Scrivener Dam is a critical piece of Canberra’s infrastructure, maintaining water levels of the iconic Lake Burley Griffin. The dam is located on the Molonglo River, on Canberra’s Lady Denman Drive and was constructed in the early 1960s.
Five flap gates provide precise control of the levels in the lake which provides amenity for many activities on the lake and flood mitigation for Canberra’s foreshore.
Like all dams in Australia, Scrivener Dam requires regular maintenance and assessment to ensure ongoing safe operation. A recent inspection identified potential for undesirable uplift forces under the stilling basin slab, triggering a more detailed assessment of dam stability.
UNSW’s WRL was commissioned by the National Capital Authority (NCA) to construct a 1:40 scale physical hydraulic model of the dam, to measure water dynamics of the dam at critical flood conditions.
Physical models remain the preferred method to assess the hydrodynamics of complex flows such as those found in dam spillways and dissipators. While numerical methods continue to advance, they are not relied upon to reproduce turbulence and other complex flow features without careful calibration to a physical model.
The project team was led by Ben Modra, Principal Engineer – Coastal and Water, one of Australia’s foremost physical hydraulic modellers with experience across dams, coastal and stormwater infrastructure, and Dr Laura Montano, a specialist hydraulic engineer at WRL.
The physical model was an exact reproduction of the structure, with WRL’s engineers using the original construction drawings, 3D LIDAR scans, and hydrographic surveys to create a CAD model of the structure and surrounding topography. This was used to scale and design the model, and generate drawings for construction within the lab space.
One of the challenges in constructing a hydraulic model of this size is achieving sub-millimetre accuracy at critical parts of the structure, within the overall model area of 25 x 9 m.
To achieve the accuracy required for the dam structure, WRL used advanced manufacturing techniques, such as 3D printing (selective laser sintering), laser cutting and etching, together with traditional crafting skills. The five flap gates required a bespoke mechanism to allow the engineers to accurately set the gate levels, with flows still flowing over the dam.
The topography was constructed using a method WRL applies to many dam and coastal models. Templates are lasercut and installed to millimetre accuracy, then back filled with aggregate, and capped with concrete to provide a robust base for the model.
The stilling basin was a critical component of the model, as the stability of the stilling basin slab was the primary driver for the modelling. High speed pressure transducers were located in the stilling basing to measure the hydrodynamic pressures on the basin floor, which will be used to assess the stability of the actual basin slab. Other instrumentation included acoustic displacement meters and piezometers to measure the water surface, and a 3D Laser scanner to assess scour potential downstream of the structure.
The accurate representation of the model behaviour and hydraulic features was confirmed by the visual comparison of model flows against videos of the prototype during large flood events occurring in December 2010 and March 2012. Flows of up to 10,000 m3/s (1,000 L/s model scale) were then tested on the model to simulate the largest flood expected on the dam. Other tests provided improved spillway rating curves, assessment of the potential for large loose rock to damage the basin and the potential effects of individual gate failure.
The results of the model study will be used by the NCA to ensure the ongoing, safe operation of the dam for decades to come.
To read more about WRL's Scrivener Dam model visit: https://www.wrl.unsw.edu.au/research/research-project-highlights/scrivener-dam-physical-model
For further information contact:
Ben Modra | Principal Engineer - Coastal and Water | b.modra@wrl.unsw.edu.au