Climate Changes, Impacts and Implications for New Zealand to 2100 Synthesis Report: RA2 Coastal Case Study The Firth of Thames and Lower Waihou River G McBride1, G Reeve1, M Pritchard1, C Lundquist1, A Daigneault2, R Bell1, P Blackett1, A Swales1, S Wadhwa1, A Tait3, C Zammit4 1 National Institute of Water & Atmospheric Research Ltd, Hamilton, New Zealand 2 Landcare Research Ltd, Auckland, New Zealand 3National Institute of Water & Atmospheric Research Ltd, Wellington, New Zealand 4National Institute of Water & Atmospheric Research Ltd, Christchurch, New Zealand *Corresponding author, email: [email protected] PSConsulting Ltd Present-day LiDAR bathymetry of the Firth of Thames intertidal zone, overlaid by mangrove distribution © All rights reserved. The copyright and all other intellectual property rights in this report remain vested solely in the organisation(s) listed in the author affiliation list. 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When quoting, citing or distributing this Report or its individual sections, please provide the full reference: McBride G, Reeve G, Pritchard M, Lundquist C, Daigneault A, Bell R, Blackett P, Swales A, Wadhwa S, Tait A, Zammit C (2016) The Firth of Thames and Lower Waihou River. Synthesis Report RA2, Coastal Case Study. Climate Changes, Impacts and Implications (CCII) for New Zealand to 2100. MBIE contract C01X1225. 50pp. Cover photo: Ray J Salisbury, www.mychillybin.co.nz 2 CONTENTS HIGHLIGHTS 5 INTRODUCTION AND BACKGROUND 6 The CCII project 6 Coastal climate change and sea-level rise 6 Projected climate changes for the Waikato region 7 CASE STUDY SELECTION AND STAKEHOLDER ENGAGEMENT 8 Range of expected impacts and implications on estuaries 8 and coasts Case study selection—stakeholder engagement 8 Final selection 10 Background to that choice – Hazards 10 Background to that choice – Habitats 10 Coastal case study description 11 METHODOLOGY 12 Models used 12 Computational hydrodynamic models 12 Sediment models 12 TopNet 13 Mangrove cover modelling 14 RESULTS 17 Firth of Thames sedimentation 17 Mangrove distribution 19 Waihou and Piako River flows 20 Waihou salinity intrusion and river flooding 20 Salinity intrusion 20 River flooding 21 3 KEY FINDINGS AND IMPLICATIONS 23 Drying and wetting in the Firth 23 Mangrove distribution in the Firth 23 River flows 23 Salinity intrusion and flooding 24 SCENARIOS, TAKING ACCOUNT OF ECONOMICS AND 25 PUBLIC POLICY Shared socio-economic pathways (SSP) 25 Shared Policy Assumptions for New Zealand (SPANZ) 25 Elements of scenario analysis 25 Scenario modelling 25 Integrated assessment 25 Scenario estimates 28 SUMMARY AND CONCLUDING REMARKS 32 FUTURE WORK 32 ACKNOWLEDGEMENTS 33 REFERENCES 33 APPENDIX 36 Future CO2 concentration scenarios 36 Computational Grid 37 Derivation of simplified tidal sedimentation model and its 38 numerical solutions Changes in water depths predicted using six down-scaled 43 GCM models TopNet calibration 44 Changes in saline intrusion predicted using six down-scaled 47 GCM models Scenario models details 47 HIGHLIGHTS Firth of Thames water levels and depths Waihou River salinity intrusion and flooding • Sea level rise (SLR) will result in increased • SLR is the main contributor to changes in flooding of shore communities saline intrusion and flooding • Water depths in the Firth over time are • Salinity intrusion up the Waihou River is projected to be relatively independent of RCP projected to extend up to a further 5 km inland values (with rate of sea level rise roughly (for a 1 metre SLR over the next century), and in tandem with sedimentation), but will is relatively unaffected by changes in low river nevertheless be maximal for RCP 8.5 (high flows greenhouse gas emissions) • Flooding in the upper part of the river (>50 km • Areas of wetting and drying in the Firth may from the Firth) is largely unaffected by SLR increase over time, particularly if more intense • The largest impact of SLR and flood interaction land uses evolve, making navigation more will occur in the lower Waihou River (between difficult Pekapeka and Puke Bridge), with up to a 0.5 m • Conversely, there would be less wetting and increase in combined flood level at Pekapeka drying were widespread reforestation to be for a 1 m SLR adopted in the contributing catchments Economic and land use scenario analysis Firth of Thames mangrove distributions projections • Agricultural commodity