Porirua Harbour: Understanding the Past to Secure a Healthy Future

Porirua Harbour has a rich history that dates back to the early 19th century, with the first European settlers arriving in the area in 1825.

The harbour was an important fishing ground for the local Māori iwi, who used it for centuries before European settlement.

Porirua Harbour's unique shape and geography make it susceptible to pollution and environmental degradation.

The harbour's water quality has been a concern for many years, with high levels of nitrogen and phosphorus contributing to the growth of toxic algae.

Porirua Harbour is a unique and fascinating body of water, with a geography that's both beautiful and fragile.

The harbour has a narrow entrance, just a few hundred metres wide, located near the suburb of Plimmerton.

Each arm of the harbour is approximately three kilometres long.

The Porirua Harbour has a fascinating history that dates back thousands of years. Approximately 10,000-14,000 years ago, the harbour formed when westward flowing rivers were drowned by rising postglacial sea levels.

The harbour's shoreline has undergone changes over the years, with one notable event being the 1855 Wairarapa Earthquake. However, according to George Eiby, the earthquake didn't significantly change the shoreline, with any changes likely coming from more recent sedimentation.

In the 20th century, the harbour underwent significant transformations with the construction of a causeway for the North Island Main Trunk railway. The new section of the Kapiti Line was opened on 7 November 1960, requiring a new Paremata Railway Station and bridge over the entrance to the Pāuatahanui Inlet.

The construction of the causeway and railway line led to the creation of three shallow lagoons on the land side of the new causeway. These lagoons were partially filled in when State Highway 1 was re-aligned and straightened in the 1970s to run alongside the rail line.

The catchment area of the Porirua Harbour is a significant part of the region's ecosystem. It includes all the areas where water flows into the harbour through streams, lagoons, and lakes. The Johnsonville, Churton Park, and Tawa catchments are all part of this larger area, with water flowing down Porirua Stream.

Greater Wellington has taken a collaborative approach to managing the catchment area through the Te Awarua-o-Porirua Whaitua. This involves working with a committee and other experts to develop proposals for land and water management. The goal is to set objectives and limits on what can be taken from or discharged into the Porirua Harbour waterways.

A team of researchers from throughout New Zealand is involved in the Whaitua process, including NIWA urban aquatic scientist Jonathan Moores and aquatic chemist Dr Jennifer Gadd. They're using a Bayesian Network model to assess the effects of different management methods and development scenarios on water quality and ecological attributes. This model will provide a comprehensive view of the impacts of policies recommended by the Whaitua Committee.

Back in 1888, Ngati Toa had the foresight to write a letter to the colonial government, warning that the harbour's health would soon decline as more and more settlers were arriving.

In the 1880s, the harbour was still in its pristine condition, but a decade later a hospital asylum was built, which resulted in waste being pumped into Porirua Stream. This had a devastating impact on the local community, with people dying of typhoid while gathering shellfish.

The construction of the hospital asylum marked the beginning of a long history of degradation for the harbour. The dumping of waste and other pollutants into the stream and harbour had a lasting impact on the environment and the people who relied on it.

In the 1950s, road construction straightened some of the coastline and truckloads of rocks and debris were dumped across flounder beds. This further degraded the harbour and its ecosystem.

The construction of the four-lane Transmission Gully motorway, new residential housing, and forestry plantations in the catchment have all contributed to ongoing changes in land use and further impacts on the harbour.

The Porirua Harbour has a rich and complex history that spans thousands of years. It formed around 10000-14000 years ago when westward flowing rivers were drowned by rising postglacial sea levels.

The harbour was home to a succession of tribes, including the Ngāi Tara people, who were succeeded by Ngāti Kahungunu, then Ngāti Ira, and finally Ngāti Toa Rangatira in the 1820s.

The 1855 Wairarapa earthquake caused tectonic uplift in the Pāuatahanui arm of the inlet, changing the shoreline and reducing navigability. Sedimentation from deforestation and ongoing urban development has continued to shape the harbour's ecosystem.

The harbour's shoreline has been altered significantly over the years, with the creation of three shallow lagoons on the land side of the new causeway when the North Island Main Trunk railway was straightened and double tracked in the 1960s.

Here are some key events that have shaped the harbour's history:

The harbour's history is a complex and multifaceted one, with both natural and human-induced changes shaping its ecosystem over time.

The harbour's inhabitants are quite fascinating. Porirua Harbour is an important estuary with habitats that are essential in the life history of many species.

