INFRASTRUCTURE & ENGINEERING ATTACHMENT A REPORT ON PUBLIC EXHIBITION – KARUAH RIVER AND STROUD FLOODPLAIN RISK MANAGEMENT STUDY AND PLAN ORDINARY MEETING 26 FEBRUARY 2020 MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report Karuah River and Stroud Flood Study Update Volume 1 - Draft Report DRAFT Level 17, 141 Walker St North Sydney NSW 2060 Australia Revision B October 2019 rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1.docm www.advisian.com MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report Disclaimer This report has been prepared on behalf of and for the exclusive use of MidCoast Council, and is subject to and issued in accordance with the agreement between MidCoast Council and Advisian. Advisian accepts no liability or responsibility whatsoever for it in respect of any use of or reliance upon this report by any third party. Copying this report without the permission of MidCoast Council and Advisian is not permitted. Cover Photo: Floodwaters flowing across Cowper Street, Stroud on the morning of 21st April 2015 (Source: Gloucester Advocate) Project No: 301015-03792 Project: Karuah River & Stroud Flood Study Update Volume 1: Report Advisian Rev Description Author Review Date DRAFTApproval 0 Draft for Internal Review LC 31/01/2018 L Collins A Draft Report – Issued for Council LC CRT 5/02/2018 Review L Collins C Thomas B Draft Report for Public Exhibition LC CRT 23/10/2019 L Collins C Thomas Chris Thomas rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1 MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report Table of Contents 1 INTRODUCTION ........................................................................................... 1 2 NUMERICAL MODEL DEVELOPMENT.......................................................... 4 2.1 Available Data ............................................................................................................. 4 2.2 Modelling Approach ................................................................................................ 4 2.3 Hydrologic Model Development ......................................................................... 5 2.4 Hydraulic Model Development ............................................................................ 5 3 MODEL CALIBRATION AND VERIFICATION ............................................. 12 3.1 Overview of Model Calibration and Verification .......................................... 12 3.2 Selection of Model Calibration Events ............................................................ 12 3.3 Model Calibration – 21 April 2015 Event ........................................................ 13 3.3.1 Event Overview ....................................................................................................................... 13 3.3.2 Rainfall Data ............................................................................................................................ 17 3.3.3 Assessment of Rainfall Return Period ........................................................................... 18 3.3.4 Stream Gauge Data .............................................................................................................. 19 3.3.5 Review of Stream Gauge Rating Tables ....................................................................... 20 3.3.6 Ocean and Port Stephens Estuary Water Levels ....................................................... 22 3.3.7 Hydrologic Model Calibration .......................................................................................... 23 3.3.8 Hydraulic Model Calibration ............................................................................................. 27 3.4 Model Verification – 8 June 2007 Event .......................................................... 35 3.4.1 Event Overview ....................................................................................................................... 35 3.4.2 RainfallDRAFT Data ............................................................................................................................ 35 3.4.3 Stream Gauge Data .............................................................................................................. 36 3.4.4 Hydrologic Model Verification ......................................................................................... 37 3.4.5 Hydraulic Model Verification ............................................................................................ 38 4 DESIGN FLOOD ESTIMATION .................................................................... 41 4.1 Introduction............................................................................................................... 41 4.2 Design Rainfall .......................................................................................................... 41 4.2.1 Design Rainfall Depths ........................................................................................................ 41 4.2.2 Design Rainfall Spatial Pattern ......................................................................................... 41 4.2.3 Design Rainfall Temporal Patterns and Critical Storm Duration ........................ 42 4.2.4 Probable Maximum Precipitation ................................................................................... 48 4.3 Design Boundary Conditions at Port Stephens ............................................ 48 rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1 page i MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report 5 DESIGN FLOOD RESULTS ........................................................................... 50 5.1 Peak Design Flood Flows ...................................................................................... 50 5.2 Peak Design Flood Levels ..................................................................................... 52 5.3 Design Flood Mapping .......................................................................................... 54 5.3.1 Flood Hazard ........................................................................................................................... 56 6 REFERENCES ............................................................................................... 58 DRAFT rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1 page ii MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Draft Report Figure List Figure 1-1 Study Location 3 Figure 2-1 Hydrologic model layout 9 Figure 2-2 TUFLOW hydraulic model boundaries 10 Figure 2-3 TUFLOW hydraulic roughness zones 11 Figure 3-1 BoM radar rainfall rate imagery 10:00 pm 20 April to 2:00 am 21 April 2015 14 Figure 3-2 BoM radar rainfall rate imagery 2:30 am to 6:00 am 21 April 2015 15 Figure 3-3 24 hour rainfall totals, 9:00 am 20 April to 9:00 am 21 April 2015 16 Figure 3-4 Cumulative rainfall plot for the period 9 am 20 April to 9 am 21 April 2015 17 Figure 3-5 Comparison of recorded rainfall and design rainfall for 21 April 2015 event 18 Figure 3-6 DPI Water streamflow data for 20 to 22 April 2015 19 Figure 3-7 Analysis of Rating Curves for the Karuah River at Booral Gauge 21 Figure 3-8 MHL ocean and estuary water level data for 20 and 21 April 2015 22 Figure 3-9 Comparison of gauged and simulated flows for the April 2015 event 25 Figure 3-10 Comparison of recorded and simulated flood levels at the Booral gauge for the April 2015 event 27 Figure 3-11 Comparison of gauged and simulated hydrographs at the Booral gauge for the April 2015 event 28 Figure 3-12 Simulated peak flood levels for the April 2015 event 32 Figure 3-13 Simulated peak flood levels at Stroud for the April 2015 event, unblocked scenario 33 Figure 3-14 Simulated peak DRAFTflood levels at Stroud for the April 2015 event, with 20% blockage of Mill Creek bridges 34 Figure 3-15 Cumulative rainfall plot for the period 9 am 7 June to 9 am 9 June 2007 35 Figure 3-16 DPI Water streamflow data for 7 to 9 June 2007 36 Figure 3-17 Comparison of gauged and simulated flows for the June 2007 event 38 Figure 3-18 Comparison of recorded and simulated flood levels at the Booral gauge for the June 2007 event 39 Figure 3-19 Comparison of gauged and simulated hydrographs at the Booral gauge for the June 2007 event 40 Figure 4-1 ARR 2016 IFD design rainfall depth spatial pattern, 1% AEP 12-hour event 43 Figure 5-1 Flood level reporting locations 53 rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1 page iii MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report Figure 5-2 Applicable model result extents and 1% AEP critical storm durations 55 Figure 5-3 Provisional Flood Hazard Categories (Source: NSW Government 2005) 56 Figure 5-4 General Flood Hazard Curves (Source: ARR 2016) 57 DRAFT rp301015-03792lc_crt190928-Karuah River & Stroud FS_Volume 1 page iv MidCoast Council Karuah River and Stroud Flood Study Update Volume 1 – Report Table List Table 2-1 Adopted Manning's 'n' hydraulic roughness coefficients 8 Table 3-1 Highest recorded flood levels for the Karuah River at the Booral gauge (since 1968) 12 Table 3-2 Review of gauged flow volumes for the April 2015 event 20 Table 3-3 Adopted WBNM hydrologic model parameters for the April 2015 calibration event 24 Table 3-4 Comparison of simulated peak flood levels with flood marks at Stroud for the April 2015 event, unblocked scenario 30 Table 3-5 Comparison of simulated peak flood levels with flood marks at Stroud for the April 2015 event, with 20%