In a project designed to mitigate flooding in the Moody Creek catchment in Northern Queensland, a large detention basin has been constructed as one of a number of measures to protect businesses and homes in the area in the event of a 20-year (Q20) flood.
Significant flood events in 2009, 2010 and 2012 resulted in the damage of a number of businesses and residences in the catchment and became a key driver for the project, an outcome of an extensive study undertaken by VDM Consultants in 2010.
The detention system consists of an earth embankment in combination with a lowEdit HTML Source flow pipe at the creek bed level and a high flow spillway to handle substantial flood events.
“It’s a detention system rather than a dam because the continuous low flow pipe positioned underneath the dam wall enables normal creek flows to pass through the dam on a continuous basis,” said Stephen Bolden, Senior Project Engineer for Cairns Regional Council. “The basin has been located geographically to fit the topography in the area, and combined with the embankment, provides for a part-time dam in the event of flooding in the creek.”
Rocla supplied the 1.8m diameter, class 6 reinforced concrete pressure pipe for the 77m long low flow pipeline, along with substantial inlet and outlet structures to control water flow. Because of the nature of the dam the pipe allows normal flow under normal circumstances and constricts flow during a flood event when its capacity is exceeded. The pipeline features a 26-degree bend near the outlet to realign it with the direction of the creek.
Under a flood environment exceeding Q20, water falls over the spillway onto baffleblocks. This resembles a series of sharp teeth, designed to dissipate the high energy of the fast flowing water at the base and avoid scouring. Rocla developed the forms and cast 32 of the units locally at their Cairns factory.
The detention system’s operation is passive with no floodgates, valves or pumping. The basin is open and grassed which means it can be enjoyed as a public park area for most of the year.
“Rocla was able to provide precast reinforced pipe units to the required specs and were able to have them pressure tested as part of the manufacturing process,” said Stephen. “Those capabilities and the associated advice meant Rocla assisted in matching the specification to the end product.”
- 31 x 2.44 lengths of 270KPa pressure pipe and 1 x 26.5 degree bend (246t)
- Pedestals for pipe transport and installation
- 32 x precast baffles (57t)