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.
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K. Chandler, D. Gill, B. Maher, S. Macnish and G. Roads
SEQWater is the major supplier of untreated water in bulk to Local Governments and industry in the South East Queensland region of Australia, through ownership of Wivenhoe, Somerset and North Pine Dams. Wivenhoe Dam (Lake Wivenhoe) is located on the Brisbane River in Esk Shire. The storage provides both flood mitigation and water supply storage to Brisbane and Ipswich. The water supply storage capacity at full supply level is 1,160 GL. An additional 1,450 GL of storage above full supply level is used for flood mitigation.
Changes to the estimation of extreme rainfall events has resulted in significant increases in the estimates of the PMF since the original design of Wivenhoe Dam. To upgrade the flood security of Wivenhoe Dam, SEQWater has formed an alliance with Leighton Contractors, Coffey Geosciences, MWH and the NSW Department of Commerce.
This paper details the alliance delivery method, the latest estimates of the PMF based on the GTSMR method and details of the two preferred options being finalised by the Alliance.
Peter D Amos, Thomas G Newson, Murray D Gillon
In September 2000, pressures being monitored in a geological fracture beneath Arapuni Dam were found to be rising significantly, indicating that a deteriorating condition was developing in the foundation. Two boreholes drilled in 1995 had intersected high water pressures within the fracture in an area close to the downstream face of the dam, posing a risk of major leakage developing from where the fracture day-lighted downstream of the dam. Lumps of clay, bitumen and lake biota, including snails and small fish, were identified discharging from the boreholes, indicating that a significant leakage path had developed. Detailed investigations, the subject of this paper, were carried out from September 2000 to confirm the extent and nature of the deterioration. A range of groundwater investigation techniques and tools were used, while the reservoir remained full, to identify the source of the leak and confirm the path it took. The investigations culminated in development of a groundwater model that described the seepage behaviour in the dam foundation. Based on the investigation information gathered, the foundation fracture bearing the high water pressure was successfully grouted in December 2001 without lowering the reservoir.
J.H. Green, D.J. Walland, N. Nandakumar
The Bureau of Meteorology has recently revised the Probable Maximum Precipitation (PMP) estimates for the Generalised Tropical Storm Method (GTSM) region of Australia. The revision process has involved the application of the more technically rigorous Generalised Southeast Australia Method (GSAM) that was previously developed for the southern part of Australia to a much larger data set of severe tropical storms. This has generally lead to an increase in the total GTSM PMP depths with a resultant increase in the Probable Maximum Precipitation Design Flood (PMPDF) and the Probable Maximum Flood (PMF).
In addition, the revision process has produced significant modifications to the temporal and spatial patterns adopted when applying the PMP depths to a dam’s catchment and these changes have also generally lead to increases in the resultant floods.
This paper discusses the rationale behind the increases in PMP depths and changes in the associated temporal and spatial patterns and presents the justification for the adoption of these more scientifically defensible estimates.
The application of the revised PMP estimates to the Keepit Dam catchment in northern NSW is presented and a comparison between the PMPDF and PMF estimates based on the original GTSM and the revised GTSM (GTSMR) made for this specific case study.
This paper presents the findings of experimental investigation of internal erosion by the process of suffusion within embankment dams and their foundations.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces. Soils susceptible to suffusion are usually described as internally unstable. Understanding of the suffusion process is important to the assessment of the risk of internal erosion in an embankment dam and its foundation. Suffusion results in a coarser soil structure, leading to increased seepage, progressive deterioration of the dam or its foundation, and a higher risk of toe instability. Suffusion within the protective filter of a dam may result in a coarser filter, rendering it ineffective in protecting the core materials from erosion.
Two types of suffusion tests, namely the downflow test and the upflow test, have been conducted at the University of New South Wales. The downflow test aims at identifying the types of soils that are susceptible to suffusion, whereas the upflow test aims at identifying the hydraulic gradient at which suffusion is initiated. This paper presents the initial findings of the downflow test. Eighteen downflow tests have been carried out on fourteen clay-silt-sand-gravel soils. The Kenney and Lau (1985, 86) method, which is commonly used for assessing the internal stability of coarse-grained soils, appears to be too conservative when used to predict the internal stability of silt-sand-gravel or clay-silt-sand- gravel soils, whereas the Burenkova (1993) method appears to provide better predictions. Further testing is required to define more accurate criteria for determining the internal stability of broadly-graded clay-silt-sand-gravel soils.
Assessment of dam safety requires estimates of extreme rainfall together with the temporal and spatial distributions of extreme rainfall. In order to satisfy dam safety requirements for dams in the west coast of Tasmania, the Bureau of Meteorology has developed the method of storm transposition and maximisation for application in this region.
Daily, as well as continuously recorded rainfall data for all Bureau of Meteorology and Hydro Tasmania sites in western Tasmania have been analysed and the most outstanding rainfall events over one, two and three-day durations in the region have been identified. Meteorological analysis of these events reveals that the most significant rainfall events in the west coast of Tasmania are caused by the passage of fronts, which are sometimes associated with an intense extratropical cyclone, with a westerly or southwesterly airstream.
A database of isohyetal analyses of the most significant rainfall events in western Tasmania has been established. These can be used either ‘in situ’ or transposed to estimate mean catchment rainfall. Storm dewpoint temperatures for the purpose of moisture maximisation have been determined.
Cumulative and incremental three-hourly temporal distributions for sites having continuous rainfall data or three-hourly meteorological observations have been constructed and design temporal distributions of extreme rainfall have been derived.
An objective method for adjusting for differences in the topography between the storm and target locations is proposed