Wi Neera to Onepoto Shared Pathway & Coastal Resilience Project

Hello

This is a quick update to let you know how this project is progressing.

We held a workshop with councillors last week (19 November) where we sought direction on a recommended option for the shared pathway and coastal resilience works, and possible funding approaches.

Based on feedback from councillors, the project team is now working to further develop a recommended option that will be presented to Council for approval at the next meeting on 16 December.

At that stage, we look forward to being able to share more information with you.

Thanks for your patience as we work through all the issues.

Update - Wi Neera to Onepoto Shared Pathway and Coastal Resilience project

Kia ora

In our last update to you we said we hoped to present Councillors with two preliminary concept options for the Wi Neera to Onepoto Shared Pathway and Coastal Resilience Project at a workshop in October 2020.  

However work done to develop and cost those options has made it clear they would exceed the currently available budget of $3.6m.  This work has helped us understand the full cost of the coastal resilience work, which is similar to that of the pathway development, and has understandably changed since we drafted budgets for the works some years ago.  

As a result, the project team, including Ngāti Toa, will now regroup to review the two preliminary concept options, alongside other possible options, which will help ensure we:

• develop the best integrated pathway and coastal resilience solution that delivers against all the objectives and requirements, and presents value for money for the City,
• meets stakeholder and community expectations, and
• meets Waka Kotahi (NZTA) co-funding requirements.

This means, we have had to readjust our planned timeframes and will now workshop a preferred option/options with Councillors at a later date and not October as originally planned.

Following Council input we will be inviting feedback from stakeholders and the community on aspects of the proposal before final design decisions are made and consent is sought.

It’s important to stress that feedback received during the life of this project has helped us shape the preliminary concept options.  The design is being guided by the themes raised during engagement with Ngāti Toa, community consultation in 2016, in feedback to the Long Term Plan, and the more recent 2019 community workshop.   

These themes include:

• A harbour edge shared pathway (as favoured by 75% of people who gave feedback on the project as part of the LTP consultation)
• Recognition that the harbour has special value and is a priority
• A preference for a ‘soft’ harbour edge resilience/restoration solution - to protect the vulnerable coastal edge 
• The need to protect and improve current harbour /harbour edge habitat and not encroach any more than necessary
• A preference to retain the Pohutakawa trees if possible
• Accessibility – a shared pathway that is suitable for all ages and all abilities
• Seating, and opportunities for pause/stopping points
• Opportunities to improve connections to the harbour
• The need for good stormwater management and filtration, and to ensure the work does not worsen flooding within Takapuwahia
• Consideration of parking and lighting-related needs
• Beach creation at Wi Neera, waka ama access and opportunities to support regatta and other water sport events
• The need for a good safety zone/separation between traffic and shared pathway users
• The need for the design to consider the safety of all users travelling along and across the corridor  
• A desire for meaningful opportunities to have a say and provide feedback on the draft proposal.

We’re aware that this project has been ongoing for some time, and we appreciate your patience as we work through the challenging and complex issues it is presenting us with.  Projects next to the sea are often complex and in this situation that is no different.  It’s really important that we get this right, by coming up with best solutions and options for the city.

We hope to have more clarity around next steps and timings in the next few months and will keep you updated.

This update (30 July, 2020) is to let you know how the Wi Neera Drive to Onepoto Shared Pathway and Coastal Resilience Project is progressing.

We are now well on the way to preparing the information required for shaping the best design solution, and the consenting process, for this project along Titahi Bay Road and Wi Neera Drive.

The project includes construction of a shared cycling and pedestrian pathway between Wi Neera Drive and Onepoto, and erosion protection works to enhance coastal resilience along the coastal edge of Titahi Bay Road and Wi Neera Drive.   

An experienced team has been assembled to ensure the design solution is appropriate for the area and to also understand what can be done to improve habitat quality within the inner harbour as a result of the project.

The design required for a comprehensive consenting process is significantly more complex than originally envisaged based on all the previous harbour-related works, and includes investigations into coastal processes, rates of erosion and sedimentation, tidal and sub-tidal ecology, and stormwater management.  

