Review of sand extraction impacts for Miller’s Bank, Kaipara Harbour

Abstract

A review of the monitoring reports for resource consent MCO623701 (CON20030623701) involving sand extraction near Millers Bank, and additional reports relating to sediment transport within the Kaipara Harbour was undertaken. The available data indicates that the Waikeri Bank and surrounding channels are mobile, and while occupying approximately the same position since the first full survey in 1852, they are not fixed features. The adjacent shoreline between Ru Point and Tauhara Creek, have undergone phases of erosion and accretion since 1852, with variable behaviour along the coast. Overall this section of coast is tending to erode. Monitoring since sand extraction began indicates that natural phenomena, particularly episodic storm conditions from the northeast quadrant, are the dominant cause of shoreline erosion. No quantitative data are available prior to sand extraction, so it is difficult to determine if there has been any change in the rate of erosion since extraction began. Limited current data, and numerical modelling, suggest that Five Fathom Channel is ebb dominated, and the channel between Waikeri Bank and the shore may be flood dominated. This would cause a recirculation of sediment around Waikeri Bank, and the morphology suggests that sediment tends to accumulate at the northern and southern junctions of the two channels. Bathymetric surveys indicate that the supply of sediment to this region is sufficient to replenish sand extracted. However, the surveys completed to date do not have sufficient resolution and datum controls to quantify volumetric changes. The variations recorded by monitoring are minor compared to historic changes, so it is unlikely that the sand extraction is having any measurable effect on flow through either channel or sedimentation in the vicinity.