Publication Abstract

Patterns of wave-orbital speed and skin friction under estuarine (fetch-limited) waves

T.J.Dolphin and M.O. Green

Sediment transport on estuarine intertidal flats is complex, due to mixed sediments (muds and sands), wetting and drying, and exposure to spatially and temporally varying tidal currents and the locally generated, fetch-limited, wave field. In comparison to open-coast settings, waves on estuarine flats have received comparatively little attention. In this paper we present the results of field experiments investigating processes that control sediment transport on an intertidal flat in a large New Zealand estuary (fetch lengths up to 25 km) under locally generated waves (wave heights of 0.2 – 1 m and periods of 2 – 6 s). The tidal range is > 4 m on springs and in channels tidal flows can reach 2 m/s. Waves control sediment entrainment as tidal currents are subcritical. During wind events, variation in the near bed wave-orbital speed at any point on the intertidal flat depends on a subtle balance between depth, wave height and period, all of which vary over the tidal cycle. As water level rises, intertidal banks of the central harbour are inundated, fetch increases, and height and period of the fetch-limited waves grow. Whilst increasing depth retards the penetration of wave-orbital motions down to the bed, increasing period and height counteract that, resulting in an increase in wave-orbital speed at the bed, wave-induced skin friction, and sediment entrainment. However, sediment flux may not co-vary with entrainment, since it depends also on the tidal current, which transports the entrained sediment, and which is not necessarily coupled with the wave field. The typical variation over the tidal cycle in the resulting sediment transport is thought to change with elevation on the flat, which has important implications for the morphological zonation on intertidal flats.

Reference:

T.J.Dolphin and M.O. Green (2009) Patterns in wave-orbital speed and skin friction under estuarine (fetch-limited) waves. Journal of Coastal Research, SI 56, 178 – 182.