Peer Review of the Christchurch Coastal Hazard Assessment Report
Kenderdine, S.E.; Hart, D.E.; Cox, R.J.; de Lange, W.P.; Smith, M.H. (2016) Peer review of the Christchurch Coastal Hazards Assessment Report. Review report produced for the Christchurch City Council, 18 August 2016, 74 pp.
Permanent Research Commons link: https://hdl.handle.net/10289/10915
This review reports findings from the peer review panel (‘the Panel’) assessment of the Tonkin & Taylor Ltd (2015) report: Coastal Hazard Assessment – Stage Two (‘the Report’). The Panel were asked to address four overarching questions in this review (‘the Review’), covering both the science and legal context of the report. These were (i) Does the report represent good science?; (ii) Are the findings still relevant in terms of new research?; (iii) Has the report taken account of relevant statutory policy documents in providing technical or expert advice (refer to Appendix E GHD TOR as a guide)?; and (iv) Is the report and its findings appropriate for its intended purpose - to inform planning for future land use decisions (all referred to as Appendix E GHD Bundle of Documents for full purpose statements)? The Panel found that the Report’s purpose went beyond the technical assessment of areas subject to coastal hazards as required under Policy 24 of the New Zealand Coastal Policy Statement (2010) (‘NZCPS’). Requiring mapping suitable to be included in the district plan should have required draft mapping. Such a technical exercise constitutes the first scientific stage in the process of eventually producing maps which are suitable for inclusion in the Proposed Christchurch Replacement District Plan (CRDP). This technical stage should be followed by adaptive planning and application of a precautionary approach identified in Policies 3, 25 and 26 NZCPS before such maps are finalised. Notwithstanding, the Panel finds that the Report could, and should, constitute a suitable and robust technical basis on which to proceed towards the next stage of development of coastal hazard maps for the district of Christchurch after recommended modifications are made. This Review outlines recommendations for work that should be completed before the results of the Report are used to inform the next stages in establishing coastal hazard provisions for the CRDP. We also make suggestions for future work that could be incorporated into the first reassessment of the coastal hazard zones (planned to occur in around 10 years’ time). To be clear, the recommendations are the only work that the Panel indicates needs to be done now. Positive aspects of the Report included spatial data as requested by CCC on areas susceptible to coastal hazards, including the future timeframes of 2065 and 2115. It included the coastal settlements located on unconsolidated shorelines within CCC’s jurisdictional boundaries. The Report also gave close attention to the documents that are directly relevant to an exercise of this technical kind, namely MfE (2008b) (shortly to be updated), and Ramsay et al (2012). The Report met with what might be considered truncated time frames for a technical exercise encompassing coastal erosion and inundation over many open coast and harbour sites. The Report did not consult the community, nor was this within its Terms of Reference. Community consultation is not a requirement in Table I of the Ramsay et al (2012) good practice guide for this technical stage but rather is a requirement for the subsequent adaptive planning stage. Page 1 of 75 The Report was based on a single IPCC scenario RCP8.5, which is commonly used in hazard assessments. However, the subsequent adaptive stage of coastal hazard zone mapping will benefit from considering a range of scenarios. Thus, the Panel recommends that additional work is conducted to produce coastal hazard assessments for more than one IPCC scenario. Regarding the open coast Coastal Erosion Hazard Zones (CEHZ), the Panel endorses a probabilistic approach, but recommends that the analysis use more appropriate probability distributions than the triangular distributions used in the Report. The open coast CEHZ is also dependent on the Waimakariri River sediment supply. The Panel recommends that the open coast CEHZ is reassessed for a range of sediment budget scenarios - with the current ‘no change’ scenario forming the middle scenario. This can readily be done with the probabilistic modelling approach. In assessing the Coastal Inundation Hazard Zone (CIHZ) for the Open Coast (New Brighton, Sumner, Taylors Mistake) and for the harbour environments of Lyttelton and Akaroa, overall the Panel conclude that the simple ‘building block bathtub’ approach used is acceptable. In assessing the harbour CIHZ, however, the Panel acknowledges that the assumption that all components concurrently reach extreme values is conservative. The Panel suggests that the likelihood of concurrence of extreme waves and coincidence of peak wave setup and wind setup be investigated for inclusion in the next (i.e. ‘10 years’ into the future) reassessment. For the Avon- Heathcote Estuary and Brooklands Lagoon CIHZ, TUFLOW hydrodynamic modelling is used in the Report, but does not consider the effects of river baseflows and concurrent rainfall. The Panel suggests that river baseflows and rainfall, and the influence of climate change on them, be incorporated in the next reassessment. The indicated coastal erosion hazard zone CEHZ for all harbour sites is not a robust indication of the likely erosion hazard for these specific areas. The Panel recommends that they are re-assessed with more attention to detail and on-ground inspections. The ‘high tide translation’ method approach should not be used and only the ‘equilibrium profile’ values considered. The Report acknowledged the occurrence of ground elevation changes with the earthquakes and appropriately uses the 2011 LiDAR survey for baseline ground elevations. The Report did not assess earthquake-induced changes in groundwater depths and should not do so as this separate exercise is in the area of responsibility of EQC. NZCPS Policy 24(1)(a)-(h) is the only NZCPS provision relevant to a technical exercise at the first stage in identifying the coastal environment potentially affected by coastal hazard risk and the likely effects of climate change including sea level rise. It was not identified in the Tonkin & Taylor Terms of Reference for the Report, and should have been. Page 2 of 75 NZCPS Objective 5 and other relevant policies – Policy 3 Precautionary approach, Policy 25 Subdivision, use and development in areas of coastal hazard risk, and Policy 27 Strategies for protecting significant existing development from coastal hazard risk, and its consequences, are all relevant to the next stages of providing mapping of such areas to a sufficient standard to be included in the CRDP – stages and mapping to be defined with the involvement of the community and stakeholders. An evaluation required under s 32 RMA is only undertaken after information provided by CCC and other experts are brought together to further inform the community, stakeholders and CCC officials on such matters as cost, benefit and the other issues identified in Appendix B. This is to inform through a process of adaptive management the evaluation report for CCC to approve under s 32 sometime in the future. The hazard maps generated from the results of the Report should be withdrawn for now until the final mapping is concluded. The hazard lines may end up being reinforced after this stage but in the meantime the Coastal Hazard Zones (CHZ) formerly established by ECan may suffice as an interim measure for the open coast, but these do not apply to most of the harbour coasts. Overall, it is recommended that CCC and ECan work more closely together to develop such information resources as are needed for the coastal hazard assessment and community considerations of adaptive management pathways as part of an integrated approach to coastal management.
Review report produced for the Christchurch City Council. Used wit permission.