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3.2.3 Construction and Demolition Impacts

Construction impacts are likely to be similar to the Duplicate Bridge in regard to piling operations. Demolition operations will be required to remove the Old Mangere Bridge. Demolition debris will need to be contained to minimise discharge to the water. Pile removal may require use of explosives at or near bed level. If used this will cause localised disturbance of harbour bed sediment, some of which will be initially suspended. This is expected to resettle on the harbour bed. Fish in the immediate area may be stunned or killed as a result of the use of explosives. However numbers are expected to be few if any.

The use of alternative methods such as vibration equipment to remove the old piles will be considered during the design phase.

3.2.4 Mitigation

The analysis above has identified that long term effects will either be similar to the existing footbridge or less. In this respect there is no need for mitigation, even when considered in combination with the duplication of the Mangere Bridge.

Construction related effects are similar to the Duplicate Bridge construction, and the mitigation measures listed in 3.1.3 are also applicable.

Section 5 covers the overall mitigation measures proposed.

3.3 Coastal Reclamation at Orpheus Drive

3.3.1 Proposed Works

The project includes areas of reclamation associated with ramp construction for Gloucester Park Interchange, realignment of Orpheus Drive, and alterations around the Manukau Cruising Club on Orpheus Drive. These are shown on Figure 1.1 and drawings 1/69/82/5104/97-98 which denote the R references. These are:

• An area of 1.2 hectares of permanent reclamation adjacent to Orpheus Drive and Onehunga Harbour Road, east from the Manukau Cruising Club for 400 metres with a width from 25 to 40 metres (R1);

• An area of 0.2 hectares of permanent reclamation adjacent to Orpheus Drive west of the Manukau Cruising Club for 150 metres with a width of up to 10 metres (R2).

• An area of 0.2 hectares of temporary reclamation (R3) with a width of up to 10 metres in width is also required which is associated with permanent reclamation R1.

The reclamation areas are effectively a continuous strip separated by the Manukau Cruising Club.

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The layout of the proposed boat ramp and surrounds at the Manukau Cruising Club will be refined during the design phase to ensure shelter and appropriate sea access.

The reclamations will be protected by rock revetments similar to the existing shoreline protection.

3.3.2 Long Term Impacts

The reclamation areas adjacent to Orpheus Drive are located west of the bridge and in a region of the Harbour open to the main body of water. The permanent reclamation area totals approximately 1.4 hectares, which is insignificant in relation to the area of the main body of water. The loss of harbour area will therefore not significantly affect the tidal prism of the harbour, and consequently will have negligible effect on channel flows.

The reclamations all take place against previously reclaimed land areas and edge protection. They therefore alter already-modified coastal boundaries. It is not expected that there will be any change in wave action or current flows at the coastline as a result of these works.

The breakwater offshore from the Manukau Cruising Club is a feature that is evident on the earliest aerial photos (1940), and was probably constructed well before that time. It has a crest level around MHWS and provided shelter to the much larger historic basin of water eastward to the edge of the tuff ring at a time when boats used the area for mooring and were drawn up onto the adjacent shoreline. The south end of the basin is completely open to tidal water exchange, and there is a gap between the north end of the breakwater and the Manukau Cruising Club building.

The proposed reclamation adjacent to Orpheus Drive in this area will provide a replacement boat ramp, located to avoid narrowing the gap between the north end of the breakwater and the Manukau Cruising Club building. This is not expected to have any impact on tidal flows, given the extent of the fully-open margin of the basin to the south. There may be some change in the location of the seabed channel scoured by water flows, but no consequence is expected to result from this other than the fact that the breakwater foundation will have to be checked during the design phase for capacity to resist toe scour, and if necessary strengthened by the placement of larger rocks (possibly relocated from the Orpheus Drive revetment).

