At Lagoon Outlets
A small tidal rise spreading tidal water over a large expanse of lagoon or inland backwater causes the influx and efflux of the tide to maintain a deep channel through a narrows no longer confined by a bank on each side, becomes dispersed, and owing to the reduction of its scouring force, is no longer able at a moderate distance from the shore effectually to resist the action of tending to form a continuous beach in front of the outlet. Hence a bar is produced that diminishes the available depth in the approach channel. By carrying out a solid jetty over the bar, however on each side of the outlet, the tidal currents are concentrated in the channel across the bar, and lower it by scour. Thus the available depth of the approach channels to Venice through the Malamocco and Lido outlets from the Venetian Lagoon have been deepened several feet (metres) over their bars by jetties of rubble, carried out across the foreshore into deep water on both sides of the channel. Other examples are provided by the long jetties extended into the sea in front of the entrance to Charleston harbour, formerly constructed of fascines weighed down with stone and logs, but subsequently of rubble stone, and by the two converging rubble jetties carried out from each shore of Dublin Bay for deepening the approach to Dublin harbour.
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