12-hours flood notice for city council in Sydney
The city of Parramatta, a district area of Sydney, Australia, has a 12 hours’ flood warning system for the Parramatta river that runs through the district. Last week, with the extreme heavy rainfall, the system allowed district authorities and inhabitants sufficient time to take preparatory actions.
Tail end of La Niña
Rainfall totals of up to 1,000 millimetres in only four days have been recorded along the coast of New South Wales, Australia. Although this weather phenomenon supposedly has come to a close, the 2020–21 La Niña still caused 1 in 20 year flood events. In areas north of Sydney flood events of up to 1 in 50 years occurred.
Sydney also had to cope with the spilling of the Warragamba Dam up the mountains in the hinterland. Because of the heavy rainfall, the dam, built for the water supply of the city, had insufficient capacity causing major flooding in Sydney’s western suburbs. This spillage did not reach the basin of the Parramatta river so the river only broke its banks where it usually does in the event of heavy rainfall.
Forecast information service
Following previous flood events, the city council of Parramatta asked Dutch consultancy firms Royal HaskoningDHV and Nelen & Schuurmans to install their Flash early warning system for the steep and highly urbanised catchment area.
The Flash system comprises a hydraulic model predicting water level rise in the river over the next 12 hours. Using a mix of inputs from recorded rainfall to rainfall forecast data from the Bureau of Meteorology, Flash produces a new flood impact forecast every thirty minutes.
Timely for appropriate action
The FloodSmart Parramatta dashboard allows emergency responders and local authorities to see all the catchment information in one place. This allows them to take appropriate action prior to a flood event including road closures, deployment of emergency response staff and sending public flood warnings.
Luckily, in this event Parramatta did not catch the full brunt of the weather. Yet, the Flash Flood Forecast technology allowed the council to understand the severity of the flooding and to stand down response staff early once the forecasted rain showed no further impact on sensitive catchment areas.