Technical Report 15

National Institute of Water & Atmospheric Research Ltd (NIWA) – Coastal Processes Assessment

Wellington International Airport Runway Extension Coastal Processes Assessment

Prepared for Wellington International Airport Ltd

March 2016

Prepared by: R.G. Bell

For any information regarding this report please contact: Rob Bell Programme Leader: Hazards & Risk Coastal & Estuarine Processes +64-7-856 1742 [email protected]

National Institute of Water & Atmospheric Research Ltd PO Box 11115 Hamilton 3251

Phone +64 7 856 7026

NIWA CLIENT REPORT No: HAM2015-079 Report date: March 2016 NIWA Project: WIA15301

Quality Assurance Statement

Reviewed by: Craig Stevens

Formatting checked by: Alison Bartley

Approved for release by: Andrew Laing

Image of Wellington International Airport and Lyall Bay in southerly sea/swell on 20 July 2015. North is due left of image. [Source: DigitalGlobe/Google Earth]

© All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client.

Contents

Executive summary ...... 8

1 Background ...... 17

2 Description of the Project ...... 19

3 Assessment methodology ...... 21 3.1 Introduction ...... 21 3.2 Desk-top analysis ...... 22 3.3 Field surveys ...... 24 3.4 Modelling approach: coastal physical processes ...... 30 3.5 Assessment of effects criteria: policies/plans/statutes ...... 35

4 Existing environment ...... 39 4.1 Coastal and geomorphic setting ...... 39 4.2 Coastal geology, sediment processes and characteristics ...... 42 4.3 Marine discharges and water/sediment quality ...... 49 4.4 Winds ...... 52 4.5 Hydrodynamic and wave processes for existing environment ...... 54

5 Effects Assessment: Operation of Project ...... 67 5.1 Description of operational effects ...... 67 5.2 Assessment of operational effects ...... 68 5.3 Relevant assessments against statutory plans/policies or guidelines ...... 92

6 Effects assessment: Construction activities ...... 94 6.1 Description of construction effects ...... 94 6.2 Assessment of construction effects ...... 97 6.3 Assessment against statutory plans/policies or technical criteria and guidelines 111

7 Mitigation and monitoring ...... 114 7.1 Overview of effects on coastal physical processes ...... 114 7.2 Mitigation or avoidance measures ...... 116 7.3 Suggested coastal monitoring conditions ...... 116

8 Acknowledgements ...... 118

9 Glossary of abbreviations and terms ...... 119

Wellington International Airport Runway Extension: Coastal Processes

10 References ...... 121

Tables Table 3-1: Locations (WGS-84) and 2014 deployment information for the Lyall Bay instruments. 28 Table 4-1: Statistics for the distribution of measured SSC (mg/L) at the optical mooring in outer Lyall Bay. 51 Table 5-1: Change in predicted wave heights locally at site P1 near The Corner as a result of the runway extension. 74 Table 5-2: Change in predicted wave heights locally at site P4 in the centre of inner Lyall Bay as a result of the runway extension. 74 Table 6-1: Maximum predicted SSC above background levels after reasonable mixing at ~150 m for discharges at D1–D3 and sediment-discharge rates of 1 and 2 kg/s. 100

Figures Figure 2-1: General layout for the proposed 350 m south runway extension into Lyall Bay to develop a 2300 m Take Off Runway Available (TORA). 19 Figure 3-1: Sea-bed sediment sampling sites in Lyall Bay, Wellington. Contaminant sampling sites were 1, 3, 5, 8, 10, 11 and 12. 26 Figure 3-2: Extent of Lyall Bay infill bathymetry survey (black dashed line) shown against grey area collected from previous surveys. 27 Figure 3-3: Site map showing oceanographic instrument locations overlying the seabed bathymetry to WVD-53. 29 Figure 3-4: Delft3D-WAVE model grid (red) of south Wellington coast superimposed on top of the Delft3D-FLOW model grid (black) that also includes Wellington Harbour. 31 Figure 3-5: ARTEMIS finite-element model grid resolution in the area around the runway extension (left) existing runway; (right) proposed runway reclamation. 33 Figure 3-6: ARTEMIS model predicted surface water level as a result of a 1.5 m wave at the Cook Strait boundary with a wave period of 12 seconds (LEFT). Google Earth image of Lyall Bay taken on 24 July 2014 for a wave height of 1–1.5 m and a period ~10 seconds (RIGHT). 34 Figure 3-7: Areas of Conservation Value in Lyall Bay area (Tarakena Bay and Tauputeranga Island) in the Regional Coastal Plan. 36 Figure 4-1: Aerial photograph looking north on completion of the airport construction (21 Jan 1959). 40 Figure 4-2: Geomorphic features in Lyall Bay incl. the 1941 shoreline. 41 Figure 4-3: Multi-beam bathymetric data at 0.5 m resolution off the existing runway in Lyall Bay obtained by NIWA (Mackay and Mitchell, 2014). 42 Figure 4-4: Surface sea-bed sediments of Lyall Bay and Wellington south coast (from Arron & Lewis 1993). 43 Figure 4-5: Sand cover (cm) and the edge of bedrock estimated from an early geophysical survey of eastern Lyall Bay in 1971. 44

Wellington International Airport Runway Extension: Coastal Processes

Figure 4-6: Historic bathymetric changes in Lyall Bay. 45 Figure 4-7: Aerial photograph of Moa Point beach showing the beach slope profile locations (transects #1-12). 47 Figure 4-8: (Right): Western end of Moa Point beach with coastal cliffs at top right; (Left): Central area of exposed rock dividing the beach into two shallow- indented coves, looking along the cuspated beach from the southern/ eastern cove. 48 Figure 4-9: Time series of suspended-sediment concentration (SSC) from the upper (8 m) and lower sensors (16 m) and significant wave height (orange) from the optical mooring in outer Lyall Bay (4-Sept to 9-Oct 2014). 51 Figure 4-10: Wind roses for the Wellington area. 53 Figure 4-11: Wind rose derived from hourly records of wind speed and direction at Wellington Airport from 1962–2004. 54 Figure 4-12: Present-day tide marks at Wellington relative to WVD-53. 55 Figure 4-13: Change in annual mean sea level at the 4 main ports of NZ since 1900. 56 Figure 4-14: Alignment of the as-built Lyall Bay outfall (white line) and sites for the Baring Head NIWA wave buoy and a recording current meter (RCM) in Lyall Bay in September 1989. 58 Figure 4-15: Lyall Bay and Moa Point circulation zones during an ebb tide (top) and a flood tide (bottom). 59 Figure 4-16: Wave refraction and diffraction patterns during a southerly-swell event on 29 April 2015. 60 Figure 4-17: Surfing waves at The Corner viewed from the stormwater outlet adjacent to the eastern carpark (30 June 2015). 61

Figure 4-18: Significant wave height (Hs) vs mean zero-crossing wave period (Tz) distribution as a % of the entire 15-year Waverider buoy record off Baring Head. 62

Figure 4-19: Hs distributions (top) full distribution (in total days per year, i.e., number of 30 minute estimates in each 0.25 m Hs bins divided by 48, with a semi-log10 scale) and (bottom) partial cumulative distribution (proportion) showing that e.g., 99% of the time waves have a Hs<4.0 m. 63 Figure 4-20: Wave conditions averaged over the 55-day SWAN simulation, in Lyall Bay. 65

Figure 4-21: Distribution of Hs from 55-day simulation (top panel) and wave roses (joint occurrence distributions for mean wave direction and height) (bottom panel) at sites 4, 6, 15. 66 Figure 5-1: Residual (net) current circulation in Lyall Bay over the ~7-week field deployment period. 69 Figure 5-2: Current circulation at mid-flood tide (HW-3 hrs) for strong southerly winds (top) and northerly winds (bottom) for the existing situation (left panels) and the proposed runway extension (right panels). 71 Figure 5-3: Lyall Bay bathymetry plots showing the point extraction sites (P1–P8) for assessment of changes in wave heights. 73 Figure 5-4: For a boundary incident wave height of 1.5 m with a wave period of 8 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. 78

Wellington International Airport Runway Extension: Coastal Processes

Figure 5-5: For a boundary incident wave height of 1.5 m with a wave period of 12 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. 79 Figure 5-6: For a boundary incident wave height of 3 m with a wave period of 8 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. 80 Figure 5-7: For a boundary incident wave height of 3 m with a wave period of 12 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. 81 Figure 5-8: Predicted changes in seabed height for the existing situation over the ~7-week field period in 2014. 85 Figure 5-9: Predicted changes in seabed height for the proposed runway situation over the ~7-week field period in 2014. 85 Figure 5-10: Net or relative change in the response of sea-bed heights with the proposed runway in place over the ~7-week simulation. 87 Figure 5-11: IPCC global mean sea-level projections from the 5th Assessment Report (AR5) for two greenhouse-gas emission pathways: RCP2.6 (severe curbs required) and RCP8.5 (business-as-usual and global population growth). 90 Figure 5-12: Longitudinal elevation profile along the centreline of the present Wellington Airport runway, from Evans Bay (left) to Lyall Bay (right). 91 Figure 6-1: Schematics of a NW decant discharge location D1 (top) and SW and S discharge locations D2 and D3 (bottom) in the perimeter rock dyke with flow of fill (yellow arrows) and options for associated fill operations. 96 Figure 6-2: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D1 (NW corner). 101 Figure 6-3: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D1 (NW corner). 102 Figure 6-4: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D2 (SW corner). 103 Figure 6-5: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D2 (SW corner). 104 Figure 6-6: Composite surface-layer maximum SSC over a 30-day simulation (top) & averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D3 (mid-south end). 105

Wellington International Airport Runway Extension: Coastal Processes

Figure 6-7: Composite surface-layer maximum SSC over a 30-day simulation (top) & averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D3 (mid-south end). 106 Figure 6-8: Predicted instantaneous plume snapshots of SSC at 24, 48, 72 and 96 hours after a 1 kg/s decant slug release for locations D1 (left panels) and D2 (right panels). 108 Figure 6-9: Predicted instantaneous plume snapshots of SSC at 24, 48, 72 and 96 hours after a 2 kg/s decant slug release for locations D1 (left panels) and D2 (right panels). 109 Figure 6-10: Predicted instantaneous plume snapshots from location D3 of SSC at 24, 48, 72 and 96 hours after a 1 kg/s (left panels) and 2 kg/s (right panels) decant slug release. 110 Figure 6-11: Frequency of SSC for the upper sensor at 8 m depth (solid line) and lower sensor at 16 m depth (dashed line) from the outer Lyall Bay optical mooring and compared with a 25 mg/L SSC limit. 113

Wellington International Airport Runway Extension: Coastal Processes

Executive summary Wellington International Airport Limited (WIAL) propose to extend the runway to 2300 m Take Off Runway Available to further enable long-haul flights, including Code E aircraft (e.g., Boeing 777 or Airbus A330) and the ensuing economic benefits for the Wellington region. After evaluating options for extending north and south, WIAL is proceeding with a 350-m extension of the runway south in to Lyall Bay (”the Project”).

Coastal environmental assessments are required to support applications for coastal permits and consents for the Airport runway upgrade in the coastal marine area (CMA). NIWA were engaged by WIAL in 2014 to undertake technical studies and an assessment of effects of the extended runway on coastal physical processes.

This AEE Report covers the assessment of effects on coastal physical processes of the extended runway into Lyall Bay, focused on coastal hydrodynamic and coastal sediment processes. It draws on comprehensive information from field and model studies in the Technical Report on Coastal Hydrodynamics and Sediment Processes in Lyall Bay (Prichard et al. 2016), the report on sediment characteristics and sediment contaminant levels (Depree et al. 2016), an analysis of the existing wave climate off Baring Head (Section 4.5) and an additional site survey of the cove east of the runway extension (covered in Section 4.2).

As effects of the runway extension on inshore waves were highlighted early on in the AEE investigations, surfing and recreational-related effects are given greater scrutiny when assessing the degree of coastal environmental impacts, with a more detailed, complementary assessment provided by DHI (2016). Consequently, this Report only describes likely changes in general wave patterns, but the effects of these changes on wave characteristics (wave steepness and peakiness, wave directionality) and water-contact recreation activities such as surfing and swimming are covered in more detail by DHI.

Other than for describing the existing environment, this AEE Report also does not cover assessments of coastal-hazard extremes (e.g., waves and storm-tide), which are considered in the Concept Feasibility and Design Report (AECOM, 2015a). However this AEE Report does include an appraisal of the implications on the Project of sea-level rise and associated coastal climate-change impacts over at least the next 100 years, as required in Section 7(i) of the RMA and Policy 24 of the NZ Coastal Policy Statement.

Assessment methodology used For this assessment, the main coastal physical processes considered in terms of potential effects from the Project are:

. Coastal hydrodynamics (changes in tidal and wind-driven currents).

. Coastal wave heights and wave refraction patterns.

. Sediment transport and coastal geomorphology (storm-scale changes to Lyall Bay and Moa Point beaches).

. Water quality (turbidity and any subsequent sedimentation from suspended-sediment discharges and potential for contaminant release arising from sea-bed disturbances from construction sites).

8 Wellington International Airport Runway Extension: Coastal Processes

. Effects of climate-change, particularly sea-level rise and coastal-storm inundation, on the Project.

There are no quantitative assessment criteria for assessing the degree of effects on hydrodynamic, wave and sediment processes and sub-tidal geomorphology from reclamation and construction effects. Therefore the assessments in this AEE Report rely on a blend of expert appraisal, supported by comprehensive field observations and modelling, as well as understanding the analogue of how the existing runway reclamation has influenced physical processes. The assessments are underpinned by the following considerations:

. Within inner Lyall Bay, the effects on waves of the runway extension are likely to be considerably more sensitive than the change in current velocity patterns, as tidal currents are generally weak.

. Surfing amenity of waves within Lyall Bay are potentially sensitive to changes in wave refraction patterns caused by the proposed runway extension. Therefore surfing and recreational-related effects are given greater scrutiny when assessing the degree of coastal environmental impacts, with a more detailed, complementary assessment provided by DHI (2016).

. Modelling of combined coastal hydrodynamic and geomorphic/sediment processes continues to be challenging and at the forefront of ongoing research, particularly over longer multi-timescales, and the considerable natural variability that is invariably present. Therefore reliance is placed on using the models as tools, with sufficiently robust physics, to assess the relative changes before and after the runway extension is in place, backed up by field data when modelling the existing situation.

. Given the predominance of sands on the sea bed in Lyall Bay with minimal fine sediments and low contamination, discharge simulations for assessing the effect of turbidity are focused on de-watering discharges from the construction operations associated with the imported fill (which may contain fine sediments), rather than modelling sea-bed disturbances that may temporarily arise from using controlled ground-treatment techniques or rock placement for the dyke.

The effects on coastal physical processes are also assessed in line with requirements of the Resource Management Act (RMA), the NZ Coastal Policy Statement (NZCPS) and the operative Wellington Regional Coastal Plan (19 June 2000).

Existing coastal environment Lyall Bay is a semi-circular, open bay on the Wellington south coast between the rocky headlands of Te Raekaihu to the west and Hue te Taka (Moa Point) to the east that is exposed to southerly swell from the Cook Strait. Lyall Bay formed when a tombolo connected the Miramar Peninsula (to the east of the Airport) to the hills to the west of Kilbirnie after four uplift events in the past 7000 years.

The construction of Wellington Airport at Rongotai from 1952–1959 reclaimed 14 hectares of the eastern Lyall Bay for an 850 m runway extension, connecting with a rocky outcrop towards Moa Point beach, which formerly acted as a natural breakwater. The reclamation at that stage extended to the spur groyne (breakwater) placed on the submerged natural rock outcrop from the headland

Wellington International Airport Runway Extension: Coastal Processes 9

and permanently occupied about a quarter of the inner Bay coastal marine area.1 The runway reclamation was subsequently extended by 180 m to its present shoreline configuration in 1971– 1972, to allow operation of DC-8s, with 10-tonne akmon units placed along the periphery for coastal- wave protection (now has 12-tonne akmons along the southern edge after storm damage in 1973).

Locally, around the terminus of the existing runway and spur-groyne, the seabed bathymetry is dominated by the submerged extension of the former rocky reef (that was reclaimed) and gravelly deposits, whereas most of Lyall Bay comprises surface sediments of fine sand (median size of 0.15 mm) or rocky platforms along the periphery of the outer Bay. The area north of spur-groyne (breakwater) and to the west of the original runway reclamation, has accreted since the reclamations, forming a shoal that provides good-quality surfing waves in The Corner. The western side of inner Lyall Bay has an extensive naturally-occurring shoal, with diminished wave energy.

In terms of the geomorphic setting and associated hydrodynamic processes, the existing environment in the eastern side of Lyall Bay has been considerably modified by the existing reclamations. The historic placement of a seawall along the back beach of Lyall Bay and associated carparks and road has also considerably modified the natural beach processes by permanently fixing the shoreline.

The morphology of Lyall Bay Beach is largely-shaped by sequences of southerly-wave events (especially in winter), resulting in temporary landward retreat of the shoreline, with the sediment removed forming an offshore bar. There tends to be a reversal of this process in summer. The cove on the east side of the runway (Moa Point beach) is sandy with a coarser surface veneer of pebbles, but the relatively-shallow beach sand cover is generally perched on an underlying rock platform. The longer-term morphological response of Lyall Bay, to climate cycles such as El Niño-Southern Oscillation episodes and longer 20–30 year Pacific-wide climate cycles, is largely unknown. There has been no routine beach-profile monitoring of Lyall Bay Beach (Dr Iain Dawe, GWRC, pers. com.) from which to understand long-term effects (e.g., from the existing runway reclamations, or from climate variability), apart from a 1979 study by the then NZ Oceanographic Institute (Pickrill, 1979), which has been relied on for assessing shorter-term (seasonal) geomorphic response of the nearshore seabed profile.

Lyall Bay receives stormwater through shoreline outfalls from five neighbouring catchments and a marine outfall off the entrance to Lyall Bay discharged treated effluent from the Wellington City wastewater treatment plant (adjacent to the Airport). Despite these discharges, the water quality is generally very good and is mirrored in the quality of the seabed-sediment samples tested, which produced very low contaminant levels.

Winds are dominated by either southerlies or northerlies, with a long-term average wind speed of 27 km/hr based on the Airport wind-station record. Southerlies can generate a substantial wind sea which is usually accompanied by swell from the long fetches to the south, whereas the local sea generated by northerlies is severely limited by the short fetch (although may be coincident with a residual swell from the south).

The tide range for Wellington is relatively low compared with the rest of New Zealand, with an average range of just over 1 m. Only around 11% of the water volume of Lyall bay is exchanged on an average tide, which explains why the tidal currents are very weak (negligible) within the inner

1 inner Bay here defined as the environment inshore of an east-west line through the spur breakwater (groyne) at the end of the original runway reclamation

10 Wellington International Airport Runway Extension: Coastal Processes

confines of Lyall Bay. Currents near the spur-groyne are typically less than 0.1 m/s with low to moderate winds, but currents reached 0.32–0.37 m/s during two southerly gales during the 2014 field deployment. Modelling shows wind-driven currents tend to flow down-wind along the shallower margins of the Bay, and into the wind through the middle of the Bay— although for strong southerlies a clockwise eddy develops in the northern section and an anti-clockwise eddy in the southern section joining to drive a flow to the westerly quarter from the spur-groyne area.

The wave climate of the south Wellington coastline and outer Lyall Bay is well described, given the long 15-year wave-buoy record off Baring Head and shorter deployments in outer Lyall Bay. The majority of waves are relatively low (<1 m) and short period (3–7 seconds), while the more highly- publicized southerly wave events of over 5 m are only episodic. Significant wave heights exceeding 3 m and 5 m occur on average for a cumulative time period of 14 days and 1 day per year respectively. From the ~7-week deployment off the spur-groyne and wave modelling during the AEE investigations, there is evidence of attenuation in wave height within the confines of inner Lyall Bay, for both average conditions and larger events e.g., 80 km/hr southerly gale of 22-September 2014 produced a significant wave height of 4.7 m compared with 6.1 m in outer Lyall Bay.

High-resolution wave modelling of the existing situation shows that wave refraction, as the wave shoals in shallower water, and wave diffraction around the spur-groyne, produce a wide range of wave directions within the inner Bay, whereas the approaching wavetrains in the outer Bay are tightly constrained around due South. The spur-groyne and associated submerged rock outcrop, acts as a hydraulic control on wave patterns in The Corner area to the west of the existing runway.

Assessment of effects of the proposed 350-m runway extension was undertaken through modelling primarily, looking at the changes relative to the existing environment as the baseline (which includes previous runway reclamations).

Operational effects on coastal physical processes Currents— Delft2D model simulations indicate that there would be little change in Lyall Bay wind- driven circulation during strong northerly winds, other than minor localised changes around the runway embankment area. Circulation during northerly winds would remain largely unaffected in the Bay by the proposed extended runway.

During strong southerly winds, the before-and-after model simulations indicate that the proposed runway extension would slightly weaken the wind-driven current through the central area of Lyall Bay that passes off the existing-runway terminus. The wind-generated current from a strong southerly wind, presently curves westwards around the eastern cove (Moa Point beach) and passes across the end of the existing runway. With the extended runway, this westward flow would be intercepted and steered to double back into the southerly wind in order to pass around the terminus of the longer extension. This longer flowpath in deeper water around the end of the extended runway, during southerlies, would slightly weaken and narrow the wind-generated flow into the central Bay (compared to the existing situation), along with a more quiescent shadow zone on the western side of the extended runway compared to present in strong southerlies. But this change would only have a minor effect on the southerly-wind-driven hydrodynamics of the central Bay and is unlikely to affect flushing characteristics or morphological change of the inner Bay. The local re- steering of the downwind flow off Moa Point beach would also produce slightly faster velocities across the end of the extended runway, than presently occurs at the same location 350–400 m off the present runway terminus, as would be expected by the presence of the reclamation in what is

Wellington International Airport Runway Extension: Coastal Processes 11

presently open water. However, current speeds across the end of the extended runway would still be less than across the existing runway terminus, due to the deeper water 350–400 m further offshore.

Waves— The high-resolution ARTEMIS wave model was used to represent the general wave patterns in Lyall Bay, assuming a regular2 (sinusoidal) wave train applied at the entrance to outer Lyall Bay. The ARTEMIS model reproduces the spatial wave-crest patterns in Lyall Bay shown on vertical satellite images, as the incident wave-train is refracted and diffracted, particularly in shoaling waters of the inner Bay. Results from modelling the before and post-construction situations, indicate that the proposed runway extension has the potential to have a more than minor effect, i.e., moderate effect, on some combinations of wave height and period in some parts of the Bay, mainly on the eastern side of Lyall Bay near the existing runway revetment and more localised along the western and eastern sides of the proposed extended runway.

In the north-eastern sector, off the western side of the present runway rock revetment, 1.5 m incident waves through the Bay entrance (8-second period) could be attenuated by a further 0.2 to 0.6 m in a zone extending out ~250 m west from the existing runway revetment. The greatest wave- height reduction in this sector would be a localised patch 80–100 m NW of the spur groyne, and conversely, a very localised increase in these 8-second wind waves immediately behind the spur groyne through diffraction. In The Corner, the wave-height reduction would be somewhat less at 0.2 to 0.4 m for the 1.5 m incident waves. The model also predicts an increase in wave heights: i) locally around the spur groyne (breakwater) and adjacent submerged rock outcrop; ii) slight increases in a narrow central part of the inner Bay of the main Beach; and iii) in a patch close to the beach towards the eastern end of Lyall Bay Beach near The Corner. This latter area arises due to the reduced wave height offshore from this part of the Beach, with waves able to propagate (shoal) slightly further inshore than is the case for the existing situation, given the same offshore wave height and period.

The largest reduction in wave height overall would occur adjacent to the west-side rock dyke of the extended-runway, as the wave peels along the rock dyke subject to dissipation by friction (drag) exerted by the accropode units and also the influence of a subtle wave-sheltering effect as the runway extension alignment of ~183S is slightly west of the predominant wave-approach direction from due south.

Predicted changes after the runway is extended, for the same incident wave height (1.5 m) at the outer entrance, but considering swell (12-second period), would have a similar spatial pattern of wave height change to the shorter 8-second wind waves. The main differences would be a somewhat smaller reduction in wave height as the swell peels along the western side of the extended-runway rock dyke and a wider shadow zone in eastern part of Lyall Bay with reductions of 0.3 to 0.6 m. The eastern-most swath of propagating swell, parallel with and ~50 m west of the existing runway revetment, is predicted to be slightly narrower and around 0.2–0.4 m lower in wave height than for the existing situation for a combination of 1.5 m incident swell height of 12 seconds. The increase in wave height around the spur groyne (breakwater) and adjacent rock outcrop would cover less area for the longer swells compared to an 8-second wave, given the same incident wave height.

The height of wave-trains of regular waves reaching the central part of Lyall Bay Beach are likely to be only slightly affected by the runway extension, otherwise elsewhere in the Bay, including the

2 Pertains to the use of a repeating regular sinusoidal wave time series used for input on the offshore boundary of wave models such as ARTEMIS (otherwise known as monochromatic waves).

12 Wellington International Airport Runway Extension: Coastal Processes

western side, the changes in the height of regular waves will be negligible. However, the DHI study found some changes in the detailed characteristics of waves in some of these areas (DHI, 2016).

The cove east of the runway (Moa Point beach) will also exhibit a reduction in wave height as a result of the proposed extension. This reduction in wave height is more appreciable inshore and in the NW corner immediately adjacent to the eastern side of the existing runway. There is also likely to be more resonant or wave-reflection behaviour within the cove after the extension has been constructed, arising from the more-enclosed basin. However within the more-enclosed cove off Moa Point beach, the likelihood of additional coastal erosion of the beach by any wave-reflection patterns is small, particularly as the wave heights will be reduced compared to present, and the sand/gravel occupies a relatively thin veneer over rock in the lower tidal and shallow sub-tidal zone, limiting any storm cut in this zone.

These results indicate the predicted scale of general changes in waves throughout the Bay, relative to the existing situation, based on regular sinusoidal wave sets. More detailed high-resolution wave modelling was undertaken by DHI, using irregular measured waves propagating through the Entrance for occasions when surfing quality was good, to further investigate the effect of the runway extension on wave heights and characteristics (e.g., wave steepness, wave peakiness). These aspects are important considerations for assessing the effects on surfing quality and safety of recreational users, are described in the assessment report by DHI (2016). The DHI results also indicate that the ARTEMIS model overestimates the effect on the reduction of wave heights behind the spur breakwater as a result of the runway extension (Dr S. Mortensen, DHI, pers. com.).

Seabed changes — Introducing waves as well as tide and wind-driven forcing into a sediment- transport model, showed the predicted net change in sea-bed heights, following the construction of the 350-m runway extension, would be no more than minor (<1 cm at the seasonal scale) over much of Lyall Bay, including the nearshore area off Lyall Bay Beach and the eastern cove (Moa Point beach). The morphology of Lyall Bay beach will continue to be dominated by cut and fill along an on/offshore seabed profile governed by sequences of southerly storm events, with any effects of the proposed runway extension on seasonal morphological timescales likely to be second-order influences.

Localised changes in sea-bed heights, within 50 m around the extended runway, are likely to be minor (order of a few cm’s at seasonal timescales), particularly after the existing sea-bed topography has adjusted to the presence of the rock dyke/apron and the associated wave interaction with these structures. The local morphological adjustments off the end of the proposed runway dyke will occur through some erosion of sand seaward of the rock apron, adjacent to the accropode dyke, and depositing the sand further offshore, especially to the south, to form a new equilibrium seabed profile. However, this is likely to be a “dynamic equilibrium” depending on varying seasonal weather conditions and swell frequency or persistence.

