Hydrodynamics and morphological adjustment of a mixed sand and gravel beach, Torere, Bay of Plenty, New Zealand

Abstract

Detailed measurements of cross-shore wave energy transformation, swash sediment transport and beach profile change under low energy conditions are reported from a mixed sand and gravel beach at Torere Bay, Bay of Plenty, New Zealand. Measurements were undertaken during a 10-day period following a storm event and document post-storm recovery of the beach. Nearshore wave spectra identify periods in which incident wave energy is dominated by swell and wind wave energy. Results show that wind wave energy observed in the nearshore is effectively removed from beachface spectra at the break-point step under lower tidal stages. Swash is driven by longer period swell energy although dissipation of short period wave energy is critical in entraining sediments on the lower beach. At higher tidal stages wind wave energy is able to propagate across the break-point step, shoal and break on the lower beachface. Tidal modulation of wave processes across the lower beachface under low incident wave conditions is found to stimulate active sediment reworking, migration of the break-point step and shoreward sediment flux to promote beach accretion.