Summary of historic estuarine sedimentation measurements in the Waikato region and formulation of a historic baseline sedimentation rate

Report: TR 2019/08

Author: Stephen Hunt

About this report

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. The implications of this guidance is that in order to put contemporary measurements of estuarine sedimentation in context, the background rate of sedimentation prior to catchment modification is required.

Historic sedimentation data has been collected using cores throughout various estuaries in the Waikato region and this report uses the historic SAR to formulate (i) an understanding of changes in SAR through time and (ii) an appropriate pre-catchment disturbance SAR. This research indicates that estuarine SAR pre-catchment disturbance was low in magnitude and varied little between sample locations both between estuaries and within the same estuary. The reason for this low variability and magnitude of SAR was the small amount of sediment available to the estuaries in the Waikato region; with little sediment availability, sedimentation rates appeared to have been low regardless of the estuarine morphology or hydrodynamic environment.

Following human settlement, both the magnitude and variability of sedimentation increased and available evidence indicates that this increase is primarily due to catchment disturbance and the associated increase in sediment supply to the estuaries. Using this data a pre-catchment disturbance SAR of 0.2 mm/yr has been determined for estuaries within the Waikato region. Based on the historical SAR and current sedimentation guidance, contemporary SAR should therefore not exceed 2.2 mm/yr in estuaries within the Waikato region.