Wickliffe Flood Investigation RM2365 v1.0 FINAL Prepared for Glenelg Hopkins CMA December 2012 Cardno Victoria Pty Ltd ABN 47 106 610 913 150 Oxford Street Collingwood VIC 3066 Australia Phone: 61 3 8415 7500 Fax: 61 3 8415 7788 www.cardno.com.au In association with: Michael Cawood & Associates Pty Ltd 8 Stanley Street Chirnside Park VIC 3116 Phone: 61 03 9727 2216 Document Control Version Status Date Author Reviewer 0.1 Draft Feb 2012 Heath Sommerville HCS Rob Swan RCS 0.2 Draft Apr 2012 Heath Sommerville HCS Rob Swan RCS 0.3 Draft May 2012 Heath Sommerville HCS Rob Swan RCS 0.4 Draft Aug 2012 Heath Sommerville HCS Rob Swan RCS 0.5 Draft Sep 2012 Heath Sommerville HCS Rob Swan RCS 0.6 Draft Nov 2012 Heath Sommerville HCS Rob Swan RCS 1.0 Final Dec 2012 Heath Sommerville HCS Rob Swan RCS Cover image: January 2011 floods for Wickliffe Source: Glenelg Hopkins CMA © 2012 Cardno Victoria Pty Ltd All Rights Reserved. Copyright in the whole and every part of this document belongs to Cardno Pty Ltd and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person without the prior written consent of Cardno Pty Ltd. Cardno LJ5786 Cardno Pty Ltd Wickliffe Flood Investigation RM2365 v1.0 FINAL EXECUTIVE SUMMARY The Glenelg Hopkins Catchment Management Authority, in partnership with the Ararat Rural City Council received State and Australian Government funding to undertake the Wickliffe Flood Investigation. Funding for the Investigation has been made available through the Victorian Coalition Government’s Flood Warning Network – Repair and Improvement initiative and the Australian Government’s Natural Disaster Resilience Grants Scheme. Cardno have undertaken the Wickliffe Flood Investigation with the assistance of Michael Cawood and Associates. Michael completed the Flood Warning System review and recommendations and developed the VICSES Municipal Flood Emergency Plan (MFEP) Appendices for this investigation. The Wickliffe Flood Investigation is primarily in response to significant flooding in January 2011. The primary aim of this work was to undertake definitive flood investigations for the Hopkins River floodplain at Wickliffe and to undertake a comprehensive analysis with all of the available data to determine appropriate flood warning and response arrangements for the township of Wickliffe, including the derivation of flood levels and extents for planning purposes. Key Deliverables The Wickliffe Flood Investigation has examined the Wickliffe floodplain in detail and the findings of this investigation are summarised within this report. The key findings from the investigation include: x Developing background information for understanding the catchment upstream of Wickliffe. x Qualification of the January 2011 flood event. x Developing the design flood events for Wickliffe including the 20%, 10%, 5%, 2%, 1%, 0.5% and 0.2% AEP events and the PMF. x Developing the Victorian Flood Database (VFD) compliant maps. x Developing flood maps for Wickliffe including maps showing depth, water surface elevation, flood contours, velocity, hazard class and flood extents. x Developing possible planning maps for implementation by the Ararat Rural City Council including the Floodway Overlay (FO) and the Land Subject to Inundation (LSIO). These maps have been developed for the pre and post removal of the old Glenelg Highway Bridge decking by VicRoads (planned to be completed prior to July 2013). x Detailed review and recommendations of the Flood Warning System for Wickliffe. x Development of the Municipal Flood Emergency Plan (MFEP) Appendices for use in conjunction with the Victoria SES documentation. x The identification of the high risk areas within Wickliffe and properties that are at risk of overfloor flooding. x Assessment of possible mitigation options and a cost / benefit assessment of these options. Background Information Wickliffe is located within the municipality of Ararat Rural City. The study area includes the majority of the residential and commercial areas of Wickliffe. The area has generally flat topography. The study area incorporates the reach of the Hopkins River between the Burdett Lane in the north to the intersection of Chatsworth-Wickliffe Road and Huckers Road in the south. Within this segment a number of smaller tributaries join the Hopkins River at or upstream of the Glenelg Highway. A flow gauge is situated at Wickliffe on the west side of the Hopkins River. There are currently no flood related controls in the Ararat Rural City Council planning scheme. The township has historically been subjected to flooding and has no flood warning system. The study area includes the plains immediately upstream and downstream of Wickliffe, however the hydrology and assessment of the catchment flood