Torrential rain, widespread flooding on Far North QLD coast
Sat 10 Mar 2018

Heavy, near continuous rain fell on Queensland's North Tropical Coast and Tablelands from 6 to 10 March thanks to an unusual conjunction of circumstances.

A stream of active, rain-producing cloud pushes onto the North Tropical Coast around Ingham and Innisfail in this visible image from Himawari-8 at 08.00 EST 8 March. The unusually long convergence line was well formed at this time and extended over 1,000km to the NE of the coast. JMA processed by BoM.

Exceptionally moist air was already being dragged into the area between a tropical low in the Gulf of Carpentaria and a high pressure ridge up the QLD coast. Within this airstream was a convergence line that moved north very slowly, where winds from slightly different directions met and were forced to rise.

The high mountains behind the North Tropical Coast caused further strong ascent and delivered 500 to 700mm and as much as 1,000mm of heavy, continuous rain. As the BoM said in a note, "When the moist air comes onshore, it hits the ranges and has only one way to go – straight up. This generates even more cloud and intensifies the rainfall."

Rain also penetrated farther inland than would be normal in this type of event, bringer stronger flooding downstream from the upper reaches of many rivers. Rain lingered in the Tully to Innisfail area until the 9^th and moved slowly north to Cairns and the Daintree from the 8^th.

The resulting floods were the fourth in QLD in the past three weeks, following those in SE QLD, Townsville and western QLD. Communities were isolated, highways and roads cut and schools closed as waters rose, with the towns of Ingham and Halifax the worst affected. Over two hundred houses in and around Ingham were inundated while ten people had to be rescued from floodwaters. The Premier Annastacia Palaszczuk, who visited Ingham, said that 90% of homes and businesses had suffered flooding or damage. Floodwaters also covered sugar and banana plantations and farms in the area leading to loss of crops and delays in processing.

Seventy-two primary school children with eight teachers and a parent were stranded at Echo Creek Adventure Camp, 17km W of Tully, being rescued after six days' isolation. A disaster declaration was issued on 9 March for Hinchinbrook Shire, which includes Ingham and Halifax, as well as Mareeba, Cassowary Coast and Yarrabah. This gave additional powers to emergency services as well as paving the way for disaster relief. [Floodlist, ABC News, Brisbane Times, news.com.au]

This article is broken into a summary of weather and river conditions in each of the main valleys followed by media and social media reaction. Firstly, this table gives a comparison of vital statistics across the main river systems, from the Herbert in the south to the Barron in the north.

Conditions by catchment

The worst flooding was in the Herbert River, especially in and around Ingham. The Herbert is by far the largest river basin on the North Tropical Coast, with a catchment area of about 9,000 sq km. The coastal plains and immediate ranges had already been soaked by 150 to 250mm of rain between 28 February and 3 March as a result of the depression that moved through Townsville on the 1^st. Ingham town had the highest total on the North Tropical Coast over those four days of 306mm.

The flooding rains in the Herbert began early on the 6^th and did not clear the catchment until late on the 9^th. Floodwaters covered much of the town and the sugar plantations and farms in the lower Herbert Valley as water at Ingham Pump Station passed the major flood level of 12.0m at 21.35 EST on Thursday 8^th and stayed above major level until 18.41 on Sunday 11^th, almost three days later. Peaks of 14.52m were reached several times on Friday afternoon 9^th, with the BoM giving the absolute peak as "about 14.7m".

Other river valleys on the North Tropical Coast experienced flooding but not to the same extent as in the Herbert. None had received the heavy rain from the Townsville depression that the Ingham area did, so their catchments were not as sodden. However, as the table above shows, they all received even heavier rain than the Herbert catchment as the convergence line moved north. Six gauges in the Tully and Johnstone Valleys had three consecutive days with totals above 200mm, while The Boulders, just west of Babinda in the Russell Valley, had the dubious distinction of four consecutive 24-hour periods from the 7^th to the 10^th with over 200mm yielding 975mm.

Next north of the Herbert catchment, in the Tully and adjacent Murray River Valleys, the highest one-day rainfall for the whole event of 409mm to 0900 9^th was recorded at Kirrama Range Alert, deep in the rainforests of the Kirrama National Park 30km SW of Tully. The Tully River at Euramo remained just below major flood level for three days to around midday Sunday 11^th. The Murray River set a new record at the Murray Flats station which reported its first flood in February 2004, but the upstream Upper Murray gauge recorded only 8.11m at 0600 9^th which has been exceeded in all but one of the nine significant flood events since its first in March 1996.

