Check valves protect Brisbane central district from floods
Flood prevention in Brisbane, Australia, is paramount to a sense of security for those living in close proximity to the Brisbane River. Kerry Olsson of Wapro AB reports why the Brisbane city council chose WaStop check valves to prevent the reoccurrence of devastating and costly flooding in its central business district after trials, tests, and analysis.
Situated inside a bend of the Brisbane River, the central business district of Brisbane, Australia, is a thriving and trendy area popular with companies, families, and students. The state capital of Queensland, Brisbane is a metropolitan area with a population of 2.3 million people. Today, Brisbane is well known for its distinct Queenslander architecture, which forms much of the city’s built heritage. It also receives attention for its damaging flood events, most notably for those that occurred in 1974 and 2011.
Flooding along the Brisbane River has the potential to be devastating, as documented following previous flood events. The central business district is located 15 kilometers (km) from the mouth of the Brisbane River, where it is in close proximity to an area of known flood risk. For much of the river’s length, its banks are relatively high but topped by a broad plain. The river’s meandering course prevents upstream floodwaters from being quickly discharged into Moreton Bay. Therefore, higher than normal flows cause river levels to rise rapidly, and once the top of the banks are breached, the floodwaters can spread over wide areas of the city.
On January 13, 2011, major flooding occurred throughout most of the Brisbane River catchment, most severely in Toowoomba and the Lockyer Creek catchment, where 23 people drowned, as well as in the Bremer River catchment and in Brisbane. Insurers received some 56,200 claims with payouts totalling US$1.93 billion, but devastation resulting from the flood swept far beyond physical losses.
In all, more than 15,000 properties were inundated in metropolitan Brisbane, with approximately 3,600 homes evacuated and more than 200,000 people affected. Commercial losses of approximately $3.02 billion were reported across the mining, agriculture, and tourism sectors. More than 19,000 kilometers of roads were damaged, around 28 percent of the Queensland rail network damaged, and three major ports significantly affected. An estimated 28,000 homes would need to be rebuilt, while vast numbers of dwellings would require extensive repairs.
During the January 2011 flood, some parts of Brisbane were affected by water that came up from the river through the drainage networks and into the city’s streets, a problem referred to as backflow flooding.
Between July 2011 and May 2012, the city council conducted technical investigations to look at the feasibility of installing backflow devices in areas where backflow flooding occurred. The investigation was required, as these devices are not suitable in all situations.
A backflow device is designed so that water flows in one direction through piped stormwater systems and minimizes water flowing back up stormwater pipes. Backflow devices are effective at preventing stormwater flooding caused by backflow but they do not guarantee protection from other forms of flooding.
Backflow devices are one of many flood mitigation tools and strategies that Brisbane’s city council is considering to help protect the city from the impacts of future flooding. Its wider flood risk management strategy also considers the role of planning measures to assist in managing all flooding risks in Brisbane. Additionally, the investigation focused on the feasibility of backflow devices along the length of the Brisbane River.
The technical investigations were carried out in two stages. Stage 1 consisted of a pre-feasibility investigation for the implementation of backflow prevention technology in three designated case study areas–the central business district (CBD), Rosalie/Milton, and New Farm–in addition to a citywide desktop review of other affected areas along the Brisbane River. Stage 2 included a technical investigation for developing detailed designs for the implementation of backflow devices in the same three case study areas. Investigations were also conducted along the length of the Brisbane River to determine stormwater systems that contributed to backflow flooding in the January 2011 floods and that could be retrofitted with backflow devices.
Backflow devices installed
In 2012, backflow devices were installed on the pre-feasibility sites, and twelve further stormwater systems were identified as high priority sites for backflow device installation. The Brisbane City Council (BCC) completed the installation of WaStop inline check valves and other valves in June 2014. With protective measures now in place, these twelve stormwater systems that were identified as high priority sites for backflow device installation will provide protection to approximately 80 percent of properties that were affected by backflow flooding in January 2011.
In the future, other feasible systems such as backflow devices will be considered in the council’s future drainage program, which is subject to its annual budget and review among citywide priorities.
Technical study results
The Stage 2 technical investigation produced data on the drainage systems that were identified as feasible for installation of backflow devices. Findings were used to calculate a relative cost-benefit ratio in order to produce a draft backflow management strategy. The anticipated installation costs were compared with benefits based on the damages potentially mitigated by backflow devices to produce the ratio. While this ratio provides information about the systems that provide the highest value, the council considered a number of factors when deciding the priority for installing backflow devices. These factors include cost-benefit analysis, the number of properties impacted by floods, the cost of installation, operational issues, previously programmed drainage network upgrades, and critical infrastructure impacted by floods.
Particular attention was given to: (1) the head loss rating curve of the device (typically relating flow rate with head loss); (2) the ability to install the device simply and effectively; (3) whether the device is best installed at an outlet or at some point further upstream; (4) reducing required monitoring and maintenance of the device; (5) the ability of the device to withstand high external pressures and high lateral velocities; (6) suitability for installation in the intertidal zone; (7) types of devices already installed within Brisbane, (8) and the procurement and installation cost of the device.
The WaStop inline check valve, manufactured by Wapro AB of Sweden, was chosen for installation in some of the most critical areas, such as in the Brisbane suburb of Bulimba. The twin DN1800-mm (NPS 72” ) WaStop check valves were chosen for their proven reliability, ease of installation, long-term cost of ownership benefits, and ability to work in silt-prone areas. Installations of a variety of smaller valves from 2011 to 2014 lead to the decision that WaStop was the valve of choice for the three large NPS 72” valves.
Brisbane City Council’s Principal Design Project Manager Drainage, Planning and Design, Chris Wilson says that by using WaStop instead of other alternatives, more than US$570,000 was saved by not needing to reconstruct the outlet structure, which would have been a major project lasting at least three to four months and involving the construction of a coffer dam in the river before the piled foundations could be started. He says, “Yes, there are cheaper backflow devices, but the savings on the civils made the WaStops very attractive. In addition, as we were simply bolting the devices to the existing structure, there was no requirement for any statutory approvals for working within the tidal zone, which saved us another 6 months project time.”
Wilson adds, “In terms of the future maintenance savings, we recently spent more than US$75,000 just to desilt the twin culverts. There will always be some silt washed down the pipes originating from the local catchment, but the WaStops will prevent the additional deposition of river silt and therefore extend the interval between future cleaning.”
Wapro installed the WaStop valves in early 2016 in an installation that went smoothly, quickly, and without incident. The benefit relative to cost for the council was substantial, and the resulting peace of mind for the residents is priceless.