Arrangements for Debriefs / After Action Reviews

Part 4 Recovery

4.3 Arrangements for Debriefs / After Action Reviews

4.3.1 As soon as possible after flooding has abated, the SES Local Controller will advise participating organisations of details of response operations after action review arrangements.

4.3.2 The SES Local Controller will ensure that adequate arrangements are in place to record details of the after action review and each item requiring further action is delegated to an organisation or individual to implement.

4.3.3 The SES Local Controller will pass the results of the debrief to the SES Sydney Western Region Controller.

4.3.4 Follow-up to ensure the satisfactory completion of these actions will be undertaken by The Hills Local Emergency Management Committee.

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CLASSIFICATION: SES-IN-CONFIDENCE THIS MATERIAL IS COPYRIGHT

for use by the general public or untrained persons, and is not a substitute for professional advice and/or training. Untrained persons should not use this map publication unless they are under the supervision of trained and qualified emergency management personnel. The State Emergency Service disclaims any liability (including for negligence) to any person in respect of anything

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Map 1

Sackville North

South Maroota Lower Portland East

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Map 2

Annex A

The Flood Threat

Introduction

1. The most severe threat, however, is flooding from the Hawkesbury River along the Shire’s northern boundary where small areas of floodplain land exist. Because this reach is estuarine within the Shire, marine influences can be important in determining the characteristics and severity of flooding.

The Hawkesbury River System

2. The Hawkesbury River catchment is one of the largest on the coast of New South Wales, with a total area of 22,000 square kilometres stretching south beyond Goulburn and west to the Great Dividing Range. About 18,500 square kilometres of this catchment are above Wisemans Ferry.

3. The major tributaries are the Mulwaree, Wollondilly, Nattai, Wingecarribee, Kowmung, Coxs, Warragamba, Grose, Colo and MacDonald rivers and South, Eastern, Webbs and Mangrove creeks. The upper and middle sections of the Hawkesbury River basin house a number of significant water storage dams, the largest of which is Warragamba Dam which controls 41%

of the total catchment. Most of the catchment, especially the western and northern portions, is made up of steep, rugged, forested country.

4. The significant sources of flooding in the Baulkham Hills Shire are the main arm of the Hawkesbury River, which flows through Sackville, and the Colo and MacDonald rivers, which together drain the northernmost portion of the total Hawkesbury River catchment. These three streams and Webbs Creek join just upstream of Wisemans Ferry.

5. Large flood flows on the Hawkesbury and Colo rivers, reaching Wisemans Ferry simultaneously and in conjunction with an oceanic storm surge, would represent a severe flood threat. Flood flows from Wisemans Ferry downstream can be of high velocity, though this would be less true if raised ocean levels and large waves retarded flood drainage.

6. A factor which acts to mitigate the severity of flooding on the lower Hawkesbury River is the existence of a major constriction, the Sackville Gorge, between Ebenezer and Wisemans Ferry.

Very large floods are ponded in the Windsor-Richmond area and can escape only slowly to the lower river via a long and narrow waterway.

7. Tsunami could also cause flooding on the islands and in other low-lying areas in the estuary of the lower Hawkesbury River, but this flood-producing mechanism is very rare.

Weather Systems and Flooding

8. Flooding on the lower Hawkesbury River can occur at any time of year. The wettest part of the year is normally summer but some of the worst floods seen on the river, including the record flood of 1867, occurred during the winter months. Many parts of the Hawkesbury catchment have received 175mm or more of rain in a 24-hour period and some have recorded falls of over 250mm in such a period.

9. The most common flood-producing mechanisms on the Hawkesbury River system are East Coast Low Pressure systems. Such systems appear on weather maps as depressions located off the New South Wales coast, usually in the cooler months and usually moving slowly southward. They can develop in other seasons, however, and can move in a northerly direction or remain stationary for several hours ( or in some cases a small number of days).

10. The most severe floods experienced on the Hawkesbury River have resulted from East Coast Low Pressure systems. Among these was the flood of record in June 1867. More recently, flooding in June 1964, March 1978, August 1986 and August 1990 resulted from similar weather systems.

11. East Coast low pressure systems can also generate oceanic storm surge conditions and large waves can occur as a result of associated gale-force and storm-force winds. Such conditions may lead to incursions of sea water onto land that is usually not flooded and to the retarding of flood flows from the Hawkesbury River. The larger floods that have occurred on the Hawkesbury River have often been accompanied by storm surges of 0.1-0.3 metres in Broken Bay. These effects are most apparent if storm surge conditions occur during times of spring or extreme high tides.

