Investigation of sorted bedforms, Tauranga Harbour, New Zealand

Abstract

Elongated bands of coarse grained ripples (sorted bedforms) are common in the nearshore swell wave environment and have been identified by numerous researchers. Although there is a general concurrence regarding the maintenance of these features, the mechanism controlling their formation and positional stability is still under discussion. In 2004-5 the current study was conducted to investigate sorted bedforms on the inner shelf off Mt Maunganui beach, which is located on the north-eastern coast of New Zealand. The morphology and evolution of these features was examined by 4 side-scan sonar surveys, spanning 9 months. The influence of wave convergence on the formation and positional stability of the sorted bedforms was investigated via a calibrated numerical wave model (Mike21-NSW). Transects from multibeam bathymetry were used to assess the influence of alongshore flow. Results from the side-scan sonar surveys indicate an alternating pattern of degradation and restoration during the monitoring period. Based upon numerical model simulations it seems likely that the positional stability of the elongated sorted bedforms at Mt Maunganui is primarily determined by localised wave height reinforcement, resulting from the focussing of wave energy by two offshore spoil mounds. Cross-sections indicate that the maintenance of these features is likely to be assisted by increased near-bottom turbulence over the coarse bedformed areas and removal of fine suspended load by superimposed alongshore currents.