Topic: NSW and SE QLD Rain Event / Flash Flooding Coffs Harbour and Sunshine Coast :28 Mar - 7 Apr 2009

[Antonio (stormboy)]

duration last night 29th to this day lunch emptied 3times

[Antonio (stormboy)]

SEVERE WEATHER WARNING!!!

Top Priority for Immediate Broadcast
NSW SEVERE WEATHER WARNING
for Flash Flooding
For people in the
Northern Rivers and Midnorth Coast
weather forecast districts.

Issued at 4:20 pm on Monday 30 March 2009

Synoptic Situation: 3:00 pm EDT Monday
A low pressure system off the northern NSW coast is moving very slowly and is forecast to develop further.

Steady rain which began this morning is expected to continue this afternoon and evening resulting in cumulative totals that may cause localised flash flooding.

Emergency services advise do not enter flood water. Stay well clear of creeks, storm drains and causeways.

Dangerous surf conditions are expected. Surf Life Saving Australia recommends that you stay out of the water and stay well away from surf-exposed areas.

For emergency help in floods and storms, ring the SES [NSW and ACT] on telephone number 132 500.

The next warning is due to be issued by 10:15 pm Monday.

This warning is also available through TV and Radio broadcasts; the Bureau's website at www.bom.gov.au or call 1300 659 218. The Bureau and State Emergency Service would appreciate this warning being broadcast regularly

[Michael Bath]

emptied how much exactly ?  It is still unclear and sounds like 3 x 120mm = 360mm !!!   Which means it must be getting more water in it than just from rain lol

Ballina Airport had 28mm to 9am and 52mm 9am to 8pm - half of which has fallen after 430pm

[Richary]

Briefly spoke to a friend's son at Woolgoolga (20km north of Coffs Harbour) tonight and he said the surf was running at about 8 foot. And him and my parents at Woolgoolga have both commented they have had some pretty heavy downpours.

[Antonio (stormboy)]

You see this is where I ask myself sometimes dose the observations on bom always show the best result NO!
Ballina 11pm 77 mm mines 103 or 105?? My friend in Evans head has one and he said it has been 90mm and my nanas in murwimbula said 155mm has fallen all since 9ish
It seems to me that the boms obs are not running right we have a swimming pool that has been overflowing all day and our yard huh well just a huge deep puddle even my dad said without looking at the rain gauge at least more than 100mm has fallen and I agree. Just my thoughts

ps- whats up with byrons obs they never work for rainfall???

[Colin Maitland]

Blue skies this morning with cloud sitting off Moreton Island, just supposed to be a few showers today increasing to rain tomorrow. But what I found amazing, and something that Richary has touch on, was the system that brought all the rain, also gave the Gold Coast and Duranbah (The Tweed, probably Fingals+ down the coast a bit,) 3.9 to 4.1 swell. There was a lot of tow in surfing yesterday. The pictures were awesome. They predicted the same for today, but looking at the wave rider buoys it seems to have dropped of this morning.

I have posted the Obs from the wave rider bouys.


The reading on these charts are the HSIG readings or average height for the recorded period, if you go into the site you will see the Hmax, or highest recorded wave for the same period ( http://www.epa.qld.gov.au/environmental_management/coast_and_oceans/waves_and_storm_tides/wave_monitoring/) click onto the site you want to look at then go to the wave hight plot to see max waves.

[Michael Bath]

The LOW is quite clear on the 2pm EDT VIS satpic attached. Winds are ESE at Yamba and N at Evans Head with pressures still falling.

There have been some very heavy falls along the Clarence Coast this morning and now in the Coffs Harbour hills, with 74mm at Red Hill in the past hour.

This weather system is supposed to move north tomorrow.

[Colin Maitland]

You really are getting a canning down there with this weather system. NSW has been issued a severe weather warning which states:

Top Priority for Immediate Broadcast
NSW SEVERE WEATHER WARNING
Flash Flooding, Damaging Surf, Locally Damaging Winds
For people in the
Sydney Metropolitan, Northern Rivers, Midnorth Coast and Hunter

Issued at 3:25 pm on Tuesday 31 March 2009

Synoptic Situation: 3:00 pm EDT Tuesday
A low pressure trough off the northern NSW coast is moving very slowly south and is forecast to develop further.

This is an update of the warning issued at 11:25 am Tuesday.
Very heavy rain may cause flash flooding in the Northern Rivers, Mid North Coast, Hunter and Sydney Metropolitan districts tonight and Wednesday. Note: rain has eased in most of the Northern Rivers district, but is expected to increase during Wednesday.

