342
10.3.5 Lake Wagonga
The Lake Wagonga estuary is situated at Narooma on the NSW south coast
(Fig. 10.22). Twin entrance jetties were constructed in 1976–1978, primarily to
improve entrance navigability for the commercial fishing fleet (MHL 1994). The
estuary comprises a steep-sided bay of area around 7 km
2
(MHL 2001); an order of
magnitude smaller than Wallis Lake and Lake Macquarie. A regular 3,250 m long
entrance channel has a depth around 2.0 m, an average width of around 100 m and
has intertidal training walls constructed of rock rubble. The spring tidal range in the
bay is around 0.7 m and on the higher spring ebb tides the peak channel velocities
approach 2 m/s (MHL 2001).
Regular tidal stage measurements are available from 1997. As shown in
Fig. 10.23, the spring tidal range has increased steadily over the period of record at
an average rate of 3.0 mm/a (R
2
= 0.84) and the bay-to-ocean spring range ratio has
been increasing annually at an average rate of around 0.0033/a (R
2
= 0.91). The
change history of the major spring tidal constituent phase lag is in Fig. 10.24,
indicating a steady reduction of around 0.2°/a (R
2
= 0.80).
The regular features of this estuary allow for a considered derivation of an
Escoffier Diagram, which is presented in Fig. 10.25. The Escoffier Diagram confirms the trend in the field data, indicating that the estuary channel is in an unstable
scouring mode.
Without limitations, such as the influence and behavior of the channel training
walls, the indications are that the channel could scour to more than double its present cross-sectional area, leading to the bay achieving almost full ocean tidal range.
At current rates and without constraint, full ocean tidal range in the bay is predicted
to be achieved within 120 years. However, taking the approach of Mota Oliviera
(1970), at an average rate of increase for a B /a O of around 0.0033/a (Fig. 10.23), it
would take some 20 to 50 years, rather than 120 years, for the equilibrium crosssectional area to be reached.
The trends, based on the relatively short 18 years’ record, currently are linear
with no indications of any decreasing rates of change to the amplitudes or phase
lags of the major spring tidal constituents. However, as indicated by the Escoffier
Diagram, once the critical flow area has been exceeded, the channel velocities are
predicted to decline and the rate of change of the tidal range in the bay also may
decline, extending the time required to reach stability.
10.4 Impacts on Coastal Processes
10.4.1 Wallis Lake
Prior to training wall and jetty construction, the entrances exemplified herein were
plagued with shifting sand shoals and, occasionally, were closed to navigation. Jetty
construction intersected the surf zone bars and eliminated the marginal flood tide
A.F. Nielsen and A.D. Gordon
10.3.5 Lake Wagonga
The Lake Wagonga estuary is situated at Narooma on the NSW south coast
(Fig. 10.22). Twin entrance jetties were constructed in 1976–1978, primarily to
improve entrance navigability for the commercial fishing fleet (MHL 1994). The
estuary comprises a steep-sided bay of area around 7 km
2
(MHL 2001); an order of
magnitude smaller than Wallis Lake and Lake Macquarie. A regular 3,250 m long
entrance channel has a depth around 2.0 m, an average width of around 100 m and
has intertidal training walls constructed of rock rubble. The spring tidal range in the
bay is around 0.7 m and on the higher spring ebb tides the peak channel velocities
approach 2 m/s (MHL 2001).
Regular tidal stage measurements are available from 1997. As shown in
Fig. 10.23, the spring tidal range has increased steadily over the period of record at
an average rate of 3.0 mm/a (R
2
= 0.84) and the bay-to-ocean spring range ratio has
been increasing annually at an average rate of around 0.0033/a (R
2
= 0.91). The
change history of the major spring tidal constituent phase lag is in Fig. 10.24,
indicating a steady reduction of around 0.2°/a (R
2
= 0.80).
The regular features of this estuary allow for a considered derivation of an
Escoffier Diagram, which is presented in Fig. 10.25. The Escoffier Diagram confirms the trend in the field data, indicating that the estuary channel is in an unstable
scouring mode.
Without limitations, such as the influence and behavior of the channel training
walls, the indications are that the channel could scour to more than double its present cross-sectional area, leading to the bay achieving almost full ocean tidal range.
At current rates and without constraint, full ocean tidal range in the bay is predicted
to be achieved within 120 years. However, taking the approach of Mota Oliviera
(1970), at an average rate of increase for a B /a O of around 0.0033/a (Fig. 10.23), it
would take some 20 to 50 years, rather than 120 years, for the equilibrium crosssectional area to be reached.
The trends, based on the relatively short 18 years’ record, currently are linear
with no indications of any decreasing rates of change to the amplitudes or phase
lags of the major spring tidal constituents. However, as indicated by the Escoffier
Diagram, once the critical flow area has been exceeded, the channel velocities are
predicted to decline and the rate of change of the tidal range in the bay also may
decline, extending the time required to reach stability.
10.4 Impacts on Coastal Processes
10.4.1 Wallis Lake
Prior to training wall and jetty construction, the entrances exemplified herein were
plagued with shifting sand shoals and, occasionally, were closed to navigation. Jetty
construction intersected the surf zone bars and eliminated the marginal flood tide
A.F. Nielsen and A.D. Gordon