Category F - Barrier enclosed lagoons or drowned valleys
Similar in some respects to Category E estuaries, these have shallow basins and narrow mouths, usually formed by a spit or sand barrier. However, Category F estuaries have complex shorelines and numerous arms leading off a main basin.
As a result of this, the extensive intertidal area of Category F estuaries tends to be cut by deep channels caused by drainage from the arms. Sand bars and shoals , which take the form of ebb and flood tidal deltas, occur at the estuary mouth on shores where there is littoral drift (movement of particles such as sand along the shore, caused by wind and waves). On shores with low littoral drift, entrances to the estuary are funnel-shaped.
The tidal prism - the difference between the mean high-water volume and the mean low-water volume of the estuary – makes up a large proportion of the basin volume.
Water inputs from the river during the tidal cycle are very small compared with the total volume of the estuary. As a result, the estuary's water movement processes are dominated by the tides.
Two-dimensional circulation, mixing and wave resuspension generated by the wind is less than pronounced than for Category E estuaries, as their narrow arms mean that the wind fetch (the length over which wind can blow) is small. This means that the main body of the Category F estuaries generally has sandy bottom sediment, with muddy bottom sediment in the upper parts of the arms.
The complexity of the Category F estuary's shape means that these estuaries are not well flushed, because water is trapped in the arms. Although the main body of these estuaries is reasonably well mixed, the upper parts of narrow arms tend to have weak stratification, and salt wedges.
Salinity in the main body of the estuary is close to the ocean's, and gets lower as one progresses further up the arms. Ocean swell can resuspend sediment in the entrance of Category F estuaries which have wider mouths at high tide, when any screening from the ebb tidal delta is minimised.
Category F estuaries are representative of features commonly called barrier enclosed lagoons or drowned valleys.