Why does the stormwater outlet have to be replaced
The existing Collaroy Stormwater outlet is a reinforced concrete pipeline and box culvert that extends from the seawall into the surf zone. It provides essential drainage of stormwater from the northern Collaroy catchment out to sea. The existing structure was built in the 1970s; however wave action has damaged the outermost sections and associated piling.
A number of options and locations for a replacement outlet have been investigated. However, after an extensive technical evaluation and community consultation process, Council resolved in June 2012 that the new structure was to be located at the site of the current outlet.
Why does the outlet have to be on the beach?
Stormwater from the Collaroy catchment naturally drains to the low area in Collaroy Street, on the western side of Pittwater Road. The pipe under Pittwater Road and the stormwater outlet replicate the historical drainage path for stormwater out to sea.
The local topography in Collaroy, with the low area to the west of Pittwater Road, and the relatively flat area through the car park, means that the outlet needs to extend well onto the beach in order to provide effective drainage. The end of the outlet has to be below the level of low area in Collaroy Street, and yet remain above the beach sand level so that it does not block.
The optimisation of these design requirements means that the new outlet will be approximately 10 m shorter that the current outlet.
Will the new outlet be bigger or smaller than the current outlet?
The local geographic area and hydraulic capacity requirements (25% greater than the current structure) for the new outlet have substantially determined the size and length of the new outlet.
The first section of the outlet will be a 2.1 m diameter pipe, (slightly larger than the current 1.8 m diameter pipe).
The new design includes a 3 m square surcharge structure, approximately 34 m from the sea wall. This structure allows drainage under extreme conditions of rainfall and high sea level, while allowing the seaward section of the outlet to be much smaller. Under normal beach conditions only the top of the surcharge structure will be visible.
The seaward section of the outlet will be a narrower but slightly taller flat top, vee bottom profile. The vee bottom profile will minimise the effects of wave forces on the structure, and reduce the risk for swimmers.
Why do the works have to be constructed during the summer?
Building the new outlet involves a series of complex construction activities on the beach and in the surf zone. The extended beach sand profile and generally small wave conditions in the summer months provide substantially better construction conditions for the works. This will result in a quicker program with fewer inclement weather delays, and a lower cost to Council.
Will I be able to swim and walk on the beach next summer?
The construction works on the beach will be confined to a narrow area including the existing outlet. An access pathway will be provided for people to walk up and down the beach, however there may be occasional short delays as materials and equipment are moved across the beach.
Swimming and walking on the beach outside of the construction area will not be impacted by the works.
What is the environmental impact of these works?
The Review of Environmental Factors assessment report concluded that the proposed works would not generate significant environmental impacts. No threatened species populations or endangered ecological communities will be affected by the proposed works.
Construction impacts including traffic, noise, sediment, and waste management, are considered to be short term, and would be mitigated by conventional management methods.
Why does the Norfolk Island Pine tree have to be removed?
The Norfolk Island pine is located between the junction of two existing stormwater drainage lines. It is necessary to remove the tree and root structure to construct the new stormwater drainage pit in the car park.
The tree is not a protected or threatened species, and is not heritage listed.
Why is Warringah Council proposing to change the existing Collaroy stormwater outlet?
Council is in the early stages of planning to upgrade the stormwater outlet at Collaroy Beach. The existing stormwater outlet is in very poor condition, having been installed in the 1970s. The stormwater outlet performs an important function – reducing flood risk by carrying stormwater away from flood prone areas and out to sea.
Why can’t the stormwater be recycled or not sent out to sea?
The quantities of stormwater that would originate in the Collaroy catchment in a moderate storm event would require a very large and expensive water storage facility to be constructed within the Collaroy Beach Reserve. Major storm events would overwhelm the capacity of any such storage.
Replacement of the Collaroy stormwater outlet with either option would not preclude the addition of water storage capacity in the future.
How will the plans for the stormwater outlet impact on the Collaroy Precinct Project?
The Southern Beach Option has significant interaction with the area covered under the Precinct Plan and if this option was to be pursued, works would need to be undertaken prior to or concurrently with precinct upgrade works.
What other options were considered and why are we not seeing them?
Council originally assessed several options, which included the Middle Beach Option, a very short outlet in the same position, a very long buried outlet and the Southern Beach Option.
Other options considered are variations of the Southern Beach Option - with different pit locations and pipe alignments to get stormwater to the outfall point. A northern outfall location was considered but was discounted from further investigation and consideration due to cost and technical constraints such as limited water flow
How is the community being consulted about the options for the stormwater outlet?
Council is consulting with the community about both options because it is important something is done about the existing outlet that is in poor condition. Neither of the two options is perfect and both will involve some trade-offs, as outlined in the discussion paper that shows the pros and cons of each option.
The community are invited to attend an open-house community information session at the beach on Saturday 24 March 2012 to meet with and provide feedback to the Project Team about the options for the stormwater outlet.
We are also meeting with a number of local groups that use the beach and others who have an interest in environmental issues within the area. Members of the community are also be able to have a say about the options in the online forum.
The Southern Beach Option involves more construction. Will this have an impact on parking and beach amenity? How long will construction take? If it were to go ahead what impacts will there be on beach access during construction?
The Southern Beach Option involves considerably more construction within the Collaroy Beach Reserve, and would significantly impact on parking and use of the Reserve. Access to the beach would still be available. Construction works for this option could take up to five months, and would be scheduled during a winter period.
The Middle Beach Option would have a shorter construction period , with less impact on the use of the Collaroy Beach Reserve, but a greater impact on the beach in the vicinity of the existing structure.
Why can the damaged exit sections of the existing outfall not just be simply removed?
