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Flooding in the Hawkesbury-Nepean Valley is influenced by the combination of several different processes and cycles within our atmospheric, hydrological and geomorphic systems.
Atmospheric systems
Atmospheric systems
El Niño - Southern Oscillation (ENSO)
El Niño - Southern Oscillation (ENSO)
El Niño and La Niña are two phases of the El Niño-Southern Oscillation (ENSO) climate pattern, which is a natural climate phenomenon. They occur when the Pacific Ocean and the atmosphere above it change from their neutral ('normal') state for several seasons.
El Niño occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become much warmer than average. As a result, a shift in atmospheric circulation occurs. Consequently, the heavy rainfall that would normally occur in Australia, moves to the central and eastern parts of the Pacific. This causes drier than average conditions for Longneck Lagoon.
In contrast, La Niña is characterised by a cooling of sea surface temperatures in the central and eastern tropical Pacific Ocean. This occurs as a result of trade winds becoming stronger which draws cooler, deep water from below. These same strong trade winds help to pile up warm surface waters in the western Pacific and to the north of Australia. As a result, the western Pacific area becomes more favourable for rising air, cloud development and rainfall. During La Niña events, Longneck Lagoon is likely to experience above average rainfall, more intense rainfall events and waterlogging which can lead to flooding.
East Coast Lows
East Coast Lows
East Coast Lows (ECL) are intense low pressure systems which occur, on average, several times each year off the eastern coast of Australia. ECLs are more common during autumn and winter but can occur all year round. ECLs can generate heavy and widespread rainfall that often leads to flooding, gale or storm force winds along the coast and adjacent waters, and very rough seas and heavy swells.
Each of the floods that occurred at Longneck Lagoon during 2021 and 2022 were preceded by an East Coast Low.
Source 2:Ask the Bureau: What is an East Coast Low?Bureau of Meteorology https://www.youtube.com/watch?v=FCgULsMi9iQ&t=1s
Hydrological systems
Precipitation and catchment functioning
A catchment area is an area of land where water collects when it rains. As the water flows over the landscape it finds its way into streams and creeks and down into the soil, eventually feeding a water body such as a river, lake or wetland.
The Hawkesbury-Nepean Catchment area is approximately 21,400 square kilometres and it includes a number of major rivers and tributaries that all eventually drain into the Hawkesbury River. This makes the Hawkesbury region, including Longneck Lagoon, susceptible to severe flooding. The largest flood on record occurred in June 1867 when flood waters reached approximately 19.7m at Windsor. Many lives were lost during this flood. The most recent flood of July 2022 was the worst flood to occur in the Hawkesbury region in 44 years reaching 13.93 metres at Windsor.
The number and intensity of floods experienced across the east coast of Australia is expected to increase with climate change. These extreme events have the potential to alter the physical and biological conditions of the Hawkesbury-Nepean Catchment and pose a greater risk to the increasing number of settlements in flood prone areas in the catchment.
Source 3: A map showing the Hawkesbury-Nepean Catchment. https://www.ses.nsw.gov.au/media/3366/geo-skills-map-1.pdf
Source 4:A map showing the average July rainfall for NSW.http://www.bom.gov.au/climate/maps/averages/rainfall/
Source 5:A map showing rainfall totals in NSW for July, 2022. The floods that occurred this month were the worst floods to occur in the Hawkesbury region for 44 years. http://www.bom.gov.au/climate/maps/averages/rainfall/
Source 6:A photograph of the toilet block during the July 2022 flood. Courtesy Longneck Lagoon EEC.
Source 7:A photograph of the Longneck Lagoon EEC classroom during the July 2022 flood. Courtesy Longneck Lagoon EEC.
Geomorphic systems
The Hawkesbury River 'bathtub effect'
The Hawkesbury River 'bathtub effect'
There are a number of major rivers and tributaries including the Colo, Coxs, Grose, McDonald, Wollondilly and Warragamba rivers that flow into the Hawkesbury River. When weather events such as East Coast Lows generate heavy, widespread rainfall across the catchment, a significant amount of water is drained into the Hawkesbury River in a short period of time.
Along the upper and middle course of the river, there are narrow sandstone gorges at Castlereagh and Sackville. During flooding, these gorges create natural chokepoints and in the case of Sackville Reach, act as hydrological bottlenecks. As a result, floodwaters from the river's tributaries back up and rise rapidly, causing deep and widespread flooding across the floodplain. This is known as the 'bathtub effect' because much like a bathtub with more water coming out of a tap that can be drained, the volume of water will continue to increase. Longneck Lagoon is located in a low point of the flood plain and is significantly impacted by these flooding events.
Watch the following video to gain a better understanding of the 'bathtub effect' and its impact on residents in the Hawkesbury and Penrith areas.
Source 9:
Photograph of sandstone cliffs at Sackville Reach. These large cliffs are the cause of the 'bathtub effect' and the extensive flooding that occurs further upstream.
https://www.sl.nsw.gov.au/stories/dyarubbin/chapter-5-gurangatty-story/5
Pre-work activity
Using the information above, create a concept map showing how the different systems contribute to flooding in the Hawkesbury-Nepean Valley.
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