On the foreshore, leading experts in coastal soft engineering and managing the effects of sea level changes, are investigating how coastal resilience designs can work to protect the harbour edge using bathymetry (seafloor) data, wave modelling and ecological analysis just offshore.  Onshore, engineering specialists are investigating possible options for the pathway – and this will draw from the results of geotechnical investigations to help ensure the design will be resilient into the future.

Two preliminary design options are currently being developed and costed.  These will be assessed at a workshop with Porirua City councillors and a preferred option identified for Council consideration and approval in October.  Confirmation of a preferred option will clarify the project scope for the concept design and consenting stages that will follow.

The team is working closely with Ngāti Toa (who are supportive of the project and will have a representative on the project team) and we will also be inviting feedback from stakeholders and the public on aspects of the proposal before final design decisions are made.

Consenting is planned for 2020-21.

Once completed, it will create a unique, naturally vegetated coastal margin that will help improve the health of the harbour, establish a stunning natural landscape for residents in Takapūwāhia, Elsdon and Titahi Bay, and connect our city.  The path will allow for coastal resilience, interaction with the harbour, and become a link between recreational activities on Wi Neera Beach and Onepoto Park. 

We appreciate your ongoing patience and interest as we work through this complex project. We will provide ongoing updates as the project develops.

• Option 1 – Narrow road lanes + harbourside path + coastal resilience + Wi Neera Beach
• Option 2 – Wide option (retaining the existing road lanes) + harbourside path + coastal resilience + Wi Neera Beach
• Option 3 – Value Engineered version of Option 1 + coastal resilience + Wi Neera Beach
• Option 4 – Project ‘Do Minimum’ – no path + only essential erosion repairs + Wi Neera Beach
• Option 5 – Value Engineered harbourside path + only essential erosion repairs + Wi Neera Beach (updated and re-costed from a 2009 feasibility report)
• Option 6 – Inland path + only essential erosion repairs + Wi Neera Beach (updated and re-costed from a 2009 feasibility report)
• Option 7 – Project ‘Do Nothing’ = Project Closes.

These options were assessed and presented to Councillors at a workshop on 19 November 2020.

The Council direction at the workshop was to ask staff to provide further information on Option 1 for consideration at a Council meeting in December 2020.

Option 1 will:

• Deliver on long-standing commitments to improve walking and cycling access from the CBD to Titahi Bay with a coordinated, quality, long-term solution
• Repair historical and current road edge erosion
• Restore the landscape quality of the harbour edge and greatly improve ecological values within the inner harbour
• Improve future resilience against erosion and the effects of sea level rise associated with climate change.

This option currently has the support of our project partners – Ngāti Toa and Waka Kotahi NZ Transport Agency (and meets Waka Kotahi co-funding requirements).

• Shared path – a 3m wide concrete shared path.
• Shoulder – a 1.2m wide shoulder to cater for on-road cyclists travelling at speed and provide space for broken down vehicles to pull over, clear of the traffic lane.
• Buffer – a 0.5m level surface between the edge of the shared pathway and the shoulder that caters for cyclists who stray off the shared path.
• Median strip– a 0.5m wide painted area to provide a separation between the north and south-bound lanes, as well as space for motorists to pass cyclists.
• Narrowed lanes – 3.2m wide traffic lanes (allowing the pathway to be located on the landward side of the Pohutakawa trees).
• Speed limit reduction to 50km/h along the full length – removing the 70km/h limits that currently apply in some sections. This will increase motorists’ travel times by approximately 20 seconds per trip.
• Crossing points – three crossing points (with raised cycle/pedestrian islands) to facilitate safe access to and from the pathway across Titahi Bay Road.