For assessing any changes to longer-term beach-nearshore morphology, it is not feasible or meaningful to scale up the sediment–transport and morphology model results to longer timescales than seasonal without having the benefit of previous multi-year beach profiling data. However an indication of the likely long-term effects of the additional runway extension in deeper water can be based on both: a) the existing analogue of the present runway embankment and associated spur groyne (breakwater) still having the dominant control on nearshore morphology on the eastern side of the inner Bay, along with; b) the wave and morphological model results for seasonal timescales (which incorporates the changes in wave heights due to the runway extension). There could be

Wellington International Airport Runway Extension: Coastal Processes 13

ongoing broader or localised changes within the shoal adjacent to the existing-runway rock revetment arising from natural variability at annual to decadal timescales, and any ongoing adjustment to the existing reclamation, within which subtle longer-term changes may arise from the proposed runway extension.

Based on the limited beach-profile dataset of Pickrill (1979), most of the seabed height variability occurred within about 600 m of the shoreline, which indicates that any morphological effects of the runway-extension Project are likely to be driven by changes in waves and currents northwards of the existing spur-groyne and associated submerged rock outcrop. Indeed, the wave modelling has demonstrated that this existing groyne (breakwater) on the rock outcrop, is acting as the hydraulic control on wave refraction and diffraction processes north of this structure, and to some extent on current flows as well.

The predicted wave patterns for the proposed runway configuration in the Technical Report on Hydrodynamic and Sediment Processes (Pritchard et al. 2015), show that largely within the nearshore zone off Lyall Bay Beach, the overall spatial pattern of wave refraction and diffraction is similar to the existing situation, albeit with reduced wave heights preferentially on the north-eastern side. Given the wave refraction and diffraction patterns and wave crest orientations are likely to be similar to the existing situation, then long term, the gradual shoaling of the NE corner of the Bay arising from the original runway reclamation and spur groyne (which governs the current wave climate in The Corner), is likely to continue with only minor effects from the proposed runway extension in deeper water further offshore. There may however be some subtle changes in the shoaling rate as it adjusts to the reduced wave heights on average after the runway extension is in place.

A fixed-term beach-profile monitoring programme could be implemented to provide more quantitative information on how the nearshore morphology responds to storm sequences over a period of a few years. However, because of the more dominant response of the nearshore-beach system to natural climate variability and southerly-storm sequences, it would be difficult to isolate the effect of the proposed runway extension (which would be situated in deeper water) from the ongoing morphological response associated with the historic runway reclamation and spur groyne (breakwater) in the nearshore environment and the seawalls that limit the back-beach response along Lyall Bay Beach during storm-cut episodes. However there may be some merit in undertaking future high-resolution multi-beam bathymetric surveys in the eastern sector in the inner Bay, building on, and comparing with, the 2014 survey by NIWA (Mackay & Mitchell, 2014).

Water quality — Leaving aside operational stormwater discharges, the permanent runway extension would have negligible effects on turbidity and hence water quality and clarity. Local changes in waves and swell during storms around the runway extension may slightly alter resuspension of finer seabed sediments, but usually turbidity increases in Lyall Bay are ubiquitous during strong wind and/or wave events, so any changes in turbidity post-construction would not be detectable.

Construction effects Water quality — Early in the construction sequence, sporadic sea-bed disturbances from activities such as ground-treatment, pile driving, placement of rock-apron material, or shallow excavations, may locally generate turbid plumes or turbid fringes around the work area. Due to low levels of contamination in present in seabed sediments within Lyall Bay, the risk of adverse effects on the water column biota from transient sediment suspension/disturbance events during construction is very low (Depree et al. 2016). The turbidity from these sea-bed disturbance activities to some extent

14 Wellington International Airport Runway Extension: Coastal Processes

can be controlled at source as outlined in the Construction Methodology Report (AECOM, 2015b) and the Erosion and Sediment Control Plan. The fixed sites used for monitoring de-watering discharges (see below) can also be used to assess the compliance of turbidity or SSC levels at ~150 m away from the general work area.

On completion of the dyke wall, forming the perimeter of the extended runway, de-watering discharges during the embankment fill operations have the potential to introduce high-turbidity plumes into the receiving waters of Lyall Bay if a fine-sediment fraction is present in the fill material. The main source of turbidity in the water column would be from any clays, muds, or silts present in the fill material, even if they are only a small percentage of material by volume.

Three alternative discharge locations in the dyke walls were assessed in the turbid-plume model simulations – two on the western side of the airport runway extension at the NW and SW corners of the dyke and one in the middle of the southern wall of the dyke. The discharge was introduced into the top layer of the 5 depth-layers in dispersion model, to cover the case of the discharge being mainly freshwater from rainfall runoff within the fill area. Other options, not modelled specifically, could include piped dredged-sediments as a slurry from an anchored barge, predominantly containing denser seawater.

Based on preliminary Delft3D-SED plume simulations using two sediment discharges of medium silt at rates of 1 kg/s and 2 kg/s, maximum suspended-sediment concentrations (SSC) can be limited to around 11–16 mg/L and 22–36 mg/L respectively above background levels at around 150 m from any of the three discharge points modelled. An upper SSC environmental limit, to apply after reasonable mixing3 at ~150 m from the discharge location, is proposed to be set to 25 mg/L (or equivalent NTU magnitude), derived from considering the limit at which sea-bird foraging is curtailed (James et al. 2016). This is reflected in the monitoring conditions summarised below.

Modelling results show that due to the slow tidal currents during fair weather, the turbid plume exhibits small footprints (relative to higher winds) with a radial dispersive pattern from the discharge point. In conditions with high winds (northerly or southerly), currents are higher, which increases both the lateral spread of plumes and reduces the SSC. Consequently, the highest concentrations in the near to intermediate areas (a few 100s of metres from the discharge) will occur in calm weather, while in the far-field areas of the Bay, the highest concentrations, albeit only 1–5 mg/L above ambient conditions, will occur during high winds when the entire inner Bay is more turbid anyway from wave activity and possibly stormwater.

Monitoring conditions and mitigation of potential effects A set of monitoring conditions associated with coastal physical processes is put forward, mainly documenting construction progress and changes to the coastal marine area, encouraging the use of clean fill material that has as low a fraction of silts, and the use of best-practice sediment and erosion control measures to control discharges to the CMA “at source” from the infill operations.

Proposed consent compliance conditions for monitoring turbidity are detailed in the monitoring conditions associated with the Erosion and Sediment Control Plan appended to the AEE report. They are based around not exceeding an environment SSC limit of 25 mg/L beyond ~150 m from a discharge location, for minimising effects on seabird foraging (Marine Ecological Report). However, at

3 “reasonable mixing” in the RMA (e.g., s107) implies the existence of a zone in which the compliance standard need not be met – but important to distinguish between the near-field mixing zone, the point of complete mixing, and the non-compliance zone (MfE, Resource Management Ideas, No. 10, Aug 1994).

Wellington International Airport Runway Extension: Coastal Processes 15

present, the background SSC can be temporarily higher than this limit during high wind and wave/swell conditions including turbid plumes from several stormwater outlets in Lyall Bay following heavy rainfall. Therefor a relative SSC limit of an additional 10 mg/L not to be exceeded beyond ~150 m from a discharge location is proposed when the background exceeds 15 mg/L.

Other than the effect of the runway extension on reducing waves in some parts of the Bay, and minor increases in wave heights in other parts (leaving aside local effects around the rock dyke perimeter), all other effects on coastal physical processes are assessed as being minor or negligible. Mitigation of the effects of changes in wave patterns and surfing quality arising from the extended runway is covered in the assessment by DHI (2016).

16 Wellington International Airport Runway Extension: Coastal Processes

1 Background Wellington International Airport operates on a constrained footprint in the coastal suburb of Rongotai. The Airport has a single runway of 1945 m Take Off Runway Available (TORA) with 90 m safety areas at each end of the runway.

Wellington International Airport Limited (WIAL) propose to extend the runway to 2300 m TORA to further enable long-haul flights, including Code E aircraft (e.g., Boeing 777 or Airbus A330) and the ensuing economic benefits for the Wellington region.

WIAL engaged URS Ltd. (now AECOM) to provide Engineering Services in the development of engineering design options and the proposed specimen design carried forward through the environmental assessment and consenting process. The AECOM component also includes incorporating aspects of hazard risk (wave overtopping and storm-tide) and marine geology into the final specimen design. This coastal hazard information is covered in the Concept Feasibility and Design Report (AECOM, 2015a) rather than this Coastal Processes Report (other than a brief summary of extreme waves and an appraisal of the effects of sea-level rise on the Project).

After initial engineering and economic feasibility studies considering both Evans Bay and Lyall Bay, the additional 350 m runway extension is proposed to be to the south into Lyall Bay.

An initial scoping investigation was carried out and reported in October 2013 (AES/NIWA, 2013) and a number of work streams identified which would involve further field work and modelling as part of an Assessment of Effects (AEE) for the proposed runway-extension project.

This work has now been completed and a description of the existing coastal processes in Lyall Bay, covering tides, currents, waves, sediment transport, coastal morphology and sediment discharges (during construction) together with predicted level of changes as a result of the runway extension, has been prepared based on existing and new information (Pritchard et al. 2016). A separate technical report was prepared by Depree et al. (2016) on seabed sediment characteristics and the level of in-situ contamination of these sediments. New work undertaken as part of this assessment has included:

. Deployment of moorings in Lyall Bay for ~7 weeks to gather information on waves, currents, and physical characteristics of the water column (light, turbidity, temperature, salinity).

. Bathymetric surveys of Lyall Bay to support modelling and engineering investigations (Mackay & Mitchell, 2014).

. Modelling of the potential impacts of the extended runway structure on regular waves, currents, sediment transport and coastal morphology.

. Modelling of the potential impacts of sediment-plume discharges during construction of the runway structure and associated fill operations on in-situ turbidity of Lyall Bay.

. Site survey of the intertidal beach of the cove east of the runway (referred to as Moa Point beach alongside Moa Point Road).

Wellington International Airport Runway Extension: Coastal Processes 17

Coastal environmental assessments are required to support applications for coastal permits and consents for the Airport runway extension in the coastal marine area (CMA) and the adjacent coastal environment.

This Report covers the assessment of effects on coastal physical processes of the extended runway into Lyall Bay, focused on coastal hydrodynamic and coastal sediment processes. It draws on detailed information from field and model studies in the Technical Report on Coastal Hydrodynamics and Sediment Processes in Lyall Bay (Prichard et al. 2016), the report on sediment characteristics and sediment contaminant levels (Depree et al. 2016) and an additional site survey of the cove east of the runway extension (covered in Section 4.2).

Potential effects of the runway extension on inshore waves, based on modelling regular (sinusoidal) waves, were highlighted early on in the AEE investigations (Pritchard et al. 2016). Subsequently, surfing and recreational-related effects have been given greater scrutiny when assessing the degree of coastal environmental impacts, with a more detailed, complementary assessment provided by DHI (2016). This Coastal Processes Assessment Report only describes likely changes in general wave patterns and height for regular (sinusoidal) waves, but the effects of the runway extension on changes in irregular wave characteristics (e.g., wave steepness, peakiness) and therefore on water- contact recreation activities such as surfing and swimming are covered by DHI (2016).

Other than for describing the existing environment, this Report does not cover assessments of coastal-hazard extremes (e.g., waves, tsunami, storm-tide), but does cover the effects of climate change, including sea-level rise, on the Project in the next 100 years, as required by the NZ Coastal Policy Statement (NZCPS).

A companion report on the assessment of coastal ecological effects was prepared by James et al. (2016).

18 Wellington International Airport Runway Extension: Coastal Processes

2 Description of the Project Wellington International Airport Limited (WIAL) propose to extend the runway to 2300 m TORA to further enable long-haul flights, including Code E aircraft (e.g., Boeing 777 or Airbus A330).

WIAL are very aware of concerns about a reclamation for a runway extension and the potential effects on the environment and have commenced planning for this extension with the first phase being preparation of resource consent applications. Based on geotechnical work, and consideration of options to the north and south, a decision has been made by WIAL to reclaim land from the CMA to the south into Lyall Bay to support the runway platform.

Providing for this increased capacity of aircraft will involve an extension of the runway and taxiway of approximately 350 m into Lyall Bay4, as shown in Figure 2-1.

Figure 2-1: General layout for the proposed 350 m south runway extension into Lyall Bay to develop a 2300 m Take Off Runway Available (TORA). Source: AECOM (2015a).

The construction methodology envisages constructing a rock dyke around the perimeter of the runway extension and building a reclaimed land platform inside the rock dyke (AECOM, 2015b). The preferred fill materials consist of dredged primarily sandy marine sediments or alternatively quarry- sourced fill material. Any potentially liquefiable and/or loose/soft soils beneath the rock dyke will need to be either improved or removed to provide a firm foundation for the dyke. The reclaimed land platform will undergo settlement before the runway and taxiway can be built upon it. The settlement process can be accelerated by placing – then removing – a surcharge fill (or over filling)

4 with an additional ~35–45 m length underwater for the revetment batter and toe rock apron in approximately 13 m water depth (MSL)

Wellington International Airport Runway Extension: Coastal Processes 19

and along with vertical wick drains, and/or using ground improvement methods to improve the reclamation fill (AECOM, 2015b).

The key elements of the Project, which could potentially affect physical processes in the coastal marine area (CMA), either during construction and/or during the operational lifetime, are:

1. Sea-bed ground treatment (e.g., stone columns), if required, to ensure sufficient strength to support the extended embankment into Lyall Bay [construction effect].

2. Perimeter rock dyke constructed on the sea bed to contain the runway embankment [construction & operational effects].

3. Wave protection armour on the seaward side of the rock dyke using various size/weight of accropode units, which are unreinforced concrete objects that interlock and better resist the action of waves on breakwaters and coastal structures [construction & operational effects].

4. Placement of gravel/sand fill material within the rock dyke to form the runway platform reclaimed from the sea – either through pumping from barges or tipped from trucks, with a de-watering discharge to the CMA of supernatant containing suspended fine sediment [construction effect].

5. Construction of runway and taxiway bridges over Moa Point Road, which currently circumvents the southern end of the present runway [construction effect].

The source of the reclamation fill material is yet to be finalized but is likely to consist of material from a quarry and/or possibly from dredging predominantly sand material from the entrance to the Harbour or elsewhere along the south Wellington coast.

Further details on the construction of the specimen runway-extension design are provided in the Construction Methodology Report (AECOM, 2015b).

20 Wellington International Airport Runway Extension: Coastal Processes

3 Assessment methodology

3.1 Introduction This AEE Report describes both the existing coastal environment of Lyall Bay focusing on coastal physical processes, sediments and the potential effects on these processes that may arise from construction and long-term operation of the reclamation to form the ~350 m runway extension.

For this assessment, the main coastal physical processes considered in terms of potential effects from the Project are:

. Coastal hydrodynamics (changes in tidal and wind-driven currents).

. Coastal wave heights and wave refraction patterns.

. Sediment transport and coastal geomorphology (storm-scale changes to Lyall Bay and Moa Point beaches).

. Water quality (turbidity and any subsequent sedimentation from suspended-sediment discharges and potential for contaminant release arising from sea-bed disturbances from construction sites).

. Effects of climate-change, particularly sea-level rise and coastal-storm inundation, on the Project.

Coastal processes will also have the potential to impact on the Project, particularly in this case from coastal storm inundation, wave forces and in the longer-term, from climate change. These aspects, apart from climate-change and coastal-storm inundation are covered in the Concept Feasibility and Design Report (AECOM, 2015a) and are factored into the overall design of the Project. An assessment is provided in this Report on the effects of climate change on coastal-storm inundation in order satisfy the requirement under the RMA to consider the effects of coastal hazards and climate change, over a period of at least 100 years as stipulated in the NZ Coastal Policy Statement.

Effects on coastal ecology are discussed in the Marine Ecology Assessment Report (James et al. 2016).

There are no quantitative assessment criteria for assessing the degree of effects on hydrodynamic, wave and sediment processes and sub-tidal geomorphology from reclamation and construction effects. Therefore the assessments in this Report rely on a blend of expert appraisal, supported by field observations and modelling, as well as understanding the analogue of how the existing runway reclamation has influenced physical processes. The assessments are underpinned by the following considerations:

. Within inner Lyall Bay, the effects on waves of the runway extension are likely to be considerably more sensitive than the change in current velocity patterns, as tidal currents are generally weak.

. Surfing amenity of waves within Lyall Bay are potentially sensitive to changes in wave refraction patterns caused by the proposed runway extension. Therefore surfing and recreational-related effects are given greater scrutiny when assessing the degree of coastal environmental impacts, with a more detailed, complementary assessment provided by DHI (2016).

Wellington International Airport Runway Extension: Coastal Processes 21

. Modelling of the combined coastal hydrodynamic and geomorphic/sediment processes continues to be challenging and at the forefront of ongoing research, particularly over longer multi-timescales, not to mention the considerable natural variability that is invariably present. Therefore reliance is placed on using the models as tools, with sufficiently robust physics, to assess the relative changes before and after the runway extension is in place, without resorting to lengthy, intensive field programmes to support very detailed calibration and verification when modelling the existing morphological response.

. Given the predominance of clean sands on the sea bed in Lyall Bay, discharge simulations for assessing the effect of turbidity are focused on de-watering discharges from the construction operations associated with the imported fill (which may contain fine sediments), rather than modelling sea-bed disturbances that may arise from using controlled ground-treatment techniques or rock placement for the dyke.

The effects on coastal physical processes are also assessed in line with requirements of the Resource Management Act (RMA), the NZ Coastal Policy Statement (NZCPS) and the operative Wellington Regional Coastal Plan (19 June 2000).

The investigations and assessment of effects on coastal physical processes of the proposed extended runway into Lyall Bay were undertaken through a mix of:

. Desk-top review of previous reports, field data and satellite imagery. The review was previously undertaken for WIAL in 2013 by Aquatic Environmental Sciences Ltd and NIWA (2013).

. Field investigations and surveys focused on bathymetric surveys, seabed sediment/contaminant sampling and deployment of in-situ instruments in Lyall Bay to measure waves, currents and turbidity.

. Numerical modelling of the effects of the runway extension, relative to the existing situation, for waves, currents, sub-tidal geomorphic change and suspended-sediment discharges during construction.

. Expert appraisal of potential effects of the Project (both construction and operational) from the simulated changes from modelling investigations, informed by the above analyses, information and the response to the historic runway extension. Potential effects are aligned with requirements in relevant statutory instruments to assess whether effects are minor or otherwise, and whether mitigation or remediation is possible and some suggested monitoring conditions.

3.2 Desk-top analysis The desk-top analysis undertaken for WIAL (Aquatic Environmental Sciences Ltd. & NIWA, 2013) covering the existing coastal environment, both Lyall Bay and Evans Bay, included a review of the following information sources:

. Previous bathymetric and seabed geological surveys.

22 Wellington International Airport Runway Extension: Coastal Processes

. Past oceanographic field deployments and analyses, primarily for AEE investigations for the ocean outfall in Lyall Bay that discharges from the Wellington City wastewater treatment plant adjacent to the Airport.

. Modelling of currents and plume dispersion in that late 1980s (e.g., Beca Carter– Caldwell Connell, 1980; Bell, 1989) and 2013 (for Opus Consultants Ltd. and Capacity Infrastructure Services Ltd.) to assess the water quality effects of the main Wellington ocean outfall in outer Lyall Bay.

. Research papers or reports on coastal physical processes, such as sediment transport pathways and beach response for Lyall Bay specifically or more generally along the south Wellington coast.

. Long-term monitoring data for winds (Met Service station at the Airport) and waves (NIWA buoy at Baring Head operated on behalf of Greater Wellington Regional Council-GWRC).

. Synthesis on past investigations on coastal hazards (winds, waves, tsunami, storm-tide and climate-change impacts).

. Marine ecological studies, fisheries models and research papers in both bays.

The desk-top review of previous investigations highlighted gaps in the available data and knowledge, which then formed the basis for implementing additional field and modelling work in 2014 to support the environmental assessment of coastal physical processes.

Studies required to fill information gaps on coastal physical processes that were identified in the desk-top review are listed below, excluding items on coastal hazards and engineering requirements, which are covered in the Concept Feasibility and Design Report (AECOM, 2015a). Succeeding actions are listed in italics after each item.

3.2.1 Relevant gaps - Sediments/Geology . Contamination remobilisation potential from disturbed sediments – collection of grab or core samples and analysis for specific chemical species and sediment grain-size and settling/floc characteristics. [Following decision by WIAL to extend the runway south, the sea-bed sediment sampling programme was only undertaken in Lyall Bay (Depree et al. 2016).]

. Establish a before (baseline) and after-construction beach-profile monitoring programme on the Lyall Bay Beach if a southern option is preferred, to monitor any changes due to reclamation and wave refraction/diffraction. [This aspect could be part of a coastal-permit monitoring condition, but it would be difficult to isolate any specific effects of the extended runway from any ongoing morphological adjustment of historical shoreline modifications and reclamations and climate variability.]

3.2.2 Gaps - Hydrodynamics . Complete full multi-beam bathymetric mapping coverage of western Lyall Bay; improve compatibility of existing multi-beam coverage in Evans and Lyall Bays undertaken in different surveys to produce a consistent dataset. [Completed in both Bays in January 2014 – see Mackay & Mitchell, 2014).]

Wellington International Airport Runway Extension: Coastal Processes 23

. High-resolution wave modelling in Lyall Bay or Evans Bay, with and without reclamation options to assess changes in wave patterns and description of the extreme wave climate in Lyall or Evans bays to inform engineering design. [Wave modelling undertaken for this assessment in Lyall Bay only – with more detailed wave modelling and assessment undertaken by DHI – the extreme wave analysis on Baring Head wave buoy data was undertaken by AECOM Ltd. for the engineering concept and design (AECOM, 2015a).]

. Assessment of any changes in potential tsunami propagation in Lyall and Evans Bays as a result of reclamation and assessment of tsunami wave amplitude to consider for engineering design. [Tsunami response is largely an operational matter for WIAL and civil defence evacuation planning.]

. Assess the extent of turbid-plume dispersion using existing models. [Plume modelling undertaken for this assessment in Lyall Bay only.]

3.3 Field surveys Based largely on the gap analysis in the desk-top review outlined above, the necessary field data requirements for the preferred Lyall Bay option were focused on:

. Collecting a representative range of sea-bed sediment characteristics and surficial- sediment contaminants.

. High-resolution sea-bed bathymetry obtained from multi-beam sounding surveys.

. Background turbidity and water column clarity measurements, which are covered in the Marine Ecology AEE Report (James et al. 2016).

. An additional field deployment for at least 1 month of wave and current-meter gauges at two sites in Lyall Bay to provide additional calibration data for modelling, especially for waves. [Note: a subsequent field deployment of three surface-following wave gauges was undertaken by NIWA in Lyall Bay in late 2015 for use by DHI in wave modelling – but otherwise not described in this Report].

3.3.1 Sea-bed sediment and contaminant characteristics Sea-bed sampling Broad-scale sea-bed sediment mapping in Lyall Bay was undertaken for grain-size analyses and contaminants in such a way as to complement the use of the same sites for the companion marine biological sampling (MacDiarmid et al. 2015). Short-core sediment extrusions underwent laboratory analysis. The analyses determined grain-size and also total organic carbon and contaminants (see below) that were also undertaken for the companion marine ecological studies.

The gridded survey plan included a total of 13 sampling sites in Lyall Bay (Figure 3-1) from which surficial sediments (0–5 cm) and subsurface sediments (typically less than 20 cm) were collected. Water depths of the sites ranged from approximately 5 to 15 m. These sites were identical to the sites also sampled for benthic biological communities. Specific sampling sites for contaminant analysis included sediments adjacent to (sites 1, 3, 5 and 8) and within the proposed area of the runway extension (sites 10, 11 and 12), a total of seven sites (Figure 3-1).

24 Wellington International Airport Runway Extension: Coastal Processes

Grain-size analyses Grain-size distributions in the sediment samples were measured using a Beckman Coulter LS13-320 dual-wavelength laser-particle sizer that measures grain-sizes in the 0.04–2000 μm size range with 5 3 1% reproducibility. An approximate volume of 0.5 cm of sediment was re-suspended in distilled water and then washed through a 2 mm sieve into the laser-sizer where it was analysed.

Standard statistics (mean, sorting, skewness and kurtosis) on the grain-size distributions measured were calculated for each core sample from the ‘percent by volume’ laser data and matched to standard sediment textural descriptions using the Folk and Ward (1957) classification scheme. Although continuous grain-size distributions are produced from the laser-particle sizer output, grain- sizes are presented primarily as percent gravel (over 2 mm diameter), sand (0.063–2 mm), mud (<0.063 mm), silt (0.002–0.063 mm) and clay (<0.002 mm).

Contaminant analyses Samples from surficial sediments and the last depth fraction to the bottom of the core (typically less than 20 cm penetration due to the sandy nature of the seafloor sediments) were double-bagged in labelled zip-lock plastic bags, chilled and shipped overnight to the NIWA Hamilton laboratory for further processing. Both whole (<2 mm size) and fine (<0.063 mm) sediment particle size-fractions were prepared for analysis of contaminants.

The samples were analysed for a range of contaminants, based on previous and ongoing Greater Wellington Regional Council (GWRC) and Wellington City Council (WCC) monitoring in Wellington Harbour. Analyses conducted on these samples included Total Organic Carbon (TOC), heavy metals and DDT-derived organochlorine pesticides. Further analyses were undertaken for sites 10 and 11 (proposed runway extension), where deeper cores were sampled in 5 cm increments to 20 cm and 0– 10 cm and 10–20 cm depth composites were used for elutriate testing (mobilisation/solubilisation of contaminants from sediments to overlying water). These elutriate sediment depth fractions at these two sites were also analysed for heavy metals.

Outputs Grain-size information was used to develop a picture of grain size distributions within Lyall Bay and complement previous studies of sediment characteristics and sea-bed morphology. From this grain- size information, settling velocities for suspended-sediment plume modelling of sea-bed disturbances, especially fine sediment fractions, could be determined, although no modelling was undertaken for disturbances due to the small fraction of fine material and limited disturbance- generating activities required (Construction Methodology Report, AECOM, 2015b). Note: the primary source of fine sediments in de-watering discharges comes from the fines in imported fill material.

Sediment contaminant analyses enabled the spatial distribution of contaminants within in Lyall Bay to be determined and assessed. The results were assessed with ANZECC and other guidelines.

5 μm = micro-metres or 0.001 mm

Wellington International Airport Runway Extension: Coastal Processes 25

Figure 3-1: Sea-bed sediment sampling sites in Lyall Bay, Wellington. Contaminant sampling sites were 1, 3, 5, 8, 10, 11 and 12. Other sampling sites shown were analysed for sediment grain-sizes. Seds = Sediment analyses, Contam = Contaminant analyses, Elutriates = Elutriation experiments (see Depree et al. 2016 for details). The location of the Lyall Bay ocean outfall is also shown as a red line. Bathymetric data are from Wright et al. (2006) and Mackay & Mitchell (2014).

3.3.2 Hydrodynamic processes Bathymetric surveys In January 2014, NIWA carried out a bathymetric survey of parts of Lyall Bay and Evans Bay on behalf of WIAL (Mackay & Mitchell, 2014). The vessel used for the hydrographic survey was the 14-m survey launch Ikatere, owned and operated by NIWA Vessels. Previous bathymetric surveys have been

26 Wellington International Airport Runway Extension: Coastal Processes

undertaken in Lyall Bay (e.g., Wright et al. 2006), so the focus was on sounding the area enclosed by the black dashed line in Figure 3-2. For engineering and geo-technical purposes, a survey was also undertaken immediately to the south of the present runway (Mackay & Mitchell, 2014; Figure 2) covering the proposed runway extension footprint and reached within 10 m of the akmon coastal protection units.

A Kongsberg EM-3002D shallow water multi-beam echo sounder was used to obtain near-complete coverage of the seabed in the survey areas, although limited to narrower swathes in shallow water 25-m apart. Depths shallower than ~2 m below Chart Datum were not able to be surveyed due to draught restrictions. Also the shallower waters along the eastern and western periphery of outer Lyall Bay were also not able to be surveyed due to submerged rocky outcrops. The results were processed according to hydrographic best practice and tidally corrected based on tidal measurements in Lyall Bay relative to surveyed benchmarks in Wellington Vertical Datum-1953.

