Northern River water for Australian cities.
Introduction
The total annual rainfall on the Australian continent vastly exceeds the needs of the small population. However, this rainfall occurs in mainly in the sparsely populated Northern tropics and in Tasmania. The populated areas in the Southern half of the continent are relatively dry and the South Eastern corner is becoming drier due to climate change. The disparity in water supply is most vividly manifest in Queensland where the Northern rivers, principally the Herbert, Tully and Burdekin, pour vast quantities (average 30 cubic kilometers annually [1]) of fresh water into the sea while the Brisbane River has run dry. In NSW the Clarence River discharges about 4 cubic kilometers annually while Sydney catchments are running dry.
The continuing water shortage in South East Queensland has prompted the Queensland Government to fund, in 2006, a $3 Million re-examination of schemes to pipe water from North Queensland to Brisbane [2]. These schemes are confounded by the high cost of pipe infrastructure and the massive energy to pump the water through pipes several hundred kilometres long. Previous estimates of the cost of a pipeline from the Burdekin River to Brisbane were $7.5 Billion for infrastructure and $250 Million for annual operation. Due to the impracticality of pumping water very long distances and faced with critical shortages, the Brisbane and Gold Coast authorities are currently planning to utilize local sources of impure water. In Brisbane the State Government is building plant and pipeline to recycle sewage water at a rate of 136 ML/day (1 ML = one million litres) to cool the Swanbank and Tarong power stations at a cost $ 1.7 Billion [3]. On the Gold Coast the Tugun desalination plant is being built to supply 120 ML of drinking water daily at a cost of $1.2 Billion [4]. These schemes, as well as being expensive are very energy intensive. Operation of the Tugun desalination plant will produce 235,000 tonnes of CO2 annually [4].
A new proposal
The Tully River discharges, each year, 3 Million ML of pristine water into the Coral Sea, or about 9000 ML/day on average [5]. To transport 120 ML/day of Tully water through 1600 km of pipeline would be monumentally expensive in infrastructure and energy. However the East Australian Current could be used to transport the water at much lower cost. This Current, Figure 1, flows from the Coral Sea down the Eastern Coast of Australia to Sydney where it diverges out into the Tasman Sea [6],[7]. It flows at about 3 km/hour and takes about 20 days to flow from Tully to Brisbane. At Tully the Current flows about 50 km off shore on the outer side of the Barrier Reef and at the Gold Coast it flows to within 30 km, it’s closest approach to the mainland during it’s journey South, Figure 2.
Fig 1. The major ocean currents in the Australian region [12]. The East Australian Current arises in the Coral Sea and flows down the coastline from Cairns to Sydney.
Fig 2. A thermal image of the East Australian Current flowing past the Gold Coast.
In this proposal large membrane containers, each holding 60 ML are filled at the mouth of the Tully then towed offshore into the Current and released. Twenty days later the containers reach Brisbane or the Gold Coast and are towed the 30 km to shore where the water is pumped into pipelines supplying the Gold Coast and Brisbane. In fact, all of the 9000 ML per day discharged by the Tully River flows past Brisbane 20 days later. This scheme just encloses 1% of it in a membrane to stop it from mixing with salt water. At any one time there would be 40 of these containers floating on the Current towards Brisbane, each container separated from the other by a distance of about 40 km, Figure 3.
Fig 3. Two 60 ML containers are towed into and out of the current each day. At any one time up to 40 containers would be floating towards the Gold Coast on the Current.
(Containers illustrated at 1500 times actual scale).
The containers would be 200 m long, 30 m wide and 10 m deep (60ML) and formed from 1mm thick fabric reinforced plastic membrane. While the volume of water contained is large (60 ML), equivalent to the volume of a ship of displacement 60,000 tonnes, the actual mass of the container itself is only about 20 tonnes. Thus, after being pumped empty at the Gold Coast, the containers can be removed from the water by rolling onto a frame then loaded by gantry onto rail cars or a barge for transport back to Tully for refilling.
