Hawkesbury-Nepean Valley Regional Flood Study
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INFRASTRUCTURE NSW HAWKESBURY-NEPEAN VALLEY REGIONAL FLOOD STUDY FINAL REPORT VOLUME 1 – MAIN REPORT JULY 2019 HAWKESBURY-NEPEAN VALLEY REGIONAL FLOOD STUDY Level 2, 160 Clarence Street FINAL REPORT Sydney, NSW, 2000 Tel: (02) 9299 2855 Fax: (02) 9262 6208 Email: [email protected] 26 JULY 2019 Web: www.wmawater.com.au Project Project Number Hawkesbury-Nepean Valley Regional Flood Study 113031-07 Client Client’s Representative Infrastructure NSW Sue Ribbons Authors Prepared by Mark Babister MER Monique Retallick Mikayla Ward Scott Podger Date Verified by 26 Jul 2019 MKB Revision Description Distribution Date 7 Final Report Public release Jul 2019 Final Draft Infrastructure NSW, local councils, Jan 2019 6 state agencies, utilities, ICA 5 Final Draft for Client Review Infrastructure NSW Oct 2018 Infrastructure NSW, local councils, 4 Final Draft for External Review state agencies, independent technical Sep 2018 review 3 Revised Draft Infrastructure NSW Jul 2018 2 Preliminary Draft Infrastructure NSW May 2018 1 Working Draft WMAwater Jul 2017 COPYRIGHT NOTICE Hawkesbury-Nepean Valley Regional Flood Study © State of New South Wales 2019 ISBN 978-0-6480367-0-8 Infrastructure NSW commissioned WMAwater Pty Ltd to develop this report in good faith, exercising all due care and attention. No representation is made about the accuracy, completeness or suitability of the information in this publication for any particular purpose. Infrastructure NSW shall not be liable for any damage which may occur to any person or organisation taking action or not on the basis of this publication. Readers should seek appropriate advice when applying the information to their specific needs. All content in this publication is owned by the State of NSW and is protected by Crown Copyright, unless credited otherwise. It is licensed under the Creative Commons International Attribution 4.0 Licence (CC BY 4.0), subject to the exemptions contained in the licence. The legal code for the licence is available at the Creative Commons link. The State of NSW allows this material to be reproduced in whole or in part, provided the meaning is unchanged and its source, publisher and authorship are acknowledged. The State of NSW asserts the right to be attributed as the commissioning agency of the original material in the following manner: © State of New South Wales 2019. Citation: WMAwater (2019). Hawkesbury-Nepean Valley Regional Flood Study, prepared for Infrastructure NSW, July 2019. For enquiries, please contact [email protected] 113031-07: HNV Regional Flood Study. Final Report (July 2019) I FOREWORD Context The Hawkesbury-Nepean Valley has the most significant flood risk exposure in NSW, if not Australia. To better manage this risk, the NSW Government released Resilient Valley, Resilient Communities: Hawkesbury-Nepean Valley Flood Risk Management Strategy (Flood Strategy) in May 2017. The Flood Strategy is a comprehensive long-term framework for the NSW Government, local councils, businesses and the community to work together to reduce and manage the flood risk in the valley. A key output of the Flood Strategy is the Hawkesbury-Nepean Valley Regional Flood Study (Regional Flood Study) – a technical document describing the existing flood behaviour of the main Hawkesbury-Nepean River from Bents Basin near Wallacia downstream to Brooklyn Bridge, and the backwater flooding associated with river flooding. Purpose The high flood risk in the Hawkesbury-Nepean Valley means that having access to the best available flood information is essential. The last regional flood studies were prepared in the mid-1990s. There have been significant changes to science and technology, as well as some changes to the valley landscape since then. There is also a need to consider the potential impacts of climate change on flooding. The Regional Flood Study was developed to provide contemporary flood risk information for the valley. Adopting best practice The Regional Flood Study is underpinned by best practice approaches and up-to-date flood guidelines as specified in: Floodplain Development Manual: the management of flood liable land (NSW Government, 2005), and associated guidance Australian Rainfall and Runoff: a guide to flood estimation (Ball et al., 2016) Managing the Floodplain: a guide to best practice in flood risk management in Australia (AIDR, 2017). This study identifies the full range of flood behaviour and flood hazard in the floodplain to inform decision-making consistent with the intent of the Floodplain Development Manual and Managing the Floodplain. How should the Regional Flood Study be used? The Regional Flood Study will be used to: help local communities understand their flood risk - providing accessible flood