• ‘Soft’ planted harbour edge – approximately 62% of the sloping coastal edge alongside Titahi Bay Road will be planted with coastal species naturally occurring around Onepoto.
• Saltmarsh – 8 – 9m wide sections of saltmarsh will be planted in a combination of oioi (which is mostly seen in the drier areas of mudflats) and sea rush (which lives where there is around four hours of seawater at high tide).
• Low rock “sill” or reefs – Located just off-shore (alongside the outer edge of the saltmarsh) low rock sills will make the shoreline more resilient by dissipating wave energy, and will also protect the neighbouring saltmarsh in the early years.
• Stormwater management and filtration – stormwater will sheet across the shared pathway and down the sloping coastal edge, into the saltmarsh which will filter and reduce sediment, heavy metals and other contaminants going into the harbour.
• ‘Hard’ (rock) harbour edge – the remaining 38% of the harbour edge will be rock armoured to protect the patches of seagrass within the inner harbour.
• Restored gravel beach – alongside Wi Neera Reserve.

• A stakeholder workshop in mid-2016 to identify project benefits and build a case for investment – resulting in development of an Investment Logic Map;
• Stakeholder and community consultation in late 2016-early 2017 on a proposal for a shared pathway and erosion protection (including ‘hard’ rock rip-rap) for the harbour edge alongside Titahi Bay Road;
• Consultation on the 2018-38 Long Term Plan seeking feedback on various pathway and coastal protection options with 73% of submitters expressing a preference to complete a pathway on the harbour-side and address the existing harbour-side erosion;
• Workshops with the community, stakeholders, local schools and Elected Members in early 2019 outlining, and seeking feedback on, a broad concept for the shared pathway and ‘soft’ coastal protection works along the harbour edge.
• Ngāti Toa engagement. Throughout the development of this project we have worked with Ngāti Toa, and a Ngāti Toa representative now sits on the project team.

People asked for:

Pathway:

• Harbour edge pathway (Submitters to the draft Long-Term Plan 2018-38 supported a harbour-side pathway and erosion repairs along the harbour edge)
• Provide crossing points (and access) onto the pathway along its length, e.g. at Te Pene Ave
• Provide a safety zone/separation between traffic and shared pathway users
• Consider cyclist and pedestrian safety in the design and potentially reduce traffic speeds
• Accessible for all ages and abilities

Harbour:

• The harbour has special value and is a priority (with the project)
• Enhance the mana of Te Awarua-o-Porirua
• Preference for a 'soft', resilient harbour edge solution - to protect the vulnerable coastal edge
• Minimise encroachment into the harbour, protect seagrass and kaimoana beds, improve harbour/harbour edge habitat
• Opportunity to improve stormwater management and filtration
• Ensure the work doesn't worsen flooding within Takapuwahia

Amenities:

• Improve connections to the harbour
• Retain the Pohutakawa trees if possible
• Consider beach creation at Wi Neera and opportunities to improve access for waka ama and other water-based events
• Consider parking and lighting-related needs, seating and pause/stopping points

Other:

• A desire for meaningful opportunities to have a say and provide feedback on the draft proposal.

The Onepoto Arm of Porirua Harbour has been extensively modified over the years. Almost all the present Onepoto harbour margin is reclaimed. The original coastline is 60-150 metres inland of the current coastline which was reclaimed during the 1950s. The rest of the Onepoto harbour arm has also been modified particularly with the development of SH1 and the railway lines, which have an ongoing impact on the health of the harbour.

The image below shows Titahi Bay road prior to reclamation

The image below shows the harbour margin pre and post the 1950s reclamation work

• Increased sediment and contaminant run-off into the harbour
• Eliminated naturally occurring coastal and freshwater wetlands
• Resulted in most small streams being culverted (piped)
• Eliminated all natural shorelines and nearshore ecosystems around most of the harbour.

• There is ongoing decline of seagrass across the harbour. Just 2% of the original seagrass beds remain in the Onepoto area in 3 small patches along the Titahi Bay Road coastline (see seagrass maps below).
• Seagrass and salt marsh vegetation are important habitats in estuaries, providing a range of ecosystem services including sediment stabilisation, erosion mitigation, elevated biodiversity and nursery and/or feeding grounds for a range of invertebrates, fish and birds.

