Sediment transport processes in embayments and drowned river valleys
1. Fine and coarse sediment from the catchment
Fine and coarse sediment enters the embayment from the catchment. The amount and type of sediment input varies regionally, depending on catchment and climatic conditions, and the volume of freshwater input.
2. Suspended sediment plume
Fine sediment (i.e., muds, clays, and some organic material) is transported into the embayment as a plume of suspended sediment, and is mixed with the low-turbidity coastal waters. Coarse grained sediment (i.e., sands and gravels) is deposited on the floor of the embayment, close to the river sediment source (Carter et al., 1996).
3. Fine sediment deposition and resuspension
A small amount of fine sediment is deposited along the edges of the embayment, transported by waves in exposed areas, and by tides in sheltered regions (Semeniuk, 1981, Carter et al., 1996). Off-channel embayments within drowned river valleys also tend to accumulate fine sediment (Taylor et al., 1999). Baffling by saltmarshes and/or mangroves (where present) enhances deposition of fine material along the fringes of the embayment (Boorman et al., 1998, Brown, 1998, Saintlan et al., 1999, Laegdsgaard, 2001). Some coarse-grained sediment is generally transported onshore by wave action (in exposed areas) and deposited along the edges of the embayment (Green et al., 2001). Some resuspension of fine sediment occurs in shallow water due to wave energy (Bulthuis et al., 1984).
4. Onshore sediment transport
Onshore sediment transport leads to the development of prograding beach barriers, and occasional extensive dune systems. Limited swamp or estuarine deposits often occur landward of these barriers (Roy et al., 1980, Thom et al., 1978). Beach systems and intertidal flats also form from marine sediment around the narrow internal margins of the embayment, and range in morphology from ‘reflective’ (steep) to ‘dissipative’ (low-gradient) beaches in sheltered and exposed regions, respectively (Wright et al., 1979).
5. Accumulation of coarse marine sediment
Accumulation of coarse marine sediment (including carbonate material such as shell), and some fluvial sediment (depending on river inputs) occurs on the smooth and typically gently sloping floor of the embayment. Because of the large entrance typical of embayments, the seabed is dominated by marine-derived sediment (such as siliciclastic and/or carbonate sand). Carbonate content in the sediment generally increases moving seaward (Taylor, 1972, Sussko et al., 1992). Along wave-dominated coasts, a coast-parallel submerged ‘sill’ may form on the seabed near the entrance of embayments, in place of a sandy barrier and marine tidal delta characteristic of coastal waterways with a higher sediment supply and shallower water depths (Roy, 1984a, Harris et al., 1992, Roy et al., 2001). Where a sub-tidal sill is present, some of the fine sediment may accumulate landward of the sill, due to a reduction in flow energy (Cooper, 2001). Along tide-dominated coasts, coast-perpendicular subaqueous tidal sand banks may occur on the sea bed (Harris et al., 1992, Wells, 1995, Fitzgerald et al., 2000).
6. Large tidal prism and exchange of marine waters
Due to a wide, unconstricted entrance and large tidal prism, the exchange of water between the embayment and the open ocean results in dilution of river-derived suspended sediment, and transport offshore (Bulthuis et al., 1984).