information including new flood maps to show flood extents and depths 113031-07: HNV Regional Flood Study. Final Report (July 2019) II inform: o the continual improvement of emergency management and evacuation plans which require a detailed understanding of how floods behave and vary o regional land use and road planning, particularly the development of a new regional land use planning framework and road evacuation master plan o local land use planning, including through local environmental plans, development control plans and other council flood plans and policies o more accurate pricing of flood risk to help asset owners make decisions about flood risk mitigation measures, and for the insurance industry to more accurately price insurance premiums. underpin the ongoing assessment of regional flood mitigation options. This Regional Flood Study supersedes previous regional studies for the Hawkesbury-Nepean River. It adopts a ‘Monte Carlo’ modelling approach, generating thousands of potential events to mimic the variability of actual floods in the valley. The new study has also used the best available guidance and information to assess the impacts of climate change on flooding. The study provides the best source of information for defining the limits of flood prone land (up to the probable maximum flood or PMF) and for regional flood evacuation planning. This Regional Flood Study covers a large geographic area and focuses on regional scale flooding. It does not include shorter-duration local catchment flooding or overland flow inundation. Where more detailed flood modelling has been undertaken for locations in the floodplain, these studies may provide a better understanding of local flood behaviour. For example, Penrith City Council’s Nepean River Flood Study# uses a more detailed model that may be more suited for the setting of flood planning levels within its study limits. The best available information may come from a range of different studies, which may overlap in footprint. For a complete understanding of potential flood behaviours, councils and other users should consider the interaction of regional and localised flooding. What is most fit for purpose may depend on location and the decision being made. Where the Regional Flood Study has revealed significant changes in flood levels, such as at Wallacia, more detailed investigations are required ahead of any decision to amend existing flood plans or policies. # The Nepean River Flood Study was adopted by Penrith City Council on 26 November 2018. 113031-07: HNV Regional Flood Study. Final Report (July 2019) III Independent review The Regional Flood Study was reviewed by a highly experienced, independent flood expert to check the validity and accuracy of the data, method and results. The review found that the methodology was appropriate for the regional scale of the study and adopted some of the most complex hydrological methods available. This represented some of the most rigorous assessment that has been undertaken in Australia. The reviewer reconfirmed the need to develop a more detailed, ‘two-dimensional’ hydraulic model to better represent the complex behaviour of flows in the floodplain. The former Office of Environment and Heritage (now part of the Department of Planning, Industry and Environment) also provided comment. Local councils in the Hawkesbury-Nepean Valley floodplain provided comment in relation to their areas of interest. Key outputs The Hawkesbury-Nepean Valley Regional Flood Study is a technical report comprising three volumes, as follows: Volume 1 – Main report Volume 2 – Appendices Volume 3 – Map book o Parts A/B – Flood levels, extents and depths o Part C – Provisional flood hazard o Part D – Hydraulic categorisation. The results of the Regional Flood Study are also being used to develop a new, interactive online flood mapping tool to make it easier for people living and working in the valley to understand their flood risk. In summary While major floods are rare, their consequences can be catastrophic. Maintaining contemporary flood information, as detailed in the Regional Flood Study, is critical for building increased awareness and resilience in the Hawkesbury-Nepean Valley – one of the most flood- exposed regions of Australia. This study has generated, for the first time, a large range of potential flood characteristics that better reflect the variability of floods that can occur in the floodplain. Simon Draper CEO, Infrastructure NSW July 2019 113031-07: HNV Regional Flood Study. Final Report (July 2019) IV EXECUTIVE SUMMARY Project overview The last regional flood studies for the Hawkesbury-Nepean Valley were prepared in 1996/1997. Since then there have been advances in the science of flood modelling and changes in the valley landscape. The understanding of the potential impacts