Canning Dam
Perth's primary water supply until 1961, still supplies 20%.
The Canning Dam, a concrete gravity structure on the Canning River near Ashendon in Armadale, Western Australia, was finished in 1940. Its reservoir became a key drinking water source for Perth. The dam's design was groundbreaking for its time, featuring advanced structural and hydraulic engineering.
Quick Facts
- Country
- Australia
- Location
- Ashendon, City of Armadale, Western Australia
- Construction began
- 1933
- Opening
- 1940
- Cost
- £A1.1 million
- Operator
- Water Corporation
Facts from the source article.
History
Development of the Canning River as a water source for Perth was first proposed in an 1896 report of the first Metropolitan Water Works Board. Engineer Thomas Hodgson surveyed and proposed the current dam site in 1897, but government funds were not allocated until the Great Depression of the 1930s. In March 1923, Premier James Mitchell announced the damming of the Canning River as part of the Hills Water Supply Scheme, which also included Churchman Brook Dam and Wungong Dam. A small pipehead dam was built 6 km downstream in 1924 as a temporary measure, but a major reservoir was soon needed. Work on the current dam began nine years later, in 1933, and was completed in 1940 at a cost of £A 1.1 million. Engineer Russell Dumas designed the dam and directed most of its construction. In 1951 a concrete-lined channel was built to divert stream flow from the Kangaroo Gully catchment. The Canning Dam and reservoir served as Perth's primary water source until the city's growth in the 1960s and the completion of Serpentine Dam in 1961. In 1975 the reservoir was connected to Perth's Integrated Water Supply System via the Canning Tunnel; before that, water flowed through the Canning Contour Channel to Gosnells. The reservoir also stores water from the Kwinana Desalination Plant, pumped via the Forrestdale Pumping Station.
Hydrology
The Canning Dam catchment lies within the Darling Scarp, part of an Archaean Shield composed largely of granite with some metamorphosed sedimentary and volcanic rock belts. The dam wall sits in a narrow east–west gorge with rock sides sloping upward from the river bed. Behind the wall, the south branch of the Canning River joins the main stream, forming a lake with three major arms extending east, south-east and south. The catchment area is 804 km²; the reservoir is at 200 m AHD, and the highest point, Mount Cooke, is at 582 m AHD. The area receives about 900 mm of rainfall per annum, mostly between May and September, with variability from 700 to 1300 mm across the catchment. Since 1975, long-term average rainfall at the dam wall has decreased by 20%, and streamflow into the catchment by about 60%—average annual inflow dropped from 52 GL (1948–1974) to 22 GL (1975–2004).
Construction
Several innovative design concepts and construction methods new to Australia were introduced at the Canning Dam, while others used at the nearby Wellington Dam were improved upon. To prevent water seepage between the foundation rock and the structure, the foundation was cut back to solid unfractured rock, and a cut-off trench was placed near the upstream face. Downstream of this trench, a rock-filled drain with open jointed pipes intercepted seepage between the rock face and concrete. An internal drainage system, considered innovative at the time, included near-vertical tubular 8-inch cut-off drains at 5-foot intervals to relieve internal seepage through the concrete. State-of-the-art materials-handling methods were used, but some labour-saving machinery worked alongside operations intended to maximise labour content. Sustenance workers handled on-site preparation, road construction, foundation excavation, clearing timber from the reservoir basin, and some concreting; skilled workers fixed formwork for concrete pours, likely at normal day labour rates. Bulk handling of cement was used for the first time in Australia, saving significantly compared to bagged cement. Construction proceeded smoothly except for a March 1934 storm that brought 130 mm of rain in under two days, flooding the dam foundation workings; pumps were installed and work resumed three days later. The dam was completed in September 1940. At completion, it was the longest concrete gravity dam in Australia and the second highest after Burrinjuck Dam in New South Wales. As of 1997, it remained the fifth highest and sixth longest among Australia's 90 large concrete and masonry gravity dams, and is the largest concrete dam in Western Australia by crest length and concrete volume.
Environmental issues
Since construction of the Canning Dam and other drinking water supply dams, water flow into the Canning River has been reduced by up to 96%. Endemic freshwater fish species in the Canning River system show low numbers due to habitat loss and reduced linkage between breeding areas from low flows, which prevent upstream migration to breeding and nursery grounds. Stagnant water has created suitable breeding habitat for the introduced mosquitofish. Dramatic flow reductions have significantly altered downstream aquatic macroinvertebrate communities. Poor flushing below the dam wall, combined with excess nutrients from fertilizers and animal waste, has caused algal blooms and eutrophication. Many river pools, important summer refuges and habitats, have been lost to sedimentation and flow regime modification from impoundments. Periodic flooding from the dam is needed to disperse seed, stimulate germination, recharge shallow groundwater tables during drought, and discourage weed growth, but such flooding is reduced during low rainfall periods.
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