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.