Processes driving estuary infilling by marine sands on an embayed coast

Detailed measurements of waves, currents and sediment transport from an int ertidal sandflat at the mouth of a partially filled New Zealand estuary in an embayment with low littoral drift are presented with a view to establish ing how marine sands are transported into the estuary. Waves mobilised sedi ments during episodic events. Early in events, when waves were energetic, s ediments were in motion throughout the tidal cycle. Under abating waves, se diment motion became intermittent, finally becoming restricted to periods a round low tide, when wave-orbital motions were able to penetrate to the bed . Three components of the total sediment flux were estimated from the data and analysed. (1) A wind-driven current, which presumably has a complicated spatial expression in the vicinity of the estuary mouth, controlled the ne t (i.e. integrated over event) direction of flux of suspended sediment entr ained by waves and advected by currents. There was no evidence of non-symme try over the tidal cycle in the suspension process that might drive a net s ediment flux; however, there is some suggestion that lag in growth and deca y of the estuary wave field over the tidal cycle systematically enhances of fshore transport. (2) Net bedload transport by waves and currents was direc ted onshore, which was guaranteed by two mechanisms. Firstly, increasing wa ve-orbital-current skewness as the tide ebbed turned bedload transport onsh ore late in the ebb tide against the current and, secondly, bedload peaked early in the flood tide when skewed waves were reinforced by the flooding c urrent. (3) Correlation between seaward-directed forced mean flow and enhan ced suspension under groups of high waves resulted in persistent offshore f lux of suspended sediment at infragravity frequencies. Suspended-sediment f lux carried by gravity waves was directed seawards on flood tides (and coun ter to the waves) and onshore on ebb tides (and with the waves), which was explained in terms of a simple vortex-ejection model. We conclude that sedi ment transport on the sandflat at the mouth of this partially filled estuar y is governed by subtle interactions between waves and currents that vary o ver the tidal cycle. Some interactions, such as those that drive bedload tr ansport, promote estuary infilling by marine sands, and others, such as tho se that drive suspended-sediment flux at infragravity frequencies, do the o pposite. (C) 2001 Elsevier Science B.V. All rights reserved.