CockburnCoast - Western Australian Planning Commission

CockburnCoast
The construction of the power station, including the two groynes
forming the intake area, caused erosion to the south that required
the construction of a rock revetment. An additional groyne (groyne
3) was constructed to the north to help reduce the rate of
sedimentation in the intake area.
Groyne 3 has caused beach accretion to the north which
counteracted the downdrift erosion to the south of the Catherine
Point groyne until 2001. Historically, seawalls and timber groynes
had also been installed to mitigate shoreline erosion impacts on the
recreation facilities, freight railway, switchyard and the ANI Bradken
foundry site.
Additional dredging, nourishment and mitigation structures have
been conducted in the Cockburn coast district structure plan project
area and the wider Owen Anchorage.
6.2.3 Coastal processes
The behaviour of the Cockburn coast shoreline is controlled by
human modifications, in combination with seasonal and inter-annual
variation in sediment transport rates and sediment supply. The
sediment transport rates are mainly governed by the sediment
supply, seasonal variation in wave conditions and wind-driven
currents, and the rate of saturation and bypassing of the structures.
The Cockburn coast is partially sheltered from ocean wave action by
the reefs and islands between Garden and Rottnest islands, and the
broad shallows of Success and Parmelia banks. These features act
to modify the prevailing wave directions on a local scale, enhancing
the significance of north-west winter storms compared with the more
frequent south-west storms. During summer and early autumn, the
prevailing wave conditions are south-south-westerly wind waves,
generated by strong sea breezes blowing along the length of the
coastal lagoon from Cockburn Sound to Owen Anchorage.
Seasonal switching of the prevailing wave directions causes
corresponding reversals in the direction of sediment transport and a
rotation in the alignment of pocket beaches. The seasonal switching
also influences the magnitude of the bypassing of Island Street
groyne to the north, Catherine Point groyne to the south, the power
station groyne 3 in both directions and the power station groyne 2 to
the south. Additional seasonal sediment transport occurs due to
aeolian (wind-driven) transport.
Table 6.1: Coastal infrastructure within the Cockburn coast district structure plan project area
Location Works Year(s)
Island Street Rock groyne 1962,
ext. 1996
Catherine Rock groyne 1959, ext.
Point 1962
South Groyne No. 3 1949, ext.
Fremantle 1960?
power station
South Groynes No. 1946/7
Fremantle 1 and 2
power station
South Rock 1948, ext.
Fremantle revetment 1949
power station
Port Coogee* Two 2006
breakwaters
Note: * Outside the district structure plan study area
Why constructed?
During three stages of South Beach
restoration. The second stage was the
construction of the Island Street groyne
with nourishment north of the structure
from offshore dredging (in 1964/65).
Constructed in 1962 to 107m, extended
80m in 1996. Additional rock revetment
protection placed on south side of the
groyne in 2006.
During three stages of South Beach
restoration. The first stage was the
proposed Constructed to 107m in 1959.
Catherine Point groyne, with behaviour
to be observed over the next three
years. Groyne extended 61m in 1962
due to saturation.
Constructed to reduce the quantity of
sediment infilling the South Fremantle
power station holding basin.
Constructed as a holding basin for the
intake water. Groyne No.1 is to the
south (~180 m long) with the discharge
pipe cut through it. Groyne No. 2 is to
the north (~165 m). Groynes were joined
by a bridge. Rapidly filled with sediment
and the intake pipe was extended
offshore. Two walls constructed within
the basin.
Constructed in response to erosion to
the south of the South Fremantle power
ptation intake groynes. Originally ~45m,
extended to ~22m.
Sheltering for Port Coogee harbour
waters.
84 district structure plan