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2016 Floodplain Management Australia National Conference

THE VALUE OF IMMEDIATE POST-FLOOD EVENT DATA

CAPTURE: ILLAWARRA AUGUST 2015 FLOOD EVENT

B Patterson1, P Hart2 I Ghetti3, A Schofield3, T Carn4 1 Royal HaskoningDHV, Newcastle, NSW 2 Metasphere, North Sydney, NSW

3 Shoalhaven City Council, NSW 4 Spatial Technologies, NSW

Introduction On the 24th and 25th August 2015 an East Coast Low deposited up to 400mm across several catchments including Shoalhaven River, St Georges Basin and Lake Conjola catchments. Flooding was experienced in several communities including Shoalhaven Heads, Sussex Inlet and Lake Conjola. Royal HaskoningDHV was commissioned to undertake a rapid post flood water level

survey to capture this important data whilst it was available. Royal HaskoningDHV

teamed up with Spatial Technologies Pty Ltd to ensure effective and rapid survey data

collection. The scope of the post flood survey was to capture as many flood levels as

possible with a 5 day survey period (2nd – 6th September 2015) in the following

catchments:

• Shoalhaven River

• St Georges/Sussex Inlet

• Lake Conjola.

Survey Methodology All survey points were collected by using a GPS receiver connected to the SmartNet network. For quality control, a maximum limit of +/- 50mm was set for elevation (AHD) recordings. Daily benchmark surveys were conducted at State Survey Marks (SSM) in the Shoalhaven region to assure survey accuracy and consistency.

Three data samples were collected at each flood mark level resulting in an averaged:

• Easting (MGA) • Northing (MGA)

• Elevation/Height (AHD) • 3D Quality Report (based on the SmartNet real-time corrections).

The types of flood levels identified and surveyed typically fell into 1 of 4 following categories. Typical photos of each category are given below:

• Debris Lines / Wrack Marks – typically small woody debris and / or seagrass

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2016 Floodplain Management Australia National Conference

• Sediment lines on fences, buildings or other fixed objects

• Water lines on fences and buildings

• Levels marked by residents

Communities From Which Flood Level Data Was Captured Flood levels were surveyed in the following communities:

Shoalhaven River Catchment:

• Shoalhaven Heads • Bomaderry

• Nowra • Greenwell Point

St Georges Basin/Sussex Inlet Catchment:

• Sussex Inlet • Erowal Beach

• Sanctuary Point

• St Georges Basin

• Basin View

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2016 Floodplain Management Australia National Conference

Lake Conjola Catchment:

• Lake Conjola

• Conjola Park

• Fishermans Paradise • Cunjurong Point • Berringer Lake

Surveyed Flood Levels A total of 71 flood levels were surveyed. Each survey point was assigned a unique 3 digit code i.e. 001; 002 etc. which is used to identify each individual survey point across the range of deliverables: survey level, maps and photos. An example output of survey point locations is given below in Figure 1. Figure 1: Survey Locations at Sanctuary Point in the St Georges / Sussex Inlet Catchment

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2016 Floodplain Management Australia National Conference

Other Flood Related Information A lot of other very relevant and useful flood event information, other than surveyed levels, was gathered, simply by being present in the flood affected communities during the immediate post event period. Much of the information was anecdotal in nature and therefore should be treated with caution, but if used carefully has significant potential value to future floodplain management in the catchments. Information gathered included:

Shoalhaven River Catchment:

• Time of Peak Flooding at Shoalhaven Heads: 12 noon on Wednesday 26th

August 2015. • Historic flood marks and levels shown on Figure 2 below from inside a

residence at Shoalhaven Heads. • Time of Peak Flooding at the Automotive Services on Jerry Bolong

Road, Shoalhaven Heads was 2pm on Wednesday 26th August 2015. Figure 2: Historic Flood Levels at Shoalhaven Heads

St Georges Basin/Sussex Inlet Catchment:

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2016 Floodplain Management Australia National Conference

• Time of Peak Flooding at Riveria Caravan Park, Sussex Inlet was 8pm on

Wednesday 26th August 2015. Flood waters first came as overland flow from the road (just a few inches deep and flowing) then from the main river – 2 different sources and directions.

Lake Conjola Catchment:

• Time of Peak Flooding at Lake Conjola Caravan Park, Lake Conjola was

7.30pm on Tuesday 25th

August 2015.

• Owner of Lake Conjola Post Office has lived there for 42 years and the 2015 event was the worst he has seen, worse even than the 1972 event. Photo below is taken from the Post Office near the time of peak water level on

Tuesday 25th August 2015. Plate 5: Lake Conjola Post Office 25th August 2015

• Time of Peak Flooding on Conjola Creek, near 90 Anglers Parade, was

4.30pm on Tuesday 25th

August 2015.

• A lady living at the end of Killarney Road, Lake Conjola described the August 2015 event as the biggest event she has seen since circa 1940. She has lived at the property all her life.

• The Owner of Kidgeridge Farm, Fishermans Paradise, measured 506mm

rain in 60 hours and 240mm in the 14 hours between 4pm Monday 24th and

6am Tuesday 25th

August 2015. The gentlemen used to be on the Lake Conjola Estuary Management Committee.

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2016 Floodplain Management Australia National Conference

The Value of the Survey and Data Collection Council plan to use the survey data and anecdotal information collected for the following valuable purposes to advance their understanding of the event and future proactive floodplain management across the affected catchments:

i. Estimate the Annual Recurrence Interval (ARI) of the August 2015 event and its relative flood magnitude position in the flood history of each catchment;

ii. Assist with recalibrating existing hydraulic models and / or calibrating new models;

iii. As a template for future post flood event surveys.