Damaging surf conditions, with waves exceeding 5 metres in the surf zone, are likely to produce significant beach erosion on the Mid North Coast, Hunter and Sydney between Smoky Cape and Port Hacking today.

Winds may average over 65 km/h in coastal areas of the Northern Rivers, Mid North Coast and Hunter, particularly with thunderstorms, with gusts in excess of 90 km/hr.

Emergency services advise do not enter flood water. Stay well clear of creeks, storm drains and causeways.

Emergency services advise you check your property regularly for erosion or inundation by sea water, and if necessary, raise goods and electrical items.

Surf Life Saving Australia recommends that you stay out of the water and stay well away from surf-exposed areas.

Emergency services advise you to keep clear of fallen power lines, stay indoors away from windows and keep children indoors.

A separate Flood Watch has been issued for NSW coastal rivers from the Queensland border to the Hunter, with Flood Warnings current for the Bellinger River, the Hastings River, Orara River and Coffs Creek at Coffs Harbour. Refer to warnings at www.bom.gov.au for updated information.

For emergency help in floods and storms, ring the SES [NSW and ACT] on telephone number 132 500.

The next warning is due to be issued by 10 pm Tuesday.

The ocean swell has been recorded as over 5M. The Brisbane wave rider Bouy (North Stradbroke Island) recently registered 4.6M.
 
Late edit: 4.6M wave swell is the HSIG reading or average swell height for that period. Just going through the records the Hmax, or highest recorded for the same period was of Brisbane was 8.1 M

[Michael Bath]

There is severe flooding in Coffs Harbour CBD right now with the creek about the same level as the November 1996 event and it could go higher. The hills have received over 300mm of rain and it's still going. The 4 hour rainfall record for NSW has been broken with 309mm at Red Hill.

I have attached the hourly rainfall figures.

Besides Coffs there is major flooding along many of the Mid North Coast rivers as the convergence zone has remained in the same area most of today. Bellingen appears to have copped some of the worst of it again (following the mid Feb flood)


MAJOR FLOOD WARNING
FOR COFFS CREEK AT COFFS HARBOUR
Issued at 6:09 PM on Tuesday the 31st of March 2009

Flood Warning Number: 4
 
Heavy rain is still being recorded upstream of Coffs Harbour township with 80
millimeters recorded at Perry Drive (upstream of Coffs Harbour township).

At 6pm the level of Coffs Creek at the Grafton St Bridge was at 5.12 metres and
still rising.  Flood levels similar to the November 1996 are expected within the
next 2 hours.

However more heavy falls are forecast over the next few hours.  At this stage,
it is not possible to predict the flood peak because of uncertainty over how
much more rain will fall.   


Predicted River Heights/Flows:

Coffs Creek - With further rain predicted rise close 5.4 metres between 7pm and
8 pm




MAJOR FLOOD WARNING
FOR THE BELLINGER VALLEY
Issued at 7:06 PM on Tuesday the 31st of March 2009

Flood Warning Number: 8

Further extreme rainfall of up to 195 millimetres has fallen in the Bellingen
Valley in the past 3 hours to 6pm.


Further moderate to heavy rain is forecast for the next 12 to 24 hours.

At this stage it is not possible to predict the flood peak because of
uncertainty over how much more rain will fall.
 
Predicted River Heights/Flows:
Thora: Reach 7 metres around midnight Tuesday  [Major Flooding] with further
rises possible.

Bellingen: Reach 8.7 metres around 10:00pm Tuesday [Major Flooding] with further
rises possible.

Repton: Reach 2.8 Metres [moderate Flooding] midnight Tuesday


Urunga: Reach 1.9 metres midnight Tuesday with minor to moderate flooding

[Antonio (stormboy)]

indeed coffs is sure suffering today just look at todays radar

See : 128km Radar Loop for Grafton, 16:00 30/03/2009 to 14:00 31/03/2009 UTC

[Harley Pearman]

I have just checked the Bureau of Meteorology Weather Stations around Coffs Harbour and 3 have recorded exceptional rainfalls for 31/3/2009 being:-

- Perry Drive Weather Station - 324 mm from 9 am to 9.30 pm which is 25.92 mm per hour for the period.
- Shepherds Lane Weather Station - 368 mm from 9 am to 9.30 pm which is 29.44 mm per hour for the period.
- Red Hill Weather Station - 414 mm from 9 am to 9.30 pm which is 33.12 mm per hour for the period.