The same gridded-dataset at 25 m spacing was supplied to the hydrodynamic/wave modellers at both NIWA and DHI.

Figure 3-2: Extent of Lyall Bay infill bathymetry survey (black dashed line) shown against grey area collected from previous surveys. Background image: ESRI World Imagery.

Wellington International Airport Runway Extension: Coastal Processes 27

Hydrodynamic/wave deployments Given the importance in quantifying the effects of changes in waves from the proposed Wellington Airport runway extension, a field deployment of instruments was undertaken in Lyall Bay over a ~7- week period from August to October 2014 to provide observational data to validate both wave and tidal models.

Figure 3-3 shows the locations occupied during the instrument deployment in Lyall Bay in 2014. The details of what instruments were deployed at each of the sites and over which period are in shown in Table 3-1.

Table 3-1: Locations (WGS-84) and 2014 deployment information for the Lyall Bay instruments.

Location Latitude Longitude Instrument Deployment Depth Mean (S) (E) (parameter measured) Period below water Water depth Surface (m) (m)

Site 1 41˚ 20.197 174˚ 48.140 ADCP (currents and waves) 18-Aug 14:00 to 11.0 11.0 (inshore) 09-Oct 09:00

Site 2 41˚ 20.932 174˚ 47.933 ADCP (currents and waves)6 N/A (offshore) DWG (waves) 04-Sep 11:00 to 7.6 19.0 09-Oct 09:00

Current velocities in the Lyall Bay were measured using Acoustic Doppler Current Profilers (ADCP). ADCPs measure currents in “bins” throughout the water column, thus providing the user with a velocity profile over the entire water column.

ADCPs were deployed on the sea bed facing the water surface, which is known as a bottom-mounted upward-looking configuration. ADCP acoustic transducers were nominally positioned 0.4 m above the sea bed.

7 Significant wave height (Hs), mean spectral period (Tm) and, where possible, peak direction of wave propagation (Dp) were measured using an ADCP and a DOBIE Wave Gauge.

The outer ADCP at Site 2 broke free during a southerly gale on or around 22 September 2014 and was not found on subsequent searches. Fortunately, a DOBIE Wave Gauge was moored adjacent to the ADCP at Site 2, so some wave information was collected (excluding wave direction). Note: a further deployment of surface-following wave gauges at three sites in Lyall Bay was undertaken by NIWA in late 2015 to provide additional high-resolution wave data for DHI to assist with assessing mitigation options to address surfing quality.

Winds prior to 1 January 2013 were obtained from the Automatic Weather Station at Wellington airport operated by the NZ Met Service (NIWA Climate Database; Agent No. 3455). The station at Wellington airport is located 4 m above mean sea level. Winds for Lyall Bay after 1 January 2013

6 The ADCP at Site 2 was lost to a storm during the deployment. No current data is available at Site 2.Wave data at Site 2 comes from the Dobie Wave Gauge. 7 Defined as the average of the top 33% (1/3) of wave heights over the measurement cycle

28 Wellington International Airport Runway Extension: Coastal Processes

were obtained from NIWA’s meteorological forecasting model (EcoConnect) as nowcasts (i.e., current conditions at or a few hours ahead of the forecast time) as well as NIWA’s nearby Baring Head weather station (at the lighthouse), with access to NZ Met Service data now restricted.

Further details are provided in Section 2 of the Technical Report on Coastal Hydrodynamics and Sediment Processes in Lyall Bay (Prichard et al. 2016).

Figure 3-3: Site map showing oceanographic instrument locations overlying the seabed bathymetry to WVD-53. The black polygon outlines the approximate area covered by the proposed runway extension.

Wellington International Airport Runway Extension: Coastal Processes 29

3.4 Modelling approach: coastal physical processes

3.4.1 Approach to hydrodynamic and sediment-transport modelling The integrated Deltares Delft3D modelling suite was used in the investigations of the changes before and after construction of the extended runway on the tide, wind and wave-driven dynamics of Lyall Bay and on the before-and-after effect of changes in current circulation and waves on sediment transport and morphology in the Bay (Coastal Processes Technical Report, Pritchard et al. 2015). The modelling approach adopted was based on the following components:

• For this present application to the south Wellington coast and Wellington Harbour, the Delft3D modelling system was used in depth-averaged (2-D) mode. The use of the depth- average 2-D mode, which still incorporates all the main forcing mechanisms, can be justified as outlined in Section 3 of the Coastal Processes Technical Report, alongside run- time constraints on running the entire model in 3-D (which included part of the deep Cook Strait waters and Wellington Harbour as shown in Figure 3-4).

• The model were primarily used as a comparative tool to determine the effects on hydrodynamics after the runway extension is constructed, relative to the present situation.

. Residual hydrodynamic circulation in Lyall Bay was investigated using forcing from tides at the open-sea boundary in Cook Strait and local wind speed and directions applied over the sea-surface of the model. The simulations were run for the same time frame as the field observational period of approximately 7 weeks in 2014.

• Combined tide and wind-driven circulation investigations in Lyall Bay focused on isolating the wind response to currents (leaving aside waves) and were forced by a sequence of 3- day wind events peaking at a speed of 22 m/s (strong gale), directed from the prevailing south and north directions. The winds were superimposed on a background of a repeating mean (average) tide applied as a boundary condition on the open-sea boundary to better isolate the change in wind-driven response in currents (before and after the runway extension).

• For assessing sediment transport and morphological change, the 2-D hydrodynamic model was forced with tide (multiple constituents), local wind speed and directions and coupled with a SWAN wave model8 on the same grid for the south Wellington coast (red–coloured grid in Figure 3-4). The simulations also covered the same time frame as the ~7-week field- measurement period. Sand transport in shallow coastal areas is a complex interaction between waves, which primarily re-suspended seabed sediments into the water column, which are then subsequently transported by currents until the sediment particles settle again. In nearshore waters, including the surf-zone, non-linear wave dynamics along can also transport re-suspended sediments, especially if waves propagate towards the nearshore zone at an oblique angle to the beach. The Delft3D-SED and Delft3D-MOR modules incorporate tidal and wind-driven hydrodynamics from the Delft2D simulation coupled with waves simulated by the SWAN spectral-wave model to combine these wave- current interactions with mobile sediments on the seabed. The sediment-transport modelling, based on fine sand with a median grain size of 0.15 mm, was then used to determine the before and after changes in sand deposition or scour within Lyall Bay. Note:

8 SWAN is a spectral wave model developed by Delft Hydraulics, that is widely used globally for broader-scale wave climate studies

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field samples had a narrow range of variability for the median sand grain size of only ±0.013 mm in terms of a standard deviation (Depree et al. 2016).

• The spatial variability of wave climate for the existing environment of Lyall Bay was also appraised using the SWAN model and compared to the field measurements on waves at Sites 1 and 2 (Figure 3-3).

• Investigation of the intensity and extent of suspended-sediment plumes from de-watering discharges during construction was supported by simulations using the 2-D hydrodynamic model coupled to a non-cohesive sediment-transport and settling model. This assumed that the water in the discharge is the same density as water in the bay, which would be the case for barged fill material sourced from dredging the seabed or from discharging wave- overtopping waters. In this case, there would be no buoyancy effect in the water column associated with the sediment plume. For discharges with substantial freshwater from stormwater runoff, further model simulations will be required to include plume-buoyancy effects, through coupling with a near-field mixing model for the discharge.

Further details of the model grids and implementation of the modelling simulations are described in the Coastal Processes Technical Report.

Figure 3-4: Delft3D-WAVE model grid (red) of south Wellington coast superimposed on top of the Delft3D- FLOW model grid (black) that also includes Wellington Harbour.

Wellington International Airport Runway Extension: Coastal Processes 31

3.4.2 High-resolution wave modelling to predict changes post runway-extension In order to locally resolve wave patterns around a structure such as the Wellington International Airport runway extension into Lyall Bay, a high-resolution wave model is required that incorporates local changes in wave processes particularly:

. Refraction – process by which the direction of a wave moving in shallow water at an angle to the seabed contours is changed (e.g., the part of the wave in shallower water moves more slowly than the part still advancing in deeper water). Refraction explains why swell and longer-period wind waves straighten up across inner Lyall Bay, even if the waves approach the outer Bay slightly to the east or west of a southerly direction.

. Diffraction – where wave energy is transmitted laterally along a wave crest, when part of a train of waves is interrupted by a barrier such as a rock revetment or breakwater of a comparable physical scale to the wave length. Diffraction is the "spreading" of waves into the sheltered region within the barrier's geometric shadow. This currently occurs in the lee of the spur groyne and associated rocky outcrop on the western side of the existing runway revetment.

. Reflection– part of an incident wave that is returned seaward when a wave impinges on a steep beach, rocky outcrop, rock revetment or other reflecting structure. This would potentially occur on the end of the extended runway revetment. However, porous multi-layer rock armour layers and akmons or accropodes provide substantial absorption of wave energy, thereby reducing reflection, which is controlled in models by a reflection coefficient.

. Wave breaking – depth-induced steepening and eventual breaking of waves as they shoal into shallower water, and depends on the wave period how far offshore they break e.g., swell waves feel the bottom in greater depths than wind sea.

There are a number of high-resolution phase-resolving9 models available globally – NIWA selected the ARTEMIS10 model (Aelbrecht, 1997), which is part of the widely-applied TELEMAC–MASCARET modelling system developed by Laboratoire National d'Hydraulique (Electricite de France).

ARTEMIS model runs on a finite element or triangular grid to solve the extended elliptic mild-slope wave equation with additional dissipative terms (Booji, 1981). It is used to describe the combined effects of diffraction, refraction, reflection and wave breaking for regular (sinusoidal) water waves propagating over bathymetry and due to interactions with lateral boundaries—like breakwaters and coastlines.

The resolution of the Lyall Bay ARTEMIS grid zoomed in on the immediate area around the end of the present runway and proposed runway reclamations is shown in Figure 3-5 to show the degree of grid resolution at around grid-cell scales of 5 m. The extent of the entire Lyall Bay grid out to the entrance in Cook Strait is shown in the Coastal Processes Technical Report (Section 5).

9 Phase resolving means there are sufficient model grid nodes or cells to fully resolve a single wave, including the crest and trough. SWAN on the other hand operates on coarser grids as it is only resolving spatial changes in wave spectral energy (and excludes diffraction). 10 Agitation and Refraction with Telemac on a MIldSlope http://www.opentelemac.org/index.php/presentation?id=19

32 Wellington International Airport Runway Extension: Coastal Processes

ARTEMIS was used for the Project to compare wave patterns of regular waves generally within Lyall Bay for the current situation to the wave patterns and heights predicted with the proposed extended runway reclamation in place. The results in the Coastal Processes Technical Report (Pritchard et al. 2015) were presented as a change in significant wave height relative to the existing runway situation, which is in line with assessment of effects relative to the existing environment.

In consideration of post-extension changes on the range of waves in Lyall Bay used by surfers, the Coastal Processes Technical Report focused mainly on wave heights of 1–3 m for longer period wind waves (e.g., 8 seconds) and the 10 – 15 second swell wave spectrum. Surfing is undertaken at ‘The Corner’ which is located to the eastern end of Lyall Bay (Figure 4-2) and also in the central part of the Bay.

DHI have undertaken complementary wave modelling and assessment specifically addressing effects on non-linear aspects of irregular waves and surfing-wave quality and public safety of beach recreational users and feasibility of a surf-break to mitigate effects on waves (DHI, 2016).

Figure 3-5: ARTEMIS finite-element model grid resolution in the area around the runway extension (left) existing runway; (right) proposed runway reclamation. Depths are scaled according to colour shading (red shallowest, blue deepest). [Horizontal grid: NZTM].

Wellington International Airport Runway Extension: Coastal Processes 33

A visual validation of the ARTEMIS wave model is shown in Figure 3-6 comparing spatial wave-crest patterns and breaking zones throughout Lyall Bay from a historic aerial photo archived in the Google Earth software. This visual comparison with the vertical view of the predicted wave fields for similar incident waves, shows a close match with a number of features, including the extent of surf zones (white areas in Figure 3-6), zones of refracting shoaling waves turning into both coasts of the outer Bay, more quiescent areas in coves and the western side of the Bay and a particularly close match with the overall refracting and diffracting wave crest patterns including in The Corner area adjacent to the existing runway revetment. There are some slight local differences in the wave-crest pattern in the predicted wave field at the tip of the spur groyne – an area of rapidly changing bathymetry associated with the rock outcrop.

Results from the ARTEMIS wave model suggest that for 1 and 1.5 m regular waves applied at the outer-Bay boundary, the wave attenuation between the two field mooring sites (Site 1–inner ADCP and Site 2, located in Figure 3-3) is between 0.03 and 0.05 m, which is within the limits of measurable accuracy of the instruments.

Figure 3-6: ARTEMIS model predicted surface water level as a result of a 1.5 m wave at the Cook Strait boundary with a wave period of 12 seconds (LEFT). Google Earth image of Lyall Bay taken on 24 July 2014 for a wave height of 1–1.5 m and a period ~10 seconds (RIGHT). Wave measurements for the latter were derived from the Baring Head wavebuoy.

3.4.3 Turbid-plume dispersion modelling During construction turbid plumes from discharged decant water are likely to occur, particularly during the infill phase once the perimeter rock dyke is in place.

The transport of suspended-sediment transport as turbid plumes was simulated in the Delft-3D modelling suite, where suspended sediment concentration (SSC) is advected (transported) by the

34 Wellington International Airport Runway Extension: Coastal Processes

flow of currents and diffused (mixed) vertically and horizontally by turbulent mixing processes, with a slow sediment-settling velocity to simulate the gradual sinking of the small sediment particles.

Sediment characteristics are parametrised in the model through a settling velocity (Stokes equation) and at the seabed, a critical bed-erosion threshold and critical deposition threshold (related to bed shear stresses at the bottom boundary layer). Flocculation or clumping of cohesive sediments can also be simulated, but was not invoked in the plume modelling scenarios, as flocculated particles tend to have a higher settling velocity than individual grains (and hence settle out quicker rather than disperse more widely).

The Delft3D-FLOW model for plume simulation was setup in 3-dimensional mode with 5 vertical depth layers, with all layers equally spaced down the water column in the model (but layers varying in actual height depending on the total water depth (e.g., in 10 m water depth, the 5 layers are all 2 m in height). The water density in each depth-layer was set to typical uniform value assuming well- mixed salinity and sea-temperature profiles in the water column, which is reasonable for the conditions during and following a moderate to strong wind events.

The hydrodynamic boundary conditions for the plume modelling were also generalised to a repeating average tide (given tidal currents are very low within the inner Bay) over a 30-day period interspersed with two southerly and one northerly wind events up to 22 m/s (strong gale) – the same sequence as used for assessing the effects of the runway extension on wind-driven currents (Section 3.4.1).

The decant water, from either pumped dredged or barged seawater-based slurries or drainage of surface rainwater during storms (for truck-tipping fill operations), was modelled for three weir/discharge locations. These were set on the western side of the rock dyke (based on advice from AECOM Ltd, Construction Methodology Report) – one at the NW corner of the extension (near the present akmon revetment) and one at the SW corner of the extended dyke, some 350 m further offshore, and a third location midway along the southern terminus of the rock dyke. In the model the discharge of turbid water is released into the top surface layer of the model, as a weir is likely to be used and also to mimic the initial buoyancy of rainwater or a freshwater discharge through a pipe below the surface.

The settling velocity of sediment in Delft3D-SED was set at 0.13 mm/s, based on Stokes settling velocity computation for a medium silt grain size of 15 µm or 0.015 mm.

Plume model simulations were undertaken for a continuous de-watering discharge and also a 12- hour pulse discharge (e.g., after a rainstorm event).

The approach adopted for this assessment was to set the sediment discharge rate to achieve two different SSC levels, based on marine ecological criteria, after reasonable mixing has occurred around 150 m from the discharge point. Then during construction, turbidity monitoring would be required to ensure these SSC limits were not exceeded.

3.5 Assessment of effects criteria: policies/plans/statutes

3.5.1 Wellington Regional Coastal Plan (2000) The Regional Coastal Plan (RCP), which has been operative from June 2000, provides objectives, policies and rules for managing activities in and above the coastal marine area (CMA), which is the marine waters and inter-tidal land below Mean High Water Springs (MHWS).

Wellington International Airport Runway Extension: Coastal Processes 35

The RCP also specifies designated areas around the Wellington region of either significant or important conservation value. The only conservation areas in the general vicinity of Lyall Bay area are Tarakena Bay (Figure 3-7) to the west of Hue te Taka Peninsula and east of West Ledge Reef, which is an Area of Important Conservation Value (AICV) marking an important waka landing place and the area around Taputeranga Island off Island Bay (Figure 3-7).

Planning Map 8D in the RCP designates water quality classes in the Wellington area, with the waters throughout Lyall Bay managed for water-contact recreation and the nearshore waters around Te Raekaihau Point and Hue te Taka Peninsula in outer Lyall Bay also managed for shellfish gathering (see Figure 4-2 for locations).

Figure 3-7: Areas of Conservation Value in Lyall Bay area (Tarakena Bay and Tauputeranga Island) in the Regional Coastal Plan. Source: Planning Map 2H, Regional Coastal Plan, GWRC.

3.5.2 NZ Coastal Policy Statement (NZCPS) The policies and objectives of the NZCPS most relevant to coastal physical processes are:

. Objective 1: “… maintaining or enhancing natural biological and physical processes in the coastal environment and recognising their dynamic, complex and interdependent nature …”

. Policy 3(1): “Adopt a precautionary approach towards proposed activities whose effects on the coastal environment are uncertain, unknown or little understood, but potentially adverse.”

. Policy 10(1–2) on reclamation: For 10(1), “Where a reclamation is considered to be a suitable use of the coastal marine area, in considering its form and design have particular regard to [Policy 10(2)]: …

36 Wellington International Airport Runway Extension: Coastal Processes

c) the use of materials in the reclamation, including avoiding the use of contaminated materials11 that could significantly adversely affect water quality …. in the coastal marine area

e) the ability to remedy or mitigate adverse effects on the coastal environment

g) the ability to avoid consequential erosion and accretion, and other natural hazards.”

. Policy 23 (Discharge of contaminants) – covered in the Marine Ecology AEE Report (James et al. 2016).

. Policy 24 (Coastal hazards and climate change) – the effects of sea-level rise combined with coastal inundation hazards are considered in this Report, with the 1% AEP12 joint storm-tide and wave setup (excluding wave runup) inundation levels for Lyall and Evans Bay obtained from a report prepared for Greater Wellington regional Council (Lane et al. 2012) combined with projected sea-level rise values from the present Ministry for the Environment coastal guidance manual (MfE, 2008).

3.5.3 Resource Management Act (RMA) 1991 Relevant aspects of the RMA in relation to coastal physical processes are outlined below.

Under section 3, actual or potential effects includes any positive or adverse effect, temporary or permanent effect, any past13, present or future effect and any cumulative effects. In this Report, actual or potential effects on coastal physical processes alone have been assessed as:

. Negligible.

. Minor14.

. Moderate (between minor and adverse).

. Major or adverse effect.

Central to the purpose of the Act (section 5) is sustainable management in managing the use, development and protection of natural and physical resources which enables communities to provide for social, economic and cultural wellbeing, while …. (c) avoiding, remedying or mitigating any adverse effects of activities on the environment.

Section 6 (Matters of national importance) and section 7 (Other matters) in general relation to coastal processes are covered in the AEE and the Marine Ecology AEE Report (James et al. 2016) and the Natural Character and Landscapes AEE Report (Boffa Miskell, 2015). Section 7(i) of the RMA also requires councils to have particular regard to the effects of climate change. For this case, the assessment is focused on the effects of coastal-storm inundation and sea-level rise on the runway,

11 Suspended-sediments or turbidity generate from discharges or disturbances can be regarded as a contaminant (RMA Section 2 definition) 12 AEP is the annual exceedance probability such an event will occur in any year - a 1% AEP is an event with a 1% chance occurring in a year (or more well known, the event with an average recurrence interval of around 100 years) 13 Past and any continuing effects from the historic runway reclamation on the CMA are not considered in this Report, as reclamations are granted in perpetuity and become land. 14 Minor is a comparative word meaning lesser or comparatively small in size or importance (Bethwaite v. Christchurch City Council (CO85/93)19

Wellington International Airport Runway Extension: Coastal Processes 37

whereas the effects of climate change on erosion around the shoreline of the extended runway will be limited due to the extensive armouring of the rock dyke around the perimeter.

Based on section 105, the consent authority must, in addition to matters in section 104(1), have regard to: a) the nature of the discharge and the sensitivity of the receiving environment to adverse effects (covered in the Marine Ecology AEE Report); b) the applicant’s reasons for the proposed choice; and c) possible alternatives methods of discharge, including discharge into any other receiving environment. These latter two matters are addressed in the AEE and in the Construction Methodology Report (AECOM, 2015a).

Section 107 for assessing discharge permits, outlines that after allowing for reasonable mixing, the discharged contaminant (suspended sediment in this case) should not produce any conspicuous change in colour or visual clarity. This aspect is largely covered in the Marine Ecology AEE Report (James et al. 2016), but is also considered in this Report in relation to monitoring turbidity during construction phase, based on the analysis of the suspended-sediment plume modelling.

38 Wellington International Airport Runway Extension: Coastal Processes

4 Existing environment This section provides a description of the existing coastal environment, relative to which the effects of introducing the proposed extended-runway reclamation are assessed. It covers the geomorphic and geographical setting followed by a description of coastal physical processes that currently operate in Lyall Bay based on previous studies and new data collected during the present AEE investigations for the runway extension.

4.1 Coastal and geomorphic setting Lyall Bay is a semi-circular, open bay on the Wellington south coast between the rocky headlands of Te Raekaihu to the west and Hue te Taka (Moa Point) to the east that is exposed to southerly swell from the Cook Strait (Figure 3-3).

Pillans & Huber (1992) provide evidence that there has been at least four uplift events of the Miramar Peninsula area over the Holocene epoch in the last 7000 years, probably relating mainly to activity on the Wairarapa Fault. Lyall Bay formed when a tombolo connected the Miramar Peninsula (to the east of the Airport) to the hills to the west of Kilbirnie. The growth of the tombolo has occurred within the last 900 years. Māori history identifies the Miramar Peninsula as a separate island (Motu-Kairangi or Miramar Island) some 900 years ago (Raukura Consultants, 2014), but the area has since been subject to tectonic uplift by two events in more recent geological time. Uplift of ~3 m occurred from an end of 15th century earthquake event 520–470 years before present (Pickrill, 1979; Pillans & Huber, 1992; Clark et al. 2015) and ~2 m in the 1855 Wairarapa earthquake (Pickrill, 1979). Sediment from the shelf in Cook Strait and to a lesser extent flanking cliffs, have progressively extended the tombolo southwards until it attained its present-day width (Pickrill, 1979).

The construction of Wellington Airport at Rongotai from 1952–1959 reclaimed 14 hectares of the eastern Lyall Bay for an 850 m runway extension, connecting with a rocky outcrop towards Moa Point beach, which formerly acted as a natural breakwater (Figure 4-1). The reclamation at that stage extended to the spur groyne (breakwater) placed on the submerged natural rock outcrop from the headland and occupied about a quarter of the inner Bay coastal marine area (i.e., inshore of an east- west line through the spur-groyne).

The runway reclamation was subsequently extended by 180 m to its present shoreline configuration Figure 4-2 in 1971–1972, to allow operation of DC-8s, with 10-tonne akmon units placed along the periphery for coastal-wave protection (Webby, 1984). During a storm in May 1972, a substantial number of akmons were displaced, so heavier 12-tonne units were placed along the southern edge and the bund crest raised by 2 m in August 1973 (Webby, 1984). There is no documented evidence of the effects that the additional 180-m runway extension has had on coastal processes, but is likely to have been modest, given the hydraulic control on waves in particular by the spur groyne (breakwater) and that it was a short extension.

Wellington International Airport Runway Extension: Coastal Processes 39

Figure 4-1: Aerial photograph looking north on completion of the airport construction (21 Jan 1959). Source Ref: WA-49119-F. Alexander Turnbull Library.

The original 1941 shoreline of Lyall Bay and the former rocky outcrop are shown in Figure 4-2, along with other features mentioned in this Report, including The Corner where surfing is popular.

The bathymetry of Lyall Bay (Figure 3-3) reflects the Pleistocene geological history, with the former steep-sided valley infilled with sands and reworked by wave activity to form a flat seabed gently sloping seaward (Pickrill, 1979). Inshore of the present Airport runway embankment, the Bay is asymmetrical, with a deeper trough extending down the eastern side and a wide shallow sandy platform extending from the western shoreline (Figure 3-3), which greatly influences the wave refraction patterns from east to west off Lyall Bay Beach.

Locally, around the terminus of the existing runway and spur-groyne, the seabed bathymetry is dominated by the submerged extension of the former rocky reef (that was reclaimed) and gravelly deposits, with the sandy bed shoaling to the north of the groyne, as shown in Figure 4-3. More recently in 2014, NIWA carried out additional multi-beam sounding surveys for the Project to cover gaps from previous bathymetric surveys and another high-resolution survey off the end of the existing runway (see Section 3.3.2).

Lyall Bay is 1.3 km wide at the southern entrance narrowing to 1.1 km in the inner Bay (post-runway extension in 1959) and is 2 km in total length over a north-south orientation. Lyall Bay encompasses a depth range from 25-30 m (MSL) beyond Hue te Taka Peninsula, 12-13 m deep in the central section before shoaling to the intertidal beach at the northern head of the Bay.

40 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-2: Geomorphic features in Lyall Bay incl. the 1941 shoreline. Blue line shows the approx. 1941 shoreline (from Pickrill, 1979) and green dotted line the rock outcrop in 1941 that joined the submerged rock outcrop the spur groyne currently sits on. [Background image: Google Earth, 8 October, 2014].

Wellington International Airport Runway Extension: Coastal Processes 41

Figure 4-3: Multi-beam bathymetric data at 0.5 m resolution off the existing runway in Lyall Bay obtained by NIWA (Mackay and Mitchell, 2014). Background imagery is sourced from ESRI http://services.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer

4.2 Coastal geology, sediment processes and characteristics

Sediments and geology Surface sediments in Lyall Bay are predominantly well sorted, fine sands overlying coarser gravels that can form exposed aprons of mega-ripples, especially in the vicinity of the rocky substrates (greywacke bedrock) that form the headlands (e.g., Arron and Lewis, 1993; Carter and Lewis, 1995). The upper sandy deposits are mobile under fair- and storm-wave activity, with pebble gravels also able to be transported under extreme wave conditions, even at water depths of 60 m (Nodder, 1991, 1994; Carter and Lewis, 1995). Offshore, in the vicinity of Hue te Taka (Moa Point), greywacke basement or boulder-sized materials associated with the basement lithologies may be within about 2 m of the sea-floor, although there are localised incidences of the upper sandy and gravel deposits being at least 7-10 m below the sediment surface (Nodder and Smits, 1994). In outer Lyall Bay, the depth to greywacke basement might be as much as 70-80 m (Davey, 1971; Rogers, 1971; Grant- Taylor et al. 1974).

42 Wellington International Airport Runway Extension: Coastal Processes

The spatial distribution of sea-bed sediments in Lyall Bay is shown in the map extract in Figure 4-4, comprising predominantly well-sorted fine sands (light shade in Figure 4-4) overlying coarser gravels and gravelly-sands (stippled shade) that can form exposed aprons on the seabed especially in the vicinity of the rocky substrates (greywacke bedrock- red shade) that form the two enclosing headlands Te Raekaihau on the west and Hue te Taka on the east (Arron and Lewis, 1993).

