Severe Flooding Isolates Wollongong: Monday 17th August 1998
by Michael Bath
The intense rainfall was caused by an almost stationary trough line. The trough was imbedded in the easterly flow from a strong blocking high SE of Tasmania. The alignment of this trough on the 17th was NNE to SSW from just east of Sydney to the Illawarra Escarpment. It was still persisting just off the coastline (in virtually the same location) on the morning of the 18th, with a spectacular line of thunderstorms on show (refer note below). Convergence along and to the east of the trough generated large cumulus clouds and some cumulonimbus. Winds forced the rain bearing clouds to move towards the SSW-SW along the trough which subsequently moved most of the heavy rainfall over the coastal strip south of Botany Bay and into the Wollongong area (click on the map for locations). Thunder was observed in the Illawarra from mid afternoon on the 17th and continued well into the evening.
During the 18th a small low developed in the trough and maintained the moist flow into the Illawarra. By the afternoon torrential rain was again falling in the Wollongong area, but this time concentrated in the more southern suburbs. Further evacuations resulted, particularly near Lake Illawarra. The rain abating during the night.
Heavy rain again returned to the Wollongong area on the morning of Wednesday the 19th August 1998.
The most intense rainfall occurred late afternoon on the 17th. Figures obtained from Darkes Forest rain gauge show that 110mm fell in 1 hour from 4.56pm. At Bulli Pass to the south, over 150mm rain fell in 2 hours from 6pm on the 17th, with 103mm in 1 hour from 6.22pm. Bulli Pass recorded 210.5mm in 3 hours from 5.06pm and a total of 357.5mm in the 12 hours from 9.55am on the 17th August.
(obtained from Bureau of Meteorology)
Illawarra 16/8 17/8 18/8 19/8 ================================================= Bellambi AWS 101 40 240 75 Berry 136/3 114 105 Bowral 22 39 61 40 Camden Ap AWS 44 58 50 0.4 Darkes Forest 116 74 271 31 Greenwell Point 74/3 117 108 Jervis Bay 16 27 88 163 Kiama 88 20 162 155 Moss Vale 27 34 45 46 Nowra 40 31 82 91 Robertson 66 78 158 90 Wollongong 80 32 316 91 Metropolitan 16/8 17/8 18/8 ======================================== Avalon 34 Belrose 64 30 20 Bankstown 50 58 19 Berowra 35 22 Bexley 81 73 Canterbury AWS 59 50 0.2 Castle Cove 63 56 27 Concord 24 Castle Hill 68/2 Cronulla 61 Epping 44/2 Frenchs Forest 67 28 26 Glenmore Park 23 34 20 Glenorie 29 33 3 Gordon 113/2 14 Granville 43 39 7 Homebush AWS 49 47 21 Holsworthy AWS 51 61 17 Horsley Park AWS 34 31 18 Liverpool 45 50 21 Lucas Heights 68 62 Merrylands 9 Mosman 47 30 52 Newport 36 23 Oatley 88 65 94 Parramatta 42 48 Penrith AWS 18 29 8 Prospect Dam 34 13 Richmond Ap 23 24 4 Riverview 62 30 15 Rose Bay 40 76 Seven Hills 29 36 Sydney 54 30 40 Sydney Ap 63 70 54 Turramurra 53 45 18 South Coast 17/8 18/8 19/8 ================================================= Araluen 43 105 75 Batemans Bay 39 125 Bega AWS 9 18 17 Bodalla 54/3 86 84 Braidwood AWS 12 60 20 Candelo 8/2 20 14 Eden 25 31 Merimbula 7 18 18 Milton 42 87 100 Moruya Heads 14 67 98 Narooma 6 41 73 Nerriga 12 26 25 Pambula 9/3 17 21 Ulladulla AWS 33 82 108
Hourly Rainfall Amounts are available for a number of sites in the affected areas. Figures shown are for Bulli Pass, Darkes Forest, Dombarton Loop, Lake Illawarra and Wongawilli. The data has been obtained from Manly Hydraulics Laboratory and permission has been obtained for its reproduction here.
A report was received during the morning of 18th August from Michael Thompson who lives at Shellharbour, south of Wollongong city (click the link for his report on the event):
"But I did see some sobering sites at Allens creek, this was the creek I saw the car bobbing around in last night. The creek is now well within its banks, but mud and debris are everywhere, along with a dozen drowned cars, with mud and grass over them from top to bottom. A couple look like they had been doing some bumping around as well. The bridge guard rails have reeds and rubbish about 2-3ft above road level, but the bridge itself is 3-4 ft above the road either side. Evidence indicates that the water was a torrent."
"One poor girl at work here, who lives beside a 'natural watercourse' at Corrimal was evacuated at 9pm last night when her house was knee deep in water, the water got to chest deep during the trip from her house to the emergency vehicles. She has not seen the house yet, and expects the worst. It is only one of numerous houses that would have been in a similar situation."
"Lake Illawarra is about 18 inches from flooding, but I'm 99% sure it will not get there. The caravan park residents are extremely lucky as only the northern creeks of Lake Illawarra hit huge flood levels. Had the same event been located 20 kms further south Mullet Creek and Macquarie Rivulet, whose catchments are larger would have been involved and the Lake would have went easily 2-3ft deeper. You can see this if you compare the Manly Hydro gauges at Bulli Pass with Wongawilli and Yellow Rock. The latter two being Mullet Creek and Macquarie Rivulet respectively."
The thunderstorms as viewed from Parramatta between 8.30 and 9.00 am. Photos by Michael Bath
Don White reports:
"Narrabeen Lagoon AWS had 43 mm in the hour from 6.45 and Mona Vale 41 mm in the hour from 6.43 am. The heaviest rain was in the Collaroy - Dee Why area - the cinema in Collaroy was flooded to a depth of 2 metres and there was major beach erosion caused by almost 1 metre of water flowing across Pittwater Rd from Collaroy Plateau and over the beach. It must be rare from beach damage on the east coast being caused by freshwater flowing from the land out to sea."
"My recorder at Collaroy had 123 mm in 90 minutes from 7.30 am when he measured to 9 am when the rain stopped. His estimate of 100 mm in 40 minutes from 7.30 could be taken as fairly reliable - he is an interested weather watcher and has been recording rain for me for 20 years."
"The 'storm' hugged the coast further south - Little Bay had 49 mm in the hours from 7.03 am as well."
This image obtained from the Bureau of Meteorology
MSL Analysis 4pm 17th August Local Time
These images obtained from James Cook University
Infrared Satellite Images at 1pm, 4pm, 7pm and 10pm on 17th August Local Time.
Updated: 14th January, 2003
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