prices vary • Under present landuses, up to 100% increase significantly over time due to changes in global in mangrove habitat is projected in suitable socio-economic conditions, population growth, areas, with a decrease of habitat in subtidal and climate change policy. areas • Primary production is estimated to increase • For a 20% reduction in sediment supply (e.g., regardless of which Representative because of reforestation) some reduction in Concentration Pathway (RCP) is modelled. suitable habitat can be expected, especially Forestry yields increase the most due to the for areas presently marginal for mangrove influence of an increase in atmospheric CO2. production • A scenario of increased sediment loads is • For a 20% increase in sediment supply (e.g., driven by deforestation and a shift to because of intensified landuses) a general pastoral land uses increase in suitable habitat is projected • A scenario of a decline in sediment loads Waihou and Piako River flows can be attributed to high carbon prices shifting marginal sheep and beef land • Projected changes in river flows are strongly to pine plantations, scrub, and linked to projected changes in precipitation fallow land. • The median change in the annual mean flow The results giving rise to these is relatively small, with generally negative highlights are given in the changes in winter and summer (decreased main body of this report; various mean flow) and positive changes in spring and technical explanations are given in the autumn (increased mean flow) Appendix. • Uncertainty in flow projections is high, corresponding to similar uncertainty in future precipitation projections for this region 5 INTRODUCTION AND BACKGROUND The CCII project The “Climate Changes, Impacts and Implications” (CCII) project is a four-year project (October 2012 – September 2016) designed to address the following question: What are the predicted climatic conditions and assessed/ potential impacts and implications of climate variability and trends on New Zealand and its regional biophysical environment, the economy and society, at projected critical temporal steps up to 2100? The CCII project brings together a strong research Across New Zealand, the average relative (local) sea- team with knowledge and modelling capabilities in level rise from 1900–2015 is 1.78 ± 0.21 mm per year climate, ecosystems, land and water use, economics, and, since 1961, is 2.14 ± 0.47 mm per year (MfE, 2016), and sociocultural research to address the environment results that are very similar to those given in Church sector investment plan priority of “stronger prediction and White (2011) for global averages. This means that and modelling systems”. projections for global sea-level rise can generally be adopted, provided allowances are made for regional The project is based around five inter-related and local differences. Research Aims (RAs) that will ultimately provide new climate change projections and advancements in In the satellite era (since 1993), global mean sea level understanding their impacts and implications for New has risen by 3.3 mm per year, attributable partly to Zealand’s environment, economy and society. The five natural climate variability and partly an acceleration in RAs are: sea-level rise due to warming of the atmosphere and oceans. Research Aim 1: Improved Climate Projections Key drivers of the rise in sea level are: Research Aim 2: Understanding Pressure Points, Critical Steps and Potential Responses • Thermal expansion from warming ocean waters Research Aim 3: Identifying Feedbacks, Understanding • Additional water mass added to the ocean from Cumulative Impacts and Recognising Limits glaciers, ice sheets and net freshwater runoff Research Aim 4: Enhancing Capacity and Increasing • Vertical land movement can substantially alter Coordination to Support Decision-making the local sea-level rise, with any land subsidence compounding the ocean rise Research Aim 5: Exploring Options for New Zealand in Different Changing Global Climates Locally, it is the local or relative sea-level rise that need to be adapted to – not the global average rate. The overall purpose of RA2 is to: Perform five case However projections of future sea-level rise are usually studies on the potential impacts of climate change based on global projections. So for New Zealand, and other key drivers on alpine, hill & high country, offsets need to