Some species are year-round residents, such as kahawai that use the harbour until they reach maturity. Sand flounder of all sizes leave only to spawn in the harbour.

Rig, a commercial species known as lemonfish, come into Porirua Harbour to mate and to have pups. These small sharks are often what you eat when you have fish-and-chips.

The harbour's sedimentation rates have improved significantly over time. Average sedimentation rates reached between 8.5mm and 15mm per year during housing development, but now it's improving.

One example of what can be achieved lies at the far end of the Pauatahanui Inlet, where a bird-rich saltmarsh has replaced what was once paddocks and a go-kart track.

Dredging might seem like a simple solution to Porirua Harbour's problems, but it's not that straightforward.

All estuaries, including Porirua Harbour, trap sediment, and dredging would only temporarily fix the issue.

Dredging would also destroy shellfish beds and other sea life.

Coastal scientists have advised against dredging until the large amounts of sediment coming into the harbour from the catchment have been significantly reduced.

The harbour's sedimentation rates are concerning, and dredging would only be a band-aid solution.

Dredging would require finding somewhere to dispose of the dredged materials, which is a significant problem, especially since some of it contains pollutants.

The harbour bar has remained relatively unchanged over the past 160 years, fluctuating by just 5cm.

Dredging would be ineffective and expensive, as sand on the bar would simply refill any gap created.

As you explore the shores of Porirua Harbour, you'll notice the sandy banks that line the water's edge. These sand banks are a haven for beachcombers, with shells and other treasures washing up on the shore.

Porirua Harbour's unique geology creates a perfect environment for shells to thrive, with the harbour's tidal flats and sandy beaches providing a home for a wide variety of shell species.

The sand banks around the harbour seem to be growing, but it's not because new sand is being added. Very little new sand is coming into the harbour, according to research.

King low tides during the day make it seem like the sand banks are spreading, because more of them are visible to people. This can be misleading, as the sand is just being redistributed naturally.

The harbour has been surveyed many times since 1847, and the earliest survey by the HMS Acheron shows that the inlet was already shoaled, or shallow, even back then. This suggests that the sand banks are not growing, but rather being rearranged.

Anecdotal evidence from local boaties and engineers agrees with the science on this one - we're just seeing the natural redistribution of sand already in the harbour.

Dead cockle shells are a natural occurrence, not related to pollutants. They're a sign of a healthy cockle population, which is good news for the harbour.

Cockle shell banks and beaches used to be widespread throughout the harbour, but development destroyed many of them. Now, big concentrations of cockles still exist in the subtidal and intertidal areas of both harbour arms.

Natural mortality and tides wash cockle shells to the harbour edges, especially in sheltered bays. This is a normal process that happens in the harbour.

The 2013 Cockle Survey showed a steady increase in cockle numbers since 1995, with an 85% increase overall.

Protecting the harbour's wildlife is crucial for its overall health. The harbour is an important home and habitat for birdlife and fish.

There are over 72 stormwater outfalls around Porirua Harbour, which can significantly harm the wildlife. Simple behavior changes can help reduce harbour pollution from common household activities and chemical use.

Cleaning up the harbour will make it even better for the wildlife that lives there. The science is telling us that the harbour's health can be significantly improved, and that's what the Te Awarua-o-Porirua Harbour and Catchment Strategy and Action Plan is all about.

By making a few simple changes, we can help protect Porirua Harbour and its wildlife.

The harbour's health is closely monitored, and so far, the scientific monitoring and observations of locals suggest the highway project is adding relatively little sediment to the harbour.

However, the harbour is still affected by sediment and pollutants, particularly from major flood events. The 2016 Cockle Survey showed a reduction in cockle numbers and size, attributed to the extra sediment entering the harbour.

Regular clean-ups by Council, school, and community groups are making a significant difference in reducing accumulated litter around the harbour, although 'mega-litter' remains an ongoing issue.

The Transmission Gully Highway is a significant infrastructure project that's changing the city's landscape.

The harbour is one area that's being closely monitored for its impact.

During construction, there may be short-term impacts on the harbour from increased sediment.

However, the finished road is expected to have long-term benefits for the harbour's health.

Best practice controls are being used during construction to minimize sediment and pollutant run-off into the harbour.

The state of the harbour and the performance of these control measures is closely monitored.

So far, the scientific monitoring and local observations suggest that the highway project is adding relatively little sediment to the harbour.

