Retarding Basin Fundamentals (April 2013) – MODULE 1 TO 4
Retarding basins are becoming increasingly important elements in urban flood planning throughout Australia but it is evident that many basins have not been implemented in line with modern day “risk management” approaches. Accordingly, as part of its Professional Development program, and in advance of its proposed Guideline on Retarding Basins due out in 2014, the Australian National Committee on Large Dams (ANCOLD) is planning a series of seminars across Australia on Retarding Basin Fundamentals. The seminars are being provided to assist basin owners, and their professional staff and advisors, to plan, implement and operate their basins in a safe and effective manner consistent with various Government and “duty of care” obligations.
The one day seminars are being presented by leading dam safety and risk management professionals including:
Richard Rodd- dams consultant with over 40 years experience in dam and basin design, construction, operation and maintenance.
Kelly Maslin – dams consultant with over 15 years experience in dams and risk management.
Norm Himsley – dams consultant and member of NSW Dam Safety Committee with over 40 years experience in dam and basin design, construction, operation and maintenance.
Includes access to the following videos:
$60.00 - $80.00
Since publication in 2003, the ANCOLD Guidelines for Risk Assessment have reached broad acceptance and use in Australia. In practice, dam owners use the principles of risk assessment to drive business investment decisions. As the guidelines undergo revision, it is timely to assess whether our practices need to evolve to more holistically consider all types of consequences, rather than our current focus on loss of life, in decision-making. This paper aims to prompt dam owners and consultants alike to re-assess our focus on loss of life in risk assessment decision-making, and whether we should more meaningfully consider alternative or broader indicators.
An industry survey was undertaken which found that large dam owners are generally happy with the current system of dam safety decision making. However, the survey responses did identify difficulties in relation to justifying investment below the limit of tolerability that are subject to ALARP principles. In a small number of cases, dam owners found it difficult to justify investment when life safety was not important.
Building on the industry survey and subsequent discussions with practitioners, this paper discusses how the current approach to risk based decision making may result in sub optimal decision making. Further it is discussed how there is an important role that economics should play in providing a universally accepted framework for assessing trade-offs and providing consistent evidence to support decision making.
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.
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
Kristen Sih, Richard Rodd
Melbourne Water currently manages over 235 stormwater retarding basins. The process of assessing the risk posed by these assets began in 2006, and at the end of 2015 full risk assessments were completed for around 30 of the basins that were estimated to pose the highest societal risk. However, when analysing the results of these risk assessments, there was some concern that the results were inconsistent and often too conservative, given the few incipient or actual failures that had been experienced.
It was found that one of the key areas causing the conservatism was poor documentation of design and construction details, and the fact that the tools used for assessing the Potential Loss of Life (PLL) were aimed at larger storages that cause much higher depths and velocities in dambreak events than these (generally) small storages. To remedy this situation, advice was sought from specialist practitioners to develop guidance notes on the assessment of PLL and failure likelihoods for retarding basins.
On the back of these guidance notes, Melbourne Water initiated an accelerated program of assessing the risk associated with 78 retarding basins over a 6 month period. This paper describes the key recommendations from the guidance notes, compares the results of the risk assessments performed pre- and post-guidance notes and provides a summary of the portfolio risk assessment outcomes, what they mean for Melbourne Water and what the organisation intends to do to manage this risk into the future.
Changes to the estimation of extreme rainfall events resulted in significant increases in the estimates of the PMF since the original design of Wivenhoe Dam. To upgrade the dam to meet these new requirements, SEQWater (owner and operator) formed an Alliance with Leighton Contractors, Coffey Geosciences, MWH and the NSW Department of Commerce.
The option selected for the upgrade works included the construction of a new secondary spillway, upgrade of the existing gravity section, radial-gated spillway, and strengthening of the dam crest.
Value management was key throughout the project ensuring the Alliance was continually looking to
improve practices, increase cost-effectiveness and create innovative solutions for design elements of the project.
On numerous occasions when the design was challenged, the Alliance made ‘best for project’ decisions to carry out additional investigations or design work to pursue alternatives. As an example, the powerful tool of Computational Fluid Dynamics was used in the analysis and design of flow deflector plates on the existing spillway, which were an alternative to the originally designed gate locking pins. The investigation and development of this alternative resulted in significant cost savings and a more effective design solution.
This paper presents aspects of the design carried out by the Wivenhoe Alliance, lessons learned, and the way continual investigations during construction provided value for money solutions.