The existing outfall exit sections and supporting piles are in poor condition, and have moved from their original position. Safety chains are retaining these sections from further dislocation. Simply removing a few pipe sections would leave the stormwater outlet without a distributed outlet structure and insufficient piling support.
The possibility of structural collapse of the outlet also exists, possibly resulting in injury to individuals. If the Middle Beach Option is selected, it may be possible to retain some of the existing outlet pipe closer to the seawall; however a full condition assessment would be required.
What is the purpose of the community consultation?
Council is consulting with the local community about both options for the Collaroy stormwater outlet. The purpose of the consultation is to listen to the community, and understand the full range of community issues when weighing up the two different options. Community input is one of a number of considerations that will drive Council’s decision about how to proceed.
Should I be worried about the existing outfall?
Council undertakes regular inspections and routine maintenance of the outfall structure. Council has erected barriers to keep people off the outfall structure and signage to warn people not to swim close to the outfall.
The NSW Beach Watch program undertakes regular water quality monitoring (every 6 days) ofCollaroyBeach. The results are shown on the Beach Watch website, and in January 2012 were within recommended limits for safe swimming. As with all Sydney beaches, the community is advised to exercise caution and not swim following high rainfall or storm events.
Are there any public health and safety issues to consider when assessing the two options for the Collaroy Stormwater Outlet?
The Middle Beach Option and the existing stormwater outlet both provide a potential risk to surfers and swimmers, due to the risk of collision in the surf zone. However, it may be possible to mitigate this risk to some extent with Middle Beach Option by reducing the length of the new structure in the surf zone, improved design, and signage.
The Southern Beach Option has a higher risk of poor water quality in the rock pool.
What is being proposed?
A detailed options study and concept design report has been prepared for Council, outlining a range of options for upgrading or relocating the stormwater outlet. From this report and previous Council investigations, there are two feasible options to consider:
Are there any differences in the environmental impact of the two options?
An assessment of the local environment indicated that the Southern Beach Option would have a relatively higher environmental impact, mainly due to the release of stormwater in to the rock shelf ecological zone, adjacent to the rock pool.
What are the two options Council is looking at for the stormwater outlet?
Warringah Council engaged a specialist consultant to assess a range of options and prepare an options study and concept design report for the Collaroy stormwater outlet project. Two of the options investigated are feasible, however there is no ‘perfect solution’ and Council understand that both options involve some trade-offs:
What is the cost of both options, and is this a consideration of Council when determining which option to proceed with?
Warringah Council takes a responsible approach to financial management. However, there are a large number of considerations with a project of this nature, and cost will be one of many criteria that Council uses to weigh up the options.
There is a considerable cost difference between the two options. Early estimates suggest the Middle Beach Option may cost just over $1.4 million and the Southern Beach Option could cost almost $7.5 million.
Were environmental impacts, such as sand subsidence, considered in the development of both options?
Yes; sand subsidence, beach levels, and water level changes caused by climate change were considered in the development of both options. The concept designs for both stormwater outlet options are for infrastructure with a 50 year life span.
How will the existing surf break at the southern end of Collaroy Beach be impacted by changes to the stormwater outlet?
The impact of stormwater outlets on beach conditions and surf breaks are very difficult to predict with any certainty. There is a possibility that the Southern Beach Option could impact on the surf break at the southern end ofCollaroyBeach under certain circumstances.
The Middle Beach Option would release water in a similar location to the current outlet , and would therefore seem to be less likely to change the surf conditions
What are the key similarities and differences between the two options Council is consulting about?
Both options have some core similarities: both have been designed to last for 50 years, taking into account climate change and beach recession and both options will improve the stormwater management system in Collaroy.
At the same time, there are some fundamental differences between the two options: the Middle beach Option will release the stormwater in the same spot as the existing outlet whereas the Southern Beach Option will release stormwater near the Collaroy ocean pool.
The Middle Beach Option does have a public safety risk, in that swimmers and surfers in the surf zone could collide with the outlet structure. Also that any unauthorised access to the structure for fishing or other reason could result in injury.
There is a considerable cost difference between the two options, with cost estimates showing that the Southern Beach Option could cost five times more than rebuilding the outfall in its present location.
How do these levels comply with health guidelines?
NSW Beach Watch indicates a safe swimming level of up to 40 cfu/100mL Enterococci units. Samples collected as part of project investigations, from both the rock pool and the ocean adjacent to the rock pool, were within this safe swimming level. However it is possible that on occasion the threshold could be exceeded. As with all Sydney beaches, the community is advised to exercise caution and not swim following high rainfall or storm events.
Will the Southern Beach option impact on water levels and water quality in Collaroy ocean pool?
Council’s consultants have carried out extensive modelling of the water quality impacts for the two options.
The results show that the relocation of the outlet from the existing location to the Southern Beach Option would increase ocean water contamination in the vicinity of the rock pool by around 50%. It is more difficult to predict how much of this contamination would enter the rock pool, as overtopping of polluted seawater would depend on sea, wind and tide conditions; however an adverse impact on water quality in the rock pool is likely. Additional cleaning of the rock pool may be required.
What were the criteria used to arrive at these two options?
In a public and dynamic beach environment – where sand and tides are constantly changing – there are a number of considerations for infrastructure. The options developed, in association with Council, will continue to be assessed against a range of criteria, including:
Can the stormwater outlet be removed from the beach?
The stormwater outlet performs an important function, and is required to reducing flood risk by carrying stormwater away from flood prone areas and out to sea. The Southern Beach Option would allow the removal of the existing structure from the beach, resulting in a substantial improvement in visual amenity.