Sediment suspension and morphological response under vessel-generated wavegroups: Torpedo Bay, Auckland, new Zealand

Waves, currents, suspended sediments and beach morphological response were measured using fast-response sensors over a 13 month period at Torpedo Bay, Auckland to evaluate the relative effects of vessel generated waves (VGW) and wind generated waves (WGW). WGW (H-s = 0.1-0.2 m, T-pk = 1-2 s) are sev erely limited by the maximum unrestricted fetch of only 2.5 km at this loca tion. In contrast, VGW reach maximum heights in excess of 0.85 m, have an a verage H-s similar to 0.3 m and periods of 2-6 s on the foreshore. The groupiness and nonlinear form of these large VGW makes them capable of entraining and suspending significant quantities of bottom sediment (concen trations reaching 10-100 g l(-1)) resulting in sustained increases of turbi dity in the nearshore region. VGW represent a significant proportion of the total energy available to transport sediment at Torpedo Bay, contributing as much as twice the sediment transport potential relative to wind-generate d waves. Sand resuspension events under non-linear (asymmetric and skewed) shoaling and breaking VGW exhibit a distinctive temporal structure. This structure i s characterised by a marked instantaneous response to sharp accelerations, high velocities and intense turbulence under the crests of asymmetric break ing waves and also by a gradual accumulation and decay of suspended sedimen t in the water column. The former feature leads to net onshore transport wh ile the latter feature leads to both a distinctive phase lag between the la rgest VGW and the event maximum suspended sediment concentration (SSC), and to the enhancement of turbidity in the nearshore. Despite short term fluctuations in bed elevation of up to +/- 10 cm in resp onse to large VGW and relatively high gross sediment transport, the net eff ect of both WGW and VGW on the sediment transport and foreshore response at Torpedo Bay appears to be insignificant.