Sediment dynamics and stability of tidal inlets

Abstract

Sediment transport patterns and equilibrium conditions of three tidal inlet systems (i.e. Christchurch, Langstone and Yangpu Harbours) are investigated. Various approaches to the study are adopted: (1) sediment transport rates are calculated on the basis of the empirical equations of Madsen and Grant (1976), Gadd et al. (1978) and Vincent et al. (1981); (2) an original model for grain size trend analysis (McLaren and Bowles 1985) is modified for use in shallow marine environments, based upon identification of "transport vectors" and a "critical vector length"; (3) a mathematical model is developed to simulate movement of natural trace sediments, from the continental shelf to the tidal basin; and (4) an analytical approach to inlet equilibrium is developed, based upon mass conservation of water and sediment. Data sets for the application of these methods are derived from geophysical surveys, analyses of sediment samples, tidal cycle observations, and long-term monitoring of water levels and suspended sediment concentrations. In the entrance to Christchurch (Harbour), significant velocity asymmetry is present, which results in the seaward transport of sediment under the combined action of currents and waves (without considering longshore drift). Net sand transport is directed, however, to landward according to the trace sediment exchange model. Significant grain size trends exist in Christchurch and Yangpu Harbours. In the entrance to Christchurch, net transport is landward, in agreement with the pattern defined by the trace sediment exchange model. Within the harbour, the trends show seaward and landward transport in the south and north, respectively. In yangpu Harbour, landward transport in the inner part of the tidal basin, seaward transport in the entrance, and clockwise circulation outside the harbour have been identified. Equilibrium conditions of the three inlet systems indicate that Christchurch is wave- dominated and characterised by a small cross- sectional area, whilst Langstone and Yangpu show tidal dominance and larger cross-sectional areas. Thus, the degree of stability of Christchurch Harbour is lower than that of the other inlets. It is concluded that: (i) application of the grain size trend analysis to marine environments requires sampling intervals large enough to reflect the real grain size trends resulting from sediment transport, but small compared with the dimensions of the sedimentary environment; (ii) the model for trace sediment exchange developed here can be used to identify net transport patterns in estuaries and tidal inlets, overcoming the difficulty of using the mere presence of the tracer as an indicator of net transport; and (iii) as a modification of the classical O'Brien (1969) and Bruun Methods, inlet equilibrium and stability conditions should be related to sediment transport patterns and other factors (in addition to the tidal prism and, in the Bruun's Method, the longshore sediment transport rate).

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