Richard R. Davidson, Michael Zoccola, Barney Davis, John W. France
Seepage barriers have become an essential element of dam safety upgrades for many aging dams. Our construction technical specifications are generally written to achieve a degree of perfection that may not be possible or practical. Many practitioners believe that grouting alone can achieve an acceptable seepage barrier through a pervious rock foundation. However, precedent from many Corps of Engineers dams has revealed that grouting can only treat those open features that the grout holes intersect. What about clay filled fractures or solution features that resist grout penetration but then erode over time? Can any grout treatment ever be considered as a permanent seepage barrier? Cutoff walls through embankment dams and their foundations are generally considered as a more permanent seepage barrier. However, do we have the means to construct a perfect seepage barrier wall, or are defects to be expected. Do these defects represent fundamental flaws that require risk mitigation? How can we verify that we have built an acceptable seepage barrier that meets the design intent?
Simon Lang, Chriselyn Meneses, Peter Hill, Kristen Sih
In Australia to date, the empirical method developed by Graham (1999) is the most widely applied approach for estimating loss of life from dambreak flooding. However, as the move to risk-based approaches of dam safety management has gathered momentum internationally, increasingly sophisticated techniques for estimating loss of life have emerged. One of these models is the United States Army Corps of Engineers (USACE) HEC-FIA model. HEC-FIA models the influence of flooding, structure characteristics, and warning and evacuation assumptions on loss of life in a spatially distributed manner. In contrast to Graham (1999), HEC-FIA also allows the user to model the loss of life for both dambreak and natural flooding.
This paper presents the results from the first Australian application of HEC-FIA to two dams in southeast Australia. The application of empirical methods developed by Graham (2004) and Reiter (2001) is also discussed.
Mark Locke and Scott Kindred
The Bulk Water Alliance (BWA) consisting of ACTEW and ACTEW-AGL, GHD, and John Holland / Abigroup, are delivering the Enlarged Cotter Dam project in Canberra, ACT. The greatly enlarged reservoir will require two central core rockfill saddle dams on a ridge adjacent to the main dam site. Construction of these two dams was completed in early 2011. The challenges of the site and the Alliance delivery model have provided opportunities for innovation in both use of materials and construction.
The dam foundations were variably weathered and fractured with some highly weathered seams extending below the cutoff trench foundation. The foundation was grouted effectively using GIN grouting and the entire cutoff trench was shotcreted to reduce the risk of piping of the dispersive core material.
The steep topography provided very limited sources of material suitable for a dam core. Potential contingency plans considered included bentonite enrichment of the low plasticity materials or a change to a concrete faced rockfill dam. The high cost of these options drove the decision to use the available residual soils from small gullies by selectively winning material with a higher fines content for use below full supply level. The lack of room on the ridge for stockpiling and conditioning of clays lead to trialling of a continuous mixer for mixing and conditioning the core which was found to be highly successful.
Filter materials were crushed sands and gravels produced from nearby commercial quarries. The materials and grading were generally high quality, with some challenges producing coarser filter materials by blending available aggregate products. A range of options were effectively adopted for placement of the filters including loader placement, trench boxes and spreading from a modified ejector dump truck.
Enlarged Cotter Dam Saddle Dams – Materials and Construction
Krey Price, Mike Harvey, Bob Mussetter, Stuart Trabant
The California Department of Water Resources, Division of Dam Safety (DWR-DSD), has determined that San Clemente Dam on the Carmel River in Monterey County, California, does not meet seismic safety standards. Several alternatives have been considered to decommission the dam and eliminate the hazard, including thickening of the 25-m-high, concrete arch structure, lowering the dam, and complete removal. At the present time, the upstream reservoir that had an original storage capacity of about 1.8 GL, is essentially filled with sediment. The 29-km reach of the Carmel River between the dam and the Pacific Ocean passes through urbanised areas within the upscale Carmel Valley; flooding and channel stability in these areas are significant concerns. The Carmel River also contains habitat for the endangered steelhead and red-legged frog that could be positively or negatively affected by the decommissioning.
After an extensive series of hydraulic and sediment transport modelling studies, two actions remain under consideration: (1) dam thickening, which will require reconstruction of the existing fish ladder and construction of an adjacent, 3-metre diameter sluice gate to prevent sediment build-up from blocking the ladder outlet, and (2) removal of the dam and rerouting the river into a tributary branch of the reservoir, which would isolate approximately 65 percent of the existing sediment deposits from future river flows and eliminate a significant fish-passage problem. Both options were modelled extensively in hydrologic, hydraulic, and sediment transport applications. Since available models do not adequately represent sediment dynamics at the sluice gate, a special sediment routing model was formulated to evaluate this aspect of Option 1. Option 2 is currently preferred by the resource agencies, since it would optimise endangered species habitat; however, this option would be three to four times more expensive than Option 1, and funding limitations may impact the alternative selection. Evaluation efforts are ongoing, along with approaches to address liability issues associated with the decommissioning actions for the privately owned facility, while optimising the benefits and costs of the selected action.
Modelling Studies to Design and Assess Decommissioning Actions for a Seismically Unsafe, Concrete Arch Dam
The enlargement of the Cotter Dam is being undertaken by ACTEW to provide a greater security of water supply to Canberra. The project involves constructing a larger, higher new dam wall immediately downstream of the existing Cotter Dam, to allow the present dam to continue functioning and supplying water while construction is underway. The project raised a number of environmental issues partly because the Cotter Dam currently supports a self-sustaining population of (endangered) Macquarie Perch, and because the Bendora Dam, upstream of Cotter Dam, contains a breeding population of (endangered) Trout Cod. Bendora Dam will not be physically affected by the works on Cotter Dam, but its operations may be altered. An ecological risk analysis was conducted to identify critical environmental risks that would need to be investigated and managed or ameliorated and management strategies were put in place to reduce risks. ACTEW have adopted an adaptive management approach to the project, but in order to implement that approach it is necessary to conduct effective monitoring of the fish populations of concern. These potentially include the two endangered species, as well as potential predators (such as cormorants) and competitors (such as trout). Power analysis has been used as a tool to evaluate whether it is feasible to monitor key populations sufficiently rigorously to be able to confidently detect a change (either an increase or decrease in a population). For Macquarie Perch and trout it should be possible to detect population changes statistically with a logistically feasible monitoring program.
2011 – Using risk analysis, power analysis and adaptive management to minimise ecological impacts of the Cotter Dam enlargement