By 1976 head loss in the 23 km long 750/900 mm diameter CLMS pipeline from Eppalock Reservoir to Bendigo had increased from 45.7 m to 98.2 m (115%) after only 12 years service. The cause was identified as increased friction from soft voluminous iron and manganese bacterial slime building up on the pipe walls and increasing the friction. Inspection of the drained pipes in the dry gave little indication of the problem since the slime consolidated to an innocuous looking thin smooth coating as it dried.
1960 studies by Tyler and Mitchell at the University of Tasmania for the Hydro-Electric Commission had shown that the micro-organisms producing these slime growths were present in all pipelines. However they required the presence of iron and manganese in the water to flourish and produce flow reduction. Remobilisation from oxygen deficient bottom sediments was shown in the 1940’s by Pearsall and Mortimer in England to be a major source of iron and manganese in reservoir water and this could be controlled if sufficient dissolved oxygen could be provided to convert the reducing conditions at the sediments to oxidising conditions.
An experimental aeration system designed by the author was operated in the 180,000 ML Eppalock Reservoir for 19 days during March 1977. This mixed the reservoir to the depth of the aerators (24 m) increasing the low 10% saturation dissolved oxygen at this depth to a high 94% saturation thereby changing chemical conditions from reducing to oxidising. As a result the iron concentration in the surface water decreased from 2.04 mg/L to 0.54 mg/L but there was little change in the pre-aeration 0.03 mg/L manganese concentration with this short period of aeration. The iron concentration in the water flowing in the pipeline changed from 1.78 mg/l to 0.57 mg/l.
The problem of pipe flow reduction from bacterial slime growth on the pipe walls is discussed in this paper and examples are given of the use of automatic reservoir aeration to overcome the problem including costs and results.
Many dam structures in Australia were designed and built in the 1950s and 60s with limited hydrological information. As a result existing spillway structures are under-sized for today’s revised probable maximum floods (PMF). Potential problems such as the generation of excessive negative pressure over spillway crest under increased flood condition could be encountered. This may cause instability or cavitation damage to the spillway. The raised flow profile may also have adverse impacts on crest bridges and gate structures.
Historically, physical models have been constructed in hydraulic laboratories to study these behaviours, but they are expensive, time-consuming and there are many difficulties associated with scaling effects. Today, with the use of high-performance computers and more efficient computational fluid dynamics (CFD) codes, the behaviour of hydraulic structures can be investigated numerically in reasonable time and expense.
This paper describes the two- and three-dimensional CFD modelling of spillway behaviour under rising flood levels. The results have been validated against published data and good agreement was obtained. The technique has been applied to investigate several spillway structures in Australia.
Cairn Curran Dam is a 44m high zoned earthfill embankment located near Bendigo in central
Victoria. The dam is owned and operated by Goulburn-Murray Water.
A risk assessment had identified that the junction between the embankment and spillway wall was a weakness in regard to the potential for piping. Initial geotechnical investigations indicated a softened zone adjacent to the foundation.
The conceptual upgrade design was to excavate the downstream slope and place filter material and a rockfill weighting berm. A 2-D slope stability analysis gave unacceptably low factors of safety for this excavation. The three dimensional nature of the embankment/spillway interface and excavation geometry was identified as an important factor in the upgrade design.
A detailed geotechnical assessment was undertaken and a geotechnical model developed that
accounted for potential softened zones adjacent to the spillway wall, along the foundation, and within the embankment.
A 3-D limit equilibrium slope stability program was utilised to analyse the 3-D factors of safety. The
program employed an extension of Bishop’s method of slices to a 3-D ‘method of columns’. A 3-D
finite element analysis was also undertaken to estimate likely deformations of the embankment and cut slope during construction.
The development of the geotechnical model and subsequent analysis allowed the upgrade works to be undertaken with confidence.
Craig Johnson, Phillip Solomon, Nihal Vitharana
Tank Hill Reservoir is located approximately 25km north-east of Warrnambool and forms part of the fresh water supply for the town. It was built in the 1930’s by the construction of an earthfill dam across the natural breach of the crater of an extinct volcano. The reservoir is an offline storage with a small natural catchment and has a nominal capacity of 770ML at Full Supply Level (FSL). The reservoir is operated by South West Water Authority (SWWA).
Previous investigations had identified instability issues associated with the dam embankment and the necessity for remedial work to increase the stability of the dam embankment. SKM undertook detailed survey and investigations and the proposed upgrade works include the construction of a downstream stabilising berm incorporating graded filters and a drainage system. The condition of the outlet works was investigated as part of the project, with some of these works found to be in poor condition with a risk to the security of supply, necessitating the design of refurbishment of the outlet works. The degree of siltation of the reservoir was also assessed, and some loss of capacity due to siltation was noted.
Detailed investigations were performed to determine the optimum configuration of the stabilising berm and to locate and test suitable construction materials. The embankment interface filters were designed to satisfy modern filter design criteria and were incorporated in the embankment drainage system. The condition of the outlet works, including the intake standpipe, three offtake valves and the outlet conduit beneath the embankment, were assessed via manual and CCTV inspections. An operation review, incorporating the proposed upgrade works within the framework of ongoing operation of the reservoir for supply to downstream customers was also prepared, as was a construction risk assessment.
This paper will present “extremely useful practical information” for dam design engineers, owners and operators where the whole spectrum of dam safety issues is required for the successful completion of remedial works design and construction.
N. Vitharana, A. Gower, G. Bell and N. Petrovic
Churchman Brook Dam is a 26m high earthfill dam with a puddle clay core and impounds a reservoir of 2.2GL. Various remedial works have been undertaken since completion of construction in 1928. In September 2000, a sinkhole in the right abutment was observed during a routine dam inspection. Following this incident, detailed site investigations were carried out. These investigations revealed that there are soft zones and possibly voids formed in the upper part of the clay core.
A comprehensive dam safety study and a risk workshop undertaken in 2002/2003 showed the dam to be deficient in aspects associated with piping, spillway adequacy and outlet works condition. A rational geotechnical model was developed for the foundation utilising triaxial test data from 1980s and recent investigations. The existing spillway chute will be upgraded with a concrete liner attached to the existing chute incorporating no-fine concrete as a free-draining medium. This paper presents the various aspects of the remedial works currently being designed.