Hazard and Risk in the Hawkesbury-Nepean Valley
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HAZARD AND RISK IN THE HAWKESBURY-NEPEAN VALLEY Annex A Supporting document (NSW SES Response Arrangements for Hawkesbury-Nepean Valley) to the Hawkesbury-Nepean Flood Plan Last Update: June 2020 Hawkesbury-Nepean Valley Flood Plan Version 2020-0.11 CONTENTS CONTENTS ....................................................................................................................................................... 1 LIST OF MAPS ................................................................................................................................................... 2 LIST OF FIGURES ............................................................................................................................................... 2 LIST OF TABLES ................................................................................................................................................ 2 VERSION LIST ................................................................................................................................................... 3 AMENDMENT LIST ........................................................................................................................................... 3 PART 1 THE FLOOD THREAT ................................................................................................................. 4 PART 2 EFFECTS OF FLOODING ON THE COMMUNITY ........................................................................ 25 Flood Islands ........................................................................................................................ 25 Low Flood Island ................................................................................................................... 26 Trapped Perimeters .............................................................................................................. 26 High Trapped Perimeters ..................................................................................................... 26 Low Trapped Perimeters ...................................................................................................... 26 Areas with Overland Escape Routes ..................................................................................... 27 Areas with Rising Road Access ............................................................................................. 27 Indirectly Affected Areas ...................................................................................................... 27 LIST OF REFERENCES ...................................................................................................................................... 41 Annex A Hazard and Risk in Hawkesbury-Nepean Valley Page 1 Hawkesbury-Nepean Valley Flood Plan Version 2020-0.11 LIST OF MAPS Map 1: Hawkesbury-Nepean River Basin (212 in the AWRC classification) ........................................................... 5 Map 2: Floodplains in the Hawkesbury-Nepean Valley .......................................................................................... 7 Map 3: Wallacia Floodplain .................................................................................................................................... 8 Map 4: Penrith / Emu Plains / Castlereagh Floodplain ........................................................................................... 9 Map 5: Richmond/ Windsor / Wilberforce Floodplain ......................................................................................... 10 Map 6: Lower Hawkesbury Floodplain ................................................................................................................. 11 LIST OF FIGURES Figure 1: Sub-catchment contributions to flooding at Windsor from recent floods ............................................ 13 Figure 2: Seasonality of Hawkesbury-Nepean floods ........................................................................................... 14 Figure 3: The 'bathtub effect’ caused by some of the natural choke points in the Hawkesbury-Nepean Valley . 17 Figure 4: Comparison of the differences in flood levels and flood risk between the Hawkesbury-Nepean River and other floodplains ............................................................................................................................. 17 Figure 5: Hawkesbury-Nepean Flood History showing Major Floods at or above 12.2m at Windsor .................. 21 Figure 6: High Flood Island ................................................................................................................................... 26 Figure 7: Low Flood Island .................................................................................................................................... 26 Figure 8: High Trapped Perimeter ........................................................................................................................ 26 Figure 9: Low Trapped Perimeter ......................................................................................................................... 26 Figure 10: Area with Overland Escape Route ....................................................................................................... 27 Figure 11: Area with Rising Road Access .............................................................................................................. 27 Figure 12: Indirectly affected area ........................................................................................................................ 27 Figure 13: Estimates of the residential properties affected by flooding in the Hawkesbury-Nepean Valley dependant on flood size (Source: HNV Strategy Database, January 2020) ............................................ 33 LIST OF TABLES Table 1: Indicative Flow Travel Times for the Hawkesbury-Nepean River ........................................................... 16 Table 2: Hawkesbury-Nepean flood levels for different probabilities at various flood warning gauges ............. 23 Table 3: The main Hawkesbury-Nepean flood risk areas and the heights at which they are expected to be flood affected in the Richmond Windsor Floodplain ....................................................................................... 28 Table 4: The main Hawkesbury-Nepean flood risk areas and the heights at which they are expected to be flood affected in other areas ........................................................................................................................... 29 Table 5: Estimated number of properties and people affected by flooding in the Hawkesbury-Nepean floodplain ................................................................................................................................................ 31 Table 6: Estimated number of residential properties flooded by Local Government Area .................................. 32 Table 7: Summary of Utilities and Infrastructure Consequences Windsor / Wilberforce Floodplain .................. 38 Table 8: Summary of Utility and Infrastructure Consequences Penrith / Castlereagh Floodplain ....................... 39 Annex A Hazard and Risk in Hawkesbury-Nepean Valley Page 2 Hawkesbury-Nepean Valley Flood Plan Version 2020-0.11 VERSION LIST The following table lists all previously versions of this Annex. Description Date September 2013 as amended June 2014 June 2014 September 2015 update Sep 2015 AMENDMENT LIST Suggestions for amendments to this Annex should be forwarded to: Deputy Commissioner Operations NSW State Emergency Service PO Box 6126, 93-99 Burrelli St, Wollongong NSW 2500 Amendments promulgated in the amendments list below have been entered in this Annex. Amendment Description Updated by Date Number Annex A Hazard and Risk in Hawkesbury-Nepean Valley Page 3 Hawkesbury-Nepean Valley Flood Plan Version 2020-0.11 The Hawkesbury River flows generally North PART 1 THE FLOOD THREAT east from Yarramundi through the main urban and agricultural areas of Richmond and LANDFORMS AND RIVER SYSTEMS Windsor. It continues through rugged The Hawkesbury-Nepean catchment is around sandstone gorge country before discharging 21,400 square kilometres in size stretching into the ocean at Broken Bay. from Goulburn in the south almost to Singleton The Wollondilly River originates in farm land in the north-west. near Goulburn and is fed by smaller tributaries The Hawkesbury-Nepean River is around 470 including the Nattai and Wingecarribee Rivers. kilometers long. It flows generally in a north The Wollondilly River flows through Sydney’s easterly direction from its source near water supply catchment area into the Goulburn, until eventually discharging into the Warragamba River which in turn flows into Pacific Ocean north of Sydney at Broken Bay. Lake Burragorang. This lake is formed by the Warragamba Dam. Water that is discharged The Hawkesbury and Nepean rivers are through the dam flows into the Warragamba actually the same river with the Nepean River which joins the Nepean River below forming the upstream portion, and the Warragamba Dam. Hawkesbury the lower half. There are a number of other large rivers and creeks that The Coxs River originates to the west of the flow into the Hawkesbury-Nepean River Blue Mountains near the Capertee Valley. It system. The major ones include the: flows in a south easterly direction, initially through farmland, then through rugged • Wollondilly River bushland valleys to the south of Blue • Coxs River Mountains townships. It eventually flows into Lake Burragorang. • Grose