Gascoyne River Flood

16 - 20 December 2010

Summary

Heavy rainfall from a monsoonal low caused major flooding of the Gascoyne River.

Meteorological Description

On 11 December, a monsoon trough developed over tropical waters to the northwest of Western Australia. The trough strengthened over subsequent days with a monsoon low developing within the trough on 15 December, about 520 kilometres northwest of Exmouth. The low slowly moved to the south southeast towards the upper west coast with strong to gale force winds well away from the centre. Heavy rainfall began in the Gascoyne River catchment during Thursday 16 December and continued until Saturday 18 December. Heavy rainfall also occurred over parts of the Wooramel, Murchison, Lyndon-Minilya and Ashburton River catchments. From 19 December the low started to move to the southwest, away from the Gascoyne coast.

Track of the Monsoonal low from 15 December to 20 December.

Rainfall

Significant rainfall occurred in western parts of the Gascoyne with large parts of the region recording over 150 mm and a few sites recording over 300 mm (see map and table below).

Western Australian Rainfall totals for the week ending 20 December 2010.

Table 1: Preliminary five day rainfall totals at selected sites in the Gascoyne from 16 to 20 December 2010 ( - indicates gauge not read). Daily rainfall is recorded in the 24 hours to 9am.

The significance of the rainfall event for the region can be put in context by the fact that December mean rainfall is in the order of 5 mm, whilst mean annual rainfall is between 200 mm and 300 mm. So several locations would have recorded their annual average rainfall in one or two days during the event.
Many locations in the west Gascoyne broke their December daily rainfall record on the 17th, while Carnarvon Airport, Meeragoolia and Shark Bay Airport broke their highest annual daily rainfall records.
Carnarvon Airport recorded 207.8 mm in the 24 hours to 9am on 17 December 2010, which is the wettest day on record for Carnarvon since records commenced in 1883, easily breaking the previous record of 119.4 mm observed at the Post Office on 24 March 1923. The average rainfall for December for Carnarvon Airport is 1.8 mm, whilst the annual average rainfall is 225.2 mm. Carnarvon also registered its wettest month on record, with a December total (to 23 December 2010) of 255.0 mm at Carnarvon Airport, breaking the previous record of 219.7 mm recorded at the Post Office in June 1895.

Infra red satellite image at 00:30 WST 17 December, about the time of the maximum rainfall rate at Carnarvon. MTSAT image courtesy of Japanese Meteorological Agency.

Impact

The Gascoyne River began responding to heavy rainfall from late Friday night to early Saturday morning, and continued to rise to become one of the highest floods on record. The damage bill is yet to be finalised, but initial estimates are in the order of 100 million dollars. Vast horticultural crops and pastoral livelihoods have been destroyed, going from drought to flood in less than 24 hours. At least 2000 cattle drowned.

Highest river levels on record occurred at three of the five river gauging stations along the Gascoyne River from Saturday 18 December to Monday 20 December 2010.

Nine Mile Bridge gauging station (opened on the 1 February 1956), in the town of Carnarvon, recorded a height of 7.80m at 04:00 WST on Monday 20 December. The previous high was 7.63m in 1960, followed by 7.6m in 2000.

Fishy Pool gauging station (opened 1 April 1964), located 110 km upstream of Carnarvon and 60 km downstream of Gascoyne Junction, recorded a peak height of 15.53m at 09:00 WST 19 December 2010. The previous record high was 12.23m during the 1980 flood.

Jimba gauging station (opened on 13 December 1984), located near the town of Gascoyne Junction, recorded it's highest peak on record of 10.76m at 19:00 WST 18 December 2010 prior to instrument failure. The previous record high was 9.5m during the 2000 flood.

Below is a hydrograph for Fishy Pool, showing the change in water level over time during from Thursday 16 December to Tuesday 23 December.

These gauging stations are operated and maintained by the Department of Water.

It is difficult to conclude whether this flood was the 'Worst Flood on Record' for the Gascoyne River. No two floods are the same. Since 1960, levee banks have been built around the town of Carnarvon and increased development and agricultural land use have occurred. Since 2000, there has been further expansion. However, it can be concluded that Nine Mile Bridge and Fishy Pool recorded their highest levels.

Flooding around Carnarvon. Photos courtesy of Andrea Bride.

Flood Watch/Warning Summary

The first Flood Watch for the Gascoyne River catchment was issued at 13:30 WST on Wednesday 15 December, indicating that the monsoon low could bring rainfall totals between 150 mm and 200 mm over the next 72 hours. The Flood Watch was updated until it was upgraded to a Flood Warning on the morning of Saturday 18 December.

The Flood Warning continued until it's cancellation on the afternoon of Tuesday 21 December.