Dawesville Channel
Encyclopedia
Dawesville Channel is a man-made channel
between the Peel-Harvey Estuary
and the Indian Ocean at Dawesville
about 80 km south of Perth
in Western Australia
. It is just south of the regional city of Mandurah
and just north of Yalgorup National Park
.
The channel was constructed to alleviate an environmental problem which had developed where much of the 136 km², two metre deep estuarine system had become eutrophic. The adjoining estuaries' ability to support the natural flora and fauna had become seriously degraded and the smell of rotting algae, particularly during the summer months had caused increasing complaints from residents. Stocks of the normally abundant fish and crabs had become depleted and recreational use was impacted. The deteriorating environmental situation became a major political and environmental issue
for the Government of Western Australia
during the mid 1980s.
The channel allows seawater from the Indian Ocean
to regularly flush in and out of the estuary using the daily tidal movements which in turn prevent the build up of algae.
, Serpentine
and Harvey River
s and their tributaries. This represents almost all of the annual rainfall, and so in the drier summer months the rainfall is all but non-existent. This is due to the region's Mediterranean climate
.
The first European settlers had found the area almost impassable for many months of the year as the flatlands above the estuary become vast floodplains. During the late 1800s, landowners and governments started constructing major drainage systems and culverts to remove the excess water to free up grazing land for livestock and pastures. Forested areas were also cleared. Since then, over 100 years of development of the low-lying sandplain depleted the moisture holding capacity of the soil and any rainfall that fell in the region quickly dissipated into the drains. Soil quality became increasingly degraded and farmers reverted to excessive superphosphate use to compensate for the loss of nutrients.
In the early 1970s, several industries, including a large piggery and sheep holding paddocks within the lower catchment poured quantities of nutrients into the river system.
A tipping point
occurred during the 1970s and 1980s when the discharge of nutrients into the estuary resulted in it becoming eutrophic. Growths of macroalgae in the form of toxic cyanobacteria nodularia spumigena started occurring on a regular seasonal basis. The combination of sunlight and stagnant, nutrient rich, heated water caused massive blooms of blue-green algae.
In the meantime, the region surrounding the estuaries, particularly the coastal strip south of Mandurah was undergoing a major population growth.
Some programs that were undertaken with various degrees of success included:
load in the water which had come from run-off associated with agricultural and industrial practices in the catchment area. They formed the conclusion that altered procedures for applying fertiliser would improve the estuary, but that a dramatic improvement in the short-term would only be achieved by cutting a new channel to the ocean.
The proposed channel would increase water exchange between the estuary and the ocean to facilitate the flushing of nutrients from the estuary out to sea, and would increase the salinity to levels that would restrict germination and growth of the blue-green algae. A daily tidal movement of about 1 metre assisted the flushing process.
The solution also included widening and deepening of the natural estuary opening into the sea near the Mandurah
township.
The channel is spanned by the four lane Port Bouvard Bridge which forms part of the Old Coast Road
which carries traffic between Perth and the southern coastal townships. Approximately 40,000 vehicles per day use the road. Major residential canal developments have been constructed off the channel including the suburb of Wannanup
and a golf course
.
Since its opening in 1994 no blue-green algal blooms have occurred.
Approximately 85,000 m³ of sand per year is mechanically moved from the south to the northern side of the channel. This is done by converting the build up in the catchment area into a slurry and pumping the mixture across the channel in one of two submerged 315 mm pipes.
