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Location
The McCoy Park flood retarding basin is situated within McCoy Park
at the confluence of Toongabbie and Blacktown Creeks approximately
7.5 km upstream of the Parramatta River. Immediately downstream,
Greystanes Creek and Pendle Hill Creek join Toongabbie Creek so
that effectively four creeks meet at what is known as the "confluence
area".
Purpose
Although the original 1976-1980 design indicated that flood levels
would be reduced as far as Lennox Bridge at Parramatta, the benefits
occur mainly in the area immediately downstream. Channel clearing
works accompanied the basin construction as far downstream as Lister
Street, Old Toongabbie with later clearing as far as the second
bridge on Old Windsor Road.
Catchment
The catchment draining to the basin wall has an area of approximately
31 square kilometres. The basin outlet is located approximately
2.5km downstream of Sierra Place flood retarding basin on Toongabbie
Creek.
Basin Wall
The basin embankment is constructed of compacted earth fill to a
maximum height of 5 metres (m) at the outlet, although the maximum
height of the embankment above its foundation level is approximately
3.5 m. The embankment crest is 800 m long and 3.0 m wide. The bank
slopes are 1 vertical to 4 horizontal. The capacity of the basin
is 400,000 m3. The outlet structure of the basin is a 13 m wide
slot with reinforced concrete side walls and base slab.
Design
The original design 100 year flood peak inflow was 319 m3/sec and
peak outflow was 301 m3/sec. The basin was designed to attenuate
(delay) the flood peak rather than to reduce the peak flow. This
would allow floodwaters in Pendle Hill Creek and Greystanes Creek
to pass before the peak discharge from the basin occurred. The current
100 year flood peak inflow is 420 m3/sec and peak outflow is 305
m3/sec.
A piped low flow system was installed during construction of the
basin. This was intended to convey dry weather flows from the two
tributaries, Toongabbie Creek and Blacktown Creek, beneath the floor
of the basin to downstream of the basin's outlet structure. However,
the pipe from Toongabbie Creek has become blocked, leading to the
creation of a natural wetland in the upper bay of the basin. Parramatta
City Council is currently investigating options for the future of
the wetland because it cannot be readily maintained in its present
form.
Construction
The detention basin was constructed by Parramatta City Council and
completed in 1987. After a section of the main embankment opposite
the end of Pendle Hill Creek was badly eroded during the August
1986 flood, this section was reconstructed. Channel clearing works
in Toongabbie Creek downstream of the basin were carried out from
1987 to 1989.
Funding
The total cost of the project was $2.2 million which was shared
between Parramatta City Council, (20%) and the State (40%) and Federal
(40%) governments.
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