Wee Waa Levee Risk Management Plan And
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NARRABRI SHIRE COUNCIL WEE WAA LEVEE RISK MANAGEMENT STUDY AND PLAN DECEMBER 2019 VOLUME 1 – REPORT Job No: EE409 Date: December 2019 Principal: SAB File: WWL_V1_Report_Rev 1.4].doc Rev No: 1.4 Authors: SAB/TDR COPYRIGHT NOTICE This document, Wee Waa Levee Risk Management Study and Plan 2019, is licensed under the Creative Commons Attribution 4.0 Licence, unless otherwise indicated. Please give attribution to: © Narrabri Shire Council 2019 We also request that you observe and retain any notices that may accompany this material as part of the attribution. Notice Identifying Other Material and/or Rights in this Publication: The author of this document has taken steps to both identify third-party material and secure permission for its reproduction and reuse. However, please note that where these third-party materials are not licensed under a Creative Commons licence, or similar terms of use, y ou should obtain permission from the rights holder to reuse their material beyond the ways you are permitted to use them under the Copyright Act 1968. Please see the Table of References at the rear of this document for a list identifying other material and/or rights in this document. Further Information For further information about the copyright in this document, please contact: Narrabri Shire Council 46-48 Maitland Street, Narrabri [email protected] (02) 6799 6866 DISCLAIMER The Creative Commons Attribution 4.0 Licence contains a Disclaimer of Warranties and Limitation of Liability. In addition: This document (and its associated data or other collateral materials, if any, collectively referred to herein as the ‘document’) were produced by Lyall & Associates Consulting Water Engineers for Narrabri Shire Council only. The views expressed in the document are those of the author(s) alone, and do not necessarily represent the views of the Narrabri Shire Council. Reuse of this study or its associated data by anyone for any other purpose could result in error and/or loss. You should obtain professional advice before making decisions based upon the contents of this document. Wee Waa Levee Risk Management Study and Plan FOREWORD NSW Government’s Flood Policy The NSW Government’s Flood Prone Land Policy is directed at providing solutions to existing flooding problems in developed areas and to ensuring that new development is compatible with the flood hazard and does not create additional flooding problems in other areas. Under the Policy, the management of flood liable land remains the responsibility of local government. The State subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist councils in the discharge of their floodplain management responsibilities. The Policy provides for technical and financial support by the State through the following four sequential stages: 1. Data Collection and Flood Study Collects flood related data and undertakes an investigation to determine the nature and extent of flooding. 2. Floodplain Risk Management Study Evaluates management measures for the floodplain in respect of both existing and proposed development. 3. Floodplain Risk Management Plan Involves formal adoption by Council of a plan of management for the floodplain. 4. Implementation of the Plan Construction of flood mitigation works to protect existing development. Use of Local Environmental Plans to ensure new development is compatible with the flood hazard. Improvements to flood emergency management procedures. Presentation of Study Results The results of the flood study investigations commissioned by Narrabri Shire Council have been presented in two separate reports: Wee Waa Levee Flood Investigation dated June 2015. Wee Waa Levee Risk Management Study & Plan (this present report) The studies have been prepared under the guidance of the Floodplain Risk Management Committee comprising representatives from Narrabri Shire Council, the NSW Department of Planning, Industry and Environment, the NSW State Emergency Service and the community. ACKNOWLEDGEMENT Narrabri Shire Council has prepared this document with financial assistance from the NSW Government through its Floodplain Management Program. This document does not necessarily represent the opinions of the NSW Government or the Department of Planning, Industry and Environment. WWL_V1_Report_Rev 1.4].doc i Lyall & Associates December 2019 Rev 1.4 Wee Waa Levee Risk Management Study and Plan TABLE OF CONTENTS Page No. SUMMARY ............................................................................................................................... S1 1 INTRODUCTION ............................................................................................................ 1 1.1 Study Background .............................................................................................. 1 1.2 Background Information ...................................................................................... 1 1.3 Overview of LRMS Report .................................................................................. 1 1.4 Community Consultation ..................................................................................... 3 1.5 Insurance Industry Consultation .......................................................................... 3 1.6 Flood Frequency and Terminology ...................................................................... 3 2 BASELINE FLOODING CONDITIONS ........................................................................... 5 2.1 Physical Setting .................................................................................................. 5 2.2 Drainage System ................................................................................................ 5 2.2.1 Namoi River Floodplain .......................................................................... 5 2.2.2 Rural Floodways ..................................................................................... 7 2.3 Town Levee ........................................................................................................ 7 2.4 Flood History ...................................................................................................... 9 2.5 Design Flood Behaviour ................................................................................... 13 2.5.1 Background .......................................................................................... 13 2.5.2 Design Flooding and Drainage Patterns ................................................ 14 2.6 Impact of Flooding on Vulnerable Development and Critical Infrastructure ........ 16 2.7 Hydrologic Standard of Existing Road Network ................................................. 16 2.8 Potential Impact of a Partial Levee Failure ........................................................ 19 2.9 Potential Impact of Raising Rural Levees .......................................................... 20 2.10 Potential Impact of Coincident Namoi River and Local Catchment Flooding ...... 20 2.11 Potential Impact of a Change in Hydraulic Roughness ...................................... 21 2.12 Potential Impact of a Partial Blockage of Major Hydraulic Structures ................. 21 2.13 Potential Impacts of Future Urbanisation .......................................................... 22 2.14 Potential Impacts of Climate Change ................................................................ 22 2.15 Economic Impacts of Flooding .......................................................................... 23 2.16 Flood Hazard and Hydraulic Categorisation of the Floodplain ........................... 26 2.16.1 General ................................................................................................ 26 2.16.2 Flood Hazard Categorisation ................................................................ 26 2.16.3 Hydraulic Categorisation of the Floodplain ............................................ 27 2.17 Council’s Existing Planning Instruments and Policies ........................................ 29 2.17.1 General ................................................................................................ 29 2.17.2 Land Use Zoning – Narrabri Local Environmental Plan 2012 ................. 30 2.17.3 Flood Provisions – Narrabri LEP 2012 .................................................. 30 2.17.4 Flooding and Stormwater Controls ........................................................ 30 2.18 Flood Warning and Flood Preparedness ........................................................... 34 2.19 Environmental Considerations .......................................................................... 35 3 POTENTIAL FLOODPLAIN MANAGEMENT MEASURES ........................................... 36 3.1 Range of Available Measures ........................................................................... 36 3.2 Community Views ............................................................................................. 36 3.3 Outline of Chapter ............................................................................................ 38 Cont'd Over WWL_V1_Report_Rev 1.4].doc ii Lyall & Associates December 2019 Rev 1.4 Wee Waa Levee Risk Management Study and Plan TABLE OF CONTENTS (Cont'd) Page No. 3.4 Flood Modification Measures ............................................................................ 38 3.4.1 Town Levee Upgrade ........................................................................... 38 3.4.2 Upgrade of Stormwater Drainage System ............................................