Infrastructure cost of the proposal
The infrastructure cost includes the cost of the fabric reinforced containers, the cost of a tug boat at Tully and another at the Gold Coast, the cost of dredging channels and the cost of the loading and unloading gantries. A 60 ML container with reinforced membrane 1 mm thick requires 20 tonnes of plastic membrane. The resin used to make plastic membrane currently costs about $3000 a tonne. So the base material cost would be only $60,000. However, extrapolation from the cost of the largest membrane bag currently manufactured in Queensland, a 136 tonne pillow tank [8] used by the Army, suggests the cost of a 60 ML container would be closer to $600,000. The cost of 60 containers, the number necessary to supply 120 ML per day via the East Australian Current, is $36 Million. Two tug boats of each of 1 MW power cost about $1 Million [9]. Loading gantries (20 tonne lift capacity), Figure 4, would cost about $2 Million. Alternatively two floating cranes could be used. A barge or a coastal trader (740 tonne capacity, 17 km/hr, $ 0.5 million) would return 16 containers to Tully on a round trip taking 8 days. Brisbane has a 14 m deep seaway and the Gold Coast Cruise Ship Terminal will have a 12 m deep seaway. However, a seaway into the Tully River would need to be dredged and maintained requiring an additional $2 million for a dredge [9]. Thus the total infrastructure cost of the proposal would be around $ 40 Million. In comparison the cost of the Tugun desalination plant is $1.13 billion.
Fig 4. Empty containers 1 rolled onto former 2 then loaded by gantry 3 onto barge 5.
Annual running cost of the proposal.
The running cost of this scheme is low relative to desalination as the water used is pristine river water. The water is transported the 1600 km by the East Australian Current for essentially no cost with only 90 km towing distance for transport between the Current and the coast. The towing cost can be estimated as follows.
The containers are cigar shaped with a length of 200 m and a cross sectional area, A, of 300 square metres. As the containers are nearly submerged it is this cross sectional area, the speed of towing, v, (2.5 m/s) and how streamlined the container is (the drag coefficient, Cd) that determines the force and power required for towing. The containers would be streamlined by shaping the fabric skin and filling to a slight over-pressure so that the container adopts the required shape - a bulbous nose and a tapering tail – similar to the shape of a submarine hull. A reasonable estimate for the drag coefficient of a large streamlined container is Cd = 0.1 [10]. Now the towing force, F, can be calculated from F = Cd(1/2rv2)A = 96000 N. This is equivalent to the force to lift 9.5 tonnes. Once the container is moving at the towing speed of 5 knots (2.5 m/s) the towing power required, P, can be found from the force times the velocity, P = 96000 x 2.5 = 240 kW. The energy, E, to tow each container the towing distance of 90 km is the force times the distance, E = 96000 x 90,000 = 8500 MJ. If the tugboat produces this towing force at 20% efficiency the tugboat will consume 5 times as much fuel energy i.e. 42,500 MJ of fuel energy. This energy corresponds to the consumption of about 1100 litres of diesel fuel. For two containers per day the fuel consumption is 2200 litres/day. The coastal trader would run a continuous 8 day cycle returning 16 containers to Tully with a fuel consumption of 1400 litres/day. Thus the daily fuel consumption required to transport 120 ML/day of water from Tully to the Gold Coast is 3600 litres.
It is useful to consider the dollar cost of energy per kilolitre of water produced. Diesel fuel oil costs about $ 1.20 per litre and for this proposal 3600 litres/day is consumed. Thus, the dollar cost for diesel fuel is just $4320 for 120 ML of water or 3.6 cents per kilolitre. The energy cost of this proposal is much smaller than the $1 per kilolitre that Brisbane residents are currently paying for residential water. Diesel fuel emits 2.7 kg of CO2 for each litre of fuel consumed. Thus, the daily delivery of 120 ML of water by this scheme involves the emission of 9.7 tonnes of CO2. An annual emission of 3540 tonnes of CO2. In comparison the operating cost of the Tugun desalination plant is projected to be $40 Million annually (mainly electrical energy cost) and the Greenhouse gas emission associated with the desalination plant is projected to be 235,000 tonnes annually [4].
Implementation of the proposal
With any radical scheme there are problems to be overcome. Having 40 containers, each as big as an oil tanker, 97% submerged and spaced at 40 km intervals up the Queensland coast may be regarded as a shipping hazard. However, it may be noted that more than 100 coal containers of similar size are currently anchored in the East Australian Current off Dalrymple Bay and Newcastle without undue hazard to shipping. As the containers of this scheme are 97% submerged they are much less susceptible to wind forces than conventional vessels. The water containers could be fitted with light and radio beacons so that the location of the containers could be monitored during their progress to within a few metres by satellite. Should a collision with a ship occur the 1 mm membrane would simply part and 60,000 tonnes of fresh water would mix in the ocean. There would be no damage to the ship (which would probably be unaware of a collision) and, if the beacons remained intact, the container could be retrieved and repaired. The 60 ML container size used in this illustration is about 400 times the volume of the largest pillow tank currently manufactured in Queensland (0.136 ML, cost $8500) [8]. Pillow tanks of 1 ML capacity are produced overseas. Pillow tanks are designed to be self supporting on land and must withstand considerable hydraulic pressure. The containers envisaged in this scheme float nearly submerged in water and are under no hydraulic stress. Thus, the membrane can be lighter and the container larger than would be the case for a land based water storage container. In fact, floating fresh water in the East Australian Current imposes almost no size restriction on the containing membrane. The practical size limit is imposed by seaway channel depths, the stress during towing into and out of the Current at either end of the journey and potential weather induced stresses during storms. The alternative of using linked pods of smaller containers is also envisaged.
