Sand Volume Change and Cross-shore Sand Transfer, Mangawhai Beach, New Zealand

Abstract

Sand volumes and cross-shore sand transport within a swath of beach and shoreface at Mangawhai, Northland, New Zealand were monitored during six weeks of swell and storm waves in order to capture any significant sand exchanges between the beach and inner shelf. The main data-collection involved repeat surveys of sand levels over 16 profiles, evenly spaced along 500 m of shore and extending from dune-toe to 10-11 m depth. The upper beach was surveyed with Emery poles while a sea -sled was used for the shoreface. The sea-sled was equipped with tilt-sensors and a wheel so that the surveyed bed profile could be adjusted for mast-tilt and bedform effects. The spatial distribution of cross-shore sand fluxes was derived by integrating the sand level changes between consecutive surveys. Reference rods in the shoreface were used to verify the sled data and to identify depth of bed disturbance. An instrumented tripod was used to measure near-bed wave-orbit al and steady-current velocities and sand concentrations and to compute sand fluxes at several points on the shoreface.

Results showed that during a swell-dominated accretionary phase, onshore sand fluxes were associated with shoreward migrating bars in the swash zone and on the upper beachface but were negligible on the shoreface. With waves generated by an extra-tropical cyclone, offshore fluxes peaked over the outer bar as it was translated seaward but appeared to pinch-out at about 8 m depth , as predicted from wave climate statistics by HALLERMEIERS’s (1981) model. This average pattern was confused at individual profiles by longshore gradients in sand transport associated with rip-channels, rhythmic topography, and non-uniform beach drainage. When averaged over the whole study swath, the differences in sand volume between surveys were small (1-4 m³m^-1) and consistent with measurement error, suggesting that the beach-shoreface was essentially a closed system, with no significant sand exchange with offshore during the monitoring time frame.