In the wider context of the south Wellington coast, Lyall Bay contains the largest area of surficial sands on the seabed followed by neighbouring Houghton Bay (Figure 4-4). Recent sampling of seabed sediments in association with the seabed contaminant survey (Depree et al. 2016) at sites shown in Figure 3-1 showed that the seabed sediments in Lyall Bay comprise moderately to well-sorted, very fine to fine sands (except for two subsamples from sites 8 and 12 which were very fine gravelly sands). Across all sites (Figure 3-1), the average sand content was 97.4% (±0.7, 1 standard deviation), with 0.1% (±0.3) gravel, 2.5% (±0.7) silt and 0% clay. The median grain-size (D50) for all sand samples was 0.154 (±0.013) mm diameter, which is categorized as fine sand.

These grain-size data obtained by Depree et al. (2016) confirm previous work undertaken in Lyall Bay and along the Wellington south coast, with well-sorted fine sands forming a uniform sandy veneer that overlies pebbly gravels, which are often exposed as aprons around many of the submerged rocky reefs along the Wellington south coast (e.g., Arron and Lewis, 1993; Carter and Lewis, 1995; Wright et al. 2006). These very gravelly sediment substrates were not sampled by the recent AEE field study, since the focus was on the sandy substrates and their association with potential contaminants and infauna that might be affected by the proposed runway extension.

Figure 4-4: Surface sea-bed sediments of Lyall Bay and Wellington south coast (from Arron & Lewis 1993). Light-shade (fine sands); stippled shade (gravels and gravelly-sands) and red shade (bedrock).

Wellington International Airport Runway Extension: Coastal Processes 43

Since long sediment cores were unable to be obtained as part of the present study (Depree et al. 2016), the thickness of the uppermost fine sandy sediment is not known precisely in Lyall Bay. Previous studies suggest that the fine sand veneer is of the order of 1–2 m thick (Nodder, 1991, 1994; pers. obs.; Nodder and Smits, 1994), thinning closer to the coast where coarse-grained gravels and rocky outcrops are exposed eventually (e.g., Arron and Lewis, 1993). Figure 4-5 shows the results in sand cover estimates (in cm) from an early geophysical survey in eastern Lyall Bay by the then DSIR Geophysics Division (Davey, 1971; also cited in Rogers, 1971), that Dr Scott Nodder (NIWA) considered in the above estimate of the sand veneer. Further information is described in the Engineering Report (AECOM, 2015b) based on recent boreholes in Lyall Bay for the Project.

Figure 4-5: Sand cover (cm) and the edge of bedrock estimated from an early geophysical survey of eastern Lyall Bay in 1971. Note: North is to the left of the image. Source: Davey (1971), Rogers (1971).

In terms of sediment mobility, Carter and Lewis (1995) have showed that fine sand (less than 0.25 mm diameter) will be typically stirred by wave activity at water depths less than 30 m for almost 50% of the time, especially in autumn through to spring, based on wave data from Lyall Bay by Pickrill (1979). Furthermore, under extreme swell wave conditions, even pebble-sized gravels (greater than 4 mm diameter) will be mobilised over water depths up to 60 m (Nodder, 1994).

Morphology of inner Lyall Bay The morphology of inner Lyall Bay was investigated by Pickrill (1979), covering long-term bathymetric changes and shoreline modifications (e.g., the runway extension) – information and aerial photos from which the 1941 shoreline was plotted in Figure 4-2.

Following the reclamation for the existing runway in 1959, shoaling occurred north of the spur groyne and adjacent to the revetment, with the largest accretion of 3–5 metres in waters which were deepest prior to reclamation as shown in Figure 4-6 (Pickrill, 1979). The map of accretion since construction of the runway reclamation (bottom panel) also indicates that the spur groyne and associated submerged rock outcrop is acting as a wave and hydraulic control, as there is no apparent accretion immediately behind (north) of the groyne/outcrop and the accretion thickness tapers off

44 Wellington International Airport Runway Extension: Coastal Processes

towards it. The modern bathymetry (Figure 3-3) also shows the shoal pinches in close to the runway revetment (e.g., dashed 5 m contour) about 150 m to the north of the groyne, matching the shadow wave diffraction zone (Figure 5-4 to Figure 5-7).

The beach and nearshore morphology and temporal changes in the morphology are a direct response to the wave climate, with the largest changes to the beach system taking place in the high-energy centre of the beach, with smaller changes (over and above the eastern shoaling) occurring at the more sheltered ends of the Beach (Pickrill, 1979). During periods of calm or low energy (including northerlies), the beach develops low berms, midway up the nearshore profiles, which is replaced by a concave planar profile during southerly storms. In winter, Pickrill (1979) observed landward retreat of the shoreline, with sediment removed forming an offshore bar, which grows and moves seaward. There tends to be a reversal of this process in summer. The annual sand budget was estimated to be at least 106,000 m3 by Pickrill (1979), with more than half of this sand accounted for by seasonal changes in morphology. The sediment-transport modelling at seasonal timescales undertaken for the Project, which incorporated one southerly-gale event, was able to mimic this offshore transport and bar formation in Lyall Bay (see Section 5.2.3).

Figure 4-6: Historic bathymetric changes in Lyall Bay. A. Changes between 1903 Penguin and 1950 Lachlan surveys; B. Changes between 1950 and a NZ Oceanographic Institute survey in 1977. [Source: Pickrill (1979)].

Wellington International Airport Runway Extension: Coastal Processes 45

Over the last 100 years, the coastal environment has been modified considerably – coastal dunes have been replaced by housing, roading, a seawall built at the head of the beach, and the eastern part of the crenulate-shape beach reclaimed for the airport runway (Pickrill, 1979). A carpark also occupies the eastern coastal margin near the runway embankment. These modifications of the back- shore have altered the planform shape of Lyall Bay Beach, and reduced the natural sediment buffering capacity, causing past issues with erosion of the seawall and overwash or wind-blown sand deposits during southerly storms. While the original vegetation is long cleared, revegetation of stretches along the foreshore and environs carried out by Wellington City Council and interest groups, provides a mosaic vegetation cover that is establishing slowly given the difficult climatic conditions (Boffa Miskell, 2015).

Eastern cove morphology A beach survey was undertaken by Nodder & Gerring (2015) on 5-February 2015 of the eastern cove (Moa Point beach) between the existing runway and Hue te Taka Peninsula (adjacent to Moa Point Road). The survey straddled one hour either side of low tide at 1335 h (NZDT), with fine weather conditions, slightly cloudy and a 20–25 knot northerly wind.

Beach slope profiles were measured using a surveyor theodolite and extendable measuring pole at twelve locations along the Moa Point beach that were either 20 m (transects #1-8) or 40 m apart (transects #8-12) as shown in Figure 4-7. At the lowest possible position on the lower beach-face, typically at or below the low-tide mark, a 1.5 m long steel probe, with a diameter of 20 mm, was hammered into the beach sediments using a fence-post rammer. The depth at which the probe could not penetrate any further was noted.

46 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-7: Aerial photograph of Moa Point beach showing the beach slope profile locations (transects #1- 12). Source: NIWA field report (Nodder & Gerring, 2015)

The Moa Point beach is located in a small, west-facing pocket bay (cove) indented at the start of Moa Point proper (Hue te Taka) and east of the present airport runway. The natural geomorphology and backshore is modified to some extent by: a) the presence of the coastal road; the buried Lyall Bay marine outfall pipeline, which comes onshore at the northern/western end of the beach; and b) two sets of stormwater discharge pipes.

Wellington International Airport Runway Extension: Coastal Processes 47

Moa Point beach can be divided into two coved sections north and south of the rocky outcrop (Figure 4-7).

Along the northern/western end of the eastern bay, the beach is backed by 2–3 m-high vertical “cliffs” in late Quaternary gravels and modern infill rubble that diminish in height along the beach to less than 1 m near the central part of the beach Figure 4-8. Here, the eroded gravels and rubble cliffs are replaced by low-lying sandy to pebbly gravel dunes covered in marram grass that extend up to the road. The eroded cliffs probably represent erosion only by the most extreme waves that occur episodically in Lyall Bay.

Figure 4-8: (Right): Western end of Moa Point beach with coastal cliffs at top right; (Left): Central area of exposed rock dividing the beach into two shallow-indented coves, looking along the cuspated beach from the southern/eastern cove. [Credit: P. Gerring, S. Nodder].

In the central part of the bay, a 40 m-wide area of exposed rocks extends offshore from the lower beach-face and includes several small stacks that are exposed sub-aerially, even at high tide (Figure 4-8; right panel). The central rock outcrops effectively separate the bay into two smaller, shallow- indented coves; the cove at the western end is about 200 m long (transects #1-8; Figure 4-7) and that at the eastern end is about 120 m long (transects #10-12). Offshore, these rocks form part of a more extensive area of subtidal reef that extends out to at least 5 m water depth along the coastline of the entire northern/western embayment (Figure 4-7).

The Moa Point beach itself comprises medium to coarse sand (0.25–2 mm diameter) and well- rounded, pebble to cobble gravel-sized sediments (>2 mm, up to ~250-260 mm diameter). The sands and gravels on beach-face form a series of beach cusps where the gravels are arranged in lobate “horns” that point seaward and finer sand-sized particles occur in the intervening “embayments” (Figure 4-8; left panel), with larger and probably coarser cusps found north of the central rock outcrop than those to the south.

Penetration depths through the surficial sediments of the lower beach-face ranged from 0.1 to 0.8 m. Shallowest penetration depths of 0.1 and 0.2 m were encountered in the vicinity of the central rocky outcrops in the middle of the bay (transects #8 and 9; Figure 4-7). The probe measurements are only indicative (as probe resistance can be affected by single boulders, consolidated gravels, as well as bedrock. However, they show that the unconsolidated mobile sediments in this eastern cove generally occupy a relatively shallow veneer on the surface, with areas of bedrock acting as a control on any excessive beach erosion. Within the beach cusps, it appears that the pebble-cobble gravels that form the cusp “horns” are a veneer that may be only a single particle thick (i.e., perhaps from ~5

48 Wellington International Airport Runway Extension: Coastal Processes

to up to 25 cm-thick since most of the gravels are pebble- to cobble-sized). This veneer overlies the underlying medium-coarse sands.

4.3 Marine discharges and water/sediment quality Lyall Bay receives stormwater from the Lyall Bay catchment, the southern parts of the Miramar Golf Course and Wellington Airport and part of Moa Point Wastewater Treatment Plant. The approximately 280 ha catchment comprises 5 urban sub-catchments, namely (WCC, 2014; Depree et al. 2016):

 Lyall Bay West (118 ha), fully developed and outer residential, 4 major stormwater outfalls and one constructed overflow (sewage).

 Lyall Bay East (29 ha), fully developed and outer residential, 2 major stormwater outfalls.

 Airport South (57 ha), airport land, 1 major stormwater outfall.

 Strathmore Park South (57 ha), fully developed and outer residential, 1 major stormwater outfall.

 Moa Point (16 ha), fully developed and outer residential, wastewater treatment discharge.

The catchments discharge through a series of outfalls at Lyall Bay Beach, Moa Point, and near the breakwater at the south end of the airport runway (see Appendix A; Depree et al. 2016). The discharge zone is therefore spread along a large exposed beach, breakwater and rocky coast, all of which are subject to relatively high energy from waves. Stormwater loads of contaminants (zinc, copper, lead and PAHs) are estimated to be around 5% of the total load from Wellington City (catchment comprises 4% of the total area). Despite some sources of wastewater (i.e., occasional partially-treated overflows), water quality is generally very good with respect to aesthetics and human health (WCC 2014). Chemical contaminants have not been previously assessed, but because of the high dilution and dispersion of the area, contaminants are not considered to have any significant effects in Lyall Bay (WCC 2014).

Outer Lyall Bay receives treated wastewater (and occasional partially-treated overflows following heavy rainfall) from the Moa Point Wastewater Treatment Plant that treats wastewater from most areas of Wellington City (except the Karori catchment). Treated and UV-disinfected wastewater is discharged into the marine receiving environment in Cook Strait via an outfall pipe that enters the CMA (buried initially) just to the east of the existing runway revetment. The outfall diffuser is located 1.9 km offshore, SSW of the existing Wellington Airport runway embankment in a water depth of 22 m (MSL), providing substantial initial dilution once the buoyant plume surfaces (Bell, 1989; WCC, 1990).

Despite sources of stormwater and wastewater (i.e., overflows), water quality is generally very good with respect to aesthetics and human health (WCC, 2014). Chemical contaminants have not been previously assessed, but because of the high dilution and dispersion of the area, contaminants are not considered to have any significant effects on marine organisms (WCC 2014).

The main findings from the recent AEE field investigation and laboratory analyses (Depree et al. 2016) of contaminants in the surficial sediments of Lyall Bay were:

Wellington International Airport Runway Extension: Coastal Processes 49

 Total extractable heavy metals (and arsenic) concentrations were consistent with background soil/rock for the Wellington region, with no measurable anthropogenic ‘foot print’ observed in the Lyall Bay surficial sediments.

 Total extractable heavy metal concentrations of arsenic, cadmium, chromium, copper, nickel, lead, zinc and mercury in Lyall Bay sediments (<2 mm fraction) were all well below ANZECC interim sediment quality ISQG guideline trigger (‘low’) values.

 Heavy metal concentrations at sites 10 and 11 (Figure 3-1) within the proposed runway extension area appear to be homogenously distributed to at least a depth of 0.2 m. Over this depth range, it is assumed that organic concentrations, similarly do not change (i.e., increase) markedly.

 DDT and associated analogues were the main organochlorine pesticides present in the surficial sediments of Lyall Bay, but the average concentration (±1 standard deviation– s.d.) of 0.24 ± 0.15 g/kg is 6-times lower than the current ANZECC ISQG ‘low’ trigger value of 1.6 g/kg and below the pending revised ANZECC trigger value of 1 g/kg. Organic-normalised (1%-OC) total DDT concentrations ranged between 0.29 and 1.09 g/kg, with an average concentration (±1 s.d) of 0.49 ± 0.30 g/kg, which is around 3-fold lower than the current ANZECC ISQG value (1.6 g/kg).

 PAH concentrations (mass-normalised) ranged from 45 to 155 g/kg in Lyall Bay sediments, with an average concentration (±1 s.d.) of 94±38 g/kg, which is around 40-times lower than the ANZECC ISQG ‘low’ trigger value of 4,000 g/kg (note that the recommended revised ‘low’ guideline value for total PAH is 10,000 g/kg). Organic-normalised (1%-OC) PAH concentrations ranged between 90 and 330 g/kg, with an average concentration of 187 ±77 g/kg, which is at least 20-times lower than current ANZECC ISQG value.

Contaminant concentrations in Lyall Bay surficial sediments are therefore very low and uniformly distributed across the study area, including at sites most likely to be disturbed by construction activities, namely, sites 10 and 11 in the area to be reclaimed (Depree et al. 2016). Mobilisation of sediment from 0–0.2 m depths from within the area of the proposed runway extension is not expected to result in any significant increase in sediment contaminant concentrations in surrounding areas.

From a contaminant perspective, the risk of adverse effects on the water column from transient sediment suspension/disturbance events during construction is very low given that water column concentrations, even after allowing for reasonable mixing, are estimated to be at least two-orders of magnitude lower than default ANZECC water quality trigger values (Depree et al. 2016).

The 5-week deployment of a mooring in outer Lyall Bay during the AEE investigations during September 2014 provided an assessment of the dynamics in optical water quality and estimates of total suspended sediment from turbidity over the early-spring month (MacDiarmid et al. 2015). The deployment captured calm periods and several storm events, with corresponding reduction in visibility range and increase in suspended sediment concentration or SSC (Figure 4-9).

This 5-week snapshot showed that the waters were typical of clear water with a blue-green hue with more of browner colour during southerly storm events due primarily to re-suspension of bottom sediments by wave orbital motions, but could also be influenced by sediment runoff following heavy rainfall further inshore. During the deployment reduced visibility (20–30 m down to <1 m) or

50 Wellington International Airport Runway Extension: Coastal Processes

increased SSC corresponded to southerly storm events with significant wave heights above around 2 m in height, or somewhat lower if long-period southerly swells occur (Figure 4-9). The events on 6 and 14 September (Figure 4-9), while somewhat above 2 m in wave height, were only generated by a limited southerly wind fetch and not accompanied by longer-period swell, with swell being more conducive to re-suspension of seabed sediments.

The highest daily rainfall was on 27-28 September (30 mm total across 2 days) and 14 mm on 13 September to 0800 hrs the next day, with little evidence of stormwater runoff during these events influencing turbidity offshore at the outer mooring. However, the turbidity within inner Lyall Bay is likely to be more affected by stormwater runoff and outlet discharges, although wave resuspension is likely to still be the main process contributing to the background turbidity.

SSC (8 and 16 m depths) and wave height from outer Lyall Bay optical mooring 40 10

35 SSC (16m) 9 SSC (8m) 8 30 Wave Ht 7 25 6 20 5

15 4

SSC (mg/L) SSC 3 10 2

5 1 height (m)wave Significant 0 0 4/09 9/09 14/09 19/09 24/09 29/09 4/10 9/10

Figure 4-9: Time series of suspended-sediment concentration (SSC) from the upper (8 m) and lower sensors (16 m) and significant wave height (orange) from the optical mooring in outer Lyall Bay (4-Sept to 9- Oct 2014). Source: Data from MacDiarmid et al. (2015) and Pritchard et al. (2015).

Although limited to a snap-shot in time, this time-series provides a realistic range of conditions including a gale southerly event (~80 km/hr on 23 September) likely to be experienced in Lyall Bay (MacDiarmid et al. 2015; James et al. 2016). Summary statistics for total suspended sediment concentrations from the top and bottom sensors on the optical mooring are listed in Table 4-1. As expected the larger values occur lower down in the water column nearer the bed where turbidity is higher from a combination of re-suspended seabed sediments and settling particles from higher in the water column. The top sensor values are the more relevant values to consider for near-surface effects (both ecological and aesthetic) and for informing turbidity monitoring requirements during construction.

Table 4-1: Statistics for the distribution of measured SSC (mg/L) at the optical mooring in outer Lyall Bay.

Statistic (for SSC in mg/L) Top sensor (8 m depth) Bottom sensor (16 m depth)

33-percentile 1.0 0.9

50-percentile (median) 1.7 1.9

mean 2.6 3.3

Wellington International Airport Runway Extension: Coastal Processes 51

Statistic (for SSC in mg/L) Top sensor (8 m depth) Bottom sensor (16 m depth)

99-percentile 12.0 18.2

maximum recorded 16.3 37.5

The maximum SSC values were recorded on 9 September 2014 during a southerly, with the next highest during the southerly gale on 23 September 2014.

An earlier study of water clarity and underwater light attenuation in Lyall Bay (Bell et al. 1992; Beca Carter-Caldwell Connell, 1980) for the marine outfall investigations at sites 1.8 and 2.5 km offshore (former is present outfall diffuser site), showed that significant penetration of short-wavelength visible light occurs in the Bay down to about 9–12 m, approaching quite clear ocean-type waters.

4.4 Winds Long-term wind and barometric pressure measurements have been collected from the following stations in the region around Wellington International Airport:

. Main weather station at the Airport operated by NZ MetService since 1960 (Agent #3445).

. A newer automatic weather station (AWS) at the south end of the runway operated by NZ MetService (Agent #10331).

. Beacon Hill above Breaker Bay, operated by Port of Wellington from 1969 to 2004 (Agent #3440).

. Baring Head station (next to the lighthouse) operated by NIWA since 1969 (Agent #18234).

The main Airport wind-gauge dataset now extends to a 54-year record and at Baring Head, a 45-year record. Wind roses from the earlier part of these records along with other Wellington wind gauges is shown in Figure 4-10, illustrating the spatial variability in distribution of wind directions.

Wind flows across central New Zealand are concentrated through Cook Strait and blow across the Wellington area almost always from either north/north-west or south/south-east, depending on location in the Harbour (Figure 4-10).

At Wellington International Airport and in Lyall Bay, winds are further influenced by local topography to be either northerly or southerly winds. Winds from the northerly quarter, which often includes when prevailing south-westerly flows occur over NZ, are more frequent than those from the south, especially in spring and summer, but many of the most severe storms have been southerlies (Quayle, 1984). Strong southerlies tend to be more frequent in winter.

A more up-to-date wind rose for Wellington Airport, covering a 43-year period up to 2004, was produced by Gorman et al. (2006) as shown in Figure 4-11. The average wind speed (based on an average speed for 10-minutes) over this long-term record was 7.4 m/s or 27 km/hr.

52 Wellington International Airport Runway Extension: Coastal Processes

The maximum recorded 10-minute wind speed is 40 m/s or 145 km/hr, which occurred during ex- tropical cyclone Gisele (aka “Wahine Storm”) on 10 April 1968 from a SSW direction. Sustained winds above 110 km/hr blew for nearly 8 hours. A recent southerly gale on 20 June 2013 reached 101 km/hr, causing damage around Wellington, closure of the Airport and generated waves that matched those during Gisele.

Calm periods with winds <1 m/s occur for just under 3% of the time.

Waves in Lyall Bay are strongly influenced by the predominant bi-modal distribution of winds, from either the south or the north, with the latter generating much smaller short-period wind waves than the waves and swell that propagate into the Bay from the south.

Figure 4-10: Wind roses for the Wellington area. Source: Quayle (1984).

Wellington International Airport Runway Extension: Coastal Processes 53

Figure 4-11: Wind rose derived from hourly records of wind speed and direction at Wellington Airport from 1962–2004. Note: direction is in meteorological convention of where the wind blows from. [Source: Gorman et al. (2006)]

4.5 Hydrodynamic and wave processes for existing environment

4.5.1 Tides, sea-level variability and volumes Wellington has a relatively small tidal range compared to other areas of New Zealand, with a spring tidal range of 1.36 m, 1.06 m average tide range and a neap-tide range of 0.76 m (Nautical Almanac, 2015/16). Lyall Bay has a slightly smaller tidal range than Wellington Port - about 0.02-0.04 m less for the average tide range (Walters et al. 2010).

Present-day tide-marks extracted from the Queens Wharf record at Wellington Port are shown in Figure 4-12 relative to WVD–1953 (Bell & Hannah, 2012). The present-day MSL is nearly 0.2 m above WVD–53, based on the average from the recent record 2006 to 2011 (and similarly for the longer epoch from 1996 up to 201415).

15 from NZ Nautical Almanac, 2015/16 published by LINZ (pp. 36–37).

54 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-12: Present-day tide marks at Wellington relative to WVD-53. Note: MHWS-10 is the level exceeded by 10% of all high tides; MLWS-10, low tide mark at which 10% of all low tides descend below, and MSL is the present mean level of the sea for period 2006-2011 [Source: Bell & Hannah (2012)].

Sea-level also varies at widely-different time scales, spanning:

. hourly to daily from weather influences including storm surge and seiching

. monthly to annual for seasonal climate effects on sea level, and

. multi-year to decadal effects from longer-term climate cycles such as El Niño–Southern Oscillation of ENSO (2–4 year episodes) and the 20–30 year Inter-decadal Pacific Oscillation (IPO), both of which generate changes in sea level across the entire Pacific.

For Wellington, storm surges could reach over 0.7–0.8 m above the predicted tide. Variability of MSL on a monthly basis from seasonal to decadal climate influences can range from -0.16 to +0.20 m based on an analysis period 1945 to 2011 (Bell & Hannah, 2012). Higher than normal sea levels occur during La Niña episodes and the negative phase of the IPO (which we have been in since 1999). Conversely, monthly sea levels are lowest during El Niño episodes combined with a positive phase of the IPO.

Wellington Harbour has experienced a long-term average rise in relative mean sea level of 2.03 ±0.15 mm/year, which is relative to the landmass (Bell & Hannah, 2012). This has resulted in a rise in MSL of 0.2 m in the last 100 years, relative to WVD–53 (Figure 4-13). This is slightly higher than the global average sea-level rise over the same century period of 1.7 ±0.2 mm/yr (IPCC, 2013) and the New Zealand-wide average of 1.7 mm/yr (Bell & Hannah, 2012) as shown in Figure 4-13.

The higher relative sea-level rise in Wellington (compared to the other main ports) arises from the landmass of the lower North Island subsiding for periods of time (Hannah & Bell, 2012; Beavan & Litchfield, 2012) due to slow-slip on regional fault systems, in between uplift events during large earthquakes in the Wellington region. Climate cycles such as ENSO and IPO will continue to drive similar seasonal and interannual variability in sea level about the mean sea level as it rises.

Wellington International Airport Runway Extension: Coastal Processes 55

Annual MSL rise above local vertical datum at 4 main ports: NZ 0.3 Wellington (WVD-53) 0.25 Auckland (AVD-46)

0.2 Lyttelton (LVD-37) Dunedin (DVD-58) 0.15

0.1

0.05

0

-0.05 Annual MSL (m; relevant LVD) relevant (m; Annual MSL -0.1

-0.15 1900 1920 1940 1960 1980 2000 2020

Figure 4-13: Change in annual mean sea level at the 4 main ports of NZ since 1900. Each series of annual MSL is relative to the relevant local vertical datum (LVD) – which for Wellington is WVD–53. Data sources: Greater Wellington Regional Council, Lyttelton Port, Port Otago, Ports of Auckland, and processed by Emeritus Prof. John Hannah (Vision NZ Ltd) and NIWA (Hannah & Bell, 2012).

The estimated average volume of water within Lyall Bay is around 27.19 Mm3 relative to WVD-53 (calculated from the ARTEMIS wave model grid area in Figure 3-6) covering a surface area of approximately 2.99 Mm2. Based on an average tide range,16 the proportion of seawater volume each tidal-cycle entering and exiting Lyall Bay to the total average volume of the Bay is only 11%, which reflects the relatively small role tides play in circulation of the Bay, particularly in the inner Zone 1 (Figure 4-15).

4.5.2 Tidal and wind-generated currents

Previous relevant studies Previous information on hydrodynamic and wave processes from several studies have provided a reasonable sound basis for understanding the coastal physical processes that operate in the Lyall Bay environs (AES/NIWA, 2013). Much of the present knowledge on tides and currents along the south Wellington coast has been derived from environmental investigations in the 1980s associated with options for a long ocean outfall, to replace the shoreline outlet discharge into Lavender Bay, from the Moa Point Wastewater Treatment Plant and for the Hutt Valley outfall a Tunnel Point on the opposite side of the Harbour Entrance (locations in Figure 4-14).

For the Wellington outfall study, current meters at 2 m and 14 m above the sea-bed were deployed 1.5 km offshore in Lyall Bay in September 1989 (see location RCM in Figure 4-14), to support hydrodynamic and plume modelling for the Fitzroy and Lyall Bay outfall study by Bell (1989) for

16 averaging spring and neap ranges for Wellington- NZ Nautical Almanac 2015/16 and subtracting 3 cm for Lyall Bay

56 Wellington International Airport Runway Extension: Coastal Processes

Wellington City Council and the then Hutt Valley Drainage Board. The measured currents and tide levels from this deployment were used to calibrate the hydrodynamic model for the present AEE (Pritchard et al. 2015). The median current speeds out beyond Hue te Taka Peninsula at the RCM site were reasonably modest at 0.075 m/s and 0.09 m/s respectively at the bottom and top mooring, reaching a maximum during wind events of 0.25-0.3 m/s. The main ebb-tide flow was NW towards Te Raekaihau at the upper mooring but more westerly nearer the seabed, which also aligns with the residual or deployment-average flow directions arising from the slightly stronger ebb-tide flows than the easterly flood-tide currents heading for the Harbour Entrance.

An earlier report on the oceanography of the area was undertaken by Beca Carter – Caldwell Connell (1980), based on drogues and dye-tracing studies to determine water circulation patterns in outer Lyall Bay and from current-meters moored off Lavender Bay and around West Ledge for a longer Lavender Bay outfall option. An even earlier report in 1971 on the feasibility of extending the Airport runway by the Ministry of Works & Development Central Laboratories, showed through drogue tracking that current velocities in inner Lyall Bay were weak (0.02–0.09 m/s), and mainly generated by surface winds, with the observation that tidal currents only seem to occur outside the Bay (Rogers, 1971).

