Shoreline changes for southeastern Matakana Island (Panepane Point) following capital dredging (2015-16)

Abstract

Surveys of the mean high water mark before and after the capital dredging of 2015/16 indicate minor shoreline fluctuations on the open coast of Matakana Island that are consistent with normal variations. The largest change has occurred at the Panepane Point spit tip, which eroded by ˜20 m between August 2015 and October 2016. This is most likely due to the changed alignment of the channels approaching the narrowest section of the tidal inlet, and is consistent with the predicted impacts in the Assessment of Environmental Impacts. Panepane Point is not expected to continue to erode further with the current channel alignment. The patterns of erosion and accretion observed for the ebb tidal delta (Matakana Banks) since the dredging are consistent with those reported before dredging, and are associated with the migration of swash bars over the swash platform of the delta in response to normal variations in wave and tidal currents. These is some evidence to suggest that the ebb tidal delta is starting to extend further offshore. The observed changes do not indicate that the ebb tidal delta is undergoing collapse following dredging. The patterns of erosion and accretion observed in the Lower Western Channel are consistent with channel migrations observed in the past, superimposed on continuing shoaling of the ebb shield of the flood tidal delta (Centre Bank). The contribution of dredging to this behaviour is currently being investigated.