Morphological controls in sandy estuaries: the influence of tidal flats and bathymetry on sediment transport |
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Authors: | Peter Edward Robins Alan G Davies |
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Institution: | (1) Centre for Applied Marine Sciences, School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB, UK |
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Abstract: | The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been
investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a
2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together
with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats
and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry
(i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value
of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar
pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests
that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment
transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining
scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit,
recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate
at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state. |
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