Peyman Bozorgmehr, Sarah McComber, David Harrigan, Erik F R Bollaert
Boondooma Dam is a concrete-faced rockfill dam with an unlined, uncontrolled spillway chute. The Acceptable Flood Capacity of Boondooma Dam is 1:60,000 AEP (equal to the Dam Crest Flood (DCF) and has a maximum inflow of 14,330 m3/s.
Significant rainfall events during 2010/11 and 2013 subjected the spillway to moderate discharges over the crest which caused significant scour to the spillway chute.
Following these events, a 3D physical hydraulic model was constructed at a 1:80 scale to investigate repair options. Originally the spillway chute was modelled using a mobile bed set up which showed that that future scour could occur. However, the model could not determine the rate and characteristics of this damage.
In order to determine how future scour may occur, the 3D model was modified using laser survey mapping of the spillway chute after each flood event. Using milled aluminium and concrete capping the model was able to accurately portray the damage profile sustained by the spillway in the 2010/11 and 2013 flood events.
Transient pressure, static pressure, water elevation, velocity and jet measurements of the model were used in a Comprehensive Scour Model to help inform how damage to the chute may progress in future flood events.
Keywords: Boondooma Dam, flood damage, 3D physical hydraulic modelling, comprehensive scour assessment
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Sarah McComber, Peyman Bozorgmehr
Boondooma Dam is a concrete-faced rockfill dam with an unlined, uncontrolled spillway chute. Construction was scheduled for completion in 1983; however a spill event occurred during the last stage.of construction Following this spill event an Erosion Control Structure (ECS) was built across the spillway chute to help mitigate any future scouring.
The spillway performed as expected during minor spill events in the 1990s and early 2000s. During the significant rainfall event of 2010/11, significant scour occurred to the spillway chute and downstream of the ECS, as a result of the spillway operation.
Following the 2010/11 flood, emergency repairs were made and long term repair solutions were investigated. However, during Tropical Cyclone Oswald in January 2013, the dam experienced the flood of record, and further scour occurred in the spillway chute.
The long term repair solution was reviewed in light of the 2013 damage. A solution is required that would satisfy the engineering problem and prevent further damage, while satisfying the commercial considerations faced by dam owners, insurers, customers and downstream stakeholders.
Keywords: Boondooma Dam, flood damage, scour damage, commercial engineering solutions.
Paul Somerville, Andreas Skarlatoudis, and Hong Kie Thio
Engineers need ground motion time histories for the analysis of the response of structures to earthquake ground shaking. In current practice, these time histories are usually spectrally matched to a uniform hazard response spectrum. At low probabilities, this spectrum is too “broadband” (i.e. large over an unrealistically broad range of periods), and envelopes a set of more appropriate design response spectra, termed conditional mean spectra. These concepts are illustrated using a site-specific probabilistic seismic hazard analysis of ground shaking in which ground motion time histories are spectrally matched to conditional mean spectra that were derived from the uniform hazard spectrum.
Keywords: Ground motion time histories, Conditional mean spectrum.
Jason Fowler, Robert Wark
Tropical Forestry Services (TFS) currently (2015) leases Arthur Creek Dam from the West Australian state government and utilises the water source to drip irrigate its Indian sandalwood (Santalum album) plantation. Arthur Creek Dam is located approximately 70 km south west of Kununurra in the East Kimberley region of Western Australia. TFS grows and processes the sandalwood to produce oil that is used extensively in the global fragrance perfume market. TFS took over the lease of the 26 m high zoned earth core and rock fill dam in 2007 and has systematically carried out remedial works to the structure to lower the f-N curve below the ANCOLD “Limit of Tolerability” and to well within the ALARP zone. This paper describes the proactive risk management approach TFS has undertaken to address dam safety issues. It also specifically describes the most recent management issue, being the outlet pipe refurbishment.
A number of dam safety issues were identified during the initial surveillance and subsequent annual surveillance inspections. Issues include insufficient spillway capacity, seepage from the right abutment and deterioration of the steel outlet pipe. The remedial works to the outlet pipe were completed in late 2014 and involved close collaboration between TFS, the contractor and the designer. The outlet pipe re-sleeving operation was complex as the dam had to remain in operation and the water level could not be artificially lowered. In addition, the original outlet pipe was asymmetrical along both the vertical and horizontal axes, close to the bulkhead gate structure. Contingency measures were employed to enable the dam to remain in operation with 3 DN 400 HDPE siphon pipes installed.
The completion of the refurbishment of the outlet pipe by sleeving the pipe reduced the risk posed by this structure by an order of magnitude. Planned future risk reduction measures include the treatment of seepage within the upper right abutment and rebuilding the crest. These actions will further reduce the risk of dam failure through piping and overtopping of the dam crest.
Keywords: risk, ALARP, outlet pipe, re-sleeving.
Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.
This paper explores the role of the Lenders’ Technical Advisor (LTA) in identifying and mitigating risks in hydropower dam projects on behalf of the project lenders. It describes the LTA services that are required to manage the pre-financial close, construction and financing periods.
There are differing types of risk in both large and small hydropower projects (contractual, commercial, participant, completion, country, technology, reputational, environmental and social, etc.) and these are discussed with regard to how the lenders may be exposed if the risk eventuates either during dam construction or in operation.
Whereas a large dam for water supply would in its own right be a major project, the dam(s) associated with large hydropower will likely represent less than 25% of the total project cost and with this imbalance comes competing drivers for the other components (tunnels, waterways, powerhouse, M&E equipment, transmission lines, substations, etc).
The paper discusses the typical process whereby a hydropower developer has procured a feasibility study and is working towards financial close — covering both large and small types, i.e. storage dams and run-of-river diversion weir types, and the noticeable trend for fast-tracked developments to make a single large step from feasibility study through to engineer-procure-construct (EPC) contracting. This scenario presents some challenges for the initial due diligence when assessing in the pre-financial close stage.
The paper draws on case studies from the Asia Pacific region to illustrate the key elements in hydropower project financing from the LTA’s perspective, together with the author’s recent and current experience on multiple hydropower projects across Asia and Africa in the run-of-river, storage reservoir and pumped storage type of plants. It also brings together findings from the author’s own recent papers on the subjects of hydropower feasibility studies, the roles of lenders, owners and advisors, and tailored for an ANCOLD audience where the focus is on the dams component of hydropower.
Keywords: Lenders’ Technical Advisor, Dams, Hydropower.