The Manukau Cruising Club and Sea Scouts have noted during consultation that the harbour bed behind the breakwater has silted up in recent years. The silt build up behind the breakwater will have resulted from the shelter the breakwater has provided. In this situation there is less wave energy to agitate the water and bring sediment into the tidal prism. Analysis will be undertaken during the design phase to refine the reclamation shape and to determine if the proposed reclamation will result in further build up of sediment. If additional sedimentation is predicted, mitigation is likely to involve monitoring and maintenance dredging if required as a result of monitoring. The final design will need to reflect this and feedback from the Manukau Cruising Club, Sea Scouts and ARC

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(Auckland Regional Council). An alternative would be to locate the boat ramp north of the Manukau Cruising Club, although there is limited protection for boat launching in this area which the breakwater provides to the existing ramp.

Shelter can be provided to the proposed new ramp by leaving the breakwater untouched and by placing the ramp slightly south of the entrance gap. The soft marine sediments deposited over the harder base material are unsuitable as a foundation for the ramp and will be removed. Unsuitable material will also be removed from the shallows between the ramp and the gap in the breakwater to ensure that there is adequate depth for boats to be launched and reach deeper water outside the breakwater. If sediment removal is necessary it is likely that the deposits will build up again over time, in which case ongoing but periodic maintenance dredging would be required.

3.3.3 Construction Impacts

The earthworks will be undertaken in accordance with the ARC TP90 requirements. The Stormwater Management Assessment, Appendix 16, outlines the general erosion and sediment control principals that will be applied on the project together with the Erosion and Sediment Control Plan in Appendix 18.

The temporary reclamation area will be constructed by forming the outer (seaward) edge as a bund or cofferdam. This will be carried out progressively along the length by excavating relatively small amounts of the soft mud on the seabed at ebb and low tides, followed by filling with suitable larger material on the rising tide and raising of the bund itself, before progressing to the next repeat cycle of this action. In this way the dispersion of sediment will be minimised. The area totals approximately 0.2 hectares, which is insignificant in relation to the area of the main body of water. The effects on tidal hydraulics will therefore be negligible.

3.3.4 Mitigation

The analysis above has identified effects that require mitigation. Mitigation measures for construction effects include appropriate construction practices and procedures e.g. use reclamation fill that does not release any contaminants into the CMA.

The long term effects of the proposed reclamation involves the permanent loss of 1.4 hectares of harbour area, the impact of this is negligible in regard to overall coastal processes. There would be local effects associated with additional sedimentation behind the breakwater. Mitigation is likely to involve monitoring and maintenance dredging if required. The project provides mitigation measures including remediation of the southern foreshore in the area of the existing Mangere Bridge by removal of debris and provision of landscaping as outlined in 3.1.4.

Section 5 covers the overall mitigation measures proposed.

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3.4 Structures over Tararata Creek

3.4.1 Proposed Works

It is proposed to widen the existing Tararata Creek Bridge by constructing a separate bridge constructed to accommodate the new motorway off ramp. The existing SH20 Tararata Creek Bridge comprises three spans made continuous over the piers with extensive rock fill and revetment work on the stream banks. The bridge widening will require extension of the rock fill and revetment in the stream inter-tidal zone. Foundations will be driven or bored concrete piles to match existing. The widened bridge structure will meet the minimum flood level clearance requirements over the Tararata Creek.

A new, separate bridge is required for the new Walmsley Road southbound off ramp alignment. In order to avoid extending the rock fill revetment for the new bridge abutments into the Tararata Creek intertidal zone, a 70 metre long bridge structure is proposed.

The proposed offramp bridge over Tararata Creek can be either a four span bridge with the central span pier piles aligned with the pier piles of the existing Tararata Creek Bridge or a three span bridge with a long central span over the tidal channel section. Single column piers on bored pile foundations are likely. This bridge structure will meet the minimum flood level clearance requirements over the Tararata Creek. The pier/pile arrangements will need to be confirmed during the design phase of the project.

3.4.2 Long Term Impacts

The proposed structures are located in a very low-energy estuarine environment as there is only a minor area of estuary upstream of the Tararata Creek Bridge. The bridge site is therefore on the edge of, rather than within, the estuary hydraulics and there will not be any long term impacts on coastal processes.

