SH8 Beaumont Bridge Replacement ––– Hydrological Assessment Contact Details Name: Lennie Palmer L3, Gartshore House, 116 Cameron Rd PO Box 646, Tauranga 3140 New Zealand Telephone: +64 7 578 2089 Mobile: +64 27 626 3848 Document Details: Date: October 2018 Reference: 6-CT012.00 Status: Third Issue Prepared by Lennie Palmer Senior Hydrologist Reviewed by Dr Jack McConchie Technical Principal - Hydrology Approved for release by Mike Davies Senior Transportation Engineer www.wsp-opus.co.nz ©WSP Opus | October 2018 Page i SH8 Beaumont Bridge Replacement ––– Hydrological Assessment Contents 1 Introduction ......................................................................................................................................................................................................... 1 2 Hydrological Analysis ................................................................................................................................................................................... 2 2.1 Flow data................................................................................................................................................................................................. 2 2.2 Flow analysis ......................................................................................................................................................................................... 3 2.3 Frequency analysis .......................................................................................................................................................................... 4 2.4 Estimation of the Q 1000 ................................................................................................................................................................. 5 2.5 Hydrographs ........................................................................................................................................................................................ 6 3 Climate change ................................................................................................................................................................................................ 7 4 Low flow analysis ............................................................................................................................................................................................ 9 5 Summary .............................................................................................................................................................................................................. 11 6 References ........................................................................................................................................................................................................... 11 www.wsp-opus.co.nz ©WSP Opus | October 2018 Page ii SH8 Beaumont Bridge Replacement ––– Hydrological Assessment Document History and Status Revision Date Author Reviewed by Approved by Status 1 August 2018 Sheryl Paine Jack McConchie Mike Davies First Issue 2 September Lennie Jack McConchie Mike Davies Second issue 2018 Palmer 3 October 2018 Lennie Jack McConchie Mike Davies Third issue Palmer Revision Details Revision Details 1 First issue of the docu ment 2 Updated climate change adjustments based on MfE 2016 Climate Effects projections for Otago region 3 Updated flow design to include Q 1000 based on Bridge Manual Amendment 3. Updated climate change based on MfE 2018 update. www.wsp-opus.co.nz ©WSP Opus | October 2018 Page iii SH8 Beaumont Bridge Replacement ––– Hydrological Assessment 1 Introduction The SH8 Beaumont Bridge across the Clutha River is to be replaced by NZTA. To support the design and construction of the replacement bridge, the peak discharges and flood hydrographs for a range of design events, including the potential effects of climate change, were determined. The Beaumont Bridge is located near the town of Beaumont, below the Roxburgh Dam in the Clutha catchment. The Roxburgh Dam is currently operated by Contact Energy Ltd. The Clutha River discharges into the Pacific Ocean, just downstream of the town of Balclutha (Figure 1.1). The Teviot River is the only significant inflow to the Clutha River between Roxburgh Dam and the Beaumont Bridge. There is one other significant inflow between the Beaumont Bridge and the coast; the Pomahaka River. Figure 1.1 Location of the Beaumont Bridge, Roxburgh Dam, and flow sites of relevance. www.wsp-opus.co.nz ©WSP Opus | October 2018 Page 1 SH8 Beaumont Bridge Replacement ––– Hydrological Assessment 2 Hydrological Analysis 2.1 Flow data To complete this analysis, several flow monitoring sites were investigated (Table 2.1). Otago