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
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John Grimston, Robin Dawson
The Ambuklao and Binga Hydro-Electric Power Projects are located in Luzon, Philippines and were privatised in early 2008 after public bidding. Ambuklao dam forms an impoundment on the Agno River. The nearest city, Baguio, is approximately 45km or 1.5hrs drive away. The key headworks feature is an embankment central core rockfill dam and reaches a maximum height of some 129 m above the bed of the Agno River. A gated spillway is located at the left abutment, with a steep chute and flip bucket. Binga dam forms an impoundment approximately 20 km downstream of the Ambuklao dam. The rockfill embankment with an inclined clay core reaches a height of about 107 m above the bed of the Agno River. The spillway is located at the left abutment.
Heavy tropical rains and typhoons can cause very high flows in the rivers leading into the Ambuklao and Binga reservoirs. PMF peak flow is 11,600 cumecs. Due to the steep slopes surrounding the reservoir and along the access roads to the Binga Dam, landslides can create a hazard in the reservoir or for emergency access to the dam. There are numerous active faults in the area, including the Abra, Digdig and Philippines Faults (the latter being one of the most active faults around the Philippines). The region around the dams is capable of and has experienced earthquakes with a magnitude of 7.8 on the Richter Scale. This was demonstrated by the 1990 earthquake (7.8 magnitude) and caused minor damage to the dam structures.
The Project owner commenced rehabilitation implementation planning immediately after purchasing the facilities aimed at reactivating the Ambuklao plant’s 75MW capacity (inoperable since 1999 due to reservoir siltation issues triggered by the 1990 earthquake) and increasing it to 105MW. Rehabilitation at the Binga plant will increase capacity from it’s current 100MW to 120MW. The overall rehabilitation works include plant, intakes, associated tunnels, etc. This paper will focus primarily on the dam and spillway related rehabilitation, studies and design including review of the PMF and spillway capacity for both dams, Ambuklao innovative upstream face rehabilitation, Ambuklao spillway studies and rehabilitation and Binga spillway works and reservoir sedimentation studies.
2011 – Refurbishment of Ambuklao and Binga Hydro Power Dams and Appurtenant Works
Paul Somerville, Hong Kie Thio
There is a large degree of uncertainty as to the true state of nature (i.e. epistemic uncertainty) regarding many aspects of seismic hazard analysis. Such differences are often highlighted by differences between alternative models put forth by different model proponents. This epistemic uncertainty is treated by giving weight to all viable alternative models through the use of logic trees in probabilistic seismic hazard analysis, rather than just using a preferred model. This paper reviews epistemic uncertainties that arise from alternative distributed earthquake source models; alternative models for the recurrence of earthquakes on those sources; alternative approaches to including active faults; alternative models for the recurrence of earthquakes on active faults; alternative ground motion prediction models for Australia; and alternative methods for incorporating site response. It also reviews alternative representations of the design response spectrum for the development of ground motion time histories.
2011 – Recent Developments in Seismic Hazard Analysis
This paper provides the insight of one practitioner into the process and application of Dam Safety Risk Assessment. The ANCOLD Guidelines on Risk Assessment provide a reasonably comprehensive outline of the key tasks involved in the risk assessment process. The intent of this paper is not to rehash the Guidelines but rather to discuss some of the practicalities of completing a dam safety risk assessment and highlight some key learning’s gained from a wide range of projects for a number of different owners.
The paper includes a brief overview of each component of the risk assessment process as well as some of the advantages and disadvantages of the various approaches to completing a risk assessment project.
2011 – Dam Safety Risk Assessment – A Practitioner’s Perspective
Stuart Richardson,Tusitha Karunaratne
Goulburn-Murray Water (G-MW) manages 16 large dams across Northern Victoria. Since January 2010 after 10 years of continuous drought a number of significant and historic maximum floods were passed through some of these dams. Although these floods are not considered extreme in a dam safety context, for downstream communities they presented very real emergency situations. There has been significant community concern regarding the impact of the floods resulting in several inquiries.
G-MW has maintained and annually reviewed comprehensive Dam Safety Emergency Management Plans (DSEP) since 1997. During 2009 G-MW began developing and documenting a systemised approach to dam’s management, operation and emergency response by developing and integrating its Operations and Maintenance Manuals, Flood Incident Management Plans and Dam Safety Emergency Management Plans. The plans have been developed to align with the Australian Inter Service Incident Management System (AIIMS) which G-MW uses as its corporate incident response framework.
This paper provides an overview of the benefits of having structured and integrated manuals and response plans for managing assets, flood and extreme events. The paper also shares G-MW’s experiences in developing this integrated management approach.
Workshop paper – Karunaratne 2011 – Management of Floods in 2010 and 2011 through Goulburn-Murray Water Dams
Alex Gower, Graeme Mannand Peter Hulcup
The Water Corporation is the principal dam owner in Western Australian with a portfolio of 70 dams. Many of these dams are more than 30 years old and were designed and constructed prior to the writing of most occupational health and safety legislation and associated regulations and standards. Achieving compliance with these regulations and standards on the older assets has led to increasingly complex procedures and increased costs to undertake what were previously routine inspection, operation and maintenance tasks. In some cases achieving compliance has become impractical and modification to the assets is required.
This paper discusses a range of different safety issues, hazards and challenges faced at dams in Western Australia. These include prevention of falls from height, rescue of injured personnel within intake towers and drainage galleries and public access on the dams. Solutions adopted to improve safety and security for operators and the public are presented.
2011 – Safer Access at Water Corporation Dams