From Monday 24 January, Porirua City is at the red setting of the traffic lights protection framework. This means some changes to the way we work and live, as protections are in place to help minimise the spread of Covid-19 in the community. Find out more
The Stormwater Bylaw adopted in August 2015 makes it illegal to tip, wash or flush common pollutants down our street and other stormwater drains. Before this, we had little control over what went into the stormwater system and then into Porirua Harbour. Stormwater does not get treated. It runs either overland or into the street drain and pipe network and straight into the nearest stream and the harbour.
You need to either collect pollutants and take them to appropriate facilities for disposal, or divert small amounts of waste run-off onto your lawn, gardens or compost.
Before you run a car wash fundraiser, contact us for advice and assistance: phone (04) 237 5089.
The other impact of the bylaw for you will be a significant contribution towards cleaning up Porirua Harbour and other sites around our coast, meaning all of us will be able to enjoy a healthier environment.
Find out what you can do to keep pollutants out of our stormwater drains: Help protect our harbour.
The reality is, both sources need to be dealt with. Wet weather road run-off is a significant contributor to harbour pollution, particularly the heavy metals zinc (from unpainted roofs and tyre wear) and copper (from brake pad wear). The Council and the NZ Transport Agency are looking for ways to deal with this.
However, road run-off arises almost solely from rainfall and the pollutants are therefore diluted by rainwater and have a greater chance of being flushed out of the harbour. By comparison, car washing generally happens when the weather is fine and the car wash waste is less diluted and also less likely to be flushed out of the harbour.
During the summer, some parts of the harbour can give off strong smells. There are two reasons for this: algae decay and smelly mud.
As an estuary, Porirua Harbour is a great place for marine algae to grow. Warm temperatures and abundant nutrients promote the growth of algae such as the bright green sea lettuce Ulva sp. Part of the algae lifecycle includes die-off and this decay creates strong odours. The strength of the odour depends on the amount of algae growth, the temperature and wind direction.
The Okowai Lagoon (the first lagoon on the right of State Highway 1 north of the city centre) gets a build-up of smelly mud in summer. The intensity of the smell depends on temperature and wind direction. We’re considering ways to improve the situation.
All estuaries, including Porirua Harbour, trap sediment. Their relatively calm environment allows accumulation of sand coming in from the outer coast and silt from the land. This means all estuaries eventually fill in. It’s the fast rate of filling that is concerning about Porirua Harbour.
Coastal scientists from agencies such as NIWA have advised against dredging until the large amounts of sediment coming into the harbour from the catchment have been significantly reduced. Otherwise, like digging a hole in the beach and going back after the tide’s been in and seeing it filled in again, the same will happen with harbour dredging. Dredging would also destroy shellfish beds and other sea life.
Another problem would be finding somewhere to dispose of any dredged materials, especially since some of it contains pollutants.
We’re putting in place a catchment-wide Sediment Reduction Plan to reduce sedimentation rates to more natural and manageable levels.
Dredging is unlikely to make much difference to harbour flushing. Target areas for dredging, for example to improve navigability, are below low tide. Technically, this means dredging wouldn't improve the ‘tidal prism’ – the water coming in and out of the harbour between high and low tide – which is critical for the flushing ability.
Any dredging work would not only remove polluted sediment but also produce a sediment plume that would re-suspend and spread the pollutants further. There would also be a problem disposing of the polluted sediments. A resource consent would be unlikely to be granted to dispose of dredged materials at sea, because they could pollute fishing grounds. Disposal to land (such as in a landfill) may require a purpose-built facility and would be very expensive.
Current best practice is to leave polluted sediment undisturbed, reduce the amount of pollutants entering the harbour, and allow future sediments to cover over and ‘seal’ the existing polluted sediments.
Comparisons of all bathymetric (underwater) surveys of the harbour since 1847 show the bar height has changed little over the past 160 years – fluctuating + or – 5cm in that time. This is consistent with the normal dynamics of a harbour bar that protects an inner harbour from extreme effects of coastal waves and storm events. Sand on a bar will also respond to removal by refilling any gap. Best practice and science advice recommends not disturbing the bar. Dredging would be ineffective, temporary and expensive.
The sand banks are all made from marine sand, meaning the sand has come from the outer harbour and open sea. Research shows very little new sand is now coming into the harbour. One reason for the apparent spread of sand banks in the harbour occurs when king low tides happen during the day and more sand bank is visible to people, so it seems they are 'growing'.
Another reason is that the sand already in the harbour is being reworked and redistributed, which is a natural part of an estuary’s dynamics. The earliest survey of the harbour by the HMS Acheron in 1847 strongly suggests that Pāuatahanui Inlet was widely shoaled even then. Anecdotal evidence from long-time local boaties and engineers agrees with the science, that we are really just seeing a redistribution of sand already in the harbour.
Meanwhile, the tongue of the main sandbank is extending into Browns Bay and this is being monitored.
Dead cockle shells are a natural occurrence, not related to pollutants. One reason Ngāti Toa Rangatira settled around the harbour was its large amounts of cockles. Cockle shell banks and beaches used to be widespread throughout the harbour. However, development for road, rail and reclamations destroyed many of these, particularly in the Onepoto Arm.
