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
Nikifor Petrovic, Sladoljub Pezerovic
Dam rehabilitation works at the Visegrad Hydropower Project on the River Drina in Bosnia and Herzegovina were completed in October 2014 after two years of very challenging and collaborative effort between the client, designer and contractor.
The successfully accomplished remedial works programme was a highly complex geotechnical intervention. The dam was constructed on a karst foundation extending up to 200 m below reservoir floor level. Rates of seepage through the foundation increased over time, from 1.4 m3/s following first impoundment in 1989, to 14.7 m3/s in 2009.
The rehabilitation works comprised:
Preparatory works (site installation, work platforms, conveyer belts, electricity and water supply, drilling and grouting equipment installation);
Site investigation works (drilling of boreholes, measurements of inclination, geo-physical carotage, downhole video, underwater camera recording);
Installation of monitoring equipment and implementation of real time recording system;
Installation of inert material into a sinkhole within the storage area and into the bore holes located upstream of the dam; and
Grouting of the foundation area using different grout mixes and grouting methods.
During rehabilitation works the main achievements were:
A total of about 37,300 m3 of inert material (granular materials with different fractions from 0 to 32 mm) was installed into the foundation cracks and caverns. This was a significant achievement due to very complex geological conditions and resulted in a seepage reduction through the foundation and improvement of the overall safety and stability of the dam.
The total consumption of grouting material was in access of 2,500 tonnes of cement, bentonite, sand and additives.
After completion of the work, seepage of water through the foundation was reduced to about 4.5 m3/s.
Keywords: Seepage, remedial works, dam, grouting, inert material.
Simon Lang, Chriselyn Meneses, Kelly Maslin, Mark Arnold
It is now common practice for dam owners in Australia to take a risk based approach to managing the safety of their large dams. Some dam owners are also using risk based approaches to manage other significant assets. For example, Melbourne Water manage the safety of their retarding basins in a manner similar to their water supply dams.
Assessing the risks posed by retarding basins using methods developed for larger dams can raise challenges. For example, the Graham (1999) approach to estimating potential loss of life (PLL) is generally applied when estimating the consequences of dam failure. However, Graham (1999) may not be the most suitable model for estimating PLL downstream of structures with relatively low heights and storage volumes (e.g. retarding basins), given the characteristics of the case histories used to develop the method.
In this paper six potential methods for estimating PLL are tested on four retarding basins in Melbourne. The methods are Graham (1999), the new Reclamation Consequence Estimating Methodology (RCEM), the UK risk assessment for reservoir safety (RARS) method, a spreadsheet application of HEC-FIA 3.0, and empirical methods developed by Jonkman (2007) and Jonkman et al. (2009). Results from the methods are compared, and comment is made about which is most suitable.
Keywords: potential loss of life, dam safety, risk analysis, retarding basins.
Aida Baharestani, Dominic Kerr
North East Water (NEW) manages two reservoirs in series on Bakers Gully Creek, approximately 1.5km south of Bright in north-east Victoria. Both dams were constructed more than 100 years ago and taken out of service in the 1970s.
The Bakers Gully dams had an unacceptable risk profile according to ANCOLD’s Limit of Tolerability.
As the dams are out of service and have no operational benefit, NEW made the decision to partially decommission the dams.
The objective of the work was to lower the consequence categories of the dams from “High C” to “Low” and increase the spillway capacities according to ANCOLD Guidelines and ultimately reduce the dam safety risks to an acceptable level.
This paper describes the different stages of the project ranging from concept design, community engagement, environmental assessment and detailed design. In particular the paper explores the complexities of balancing in cost and public safety with community and ecological values.
Keywords: Dam decommissioning, Community engagement, Severity of damage and loss
T. I. Mote, M.L. So, N. Vitharana, and M. Taylor
This paper explores the sensitivity of selection of earthquake design magnitude to liquefaction triggering in Australia for ground motions typically used for dams. The low seismicity of Australia creates a situation where liquefaction triggering is marginal at design hazard levels and this low level of seismic hazard makes the liquefaction trigger analysis very sensitive to the derivation of the seismic inputs. A methodology is presented that couples the probability of liquefaction triggering with the distribution of earthquake contribution to the hazard from the magnitude-distance deaggregation. The results show that for the “typical” soil profile and input ground motions approximately equivalent to the maximum design earthquake for Australia, the probability of liquefaction triggering varies significantly with the design magnitude selected. Using the maximum credible earthquake or mean magnitude may provide significantly different liquefaction triggering implications. Combining the probability of liquefaction triggering with the contribution of varying magnitudes to calculate liquefaction probability is a useful method to understanding the sensitivity of liquefaction to design magnitude.
Keywords: Liquefaction Assessment, Design Magnitude, Probability of Liquefaction, Magnitude-distance deaggregation, Australia
This paper discusses the common environmental issues and requirements project lenders have when financing hydropower dam projects in developing countries. The environmental specialist’s role, as part of the Lender’s Technical Advisor team, is discussed throughout the main phases of project finance (credit approval, financial close, lending/construction and loan repayment/operation). Further, how environmental issues are reviewed and monitored, thereby minimising reputational risks to the lenders are outlined.
Lenders typically consider hydropower dam financing, especially reservoir schemes, as high reputational risk loans. Finance is usually syndicated and although most international lenders are Equator Principles signatories or use the International Financing Corporations (IFC) Performance Standards, some lenders have additional environmental guidelines and requirements to enable financing. These differences are discussed.
Common environmental concerns include loss of habitat of endangered and/or threatened species, changes to river flows, erosion and sediment control during construction, and the minimisation and disposal of project wastes.
These issues are discussed drawing on the author’s experience in monitoring environmental issues of hydropower projects in Asia Pacific and Africa, including both smaller run-of-river schemes and larger storage reservoir projects.
Keywords: Environment, impacts, project financing, concerns, lenders, lenders technical advisor.