The Narrandera Shire Council in NSW is currently undertaking works to improve the main drainage siphon and flood pump which carries stormwater from the Narrandera township to the Murrumbidgee river.
The existing siphon and flood pump was designed to manage local stormwater during flood events, by closing the flood gate and pumping any stormwater from the town into the river.
During the Murrumbidgee river flood of March 2013 Council staff noticed a major problem with the earth bank at the pump. Mr Frank Dyrssen, Council’s Director of Technical Services said “The river had risen to over 9.5m and staff were using a tractor to power the pump at the siphon to pump stormwater out of town. On visiting the site I noticed major leakage and sagging of the earth bank at the pump. I was quite worried about this, if the earth bank were to fail, flood water would go rushing back into town flooding a number of residences to the same level as the river, including most of those along Audley Street and Leeton Road. If this scenario was to eventuate it would have been a disaster!”
“The existing earth bank was held up with railway sleepers held in place with railway rails, this structure was starting to move and the soil behind the sleepers was leaking rapidly.
“To eliminate the risk of the flood entering the town Council staff and volunteers installed timbers into the siphon structure to form a gate to block the flood water should the earth bank fail”
“This solved one problem but created another because local stormwater couldn’t get to the pump, so we had to hire pumps and use them on the Northern side of the canal.”
“Even without the problem of the sagging earth wall the site was far from ideal, our staff need to move a tractor to this location to power the pump and the pump needs to be manned throughout its operation which is usually days on end. There are many trip and fall hazards at this site, it was far from a safe place to work, especially at night.”
“The solution was to install a concrete pipe between the siphon and the pump structure and backfill the area with earth, this way we aren’t relying on a narrow earth bank for flood protection.”
Council engineering staff designed the solution and the Council Works Department have installed the pipe and carried out the concrete works to fit it in place.
Mr Frank Dyrssen said “A few people have asked me what the giant pipes stored next to the Newell Highway were for, they are for the siphon improvement job.”
“The staff have done a great job on this project, the project budget is $50,000 to date we have spent $29,000 however we still have some minor works left to do. This is cheap insurance for the protection that this project provides.”
“These works are funded from the drainage levy, residents have probably noticed a $25 per annum charge per residential lot on the rate notice, we use those funds for this type of work.”