• Key environmental issues for Porirua Harbour are sedimentation, pollution, and habitat loss
• Sedimentation is measured using a range of methods including:
  • analysing sediment samples for mud content and the types of animals living in the sediment;
  • measuring the sediment depth over buried sediment plates at 18 harbour sites, and
  • harbour-wide bathymetry surveys every five years.
• We also monitor the amount of sediment entering the harbour at three continuous turbidity monitoring sites in the streams of the three largest catchments (Horokiri, Pauatahanui and Porirua streams)
• Pollution from wastewater, stormwater and land run off is measured by taking water and sediment quality samples in the streams (monthly) and harbour (five yearly)
• Habitat loss is measured by mapping the extent and condition of key habitats, such as seagrass and saltmarsh, mud and sand, in the intertidal areas every five years.

• Results of some sediment quality monitoring are reported on the LAWA website Land, Air, Water Aotearoa (LAWA) – Estuaries
• Excessive sediment inputs remain the biggest stressor for the harbour.
• The intertidal areas of the harbour are in reasonable health though they experience pulses of mud deposition following heavy rain.
• When the tide comes in, this washes the mud into the subtidal areas of the harbour and so the subtidal areas are very muddy and in poor health.
• If we can reduce the amount of land-derived mud entering the harbour then we will also reduce pollution (because metals such as zinc and copper ‘stick’ to mud) and improve outcomes for habitat restoration efforts, because new and existing plant won’t get smothered.
• The volume of sediment entering the harbour changes a lot from year to year depending on what activities are happening in the catchment and how much it rains.
• Based on monitoring we have observed as little as 3,000 tonnes of sediment flowing from the three largest catchments in 2014, to as much as 22,000 tonnes in a very rainy 2016 from the three largest catchments.
• We estimate that natural pre-human sediment loads to the harbour would have been less than 2,000 tonnes per year, if the catchments were completely under native forest.
• We estimate based on modelling that 25% of all sediment entering the harbour in any year is flushed out to the open coast, and so lost from the harbour.
• While estuaries do naturally infill slowly over time, human activities have obviously accelerated this process and we urgently need to reduce sediment inputs so the harbour can recover naturally
• So given sediment loads vary a lot from year to year it is more useful to talk about the relative changes we are observing, rather than give actual numbers.
• I suggest saying that “sediment inputs to the harbour far exceed what the harbour can sustain. In years when it rains a lot, we have observed a more than ten-fold increase in sediment entering the harbour from the surrounding catchments”
• These excessive sediment inputs reduce estuary function, reduce biodiversity, smother valuable habitats, and make the harbour a muddy and unhealthy environment not suitable for plants and animals, or people.

• Implement the range of sediment related actions in the Te Awarua-o-Porirua Whaitua Implementation Plan, which address sediment generated by streambank erosion, forestry, erosion-prone land and urban/rural earthworks
• Restore wetlands throughout the catchments to reduce water flow and sediment entering the streams. This requires creating and restoring wetlands at a range of places in a stepped fashion throughout high sediment-yielding catchments, rather than just at the bottom of a catchment.
• Set caps on open areas of earthworks. We talked a little about this when we meet, Jenny, and while I can’t offer any more on this right now, please note this is the subject of ongoing conversations within GW about the policy, regulation and compliance settings for this. This could be an important tool in the tool box but needs further analysis.

• All of our freshwater and coastal reports can be found here Greater Wellington Regional Council — Water monitoring (gw.govt.nz)
• Access to interactive reports from our turbidity programme in the Porirua Whaitua can be found here 2020/21 Te Awarua-o-Porirua Harbour catchment sediment monitoring | Greater Wellington Regional Council (gw.govt.nz)

These comments are from a Greater Wellington Regional Council, Environmental Scientist.