Extensive 'browning' of the harbour has occurred due to significant floods and hill-country erosion.

As we work towards a cleaner and healthier harbour, it's natural to wonder when we'll start seeing positive changes. Big reductions in sediment and pollutants entering the harbour will take decades, but we're already seeing some encouraging signs.

In the past, cockle numbers and size in Pāuatahanui Inlet have been steadily increasing since 1995, indicating reduced sedimentation and improved ecological health. However, the 2016 cockle survey showed a reduction in cockle numbers, but this is not a long-term negative trend.

Annual surveys of recreational users show a high satisfaction with their harbour and water quality experience, despite still recognizing pollution and sediment issues. Some people believe the harbour's condition is better than a decade ago.

Regular clean-ups by Council, school and community groups are making a significant difference in reducing accumulated litter around the harbour. The reduction in litter is noticeable, but 'mega-litter' remains an ongoing issue.

The restoration and cleanup of Porirua Harbour is a top priority for the community, with a significant boost from the Government's Jobs for Nature fund. This $3 million grant over five years will create 30 full-time equivalent jobs and support the "Restoring the mauri of the Porirua Harbour" project.

Key initiatives include planting close to 630,000 plants over 92 hectares, installing 20 kilometers of riparian fencing, and controlling pests and weeds over 880 hectares. This will not only improve the harbour's ecology but also enhance its recreational value as a regional and local resource.

The Porirua City Council and its partners have developed an integrated catchment management plan, which will accelerate riparian planting, education, and community capacity building. This plan is a crucial step towards restoring the harbour's health and mauri, or life essence, as Ngāti Toa Rangatira chairman Callum Katene emphasizes.

Here are some key actions being taken by the Council to restore and clean up the harbour:

Implementation of a catchment-wide plan to reduce sediment run-off

Enhancement of the Porirua Stream mouth estuary

Targeted education programme for schools, businesses, and the community

Reviewing planning controls to reduce sediment and pollutant run-off

Improved litter management

Research and monitoring to ensure improvement activities are working

The Greater Wellington region is using science to encourage restoration of the harbour. They've installed turbidity meters in major streams to establish baselines for sediment flows and contamination.

By tracking the annual accumulation of mud and sand, the team can use this scientific evidence to encourage local authorities to put stricter rules on developers and stormwater discharges. This is a crucial step towards reducing pollution and protecting the harbour's ecosystem.

A good indicator of changing ecosystem health is the size of the cockle population in the harbour. In the 1970s, the intertidal population was estimated to be over 500 million, but it had more than halved by the 1990s due to subdivision development.

Since then, the cockle population has been on the increase again, and surveys are carried out every three years to keep track of changes. This data is helping the team understand the impact of human activities on the harbour's ecosystem.

Some of the species found in the harbour or using it during certain times of the year include yellow-eyed and grey mullet, spotties, mottled triplefin, kawahai, flounders, eagle rays, and rig. This list highlights the importance of preserving the harbour's biodiversity.

Here are some of the species found in the harbour, grouped by category:

The harbour is a well-used regional and local recreational resource and the centrepiece of Porirua City. It's a great place to spend time with family and friends, and cleaning it up will only make it better.

There are areas of the harbour that aren't in good condition, but overall it has a sound ecology. This means it's a healthy environment for plants and animals.

Cleaning up the harbour will improve the health of the harbour generally, including the bad bits. This is the goal of the Te Awarua-o-Porirua Harbour and Catchment Strategy and Action Plan.

The harbour is an important home and habitat for birdlife and fish. It's a vital part of the ecosystem and deserves our care.

The restoration of Porirua Harbour is a significant project that's making great progress. Porirua City Council, Ngāti Toa, and Sustainable Coastlines have received $3 million over five years to restore the harbour's health.

This funding will create 30 full-time jobs and allow for the planting of nearly 630,000 plants over 92 hectares. The harbour will also get 20 kilometres of riparian fencing and 880 hectares of plant and pest control.

The project is a long time coming, as Porirua Harbour has struggled with sedimentation and contaminants entering its waterways. Large-scale developments have threatened the sensitive ecology and wildlife in the streams and harbour.

Ngāti Toa Rangatira chairman Callum Katene says the iwi's identity is tied closely to their natural environment, particularly the harbour. They have an intergenerational memory of the harbour's former health and are working to recover it.

Restoring the harbour's "mauri" or life essence is a key goal of the project. This means recovering the harbour's natural balance and health.