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location
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aerial
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Location
The Sierra Place flood retarding basin is situated within Sierra
Place Reserve on Toongabbie Creek Baulkham Hills, 10 km upstream
of the Parramatta River. Access to the main basin wall is from Sierra
Place while access to the secondary levee/wall Vis from Valerie
Avenue Baulkham Hills.
Purpose
An existing flood retarding basin built in 1990 was amplified by
the Trust in 2001 to prevent the basin overtopping in the 100 year
flood and to reduce the risk of McCoy Park basin downstream overtopping
in the 100 year flood. The work will reduce property flooding along
the upper reaches of Toongabbie Creek, notably at the "confluence"
area 2.4 km downstream where 309 properties and 44 dwellings were
subject to 100 year flooding. Ancillary benefits include better
access paths, landscaping (including a landscape masterplan for
the reserve), weed removal and bush regeneration in the basin area.
Catchment
The catchment draining to the basin wall has an area of 1156 hectares.
The basin wall is located approximately 1.5km upstream of McCoy
Park flood retarding basin.
Basin Wall
The basin wall originally was constructed of an earth embankment
approximately 90m long with a maximum height of 7m. The grass spillway
has an underlying scour protection consisting of a layer of Roller
Compacted Concrete (RCC) up to 0.9m thick. The embankment has a
slope of 1V to 4H. Twin culverts each 3.3m x 2.7m were designed
to carry the 1% Annual Exceedance Probability (AEP) flows while
the embankment was designed to safely pass the 0.5 Probable Maximum
Flood (PMF). A secondary levee/wall was also constructed upstream
to protect low-lying properties. The recently completed amplification
works involved the construction of a 2.5m high wall on the main
embankment with a length of approximately 150m. The secondary wall
was raised by 1.5m.
Design
The original construction included a dissipater structure downstream
of the culvert and a hooded culvert upstream, which was designed
using a physical model. Raising the main wall also involved filling
the adjoining privately owned golf driving range which was carried
out previously in conjunction with excavating a high level floodway
on the creek downstream of McCoy Park at Toongabbie.
Construction
The original basin wall was built in 1990. The reconstructed wall
was built between September 2000 and April 2001. The new wall was
cast in situ with dowel bars drilled into the RCC on the main wall.
The top 350 mm of the existing secondary wall was broken out and
new reinforcing steel was welded to the existing steel bars before
the concrete was poured.
Funding
The cost of the amplification works is approximately $1.25 million.
Funding has been provided by the NSW Department of Land and Water
Conservation (66%) and the Trust (33%).
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