A schematic of the tidal-current circulation in Lyall and Lavender Bays was developed in WCC (1990) from the information from these two previous 1980 and 1989 oceanographic studies and is reproduced in Figure 4-15 for ebb-tide flows (top panel) and incoming flood-tide flows (bottom panel). Tidal currents weaken considerably once inside the confines of Lyall Bay marked as Zone 1 in Figure 4-15.

A recent 2013 project for Opus Consultants and Capacity infrastructure Services Ltd. on the Lyall Bay outfall discharge was based on modelling the hydrodynamics of the south Wellington coast including Lyall Bay using a depth-averaged Delft 3D model grid that also included Wellington Harbour. This hydrodynamic model was further refined as part of the present Project, with new bathymetry from Lyall Bay (Mackay & Mitchell, 2014) included in a higher-resolution grid and a SWAN wave model grid by Pritchard et al. (2015) as shown above in Figure 3-4. The generally low currents within inner Lyall Bay, where regular tidal currents are very weak, explained why plume dispersion from the Lyall Bay marine outfall is more influenced by lateral mixing and spreading rather than dominated by transport processes such as stretching and shearing from higher currents and spatial gradients in velocity.

Wellington International Airport Runway Extension: Coastal Processes 57

Figure 4-14: Alignment of the as-built Lyall Bay outfall (white line) and sites for the Baring Head NIWA wave buoy and a recording current meter (RCM) in Lyall Bay in September 1989. Image source: Google Earth.

AEE investigations Measurements from the ADCP current meter moored during the 2014 field programme at Site 1 near the spur groyne (Figure 3-3) indicated that tidal currents were statistically inseparable from back ground noise in the velocities measured. Hence, tidal currents were basically not able to be detected within the accuracy performance of the ADCP.

Further out near the outer entrance to Lyall Bay, where the tidal currents of Cook Strait are stronger (average of 0.1–0.3 m/s), the comparisons between the observed and predicted tidal currents by the hydrodynamic model indicated good agreement, indicating the hydrodynamic model is performing well.

A summary of the current circulation zones in Lyall Bay from an earlier study for the marine outfall is shown in Figure 4-15, which still largely applies with the new current-meter data now available from inner Lyall Bay near the runway.

58 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-15: Lyall Bay and Moa Point circulation zones during an ebb tide (top) and a flood tide (bottom). Solid back lines represent ocean outfall options for the 1990 wastewater treatment plant study. Source: From Fig. 5.3 of Wellington City Council (1990) after Bell (1989).

Current-meter observations from Site 1 (Figure 3-3) during the 2014 field programme show that substantial wind-driven currents can be generated within Lyall Bay, in comparison with the weak tidal currents, especially during strong winds (Pritchard et al. 2015). Currents at Site 1 were typically less than 0.1 m/s, but during the two southerly-gale events, currents reached 0.32–0.37 m/s. There was a considerable degree of scatter in current directions with time and with depth through the 10- m water depth, which are probably influenced by the proximity to Site 1 of the submerged rock outcrop off the spur groyne (Figure 3-3).

Wellington International Airport Runway Extension: Coastal Processes 59

Wind fields during the 2014 field deployment were more dominated by southerly winds, with one event reaching 80 km/hr. However, the longer term climatic records of the area (Section 4.4 above) show that strong northerly winds also occur with similar or higher frequencies.

To investigate wind-driven circulation inside Lyall Bay, a time series of idealised northerly- and southerly-directed winds peaking at 80 km/hr were modelled for both the existing and the proposed runway extension scenarios, superimposed on a repeating average lunar tide cycle to focus more on wind circulation. The circulation patterns of wind-driven currents for both strong southerly and northerly winds is discussed in Section 5.2.1. Of particular note is the strongest wind-driven currents are downwind along the shallower waters around the periphery of Lyall Bay with a slower, but broader return flow upwind in the deeper central waters of the Bay, although the southerly-wind case generates a more complex twin eddy-circulation system (Figure 5-2).

4.5.3 Waves and swell Lyall Bay and the southern terminus of the Airport runway is very exposed directly to the south and hence can be subjected to large waves generated by southerlies in the Greater Cook Strait region through to long-period swell from remote storms in the Southern Ocean (e.g., Figure 4-16). A recent example was the damaging swell in Lyall Bay on 15–16 June 2015 on a fine sunny day, generated by a deep low-pressure system well to the southeast of New Zealand. Complex wave patterns occur locally around the existing runway embankment and the associated spur groyne, with wave diffraction occurring around the latter (Figure 4-16) and wave refraction further bending the shoaling waves into The Corner area.

Figure 4-16: Wave refraction and diffraction patterns during a southerly-swell event on 29 April 2015. [Credit: DigitalGlobe (2015), Google Earth].

When swell is present, surfing is popular in Lyall Bay, particularly in The Corner (Figure 4-17) but also off the middle of Lyall Bay Beach, depending on wave conditions.

60 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-17: Surfing waves at The Corner viewed from the stormwater outlet adjacent to the eastern carpark (30 June 2015). [Credit: Dave Allen, NIWA].

Long-term wave climate for south Wellington Coast NIWA operates a Datawell Waverider buoy off Baring Head on behalf of the Harbourmaster at GWRC. The buoy is moored in 45 m water depth approximately 1.6 km offshore from Baring Head (Figure 4-14). A buoy has been operating in this location since 1999 apart from some brief periods of equipment failure, and has provided a 15-year record of wave conditions at the Baring Head.

17 Wave statistics: significant wave height (Hs), mean zero-crossing period (Tz), and the maximum individual wave height (Hmax), have been derived for 30 minute intervals from the 20-minute measurement bursts during the 15-year Baring Head record. While the Waverider at Baring Head is 10 km from the southern end of the runway, both locations have similar exposure to southerly swell, but the airport site is more protected from swell with any easterly component. The significant wave height (Hs) versus wave period (Tz) distribution (Figure 4-18) shows that the majority of waves measured off Baring Head are relatively low (0.1–1 m) short-period (3–7 secs) waves and that the more highly publicized wave events of over 5 m are relatively rare.

17 Hs or significant wave height is a derived representative value related to the variance of the wave height and is the average of the top 33% of waves measured each 20-min period. One can expect occasional individual waves of as much as 2Hs, perhaps every few hours.

Wellington International Airport Runway Extension: Coastal Processes 61

Figure 4-18: Significant wave height (Hs) vs mean zero-crossing wave period (Tz) distribution as a % of the entire 15-year Waverider buoy record off Baring Head. [Credit: Craig Stevens, NIWA]

Focusing on significant wave height (Figure 4-19), although distorted by the vertical log scale, it is clear that the majority of waves are in the range where significant wave heights are <2.0 m. Wave heights exceeding 3 m and 5 m only occur on average for a cumulative time periods of 14 days and 1 day per year (3.8% and 0.3% of the time) respectively as shown in bottom panel (Figure 4-19). Note that individual wave heights up to twice the significant wave height will occasionally appear.

Persistence of calmer wave conditions will be an important consideration for construction windows at critical phases of the runway-extension reclamation. An analysis of the wave dataset from off Baring Head shows there are on average 4–5 events with significant wave height remaining below 1.5 m (arbitrary threshold) over a 5-day window or longer every quarter. However, there is an identifiable reduction in number of occasions when waves are below a wave-height threshold in the winter and early spring quarter (July–September).

62 Wellington International Airport Runway Extension: Coastal Processes

Figure 4-19: Hs distributions (top) full distribution (in total days per year, i.e., number of 30 minute estimates in each 0.25 m Hs bins divided by 48, with a semi-log10 scale) and (bottom) partial cumulative distribution (proportion) showing that e.g., 99% of the time waves have a Hs<4.0 m. [Credit: Craig Stevens, NIWA].

Lyall Bay wave climate Some shorter records provide further insights on the wave climate within Lyall Bay in comparison.

The then Ministry of Works & Development Central Laboratories deployed two Waverider buoys for ~20 months from October 1978 through to mid-1980 in outer Lyall Bay (near the site marked RCM in Figure 4-14) and further to the east near the end of West Ledge. Some of the data is reported in Beca Carter–Caldwell Connell (1980; Fig 7-6) – and a summary of the deployment in a Central Laboratories report (Valentine, 1980). Significant wave height (Hs) in outer Lyall Bay was above 1.48 m for 50% of the time (median) and exceeding just under 3.0 m for 5% of the time and exceeding 4.1 m for 1% of the time (Valentine, 1980). Comparing these more limited outer Lyall Bay measurements with the frequency-wave height from the long-term Baring Head record (Figure 4-19; e.g., 0.95 is just under 3 m and 0.99 is 4.1 m) shows that wave heights in outer Lyall bay are similar in general to waves at the Baring Head buoy site. Zero-crossing wave periods also show similar distributions with significant wave height at both sites.

For extreme wave heights, significant wave height reached was 7 m in outer Lyall Bay (on 3–4 January 1980 during the deployment in 1978–80), and over 9 m in the much longer Baring Head for the 20 June 2013 event, with a maximum wave height of 15 m for the same storm (Pritchard et al. 2015). Such high waves would break before reaching the end of the existing runway, with physical model studies showing the highest waves would reach 5–5.5 m based on a wave period of 11.5 seconds (Webby, 1984).

Wellington International Airport Runway Extension: Coastal Processes 63

A recent ~7-week deployment of an ADCP to the SW of the spur groyne (breakwater) and a wave- pressure gauge further out in the Bay (Figure 3-3) for the present AEE investigations from August to

Oct 2014 (Pritchard et al. 2015). At Site 1 (Figure 3-3) off the runway, significant wave heights (Hs) were above 2 m on seven occasions due to the preponderance of southerly-wind events, with a maximum significant wave height of 4.7 m and mean wave period of 11 seconds (i.e., swell) recorded on 22-Sept-2014 during a southerly gale reaching 80 km/hr wind speeds (Pritchard et al. 2015). During northerly winds, the significant wave height generally dropped to <0.5 m, but sometimes a ground swell from the south overrides the local wind sea e.g., significant wave height reached 2.5 m in Lyall Bay on 25-Sept-2014 during a northerly wind of 20-35 km/hr. Wave direction at Site 1 were almost completely dominated (90%) by wave from due South with a smaller proportion of measured waves from the SSE direction. Assessment and subsequent wave modelling (e.g., Figure 4-20) and aerial images indicate that there is around an 8° bias towards the east in the measured wave- direction, as waves at Site 1 tend to arrive in a very narrow directional window from due south or slightly west of south (Pritchard et al. 2015).

Further out in Lyall Bay at Site 2 (Figure 3-3), the same southerly gale on 22-Sept-2-14 produced higher significant wave heights peaking at 6.1 m at a similar wave period, and compared more closely with concurrent measurements at the Baring Head wave buoy of over 6 m (6.5 m peak). By inference comparing the peak waves during the southerly gale at Sites 1 and 2, the wave heights attenuate somewhat as they propagate and shoal within the inner part of Lyall Bay.

As part of the AEE investigations, wave modelling using SWAN was undertaken for a 55-day period covering the 2014 instrument deployment (Section 3.4.1). Detailed results of the spatial distribution of wave heights and period are provided in Section 4 of the Coastal Processes Technical Report (Pritchard et al. 2015), but a summary of the main wave patterns in Lyall Bay are repeated here.

Wave statistics output from the SWAN model were averaged over the 55-day simulation period in

2014. While a simple time average was taken for significant wave height to produce a mean Hs over this period, a mean value of mean wave direction was obtained by vector averaging, weighted by wave height. The spatial pattern of average significant wave height and direction are shown in Figure 4-20, which shows a similar pattern of spatial variability in wave height to what was seen in the two strong southerly events during the field deployment. Attenuation of wave heights is more pronounced on the western side of the bay where depths are shallower, with a smaller degree of attenuation through the deeper waters to the area off the existing runway, as discussed above. The SWAN model results also show localised amplification of average wave height off the spur groyne and submerged rock outcrop, as frequently observed. The waves in the cove on the eastern side of the runway are substantially dampened and refracted as they approach the curved shoreline.

Normally to establish a long-term wave climate, several years data is required. For Lyall Bay, the 55- day wave model simulation undertaken is shorter than would be required to produce long-term wave climate statistics for Lyall Bay, but the nearby Baring Head gauge provides the longer-term wave climate for the general area. Also the 55-day field deployment in 2014 produced a wide range of wave conditions including an infrequent southerly-gale event. Therefore summary wave statistics from the 55-day simulation can be primarily used to quantify the spatial variability of waves within Lyall Bay, with the longer-term temporal variability related to the Baring Head wave buoy statistics outlined above.

64 Wellington International Airport Runway Extension: Coastal Processes

Significant wave height and mean wave direction are shown in Figure 4-21 from three representative sites in Lyall Bay (shown in Figure 4-20), out of the 18 sites presented in the Coastal Processes Technical Report (Pritchard et al. 2015). These results show a shift in significant wave height distribution at inner Bay sites (6–mid inner Bay, 15- off runway) by about a 0.25 m histogram bin relative to the location in the outer Bay (site 4), due to attenuation of wave in the confines of the Bay. Mean wave directions vary spatially as a result of wave refraction processes e.g., wavetrains have a more SSW wave approach off the runway than from due south further west in Lyall Bay off the spur groyne (also in Figure 4-16).

Wave refraction and diffraction patterns for the existing situation in Lyall Bay are presented in Section 5.2.2.

6

15

4

Figure 4-20: Wave conditions averaged over the 55-day SWAN simulation, in Lyall Bay. Mean significant wave height is shown by the colour scale, and contours at 0.1 m intervals. Black arrows show mean wave direction. Sites marked in grey relate to the next Figure. [Credit: Richard Gorman, NIWA].

Wellington International Airport Runway Extension: Coastal Processes 65

Figure 4-21: Distribution of Hs from 55-day simulation (top panel) and wave roses (joint occurrence distributions for mean wave direction and height) (bottom panel) at sites 4, 6, 15. Sites marked in Figure 4-20 – Site 4 is same site where the outer wave instrument was deployed. [Credit: Richard Gorman, NIWA].

4.5.4 Coastal storm inundation and sea-level rise Storm tides occur when the predicted tide level is raised by low barometric pressure and onshore winds that occur during storms. Based on an analysis of historic storm-tide events in the long-term tide-gauge record from Queens Wharf (Wellington Harbour), and modelling of some of the highest events, Lane et al. (2012) determined joint combinations of storm-tide and wave setup at the shore. Wave runup, which varies much more locally, depending on the beach slope and type of the natural or artificial back shore e.g., dunes, sea walls, rock revetments) was not considered for the effects on inundating the runway generally in terms of both present-day and with climate change, but wave runup and overtopping has been factored into the design of the rock-dyke and sizing the armouring (AECOM, 2015b).

The present-day joint 1% annual exceedance probability (AEP) coastal inundation levels, which are equivalent to an average recurrence interval of 100 years, were obtained from Lane et al. (2012). These combined storm-tide and wave setup levels are estimated to be 1.47 m (WVD-53) for Evans Bay and higher at 1.71 m (WVD-53) for the more-exposed Lyall Bay, mainly due to the higher wave setup for the latter. To convert these 1% AEP events into the same probability of occurrence for the future, an appropriate sea-level rise (SLR) is added on (including local landmass effects), and any additional component for changes in waves and storm surges arising from climate change.

The implications of the effects of storm-tide and wave set-up inundation and climate change are considered in Section 5.2.5.

66 Wellington International Airport Runway Extension: Coastal Processes

5 Effects Assessment: Operation of Project

5.1 Description of operational effects It is proposed that the reclamation works for the runway extension will be undertaken in several steps, as detailed in the Construction Methodology Report (AECOM, 2015b).

The permanent aspects of the works relevant to coastal physical processes:

. Stone blanket (or filter) of approximately 5 m width on the seafloor around the outer periphery of the wave–protection structure to protect the structure from toe scour and provide a transition between the large rock and accropode elements and the natural mobile sediments of the adjacent seabed. The outer perimeter of the permanent works is approximately shown in Figure 2-1.

. Toe structure approximately 15 m wide and 1–2 m thick between the sloping revetment and stone blanket comprising larger rocks in the outer section transitioning to smaller accropodes at the base of the main revetment.

. A 1:1.5 sloping revetment (or dyke) and topped off with an approximately 10 m berm comprising large accropodes. This structure will act as the main protection against large breaking waves and absorb much of the incident wave energy.

. The lineal distance offshore of the proposed reclamation is approximately 350 m south into Lyall Bay from the present wave-protection revetment, with an additional ~35– 45 m length underwater for the revetment batter and toe rock apron in approximately 13 m water depth (MSL).

. The total proposed permanent reclamation area of the present CMA is 10.8 ha.

. The orientation of the runway-extension reclamation will be slight west of south at approximately 183S, compared to the predominant wave-approach direction from due South.

. The southern end of the current runway is 8 m above the Wellington Vertical Datum- 1953 (WVD-53), or 7.8 m above present mean sea level (MSL), which is currently at 0.2 m above WVD-53 (Section 4.5.1). The southward runway extension will continue an upward incline to reach a height of around 9 m WVD-53 at the southern end.

The 3-D profile of these structural elements surrounding the proposed reclamation have been incorporated into the fine-resolution grid used for wave modelling (right-hand panel; Figure 3-5) and to a lesser extent resolved by the schematized grids used for assessing changes to flows and sediment-transport patterns.

The estimated water volume of Lyall Bay displaced by the proposed extended reclamation is around 995,000 m3, based on a comparison of the volume for the two ARTEMIS model grids (Figure 3-6) before and after the extension relative to WVD-53, which is around 3.5% of the still-water volume of Lyall Bay out to the end of the rock outcrop off Te Raekaihu Point.

Wellington International Airport Runway Extension: Coastal Processes 67

Following completion of construction of the Project, these elements of the overall structure, particularly the footprint and 3-D profile shape of the outer “surfaces” acting as an obstacle to flows and waves, will lead to permanent (operational) effects of varying degrees on coastal physical processes. Any ongoing effects of the present runway and wave-protection revetment are not considered, as these elements form part of the existing environment.

The permanent effects of the Project on coastal physical processes considered were changes in (relative to the existing situation):

. Tidal current-flow patterns.

. Flow circulation by wind-generated currents.

. Spatial wave and swell patterns.

. Sediment transport (short-term and seasonal) and associated morphological response.

. Long-term morphological change for Lyall Bay Beach.

As the potential effects on inshore wave patterns were highlighted early in the AEE investigations, surfing and recreational-related effects are given greater scrutiny when assessing the degree of coastal environmental impacts. Subsequently, a more detailed, complementary assessment has been provided by DHI (2016). This AEE Technical Report only describes likely changes in general wave patterns, but the effects of these changes on water-contact recreation activities such as surfing and swimming, along with possible mitigation options are covered by DHI (2016).

This Report also does not cover assessments of the effects of the Project on coastal-hazard extremes (e.g., waves and storm-tide) or climate-change effects, such as sea-level rise, which a covered in the Concept Feasibility and Design Report (AECOM, 2015a) and the AEE.

5.2 Assessment of operational effects The follow sub-sections outline the key findings from the Technical Report on Coastal Hydrodynamics and Sediment Processes in Lyall Bay (Prichard et al. 2016) and determine the degree of effects in each case.

5.2.1 Tidal and wind-driven currents The 350-m extension of the proposed reclamation will jut out further into Lyall Bay than the present runway which blends in more with the coast along the cove to the east adjacent to Moa Point Road (Figure 4-2). Consequently, the extended reclamation will partially block current flows locally potentially affecting local circulation.

The hydrodynamic modelling of the before and after runway extension simulations highlighted the following predicted changes to current flows.

Tides only Within inner Lyall Bay, the predicted tidal currents by the model are very weak both for the existing situation, mirroring the measurements at Site 1, and for the proposed extension. Consequently, the proposed runway extension will have negligible effect on tidal currents (excluding the wind component). In any case intermittent wind episodes dominate the generation of current flows in Lyall Bay. Note: the effects of a new reclamation would be much more significant if tidal currents

68 Wellington International Airport Runway Extension: Coastal Processes

were much stronger, generating flow barriers to day-to-day flushing flows, accelerating speeds around the end of the reclamation and generation of regular eddies in the lee of the structure not previously present.

Residual circulation (winds and tides) Residual or net current circulation is the current pattern remaining after the to and fro tidal and wind-generated movements have been averaged out, and is indicative of flushing and general circulation within an embayment such as Lyall Bay. The residual is generated by vector-averaging the predicted velocities spatially across the model domain (taking into account both speed and direction) over a given period – in this case the ~7-week field deployment period from August to October in 2014 (Section 3.3). This period included a strong southerly gale of 80 km/hr on 22 September, but less stronger northerlies.

The modelled magnitude and direction of residual circulation within Lyall Bay both before and after the proposed runway construction is presented in Figure 5-1.

The circulation in the outer part of Lyall Bay beyond the end of Hue te Taka Peninsula and Te Raekaihu Point is governed by the stronger ebb-tidal currents that start to enter the Bay and exit back out into Cook Strait on the west side (Figure 5-1). The residual (net) current analysis indicates the existence of two weak residual eddy-like circulation patterns within Lyall Bay with small net- current amplitudes of order <0.06 m/s.

In the mid-section of the Bay (up to the end of the present runway), a residual anti-clockwise circulation occurred for the sequence of winds (mainly southerly) and tides during the field deployment period. This residual current pathway heads towards Moa Point and the eastern shoreline before turning west at the end of the present runway embankment (Panel A; Figure 5-1). The introduction of the proposed runway extension (panel B; Figure 5-1) would partially interrupt and re-direct this westerly net flow inshore beside the present runway, given the same set of tidal and wind conditions, and would form two more quiescent zones (negligible net currents) either side of the proposed runway extension. This would provide less general flushing of the cove east of the proposed runway extension, but given it is still open and exposed to waves, this effect on coastal physical processes would be minor.

Figure 5-1: Residual (net) current circulation in Lyall Bay over the ~7-week field deployment period. Existing environment (A) and proposed runway extension included (B).

Wellington International Airport Runway Extension: Coastal Processes 69

Further inshore, off Lyall Bay Beach, a weak residual clockwise circulation occurs, generating a weak north-eastwards net transport in the nearshore towards The Corner and southwards along the existing runway embankment for the field deployment period (Panel A; Figure 5-1). Along with wave diffraction around the spur groyne and rock outcrop (covered later), this albeit weak net drift towards the NE corner of the Bay could partially explain the overall seabed accretion that has occurred in the eastern side of the Bay adjacent to the runway embankment (Figure 2; Pickrill, 1979). The proposed runway extension would have a negligible effect on this inner Bay circulation cell (panel B; Figure 5-1).

The inter-comparison of results from these net (residual) circulation simulations indicate that overall the proposed runway extension is likely to have only a minor localised effect on the residual circulation within the middle section of Lyall Bay and a negligible effect on residual circulation in the nearshore area off Lyall Bay Beach.

Strong-wind current circulation The current-meter observations show that substantial wind-driven currents can be generated within Lyall Bay, relative to tidal currents, especially during strong winds.

Wind fields during the 2014 field deployment were more dominated by southerly winds, with one event reaching 80 km/hr. However, analysis of the longer term climatic records of the area show that strong northerly winds also occur for similar frequencies (Section 2; Pritchard et al. 2015).

To investigate wind-driven circulation inside Lyall Bay, a time series of idealised northerly- and southerly-directed winds peaking at 80 km/hr were modelled for both the existing and the proposed runway extension scenarios, superimposed on a repeating average lunar tide cycle to focus more on wind circulation. The results of one snap-shot at mid-tide of the simulations for both before and after the runway extension is built is shown in for strong southerly winds and for similar-strength northerly winds in Figure 5-2.

In response to strong southerly-wind conditions, the most intense wind-driven currents flow north (down-wind) in the shallow water along the periphery of both the western and eastern shores of Lyall Bay – more so on the former (top-left panel; Figure 5-2). At the head of the Bay, the currents then turn clockwise and flow eastwards along nearshore zone, then south along the existing-runway embankment before being deflected WSW by the spur groyne and rock outcrop into the centre of the Bay. This clockwise eddy then converges with counter-clockwise eddy that extends from the eastern shore of the outer Bay at Moa Point and westwards across the end of the existing runway out into the central Bay. This mid-bay eddy would be partially intercepted by the proposed runway extension (top-right panel; Figure 5-2) that forces the down-wind drift into the cove at Moa Point beach to turn into the wind around the runway extension, slightly weakening the flow into the central Bay.

Based on a visual comparison of the modelled southerly-wind scenario for before and after the runway is extended (top panels; Figure 5-2), there is likely to be only relatively localised effects on the wind-generated current flows during southerlies caused by the extended runway embankment from local re-steering of the downwind flow along the eastern Bay and slightly faster velocities across the end of the extended runway than presently occur at the same location 350–400 m offshore from the existing runway. A stone blanket on the seabed around the periphery of the proposed revetment will protect any mobilising of seabed sediments due to this slight higher wind-generated current across the southern end in proximity to the structure.

70 Wellington International Airport Runway Extension: Coastal Processes

Figure 5-2: Current circulation at mid-flood tide (HW-3 hrs) for strong southerly winds (top) and northerly winds (bottom) for the existing situation (left panels) and the proposed runway extension (right panels).

In response to strong northerly-wind conditions, the most intense wind-driven currents flow south (down-wind) in the shallow water along the periphery of both the western and eastern shores of Lyall Bay – more so on the unimpeded western side (bottom-left panel; Figure 5-2).

The down-wind outflows along the eastern and western sides of the Bay cause waters to be drawn in from Cook Strait through the central Bay to maintain water volume equilibrium. This is seen as a tongue of inflowing water directed up-wind to the north. At the head of Lyall Bay, the inflowing tongue of water then bifurcates and forms a dipole circulation. The flow on the west side of the Bay turns counter-clockwise and merges with the wind-driven southerly outflow, and the flow on the eastern side forms a clockwise eddy that eventually converges with the northerly inflow in the central Bay. The eastwards flow into The Corner area (see location in Figure 4-2), is in the same direction as for southerly winds, although considerably weaker, thus both northerly and southerly winds contribute to the easterly residual current in the NE corner of the Bay. This pattern fits with

Wellington International Airport Runway Extension: Coastal Processes 71

the seabed accretion that has occurred in this area since the runway was extended into Lyall Bay in the late 1950s (Figure 2; Pickrill, 1979).

Summary of effects on wind-driven current patterns The Delft2D model simulations indicate that there would be little change in Lyall Bay wind-driven circulation during strong northerly winds, other than minor localised changes around the runway embankment area, comparing both bottom panels in Figure 5 2. Circulation during northerly winds would remain largely unaffected in the Bay by the proposed extended runway.

During strong southerly winds, the before-and-after simulations indicate that the proposed runway extension would slightly weaken the counter-clockwise eddy-driven outflow through the central area of Lyall Bay that passes off the existing-runway terminus. The wind-generated current from a strong southerly wind, presently curves westwards around the eastern cove (Moa Point beach) and passes across the end of the existing runway. With the extended runway, this westward flow would be intercepted and steered to double back into the southerly wind in order to pass around the terminus of the longer extension. This longer flowpath around the end of the extended runway, during southerlies, would slightly weaken and narrow the wind-generated flow into the central Bay (compared to the existing situation), along with a more quiescent shadow zone on the western side of the extended runway compared to present (top panels; Figure 5-2). But this change would only have a minor effect on the southerly-wind-driven hydrodynamics of the central Bay and is unlikely to affect flushing characteristics or morphological change of the inner Bay. The local re-steering of the downwind flow off Moa Point beach would also produce slightly faster velocities across the end of the extended runway, than presently occurs at the same location 350–400 m off the present runway terminus, as would be expected by the extension further offshore. However, the current speeds across the end of the extended runway terminus would still be less than across the existing runway terminus, due to the deeper water 350-400 m further offshore.

