On 21 December 2011 a tropical low formed within an active monsoon trough over the Arafura Sea near the Northern Territory. The tropical low intensified over 23-24 December. At 3:30am on Christmas Day, the low – approximately 60 kilometres north of the Cobourg Peninsula – was named Tropical cyclone Grant.
Grant was declared a Category 2 tropical cyclone at 3.30pm, making landfall on 9.00pm. Once over land it weakened rapidly and was downgraded below tropical cyclone intensity on 26 December.
Grant reached wind speeds of up to 140 kilometres per hour and brought torrential rainfall to parts of the Cobourg Peninsula. Many trees were brought down in the community of Minjilang and the surrounding outstations and power supply was interrupted. Coastal erosion and flooding of low-lying coastal areas was reported between the Cobourg Peninsula and Nhulunbuy.
Natural Disaster Relief and Recovery Arrangements (NDRRA) assistance was made available for the Local Government Areas of West Arnhem Shire, East Arnhem Shire, Katherine Municipality, Roper Gulf Shire, and Victoria Daly Shire.
Derailing and chemical spill at Edith River
Overnight on 26 December and during the early morning of the 27 December, heavy rains fell over the Edith, Cullen and Fergusson River catchments, including 385 millimetres at Edith River Falls in 24 hours. Those rivers experienced flash flooding as a result.
The rapid rise of the Edith River on the morning of 27 December damaged the Edith River railway bridge, causing a 20 wagon freight train to derail while crossing over it. The train was carrying 1,500 tonnes of copper concentrate from OZ Minerals' Prominent Hill copper mine in South Australia when it derailed. Up to 1,200 tonnes of the copper oncentrate spilled into the Edith River, containing particles of both arsenic and uranium. The two drivers involved in the crash were airlifted to Katherine for treatment of minor injuries.
Over the following months, the Department of Lands, Planning and the Environment monitored water quality. Northern Territory Environment Department Chief Executive Jim Grant said copper concentrate is an environmental hazard and a threat to aquatic life; although it contains arsenic and silica, it is not rated as highly toxic.
The Stuart Highway also sustained significant damage for two days.