337
0.13 m and the maximum spring tide channel velocities are around 1 m/s (Watterson
2010).
Jetties were constructed from 1878 to 1887 (MHL 1994). Utilizing the available
tidal records since 1990, the spring tidal range in the bay has increased steadily at
an average rate of 1.7 mm/a (R
2
= 0.96; Fig. 10.19) with the bay-to-ocean spring
tidal range ratio increasing steadily at a rate of 0.0015/a (R
2
= 0.94).
Figure 10.20 shows a decreasing trend in the phase lag of the major spring tide
constituent of 0.5°/a. These data are strong indicators of an unstable scouring mode.
If it is assumed that the tidal range grew steadily between 1887 and 1990 to reach
0.09 m from an initial value of zero, assuming the inlet being closed in 1887, the
average rate over that period could not have been greater than 0.9 mm/a, suggesting
that the current rate represents an increasing trend.
An Escoffier Diagram for Lake Macquarie is presented in Fig. 10.21, which indicated an unstable scouring mode. Without geomorphologic and/or anthropogenic
constraints, based on the O’Brien (1931) equilibrium cross section, the channel area
could continue to scour, increasing some fivefold, allowing for the tidal range in the
bay to reach around 80% of the full ocean tidal range. At current rates of change,
this could take a further some 450 years. However, taking the approach of Mota
Oliviera (1970), it would take 350 to 400 years for the equilibrium cross-sectional
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
100
1 ,000
10,000
Maximum Channel Velocity (m/s); a
B /a
O
Channel Cross-Sectional Area (m 2 )
1990 Escoffier Curve
O'Brien P/A Relationship
Jarrett P/A twin jetties Pacific coast
aB/aO
1990 Position
2015 Position
Fig. 10.16 Escoffier diagram for Wallis Lake indicating an unstable scouring mode. The O’Brien
and Jarrett P/A relationships indicate a tenfold increase in channel area for the estuary to reach
equilibrium
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…
0.13 m and the maximum spring tide channel velocities are around 1 m/s (Watterson
2010).
Jetties were constructed from 1878 to 1887 (MHL 1994). Utilizing the available
tidal records since 1990, the spring tidal range in the bay has increased steadily at
an average rate of 1.7 mm/a (R
2
= 0.96; Fig. 10.19) with the bay-to-ocean spring
tidal range ratio increasing steadily at a rate of 0.0015/a (R
2
= 0.94).
Figure 10.20 shows a decreasing trend in the phase lag of the major spring tide
constituent of 0.5°/a. These data are strong indicators of an unstable scouring mode.
If it is assumed that the tidal range grew steadily between 1887 and 1990 to reach
0.09 m from an initial value of zero, assuming the inlet being closed in 1887, the
average rate over that period could not have been greater than 0.9 mm/a, suggesting
that the current rate represents an increasing trend.
An Escoffier Diagram for Lake Macquarie is presented in Fig. 10.21, which indicated an unstable scouring mode. Without geomorphologic and/or anthropogenic
constraints, based on the O’Brien (1931) equilibrium cross section, the channel area
could continue to scour, increasing some fivefold, allowing for the tidal range in the
bay to reach around 80% of the full ocean tidal range. At current rates of change,
this could take a further some 450 years. However, taking the approach of Mota
Oliviera (1970), it would take 350 to 400 years for the equilibrium cross-sectional
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
100
1 ,000
10,000
Maximum Channel Velocity (m/s); a
B /a
O
Channel Cross-Sectional Area (m 2 )
1990 Escoffier Curve
O'Brien P/A Relationship
Jarrett P/A twin jetties Pacific coast
aB/aO
1990 Position
2015 Position
Fig. 10.16 Escoffier diagram for Wallis Lake indicating an unstable scouring mode. The O’Brien
and Jarrett P/A relationships indicate a tenfold increase in channel area for the estuary to reach
equilibrium
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…