Regional Council (ORC) maintain flow sites at several locations within the Clutha catchment; including at Balclutha, and on the Teviot and Pomahaka Rivers. Contact Energy Ltd hold data for the Total Discharge from Roxburgh Dam, and flow in the Clutha River approximately 2km downstream of the dam. Contact Energy have given permission to use these data. Therefore, while there is no flow data for the Clutha River at the Beaumont Bridge, there are records on the Clutha River near the Roxburgh Dam and at Balclutha towards the coast (Figure 2.1). Table 2.1 Flow sites on the Clutha River. NNName Start date End Date Gaps (days) Time ---step Clutha at Balclutha 6 Jul 1954 31 May 2018 138 15 -min Pomahaka at Burkes Ford 4 Aug 1961 24 May 2018 723 15 -min Teviot at Bridge Hut Road 16 Mar 1994 27 Oct 2004 181 15 -min Clutha at Below Roxburgh Dam 28 M ar 2001 26 -Apr -2018 2.5 15 -min Roxburgh PS at Total Discharge 1 Aug 1965 1-Jun -2018 0 3-hourly 5000 4500 4000 3500 3000 2500 Flow Flow (m³/s) 2000 1500 1000 500 0 1955 1965 1975 1985 1995 2005 2015 Clutha at Balclutha from 6-Jul-1954 17:00:00 to 31-May-2018 13:35:00 Roxburgh Power Station at Total Discharge from 6-Jul-1954 17:00:00 to 31-May-2018 13:35:00 Clutha at Below Roxburgh Dam from 6-Jul-1954 17:00:00 to 31-May-2018 13:35:00 Figure 2.1 Comparison of the three flow records for the Clutha River. Flows in the Clutha River have been modified by the Roxburgh Dam since it was commissioned in 1956, and the Clyde Dam since 1992. While the dams have a significant effect on the overall flow regime and flow duration curve of the Clutha River downstream, they have less effect on the frequency and magnitude of large flood events. The flood signature of the Clutha River appears to be largely unaffected by dam construction (Figure 2.1). Since it is the furthest downstream, the Roxburgh Dam is likely to have the greater influence on the frequency and magnitude of floods experienced in the lower river i.e. near Beaumont. However, since any effects of the Roxburgh Dam have apparent since at least 1956, those effects are inherent in the instrumental flow records further downstream. Any effects of the dam are therefore included www.wsp-opus.co.nz ©WSP Opus | October 2018 Page 2 SH8 Beaumont Bridge Replacement ––– Hydrological Assessment in any analysis of the annual flood maxima. This is appropriate since these effects are likely to also persist into the future. 2.2 Flow analysis Since there is no flow gauge near the Beaumont Bridge, the flows at both Roxburgh Dam and Balclutha were investigated to determine their suitability for estimating the flows at Beaumont Bridge. Several flood events, when all three records could be compared, were analysed; 2013, 2009 and 2006 (Figure 2.2, Figure 2.3 & Figure 2.4). These figures show that the Total Discharge from Roxburgh Dam and the Clutha River Below Roxburgh Dam records are very similar, as expected. The Total Discharge record is ‘smoothed’ as it is a 3-hour average, whereas the Below Roxburgh Dam gauge records instantaneous flows at 15-min intervals. The Balclutha record, also 15-min data, shows a similar pattern to those sites upstream, but is lagged by ~12-hours. While the Balclutha recorder is much further downstream, with a 25% larger catchment area, the flows are similar to those upstream at Roxburgh Dam. Both the 1995 and 1999 floods show that the flows at Balclutha were ~10-15% larger than at Roxburgh Dam. This pattern is observed in many of the flood flows, but not all. 2000 1800 1600 1400 1200 1000 Flow Flow (m³/s) 800 600 400 200 0 1-Jan-2013 6-Jan-2013 11-Jan-2013 16-Jan-2013 21-Jan-2013 26-Jan-2013 Roxburgh Power Station at Total Discharge from 1-Jan-2013 00:00:00 to 1-Feb-2013 00:00:00 Clutha at Below Roxburgh Dam from 1-Jan-2013 00:00:00 to 1-Feb-2013 00:00:00 Clutha at Balclutha from 1-Jan-2013 00:00:00 to 1-Feb-2013 00:00:00 Figure 2.2 January 2013 flood. Comparison of the dam outflow, Clutha River below Roxburgh Dam, and the Clutha River at Balclutha. 2000 1800 1600 1400 1200 1000 Flow Flow (m³/s) 800 600 400 200 0 10-May-2009 15-May-2009 20-May-2009 25-May-2009 30-May-2009 4-Jun-2009 Roxburgh Power Station at Total Discharge from 10-May-2009 00:00:00 to 10-Jun-2009 00:00:00 Clutha at Below Roxburgh Dam from 10-May-2009 00:00:00 to 10-Jun-2009 00:00:00 Clutha at Balclutha from 10-May-2009 00:00:00 to 10-Jun-2009 00:00:00 Figure 2.3 May 2009 flood. www.wsp-opus.co.nz ©WSP Opus | October 2018 Page 3 SH8 Beaumont Bridge Replacement ––– Hydrological Assessment 1800 1600 1400 1200 1000 Flow (m³/s) Flow 800 600 400 200 20-Nov-2006 25-Nov-2006 30-Nov-2006 5-Dec-2006 10-Dec-2006 15-Dec-2006 Roxburgh Power Station at Total Discharge from 20-Nov-2006 00:00:00 to 20-Dec-2006 00:00:00 Clutha at Below Roxburgh Dam from 20-Nov-2006 00:00:00