Unprecedented rainfall across tropical QLD leads to widespread flooding
Fri 8 Feb 2019
Massive rainfall in an unrelenting, stationary monsoonal trough across tropical north QLD brought severe flooding to the Townsville region in early February with over 3000 houses inundated. The rain also extended west across the Gulf Country to NW QLD where it was initially hailed as a godsend after a dry year that followed six dry or average years since 2011, but like the rain on the coast it went on and on, turning the country into an inland sea, killing an estimated half million cattle.
In and around Townsville, rain increased from Sunday 27 to Tuesday 29 January as the monsoonal trough moved south into the area. On Wednesday 30 and Thursday 31, rain increased with gauges recording between 100 and 200mm each day. This culminated in four straight days of increasingly torrential rain with most gauges reporting between 200 and 250mm in the 24 hours to 09.00 on Monday 4 February. Most of Townsville's suburbs reported 1.4 to 1.6 metres of rain over the 12 days to 4 February.
The heaviest falls were in the mountains south of the city, where the Ross River Dam is located, and those to Townsville's northwest where several stations reported over 2m of rain in 12 days including Upper Bluewater 2357mm and Paluma (Ivy Cottage) 2334. Townsville's main flood risk is from the Ross River in its central and southern suburbs and the Boyle River in the west. The Ross River Dam was built both as a water supply and to offer flood mitigation for the city. As the incessant rain continued, the spillway gates were progressively opened higher and higher. Then, in the six hours to 1500 on Sunday 3 February, Woodlands Alert, a raingauge on the western side of Lake Ross, reported 302mm, helping push the level of the impoundment to just under 250% of its normal water level. With no more flood storage available the decision had to be made to follow the flood operations plan and open the gates to their maximum. The discharge, which had been high before, was now increased to the full 1900 cubic metres of water per second flowing downstream into Townsville suburbs.
The area where floods were entering houses in Townsville gradually increased during the early days of February, but reached a maximum as the gates were opened. This level of water in Ross River Dam and extent of flooding had not been experienced before, so, with the BoM predicting further very heavy rain overnight into Monday, authorities were uncertain as to how widespread the flooding would be. Worst-case scenario modelling indicated that up to a quarter of the city's 82,000 homes could be affected by floodwater, so authorities voiced fears that 10,000 to 20,000 houses may be affected. Extraordinarily strongly-worded warnings were issued by the BoM and QLD Police.
Fortunately, however, rain eased. As of 11 February, damaged houses around Townsville were still being identified, but Townsville City Council reported that 3,021 had suffered damage in the flooding: 1879 with minor damage, 1009 moderate and 133 with severe damage.
Gulf country and NW QLD
Townsville received its record rain from a stationary monsoon trough that pushed constant rain in from the Coral Sea, day after day. The same monsoon trough lay across Queensland's Gulf Country, but with the addition of a deep low, shown in this weather map for 04.00 EST on Sunday 3 February. The monsoon trough brought widespread rain across the Gulf Country which also pushed south across the low divide into rivers, such as the Diamantina, that flow SSW towards Lake Eyre. However, the deep low north of Mt Isa, which barely moved over eight days from Wednesday 30 January to Wednesday 6 February, brought rain of an intensity seldom seen in the area. Several towns and cattle stations recorded daily totals of 200 to over 300mm.
The heaviest rain fell over eight days from 29 January to 5 February with the heaviest totals in the Leichhardt and Flinders River Basins. Because rain fell in thunderstorms and heavy rain areas around the low, totals were extremely variable with some locations recording 8-day totals only around 200mm while others exceeded 600mm due to several days of torrential rain. Gereta Station, 85km NNE of Mt Isa, which had the misfortune to be almost directly under the low for most of the time, recorded 720mm over the 8 days, 232mm of which fell in 24 hours to 09.00 on the 3rd.
Jubilation at the breaking of the drought in NW QLD soon turned to dismay as inland seas grew larger, stranding and bogging cattle and making it nearly impossible to drop food to them, or for them to reach the food anyway. Cattle caught up against fences in deepening water soon died of exposure as estimates of livestock losses mounted. Around half a million are believed to have died.