Although Tully is famed as the town of The Golden Gumboot, the next river north, the Johnstone River through Innisfail, is a strong contender for the title with its waters rising on the slopes of Mount Bartle Frere, Queensland's highest and Australia's wettest mountain. The top station of the cable car on the neighbouring Mount Bellenden Ker averages just over 8m of rain annually and holds the record for most rain in a year with 12,461.0mm falling in 2000.

With a combined catchment of about 1,600sq km, the two branches of the Johnstone which meet in the town of Innisfail drop rapidly through steep gorges onto a narrow coastal plain. This makes them very responsive to rain, with quick rises and falls as happened in this case. Unlike the other rivers which had broad flood peaks lasting several days, the readings at Innisfail's McAvoy Bridge yo-yoed, initially rising over 4.5m in just 5½ hours on the Friday morning 8 March. The sudden arrival of rainwater from upstream saw the river rise 2m in 4½ hours on Saturday evening 9 March, to its peak of 8.90m early the next morning, its second highest level since the maximum recorded level of March 1967.

Next to the north, in the small valleys of the Russell and Mulgrave Rivers, the short, relatively sharp falls of the rivers from the mountains again made for quick flood responses to rainfall. The highest rainfall registered for the event, 1,005mm just west of Babinda, pushed the Russell River to its highest level since March 2004 while farther north on the Mulgrave the town of Gordonvale had two peaks into the major category on Thursday 8^th afternoon and again early Saturday 10^th morning.

By Friday 9^th and Saturday 10^th, in the Barron River Valley, the second-largest catchment on the North Tropical Coast mostly in the Atherton Tablelands, the convergence system was waning and heavy rain becoming more scattered. Although the Barron put on a spectacular display at the famous Barron Falls (see below), the river failed to reach minor flood level at Cairns Airport.

Media and social media reports

With floods affecting or threatening so many towns and food-producing locations, media interest was intense, while video, photos and commentary were the main topics of social media traffic. Here is a selection:

• The ABC, as the country's official emergency broadcaster, provided a constant flow of news nationally and vital information to people affected locally. Some of its meatier articles, usually well supported by videos, photos and social media clips, were:
  • The situation as the Herbert River peaked in Ingham
  • This extraordinary video of floodwaters racing through an Ingham service station
  • A story on the stranded school children on 9 March
  • Planning to evacuate the children, and stories from flood victims as water began to fall
  • Lengthy helicopter video of flooding around Tully . Note the number of plantation crops under water.
  • The evacuation of school children, with help from local farmers, on 11 March after six days stranded. The main Bruce Highway was still cut, and was finally reopened to all traffic on 12 March.
  • ABC Rural gave this detailed description of the plight of farmers across the region, and two further articles on damage to sugar and banana plantations and their infrastructure, as well as sunflower crops. About 20% of Wilmar Sugar Mills' 600km railway network was estimated to be flood affected.
• On to other media. The flooding received international coverage, with this Reuters report bemused by the unexpected dangers our tropical flooding brings as wildlife struggles to survive or seeks out food from exciting new sources. There were plenty of social media posts about snakes deciding your abode was safer than its, reports of Bull Sharks near Halifax, and Rita Russo via Oz Cyclone Chasers shared a pic of a croc cruising in the backyard. Andrea Gofton also toyed with the idea of helping out this large spider precariously situated above a road while waters rose.
• Once it was clear Ingham would be isolated, supermarket shelves emptied rapidly. [ABC North QLD]
• This is Far North QLD. What do you do when the road's flooded and you can't get down to the local for your Friday afternoon beer? Take your tinnie! [Abbey Gardiner and Cairns Post via Oz Cyclone Chasers] Being sure not to exceed the speed limit, of course. [Karli Casanovas heading SE of Ingham] Or you can just go fishing. [ABC North QLD]
• Chris and Tracey Castles caught aerial views of Ingham, posted on Facebook and as part of 7 News and WIN News reports.
• There were numerous videos posted of drivers who had exaggerated expectations of the amphibious qualities of their vehicles. As the waters rose in Ingham, some made it but others were pushed or towed out. The many washaways like this meant that you never knew what lay under the surface of the water [ABC, ABC N QLD]
• Most spectacular of all was the sight of Barron Falls near Cairns. Since 1958, the upstream Tinaroo Falls Dam has reduced the falls to a trickle at most times, but massive amounts of rain on the eastern side of the Atherton Tablelands converted the trickle to an awe-inspiring torrent. Among the many videos posted on social media were these by Mod Films and drone footage by Mike Gaia. The ABC also ran a well-illustrated story on the falls, including "before" shots of their normal trickle.
• Equally spectacular was this aerial coverage of the Herbert River Falls, 100km NW of Ingham, perhaps best not watched if you're feeling queasy.