Russell Cuerel, Richard Priman, Michel Raymond, Ian Hanks
Following significant flood events across Queensland over the last five years causing significant damage in South East Queensland, Bundaberg Burnett region, St. George in the south west and more recently in Central Queensland in the Callide Valley, there has been renewed interest in finding solutions to flooding issues.
Increasing the available flood storage within a catchment is a well-known method of improving flood mitigation outcomes for developed areas. In many basins/catchments, potential flood storage development options (new storages or augmentations to existing storages) can be identified by reviewing previous water supply investigations and flood studies and by scanning topographic mapping. From such site identification there will often be numerous combinations of possible flood storage development options to consider because of the number of tributaries which may contribute to major flood events.
This paper outlines a methodology to screen, within a relatively short timeframe and at relatively low cost, a large number of identified flood storage development options and combination development scenarios and shortlist for more detailed analysis. The screening process is heavily reliant on hydrologic assessments to rapidly short-list scenarios for assessment and then relies on traditional engineering and economic assessments to do the fine tuning of the analysis.
Keywords: flooding, damages, impacts, flood storage, flood mitigation, dams, benefit-cost ratio.
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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.
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
Monique Eggenhuizen, Eric Lesleighter, Gamini Adikari
St Georges Dam is located on Creswick Creek approximately 2km southeast of the township of Creswick and 135km northwest of Melbourne. The reservoir, located within the Creswick Regional Park and originally constructed to supply water for the Creswick quartz crushing plant in the 1890s, has since been established as a popular recreational storage and is the responsibility of Parks Victoria. The dam is approximately 16m high and located across a relatively steep gully. The embankment consists of earthfill with an upstream face of rock beaching and a grass covered downstream face. The primary and secondary spillways are cut into the right and left abutments respectively.
At the completion of a detailed design review, St Georges Dam was assessed to be within the top three of Parks Victoria’s dams portfolio in regards to Public Safety Risks. The detailed design review assessed that the risk position for the dam plotted within the unacceptable region of the ANCOLD Guidelines for the static, earthquake and flood failure modes. As such, upgrade measures were considered to be required. In 2010 and 2011, a number of significant flood events emphasised the importance of upgrade works at this dam, particularly in regards to upgrading the spillway capacity, and consequently Parks Victoria assigned these works a high priority.
SMEC was engaged to design the upgrade works for the dam. A number of arrangements to increase the spillway capacity of the dam were considered, with the most cost effective option being assessed to be a secondary spillway over the dam embankment in the form of a rock chute.
This paper describes the decision making process associated with the option selection and the methodology for designing the overbank spillway which utilised the findings in ‘Riprap Design for Overtopping Flows (Abt & Johnson, 1991), and US Army Corps of Engineers, Waterways Experiment Station, publications of standard riprap gradations and computer program CHANLPRO.
Keywords: Embankment Dams, Spillway, Rock Chute, Erosion Protection
David Stewart, Shane McGrath & Siraj Perera
Dam safety in Victoria is overseen by the Department of Environment, Land, Water and Planning on behalf of the relevant Minister and under the Water Act. For each of the 19 state-owned Water Corporations, Government has issued a Statement of Obligations which describes all responsibilities of the Corporation, including specific reference to dam safety management and ANCOLD Guidelines.
These Corporations report annually to the Department on their compliance with all their obligations, including dam safety management. In late 2014, 13 Water Corporations along with the Department commissioned a comparative benchmarking study of dam safety management practices across the state. This work was facilitated by the VicWater Dams Industry Working Group. The study used a rapid assessment method against 14 separate criteria for dam safety management, based on the Statements of Obligations, guidance notes developed by the Department, ANCOLD Guidelines, the ICOLD Draft Bulletin on Dam Safety Management, good governance principles and examples of best practice from other jurisdictions.
The study involved assessment of background data, site inspections and discussions with various individuals of each owner, including a range of field staff, dam safety staff, Executive Managers, Managing Directors and Board Directors. The benchmarking study covered 142 dams of Significant, High and Extreme Consequence Category throughout Victoria.
The results of the benchmarking study have been extremely useful for individual dam owners and for the Department to understand areas where good practice is in place and also where there is potential for improvement of individual programs. The study also provides a measure of assurance of the current status of dam safety management practices and areas where regulatory practices could be better focused. It also reinforced the importance of strong industry networks such as ANCOLD and VicWater for knowledge transfer, capacity development and sustainability of dam safety management practices.
This paper presents the methodology used for the benchmarking study and its broader findings. It also highlights good practice considerations for dam owners, regulators and other dam safety practitioners.
Keywords: Dam Safety Management, Governance, Benchmarking
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.