The images below show the current state of the seagrass beds

• The 1950s reclamation along the harbour edge is eroding slowly, particularly during major storms, and this is resulting in 1.5 – 2m high erosion escarpments (see images below)
• The Council has been looking for a way to address the erosion.
• Rates of erosion along the harbour edge differ due to different types of reclamation material and wave velocity.
• A recent harbour edge erosion assessment has shown that erosion repairs are required to approximately 44% of the harbour edge at some point within the next 1 – 10 years. The worst-affected areas are at the Onepoto end, south of Te Pene Ave and adjacent to Takapuwahia Reserve & Te Hiko St.
• We remain just one storm away from further significant damage to the harbour edge.

The images below show the current erosion issues along the Titahi Bay Road foreshore

Localised and ongoing erosion mitigation works will be required to repair and protect the road if we don't implement the preferred option.

Given the current erosion damage, rock armour protection would need to be installed along approximately 511m of the coast where the current bank edge is within 2m of the Titahi Bay Road edge.

Ongoing maintenance would be required along the length of the harbour edge, and it would be just one storm away from substantial, further damage that could potentially undermine the road.

Hard patch repairs could have a flow-on impact leading to erosion in neighbouring areas. This is known as ‘end effects’.

Installing rock rip rap would not provide any ecological benefits for the harbour and would not allow for the reinstatement of nearshore habitats.

Resource consent and coastal permits would be required from Greater Wellington Regional Council for reclamation and discharge and ongoing repairs over time.

Example of ‘patch’ erosion repairs

Hard coastal engineering would protect the shoreline from the effects of erosion and sea level rise and would be long lasting.

Ongoing repairs could be required to the seawall following storm events.

Hard engineering would not provide any ecological benefits for the harbour and would not allow for the reinstatement of nearshore habitats.

The ease of consenting would depend on the footprint/reclamation required and a detailed assessment of the proposal. Hard engineering does present a more challenging consenting pathway (than for soft engineering). It is noted that both the proposed Porirua City District Plan and GWRC plans favour soft engineering (rather than hard) as a primary method to reduce damage from sea level rise and coastal erosion, protect coastal margins and preserve natural character.

Hard engineering works are harder to stage and in the longer term more costly.

The imagae below shows an example of harbour edge hard engineering alongside Portobello Road in Dunedin

The diagram below shows a typical cross section of a hard engineering approach

Soft engineering in coastal areas is used as a cost-effective way to protect shorelines from erosion and the effects of sea level rise associated with climate change. It also reduces the effects of flooding on marine environments and helps increase harbour health.

Small rock reefs (chenier) are placed just offshore and then saltmarsh is planted in between the shore and the reef. Natural examples can be seen around Pauatahanui Inlet where natural shell banks create the “rock reefs”. The rock reef and the saltmarsh dissipate wave energy. The larger the saltmarsh, the greater the wave energy dissipation. The rock reef is also called a sill.

The coastline along Titahi Bay Road is ideal for this type of coastal engineering (which has been used successfully around New Zealand and has worked since the early 1990’s). See more information about the benefits of soft engineering in the FAQ section below.

The advantages of this approach are that it creates a natural harbour margin that would protect the shoreline from the impacts of sea level rise and storm damage. It would also stabilise sediment entering the harbour and create habitat and be a food source for shellfish, fish, invertebrates and birds.

While more expensive to construct initially than patch repairs, it would cost about the same as hard engineering upfront. Longer term it would be cheaper to maintain, with repairs possibly only required after heavy storms. The ease of consenting would depend on the footprint/reclamation required. This approach could be staged.

The image below shows the shoreline of Pauatahanui Inlet, which is protected by soft coastal engineering - a wetland behind a chenier (a sandy beach ridge)

The image below shows a typical cross section of a soft engineering approach

Early consultation

Options development

• Ecological assessments
• Transport and road safety assessments
• Engineering assessments
• Options development and analysis (Multi criteria assessment)

Information on options for Council

Preliminary concept design and cost estimates for Council decision

Next steps:

• Stakeholder and community engagement on the preliminary concept design through to 24 February 2021 – we are at this stage
• Landscape concept plan - 2021
• Consent-level design for the preferred option- 2021
• Technical assessments (including Cultural Impact Assessment) - 2021
• Resource consent applications - 2021
• Applications for external funding - 2021/22

How many options have been considered for this project?

