Sediment accumulation rates in Whāingaroa (Raglan) Harbour and the Firth of Thames, 2002/2003 to 2021

Report: TR 2022/22

Author: Stephen Hunt (Waikato Regional Council)

Sediment accumulation rates (SAR) have been monitored by Waikato Regional Council at sites across Whāingaroa (Raglan) Harbour and the Firth of Thames using sediment plates since 2002/2003. Sediment depths have been periodically measured over each plate and the changes in sediment depth have been analysed to calculate SAR. Sedimentation (accretion) is occurring at 7 sites within Whāingaroa Harbour and at 11 sites within the Firth of Thames.

In Whāingaroa Harbour, SAR at 2 plates, both of which are at the same monitoring site, exceeded the ANZECC guideline value of 2.2 mm/y estimated for the Waikato region. In the Firth of Thames, SAR at 7 plates across 2 of the 4 monitoring sites exceeded the same ANZECC guideline value. There are also sites displaying ongoing trends of erosion in both Whāingaroa Harbour and the Firth of Thames. The reason for this erosion is not clear but it may be due to reworking of relict sediment deposited following initial catchment clearance. This reworked sediment could be deposited elsewhere in the estuaries or exported out of the estuary to the wider Coastal Marine Area. For example, a known area of rapid sediment deposition is the southern Firth of Thames mangrove forest.

Low SARs are not necessarily an indicator of good estuarine health without consideration of the wider environmental context. It is necessary to consider other environmental pressures in the historical context of each estuary to provide a more comprehensive description of the health of an estuary. Despite the evidence of low SAR at some locations as presented in this report, there is evidence that sites in the Waikato region are exhibiting a decline in estuarine health.