The effects of spatially complex inner shelf roughness on boundary layer turbulence and current and wave friction: Tairua embayment, New Zealand

A field study of bedforms, associated hydraulic roughness, and turbulence was conducted on the inner shelf off the east coast of New Zealandʹs North Island under conditions that included two significant storm events. Sharply contrasting rough and smooth beds were characterized via field mapping and deployment of instrumented benthic tripods. Rough areas of coarse sand exhibited ripples with heights and lengths of ∼25 and ∼100 cm, while smooth areas supported smaller ripples with heights and lengths of ∼5 cm and ∼20 cm. Contacts between the two surfaces were sharp and maintained their position. Roughness contrasts were enhanced significantly during storms, which simultaneously accentuated migrating orbital ripples over the coarse bed and replaced ripples on the fine sediment bed with smoother hummocky features. Spectra of the fluctuating vertical velocity components, w′, from both smooth and rough sites showed good fits to −5/3 slopes within the inertial sub range enabling independent estimates of wave-averaged bed stress to be made via the inertial dissipation method (IDM). We also utilized the vertical fluctuation data to obtain alternative estimates of the wave friction factor, fw, following Smyth and Hay (J. Phys. Oceanography 32 (2002) 3490); SH. These two methods yielded generally similar results. Under high wave conditions, fw estimated via IDM averaged 0.027 at the rough site and 0.0045 at the smooth site while the SH method gave respective values of 0.027 and 0.013. Under low-energy conditions, fw from IDM averaged 0.0082 at the rough site and 0.012 at the smooth site, while the SH method yielded mean values of 0.0080 and 0.016. Thus, fw was much larger at the rough site than at the smooth site during storms but smaller at the rough site during fair weather. During storms, structured vortices with frequencies at the first harmonic of the swell waves formed over the rough surface and penetrated above the wave current boundary layer causing retardation of mean currents. Such storm-induced vortices were only intermittently present over the smooth surface. The application of Nielsenʹs (J. Geophys. Res. 86 (1981) 6467) roughness model produced some qualitatively similar trends in fw, although predicted fw was larger than observed values at the rough site. Additionally, low modelled fw over the smooth bed during high energy was based on plane bed theory rather than the inferred hummocky bed.