5.2.2 Waves and swell: spatial changes Chapter 5 of the Technical Report on Coastal Hydrodynamics and Sediment Processes in Lyall Bay (Prichard et al. 2016) describes in detail the various effects of the proposed Wellington International Airport runway extension on Lyall Bay wave patterns from the modelled results, focusing on the changes relative to the present shoreline and runway configuration. The following sub-section outlined the main results for assessing effects on waves from the proposed runway extension.

Changes at locations within inner Lyall Bay Results from the ARTEMIS wave model were initially extracted from 8 point locations (“P”) throughout the Bay as shown in Figure 5-3.

These results provide predictions of changes in wave height of regular waves at a limited set of locations, but across the full range of wave heights and wave periods applied to the outer Cook Strait boundary of the model, to compare before-and-after construction wave scenarios. The majority of locations were focused on the eastern side of the Bay, including near The Corner (“P1”). However, as will be shown below, extracting results from specific points only provides part of the picture – which complements the analysis of the full spatial changes in wave heights for a sub-set of four particular wave conditions.

72 Wellington International Airport Runway Extension: Coastal Processes

Figure 5-3: Lyall Bay bathymetry plots showing the point extraction sites (P1–P8) for assessment of changes in wave heights. Lines show the extraction transects (T1–T8) discussed in Coastal Hydrodynamics and Sediment Processes technical report.

Table 5-1 and Table 5-2 shows the absolute change in significant wave height locally for only site P1 (near The Corner) and P4 in the centre of inner Lyall Bay (Tables for other sites are in Pritchard et al. 2015). The Tables are ordered as a matrix (rows and columns) of wave period and wave height applied at the outer-Bay boundary (not the local wave height), with the contents of the matrix listing the change in wave height at the specific location (P1 or P4) after the runway extension is in place, relative to the existing situation. The changes have been colour-coded according to the scale on the right-hand side. A negative value for change implies a reduction in wave height by that amount listed in the Table.

At site P1 (near The Corner) the proposed airport runway extension would result in the greatest change on wave heights (Table 5-1) for incident significant wave heights of 3–5 m and periods of 5– 13 seconds (i.e., mainly steep wind-sea or medium-range swell), with reductions of over 0.4 m in wave height for these combinations (dark green shading). The relative change at P1 for these combinations would be around 22 to 40% reduction of the wave height relative to the existing situation at P1, with the highest reductions for steep, short-period, local wind sea of 5–7 seconds. For surfing wave conditions of 1–3 m wave heights at longer 10–15 second periods, the reduction is lower, with a 16–17% reduction at P1 for incident waves of 1–1.5 m for those longer wave periods.

At site P4, in the middle of inner Lyall Bay (Figure 5-3), there is no appreciable change predicted in wave heights as a result of the proposed airport runway extension, across the range of incident wave heights and periods applied at the offshore boundary of the model (Table 5-2).

Overall, the main changes in wave height of regular waves predicted for the eight selected locations (Figure 5-3) occur for sites near or “downstream” of the proposed runway extension, with smaller changes in wave heights across the wider inner Bay for wind-waves but less so for swell.

Wellington International Airport Runway Extension: Coastal Processes 73

Table 5-1: Change in predicted wave heights locally at site P1 near The Corner as a result of the runway extension. Axes list the wave period and wave height applied at the Cook Strait boundary of the model, and not the local wave conditions at P1. Negative change is a reduction in wave height.

Table 5-2: Change in predicted wave heights locally at site P4 in the centre of inner Lyall Bay as a result of the runway extension. Axes list the wave period and wave height applied at the Cook Strait boundary of the model, and not the local wave conditions at P4. Negative change is a reduction in wave height.

74 Wellington International Airport Runway Extension: Coastal Processes

Changes in wave heights spatially within Lyall Bay This sub-section focuses on the overall spatial pattern of changes in wave heights for regular waves and refraction and diffraction of wave crests. A limited set of results is presented for the more common sets of waves, covering incident wave heights of 1.5 and 3 m and periods of 8 and 12 seconds, spanning wind-waves (period 8 seconds) and the combination of 1.5 m and 12-second swell and a higher 3-m swell.

Spatial plots of the wave heights (Figure 5-4 to Figure 5-7) are presented side-by-side to compare the ARTEMIS model results for the existing environment with those for the proposed runway extension in place (top panels), followed underneath by a spatial map of the differences in wave heights that are predicted to arise from the extended runway, relative to present.

Figure 5-4 shows the spatial variability in wave height comparing present versus runway extension and the predicted change in wave height for a 1.5 m boundary incident wave height with a period of 8 seconds. The main features of the spatial wave pattern for this combination for the existing environment (left-hand top panel) are:

. the zone of highest waves inshore occur in the central part of the Bay and the beach (between transect lines marked T1 and T2), with little attenuation in wave height from the southern entrance to the Bay

. high attenuation in wave height through refraction in the western part of inner Lyall Bay, where depths shoal quickly (blue and yellow areas of Figure 5-4)

. in the eastern sector north of location P2, including The Corner, the wave field spreads out behind the spur groyne and rock outcrop due to both diffraction and refraction, with attenuation of wave heights e.g., 0.7 to 1.2 m for an incident offshore wave of 1.5 m

. substantial attenuation of wave heights within the cove adjacent to Moa Point Road to the east of the existing runway (e.g., location P5) in the wave shadow of Hue te Taka reducing 1.5 m incident wave heights to around 0.1 to 0.5 m.

These same features are also evident in the spatial pattern of the mean significant wave height simulated by the SWAN model for the 2014 field-deployment period in Figure 4-20 described in Section 4.5.3 of this Report.

Turning to the spatial wave patterns predicted after the runway has been extended, the main changes shown for a 1.5 m incident wave height and 8-second period are (mainly bottom panel of Figure 5-4):

. the zone of highest waves in the central part of Lyall Bay and the beach (between transect lines T1 and T2) would largely be unaffected – with only small differences due to the runway extension, but with some patches of increases wave height for the given scenario (grey areas of Figure 5-4)

. the greatest change from the existing situation would be a reduction of 0.7–0.8 m in wave height along the western side of the proposed extension seaward of the existing spur breakwater (Figure 5-4). This reduction is due to: a) the frictional (drag) effects on the wavetrains as they pass alongside the accropodes protecting the new extension,

Wellington International Airport Runway Extension: Coastal Processes 75

and b) a partial shadow zone, behind the outer-most SW tip of the runway extension, created by the slightly-west of south alignment of the runway revetment

. in the eastern sector of the Bay north of the spur groyne (breakwater), the waves would be attenuated by a reduction of 0.2 to 0.6 m in a zone extending out ~250 m from the existing runway revetment. The greatest wave-height reduction would be 80– 100 m NW of the spur groyne, with a local increase (relative to present) in these 8- second wind waves immediately behind the spur groyne through diffraction. In The Corner, the wave-height reduction would be somewhat less at 0.2 to 0.4 m

. a localised increase in wave height around the spur groyne (breakwater) and submerged rock outcrop (grey areas of Figure 5-4)

. a patch of increased wave height (relative to present) near the beach towards the eastern end of Lyall Bay Beach (grey area of Figure 5-4 near The Corner). This arises due to the reduced wave height offshore from this part of the Beach, able to propagate (shoal) slightly further inshore than is the case for the existing situation, given the same wave height and period offshore

. reduction in wave heights within the cove immediately adjacent to the east side to the extended-runway revetment

. negligible changes in wave height in the rest Lyall Bay including the western side, outside the proximal areas near the runway (existing and proposed extension).

Figure 5-5 shows the spatial variability in wave height comparing present versus runway extension and the predicted change in wave height for a 1.5 m boundary incident swell with a longer period of 12 seconds. The differences in features of the spatial wave pattern for this swell combination for the existing environment (left-hand panel), compared with the 8-second wind waves are:

. the zone of highest waves in the central part of the Bay and the beach exhibits a wider spread than for the 8-second wavetrains (top-left panel; Figure 5-4)

. comparatively higher waves propagate into The Corner than for shorter-period wind waves, with a 60-65 m swath of higher waves propagating into The Corner parallel and some 50 m off the existing runway revetment, east of location P1 (Figure 5-5)

. swell is refracted and diffracted around the lee of the spur groyne and associated rock outcrop, leading to less of a shadow zone than for shorter-period wind waves.

The bottom panel of Figure 5-5 shows the predicted changes, after the runway is extended, would show a similar spatial pattern of changes in wave heights as for the 8-second waves (Figure 5-4). The main differences would be a somewhat smaller reduction in wave height as swell peels along the western side of the extended revetment and a wider shadow zone in eastern part of inner Lyall Bay with reductions of 0.2 to 0.6 m. The eastern most swath of propagating swell, adjacent to the existing runway revetment, is predicted to be slightly narrower and around 0.2–0.4 m lower in wave height than for the existing situation for a 1.5 m incident wave height. There would also be a wider area of lower wave heights centred on site P1 (Figure 5-5). The patches where wave heights are predicted to increase, would cover a smaller area around and in the lee of the spur groyne, but a slightly wider area in the central inner-Bay area and in the nearshore Beach area just off the breaker zone (grey areas in Figure 5-5).

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Figure 5-6 shows the spatial variability in wave height comparing present versus runway extension and the predicted change in wave height for a higher 3 m boundary incident swell with a period of 8 seconds. The spatial wave pattern for this higher wind-wave combination for the existing environment is similar to that predicted for the 1.5 m wave of the same wave period (Figure 5-4). The only obvious difference is the breaking zone is further offshore as expected for higher waves.

The predicted spatial changes in wave height arising from the runway extension are also similar to the smaller 8-second wave (compare lower panels of Figure 5-4 with Figure 5-6), but with a smaller increase in wave heights immediately behind the spur groyne for the higher waves.

Figure 5-7 shows the spatial variability in wave height comparing present versus runway extension and the predicted change in wave height for a 3 m boundary incident swell height with a period of 12 seconds. Again, the spatial pattern of changes in wave height is similar to that predicted for the 1.5 m wave of the same wave period (Figure 5-5), apart from breaking further offshore and less of an increase in wave height immediately behind the spur groyne.

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Figure 5-4: For a boundary incident wave height of 1.5 m with a wave period of 8 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. For changes, positive values (grey) represent an increase in wave height, negative values (green) are a decrease in wave height.

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Figure 5-5: For a boundary incident wave height of 1.5 m with a wave period of 12 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. For changes, positive values (grey) represent an increase in wave height, negative values (green) are a decrease in wave height.

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Figure 5-6: For a boundary incident wave height of 3 m with a wave period of 8 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. For changes, positive values (grey) represent an increase in wave height, negative values (green) are a decrease in wave height.

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Figure 5-7: For a boundary incident wave height of 3 m with a wave period of 12 seconds: (Top) Spatial variability of wave height for existing situation (left) and with the proposed runway extension (right), and (bottom) map of changes in wave height after the extension relative to present. For changes, positive values (grey) represent an increase in wave height, negative values (green) are a decrease in wave height.

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Overview of spatial variability of effects on wave heights

The high-resolution ARTEMIS wave model reproduces the spatial wave-crest patterns in Lyall Bay shown in vertical satellite images, as the incident wave-train is refracted and diffracted, particularly in shoaling waters of the inner Bay. Results from modelling the before and post-construction situations, indicate that the proposed runway extension has the potential to have a more than minor effect on some combinations of wave height and period in some parts of the Bay, mainly on the eastern side of Lyall Bay near the existing runway revetment and more localised along the western and eastern sides of the proposed extended runway.

In the north-eastern sector of inner Lyall Bay, in the lee of the spur groyne (breakwater), 1.5 m incident waves of 8-second period propagating into Lyall Bay from Cook Strait would be attenuated by a further 0.2 to 0.6 m in a zone extending out ~250 m from the existing runway revetment (bottom panel; Figure 5-4). The greatest wave-height reduction in this NE sector would be 80–100 m NW of the spur groyne, and conversely, a local increase in these 8-second wind waves immediately behind the spur groyne through diffraction (bottom panel; Figure 5-4). In The Corner, the wave- height reduction would be somewhat less at 0.2 to 0.4 m.

The wave model also predicts an increase in wave heights: i) locally around the spur groyne (breakwater) and adjacent submerged rock outcrop; ii) slight increases in a narrow central part of the inner Bay of the main Beach; and iii) in a patch close to the beach towards the eastern end of Lyall Bay Beach near The Corner. This latter area arises due to the reduced wave height offshore from this part of the Beach, with waves able to propagate (shoal) slightly further inshore than is the case for the existing situation, given the same offshore wave height and period.

Overall, for regular waves, the largest reduction in the wave height would occur adjacent to the west side of the proposed extended runway, as the wave peels along the revetment subject to dissipation by friction (drag) exerted by the accropode units and also the influence of a subtle wave-sheltering effect as the runway extension alignment of ~183S is slightly west of the predominant wave- approach direction from due south.

After the runway is extended, predicted changes for the same offshore wave height (1.5 m), but incident swell of 12-second period, would show a similar spatial pattern of wave height change to the shorter 8-second waves (comparing Figure 5-4 with Figure 5-5). The main differences would be a somewhat smaller reduction in wave height as the swell peels along the western side of the extended revetment and a wider shadow zone in eastern part of Lyall Bay with reductions of 0.3 to 0.6 m. The eastern most swath of propagating swell, parallel with and 50 m off the existing runway revetment north of the spur breakwater, is predicted to be slightly narrower and around 0.2–0.4 m lower in wave height than for the existing situation for a surfing combination of 1.5 m offshore swell height of 12 seconds. The increase in wave height adjacent to and in the lee of the spur breakwater would affect less area for the longer swells compared to an 8-second wave, given the same incident wave height offshore.

The height of wave-trains reaching the central part of Lyall Bay Beach are likely to be only slightly affected by the runway extension, otherwise elsewhere in the Bay, including the western side, the changes will be negligible.

The cove east of the runway (Moa Point beach) will also exhibit a reduction in wave height as a result of the proposed extension. This reduction in wave height is more appreciable inshore and in the NW corner immediately adjacent to the existing runway. There is also likely to be more resonant or wave-

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reflection behaviour within the cove after the extension has been constructed, arising from the more-enclosed basin.

The above results indicate the predicted scale of general changes in waves throughout the Bay, relative to the existing situation, based on a regular sinusoidal wave set. More detailed high- resolution wave modelling was undertaken by DHI, using irregular measured waves propagating through the Entrance, for occasions when surfing quality was good, to further investigate the effect of the runway extension on wave heights and characteristics (e.g., wave steepness, wave peakiness). These aspects are important considerations for assessing the effects on surfing quality and safety of recreational users, are described in the assessment report by DHI (2016). The DHI results also indicate that the ARTEMIS model overestimates the effect of reduction of wave heights behind the spur breakwater as a result of the runway extension (Dr S. Mortensen, DHI, pers. com.).

5.2.3 Sediment transport and associated morphological change

Short-to-seasonal timescales Sand transport in shallow coastal areas is a complex interaction between waves, which primarily re- suspend seabed sediments into the water column, which are then subsequently transported by currents until the sediment particles settle again. In nearshore waters, including the surf-zone, non- linear wave dynamics along can also transport re-suspended sediments, especially if waves propagate towards the nearshore zone at an oblique angle to the beach. The Delft3D-SED and Delft3D-MOR modules incorporate tidal and wind-driven hydrodynamics from the Delft2D simulation coupled with waves simulated by the SWAN spectral-wave model (at a less finer resolution than the ARTEMIS model grid) to combine these wave-current interactions with mobile sediments on the seabed.

To provide some context in the Lyall Bay situation, the fine sand largely present on the seabed of median diameter of 0.15 mm, would require a current of ~0.16 m/s acting alone to mobilise these sediments. Based on the currents measured at Site 1 (Figure 3-3), only the two highest southerly events over the ~7-week field programme would have been capable of mobilising sediments in the absence of waves. On the other hand, wave orbital velocities are more efficient at mobilising these fine sands, and increases dramatically as the wave height and wave period increases. Taking the example of a 1 m wave height, sands in Lyall Bay would be mobilised in water depths up to 24, 31 and 35 m for wave periods of 8, 10 and 12 seconds. Sediment transport from one location to another only occurs when mobilised sand, usually by waves, is carried along by a current – often in a series of short steps or hops, before it re-settles on the seabed when wave conditions abate.

In this section we present results in context of the relative change between the start and end of the ~7 week instrument deployment period in August to October 2014, which included a strong southerly gale of 80 km/hr that would have generated substantial sediment transport and morphological change through wave processes, besides other southerly events during the field- monitoring programme. Such an event would be at a recurrence interval of a season through to an annual time scale. Northerly winds are unlikely to generate any substantial sediment transport acting alone, leaving aside the higher currents of 0.4–0.5 m/s along the rocky west coast and outer east coast of the Bay (lower panels; Figure 5-2) and wave energy is much more subdued at short wave periods (e.g., 2-4 seconds) with a very short fetch. Consequently, under northerly-wind conditions, re-suspension of sand by wave orbital motions is limited, even though northerly winds can produce wind-generated currents in the Bay that could otherwise transport wave-suspended sands.

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The Delft3D-SED model was forced by tide, wind and SWAN wave boundary conditions (Chapter 4; Technical Report on Hydrodynamic and Sediment Processes; Pritchard et al. 2015) derived for the field deployment period in 2014 and applied equally to both the pre and post runway extension bathymetry in Lyall Bay to ascertain the difference in morphological response over the ~7-week period, including the southerly gale event.

Sediment-transport and associated morphological models require intensive field measurements of seabed changes and bed shear stress thresholds for erosion and deposition under a wide range of wave and current combinations, to calibrate the various model parameters to best match the time- varying sea-bed changes. The required fieldwork would have been a costly and extensive undertaking and in our opinion is unwarranted when comparing the before and after construction situation, where widely-used sediment transport formulae can suffice for determining relative change. We have therefore not undertaken an exact sediment-transport calibration, but have used an un- calibrated sand-transport model to approximate the relative morphological change both with and without the proposed runway extension.

The default values for sediment transport for the median sand size in Lyall Bay (fine sand of 0.15 mm median grain size) in the Delft3D-SED model were invoked, which are based on the widely-applied sediment-transport formulations of van Rijn (1984a; 1984b). So while the adoption of this sediment- transport formulae may not necessarily have reproduced the exact seabed erosion and accretion changes that would have occurred over the field-deployment period, applying the same formulae, parameters and all other model forcing (waves, winds, tides) to both existing and post runway extension situations, sets up a comparable pair of simulations where the relative changes between the simulations is directly related to the effects of the runway extension.

Sea-bed response comparing pre and post runway extension The results of running the coupled Delft3D-SED and Delft3D-MOR modules, including waves from the SWAN model embedded in Delft3D-SED, over the ~7-week simulation period are presented in Figure 5-8 for the existing situation and Figure 5-9 for the proposed runway extension bathymetry included (with all other aspects of the model simulation kept the same). The plots show the areas of net erosion, net deposition or negligible change over the ~7-week period, rather than numerical changes in sea-bed height. In most cases the seabed height of erosion of deposition is of the order of cm down to mm, but could be decimetres where cross-shore processes such as storm-cut and offshore bar formation occurs. Since the sediment-transport model uses generally-applicable sediment- transport parameters, the absolute magnitudes of sea-bed height changes predicted by each model run may not be accurate for the local conditions, but we focus instead on the relative change in deposition and erosion patterns with and without runway extension using the same default sediment-transport parameters in both before and after simulations.

Both simulations shown below predict that over the 2014 field-deployment period for both pre- and post-runway extension, an erosional storm cut occurs close inshore at Lyall Bay Beach with sand migrating offshore and depositing further offshore in the form of a sand bar.

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Figure 5-8: Predicted changes in seabed height for the existing situation over the ~7-week field period in 2014. Categorized as erosion (E), deposition (D) or negligible change (0).

Figure 5-9: Predicted changes in seabed height for the proposed runway situation over the ~7-week field period in 2014. Categorized as erosion (E), deposition (D) or negligible change (0).

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The erosion band in shallow water adjacent to, and including, the lower beach was mainly due to near-bed wave orbital velocity and current-wave interaction increasing bed shear stress, particularly during the strong southerlies, especially the southerly gale on 22–23 September 2014, when the significant wave height reached 4.7 m, with a mean period of 11 seconds off the spur groyne (Section 2; Technical Report on Hydrodynamic and Sediment Processes; Pritchard et al. 2015). Furthermore, the incident wave field for the ~7-week simulation was mainly from the south and propagated inshore into Lyall Bay, frequently having the energy to mobilise the seabed.

Wind-generated currents during southerlies mainly play a transporting role for wave re-suspended sediments, particularly along the eastern and western perimeters of Lyall Bay where higher currents are predicted as shown in the top panels of Figure 5-2. The predicted erosion areas along the western and outer eastern perimeters by the sediment-transport model (Figure 5-8 and Figure 5-9) can be explained by the combination of wave re-suspension and transport by both wind-generated currents and non-linear wave motions in the shallow near-shore waters. The simulated effect of forcing with a northerly wind alone, coupled with local wave generation through the SWAN model (not presented), on sand transport throughout Lyall Bay was negligible. This is largely due to the very limited fetch within inner Lyall Bay and then only limited to local wind wave generation (i.e., no residual southerly swell included), which can only mobilise seabed sediments in shallow waters.

A shore-parallel band of deposition further offshore from Lyall Bay Beach is indicative of the development of an offshore bar 150–250 m from the shoreline, which was built by sand eroding from the nearshore zone in the sediment model and migrating further offshore during the ~7-week period of mainly southerlies. This cut and fill behaviour of the beach profiles at Lyall Bay was also observed in a previous study by Pickrill (1979), especially in winter when southerly swells are more frequent, with a bar forming between 200–250 m offshore. Therefore there is some confidence in the sediment-transport model predictions that the overall behaviour of the seasonal storm cut and fill pattern off Lyall Bay Beach is being reasonably replicated for an active southerly-wind period with the attendant energetic wave dynamics.

The relative change in erosion or deposition patterns is shown in Figure 5-10 where the sea-bed heights predicted for the runway-extension were subtracted from the sea-bed heights for the existing situation. In Figure 5-10, erosion (E) means the sea bed would have eroded more than for the existing situation, and deposition (D) means the sea bed would have accreted more than for the existing situation, white areas (0) mark areas predicted to have negligible or undetectable change to seabed heights with the runway extension in place.

The predicted net change in sea-bed heights, following the construction of the runway extension, would be small (<1 cm at the seasonal scale and hence difficult to detect) over much of Lyall Bay (Figure 5-10), particularly within the inner Lyall Bay, the main beach, the eastern cove (Moa Point beach) and the areas along the western and outer-eastern perimeter of the Bay. The main changes in sea-bed heights are localised around the extended-runway (order of a few cm’s at the seasonal time scale), where the existing sea-bed has to adjust to the presence of the rock dyke and the associated wave interaction with the structure, which it does by eroding sand from adjacent to sections the dyke and depositing the sand further offshore, especially to the south, to form a new equilibrium sea-bed profile.

Under the seasonal time scales and environmental conditions that were simulated, the model predicts that there may be some localised deposition on the SW and SE corners of the dyke. This is not to say that over much longer periods, varying seasonal weather conditions and swell frequency

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that such sand deposits next to the runway-extension rock dyke will not re-mobilise and move e.g., a cut and offshore fill response or alongshore drift around the end of the revetment, given the high wave exposure of this area.

The model also predicts narrow bands of both increased erosion and deposition in the order of cm to the SW of the extended runway, which are likely to be due to changes in wind-generated current patterns during southerlies (e.g., right-hand top panel of Figure 5-2) transporting wave re-suspended sediment, rather than changes in waves (which are shown to be minimal in this area as shown by the high-resolution ARTEMIS modelling discussed above).

Figure 5-10: Net or relative change in the response of sea-bed heights with the proposed runway in place over the ~7-week simulation. Categorized as erosion (E), deposition (D) or negligible change (0) for the proposed runway extension compared to the existing situation.

Overall predicted seasonal morphological changes (summary) Introducing waves as well as tide and wind-driven forcing into a sediment-transport model, showed the predicted net change in sea-bed heights, following the construction of the runway extension, would be no more than minor (<1 cm) over seasonal timescales over much of Lyall Bay, including the nearshore area off Lyall Bay Beach and the eastern cove (Moa Point beach). The morphology of Lyall Bay beach will still be dominated by cut and fill along an on/offshore seabed profile governed by southerly storm events, with any effects of the runway extension on seasonal morphological timescales likely to be second-order influences.

Localised changes in sea-bed heights, within 50 m around the extended runway, are likely to be minor (order of a few cm’s at seasonal timescales), particularly after the existing sea-bed topography has adjusted to the presence of the rock dyke and the associated wave interaction with the structure. The local morphological adjustments off the end of the proposed runway dyke will occur through some erosion of sand seaward of the rock apron, adjacent to the rock/accropode dyke, and depositing the sand further offshore, especially to the south, to form a new equilibrium seabed

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profile. However, this is likely to be a “dynamic equilibrium” depending on varying seasonal weather conditions and swell frequency or persistence.

Under the seasonal time scales and environmental conditions that were simulated, the sediment- transport model predicts that there may be some localised deposition on the SW and SE corners of the dyke. This is not to say that over much longer periods, varying seasonal weather conditions and swell frequency that such sand deposits next to the runway-extension rock dyke will not re-mobilise and move, given the high wave exposure of this area. Note: the terminus of the existing runway revetment is positioned in an area of submerged rock outcrop (Figure 3-3) so the existing in-situ morphological analogue of the local seabed response built up over decades is not easily transferable to the local seabed south of the extended runway dyke, where a mobile sand cover will be present without surface rock outcrops. However, this will be a local effect and any resulting scour effect on the structure can be managed by transitioning to natural seabed with rock aprons in front of the rock dyke.

Long-term morphological effects The longer-term morphological response of Lyall Bay, to climate cycles such as El Niño-Southern Oscillation episodes and longer 20–30 year Pacific-wide climate cycles, is largely unknown.

For assessing any changes to longer-term beach-nearshore morphology, it is not feasible or meaningful to scale up the sediment–transport and morphology model results to longer timescales than seasonal without having the benefit of a multi-year beach profiling dataset for Lyall Bay (confirmed by Dr Iain Dawe, GWRC, pers. com.). However an indication of the likely long-term effects of the additional runway extension in deeper water can be based on both the existing analogue of the present runway embankment and associated spur groyne (breakwater) still having the dominant control on nearshore morphology on the eastern side of the inner Bay, along with the wave and morphological model results for seasonal timescales (which incorporates the changes in wave heights).

Based on the limited beach-profile dataset of Pickrill (1979), most of the seabed height variability occurred within about 600 m of the shoreline, which indicates that any morphological effects of the runway-extension Project are likely to be driven by changes in waves and currents northwards of the existing spur-groyne and associated submerged rock outcrop. Indeed, the wave modelling has demonstrated that this existing groyne on the rock outcrop, is acting as a critical hydraulic control on wave refraction and diffraction processes north of this structure, and to some extent on current flows as well.

The predicted wave patterns for the proposed runway configuration (Figure 5-4 to Figure 5-7) and further plots on the free surface elevations of wave crests and troughs in Chapter 5 of the Technical Report on Hydrodynamic and Sediment Processes (Pritchard et al. 2015), show that largely within the nearshore zone off Lyall Bay Beach, the overall spatial pattern of wave refraction and diffraction is similar to the existing situation, albeit with reduced wave heights preferentially on the eastern side. Given the wave refraction and diffraction patterns and wave crest orientations are likely to be similar to the existing situation, then long term, the gradual shoaling of the NE corner of the Bay arising from the original runway reclamation and spur groyne (which governs the current wave climate in The Corner), is likely to continue with only minor effects from the proposed runway further offshore. There may however be some subtle changes in the shoaling rate as it adjusts to the reduced wave heights on average after the runway extension is in place, but the more extreme episodic southerly storms will continue to modulate the shoal in the NE corner of Lyall Bay behind the spur breakwater.

