our weather this winter and spring, with some very much above average rainfall leading up to this event. Catchments were saturated, or near saturated throughout, and the reality is even falls of 30mm across catchments were sufficient to cause rivers to burst their banks. Here in the Clare Valley, we had 20-40mm from the storms on Wednesday, and a further 50-70mm on the Thursday. On top of the previous events in September, we ended up with a whopping 220mm in Clare itself, the biggest month of rainfall I have seen in all my years of watching weather. Whilst not directly comparable due to BoM closing the long running rain gauge at Clare post office (what a huge shame) in 1994, and Clare High School taking over (a much drier site), and the possibility of southern Clare being slightly wetter than the town centre, we still easily surpassed the all time September record going back to 1862. The old record was 197.2mm in  1972 (how’s the synchronicity there!). That is to say, we experienced the wettest September since European settlement in Clare.

The 6 month period leading up to this flood was exceptional, with outback South Australia receiving hugely unseasonable rainfall. Cloud band after cloud band extended down from NW Australia, it was an incredible wet phase we had entered.

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The trippling of mean rainfall amounts in outback is in a big part due to low mean rainfalls. But this does show just how much extra moisture we had received.

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Rainfall deciles for the period from July to September (inclusive). As you can see, for the eastern half, widespread deciles above 9 were observed, with some areas experiencing record rainfall.

Now for the year by year comparison for Clare, where I have cherry picked some of the wettest ever years, to see how they compare.

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Here we have Neagles Rock, which records extremely similarly to my address, and is the closest current station to the old rainfall site. (around 1km south-west)

And historical rainfall.

021014_136_13_549494990137925545 021014_136_13_1757208659477602761 021014_136_13_6998970273920088895 021014_136_13_7986461604411927456 021014_136_13_8407015582974396254

As you can see, we are currently running level with many of these years, and therefore have potential to possibly exceed the all time yearly record.

Now for some pics, demonstrating just how much water was around! It’s hard to get a grasp of how deep some of these are, many streams went 1 metre over their banks. We ended up with 102mm in about 36 hours in my gauge in Clare.

The SES issue an emergnency warning for flooding as the Hutt River laps at its banks, and a huge effort of sandbagging is underway.

The SES issue an emergency warning for flooding in Clare as the Hutt River laps at its banks for hours on end, and a huge effort sandbagging is underway. The main street can be seen in the background. This bridge, which is sitting 1 metre under, and sits a further 1.2m or so above the creek bed was quite badly damaged.

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To both the north and south of Clare, the Hutt had broken its banks. This vineyard near the caravan park was well under water.

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A bit of shortsighted planning here. The pipe under the bridge couldn’t handle the water from this small tributary, which instead banked up behind the bridge and was redirected straight through the visitor centre.

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The Hill River, the Hutt Rivers twin which runs parallel to the Hutt, some 8km or so east, had burst its banks through Kirribilly. This river has a notorious history of killing livestock due to its wide floodplain. The actual channel is near the trees. It would rise higher than this overnight.

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The Hutt River north of Clare was bursting wide across its floodplain, the main channel is all the way back under those redgums in the distance.

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A floodway in Clare which I have always used to gauge the height of the flows, I consider it running high when it is near the tree submerged in the middle there. Being the lowest ford in Clare, it’s always the first to go under.

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The Horrocks Highway north of Clare, near the conjunction of the Hutt River and Armagh Creek (the Hutt’s biggest tributary) had formed a bit of an inland sea over the highway for many kilometres. The cars headlights are the Bridge over the Armagh Creek just before the racecourse.

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The Hutt near foodland, running right up to the bridges, as soon as it spills it starts flooding property, we got very lucky!

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The Hutt south of Sevenhill, flooded over Mintaro Road.

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Farrell Creek on Friday morning. It had been significantly higher the night before as it was too deep for me to cross the road but had lowered considerably by morning. I wondered why this particular creek was so swollen…Mintaro received 69mm to 9am, which is right near its source. If you saw this little creek beforehand, you would never have expected it could flood this wide!

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A crossing on the Broughton River west of Spalding. Very deep in the middle there and was flowing with rapids. The conjunction of the three north draining systems from the Clare Valley (Hutt, Hill Rivers and Farrell Creek), plus likely Booborowie Creek all contributing to this huge volume of water.

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Hutt River near Spalding, swollen from all the floodwaters of the northern Clare Valley.

The flow gauge at the caravan park went into Moderate Flood. After we had already had three minor floods already this year!

The flow gauge at the caravan park touched the Moderate Flood level. We had already had three minor floods this year! Non-flow is around 0.84, these values above this number indicate flood rankings. 1.1m = minor, 1.5m = moderate, 1.8m = major. This is from memory when they readjusted the base level but didn’t add the flood levels back. 

Some videos to get an idea of the water motion. Some amazing turbulence in the bigger rivers.

Whilst floods like this may be considered bad from a human perspective, from an ecological perspective they are the boom times. Changing the habitats in rivers, washing away salinity (at first at least, rising water tables can bring higher salinity after wet years), creating new generations of river red gums, dumping fertile sediment across flood plains, improving water quality and one which often goes unnoticed, allowing native fish migration through the rivers. In South Australia, we actually have a few species which can migrate out to sea during these flood events, and up and down river channels to spawn and without the occasional flood event would have likely already have gone extinct.

I also remember a conversation, and talks about how Clare has been flood proofed from the amount of dams we have. Almost every last dam was full before this flood, and some were nearly destroyed by this flood. The reality is they provide next to no protection, and in many cases can often exacerbate events if one happens to give way. During dry times however, they are hugely detrimental to streamflow and consequently river health. A bit preachy, but this flood really did remind us how ineffective at flood mitigation during large flood events they really are.

Regardless of all this, I look forward to when streamflow returns to low flows, I can check out some of the bigger waterholes and see what life we have. It’s been an awesome wet season, which will likely soon come to an end. But for now it is as green as ever despite the 30 degree days nearing.

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3 Responses to Strong low (972mb) with flooding!, 28-30 September 2016

  1. Michael Owen says:

    Incredible pictures and a great summary of the weather events Mark. Well done.

  2. You’ve compiled an exceptional report and historical record there Mark! And I’m sure put a lot of time and work into creating it! I do hope it become embedded into the permanent record of the most important weather events in South Australia. Your videos abundantly show the power of flooding South Australian rivers, and the turbulence as you point out. Danger!

    • Michael Owen says:

      I agree – apart from Mark’s excellent photographic skills he has a deep understanding of how the weather events form and their impact on the environment.

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