A weather station just to the south being Coffs Harbour MO has recorded 91 mm in the same period.

Further a fall of 317 mm has occurred at Girralong and a fall of 271 mm has occurred at Bellingen. There have been big falls at Wooli (202 mm), 168 mm at Glenreagh and 128 mm at Dorrigo during the same period as well.

The area affected by the February floods have been affected again by this rain event.


Harley Pearman

[enak_12]

ok what a day! I just got home from a mates..I was at the park beach plaza in Coffs until 4pm today and left to find SES and police directing traffic through detours I eventually wound up stranded up shepherds lane (just a couple of kilometers from Red Hill) for those who know coffs as my estate was under water and the road back out within 10-15 mins in either direction below was cut off..The photos below are from my house, thanks to my brother for taking them..it is really surreal driving through coffs tonight there are a lot of cars just left up gutters and beside the road some with hazard lights still flashing and no one around and parts of town are blacked out still, this was a crazy flash flood my brother took photos from home and the front and back yard around my house was just a river of water we were really lucky as it just began to lap at our front door when it started to subside..

front yard


backyard

[Richary]

Looked pretty nasty on the news tonight up there. My parents at Woolgoolga have recorded 150mm so far from it, not nearly as much as the areas a little bit south. Thos Red Hill figures are amazing with 444 mm for the 24 hours.

[Harley Pearman]

Auburn Council's Drainage and Development Engineer has calculated the Coffs Harbour rainfall event at a 1 in 100 year event based upon the following data from the following Bureau of Meteorology weather stations:-

Coffs Harbour - 31/3/2009 to 1/4/2009 (9 am to 9 am being a 24 hour period):

Perry Drive - 351 mm which is 14.62 mm per hour for that whole period.
Red Hill - 444 mm which is 18.5 mm mm per hour for that whole period.
Shepherds Lane 402 mm which is 16.75 mm per hour for that whole period.

Much of this as per my previous post had fallen in a 12.5 hour period from 9 am to 9.30 pm 31/3/2009.

Siva explained to me that the calculations are made for the whole 24 hour period and rainfall intensity during that period will vary. For the 24 hour period, the event rates as a 1 in 100 year event.

The exceptional falls are localised. In Coffs Harbour the three weather stations that recorded it are located in the north and west of the city. There is a range of hills running east to west along the northern side of Coffs Harbour and the exceptional falls have occurred along the southern slopes of that range (Orographic rain).

Areas south of Coffs Harbour had 161 mm.

Another area where big falls occurred was the Bellingen Valley and surrounds such as:-

Girralong 478 mm
Bellingen 395 mm
Dorrigo 264 mm.

Other weather stations in the vicinity have recorded:-

Spicketts Creek 401 mm
Cooks Creek 304 mm
Kooroowi 274 mm
Bowra Sugarloaf 253 mm

Interestingly Point Lookout at 1,563 metres in elevation to the west had a little over 100 mm for the period 31/3/09 to 1/4/09 being 9 am to 9 am.

It appears that the topography of the valley has again played a big part in the amounts that have fallen at specific weather stations. The heaviest falls in the valley have occurred in a small area around the town of Bellingen, SW of town (Cooks Creek), areas along the Kalang River including Spicketts Creek (SE) and nearby hills.

Areas outside the valley had much lighter totals.


Harley Pearman

[Richary]

I came across this site on the Bureau of Meteorology website recently. It allows plots of Intensity-Frequency-Duration for rainfall events based on historical data. Basically you can see in how often a rainfall event of a certain duration and intensity will happen.

http://www.bom.gov.au/hydro/has/ifd.shtml

This system tells me that for the coords given for Coffs Harbour MO (I can't find Red Hill on my maps) that 97mm for an hour is a 100 year event. However 14.77mm/hour for 24 hours is also a 100 year event, which equates to under 360mm for the day so Coffs has certainly exceeded that with some of the readings. 444mm per day for Red Hill = 18.5 mm/hr.

Using Excel to extrapolate these figures shows the formula for this graph is years = 0.1601 times e to the power of (0.4434 times mm/hr). Plugging 18.5 mm/hr into that formula shows it is a once is 575 year event. (e is one of those weird numbers and is approximately  2.71828)

Interestingly on an annual basis you could expect a rainfall rate of 111mm/hr over a 5 minute period, or about 9mm falling in 5 minutes.