Flooding

The monsoonal climate means that streams and rivers including the Gilbert River are typically dry from April to October. During the monsoon season, stream and river crossings commonly flood, cutting off access.

Notable high flows causing overbank floods occurred in 1956, 1965, 1974 and 2009. Peak flood heights are available from BOM from the early 1970s.

A number of houses are elevated to above 1974 flood levels, including Riverview, Blanncourt, and Rockfields.

Flood monitoring

People commonly call their neighbours to ask whether rain has occurred in an upstream tributary and therefore whether stream crossings might be flooded out, while upstream neighbours also notify downstream properties when the river is seen to flow, especially at the start of the wet season or where river heights reach a level where landholders might need to enact their flood preparations (about 3.5 m at Greenhills).

The Bureau of Meteorology receives river level and rainfall reports from a number of stations, including Green Hills and Riverview. The BOM considers a minor flood level to be 4m for both Green Hills and Riverview, and a major flood level to be 10m at Green Hills, 8.5m at Riverview, and 9.5m at Rockfields. Reports are also made from Cobbold Gorge, North Head, and Gilberton upstream, and Malacura on the Langdon river.
Water levels in the Gilbert can rise rapidly, e.g. estimated 1m/hour over three hours in 2009. Water levels in some tributaries can also rise rapidly, e.g. 6m in one day in the Little River following a rainfall event of about 80 mm in 2021.
Flood gauge near the Gilbert River
Flood gauge, April 2022

Flood planning

The Green Hills dam detailed business case identified a flood risk with a dam break. Although the dam was suggested to be constructed using a method that could be overtopped by flash floods, reducing damage to the structure, if the dam were to fail up to nine properties would likely be affected. Dam failure would incur loss and damages worth more than $200 million. The potential dam consequence hazard category was determined to be high risk, requiring a acceptable flood capacity of 1 in 20,000 annual exceedance probability.

The Detailed Business Case developed a 2D TUFLOW hydraulic model to simulate sunny day and flood dam failure scenarios. The hydraulic model extends from just upstream of the proposed upstream dam site to 140 km downstream of the proposed dam wall.

The Queensland Floodplain Assessment Overlay highlights potential inundation areas.

It is suggested that increasing sand and trees in the Gilbert may mean that a 1974-level flood could occur at lower flow levels in future.

Observations

Resources