Erosion in Wai O Taiki Bay

A 2016 Tonkin & Taylor report on the erosion of the banks of the Tamaki Estuary in Wai O Taki Bay suggested a 3-5 m/100 year range rate of retreat. I took some photos in 2013 so wanted to see if the prediction looked right for the last seven years.

The 2021 photo has more grass and that the failing fence is not keeping people off the closed track.
The 2021 photo has more exposed bedrock showing a loss of soil. The flax bush is now very exposed to elements and will likely die in the next few years. Soil loss is apparent adjacent to the unfinished seawall.
The 2021 photo shows how the harder substrate has changed its shape. Roosts of dead pine rotting and stumps falling down slope. Wilding pine growth.
There is an increased distance between unfinished seawall and clay bank in the 2021 photo. Loss of clay bank. Subsidence affecting stability of trees.
Since 2013 more bedrock has been exposed adding about a tonne of clay sediment to the Estuary. Wilding pines and gorse are the dominant species. An exposed root at the top of the bank indicates 20-50cm retreat.
The 2021 photo shows an increase in mangrove coverage and a reduction in flax.
Note these images are over a different time period. Omaru Island is clearly eroding (source data on GeoMaps).

My best guess would be about 30cm of loss over the seven year period suggesting the prediction to be about right.

The report suggested a ‘Do nothing’ option which was adopted by the Local Board.  The exposed banks are at least 100m long and average about 2m high. If we lost 30cm of bank over the last seven years that’s 60 cubic meters of clay and top soil. Wet excavated clay weighs 1,826 kg/m³, so more than 100 tones of life choking sediment dumped into the Tāmaki Estuary and the Hauraki Gulf (on our watch).

This report has been submitted to the Maungakiekie-Tāmaki Local Board.

Update September 2022. I was invited to present this report to the Local Board a year later. It was well received and has been recorded in their minutes.