Seven options have been assessed for this project, including:

• Option 1 – Narrow road lanes + harbourside path + coastal resilience + Wi Neera Beach
• Option 2 – Wide option (retaining the existing road lanes) + harbourside path + coastal resilience + Wi Neera Beach
• Option 3 – Value Engineered version of Option 1 + coastal resilience + Wi Neera Beach
• Option 4 – Project ‘Do Minimum’ – no path + only essential erosion repairs + Wi Neera Beach
• Option 5 – Value Engineered harbourside path + only essential erosion repairs + Wi Neera Beach (updated and re-costed from a 2009 feasibility report)
• Option 6 – Inland path + only essential erosion repairs + Wi Neera Beach (updated and re-costed from a 2009 feasibility report)
• Option 7 – Project ‘Do Nothing’ = Project Closes.

The options were assessed through a two-step Multi Criteria Assessment process (explained in more detail below).

MCA is a tool to assist and document decision-making where there are multiple options.

It helps carry out a comparative analysis of options (against consistently applied criteria) to decide which best meets a range of objectives set by stakeholders and the community for a project.

The MCA criteria were based on desired project outcomes, Waka Kotahi investment objectives, and community feedback from earlier engagement – and these were reviewed by Waka Kotahi and Ngāti Toa representatives. The assessment was carried out in two steps including:

• An initial ‘sieve’ – where all seven options were assessed against 8 broad criteria, and
• A detailed assessment of the two highest scoring options (Options 1 and 3) and the project ‘Do Minimum’ option (option 4) against 14 detailed criteria.

Options 1 and 3 scored the highest against the initial ‘sieve’ criteria (step 1) and progressed to step 2 alongside the project ‘Do Minimum’ option (option 4 - which included only essential erosion repairs, rather than full coastal resilience). Option 1 (the preferred option) was the highest scoring option in the detailed assessment (step 2), followed by Option 3 and option 4.

The Council decided to proceed with Option 1 (the preferred option). Council believe this is the best solution to deliver on the project objectives and will also:

• Deliver on long-standing commitments to improve walking and cycling access from the CBD to Titahi Bay with a coordinated, quality, long-term solution
• Repair historical and current road edge erosion
• Restore the landscape quality of the harbour edge and greatly improve ecological values within the inner harbour
• Improve future resilience against erosion and the effects of sea level rise associated with climate change.

The option also has the support of our project partners – Ngāti Toa and Waka Kotahi (and meets Waka Kotahi co-funding requirements).

A preliminary concept design is a more detailed look at an option after all options have been looked at by stakeholders, experts and funders.

At this point, the preliminary concept design (and initial cost estimate) includes the basic elements for the shared pathway and coastal resilience, such as:

• A 3m wide concrete pathway
• A 1.2m (narrowed) traffic lanes
• A reduction in traffic speeds to 50kmph along the length of Titahi Bay Road
• A 1.2m shoulder and a 0.5m buffer between the southbound lane and the shared pathway
• Three crossing points
• Coastal resilience works including:
  • A ‘soft’ planted harbour edge alongside 62% of the Titahi Bay Road edge
  • A ‘hard’ (rock) coastal edge in areas where there is seagrass in close proximity to the foreshore
  • Saltmarsh wetland
  • Low rock sills (or reefs)
  • A restored (gravel) beach along Wi Neera Drive
• Improved stormwater management and filtration
• Improved lighting

‘Nice to have’ elements like seating and pause points (areas with signage or a seat for users to take a break) aren’t included at this point as we’ve tried to keep the costs down and don’t yet know how much external funding we’ll be able to source. These elements will be provided for as we develop the landscape concept and work through the detailed design processes in order that they can be added in future, as funds permit.

There is no provision for parking along the length of the shared pathway. Users would be required to park at either end, alongside Wi Neera Reserve, or on the west side of Titahi Bay Road.

While some aspects of the concept (such as the harbour-side route, pathway width and ‘soft’ coastal resilience) are unlikely to change, there’s more work to be done on other aspects such as the buffer zone (and treatments within that area), the location of crossing points, and lighting (through the developed design stage). Seating, cultural elements and pause points will be provided for as we develop the landscape concept plan and work through the detailed design process so they can be added later as funds permit.

