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Abstract

Models of settling lag in coastal and estuarine settings
Pritchard, D. and Hogg, A.J.

The two principal processes driving coastal and estuarine sediment transport — tides and waves — are oscillatory flows which involve no net movement of water when averaged over the period of the fluid motion. It is well-known, however, that such flows can lead to the net movement of sediment in a particular direction. For fine sediment, which is transported largely in suspension, the process depends on "lag" effects associated with the finite time which it takes the quantity of sediment in suspension to adjust to changes in the fluid velocity. Especially when this response time is comparable to the period of the motion, the character of these lag effects can be rather complex, and is still less well understood than the processes leading to net bedload transport.

This paper draws on recent mathematical studies of fine sediment transport on muddy coasts by Pritchard & Hogg (2003a,b). We compare four different oscillatory flows (two corresponding to tidal currents and two to infragravity waves), and discuss the net transport patterns which emerge. Common themes and differences are identified, and related to the classic conceptual descriptions of settling lag due to van Straaten & Kuenen (1957) and to Postma (1961). The patterns identified may provide a basis for the time-averaged descriptions required by long-term morphodynamic simulations, as well as guiding the interpretation of field measurements of suspended sediment flux.