Channel (geography)
In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...
between the Peel-Harvey Estuary
Peel-Harvey Estuary
The Peel Harvey Estuary is a natural estuary which lies roughly parallel to the coast of Western Australia and south of the town of Mandurah. The strip of land between the Indian Ocean and the estuary carries the Old Coast Road and to the east is the Forrest Highway which is the main thoroughfare...
and the Indian Ocean at Dawesville
Dawesville, Western Australia
Dawesville is a suburb of Mandurah, and is located between the Harvey Estuary and the Indian Ocean.It is also home to the largest inland waterway in Western Australia - the Harvey Estuary and the Peel Inlet which combine to produce an array of fishing and crabbing options in the summer months...
about 80 km south of Perth
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....
in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
. It is just south of the regional city of Mandurah
Mandurah, Western Australia
Mandurah is the second-largest city in Western Australia and is located approximately south of the state capital, Perth.The city attracts a large number of tourists, including many international visitors...
and just north of Yalgorup National Park
Yalgorup National Park
Yalgorup National Park is a national park in Western Australia , 105 km south of Perth, and directly south of MandurahThe park is located on the western edge of the Swan Coastal Plain and contains a chain of about ten lakes, the name rises from the two Noongar words Yalgor meaning lake and up...
.
The channel was constructed to alleviate an environmental problem which had developed where much of the 136 km², two metre deep estuarine system had become eutrophic. The adjoining estuaries' ability to support the natural flora and fauna had become seriously degraded and the smell of rotting algae, particularly during the summer months had caused increasing complaints from residents. Stocks of the normally abundant fish and crabs had become depleted and recreational use was impacted. The deteriorating environmental situation became a major political and environmental issue
Environmental issue
Environmental issues are negative aspects of human activity on the biophysical environment. Environmentalism, a social and environmental movement that started in the 1960s, addresses environmental issues through advocacy, education and activism.-Types:...
for the Government of Western Australia
Government of Western Australia
The formation of the Government of Western Australia is prescribed in its Constitution, which dates from 1890, although it has been amended many times since then...
during the mid 1980s.
The channel allows seawater from the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
to regularly flush in and out of the estuary using the daily tidal movements which in turn prevent the build up of algae.
Background
Between May and October of each year, about 1000 mm of rain fall into the catchment areas of the MurrayMurray River (Western Australia)
The Murray River is a river in the southwest of Western Australia which played a significant part in the expansion of Aboriginal settlement in the area south of Perth after the arrival of British settlers at the Swan River Colony in 1829....
, Serpentine
Serpentine River (Western Australia)
The Serpentine River is a river in the South West region of Western Australia.The river rises in the Darling Scarp below Bowerling Hill and flows westward crossing Albany Highway North of North Bannister. The river continues North West through the Youarling State Forest then the Serpentine National...
and Harvey River
Harvey River
The Harvey River is a river in Western Australia and is the southernmost of the three major waterways which drain into the Peel-Harvey Estuary, with its delta in the southern extreme of the Harvey Estuary. It is about 90 km in length, rising near Mount Keats...
s and their tributaries. This represents almost all of the annual rainfall, and so in the drier summer months the rainfall is all but non-existent. This is due to the region's Mediterranean climate
Mediterranean climate
A Mediterranean climate is the climate typical of most of the lands in the Mediterranean Basin, and is a particular variety of subtropical climate...
.
The first European settlers had found the area almost impassable for many months of the year as the flatlands above the estuary become vast floodplains. During the late 1800s, landowners and governments started constructing major drainage systems and culverts to remove the excess water to free up grazing land for livestock and pastures. Forested areas were also cleared. Since then, over 100 years of development of the low-lying sandplain depleted the moisture holding capacity of the soil and any rainfall that fell in the region quickly dissipated into the drains. Soil quality became increasingly degraded and farmers reverted to excessive superphosphate use to compensate for the loss of nutrients.
In the early 1970s, several industries, including a large piggery and sheep holding paddocks within the lower catchment poured quantities of nutrients into the river system.
A tipping point
Tipping point
In sociology, a tipping point is the event of a previously rare phenomenon becoming rapidly and dramatically more common. The phrase was coined in its sociological use by Morton Grodzins, by analogy with the fact in physics that adding a small amount of weight to a balanced object can cause it to...
occurred during the 1970s and 1980s when the discharge of nutrients into the estuary resulted in it becoming eutrophic. Growths of macroalgae in the form of toxic cyanobacteria nodularia spumigena started occurring on a regular seasonal basis. The combination of sunlight and stagnant, nutrient rich, heated water caused massive blooms of blue-green algae.