We’ll progressively refine the details as we move through the resource consent and design processes and will be able to share more details and invite feedback on those elements in due course.

A shared pathway is a widened footpath (separated from the road) that may be used by pedestrians, cyclists, riders of mobility devices and riders of other non-motorised devices, at the same time.

Value engineering is a systematic, organised approach to providing necessary functions in a project at the lowest cost. It promotes the substitution of materials and methods with less expensive alternatives, without sacrificing functionality. That is – it’s a way of finding cheaper ways to achieve the same project outcome.

Current erosion damage is such that rock armour protection would need to be installed along approximately 511m of the project site where the current bank edge is within 2m of the Titahi Bay Road edge.

While ‘patch’ repairs would be cheaper initially, ongoing maintenance would be required along the length of the harbour edge, and it would remain just one storm away from substantial, further damage that could potentially undermine the road. ‘Hard’ patch repairs could also have a flow-on impact leading to erosion in neighbouring areas – known as ‘end effects’.

Four of the seven options we considered included essential erosion repairs (i.e. patching) to the harbour edge, instead of coastal resilience. As these options did not score well against the MCA criteria (or the other options) they didn’t proceed beyond the stage one initial ‘sieve’.

It’s also noted that this approach would not provide any ecological benefits for the harbour or allow for the reinstatement of nearshore habitats.

The recurring stumbling block has been the mis match between cost and available budgets.

The current (inflation-adjusted) budget in the LTP 2018-38 is $3.6m. (This includes $3.2m for the shared pathway and coastal resilience along Titahi Bay Road and $0.4m for Wi Neera Beach.)

The preferred option has an estimated capital construction cost of $10.7m (including 30% contingency).

Estimated construction costs for the key elements are: $4m for the pathway, $6.1m for coastal resilience alongside Titahi Bay Road, and $0.6m for Wi Neera Beach improvements.

The final cost won’t be known until we have obtained resource consent, completed detailed designs and expert costing reviews, and we have a preferred tenderer.

Council’s contribution of $3.6m to the project, will be considered as part of the Long-Term Plan. Further funding is being sought from Waka Kotahi NZ Transport Agency ($3.9m) and external funding sources ($3.2m). Applications will start in the new year.

We have a commitment in principal from Waka Kotahi. Further funding sources will be investigated over the next 12 to 18 months.

Yes. While the $10.7m does include 30% contingency, the cost estimates are preliminary and based on preliminary concept drawings. These costings will require an expert costing review, and at a later stage we may seek contractor input to explore potential construction-related efficiencies. The true final cost won’t be known until the tender for construction closes.

We have a Funding Plan which includes how we aim to request additional funding from other funding organisations.

If Waka Kotahi and external funding can’t be secured to meet the budget shortfall, the project will be closed, and the capital funding returned to the Transport renewal programme, and to City Partnerships to complete Wi Neera Beach (subject to resource consent). Coastal erosion issues along Titahi Bay Road would be managed through the ‘business as usual’ Transport maintenance programme.

Waka Kotahi (NZ Transport Agency) is one of our project partners and has been closely involved in, and helped shape, the project to date.

This project is stage 2 of the 4-stage shared pathway project (between the Porirua CBD and Titahi Bay) endorsed by Waka Kotahi in 2019. We are working closely with our Waka Kotahi Investment Advisor through the concept design process to further maximise the funding opportunities available towards the developed design and capital works stages of the project.

To date, Waka Kotahi has been very supportive of the harbour-side pathway proposal, along with the coastal resilience works to restore the harbour edge and protect the road.

Yes, feedback received over the years from Ngāti Toa, the community and other stakeholders has helped shape the preliminary concept design for the preferred option.

Yes, we have undertaken a safety audit of the preliminary concept design for the preferred option to ensure it is safe for all users. The traffic speed on Titahi Bay Road would also be dropped to 50kmph along the full length to improve safety and reduce the risk to people crossing the road and users of the shared path.

