When Clarrie Hall Dam was commissioned in the early 1980s it complied with the engineering standards of the day. Since the dam was constructed, the ability to predict rainfall and understand extreme weather patterns has improved.
The NSW Dams Safety Committee establishes safety requirements for dams in NSW. With the improved prediction of weather events the committee has upgraded the safety standards for dams in NSW.
Clarrie Hall Dam is extremely safe for day to day operations and it can pass floods many times greater than the largest flood on record. However the dam needs to be upgraded to meet the new standards established by the NSW Dams Safety Committee to pass floods which may occur in extreme rainfall events. Such events have an extremely small probability of occurring.
The proposed upgrade of the spillway would ensure the safety of public and property downstream of Clarrie Hall Dam.
The spillway upgrade works would involve widening and extending the entrance of the existing spillway chute. A parapet wall across the dam crest would be raised to prevent water from flowing over the dam wall in an extreme flood event and enable the water to be released safely through the spillway. The parapet wall would not increase the water storage capacity of the dam and the dam storage will remain at current levels.
There would be no spillway crest raising and no change to the storage’s full supply level or the storage inundation area. A parapet wall across the dam crest would be installed to accommodate flood waters and to provide increased freeboard (the area between the top water level of the dam and the top of the dam structure). A conceptual footprint and draft concept drawings of the proposed new spillway can be viewed in the available downloads section in the top-right of the page.
Prior to undertaking the upgrade, concept designs and detailed designs are required to be prepared. The design of the upgrade works are being developed with a focus on minimising environmental disturbance and disturbance to the water supply.
No field investigations are envisaged which will cause disturbance to the dam site for the development of the designs, as there is sufficient site data from previous investigations.
A Review of Environmental Factors (REF) has also been prepared to assess the construction and operational environmental impacts of the proposed works. The REF was placed on public exhibition in September 2011 to provide people with an opportunity to comment on the proposed works. Planning approval has been granted and the REF was determined in December 2011.
The on-site construction works would be contained to the immediate vicinity of the dam wall. The picnic facilities and access adjacent to the dam wall would need to be closed while construction is occurring.
The spillway upgrade work would take approximately 6 months to construct. Subject to obtaining the remaining approvals, construction would be anticipated to commence in 2013.
Construction access would be gained from the right hand side of the dam wall via the existing road.
As part of the investigation into the design of the spillway upgrade, consideration is being given to how the spillway upgrade would be constructed. The construction of the spillway requires land area to store materials and equipment, to contain temporary construction site offices, park vehicles and for concreting works.