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along with classic estuary stability theory, informs the hydraulic and ecological
impacts of jetty and training wall construction and their future prognosis.
This chapter outlines theories related to the hydraulics of tidal channels and bays
in communication with the ocean, the impacts of training walls and jetties on estuarine hydraulics, sediment transport, channel stability and the subsequent impacts on
the marine ecology of the associated estuaries. The impacts are illustrated with three
well-documented examples from two large and one smaller estuary on the Australian
eastern seaboard.
10.2 Estuary Characteristics
10.2.1 Conceptual Model
The particular situations described herein apply only to specific estuary types and
do not include large drowned river valleys or long narrow rivers that, generally, are
not as susceptible to these processes. The natural elements that characterize the
behavior of the estuaries of interest include an infinitely large water source (the
ocean) with a periodic tide of reasonable amplitude connected to a relatively large
coastal bay or lagoon that has the potential to generate a large tidal prism, by a relatively narrow and shallow erodible sandy channel; the ocean entrance of the channel
being exposed to waves and the ingress of littoral drift.
In their natural state, on shores experiencing relatively high rates of littoral drift
transport and in the absence of any significant precipitation, such estuaries trend
towards closure (Brown 1928; Bruun 1978; Gordon 1990). However, flood events
and, sometimes, large spring tides may scour the surf zone bars, the entrance channel and shoals, thereby removing the littoral drift that had been deposited over time
and had choked the flow. The resulting reduction in hydraulic impedance at the
entrance produces greater tidal discharges and, hence, the tidal ranges in the bay
increase for a time. However, during ensuing dry periods, waves and currents reestablish the entrance bars and move littoral drift back into the entrance, resulting in
the reformation of the marginal shoals, thereby increasing the impedance to the
penetration of the tidal wave with the resulting loss of hydraulic efficiency to drive
flows to and from the bay. The tidal range in the bay reduces progressively until
either the entrance closes or the tidal flushing and occasional rainfall events are sufficient to keep a channel open, albeit in a shoaled state.
The action that alters fundamentally the natural cycle described above is the
construction of entrance jetties that both increase depths over the offshore bars and
limit or eliminate the ingress of littoral drift to the entrance channel. This produces
a more hydraulically-efficient entrance allowing greater penetration of the tidal
wave into the estuary. This increase in hydraulic conveyance depends upon the
degree to which the works modify the behavior of the entrance bar. Experience on
the Australian eastern seaboard has shown that a single jetty tends to have only a
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…
along with classic estuary stability theory, informs the hydraulic and ecological
impacts of jetty and training wall construction and their future prognosis.
This chapter outlines theories related to the hydraulics of tidal channels and bays
in communication with the ocean, the impacts of training walls and jetties on estuarine hydraulics, sediment transport, channel stability and the subsequent impacts on
the marine ecology of the associated estuaries. The impacts are illustrated with three
well-documented examples from two large and one smaller estuary on the Australian
eastern seaboard.
10.2 Estuary Characteristics
10.2.1 Conceptual Model
The particular situations described herein apply only to specific estuary types and
do not include large drowned river valleys or long narrow rivers that, generally, are
not as susceptible to these processes. The natural elements that characterize the
behavior of the estuaries of interest include an infinitely large water source (the
ocean) with a periodic tide of reasonable amplitude connected to a relatively large
coastal bay or lagoon that has the potential to generate a large tidal prism, by a relatively narrow and shallow erodible sandy channel; the ocean entrance of the channel
being exposed to waves and the ingress of littoral drift.
In their natural state, on shores experiencing relatively high rates of littoral drift
transport and in the absence of any significant precipitation, such estuaries trend
towards closure (Brown 1928; Bruun 1978; Gordon 1990). However, flood events
and, sometimes, large spring tides may scour the surf zone bars, the entrance channel and shoals, thereby removing the littoral drift that had been deposited over time
and had choked the flow. The resulting reduction in hydraulic impedance at the
entrance produces greater tidal discharges and, hence, the tidal ranges in the bay
increase for a time. However, during ensuing dry periods, waves and currents reestablish the entrance bars and move littoral drift back into the entrance, resulting in
the reformation of the marginal shoals, thereby increasing the impedance to the
penetration of the tidal wave with the resulting loss of hydraulic efficiency to drive
flows to and from the bay. The tidal range in the bay reduces progressively until
either the entrance closes or the tidal flushing and occasional rainfall events are sufficient to keep a channel open, albeit in a shoaled state.
The action that alters fundamentally the natural cycle described above is the
construction of entrance jetties that both increase depths over the offshore bars and
limit or eliminate the ingress of littoral drift to the entrance channel. This produces
a more hydraulically-efficient entrance allowing greater penetration of the tidal
wave into the estuary. This increase in hydraulic conveyance depends upon the
degree to which the works modify the behavior of the entrance bar. Experience on
the Australian eastern seaboard has shown that a single jetty tends to have only a
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…