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10.1 Introduction
Continuously, natural inlets on littoral drift shores comprising entrance bars, shoals
and channels are in a state of flux, changing in response to variations in the controlling hydrodynamic forces such as floods, variations in the spring-neap tidal range,
varying wave climates and rates of littoral drift transport. Typically, they scour during flood events but, subsequently, trend towards closure as littoral drift reforms the
entrance bars and shoals, potentially closing the inlet. For much of the time the
wetlands associated with natural inlets are subjected to small tidal ranges, reflecting
shoaled entrances, which control the extent and diversity of the wetland ecology.
The sensitivity of estuarine responses to such variations depends primarily on the
size of the estuary. The most noticeably sensitive estuaries are the small bays and
lagoons, the ocean entrances of which, generally, are closed but can be opened
abruptly to wave and tidal forcing following floods. Such small estuaries are the
least stable when open (Brown 1928; Bruun 1978; Gordon 1990). On the other
hand, large estuaries, while open to the ocean for much, if not all, of the time and,
hence, exposed to a greater range of variable hydrodynamic forcing, often have a
tidal discharge and a channel cross-sectional area that appear to fluctuate about
stable average values.
For the larger estuaries, such as those commonly used for recreational boating or
commercial fishing, ever-changing bars, shoals and channels present uncertainty
and risks to navigation. Further, shoaled entrances can result in the backup of floodwaters causing inundation of waterfront properties. Often the response to navigational and flooding issues has been to construct entrance channel improvements
such as training walls and jetties. In most cases these works have achieved their
intended results, often spectacularly, but many have been implemented without an
understanding of the potential long term impacts. Training walls and jetties can alter
estuarine hydraulics significantly, increasing hydraulic conveyance, inducing scouring of the channels, changing tidal planes and, hence, changing the environmental
conditions of the associated wetlands.
Large estuaries respond slowly to perturbations at their entrances and the signature of any change to their stability may go undetected for many years to decades.
However, once set in motion, a change to the dynamic stability of a large estuary,
which may have been occasioned by jetty construction, for example, or from a rising mean sea level, will be difficult to predict both in the degree of change and time
to reach a new state of dynamic equilibrium.
As a result of technological advances in water level data loggers there is now a
large and growing body of empirical data that allows for a closer examination and
definition of estuarine hydraulics and inlet stability. Hourly water level recordings
allow the determination of high-resolution, objective, statistical estimates of the
tidal constituents on an annual basis that can be used to define accurately the relevant parameters of tidal range, phasing, prism and levels that are used in estuary
stability theories. Examining the time histories of amplitude and phase of tidal constituents within estuaries where jetties have triggered unstable scouring modes,
A.F. Nielsen and A.D. Gordon
10.1 Introduction
Continuously, natural inlets on littoral drift shores comprising entrance bars, shoals
and channels are in a state of flux, changing in response to variations in the controlling hydrodynamic forces such as floods, variations in the spring-neap tidal range,
varying wave climates and rates of littoral drift transport. Typically, they scour during flood events but, subsequently, trend towards closure as littoral drift reforms the
entrance bars and shoals, potentially closing the inlet. For much of the time the
wetlands associated with natural inlets are subjected to small tidal ranges, reflecting
shoaled entrances, which control the extent and diversity of the wetland ecology.
The sensitivity of estuarine responses to such variations depends primarily on the
size of the estuary. The most noticeably sensitive estuaries are the small bays and
lagoons, the ocean entrances of which, generally, are closed but can be opened
abruptly to wave and tidal forcing following floods. Such small estuaries are the
least stable when open (Brown 1928; Bruun 1978; Gordon 1990). On the other
hand, large estuaries, while open to the ocean for much, if not all, of the time and,
hence, exposed to a greater range of variable hydrodynamic forcing, often have a
tidal discharge and a channel cross-sectional area that appear to fluctuate about
stable average values.
For the larger estuaries, such as those commonly used for recreational boating or
commercial fishing, ever-changing bars, shoals and channels present uncertainty
and risks to navigation. Further, shoaled entrances can result in the backup of floodwaters causing inundation of waterfront properties. Often the response to navigational and flooding issues has been to construct entrance channel improvements
such as training walls and jetties. In most cases these works have achieved their
intended results, often spectacularly, but many have been implemented without an
understanding of the potential long term impacts. Training walls and jetties can alter
estuarine hydraulics significantly, increasing hydraulic conveyance, inducing scouring of the channels, changing tidal planes and, hence, changing the environmental
conditions of the associated wetlands.
Large estuaries respond slowly to perturbations at their entrances and the signature of any change to their stability may go undetected for many years to decades.
However, once set in motion, a change to the dynamic stability of a large estuary,
which may have been occasioned by jetty construction, for example, or from a rising mean sea level, will be difficult to predict both in the degree of change and time
to reach a new state of dynamic equilibrium.
As a result of technological advances in water level data loggers there is now a
large and growing body of empirical data that allows for a closer examination and
definition of estuarine hydraulics and inlet stability. Hourly water level recordings
allow the determination of high-resolution, objective, statistical estimates of the
tidal constituents on an annual basis that can be used to define accurately the relevant parameters of tidal range, phasing, prism and levels that are used in estuary
stability theories. Examining the time histories of amplitude and phase of tidal constituents within estuaries where jetties have triggered unstable scouring modes,
A.F. Nielsen and A.D. Gordon