Category Archives: natural disasters

Disasters that are attributed to natural causes

Tropical Cyclone Seroja at landfall – How strong was the storm? 12/4/2021

Published by:

Tropical Cyclone Seroja at landfall - How strong was the storm? 12/4/2021

Tropical Cyclone Seroja has made landfall very close to the town of Kalbarri in Western Australia and has caused some significant damage to the town. In addition, the regional city of Geraldton was also impacted by gale force and damaging winds including a period of heavy rain as the storm passed over.

Tropical Cyclone Seroja at landfall - How strong was the storm? 12/4/2021
Tropical Cyclone Seroja at landfall - How strong was the storm? 12/4/2021

The storm was travelling unusually fast at landfall and reached a speed of at least 55 km/h. It had a relatively narrow track as it traversed over land towards the south east.  

Media reports have indicated that peak wind gusts reached 170 km/h and that the storm was a Category 3 system upon landfall.

While verified data is still being processed and updated, a number of weather stations within proximity of the storm have recorded the following peak wind gusts:-

Geraldton Airport - 120 km/h at 8.52 pm and 8.57 pm. However, not recorded in the detailed notes, a single peak gust of 121 km/h was recorded at 9 pm according to data taken from “Australian Weather News”.

Furthermore, significant peak wind gusts at Geraldton Airport occurred mainly between 8.23 pm and 9.08 pm on Sunday evening.

Morawa Airport - A single peak wind gust of 119 km/h occurred at 11.26 pm.

Shark Bay - Maximum peak wind gusts of 91 km/h occurred.

According to observations taken from the following sites (Taken from “The Australian Weather News” 12/4/2021)

Binnu Agricultural Station - A peak wind gust of 122 km/h occurred at 8.04 pm.

Morawa Agricultural Station - A peak wind gust of 132 km/h occurred at 11.21 pm.

At the present time, additional verified data is not available and the above represents the highest identified wind gusts known as at 8 pm 12/4/2021 (Sydney Time - New South Wales).

Tropical Cyclones across the world are categorized according to their location. For example, across the Atlantic Ocean, Eastern and Central Pacific, the Saffir Simpson Scale is used and CIMSS models use the scale. Generally the National Weather Service (USA) uses the scale to determine the strength of a tropical storm.

Tropical Cyclone Seroja was determined to peak as a Category 1 system at landfall on the CIMSS models. A Category 1 storm has sustained peak wind gusts of between 119 km/h and 153 km/h and based on available data, this storm would fit the Category 1 storm system. If verified peak gusts of 170 km/h did occur, then it is possible that the storm reached Category 2 on the Saffir Simpson Scale.

Note - A Category 2 storm has maximum sustained peak wind gusts of between 154 km/h and 177 km/h on the Saffir Simpson Scale.

The Bureau of Meteorology uses a different scale for measuring the strength of tropical cyclones which makes a comparison somewhat confusing. The scale is called the “Australian Tropical Cyclone Intensity Scale” which was introduced in 1989/1990 and under this scale:-

A Category 1 storm has sustained winds of 63 to 88 km/h and with peak wind gusts of 91 to 125 km/h.

A Category 2 storm has sustained winds of 89-117 km/h with peak wind gusts of between 126 km/h and 166 km/h.

A Category 3 storm has sustained winds of 118-157 km/h with peak wind gusts of between 167 km/h and 225 km/h.

A Category 4 storm has sustained winds of 158-198 km/h with peak wind gusts of between 226 km/h and 280 km/h.

A Category 5 storm has sustained winds exceeding 198 km/h with peak wind gusts greater than 280 km/h.

Hence on the Saffir Simpson Scale, this storm appears to have rated Category 1, possibly Category 2 but under the “Australian Tropical Cyclone Intensity Scale”, the storm would have fit as a Category 3 system.

The Australian system for determining the strength of a tropical cyclone has lower thresholds than that of the Saffir Simpson Scale.

Hence, depending on what scale is used, the strength of Tropical Cyclone Seroja will vary and can be confusing.

The storm has all but decayed and for the 24 hours till 9 am 12/4/2021, a narrow band of rain of between 50 mm and 74 mm occurred along its path. The highest verified rainfall along its path for the is 24 hours is 76 mm at Binnu West.

Source

1 - Australian Weather News 12/4/2021.

2 - Bureau of Meteorology 12/4/2021 - Australian Tropical Cyclone Intensity Scale 1989/1990.

3 - National Weather Service - Saffir Simpson Hurricane Scale.

Addendum to topic - Peak wind gusts

Further to the above topic, further data has been identified in which a peak wind gust of 139 km/h occurred at Binnu West at 9.47 pm Sunday and the Bureau of Meteorology has identified a peak wind gust to 170 km/h at Meanarra Tower near Kalbarri at 7.03 pm on Sunday. They appear to be the higher wind gusts not appearing in the day to day data sets from surrounding weather stations.

Hawkesbury River Flood – The aftermath – 26 March 2021

Published by:

Hawkesbury River Flood - The aftermath - 26 March 2021

Floodwaters are now receding and the extent of the damage is becoming obvious. My wife and I returned to Windsor Friday afternoon 26 March 2021 to view the water levels. Floodwaters at that time were still high but receding and the new Windsor Bridge was clearly visible again.

Much rubbish, litter, tree branches and furniture items had washed downstream and unlike last Sunday, the floodwaters now had a distinct brown muddy colour and an odour.