12. Less commonly, flooding could also occur as a result of monsoonal troughs stretching from northern Australia or from the southward passage of ex-tropical cyclones originating in the Coral Sea. Both these mechanisms are associated with the summer and early autumn seasons;

in February 1990, rain from an ex-tropical cyclone (Nancy) caused a rise on the Hawkesbury River. In the winter months, the passage of series of cold fronts moving across the Hawkesbury catchment could also produce flooding.

13. Flash flooding from the passage of thunderstorms can also occur, especially in the warmer months. Such flooding can cause small creeks and artificial drains to surcharge but thunderstorm activity does not cause flooding on the Hawkesbury River or its larger tributaries.

Flood History

14. Floods have been quite frequent along the lower Hawkesbury River. The following table indicates the heights reached at four different locations in some of the most serious events recorded. It is noteworthy that the Hawkesbury, the Colo and the MacDonald rivers have made very different contributions in individual events. The 1867 flood was probably largely generated over the main stem of the river upstream of Sackville and over the Grose River catchment, with no great contribution from the Colo River or the MacDonald River. This appears also to have been the case in 1961 and 1990. The 1978 flood, however, resulted

mostly from a comparatively rare large event on the Colo River with significant contributions from the Hawkesbury and the MacDonald rivers. This was also the pattern in 1964 and in the lesser event of 1988.

Table 1: Recorded Flood Peaks, Various Events (m AHD)

Month, Year Windsor Sackville Colo River MacDonald River

Wisemans Ferry

(Webbs Creek)

June 1867 19.6 16.1 NA NA 6.0

June 1949 12.1 8.4 17.4 8.8 3.1

Nov 1961 15.0 10.4 9.3 3.8 3.2

June 1964 14.6 11.0 14.6 10.4 4.2

March 1978 14.5 10.7 20.7 11.2 4.8

July 1988 11.0 NA 17.5 7.9 2.8

Aug 1990 13.5 10.0 NA NA 4.3

15. Many more floods than these have occurred on the lower Hawkesbury River. Flooding was experienced in 1933, 1934, 1938, 1942, 1947, 1949, 1952, 1956, 1975, 1986 and 1992, at least, in addition to the dates cited in the table above. Not all of these were serious floods at Windsor.

16. Rates of flood rise are not high on the lower Hawkesbury River.

Design Flood Levels

17. Design flood levels are as shown in the table below in AEP (Annual Exceedance Probability) and ARI (Average Recurrence Interval (in years)) terms. Note that the AEP is a measure of the probability of occurrence of a flood of a particular level: a 1% flood has, each and every year, a 1% chance of being experienced at a specified location. At Wisemans Ferry there is a 1% chance each year of a flood reaching or exceeding 6.7 metres AHD at the Webbs Creek gauge and a 20% chance of a flood reaching or exceeding 3.2 metres.

18. These two levels can be expected (on average) to be experienced about once every 100 years and once every 5 years respectively. Note, however, that this does not mean that such floods will occur exactly once in the specified period: there are numerous recorded cases in New South Wales of floods of low AEP levels occurring two or three times in quick succession at a particular location.

19. Equally, very long periods may elapse between occurrences of particular levels of flooding.

An indication of this is that the 20% AEP (5-year ARI) flood level at Wisemans Ferry has not been reached or surpassed for more than 10 years.

Table 2: Design Peak Flood Heights (m AHD), Relevant Gauges

Location 20% AEP

(5-year ARI) 5% AEP

(20-year ARI) 2% AEP

(50-year ARI) 1% AEP (100-year ARI)

Wisemans Ferry (Webbs Creek) 3.2 4.4 5.6 6.7

Gunderman 2.4 3.3 4.3 5.2

Spencer 1.7 1.9 2.3 2.7

Brooklyn 1.5 1.5 1.6 1.7

20. Referring to this table and the previous one it would appear that the 1961 flood at Wisemans Ferry was of the order of the 20% AEP flood. The 1978 event (thought to have been a 1%

AEP flood on the Colo River and a 3% AEP flood on the Hawkesbury River at Windsor) was roughly a 3-4% flood at Wisemans Ferry. The 1867 flood approximated a 1.5% (once-in-65- years) event there despite being a 0.4% (once-in-250-years) flood at Windsor.

