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Recognising that one of the consequences of development is sedimentation, Auckland Council is developing a long-term regional monitoring programme starting in the Whangateau and Mahurangi Harbours.
Council says the most impacted estuaries are those close to urban centres where there has been the longest history of land use change and intense development, such as the sub-estuaries of the Manukau and Waitematā Harbours.
Nonetheless, the southern Kaipara and Mahurangi Harbours are also ecologically degraded in some locations due to excess sediment from rural land uses, and most smaller east coast estuaries are exhibiting trends of increasing fine sediments and mud, especially Okura, Ōrewa, Tūranga and Waikopua.
Over time, sedimentation leads to increasingly muddy sandflats and a decrease in ecological health as fewer species can tolerate the muddier conditions.
Council has been monitoring mud content and the composition of invertebrate communities in estuary sandflats for several decades. The findings from this monitoring are summarised in a report titled The Health of Tāmaki Makaurau/Auckland’s Natural Environment in 2020.*
The monitoring shows that sedimentation is currently the biggest problem faced by Auckland’s estuaries and harbours. Every monitored estuary and harbour has been affected to some extent by increases in sedimentation, as indicated by the amount of mud present in the sandflats and/or changes in the invertebrate community.
However, Council believes more targeted monitoring is needed to understand the current rates of sedimentation and to set appropriate reduction targets.
The establishment of monitoring sites in Whangateau was due to start this month and would be followed by the establishment of sites in the Mahurangi.
Roughly 12 sediment plates will be installed across six monitoring locations at Whangateau, aligned initially to sites where Council currently conducts ecological monitoring, which takes place every six months in April and October. These locations span the entire harbour, covering the main body, the Omaha River arm, and adjacent to Tramcar Bay.
Council hopes the Whangateau monitoring will provide it with an understanding of the sedimentation dynamics in medium sized estuaries that receive relatively small sediment loads from their catchment and have high tidal flushing.
Plates will be left to settle for six months, and then the first sediment depth measurements will be taken.
Measurements will be taken roughly every four months for the initial two years of the programme and will then be reduced to annually in October, to align with ecological monitoring.
As this programme aims to track long-term changes in sediment accretion, there is no date for completion and plates will remain in place until it is decided monitoring is no longer required.
It is expected the first full analysis of sediment accretion rates will be published in 2027.
By then, Council will have five years’ worth of data and will be in a position to present a robust assessment of the state of sedimentation, along with some trends, having reviewed the performance of the plates in the first few years of monitoring.
The programme will complement information Council already has on sediment muddiness derived from sediment grain size analysis, which already shows where sediment is affecting ecological health.
The project could lead to increased regulatory compliance activities in the short term, better land use planning mainly in the medium term, and advocacy to central government for better legislative tools to prevent adverse environmental outcomes.
*The Health of Tāmaki Makaurau/Auckland’s Natural Environment in 2020 report is available to read here.
