Floods and Water Quality

The number and intensity of flood events experienced across the east coast of Australia is expected to increase with a changing climate. These extreme events are likely to alter typical physical and biogeochemical patterns in estuarine systems and pose greater risk to settlements in flood prone areas.

Floods are difficult to monitor as they are unpredictable and physical measurement can be hazardous. Despite these challenges, we know floodwaters are typically polluted with high loads of fine sediment, nutrients, heavy metals, pathogens (from sewage) and other particles which enter waterways following removal of vegetation and increased urban development in the catchment. Sedimentation and nutrient recycling tend to occur as water flow slows down when the floods recede. With large quantities of freshwater entering an estuary during a flood, a freshwater layer tends to form over the saltier estuarine water resulting in increased stratification within the water column limiting nutrient and phytoplankton mixing.

Hawkesbury River, Sydney - March 2021 Flood.
European Union, Copernicus Sentinel-2 Imagery

Hawkesbury River, Sydney - March 2021 Flood.
European Union, Copernicus Sentinel-2 Imagery

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March 2021 Flood Event

Since 2004 Hornsby Shire Council has deployed ‘smart’ instrumentation along the Lower Hawkesbury estuary. These water quality monitoring probes are positioned at six locations to capture real-time information including salinity, temperature, chlorophyll-a and turbidity. The probes were active during the March 2021 flood event and provided valuable data recorded at a intervals of 15-30 minutes. This page provides a summary of the changes in water quality observed throughout the one in 100-year flood event.

As shown below, a series of satellite images represent the spatial extent of flood waters moving seawards before, during and following the March 2021 event. These images enable us to understand the physical extent of the flooded waters impacting flood prone areas and visualise the sediment transport occurring within the river system including the plume leaving the estuary. The monitoring information collected by the probes provides detailed measurements of the processes along the estuary as shown in the section below.

Flood debris swept downstream resulting in major cleanup efforts.
Hornsby Shire Council

March 2021 Flood Event - Water Quality Changes

The plots presented below capture the rapid changes in water quality along the Lower Hawkesbury estuary for the period between 1^st March 2021 and 3^rd May 2021. Significant rainfall occurred between the second and third week of March with 450 mm recorded by the Hornsby Rainfall Gauge on the 23^rd of March (note Windsor had 345 mm). Water levels in the river started to rise on the 19^th of March, Warragamba Dam reached full capacity on the 20^th March and commenced spilling with discharges of 400,000 to 200,000 ML/day. Floodwaters peaked at 12.9 m AHD in Windsor and 3.9 m AHD in Wisemans Ferry on the 24^th March. Whilst the elevated water levels associated with the flood had ended by the first week of April, we continued to see lower than usual salinities present in the Wisemans Ferry area for the following two weeks period.

Explore the water quality data captured during the March 2021 flood event using the map animation and plots below. You can select a water quality variable (Salinity, Temperature, Turbidity, Chlorophyll-a) that will be represented in the map animation and plots accordingly. Click and drag your mouse within the plots to zoom in or double click inside the plot to zoom out– (see Tutorial icon on Home Page). Temperature data is collected at different depths through the water column at Calabash Bay, Berowra creek, as this station is monitoring conditions in an unusually deep hole. An interactive plot will be displayed below when the variable Temperature is selected. The plot shows the water column stratification that occurred during the floods and how it lasted a month before it became well mixed.

Highly turbid waters and flood debris swept downstream resulting in major cleanup efforts.
Hornsby Shire Council