America's Cup, Wynyard Hobson Coastal Processes & Dredging
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Report America’s Cup, Wynyard Hobson Coastal Processes & Dredging Technical Report Resource Consent Application, Wynyard Hobson Prepared for Panuku Development Auckland and Ministry for Business, Innovation and Employment Prepared by Beca Ltd and Tonkin & Taylor Ltd April 2018 America's Cup Wynyard Hobson: Coastal Processes and Dredging Technical Report Revision History Revision Nº Prepared By Description Date 1 Jennifer Hart, Richard Reinen- Final Draft Issue for Comment 6.04.2018 Hamill, Stephen Priestley 2 Jennifer Hart, Richard Reinen- Issued for Consent 11.04.2018 Hamill, Stephen Priestley Document Acceptance Action Name Signed Date Prepared by Jennifer Hart 11.04.2018 Reviewed by Stephen Priestley 11.04.2018 Approved by Stephen Priestley 11.04.2018 on behalf of Beca Ltd © Beca 2018 (unless Beca has expressly agreed otherwise with the Client in writing). This report has been prepared by Beca on the specific instructions of our Client. It is solely for our Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which Beca has not given its prior written consent, is at that person's own risk. America's Cup Wynyard Hobson: Coastal Processes and Dredging Technical Report Executive Summary Overview 1. The maritime-based works comprise new wharves (piles and deck), piled breakwaters, wave attenuators, dredging, and underwharf services. Some of these related activities will result in discharges to the Coastal Marine Area (CMA). The proposed works for the America’s Cup are located adjacent to Viaduct Harbour, and the project is referred to as Wynyard Hobson. This report relates to an application for resource consents for Wynyard Hobson. 2. To facilitate these works, the vehicular ferry will be relocated from the proposal area. A Ferry and Fishing Industry Relocation Facility (FFIRF) is the subject of a separate application for resource consents (and is not addressed further within this report). Assessment Methodology 3. The predicted adverse effects on coastal processes are associated with changes to the basin flushing time, wave reflection outside of the Project Areas, and sedimentation. The predicted beneficial effects are more calm water for mooring a range of vessels. 4. Parallel hydrodynamic models were developed (by Tonkin & Taylor and Beca) and comparison of results/interpretation were deliberated on by the modelling teams. Wave/wake modelling was undertaken by Cardno who have previously conducted wave modelling within the lower Waitematā Harbour. Models were verified by measured data, some of which was collected as part of this project. The interpretation of the potential adverse effects relating to changes in tidal currents, flushing periods, and waves/wakes was a joint assessment by Tonkin & Taylor and Beca. 5. Project objectives for moored boat tranquillity were established from internationally recognised performance criteria and with reference to the host syndicate. Residual wakes are more critical than residual wind waves in the moored areas, with the objective of significant wake heights being less than 0.1m. Existing Environment 6. The affected coastal marina areas (CMA) (referred to as the Project Area) are in the lower Waitematā Harbour. 7. The Project Area is adjacent to the main tidal channel. Tidal velocities are less than 0.2 m/s and sedimentation rates are typically around 35mm/year averaged over the whole Viaduct Harbour. Water quality in terms of total suspended solids is generally good. 8. The natural, relatively high tidal range of 2.5m (median) results in good to fair flushing of Viaduct Harbour. 9. The Project Area is subject to diffracted wind waves from West and East and direct wind waves to the North. Vessel wakes are a regular feature and problematic for some moored vessels within Outer Viaduct Harbour and Westhaven Marina (due to vessel excitation). 10. Relevant coastal hazards include sea level rise, storm surge (extreme storm tides) and associated wave overtopping, and tsunami. America's Cup Wynyard Hobson: Coastal Processes and Dredging Technical Report Proposed Works 11. The approximate areas and dredging volumes for the Wynyard Hobson facilities as shown in Civil Drawings 1 to 11 are given below. No reclamation is proposed. 2,000m2 of temporary wharf structure and 9,000m2 of permanent wharf structure including wave panels. 3,200m2 of permanent piled, decked breakwater including wave panels. 600m2 of pile footprint at the seabed. 5,000m2 of floating pontoons. 78,000m3 of dredging. Potentially 9,000m3of borehole corings temporarily deposited on the seabed for later dredging. Potential Environmental Effects 12. The proposed layout will change tidal currents, basin flushing times, wave/wake reflection and sedimentation. 