3.4.3 Construction Impacts

Construction of the foundations and the reinforced concrete pier columns and cap beams for both the new Walmsley Road Offramp bridge and the widening of the existing Tararata Creek Bridge will be carried out from temporary staging. This temporary staging, to access each pier position will be provided by temporary jetties from the foreshore at each end of the bridge site. These temporary jetties will be supported on temporary piles driven progressively as the jetty is extended from the creek bank. The temporary jetties will also be used for lifting the precast, prestressed concrete beams into position. The temporary jetties will also need to be extended under the existing bridge to allow the central bridge beams to be strengthened.

Construction impacts are likely to be similar to the Duplicate Bridge in regard to piling operations. Demolition operations will be required to remove the existing deck edge. Demolition debris will need to be contained to minimise discharge to the water.

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3.4.4 Mitigation

Mitigation measures for construction effects include appropriate construction practices and procedures. e.g. carrying out the most disturbing work at lower tide levels on the drying mud flats. The long term effects of the proposed embankment are negligible in regard to coastal processes. Section 5 covers the overall mitigation measures proposed.

4 Related Coastal Issues

4.1 Navigation and Port Requirements

The design of the existing Mangere Bridge allowed for the then proposed eastward extension of the Onehunga wharf, beneath and beyond the bridge. For this reason the existing bridge has an elevated deck and two longer than standard centre spans to provide navigation and operational clearances for the extended wharf. The proposed extension at Onehunga Wharf has not been carried out. Ports of Auckland have advised that they have no immediate plans to extend the Onehunga Wharf eastward and no requirements for shipping access under the new bridge. Discussions with the Ports of Auckland were undertaken in 2003, 2005 and are ongoing.

There is therefore no navigation, ship impact, or other shipping requirements to be taken account of in the design of the duplicate bridge. The only vessels passing under the bridge will be small pleasure craft, which will not cause significant collision impact loads on the bridge substructure. The proposed Mangere Footbridge will, however, require protection from ship impact.

The piers of the Duplicate Bridge are to be placed in zones to avoid obstructing the long spans of the existing bridge and so minimising the impact on access under the bridges. The Harbourmaster for the Harbours of Auckland noted during discussions in 2003 that the original navigation clearance for Mangere Bridge was no longer required. This was discussed further during ongoing consultation in April 2006.

There may be localised changes in current flows at the piers which could affect manoeuvrability of small boats. The model testing will assist in design refinement during the design phase to mitigate such effects.

4.2 Sea Level Rise Implications

Global sea level has been rising at a rate of 1-2mm per year over the last 100 years. The rise relative to a land mass such as New Zealand, and particularly specific locations on the coastline, depends on a number of factors that produce upward or downward movement of the land itself. However the above range is consistent with analysis of relative rise at various locations on the New Zealand coastline.

The rate of rise is expected, under most evaluations of the chain of events and processes involved, to increase. The Intergovernmental Panel on Climate Change's (IPCC's) Third

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Assessment Report - "Climate Change 2001: The Scientific Basis" (IPCC 2001) used many greenhouse gas emission scenarios in various climate models to predict the amount of sea level rise over the next century. A sea level rise of 0.09 to 0.88 m is projected from 1990 to 2100 (Table 4.1).

Low (m)

Mid (m)

High (m)

2040 .03 .12 .30

2100 .09 .48 .88

Table 4.1: Total predicted global MSL rise for 2040 and 2100 (in cm) (IPCC, 2001).

The above figures represent the best estimates given in the majority of scientific publications. However, a number of scientists feel that low rises are more likely as all factors have not been included in the models, whereas others consider that sudden disastrous changes could take place leading to rises of many metres.

It is difficult to determine the allowance that should be made in planning and design for future rise. In particular it is difficult to strike a balance between additional capital expenditure to be incurred now, that might turn out to have been unnecessary, compared with leaving the expenditure until precise requirements are better understood.

The reclamations included in this project should be left in the latter category. These areas are part of a large coastline over the whole estuary (and around the country) which will all be affected by the same sea level rise. Impacts on the project reclamations will be a minor component of those on the whole of the surrounding coastline, and the possible need to re-route transport links and access to the coast. At the same time, reclamations have the flexibility to be raised at any time by increasing platform level (with fill) and increasing the armour height on the sea face. This is the approach that would be taken now if a greater allowance for future rises was to be required, increasing the levels above the adequate freeboard that the coastline now has against flooding. Instead, it would be much less costly and more appropriate for design to deal with changes as they occur.