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A fixed-term beach-profile monitoring programme could be implemented to provide more quantitative information on how the nearshore morphology responds to storm sequences over a period of a few years. However, because of the more dominant response of the nearshore-beach system to natural climate variability and southerly-storm sequences, it would be difficult to isolate the effect of the proposed runway extension (in deeper water beyond the spur groyne) from the ongoing morphological response associated with the historic runway reclamation and spur groyne (breakwater) and the seawalls that limit the back-beach response along Lyall Bay Beach during storm-cut cycles.

5.2.4 Water quality and clarity Leaving aside operational stormwater discharges, the permanent runway extension would have negligible effects on turbidity and hence water quality and clarity. Local changes in waves and swell during storms around the runway extension may slightly alter resuspension of finer seabed sediments, but usually turbidity increases in Lyall Bay are ubiquitous during strong wind and/or wave events, so any localised changes in turbidity post-construction would not be detectable.

5.2.5 Sea-level rise and climate change effects This section provides an appraisal of the implications on the runway project of sea-level rise (SLR) and climate-change effect on waves and storm surges in the next 100 years.

Policy 24 of the NZCPS states that the potential effects of coastal hazards and the additional effects of climate change should be assessed “taking into account national guidance and the best available information of the likely effects of climate change on the region or district”.

The current national guidance is the Guidance Manual for Local Government on Coastal Hazards and Climate Change, published by the Ministry for the Environment (MfE, 2008). The guidance is based around a risk-based framework, where the consequences for any project or plan change should be investigated for a range of sea-level rises, starting with 0.5 m (by 2095) and at least considering at least 0.8 m (2095). Given the high-value infrastructure involved with the Wellington Airport runway, that latter higher value should be the least SLR considered for assessing the effects of climate change. For this assessment, the effects of climate change are only considered for the runway and apron areas, but not the effects on local and state roads servicing the airport, which are lower lying (as outside the scope of the Project).

NZCPS in Policies 10 (Reclamation) and 24–25 (Coastal Hazards) stipulates planning timeframes of “at least 100 years” need to be considered, which effectively is out to at least 2115. Therefore, applying the timeframe extension in MfE (2008; p. 20) of a SLR allowance of 10 mm/yr (or 0.2 m in the two decades from 2095 to 2115), the equivalent value of SLR to at least consider is 1.0 m by 2115.

The latest Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report of Working Group I (Physical Sciences) was released in late 2013 (IPCC, 2013), with slightly higher sea-level rise projections previously, using different Representative Concentration Pathways (RCPs) depending on how carbon emission trajectories unfold. The plausible range of projections is for an increase of 0.5 to 1 m in global-mean sea level by 2100, with an additional caveat of several decimetres for accelerated ice-sheet contributions. The relativity of a 1 m SLR by 2115 with the latest IPCC projections for two RCPs (the lowest–RCP2.6 and the highest–RCP8.5), along with the other MfE guidance tie-points for 2095 are shown in Figure 5-11.

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NZ MfE Guidance tie-points 2115

2095 RCP 8.5 BAU emissions

RCP 2.6 Severe curbs on emissions

Figure 5-11: IPCC global mean sea-level projections from the 5th Assessment Report (AR5) for two greenhouse-gas emission pathways: RCP2.6 (severe curbs required) and RCP8.5 (business-as-usual and global population growth). Historic data for annual MSL sourced from Greater Wellington RC and Ports of Auckland. Light-yellow SLR tie-points for 2095 are from the present 2008 MfE guidance manual, with bright yellow dotes extended to 2115. Dashed lines are the likely range of projections for the relevant RCP (with a 33% chance it could be outside this range).

There are two further components of SLR that need to be considered for a particular location:

. The wider regional ocean difference in SLR projections compared to the global mean value. For the NZ area of the SW Pacific, an additional small contribution is likely of up to 0.05 m (Ackerley et al. 2013), which has already been factored into the MfE (2008) guidance manual values.

. Local rate of vertical landmass movement. Only over the past decade has vertical land movement been measured around New Zealand by continuous GPS (cGPS) instruments by GeoNet (operated by GNS Science and Land Information NZ). The cGPS station at Wellington Airport (WGTN) shows an average subsidence of 2 mm/yr over a 10.5–year record (Beavan & Litchfield, 2012), which is in line with the general pattern of subsidence in the lower North Island of 1–3 mm/yr (Beavan & Litchfield, 2012), due to slow-slip events on regional faults. Local subsidence means the relative SLR is higher locally than the rise of the general ocean level around New Zealand.

For regional subsidence, it is not clear from the short cGPS records whether the slow-slip would be continuous (until the next major earthquake uplift), but processing of the long-term annual MSL trends for Wellington (Bell & Hannah, 2012), showing a more rapid increase in the trend of relative SLR in the last two decades from the Queens Wharf record, would indicate that it may be episodic. For this assessment, a conservative approach is adopted by assuming the present subsidence continues over the next 100 years, which means an additional 0.2 m should be added to the 1 m SLR, leading to an allowance of 1.2 m SLR by 2115.

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Storm-tide and wave set-up elevations were derived from Lane et al. (2013). The joint 1% AEP water levels for Evans Bay and Lyall Bay are 1.47 m and 1.71 m respectively above WVD-53. This includes an offset for the present-day MSL relative to the zero of the WVD-53 datum. Applying a 1.2 m SLR by 2115, means at the Evans Bay end, the runway needs to be above ~2.7 m WVD-53 and ~2.9 m WVD- 53 at the Lyall Bay end, to avoid direct coastal-storm inundation around a high-tide period by a 1% AEP storm-tide and wave setup event in a 100 years from now.

Climate change may also affect storm surge heights (by changes in wind strength and barometric pressure) and wave heights (by changes in wind strength). Research by NIWA in a recent Waves and Storm-Surge Projections (WASP) project has shown that increases in storm surge height are unlikely to be significant (for the two climate-change scenarios A2 and B2 used), with results projected out to 2070–2099. For waves/swell, the projections for the 99-percentile significant wave height range from a change of –0.6% (B2 scenario) and +2.9% (higher-emission A2 scenario). Taking the higher-emission scenario results, an additional 0.15 m was added to the joint 1% AEP water level cover for wave/swell height increases. Adding up all components, to minimise the effects of coastal storm inundation elevated by sea-level rise and a modest increase in wave height in the next 100 years would require the runway to be at least 3.05 m above WVD-53 at the Lyall Bay section (and less off Evans Bay). This analysis doesn’t include the contribution of wave runup and overtopping water volume (indirect inundation), which can be reduced through the provision of sufficient drainage capacity.

A north-south longitudinal profile of the elevation (WVD-53) of the centreline of the present runway is shown in Figure 5-12, derived from a recent LiDAR18 digital elevation model, from an aerial laser- scanning survey undertaken by Greater Wellington RC in 2013. The lowest point of around 4.6 m WVD-53 is around 1300 m from the northern end of the runway embankment, due east of the carpark in the NE corner of Lyall Bay, before rising on the southern incline to around 8 m WVD-53 at the present Lyall Bay terminus. The minimum runway level at present, and the proposed extended runway continuing higher up the southern incline, will therefore be resilient to coastal-storm inundation and climate-change effects (excluding wave overtopping) out to 100 years from now assessed against the 3.05 m elevation shown by the heavy line in Figure 5-12.

N S

1% joint AEP water level + SLR

Figure 5-12: Longitudinal elevation profile along the centreline of the present Wellington Airport runway, from Evans Bay (left) to Lyall Bay (right). Elevations are relative to WVD-53 and sourced from GWRC LiDAR data. Blue line shows the estimated water level (3.1 m WVD-53) required to in most scenarios to avoid coastal storm inundation of the runway in next 100 years (based on a relative sea-level rise of 1.2 m by 2115).

18 Light Detection And Ranging – an accurate aerial laser-scanning survey technique to measuring surface elevations

Wellington International Airport Runway Extension: Coastal Processes 91

Parts of the wider air-side area, such as swales between the runway and western rock revetment near the dip in the runway, are somewhat lower at elevations of 3–4 m WVD-53, but most of the swales would be largely above the 3.05 m elevation for the 1% joint AEP and climate change contributions, mainly SLR.

5.3 Relevant assessments against statutory plans/policies or guidelines For the level of operational effects described above for coastal physical processes, a change in wave heights for some wave conditions is the only effect where there is potential in some parts of Lyall Bay for more than minor effects i.e., moderate effects on wave physical processes per se, with other “downstream” effects on recreational activities such as surfing and swimming, which are appraised in the assessment report by DHI (2016).

For completeness, the operational effects on coastal physical processes are assessed against the relevant elements of plans, policies and the RMA, as outlined in Section 3.5.

Regional Coastal Plan: Conservation Areas The only conservation area in the general vicinity of Lyall Bay area is at Tarakena Bay (Figure 3-7) to the west of Hue te Taka Peninsula and east of West Ledge Reef, which is an Area of Important Conservation Value (AICV) in the Regional Coastal Plan (RCP) for Wellington. The operational effects on coastal physical processes, including waves, will be undetectable at this Conservation site.

Similarly, operational effects on turbidity will be negligible on the waters throughout Lyall Bay managed for water-contact recreation and the nearshore waters around Te Raekaihau Point and Hue te taka Peninsula in outer Lyall Bay managed for shellfish gathering.

NZCPS The policies and objectives of the NZCPS (underlining added for emphasis) relevant to be considered in relation to coastal physical processes are (and the degree of effects):

. Objective 1: “… maintaining or enhancing natural biological and physical processes in the coastal environment and recognising their dynamic, complex and interdependent nature …”. [Overall the dynamic functioning of physical processes in Lyall Bay such as tides, currents, flow circulation, sediment transport and wave refraction patterns will be largely unaffected, but locally in the area around the runway extension in the NE area of the Bay, there will be changes to wave heights. Based on an assessment of historic effects from the existing runway reclamation, numerical modelling, expert opinion, and the effects of coastal physical processes around the extended-runway reclamation, there will be only negligible or minor changes apart from more than minor changes (i.e., moderate) to wave heights localised around the proposed runway extension and extending into the NE corner of the Bay.]

. Policy 3(1): “Adopt a precautionary approach towards proposed activities whose effects on the coastal environment are uncertain, unknown or little understood, but potentially adverse.” [Through numerical modelling of the before-and-after situations, assessing the existing analogue of the historic runway extension and expert opinion, the effects of coastal physical processes are well understood and known. DHI (2016) cover the assessment of effects of changes in wave heights, surfing characteristics and uncertainties on water-contact recreational users.]

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. Policy 10(2) on reclamation: For Policy 10(1), “Where a reclamation is considered to be a suitable use of the coastal marine area, in considering its form and design have particular regard to: …

c) the use of materials in the reclamation, including avoiding the use of contaminated materials that could significantly adversely affect water quality …. in the coastal marine area; [Not an issue for the operation of the extended runway – fine sediments (the main contaminant) and turbidity and the low levels of contaminants in the seabed sediments are covered in the next Chapter under construction effects]

e) the ability to remedy or mitigate adverse effects on the coastal environment; [DHI (2016) consider possible measures to mitigate or remedy the potential effects on water-contact recreational users, especially surfers]

g) the ability to avoid consequential erosion and accretion, and other natural hazards.” [There will only be minor changes on seabed erosion and accretion in proximity to the proposed runway extension, with the design provision of the rock aprons around the periphery the revetment structure, mitigating any potential effects from adjacent seabed scour and undermining the works. The NW corner of the Moa Point beach of the runway extension is more likely to accrete, than erode, as this area will become less exposed to waves and the wind-generated currents from strong southerlies will no longer flow west across this area as they do now. However, there is only a limited sand/gravel cover above the underlying rock structures in the eastern cove, which will limit the extent of any change in accretion and erosion. Changes in erosion and accretion processes in the nearshore area of Lyall Bay Beach are likely to be minor (and negligible in the western half of the Beach) after construction of the runway extension. These minor changes must also be appraised in the context of a highly-modified shoreline environment with possibly slow morphological adjustment ongoing from the existing runway and the landward limit imposed by the peripheral seawalls]

. Policy 24 on coastal hazards and climate change: Identify areas in the coastal environment potentially affected by coastal hazard risks over at least 100 years, including the cumulative effect of sea-level rise, storm surge and wave height under storm conditions. Take into account national guidance and the best available information on likely climate-change effects on the region. The analysis in Section 5.2.5 shows that coastal storm inundation (based on a 1% AEP or 100-year average recurrence interval event), a relative sea-level rise of 1.2 m (includes a 0.2 m regional subsidence contribution) and a 0.15 m contribution for increases in waves and storm- surge height by the end of this century, would not endanger the present runway and air-side areas adjacent to the runway, and even less so for the extended runway which will be profiled to continue the incline towards the southern end of the runway. Wave overtopping volume (when direct sea inundation does not occur) was not considered, but can be managed through sufficient drainage capacity and porosity of the rock-dyke berms.

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6 Effects assessment: Construction activities The three potential phases of construction where effects (other than occupation of the sea bed and reclamation) may arise are:

. Disturbances of the seabed in the CMA from activities that impinge on the seabed, including installing temporary marine platform piles, trenching, drilling and installation of stone columns or other similar in-ground strengthening works, placement of the peripheral rock apron on the seabed, and protection for the existing ocean-outfall pipe. These activities have the potential to sporadically suspend any fine material from the seabed leading to turbidity plumes.

. During initial construction of the rock dyke, some additional turbidity could be generated by waves winnowing fine sediments (silts, dust) from the various gravel and rock filter layers that need to be placed to support the final outer protective layer of accropodes.

. Discharges of decant water from the infill operations for the embankment within the rock dyke, likely to contain a fraction of fine sediment with either seawater or rainwater (or a mixture of both), resulting in a dispersing suspended-sediment plume in the bay.

This Chapter outlines the assessment of these two types of effects (discharges and disturbances) during the construction phase, based generally in accordance with the methods set out in the Construction Methodology Report (AECOM, 2015b). However, the prime focus is on assessing the de- watering discharges, as they are likely to persist for several months (depending on the infilling method used) and potentially can affect turbidity within the Bay, compared to the more sporadic and localised sea-bed disturbances.

6.1 Description of construction effects Turbid discharges from the infill operations Fine-grained sediments introduced into the water column, either artificially or naturally re- suspended from the seabed through high wave activity, are well known to cause temporarily- elevated turbidity and reduced water clarity, with slow settling rates compared with sand particles, followed by a more gradual return to clearer conditions.

De-watering discharges during the embankment fill phase of the runway extension project have the potential to introduce high-turbidity plumes into the receiving waters of Lyall Bay if a fine-sediment fraction is present in the fill material (whether initially in the dyke gravel/rock filter layers or during the longer infill operation for the raised embankment within the rock dyke). The main source of turbidity in the water column would be from any fine sediments (clays, muds, silts, rock-dust) present in the fill material, even if they are only a small percentage of material by volume.

For the more temporary phase when the gravel and rock filter layers are placed, any fines will be rapidly winnowed largely by waves (with currents being generally slow) from the surface of the gravel and rocks, or from the general fabric of the coarse material as placed in-situ. This can generate a turbid fringe around the perimeter of the worked sections of the dyke, but are likely to be localised, occur mainly during wave events when the natural turbidity levels are higher, and once winnowed from the filter material, would be unlikely to form an ongoing source of turbidity. The various phases of constructing the rock-dyke were not modelled, as the weir discharges for the longer-period

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embankment fill operation are more likely to be the limiting case for any turbidity effects in Lyall Bay, and in any case, the selected turbidity monitoring sites can be used to actively manage some dyke/filter sections during the filter-placement phase.

For the main infill operation to create the runway apron, outlet weir(s) or pump/pipe outlets are formed in the rock dyke for the removal of decant water from the reclamation as the filling progresses. Discharge locations will most likely be on the west side of the rock dyke at the northwest (NW) or southwest (SW) corners, with a further scenario in the centre of the southern terminus rock dyke, although the locations of discharges can be varied to match the contractors approach to performing the reclamation (AECOM, 2015b). Possible locations for a weir (or decant discharge outlet) in the dyke walls on the western side of the airport runway extension are shown in Figure 6-1 labelled D1 to D3. Also shown are options for undertaking the fill operation, including truck dumps (e.g., a NW or S discharge), barge entry (probably a NW discharge only) or piped dredged-sediments as a slurry from an anchored barge (any of the discharge locations).

As with most reclamation construction projects in the coastal marine area, the type of fill material (especially the proportion of fine silts and clays) and the exact construction methodology and erosion and sediment control measures to be deployed are not prescribed in detail – but rather the selected contractor would design the construction plan, fill flow and sequencing generally in accordance with the construction methodology as outlined in AECOM (2015b). Further the additional contribution and sequencing of rainwater from fronts and storms is not able to be predicted at this juncture. Therefore at this stage it is not possible to quantify the sediment discharge rate of fine-grained material (e.g., silts, clays) with time of the decant discharge(s). This situation is usually managed by way of an adaptive management approach, setting an environmental turbidity limit (ETL) not to be exceeded (after allowing for reasonable mixing – s107(1) RMA) while monitoring the receiving waters at a few locations during the construction phase. Such a limit would be achieved by the contractor putting in place sufficient sediment and erosion control measures, or altering the fill sequencing and spatial distribution to keep the turbidity levels below the agreed ETL. This could also apply to the initial phase of placing the rock and gravel filters material for the rock dyke as well as the longer embankment in-fill phase, where de-watering discharges will be required.

The potential dispersal footprint of suspended-sediment discharges from three discharge outlet options during the fill construction phase were generically modelled using the Delft3D-SED cohesive sediment module. The decant water may either be saltwater, from pumped dredged or barged fill slurries, or freshwater generated by rainfall within the in-fill area (e.g., if trucked quarry fill was to be used) or a mixture of both freshwater and saline water. To cover for these different situations, the sediment-laden discharge in the dispersion model simulations was placed in the surface (top) layer in the five depth-layer dispersion model, and both a continuous and a pulsed discharge (e.g., following a rainstorm) were simulated. It was assumed that the discharge will have negligible momentum when entering the receiving waters i.e., it is not discharged as a high-velocity jet, but a more slowly- introduced inflow.

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D1

D3

D2

Figure 6-1: Schematics of a NW decant discharge location D1 (top) and SW and S discharge locations D2 and D3 (bottom) in the perimeter rock dyke with flow of fill (yellow arrows) and options for associated fill operations. Source: adapted from diagram supplied by AECOM Ltd.

Releases from disturbances from the initial seabed activities Disturbances of the seabed in the CMA will occur from construction activities, including:

. new temporary marine support and berthing/mooring structures

. drilling and installation of stone columns or other similar in-ground strengthening works

. subgrade preparation for the rock dyke and placement of the rock apron on the seabed, and

. protection and/or relocation of the ocean outfall from the Moa Point wastewater treatment plant.

These sporadic activities have the potential to locally disturb and suspend fine-grained seabed sediments (sands will quickly settle out) and any resident low-level contamination present in the seabed sediments, leading to turbidity plumes.

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Construction of temporary marine structures will involve pile driving activities that will impact a small footprint of the seabed that is unlikely to cause turbidity plumes. Stone column installation typically uses a vibrating probe with compressed air that may result in some turbidity issues; however, as the sediments are primarily sands and gravels, the sediment releases are anticipated to be minimal. The outfall-protection work and construction of the rock dyke will cause local disturbances to the seabed (AECOM, 2015b).

Potential mitigation measures to minimise turbidity plumes include deploying silt curtains around the working areas of the marine equipment, using construction equipment and methodologies that minimise excessive disturbances of the seabed, and using clean stones/rock for the stone columns and rock-dyke (AECOM, 2015b).

6.2 Assessment of construction effects

6.2.1 Setting up the sediment-plume models The Delft3D-SED cohesive sediment module was used to generate sediment-plume simulations for the de-watering discharges that did not exceed two different environmental turbidity limits within 100–150 m of the discharge outlet. A representative slow settling velocity of the suspended sediment in the receiving waters was set at 0.13 mm/s for medium-silt particles (grain size = 15 µm or 0.015 mm), representing the types of fine-grained sediments that may be present in and winnowed from the placed fill material.

One of the key input parameters for the sediment-plume model is the sediment discharge rate in kg/sec of the fine-grained sediment fraction of the suspended material, leaving aside the discharge of sand-sized material which will settle relatively quickly and only have a small contribution to changes in receiving-water turbidity. As the fine sediment content of the fill material is not precisely known, nor the water discharge rates and temporal variability, sediment-discharge rates are not able to be pre-determined. Therefore, plume model simulations with varying sediment discharge rates were undertaken until the relevant SSC limit was not exceeded within 100–150 m of the discharge outlet.

Two environmental SSC limits were selected to safeguard different marine ecological effects (Dr Mark James, Aquatic Environmental Sciences Ltd., pers. com., and James et al. 2016).

An upper limit on in-situ SSC could be around 40 mg/L (or equivalent NTU turbidity-sensor magnitude) derived from considering both a suggested benthic-ecological environmental limit of 35 mg/L (James et al. 2016). This limit is also close to the natural environmental maximum of 38 mg/L (MacDiarmid et al. 2015) measured at the bottom sensor of the outer Lyall Bay optical mooring during the ~7-week field deployment (only ~16 mg/L at the top sensor), although inshore in Lyall Bay, the SSC during the field deployment may have been similar or higher with more intense wave stirring on the seabed in shallower waters)19. A sediment-discharge rate of ~2 kg/s of medium-silt material was needed in the plume model to ensure no exceedance of a SSC limit of 40 mg/L above a background of zero in the surface layer, after a mixing distance of around 100–150 m from the outlet.

Simulations were also undertaken for a lower-threshold SSC of 25 mg/L, which is the suggested limit for foraging seabirds (James et al. 2016), and while at the outer mooring in Lyall Bay the maximum reached of only ~16 mg/L at the top sensor (8 m depth) did not exceed this limit (but the 16-m deep

19 The synoptic survey during the field deployment in 2014 was necessarily carried out during calm, clear conditions and showed TSS concentrations of 0.5 mg/L at the outer-bay mooring increasing to about 1 mg/L inshore (James et al. 2015).

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sensor did), it is expected that natural levels of SSC could rise to around 30–40 mg/L within inner Lyall Bay during high wave events in Lyall Bay, particularly if substantial turbid stormwater discharges coincide with a significant wave event. A sediment-discharge rate of around 1 kg/s of medium-silt material was needed in the model to ensure no exceedance of a SSC of 25 mg/L at around 100– 150 m from the outlet. The fine-sediment discharge rates were only approximate estimates to generate plume simulations where the SSC did not exceed the relevant SSC limits within the near field area around the discharge location and hence achieve much lower SSC in the wider Bay.

The hydrodynamic conditions for the sediment-plume modelling were also generalised to a repeating average tide (given tidal currents are very low within the inner Bay) over a 30-day period interspersed with two southerly and one northerly high wind events, as also used for isolating the effect of the runway extension on wind-generated currents in Lyall Bay (Hydrodynamic and Sediment Processes Technical Report; Pritchard et al. 2015).

The outputs from the suspended-sediment plume simulations for each of the three decant-discharge locations (D1 in the NW corner; D2 in the SW corner, D3 in the mid-section of the southern end) were as follows:

 Composite maps of the maximum surface-layer SSC, reached in each model cell at any point during the sediment-plume simulations resulting from a continuous flux of 1 kg/s and 2 kg/s of medium-silt sediment over a 30-day period. Note: these SSC maps are composites of the maximum that could occur at different times for different locations, not instantaneous snapshots of the plume at any one time. These provide an upper envelope of SSC for each location or cell in the plot.

 Maps of the mean surface-layer SSC for periods of the sediment-plume simulation when calm, southerly and northerly winds occurred, averaging the time series from each model cell resulting from a continuous flux of 1 kg/s and 2 kg/s of medium-silt sediment over a 30-day period. Note: these SSC maps are an averaged snapshot of the plume over a period of at least 12 hours when the relevant wind conditions occurred in the simulation (and longer at several days for calm conditions). These averaged SSC plots for a subset of the simulation period show the effect of different wind conditions on the predicted SSC from the discharge alone (background = 0).

 Simulated time series of the surface-layer SSC extracted from the 2 field mooring locations in Lyall Bay (outer mooring site C and the inner site 1 to the south-west of the spur breakwater).

 Simulated instantaneous 12-hour snapshots of surface SSC in a sediment plume following a 12-hour pulse discharge of a 1 kg/s and 2 kg/s sediment flux commencing at the peak of a 22 m/s southerly gale event, and from the 2 decant-discharge locations.

The background SSC in the simulations was set to zero, so these results for the near-surface layer apply to the situations when the Lyall Bay waters are reasonably clear (e.g., calm or fair weather northerly conditions). From the 2014 field deployment in the outer Bay, the turbidity was low for the majority of the time (based on the top sensor at 8 m depth), where the measured 33-percentile SSC was 1 mg/L, the median 1.7 mg/L and for 99% of the time, SSC was <12 mg/L (data analysis; MacDiarmid et al. 2015). Higher SSC levels occurred during southerly wind events when significant wave heights in the outer Bay exceeded 2 m, especially if accompanied by longer-period swell (Figure 4-9), with turbidity generated by wave-stirred fine sediments winnowed from the seabed.

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The lower threshold displayed in the colour-scaled plots of SSC from the model simulations was set to around 3 mg/L, as unlikely to be ecologically meaningful below this level (James et al. 2016) – while the accuracy of in-situ monitoring of SSC is usually around 1 mg/L.

6.2.2 Plume-modelling results Continuous discharge Results of the various plume-model simulations are set out on a page for each discharge location (D1–D3) for a particular sediment discharge rate (1 and 2 kg/s). The page layout for Figure 6-2 to Figure 6-7 includes:

. (top-left): composite envelope for the maximum SSC in each cell at any time for the entire simulation

. (bottom panels): SSC averaged over calm, northerly and southerly wind conditions (left to right)

. (top-right): time series of predicted SSC for the outer field-deployment site C (lower trace) and the inner Lyall Bay site 1 (just to the south-west of the spur breakwater), with approximate locations shown as a star and triangle respectively in the adjacent maximum SS composite plot (and also in Figure 3-3).

The main findings in relation to assessment of effects of sediment-plume discharges are:

a) Discharges during calm weather show a classic radial dispersion pattern (bottom left-hand plots) arising from the very weak tidal currents within the inner Bay – so basically slower spreading by diffusion processes. At each location there is a small tidal influence on SSC, a shown in the time series plots (top-right panels).

b) The wind events in the simulations show the plume is elongated more than the calm- conditions as wind-generated currents add a transport or advection component to the dispersion of the plume (bottom middle (northerly) and bottom-right (southerly) panels).

c) Arising from the addition dispersion by the currents, the SSC arising from the discharge would decrease (relative to calm conditions) during wind events in the near-field (0–150 m) and the intermediate zone out to about 600-800 m from the discharge. Further out in the far-field, SSC would be slightly higher during windy periods than for calm conditions, but at low levels (generally < 3–4 mg/L), due to the additional plume excursion from the wind- generated currents.

d) Mostly the increase in SSC from a sediment plume is linear function of the sediment discharge rate e.g., doubling the rate from 1 to 2 kg/s of fine sediment, generally doubles the SSC generated at most sites, although there are some differences in the near field (see Table 6-1).

e) Of the 3 discharge locations, the discharge at the NW (D1) location has the most effect on inner Lyall Bay due to its closer proximity, but SSC would only reach a maximum of <3 mg/L and 6-8 mg/L above the background SSC for sediment discharges of 1 and 2 kg/s respectively, based on the 30-day simulation with high wind events (Figure 6-2 and Figure 6-3). Conversely, a sediment discharge at the southern terminus of the rock dyke (D3) would have a negligible contribution to turbidity levels in inner Lyall Bay, but would generate higher SSC

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in the Moa Point Beach area and associated fringing reefs, with highest values during calm conditions and lowest during a southerly-wind event (Figure 6-6 and Figure 6-7).

f) After discharge, a mixing distance of 150 m is deemed to be sufficient to allow for reasonable mixing (and extend beyond the area for any barge or machinery activities that could interfere with a monitoring station), before compliance through a monitoring limit is considered. Table 6-1 shows the maximum SSC values estimated from the plume simulations to be generated at ~150 m from the discharge for the 3 discharge locations and 2 sediment discharge rates for relatively calm wind/wave conditions.