Feedback received on this project during community consultation on the 2018-38 Long Term Plan showed a clear and strong community desire for a cycle and pedestrian pathway linking Titahi Bay – with 73% of submitters on the project in Long-Term Plan expressing a preference for a harbour-side pathway.

Yes. One of the seven options assessed through the MCA process included a pathway on the landward side of Titahi Bay Road, with essential erosion repairs along the harbour edge. This option didn’t score as highly as other options through the MCA process and wasn’t progressed beyond the first (initial ‘sieve’) stage.

An audit of the trees along this stretch of Titahi Bay Road has revealed that some are diseased and will need to be removed – however our aim is to keep as many trees as possible.

Soft engineering in coastal areas is used as a cost-effective way of protecting shorelines from erosion and the effects of sea level rise associated with climate change. It also reduces the effects of flooding on our marine environments and helps increase harbour health.

To make a shoreline resilient, we create small rock reefs or “sills” which are placed just offshore and then plant saltmarsh between the shore and the reef. The rock reef and the saltmarsh dissipate wave energy and protect the shoreline. The larger the area of saltmarsh, the greater the wave energy dissipation. Natural examples can be seen around Pauatahanui Inlet where shell banks create the “rock reefs”.

The low tidal mudflats along Titahi Bay Road are ideal for this type of coastal engineering (which has been used successfully around New Zealand and has worked since the early 1990’s).

The present day Titahi Bay Road shoreline was formed by reclamation in the 1950’s and in many areas lies 60-150m seaward of the original natural shore. Just 2% of the original saltmarsh area (which was once extensive) is left in the Onepoto arm of the harbour, two thirds of which is now rock armoured.

If a soft engineering approach is taken, harbour water quality would be improved through stormwater filtration by the saltmarsh created along the harbour edge. These saltmarshes would reduce sediment, heavy metals and other contaminants going into the harbour and provide a significant habitat for our bird species which are currently declining. Salt marsh is a large food source for our coastal ecosystems in Porirua.

Capturing sediment

The present day Titahi Bay Road shoreline was formed by reclamation in the 1950’s and in many areas lies 60-150m seaward of the original natural shore. Two thirds of the coast around our harbour is rock armoured. Just 2% of the saltmarsh is left in the Onepoto arm of the harbour.

By replacing rock with salt marsh we can capture sediment and stop it from damaging other parts of habitat in the harbour.

Salt marsh is also a large food source for our coastal neighbours in Porirua. Each year, over 400 tonnes of food is provided through the salt marsh to our coastal species in Pauatahanui Inlet. The saltmarsh in Onepoto will also provide a significant habitat for our bird species which are currently declining.

This length of saltmarsh means a massive improvement for harbour health.

Water sensitive design

For this project we are using water sensitive designs to manage water. What this means is using swales and saltmarsh (coastal wetlands) to slow down and treat stormwater before it gets into the harbour. Swales and wetlands are natural wastewater treatment plants.

Saltmarsh as a wetland treatment for contaminants

Contaminants like heavy metals and organic compounds from our cars and trucks are deposited on our roads and then washed into stormwater when it rains. The contaminants have a significant, detrimental impact on harbour health.

Saltmarsh is a coastal wetland. As a wetland, it can filter out contaminants in our stormwater. The marsh absorbs heavy metals and stores them, which means they are no longer deposited on the tidal mudflats where lots of our harbour life lives. The saltmarsh also slows down the effects of heavy rain and flooding caused by stormwater.

Grassy swales

Along the shoreline next to the pathway we may also put in swales which are u-shaped areas to hold and filter water. The swales can be either grass or wetland plants to filter out contaminants and slow down flooding.

The slopes next to the shared pathway will be planted with coastal species naturally occurring around Onepoto. The salt marsh formed will turn into a landscape similar to that along Grays road at Kakaho Estuary (see below). Pause points or viewing areas will allow people to see the saltmarsh close up and in time, the many types of coastal bird species that people enjoy around Pauatahanui Inlet. Two types of marsh plants can be used – oioi (which is a lovely golden brown colour and is mostly seen in drier areas of mudflats) and sea rush (which is a blue grey colour and lives where there is around four hours of seawater at high tide).

Pauatahanui estuary