Big concentrations of cockles still exist in the subtidal and intertidal areas of both arms of the harbour and it still has the highest concentration of cockles of any estuary in New Zealand. Natural mortality and the action of tides and waves washes cockle shells to the harbour edges, particularly sheltered bays.
The 2013 three-year Cockle Survey by the Guardians of Pāuatahanui Inlet showed a steady increase in cockle numbers since the first survey in 1995. The numbers have increased 85 per cent since then, and 21 per cent since the previous survey in 2010. This trend indicates reduced sediment entering the inlet, and a healthier harbour. The most recent, December 2016 survey experienced a reduction in cockle numbers for the first time since the cockle counting began. This is is attributed to the large amount of sediment flushed into the harbour by several major floods over the 2015/2016 period. One of those floods occurred just a few weeks before the scheduled December cockle count. Too much sedimentation smothers and kills shellfish.
There’s no doubt that Porirua Harbour has a sediment problem – most of this is washing in from the land. Floods carry a lot of sediment and turn the harbour brown. This is common in most estuaries and even the open coast.
The times when parts of the harbour go brown and there has been no flooding are due to wave action stirring up mud and fine silt on the bottom of the harbour and keeping them in suspension until the wind and waves die down.
As a general rule, it’s safe to swim in all beach areas. Places like Plimmerton Beach and Karehana Bay, as well as Dolly Vardon Reserve at Mana, are some of the most popular and safe beaches in the Wellington region. These are closer to the harbour entrance and open coast, where the water quality is higher.
The harbour is widely used for water sports such as waka ama, sailing, powerboating, kayaks, paddleboarding and wind surfing. The harbour is considered a protected and safe environment for water sports, particularly for learners. The recent 6-day National Sea Scout Regatta held in the Onepoto Arm over Christmas/New Year, was not only a great success (Great weather!), but there were also no reports of sickness or infection from contact with or swimming in the harbour.
Some areas are signposted with health warnings. These areas, however, are either unattractive for swimming or adjacent to stormwater pipes originating from industrial zones, particularly at the southern end of the Onepoto Arm.
Swimming is also discouraged for three days after storms/rainfall events, which can wash pollutants off roads and into the harbour. The pollutants are usually flushed out of the harbour within three days.
Greater Wellington Regional Council has produced a useful pamphlet, "Is it safe to swim in Porirua Harbour?" which will give you more details.
While the fishing in the harbour is not as good as in the past, there are definitely fish in the harbour. Fishers particularly like the entrances to both arms of the harbour. Shellfish are also safest to take from these areas, but never within 3 days after any rainfall which flushes additional contaminants into the harbour.
Fish taken from the harbour entrance are generally safe to eat. Testing has found no concerning levels of chemical pollutants. We would advise against fish or shellfish from around stormwater outfalls, particularly next to Porirua Stream and the city centre, where there is a danger from faecal and other bacterial and viral matter. Keep to the harbour entrance and outer harbour.
There may be short-term impacts on the harbour from increased sediment while the highway is built. However, evidence suggests the finished road will have long-term benefits for the health of the harbour. Using best practice controls during construction and then the completed highway will minimise 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 the observations of knowledgeable locals suggests that the highway project is adding relatively little sediment to the harbour. Extensive 'browning' of the harbour has occurred because of the unusual recurrence of significant floods and resultant increase in hill-country and stream bank erosion.
There are a few known areas of the harbour that aren’t in good condition, but otherwise the harbour has a sound ecology, is a well-used regional and local recreational resource, and remains the centrepiece of Porirua City. The harbour is an important home and habitat for birdlife and fish. Cleaning up the harbour will make it even better. Clearly, we all love the harbour and the community wants it cleaned up.
The science is telling us that the health of the harbour generally, including the bad bits, can be significantly improved – which is what the Te Awarua-o-Porirua Harbour and Catchment Strategy and Action Plan is all about.
We’re implementing the Te Awarua-o-Porirua Harbour and Catchment Strategy and Action Plan, which was developed with the the Iwi, other councils, local, regional and national agencies, and the community. Key parts of the strategy include:
Big reductions in sediment and pollutants entering the harbour will, in some instances, take decades as we control erosion, increase vegetation in the catchment and reduce pollution sources.
Until the 2016 Cockle Survey, Pāuatahanui Inlet showed a steady increase in cockle numbers and size since 1995. This indicated reduced sedimentation and improved ecological health in the inlet. However, the 2016 survey indicated the first cockle number reduction since the surveys started. This is attributed to the extra sediment entering the harbour from several major flood events over 2015/2016, and not indicative of any long-term negative trend.
Annual surveys of recreational users still show a high satisfaction with their harbour and water quality experience, while still recognising there are pollution and sediment issues. Some of those surveyed believe the condition of harbour is much better than a decade ago.
We’ve also seen a big reduction in the amount of litter seen around the harbour edges. Regular clean-ups by Council, school and community groups are making a significant difference to reducing accumulated litter around the harbour, though 'mega-litter' (shopping carts, road cones and car tyres) is an ongoing issue.