Accordingly the Orpheus Drive reclamation simply ties in with the existing Orpheus Drive level, and varies between 3.4m and 3.6m reduced level (RL). This compares with a Mean High Water Spring level of 2.05m RL and a highest recorded tide level at Onehunga of 2.74m RL. The freeboard is adequate for current conditions. The level of the roadway itself can be adjusted in future years by resurfacing without milling out the existing pavement. The reclamation revetments can be raised by adding to the rock layers.

The new bridges, which are expected to have a 100-year life, require a different approach. The motorway level below Rimu Road Interchange Bridge is at a low point. A vertical

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clearance of 5.4m is proposed for the replacement structure. This can allow for a further 300mm increase in motorway level to accommodate sea level rise before the limiting clearance of 5.1m is reached. The freeboard under the New Mangere Footbridge will be slightly higher than that of the existing Old Mangere Bridge to allow for long term sea level rise. The values in Table 4.1 will be considered in setting bridge levels in the design stage.

5 Proposed Mitigation The following mitigations measures are proposed:

Construction Works:

• Where the Project passes through the CMA construction impacts shall be minimised;

• Utilise designs that incorporate construction methods that minimise the potential impact upon the environment;

• Remove all spoil from excavations for bridge piers and reclamation foundations from the CMA;

• Restore the seabed to natural levels when temporary reclamations are removed;

• Minimise the use of construction machinery outside the footprint of works that are constructed within the CMA;

• The access areas shall be constructed and located so as to minimise the impact on saline vegetation where practicable;

• Any works within or adjacent to the CMA likely to generate suspended sediment shall be monitored for the spread of sediment. Visual detection of widespread additional turbidity more than 200m outside the construction area will be a trigger point for the temporary cessation of work or for changing to better work practices. These include in addition to normal best work practice; carrying out the most disturbing work at lower tide levels on the drying mudflats; and

• Within 6 months of completion of the works, any temporary structures, and equipment shall be removed and the foreshore and seabed reinstated.

Permanent Works:

• Remediation of the southern foreshore in the area of the existing Mangere Bridge by removal of debris and provision of landscaping to establish the existing foreshore;

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• Removal of the temporary reclamation at the north end of the Duplicate Bridge to MSL only in order to provide an environment for the adjacent salt water marsh to extend on to.

• In regard to the Duplicate Bridge and Mangere footbridge, a numerical model study of tidal flows under the bridges, including the ‘before’ and ‘after new bridge’ situation and tidal exchange in the Mangere Inlet, shall be carried out during the design phase. This shall include bathymetric information of the existing seabed and future sea bed levels. This will provide input into finalising the layout and shape of the pier foundations to mitigate localised long term effects;

• In regard to the proposed reclamations, the design phase will need to finalise layouts and ensure that the minimum amount of land is reclaimed both for temporary and permanent reclamations and shall limit the extent of the proposed temporary reclamation adjacent to the northern end of the proposed Duplicate Bridge to avoid an adjacent area of high intrinsic value. Suitable fencing shall be provided to avoid the encroachment of construction plant onto this area;

• The project should provide mitigation measures for long term effects by including remediation of the southern foreshore in the area of the existing Mangere Bridge by removal of debris and provision of landscaping. Also the temporary reclamation south of Onehunga Harbour Road is proposed to become an extension of the adjacent salt marsh after construction; and

• Use reclamation fill that does not release any contaminants into the CMA.

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6 Conclusions

The following conclusions are made in regard to coastal processes associated with the Project:

• In regard to the Duplicate Bridge, there is unlikely to be a shielding effect from pier alignment but the pier layout and shape should nevertheless be designed to provide the least total obstructive face area from hydraulic considerations.

• As a result, adopting the pier arrangements of the existing bridge for the new bridge has no advantage in terms of coastal processes.