Table 6-1: Maximum predicted SSC above background levels after reasonable mixing at ~150 m for discharges at D1–D3 and sediment-discharge rates of 1 and 2 kg/s. These maximum SSC contributed by the proposed discharges apply during calmer wind and wave conditions – but reduce for higher wind events.

SSC (mg/L) above background at ~150 m

Sediment discharge of Discharge site D1 (NW) Discharge site D2 (SW) Discharge site D3 (S) medium silt

1 kg/s 14–16 13–14 11–13

2 kg/s 33–36 22–24 22–23

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S N S

Figure 6-2: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D1 (NW corner). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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S N S

Figure 6-3: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D1 (NW corner). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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S N S

Figure 6-4: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D2 (SW corner). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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S N S

Figure 6-5: Composite surface-layer maximum SSC over a 30-day simulation (top) and averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D2 (SW corner). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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S N S

Figure 6-6: Composite surface-layer maximum SSC over a 30-day simulation (top) & averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 1 kg/s sediment flux discharged from site D3 (mid-south end). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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S N S

Figure 6-7: Composite surface-layer maximum SSC over a 30-day simulation (top) & averaged SSC for calm, northerly & southerly winds (bottom) reached in each model grid cell for a continuous 2 kg/s sediment flux discharged from site D3 (mid-south end). Note: different SSC scales between the top and bottom plots. Star = site 1, triangle = site C.

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Pulsed discharge If the method of infilling the reclamation behind the rock dyke is undertaken through truck dumping, the sediment discharge may be more intermittent, or with much higher flows during rainstorm events. Therefore further simulations were undertaken for a pulse discharge of 1 and 2 kg/s over a 12-hour period, starting at the peak wind speed of the southerly wind used in the 1-month continuous discharge simulation, before reverting to calm conditions.

Snapshots of the plume SSC are shown in Figure 6-8 to Figure 6-10 for 24, 48, 72 and 96 hours after a 1 and 2 kg/s decant pulse release has commenced, noting the discharge ceases after 12 hours. Note: Figure 6-10 presents both sediment discharge rates for the D3 (southern) discharge location. The plots have the same SSC scale as that for the maximum composite SSC plots in the previous section.

As expected, the plume from a pulse discharge would have a smaller areal extent than for the continuous discharge. For the pulse discharge, the greatest extent and peak concentrations in the intermediate and far field for the times shown, occur mostly at 48 hours after the discharge commences and 36 hours after it ceases. Thereafter the plume dissipates through ongoing slower mixing during calm conditions as present in the simulation.

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Figure 6-8: Predicted instantaneous plume snapshots of SSC at 24, 48, 72 and 96 hours after a 1 kg/s decant slug release for locations D1 (left panels) and D2 (right panels). Minimum SSC cut-off is 3 mg/L.

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Figure 6-9: Predicted instantaneous plume snapshots of SSC at 24, 48, 72 and 96 hours after a 2 kg/s decant slug release for locations D1 (left panels) and D2 (right panels). Minimum SSC cut-off is 3 mg/L.

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Figure 6-10: Predicted instantaneous plume snapshots from location D3 of SSC at 24, 48, 72 and 96 hours after a 1 kg/s (left panels) and 2 kg/s (right panels) decant slug release. Minimum SSC cut-off is 2 mg/L.

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Further refinement of the construction methodology, including the key roles of infilling procedures and erosion and sediment control measures to reduce the turbidity in the discharge “at source”, will be undertaken by the appointed contractor (AECOM, 2015b). The turbidity plume simulations above, in tandem with turbidity monitoring against appropriate turbidity limits during construction, will provide a mechanism to adaptively manage the effect of suspended-sediment discharges during the construction phase.

Disturbances Contaminant concentrations in Lyall Bay surficial sediments are very low and uniformly distributed across the study area, including at sites most likely to be disturbed by construction activities, namely, sites 10 and 11 in the area to be reclaimed by the runway extension (Depree et al. 2016). Mobilisation of near-surface sediments from within the area of the proposed runway extension is not expected to result in any significant increase in sediment-contaminant concentrations in surrounding areas (Depree et al. 2016), including within the cove at Moa Point beach.

From a contaminant perspective, the risk of any adverse effects on the water column from transient sediment suspension/disturbance events during construction is very low given that water column concentrations, even after allowing for reasonable mixing, are estimated to be at least two-orders of magnitude lower than default ANZECC water quality trigger values (Depree et al. 2016).

Further, erosion and soil control measures to be applied in the construction methodology e.g., containment of turbidity generated during drilling and placement of stone columns, trenching during calmer-wave periods etc., will further reduce the environmental risk from sea-bed disturbances.

6.3 Assessment against statutory plans/policies or technical criteria and guidelines

6.3.1 Sea-bed disturbances The two conservation areas designated in the Regional Coastal Plan (RCP) in the general vicinity of Lyall Bay area – Tarakena Bay and Taputeranga Island, off Island Bay (Figure 3-7) – are unlikely to be affected by water-borne contaminants including turbidity from seabed activities during construction. Rather these processes will be limited locally to the area around the reclamation works due to the low in-situ contaminant levels in the surface sediment and dispersal and settling of any fine-sediment fraction present to the outer Bay would be negligible, with no effect beyond Lyall Bay.

Planning Map 8D in the RCP designates the water quality class in the waters throughout Lyall Bay to be managed for water-contact recreation, with the nearshore waters around Te Raekaihau Point and Hue te Taka Peninsula in outer Lyall Bay also managed for shellfish gathering. As water quality for both these types of activity relate primarily to the presence of water-borne pathogens in relation to public health of users, sea-bed disturbances in the limited area of construction for the proposed runway extension would not expose these waters to any additional public-health risks.

Policy 10(2)(c) of the NZCPS states having particular regard to the use of materials in the reclamation, including avoiding the use of contaminated materials that could significantly adversely affect water quality in the coastal marine area. This mainly relates to fill material – covered in the next sub- section in regard to turbidity. Management of sea-bed and introduced material associated with ground-treatment and trenching methods is covered in the Construction Methodology Report (AECOM, 2015b).

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6.3.2 Turbidity and discharges to the CMA With contaminant risks to the CMA negligible, the remaining environmental risk from construction activities associated with the runway extension into the CMA is the generation of suspended- sediment discharges, which could potentially result in excessive turbidity locally, if not managed effectively at source.

Section 107 of the RMA for assessing discharge permits, outlines that after allowing for reasonable mixing, the discharged contaminant (suspended sediment in this case) should not produce any conspicuous change in colour or visual clarity, nor any significant adverse effects on aquatic life. Policy 23(1)(e) of the NZCPS, in relation to discharge of contaminants, required particular regard to using the smallest mixing zone necessary to achieve the required water quality. A mixing distance of 150 m from the discharge point is deemed to be a reasonable mixing length and pragmatic distance offshore from the rock-dyke construction activities and barge/vessel movements to avoid interference with the monitoring equipment. Compliance should be determined from monitoring turbidity or calibrating the optical sensor to generate equivalent SSC levels.

Numerical modelling of suspended-sediment plume dispersion (Section 6.2) predicts the magnitude and extent of turbidity above background levels for a range of discharge location and sediment-load scenarios. However, to assess the effects of discharges to the CMA, as per section 107 of the Act, requires a determination of an upper threshold in-situ turbidity levels before adverse effects occur on fauna and flora, and in so doing, cover off what constitutes a conspicuous change in colour or visual clarity in the Lyall Bay environment. These aspects are covered in the Marine Ecology AEE Report (James et al. 2016), but monitoring to a turbidity limit needs to be in tandem with best- practice sediment and erosion control measures (including fill-material specification and sediment- settling techniques) during construction – particularly the infilling operation by the contractor once the rock dykes are in place.

A suggested and appropriate SSC limit has been set to 25 mg/L, not to be exceeded beyond ~150 m of the discharge, which is the SSC at which sea-bird foraging is curtailed (James et al. 2016). Little data are available on SSC levels within inner Lyall Bay (other than a few fair-weather samples; MacDiarmid et al. 2015), but turbidity and SSC was measured continuously at the optical mooring in outer Lyall bay at site C by two sensors at 8 m and 16 m depth (Figure 3-3) during the 2014 field deployment that included a southerly gale. This data is plotted in Figure 6-11 in the form of a frequency plot (ascending order) and compared to a 25 mg/L SSC limit (blue line). It shows for the clearer waters of the outer Bay that this limit is not exceeded naturally in the upper water column, with a maximum of ~16 mg/L recorded during a southerly wave event (but is exceeded infrequently nearer the seabed where turbidity is higher due to wave re-suspension processes). However, the inner Bay is likely to have higher SSC levels, than those measured in the outer Bay, during southerly wave events when waves break in shallower waters and/or following rainstorms, when the stormwater system discharges through various outlets into the Bay (Section 4.3 and Appendix A; Depree et al. 2016).

The levels of SSC predicted from the modelling (relative to background concentrations) at ~150 m from the discharge, as listed in Table 6-1 for both a 1 and 2 kg/s sediment discharge, show maximum SSC from the discharge (excluding the background) would be <25 mg/L except for sediment discharges towards 2 kg/s from location D1. These maximum results, which would occur for calmer conditions, indicate that if the sediment discharge (equivalent to medium-silt) of between 1 and 2 kg/s occurs (with exception of somewhat less than 2 kg/s for site D1), then certainly under calm

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conditions when the background SSC is only 1–3 mg/L, it is unlikely to have a significant effect on benthic biota beyond ~150 m from the discharge point based on a 25 mg/L environmental limit (James et al. 2016). In the wider inner Bay, the average contribution to SSC from a 1 kg/s discharge would be <2–3 mg/L with a negligible contribution on the western side of Lyall Bay, rising to an average SSC contribution of <4–5 mg/L for a 2 kg/s discharge of silt.

Chapter 7 covers the technical issue of what background SSC or turbidity to allow for when discharging a sediment plume during the temporary construction phase.

Frequency of occurrence of SSC in outer Lyall Bay (4-Sept to 8-Oct 2014) 40 SSC (8m) 35 SSC (16m) 30 Sea-bird related ETL = 25 mg/L (SSC) 25

20

15 SSC SSC (mg/L)or FTU 10

5

0 0 10 20 30 40 50 60 70 80 90 100 % frequency below concentration

Figure 6-11: Frequency of SSC for the upper sensor at 8 m depth (solid line) and lower sensor at 16 m depth (dashed line) from the outer Lyall Bay optical mooring and compared with a 25 mg/L SSC limit. Note: turbidity will be higher within inner Bay due to both higher wave-induced resuspension that occurs naturally and from plumes emanating from stormwater outlets. [Data source: MacDiarmid et al. 2015]

Policy 10(2)(c) of the NZCPS states having particular regard to the use of materials in the reclamation, including avoiding the use of contaminated materials that could significantly adversely affect water quality in the CMA. This encourages the use of clean fill material that has as low a fraction of silts and clays as practically possible and minimal chemical-contaminant levels and a consent condition could specify, for example:

. All imported fill material to be used in the reclamations, rock dykes, groynes and temporary fill/surcharge shall be in accordance with the Ministry for the Environment “cleanfill” definition, as detailed in Publication ME418 “A Guide to the Management of Cleanfills, 2002”.

Based on the proposed construction methodology put forward (AECOM, 2015b), there are no other potential effects arising from construction that would cause greater than minor effects on coastal processes, other than the reclamation itself removing a small portion of the water compartment of Lyall Bay and any extreme wave-storm that could damage and move sections of the rock dyke (which would need to repaired quickly anyway for construction to proceed).

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7 Mitigation and monitoring This Section outlines the effects on coastal physical processes, but assessments of on-site or off-site measures to mitigate or remedy any of the effects are covered in the AEE report by Mitchell Partnerships and AECOM (2015a,b), but with very limited options for avoidance of some effects (given the spatial coverage of the proposed reclamation of the CMA).

7.1 Overview of effects on coastal physical processes Waves–During the operational (permanent) phase of the extended runway south into the CMA of Lyall Bay, the only assessed change in coastal physical processes that is potentially more than minor is the reduction in some combinations of wave heights and period in parts of Lyall Bay, particularly in the NE area of Lyall Bay adjacent to the existing runway revetment, and to a lesser extent on the cove (Moa Point beach) east of the runway extension.

In the north-eastern sector of inner Lyall Bay, 1.5 m incident waves propagating into Lyall Bay from Cook Strait would be attenuated by a further 0.2 to 0.6 m (8-second wave) and 0.2 to 0.4 m (12- second wave) than present in a zone extending out ~250 m to the west from the existing runway revetment. The greatest wave-height reduction would be a localised patch 80–100 m NW of the spur groyne, and conversely, a slight local increase in these 8-second wind waves immediately behind the spur groyne (breakwater) through diffraction.

The wave model also predicts slight increases in wave heights for the following areas: i) adjacent to and in the immediate lee of the spur groyne and submerged rock outcrop; ii) slight increases in a narrow central part of the inner Bay of the main Beach; and iii) in a patch close to the beach towards the eastern end of Lyall Bay Beach near The Corner. There would be no increase in wave heights on Moa Point Beach itself – only a slight increase adjacent to the eastern rock dyke. However, these effects of increased wave heights from the runway extension on wave heights are likely to be minor and not readily distinguishable against the background day-to-day up to storm variability in wave conditions. These patches of increased wave heights for smaller waves are also unlikely to occur for the more extreme waves in the Bay under gale-force southerly storms as larger waves will shoal and break further out in the Bay in deeper water. For example, previous physical model studies have shown the highest waves would only reach 5–5.5 m just off the existing runway dyke, thereafter reaching saturation due to wave breaking, yet higher waves of 7 m or more are possible in outer Lyall Bay (Section 4.5.3).

The largest reduction in wave height overall would occur adjacent to the western-side rock dyke of the extended runway, as the wave peels along the new rock dyke being subject to dissipation by friction (drag) exerted by the accropode units and also the influence of a subtle wave-sheltering effect as the runway extension alignment of ~183S is slightly west of the predominant wave- approach direction from due south. However, the effects of this reduction will only have minor effects on seabed sediment transport and morphology change outside the peripheral rock aprons.

The height of wave-trains reaching the central part of Lyall Bay Beach are likely to be only slightly affected by the runway extension, otherwise elsewhere in the Bay, including the western side, the changes will be small.

The cove east of the runway (Moa Point beach) will also exhibit a reduction in wave height as a result of the proposed extension. This reduction in wave height is more appreciable inshore nearer the beach and in the NW corner immediately adjacent to the existing runway. So while waves will be

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smaller, there is also likely to be slightly more resonant or wave-reflection behaviour within the more-enclosed cove after the extension has been constructed, creating a more-enclosed basin in this area. However this cove, which will be more enclosed than present, will still be dynamically flushed, but less vigorously than present as it will still be exposed to refracting waves and southerly-wind directed currents directed into the cove.

The above assessments apply only to regular sinusoidal wavetrains, based on the ARTEMIS model, to provide an overview on the general effects on waves within Lyall Bay. The ARTEMIS is known to overestimate wave reductions in the lee of a diffracting breakwater. More detailed high-resolution modelling of irregular realistic waves and inherent characteristics, such as wave steepness and peakiness, has been undertaken by DHI for assessing in more detail the effect of the runway extension on surfing and swimming of the physical changes in wave characteristics specific to these water-contact recreational activities (DHI, 2016).

Seabed changes – The morphology of Lyall Bay beach will continue to be dominated by cut and fill along an on/offshore seabed profile governed by sequences of southerly storm events, with any effects of the proposed runway extension on seasonal morphological timescales likely to be second- order influences.

Localised changes in sea-bed heights, within 50 m around the extended runway, are likely to be minor (order of a few cm’s at seasonal timescales), particularly after the existing sea-bed topography has adjusted to the presence of the rock dyke and the associated wave interaction with the structure.

There could be ongoing broader or localised changes within the shoal adjacent to the existing- runway rock revetment arising from natural variability at annual to decadal timescales, and any ongoing adjustment to the existing reclamation, within which subtle longer-term changes may arise from the proposed runway extension.

Currents (tide and wind) – the effects of the runway extension on currents and circulation patterns will be no more than minor, and largely localised – influenced by the reclamation planform. There would be no change in currents in the outer Bay, including the currents that flow over the ocean outfall diffuser that discharges treated wastewater from the plant at Moa Point adjacent to the Airport. Therefore there would be no change to dispersion patterns from the ocean outfall.

Water quality – Disturbances of the seabed sediments and discharges from de-watering the in-fill works have the potential to generate turbidity – but low contaminant levels in seabed sediments in Lyall Bay means any contamination effects from disturbances will be negligible.

The exact sedimentary characteristics of the fill for the reclamation will not be known until a contractor is appointed and the detailed construction methodology refined. Therefore, there will need to be an adaptive management approach to monitoring the turbidity generated by the decant discharges. Compliance or additional mitigation can be assessed against an appropriate environmental turbidity limit monitored at 150 m at the end of the near-field mixing area around the discharge point, coupled with sediment and erosion control measures within the infill work area to reduce the release of fine sediments.

In the wider inner Bay, the average contribution to SSC from a 1 kg/s discharge would be <2–3 mg/L with a negligible contribution on the western side of Lyall Bay, rising to an average SSC contribution of <4–5 mg/L for a 2 kg/s discharge of silt. The contribution to turbidity in outer Lyall Bay, from de-

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watering discharges during the temporary construction phase, will be undetectable, particularly at the ocean outfall diffuser site, and therefore not affect the dispersal of the plume from the wastewater treatment plant discharge.

7.2 Mitigation or avoidance measures The implications and down-stream effects of the predicted changes in wave patterns and heights, such as the effects on surfing quality and safety of recreational beach hazards, are described in the assessment report by DHI (2016).

Sediment and erosion control measures to mitigate seabed disturbances and turbid discharges to the CMA are generally outlined in the Construction Methodology Report (AECOM, 2015b).

7.3 Suggested coastal monitoring conditions The following coastal monitoring conditions are recommended to:

• Gather additional data to document physical changes to the existing environment (e.g., bathymetry, beach profiles), which should be analysed to attribute changes where possible and a report provided.

• Good-practice construction or post-construction house-keeping.

• Visual observations to ensure effects are no more than minor as works proceed (especially in relation to the decant discharges during construction).

• Compliance monitoring as part of an adaptive management approach e.g., to ensure changes to turbidity in Lyall Bay (after reasonable mixing) are within acceptable limits.

For example, monitoring conditions for coastal physical processes could include (excluding turbidity monitoring): C.1 – WIAL shall provide to the Greater Wellington Regional Council (GWRC), plans and drawings (including dimensioned, cross sections, elevations and site plans of all areas of proposed reclamation (including associated permanent and temporary CMA occupation), permanent structures and temporary structures) at least X [to be determined] working days before the proposed date of commencement of the construction of the reclamation or temporary structures.

C.2 – Construction in the CMA of Lyall Bay shall be undertaken generally in accordance with the construction methodology detailed in the application, specifically the Engineering Report and the Construction Methodology Report (AECOM, 2015a, b).

C.3 – The WIAL shall notify the Greater Wellington Regional Council in writing within 10 working days of the completion of each discrete construction phase e.g., ground-treatment works, dykes, reclamation fill, structures.

C.4 – The WIAL shall supply to the Greater Wellington Regional Council and the LINZ Hydrographic Services Office and LINZ Topographic Services Office (Chief Hydrographer, National Topo/Hydro Authority, Land Information New Zealand, Private Box PO Box 5501, Wellington 6145), a complete set of “as built” plans, final topographic and bathymetric data, and appropriate certification confirming that the new reclamations, structures, revetment works have been built in accordance with sound engineering practice, within 60 working days of the completion of the reclamation works.

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C.5 – The WIAL shall maintain the site in good order and shall, as far as practicable, remedy all damage and disturbance caused by plant, vehicles and equipment to the foreshore during construction, to the satisfaction of the Greater Wellington Regional Council.

C.6 – The WIAL shall ensure the removal of all equipment, erosion and sediment control measures, surplus soil, sediment and construction materials from the CMA within XX working days following the completion of the construction works, to the satisfaction of the Greater Wellington Regional Council.

C.7 – Cleanfill: All imported fill material to be used in the reclamations, rock dykes, groynes and temporary fill/surcharge shall be in accordance with the Ministry for the Environment “cleanfill” definition, as detailed in Publication ME418 “A Guide to the Management of Cleanfills, 2002” or subsequent updates.

C.8 – The WIAL shall maintain a log recording the sources of fill material imported onto each reclamation or temporary and permanent occupation site. This log shall be made available to the Greater Wellington Regional Council for inspection on request.

C.9 – Repeat bathymetric survey in eastern Lyall Bay 2 years after construction of the rock dyke. Hydrographic survey report to compare with the 2014 survey and ascertain any anomalous changes in seabed heights or accretion/deposition patterns. A copy of the Report to be supplied to GWRC.

7.3.1 Turbidity compliance monitoring In Chapter 6, a SSC limit of 25 mg/L was suggested as the level at which sea-bird foraging is curtailed. This is an absolute concentration – which means when the limit is exceeded naturally, as may occasionally occur in inner Lyall Bay (but less likely in the outer Bay), which would mean no discharge could occur until the natural background levels decreased below the SSC limit. An alternative is to build in an allowance for the discharges during the temporary construction phase, of up to 10 mg/L above background SSC (monitored at a control site), once the background SSC level exceeds 15 mg/L. This latter background level is only exceeded for 0.15% of the time in the outer Bay (based on the top sensor over a ~5-week period monitoring at 15-minute intervals), but is expected to be exceeded more often at present (without the construction discharge) within the inner Bay, where monitoring of the discharge would occur. Therefore imposing this additional element (of allowing up to 10 mg/L above background when the natural background reaches 15 mg/L) to the condition would still only apply for a small proportion of the time for the inner Bay (maybe 1–5% of the time) and in such cases, sea-bird foraging would begin to be curtailed anyway by increased turbidity during southerly- wave events (e.g., see Figure 4-9).

Proposed consent compliance conditions for monitoring turbidity are detailed in the monitoring conditions and Erosion and Sediment Control Plan appended to the AEE report.

In addition to the monitoring of turbidity in the CMA, the Erosion and Sediment Control Plan, outlines plume intensive monitoring shall be undertaken on a single occasion for each dewatering discharge location during fair-weather conditions. This survey data can be used to confirm the efficacy of the plume modelling results used to set the consent condition, including consideration of the assumed conditions for the simulations, relative to actual construction conditions e.g., type of fill material, % of fines, infill method and type of discharge structure (e.g., weir, pipe).

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8 Acknowledgements This report summarises material from a number of sources, particularly NIWA technical reports prepared by my colleagues for WIAL to support the AEE.

I especially acknowledge the input of Craig Stevens (Baring Head wave climate) and Scott Nodder and Peter Gerring (Moa Point beach survey), Dr Mark Pritchard (sediment plume modelling) along with other NIWA colleagues who contributed material and carried out modelling and field studies (Pritchard et al. 2016; Depree et al. 2016; MacDiarmid et al. 2015).

Thanks to Dr Mark James (Aquatic Environmental Sciences) who coordinated the coastal environmental studies on behalf of WIAL and provided insightful comments on the report.

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9 Glossary of abbreviations and terms ADCP Acoustic Doppler Current Profiler – measures currents throughout the water depth and waves. AEE Assessment of Effects Report (accompanies applications for consents). AEP Annual Exceedance Probability – the chance (%) of an extreme coastal-storm event reaching or exceeding a given water level in any year. A 1% AEP level is equivalent to an event with an average recurrence interval of 100 years. accropodes A single-layer artificial heavy armour unit developed by SOGREAH in 1981, later modified by chipping away excess materials and adding friction features in the form of small pyramids to increase interlocking. akmons Heavy unreinforced pre-cast concrete shapes that interlock to form a robust coastal-wave protection layer, as used at the southern end of the existing runway into Lyall Bay. ANZECC guidelines Australia NZ Environment Conservation Council produced guidelines on freshwater and marine water quality in 2000 (http://www.mfe.govt.nz/fresh- water/tools-and-guidelines/anzecc-2000-guidelines) ARTEMIS A high-resolution phase-resolving wave model that is part of the TELEMAC model suite that simulates regular waves. Coastal environment Includes elements of coastal areas influenced by coastal processes on land above MHWS, as well as the CMA, as defined in Policy 1 (NZCPS). CMA Coastal marine area – as defined in s. 2 of the RMA. Delft3D The widely-used Deltares hydrodynamic, sediment and water quality suite of models (Deltares, 2011). GWRC Greater Wellington Regional Council. Irregular waves Non-repeating wave shapes that are not regular (sinusoidal) and can comprise interacting wavetrains of different wave periods from different directions – more akin to time series of continuously measured waves. mean spectral wave Average of all wave periods in a time-series representing a certain sea state.

period (Tm) mean wave direction The average wave direction, which is defined as the mean of all the individual wave directions in a time-series representing a certain sea state. MHWS Mean High Water Spring – the average high-tide level during spring tides. MSL Mean Sea Level – the actual mean level of the sea (rather than a datum). NZCPS NZ Coastal Policy Statement (2010) – the mandatory coastal policy statement required under s. 56–58 of the RMA. peak direction of wave Direction of travel for waves at the wave period at which the wave energy is

propagation (Dp) highest (maybe for wind sea or a swell component of a mixed sea).

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Phase-resolving wave A wave model with grid cells of high enough resolution that have several cells model resolving a single wave length, and therefore resolve refraction and diffraction processes. Usually based on the wave mild-slope algorithm and waves applied at the seaward boundary are usually monochromatic (waves of constant height and period with time). Reasonable mixing “reasonable mixing” in the RMA (e.g., s 107) implies the existence of a zone in which the compliance standard need not be met – but important to distinguish between the near-field mixing zone, the point of complete mixing, and the non- compliance zone (MfE, Resource Management Ideas, No. 10, Aug 1994). Policy 23(e) of the NZCPS states that the smallest mixing zone necessary should be used to achieve the required water quality. Regular waves Pertains to the use of a repeating regular sinusoidal wave time series used for input on the offshore boundary of wave models such as ARTEMIS (otherwise known as monochromatic waves). RMA Resource Management Act 1991 and subsequent amendments. significant wave height The average of the highest 1/3 of wave heights over a measured or simulated

(Hs) period. Akin to the average an observer would perceive, which is usually biased towards noticing the larger waves. SLR Sea-level rise. SSC Suspended-sediment concentration. SWAN A widely-used Deltares spectral wave model (Deltares, 2011). swell Generally waves with periods greater than 8–10 seconds for medium-distance swells and above 10 seconds for longer-distance ground swells, reaching up to 20 seconds for trans-Pacific swell. wave diffraction Diffraction is the process by which the waves are bent around into the lee zone behind a structure or barrier (e.g., rock revetment or groyne) by energy transmitting laterally along the wave crests. wave refraction Process where wave crests arriving at an angle (rather than parallel) to the seabed contours in shallow water are changed, with the part of the wave in shallower water moving more slowly than the part still advancing in deeper water. A consequence of this is that the wave fronts tend to become aligned with the depth contours as they shoal. WCC Wellington City Council. WIAL Wellington International Airport Ltd. WVD-53 Wellington Vertical Datum–1953 (regional vertical survey datum). Zero-crossing mean Average of wave period analysing wave water-surface measurements extracting

wave period (Tz) times between the up-crossing (when the water surface just rises above the average sea level) and the next down-crossing (when water surface dips below the sea level). Usually lower than the mean spectral wave period, due to the large number of short-period, low-energy waves.

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124 Wellington International Airport Runway Extension: Coastal Processes