21. It appears, therefore, that a flood as rare and as large as the 1% AEP event has not been experienced in European history at Wisemans Ferry, or indeed on the lower Hawkesbury River anywhere in the Hornsby Shire, despite records which go back well over a century.

Extreme Floods

22. Bigger floods than have been seen by present residents, or in recorded history, must be thought of as being inevitable. Indeed, flooding much higher than would be expected in the 1% and 0.5% AEP events must be expected to occur, though it will not happen frequently.

23. A scientific estimate of the worst flood possible at a location is the Probable Maximum Flood (PMF). Such a flood would result from an extreme rain event occurring on an already saturated catchment. PMF values for gauges of relevance to the Hills and Hornsby Shires are shown in Table 3.

Table 3: PMF Levels (m AHD) at Relevant Gauges

Location PMF

Wisemans Ferry (Webbs Creek) 16.3

Gunderman 14.0

The PMF is estimated to be a 1 in 100,000 AEP event. It is most unlikely to be seen by present residents, therefore. Floods bigger than the 1867 event at Wisemans Ferry but considerably lower than the PMF could be experienced, however, and would have very severe consequences.

Remedial works at Warragamba Dam, involving the construction of a large auxiliary spillway, mean the dam is now considered to be safe from failure. This spillway operates only in extremely rare and large floods, however, and it is likely that a flood large enough to cause water to flow down the spillway would create a flood level at Wisemans Ferry of more than 6 metres. Heavy rain downstream of the dam or over the Colo or MacDonald rivers in the same event would likely create higher flood levels.

Annex B

Effects of Flooding on the Community

General

1. Most floods on the lower Hawkesbury River in the Hornsby Shire have only nuisance impacts, but rare severe floods do have serious consequences. In the lesser events the main effect is that the Singleton Rd between Wisemans Ferry and Singletons Mill is cut at several low points, usually for only short periods, and low-lying parts of the Laughtondale and Veeponic Caravan Parks are inundated. Two car ferries (Webbs Creek and Wisemans Ferry) are closed in low- level floods, cutting access from Wisemans Ferry to the Hawkesbury Shire and Gosford City.

Closure of these ferries occurs when a gauge height of 3.0 metres is reached on the Hawkesbury River at Windsor, or when significant flooding occurs on the Colo or MacDonald rivers. This effects school bus routes and other traffic. The RTA and SES advises the principal of Wisemans Ferry School and the superintendent of the Wisemans Ferry Pre-School of impending ferry closure.

2. In larger floods the closure of Singleton Rd lasts for longer periods, potentially resulting in a need for resupply of people who are cut off, and serious problems of inundation begin to occur. The whole of the Laughtondale Caravan Park is threatened as are a number of dwellings east of Laughtondale and between Singleton Rd and the river. In Wisemans Ferry, the kiosk at the ferry ramp can be inundated as can the changing shed/amenities block at the Showground and, in relatively uncommon events, the Wisemans Ferry Bowling Club. In August 1990, flood waters almost reached the floor level of the bowling club.

3. Flooding of low-lying portions of Brooklyn and vicinity and on the islands in the estuarine reach of the lower Hawkesbury River can also occur. Properties could be affected, especially in the more severe events, by a combination of high river levels and elevated ocean levels.

Under such conditions it is possible that vehicular access to Brooklyn via Brooklyn Rd would be restricted to high clearance vehicles.

4. It should be noted that the effects of flooding are not confined to inundation or to isolation alone. Debris loads carried by flood waters can be very damaging, especially in the case of sever floods or floods occurring after long periods in which few floods have occurred. Boats, jetties, boatsheds and other structures (including dwellings) can be seriously damaged by water and debris and ferries have been know to break free of their moorings and caravans to be washed away.

Severe Floods

5. Floods approximating the severity of the record 1867 event would inundate all of the Wisemans Ferry Country Retreat and the lower-lying shops and dwellings in the town. Such floods could last for some days and would probably require substantial resupply operations for residents who live along Singleton Rd. Some 20-30 dwellings above flood reach could be involved if they could not gain access to shops and other facilities by roads or fire trails to the south. Much of Wisemans Ferry would have to be evacuated.

6. It should be noted that the 1867 event peaked somewhat lower than the 1% AEP flood level at Wisemans Ferry and that more serious impacts must be expected in rarer, higher floods.