Hawkesbury River Flood - The aftermath - 26 March 2021
Hawkesbury River Flood - The aftermath - 26 March 2021

Roads were still closed including the Windsor Bridge with assessors viewing the bridge for damage and with cleaning crews undertaking a mammoth cleaning task.

Floodwaters rose at least 2 metres in height along the Windsor Riverwalk and much scouring of soil had occurred. A distinct brown mark had been left on trees to show how high the waters came.

Hawkesbury River Flood - The aftermath - 26 March 2021

The cleanup will take some time and insurance costs will take time to tally. Preliminary estimates would suggest that this event will exceed $1 billion but it is too early to verify this. This event has been declared a catastrophe.

Hawkesbury River Flood - The aftermath - 26 March 2021

The extent of the flooding on Thursday can clearly be seen on this NASA Worldview image at a scale of 10 km from Earth orbit. The flood is seen as a muddy brown river with the floodwaters across North West Sydney clearly visible.

Hawkesbury River Flood - The aftermath - 26 March 2021

The photos attached show some of the cleanup that is needed at Windsor Bridge and other locales and there is a need to remove debris and even entire trees that had toppled over.

The rain event that caused this was prolonged across 5 or 6 days which is unusual and many coastal localities received more than 300 mm of rain with some areas receiving more than 400 mm of rain.

There are exceptions to this including Mt Seaview which received 815 mm of rain in 5 days during this event, Kempsey has received 819 mm of rain in 19 days this month including 758 mm of rain during the event.

All photos attached to this event were taken Friday afternoon 26 March 2021 around Windsor.

Hawkesbury River Flood - The aftermath - 26 March 2021

Hawkesbury River Major Flooding – 21 to 24 March 2021

Published by:

Hawkesbury River Major Flooding - 21 to 24 March 2021

The Hawkesbury River basin in western Sydney at the base of the Blue Mountains passes through Penrith, Richmond and Windsor. Within this area, a number of flood plains exist that are prone to flooding and there have been numerus floods recorded over the years.

The topography of the river basin contributes to the flooding in which there are five tributaries but only one outlet to let the water out.

Since 1961, there have been 29 floods recorded with the 1961 event being the largest since the construction of Warragamba Dam.

Perhaps the most significant flood according to known records is the 23 June 1867 event where a flood peak of approximately 19.7 metres was recorded at Windsor. Windsor, Richmond, and Pitt Town at that time became small islands according to records. It is also understood that 12 people from the Eather Family were killed during this event.

The largest flood since the construction of Warragamba Dam was the November 1961 event in which waters reached at least 14.2 metres (Some articles suggest up to 14.5 metres) in height at Windsor.

The current flood along the river while a significant event (Most likely a 1 in 50 year event) does not and will not reach the levels of 1961 or 1867.

The current flood peak appears to have passed with the river reaching a maximum flood peak of 12.48 metres at Windsor SWC and 12.93 metres at Windsor PWD and there is evidence to suggest that the river level may have started to fall although slowly.

Hawkesbury River Major Flooding - 21 to 24 March 2021

It is identified on the attached graph that the major flood level at Windsor Bridge is 12.2 metres and as at 7.16 pm Wednesday evening, the river has been at major flood level for over 57 hours. This is a considerable amount of time for the river to be at major flood level.

The new Windsor Bridge which was thought to be high enough has failed its first major test with the bridge being submerged especially across the western side.

The images attached to this post were taken on Sunday afternoon prior to the maximum flood peak occurring.

Hawkesbury River Major Flooding - 21 to 24 March 2021
Hawkesbury River Major Flooding - 21 to 24 March 2021

This is certainly a major event with significant damage across the region to homes, infrastructure and cropping land and it is known that insurance will become an issue over coming days as claims are made.

Hawkesbury River Major Flooding - 21 to 24 March 2021
Hawkesbury River Major Flooding - 21 to 24 March 2021

On Tuesday 23 March, Cumberland City Council engineers explained to me in graphic detail the true potential for flooding for this area using the latest available flood maps. It is clear that higher floods have the potential to engulf much more of Richmond and Windsor urban area than that seen at the present time and it must be emphasized that this appears to be a 1 in 50 year event.

Heavy rain has again been a feature although a significant clearing trend passed over Sydney after 3 pm on Tuesday afternoon with the rain ceasing and cloud clearing out to sea. This will allow a cleanup to commence as flood waters recede.

For the 24 hours to 9 am 22/3/2021 the most significant rainfalls were:-

Sydney region

Kurrajong Heights - 147 mm

Colo Heights - 105 mm

Shanes Park - 90 mm

Penrith - 88 mm.

North Coast

Mt Seaview - 203 mm.

Woolgoolga - 182 mm.

Kempsey - 180 mm.

Elands - 174 mm.

Utungun - 172 mm

Coffs Harbour - Up to 182 mm.

Willina - 166 MM.

Such scenes are repeated up and down the New South Wales coast and even into south east Queensland with significant flood events on other north coast rivers occurring simultaneously.

For the 24 hours to 9 am 24/3/2021, the most significant rainfalls were limited to the far south coast including:-

Mt Darragh 162 mm.

Bodalla 127 mm.

Brogo Dam 116 mm.

Hawkesbury River Major Flooding - 21 to 24 March 2021
Hawkesbury River Major Flooding - 21 to 24 March 2021

Rainfall across Sydney eased considerably Tuesday with the highest total occurring at Stone Quarry Creek being 74 mm  and up to 35 mm around Campbelltown in the south west and at Portable to the north west.

The rainfall has ceased across most of New South Wales and affected areas which will now allow for a cleanup to commence which will take considerable time.

s2Member®