13. The effects of the proposed development on tidal flows and currents are considered to be no more than minor because of the localised extent and limited actual scale of these changes. 14. Basin flushing times are longer but predominately tend to stay in similar good / fair categories as the existing flushing. The exception is the Inner Viaduct Harbour where it changes from good to fair. All the increases are predicted, however, to remain within the “fair” or “good” flushing band. Changes in the flushing times represent a moderate/minor adverse change compared with the beneficial change of berth tranquillity for the Outer Viaduct Harbour and Wynyard Wharf South water space. Flushing is only one of the inputs to the assessment of effects on water quality that is addressed in Assessment of Coastal Environmental Effects Associated with the Development of America's Cup Facilities for the Wynyard Hobson Option (Golder, 2018). The Golder report includes the description of existing water quality, incorporating the results of recent sampling, a discussion of water quality considerations and potential consequences, and conclusions regarding the effects of the proposed development on water quality. The Golder report conclusions, which consider water quality in relation to the discharge of stormwater, algal growth and overall ecology are summarised below: Stormwater discharge and TSS: TSS concentrations from the Halsey Street stormwater outfall should not result in prolonged water clarity changes within the Inner Viaduct Harbour, however a very large rain storm event is likely to result in some clarity changes over a period of 24 hours or more. At the Freemans Bay stormwater outfall beneath North Wharf the stormwater is untreated and known to have poor clarity at times. Currently, following a period of mixing, it is likely that some clarity changes will persist off North Wharf and alongside Wynyard Wharf. Post-development, based on increased flushing times, clarity changes will be expected in this area for a period of at least 24 hours. Stormwater discharge and microbiological water quality: Overall, no significant changes in microbiological water quality are expected within the Inner Viaduct Harbour from discharges from the Halsey Street stormwater outfall. Discharge from the Freemans Bay stormwater outfall beneath North Wharf currently results in poor microbiological water quality in the area of North Wharf and north along Wynyard Wharf after rain events. Post construction, water quality in the Karanga Plaza steps area should be considered to be poor quality for 24-48 hours following a rain storm discharge from the Freemans Bay stormwater outfall. There is on-going work by Auckland Council and Watercare to address Freemans Bay stormwater outfall quality. Algal growth: Increase in biomass is not anticipated to be observable to the casual observer. America's Cup Wynyard Hobson: Coastal Processes and Dredging Technical Report Overall ecology: Reduction in water movement can have the potential to influence the composition of biological communities on basin walls, piles and floating pontoons. An Inner Viaduct Harbour Environmental Management Plan should be prepared and implemented, covering water quality monitoring and management in the Inner Viaduct Harbour. Recommendations on the plan are set out in the Golder report. 15. There will be a similar or slightly less degree of reflection outside the Project Area, in Freemans Bay and the Maritime Museum basin but the increase in maximum wave height is typically less than 0.1m for the recommended option. Any potential adverse effects relating to this small increase are expected to be difficult to detect as the sea state is relatively energetic under present conditions. 16. Sedimentation will increase in the more tranquil basins/harbours, at a similar scale to existing sedimentation rates. Potential adverse effects are therefore expected to be minor and to be managed using existing maintenance dredging practices in a similar manner to present sedimentation. 17. Dredging will create sediment plumes which become more diffuse away from the dredger. Assessment undertaken for this report and previous dredging campaigns have both shown effects from dredging plumes to be less than minor with localised and temporary increases in suspended sediment estimated to be less than 1% of the total sediment flux per tide. The depth of material deposited at any single location from the plume is expected to be undetectable. 18. Coastal hazards and climate change have been considered for the Wynyard Point structures and measures identified to address them. Overall Conclusions 19. For Wynyard Hobson, the main beneficial effect is increased calm water-space