characteristics incorporates the full upstream catchment that provides Cardno LJ5786 Page iii Cardno Wickliffe Flood Investigation RM2365 v1.0 FINAL flow to Wickliffe. Wickliffe lies on the Hopkins River and has a catchment area of approximately 1,347 km2. This upstream area includes the Grampians National Park and the western uplands areas around Ararat. The elevation for the Wickliffe catchment and for the full Hopkins River is shown in Figure 1.2. The highest elevation within the catchment is Mt William which has a peak elevation of 1,167 mAHD. The township of Wickliffe is at approximately 200 mAHD. The average annual rainfall across the catchment is relatively uniform with rainfall varying from 550 to 600 mm from Wickliffe to the upper catchment. Hydrology The catchment topography is varied with the upper reaches including parts of the Grampians National Park which are very steep. However, the majority of the catchment is flat with the Hopkins River meandering across a low grade floodplain. Antecedent conditions within the catchment play an important role in the losses within the catchment due to the flat nature and multiple storages along the tributaries of the Hopkins River. Table i shows a summary of the streamflow information used in the flood investigation. There are only two streamflow gauges on the Hopkins River upstream of Wickliffe. Table i Streamflow data used in the Wickliffe Flood Study Gauge Gauge Name Source Flow type Start Date End Date Missing No. Periods ‘Red Book’ Monthly Inst. max Jul-1920 Dec-1981 1934 - 1942 236202 Hopkins River at Wickliffe Data Warehouse Daily Inst. max May-1964 Aug-2011 - 236219 Hopkins River at Ararat Data Warehouse Daily inst. Max Jun-1989 Nov-2011 - Flood frequency assessment was undertaken on the Wickliffe streamflow gauge to determine the peak flow rates associated with the design Annual Exceedence Probability (AEP) events. The design events were simulated using the method specified in Australian Rainfall and runoff (AR&R, 1987) and using the rainfall runoff program RORB. The Probable Maximum Flood (PMF) was developed using the Generalised Southeast Australia Method (GSAM) in accordance with the Bureau of Meteorology (BoM, 2003). The peak design events are summarised in Table ii for the main Wickliffe gauge and for the tributary designated as the Floate Lane Tributary to the North West of Wickliffe. Table ii Wickliffe RORB design results Floate Lane Wickliffe Tributary CL ARI (years) AEP IL (mm) Critical (mm/hr) Peak flow RORB Peak Peak Flow Rainfall target (m3/s) flow (m3/s) (m3/s) Duration 1 in 5y 20% 15 1.7 65 67.0 18h 3.6 1 in 10y 10% 19 1.8 85 85.3 24h 4.4 1 in 20y 5% 22 2.25 110 109.8 24h 5.6 1 in 50y 2% 25 2.7 152 152.1 18h 7.5 1 in 100y 1% 29 3 194 196.0 72h 9.1 1 in 200y 0.5% 33 3.2 246 249.1 72h 11.5 1 in 500y 0.2% 37 3.7 336 338.4 18h 15.0 PMF PMF 37 3.7 N/A 4,600 36h 30.0 Climate change was assessed with the maximum increase in rainfall intensity explored at 32%. The 10% AEP climate change peak flow is predicted to increase to 231 m3/s which is higher than the current 1% AEP peak flow rate. The 1% AEP climate change event has increased from approximately 200 m3/s up to 537 m3/s which is Cardno LJ5786 Page iv Cardno Wickliffe Flood Investigation RM2365 v1.0 FINAL higher than the current 0.2% AEP flood event. Table iii shows the summary of the predicted climate change impacts. Table iii Climate change scenarios run through the hydraulic model 32% increase rainfall intensity at Hopkins River at Hopkins River at Wickliffe Wickliffe AEP (%) Design Flow CC Design Flow Increase (% from Peak Duration Peak Duration Rate (m3/s) Rate (m3/s) current) 10% 85.3 24h 231.7 18h 272% 1% 196.0 72h 537.4 72h 274% The 270% increases in peak flow rates are thought to be a result of the large catchment size and the sensitivity of the loss rates throughout the catchment. For the purpose of this investigation the increases in peak flow rates have been accepted as the applied method is the accepted approach to modelling climate change impacts. The resulting flood extents indicate that climate change has the potential to have a significant impact of the peak flow rates for the Hopkins River at Wickliffe. Qualification of the January 2011 Event A specific focus of this investigation was to explore and define the recurrence interval for the January 2011 flood event at Wickliffe. The January 2011 flood event is the largest flood event recorded at the Wickliffe gauge since 1920 when gauged records began. The peak level reached at the Wickliffe gauge was 5.89 m recorded on the 15th January 2011. The January 2011 event occurred following unusual high intensity rainfall which led to large volumes of rainfall falling from the 10th to the 14th January 2011.