• The combined facing areas of the foundations of each bridge in the water will present a larger obstacle to the tidal flow than the existing bridge only. However the existing bridges long spans and the main channel will not be further obstructed by the duplicate bridge.

• There will be some effect on tidal flows. However it is most unlikely that this will affect the flushing process in the eastern basin, and will therefore not lead to reduced water quality. The more likely local effect will be turbulence around the piers.

• Mangere Footbridge and fendering structure will not have any significant impact on tidal flows, and hence on the flushing process.

• Construction impacts are likely to be limited to minor disturbance of the seabed while boring or driving the piles.

• In regard to proposed reclamations, there will be a very minor effect on tidal flows behind the breakwater. The observed sedimentation behind the breakwater will have resulted from the shelter the breakwater has provided.

• The reclamations all take place against previously reclaimed land areas and edge protection. They therefore alter already-modified coastal boundaries. It is not expected that there will be any change in wave action or current flows at the coastline as a result of these works.

7 Recommendations

The following recommendations are made in regard to coastal processes associated with the Project and will be incorporated into the Project:

• In regard to the Duplicate Bridge and replacement footbridge, a numerical model study of tidal flows under the bridges, including the ‘before’ and ‘after new bridge’ situation and tidal exchange in the Mangere Inlet, be carried out during the design

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phase. This will provide input into finalising the layout and shape of the pier foundations to mitigate localised long term effects.

• A photo record should be made of the upper estuary before and after construction as a coastal baseline record.

• In regard to the proposed reclamations, the design phase will need to finalise layouts of the permanent reclamation and breakwater alterations. Analysis should be undertaken during the design phase to refine the reclamation shape, determine if the reclamation will result in further sediment build up and determine monitoring and maintenance dredging requirements.

• Mitigation measures for construction effects should include appropriate construction practices and procedures, as outlined in section 5. These work practices and procedures are documented in Volume 2, Technical Appendix 19, Draft Environmental Management Plan.

• The project should provide mitigation measures for long term effects by including remediation of the southern foreshore in the area of the existing Mangere Bridge by removal of debris and provision of landscaping. Also, the temporary reclamation south of Onehunga Harbour Road is proposed to become an extension of the adjacent salt marsh after construction.

• It is proposed to carry out work with the most disturbance at lower tide levels on the drying mudflats. If this is not practicable, work producing a significant sediment plume will cease at the middle 2-3 hours of high tide and, where necessary, silt screen barriers shall be provided.

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8 References

R. G. Bell, S. V. Dumnov, B. L. Williams, M. K. N. Greig, Hydrodynamics of Manukau Harbour, New Zealand, New Zealand Journal of Marine and Freshwater Research, Vol. 32: 81-100, 1998.

R. G. Bell and A. K. Senior, Upper Waitemata Harbour Bridge Duplication Investigations: Effects on the Coastal Marine Area, NIWA Client report CWL01201/1, December 2000

R. A. Heath, A Review of the Physical Oceanography of the Seas Around New Zealand, New Zealand Journal of Marine and Freshwater Research, Vol. 19: 79-124, 1985

Texas Department of Transportation, Hydraulic Design Manual parallel bridge backwater adjustment, March 2004.

IPCC (2001) Climate Change 2001: The Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.

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APPENDIX A

Photographs

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Photograph 1 : Existing Mangere Bridge looking south from the north abutment

Photograph 2 : Mangere Bridge looking north from south abutment on line of proposed duplicate bridge

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Photograph 3 : Old Mangere Bridge looking south from the Mangere Bridge Walkway

Photograph 4 : Looking northwest along Orpheus Drive to the Manukau Cruising Club, across the area of proposed reclamation. This will extend up to 41m from the existing foreshore, approximately in line with the end

of the breakwater at its widest.

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Photograph 5 : Tararata Creek looking east across the motorway bridge

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Photograph 6: Southern foreshore from Mangere Bridge looking West.

Photograph 7: Looking towards South Abutment of Mangere bridge from reclamation

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Photograph 8 : Southern Abutment of Mangere Bridge showing exposed materials and debris