In the meantime, the region surrounding the estuaries, particularly the coastal strip south of Mandurah was undergoing a major population growth.
Attempts at resolving the algae problem
For over twenty years, government and agricultural industry groups battled with the issue, largely unsuccessfully. The scale of the problems seemed daunting and because there seemed to be numerous causes, and most stakeholders blamed each other. As a result, the problem got worse and the issue became politicised.Some programs that were undertaken with various degrees of success included:
- development of a "sandplain" superphosphate by CSBP which retained its nutrients for much longer
- large-scale top-dressing of farm land with bauxite tailings taken from ALCOA's mines
- replanting cleared land
- refurbishment of waterland fauna habitats (using work-for-the-dole labour)
- removal of some of the large scale industries including the piggery
- educating farmers on better farming practice, including restrictions placed on use of phosphate-laden fertilisers.
- harvesting of algae on the estuary foreshores (using graders) and trucking the collected material off-site. Other automated mechanical harvesting systems were also tried.
A solution
The solution was proposed after a number of studies including those of environmental scientists Professor Arthur McComb and Ernest Hodgkin who identified that the cause of the blooms were the phosphorusPhosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
load in the water which had come from run-off associated with agricultural and industrial practices in the catchment area. They formed the conclusion that altered procedures for applying fertiliser would improve the estuary, but that a dramatic improvement in the short-term would only be achieved by cutting a new channel to the ocean.
The proposed channel would increase water exchange between the estuary and the ocean to facilitate the flushing of nutrients from the estuary out to sea, and would increase the salinity to levels that would restrict germination and growth of the blue-green algae. A daily tidal movement of about 1 metre assisted the flushing process.
The solution also included widening and deepening of the natural estuary opening into the sea near the Mandurah
Mandurah, Western Australia
Mandurah is the second-largest city in Western Australia and is located approximately south of the state capital, Perth.The city attracts a large number of tourists, including many international visitors...
township.
Construction
After several years of deliberations, a decision was made to build the channel at an estimated cost of $37 million. The channel's construction commenced in 1990 and was completed in April 1994. It is about 2.5 kilometres long, 200 metres wide and between 6 and 6.5 metres deep. Construction involved the excavation of about 4.5 million cubic metres of material. In addition to the excavation work, construction of breakwaters, training walls, a bridge and installation of a sand bypassing system was undertaken.The channel is spanned by the four lane Port Bouvard Bridge which forms part of the Old Coast Road
Old Coast Road
Old Coast Road is a Western Australian highway linking Mandurah, WA's second-largest city located 75 km south of Perth, with Bunbury, WA's third-largest city. It is signed as Route 1 for most of its length and is long.-Overview:...
which carries traffic between Perth and the southern coastal townships. Approximately 40,000 vehicles per day use the road. Major residential canal developments have been constructed off the channel including the suburb of Wannanup
Wannanup, Western Australia
Wannanup is a residential suburb in Western Australia, located southwest of Mandurah and south-southwest of the state capital, Perth. It is surrounded on three sides by water — the Indian Ocean to the west, the Harvey Estuary to the east and the Dawesville Channel to the south — and is home to...
and a golf course
Golf course
A golf course comprises a series of holes, each consisting of a teeing ground, fairway, rough and other hazards, and a green with a flagstick and cup, all designed for the game of golf. A standard round of golf consists of playing 18 holes, thus most golf courses have this number of holes...
.
Since its opening in 1994 no blue-green algal blooms have occurred.
Sand dredging
As part of the construction, a sand trap immediately to the south of the sea opening was incorporated into the design to capture sand build up from the natural south to north movement along the coast caused by the predominant south-westerly winds. If the sand was not captured and mechanically moved, the channel would quickly close as there is insufficient water flow out of the channel to compensate for the build up.Approximately 85,000 m³ of sand per year is mechanically moved from the south to the northern side of the channel. This is done by converting the build up in the catchment area into a slurry and pumping the mixture across the channel in one of two submerged 315 mm pipes.