Flood Heights and Effects

Approximate Flood Effects at Different Heights (Webbs Creek gauge), Wisemans Ferry to Singleton Mill

Height (metres) Effect

1.2 Beginning of inundation of Singleton Rd and Laughtondale Caravan Park.

1.5 Beginning of closures along Singleton Rd (at greater heights, closure occurs at additional locations).

2.4 Dwellings between Singleton Rd and the river threatened by inundation.

3.0 Approximate height at which access road to Laughtondale Caravan Park is cut.

4.3 (Peak height 18/7/1990). Water almost up to floorboards of Wisemans Ferry Bowling Club; several evacuations from vicinity of town.

4.8 (Peak height 21/3/1978).

6.0 (Peak height 23/6/1867). All low-lying parts of Wisemans Ferry inundated along with several properties along Singleton Rd.

Annex C

Gauges Monitored by The Hills Local Headquarters

Stream Gauge Name

Gauge Number

Flood Classification (metres)

1% AEP PMF

Flood of Record (metres)

Flood Warnings

by CBM

Local Flood Advices provided

by SES Minor Moderate Major

Hawkesbury River

Windsor

Bridge * 212903 5.8 7.0 12.2 17.3 26.0 19.4 (1867) Yes

Lower Portland (Ferry) *

212407 4.6 6.1 7.6 8.8 11.5 (1867) Yes

Webbs Creek (Wisemans

Ferry) 212408 - 3.5 4.2 6.7 16.3 6.0 (1867) Yes

Notes:

1. The SES holds a Flood Intelligence Card for the gauges marked with an asterisk *.

Annex D

Dissemination Options for SES Flood Information and Warning Products

The Sydney Western Region Headquarters distributes SES Flood Bulletins, SES Evacuation Warnings and SES Evacuation Orders to the following regional media outlets and agencies:

Television Stations ABN Channel 2 ATN Channel 7 TCN Channel 9 Channel 10 NSW SBS TV

Sky News Australia

NHK Sydney (Japan Broadcasting Corporation) Commercial Radio Stations

2CH 2UE 2GB 2SM

702 ABC Sydney Radio National SBS Radio Sydney 96.1 The Edge FM Mix 106.5 FM Nova 96.9 FM 2 Day FM

Triple J (National) Triple M Sydney WS FM 101.7

Hawkesbury Radio 89.9 FM Newspapers and other Print Media

News Limited Fairfax Limited

Sydney Morning Herald The Australian

Daily Telegraph Sunday Telegraph Sun Herald Other Agencies

All other agencies listed under this plan will be sent flood bulletins.

Annex E

Template Evacuation Warning, Evacuation

Order and All Clear

Flood Evacuation Warning

[name] SES Region Headquarters

[Enter address] Telephone: (02)

[########]

Fax: (02) [########]

Issued [day] [date] at [time in civilian format (am,pm)] Email: [#########]

Radio stations are asked to immediately broadcast this message and repeat it.

Use of the Standard Emergency Warning Signal (SEWS) with this message is authorized.

Flood Evacuation Warning for [Enter location/s]

Authorised By: [ (name and operational position title) ]

As a result of the flood level predicted by the Bureau of Meteorology for [ location ] at [date/time] the State Emergency Service recommends that residents within the nominated areas should prepare to evacuate within the next [number] hours.

Residents should monitor the situation and be prepared to evacuate when instructed to do so. A Flood Evacuation Order will be issued by the SES if evacuation is required.

You can choose to go to friends or relatives. Alternatively, evacuation centres will be established at [ location/s ] where you can obtain temporary accommodation and other help.

To prepare for possible evacuation you should:

 Raise belongings by placing them on tables, beds and benches. Put electrical items on top.

You may be able to place light weight items in the roof space.

 Collect together medicines, personal and financial documents, mementos and photos

 If possible, check to see if your neighbours need help

 Make arrangements for care of pets or other animals, or take your pets with you when you evacuate

 Take three days’ supply of clothing and medicines

 Find out where to turn off the electricity and gas

 Continue to listen to a local radio station for updates

Don’t walk ride or drive through floodwaters – this is the main cause of death and injury during floods

For emergency assistance telephone the SES on 132 500 Web site: www.ses.nsw.gov.au

End SES Flood Evacuation Warning [Enter next update and currency details]

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