Sediment Suspension and Morphological Response under Vessel-Generated Wave Groups: Torpedo Bay, Auckland, New Zealand

Abstract

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 severely limited by the maximum unrestricted fetch of only 2.5 km at this location. In contrast, VGW reach maximum heights in excess of 0.85 m, have an average H_s ~0.3 m and periods of 2-6 s on the foreshore.

The groupiness and nonlinear form of these large VGWmakes them capable of entraining and suspending significant quantities of bottom sediment (concentrations reaching 10-100 g ] ¹) resulting in sustained increases of turbidity in the nearshore region. VGWrepresent 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-generated waves.

Sand resuspension events under non-linear (asymmetric and skewed) shoaling and breaking VGW exhibit a distinctive temporal structure. This structure is characterised by a marked instantaneous response to sharp accelerations, high velocities and intense turbulence under the crests of asymmetric breaking waves and also by a gradual accumulation and decay of suspended sediment in the water column. The former feature leads to net onshore transport while the latter feature leads to both a distinctive phase lag between the largest 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 response to large VGW and relatively high gross sediment transport, the net effect of both WGW and VGW on the sediment transport and foreshore response at Torpedo Bay appears to be insignificant.