This scheme could be trialed by utilizing the much smaller pillow tanks currently commercially available from Queensland suppliers. If successful these smaller tanks could be progressively scaled up towards the size discussed above. The projected infrastructure cost of the scheme ($ 40 million) is about 30 times lower than the cost of the equivalent supply desalination plant. This huge cost difference is primarily due to the use of pristine river water, the 1600 km transport of which is free for most of the journey. The principal unknown is whether 60 ML containers can be manufactured and whether the towing, handling, emptying and filling such a large container of water is feasible. However, it may be noted that towing icebergs of similar or even greater mass is common practice in the Arctic oil industry [11]. In 1996 a US based company [13] towed 3 ML of water in a membrane container on a 160 km voyage as a demonstration of their water bag technology. The same company is currently envisaging large scale water transport in Australia with linked pods of 17 ML containers.
Conclusions
The East Australian Current that flows 2000 km from the Northern tropics to Sydney carries with it the outflow of the Northern rivers. Enclosing only a small percentage of this river water in large membrane containers and allowing the filled containers to float with the Current provides an almost free method of delivering drinking water to the major East Coast cities. Preliminary cost estimates for the supply of 120 ML per day to the Gold Coast indicate this method of water supply may be 30 times less expensive to implement than an equivalent supply by desalination plant and that the method may emit 60 times less Greenhouse gas. These figures suggest that the proposal may also be much less expensive than a pipeline from the Burdekin River to Brisbane. It may be useful to include consideration of this proposal within the scope of the pipeline feasibility study currently being made by the Queensland Government [2]. The above discussion has focused on the supply of water to South East Queensland where the need is acute. However, as the East Australian Current also connects the Clarence River to Sydney, a similar scheme could supply river water to Sydney. Recently the NSW Government approved the construction of a $1.76 Billion desalination plant at Kurnell to supply 125 ML/day to Sydney. In view of the fact that the drying trend in South East Australia is related to climate change the use of high emission desalination plants would seem to be very counterproductive. Australia is a country of drought and flood, of El Nino followed by La Nina. This current drought has led to the investment of huge sums in constructing desalination and recycling plants. The drought will end. When it does these expensive plants will sit idle. The investment cannot be deployed elsewhere. Conversely, the infrastructure of this proposal, the water containers, the tugs, the floating cranes, the dredge and the coastal trader can be usefully deployed elsewhere. The technology of this proposal could be readily exported to anywhere in the world where there is a need for fresh water and there is a favourable ocean current connecting to a river supply however distant that supply may be.
Acknowledgement
The author gratefully acknowledges useful suggestions from Dr Martin Gellender, EPA, Brisbane and Bob Swinton, Technical Editor of Water, AWA.
References
[1] River inputs of nutrients and sediment to the Great Barrier Reef, Furnas et al. www.gbrmpa.gov.au/__data/assets/pdf_file/0018/4275/ws023_paper_03.pdf Table 2.
[2] Pipeline Study, www.abc.net.au/news/newsitems/200607/s1694456.htm
[3] Water Recycling Project, www.qwc.qld.gov.au/About+QWC
[4] Gold Coast Desalination Project, www.goldcoastwater.com.au
[5] Nutrients and suspended sediments in the Tully River, Mitchell et al, www.actfr.jcu.edu.au/idc/groups/public/documents/technical_report/jcudev_015426.pdf Page 41.
[6] East Australian Current www.srfme.org.au/news/images/22mar_eezcurrents.gif
[7] East Australian Current www.marine.csiro.au/remotesensing/spectacular-images/91092916.png
[8] Pillow Tanks www.fabricsolutions.com.au/pillow-tanks.htm
[9] Tugboat, coastal trader and dredge cost www.maritimesales.com
[10] Drag Coefficient: Tim Gourlay, Private Communication.
[11] Towing Icebergs www.athropolis.com/arctic-facts/fact-berg-tow.htm
[12] Australian Sea Currents www.srfme.org.au/news/images/22mar_eezcurrents.gif
[13] Spragg and Associates www.waterbag.com
Ian Edmonds
Dr Edmonds is a physicist who operates an R&D company (www.solartran.com.au) that develops sustainable technologies for the building industry. ian@solartran.com.au
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