Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Regional Estuary Monitoring Programme (REMP) intertidal sedimentation measurements, results and review of methodologies Technical Report

Hunt, S.
Publisher / Organisation
Waikato Regional Council
Sedimentation within estuaries is a natural process but excessive sedimentation can lead to poor ecological health, with current guidance suggesting that sediment accumulation rates (SAR) should not exceed 2 mm/yr above pre-catchment disturbance SAR. To track patterns of contemporary intertidal sedimentation WRC has been monitoring SAR in the Firth of Thames and Whaingaroa (Raglan) Harbour since 2003 as part of an estuarine State of the Environment (SoE) monitoring programme. The programme measures sediment depth above plates (concrete pavers) buried in the intertidal flats. The purpose of these measurements is to track SAR in each estuary and to pair the measured SAR with monitoring of ecological health. This report analyses the WRC SAR monitoring to assess the suitability of the methodology including spatial distribution of plates, temporal distribution of measurements and longevity of the plates. Sedimentation rates at each monitoring site have also been derived from the measurements. This purpose of this analysis is to determine if the sedimentation monitoring programme has been effective and to establish principles around which a standardised sedimentation monitoring programme can be designed in the future, for the purposes of both SoE monitoring and for implementation of monitoring set out in Sea Change 2016 (a spatial plan for the Hauraki Gulf). The analysis found that sedimentation rates are highly variable, with areas of both erosion and accretion ranging between -11.52 (erosion) and 5.59 (accretion) mm/yr in the Firth of Thames and -3.98 (erosion) and 7.04 (accretion) mm/yr in Whaingaroa (Raglan) Harbour. This spatial variation in SAR can be tentatively related to the spatial distribution of hydrodynamics in each estuary including the presence or absence of wind waves, the patterns of residual circulation and magnitude of sediment supply. Sedimentation plates are effective at measuring annual rates of SAR providing the plates are set out with an appropriate spatial array that take into account the spatial distribution of hydrodynamics and are measured annually at regular intervals for at least ten years. Simple guidelines for installing the plates and a standardised methodology are also outlined in this report.