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Peter Woodman, Andrew Northfield, Tim Kallady

Currently there is little guidance available on how itinerants on roads should be included in a consequence assessment. The methods available are often subjective which can lead to itinerants on roads either being ignored or insufficiently considered. A fact that can in turn lead to consequence categories being inappropriately assigned to the asset being assessed or risks being under or over estimated. Consideration of these itinerants is especially important for smaller dams or retarding basins in urban areas where often the Potential Loss of Life (PLL) in buildings is small but there are major roads carrying a large Population At Risk (PAR) through the inundation extent, which experience flooding of sufficient severity to pose a threat to life.

This paper looks at how the method used to assess itinerants on roads can affect the consequence category assigned to an asset and/or the risk of the dam or retarding basin. It will draw on a number of recent assessments undertaken for retarding basins within Melbourne and make comment on a possible approach to consider itinerants on roads in the future.

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2016 Papers

2016 – Roads Can Drive Your Risk! A Look at the Importance of Itinerants on Roads in your Consequence Assessments
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Peter Woodman, Andrew Northfield, Tim Kallady

Currently there is little guidance available on how itinerants on roads should be included in a consequence assessment. The methods available are often subjective which can lead to itinerants on roads either being ignored or insufficiently considered. A fact that can in turn lead to consequence categories being inappropriately assigned to the asset being assessed or risks being under or over estimated. Consideration of these itinerants is especially important for smaller dams or retarding basins in urban areas where often the Potential Loss of Life (PLL) in buildings is small but there are major roads carrying a large Population At Risk (PAR) through the inundation extent, which experience flooding of sufficient severity to pose a threat to life.

This paper looks at how the method used to assess itinerants on roads can affect the consequence category assigned to an asset and/or the risk of the dam or retarding basin. It will draw on a number of recent assessments undertaken for retarding basins within Melbourne and make comment on a possible approach to consider itinerants on roads in the future.
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2016 Papers

2016 – When is a Hazard not a Hazard? A Review of Fatality Factors for Dambreak Consequence Assessments.
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Peter Woodman, Andrew Northfield, Hench Wang

Current empirical approaches assume different fatality factors for the ‘fail’ and ‘no fail’ scenarios even when the same hazard is experienced by a property. This approach can lead to some inconsistencies particularly for small dams and retarding basins. This paper looks at the base data behind the current fatality factors and explores possible alternatives to the current approach. The paper will rely on a number of examples from a recent investigation undertaken by GHD for Melbourne Water on a number of their retarding basins.
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2017 Papers

2017 – Observations from Five Australian Applications of HEC-LifeSim to Estimate Potential Loss of Life from Dam Failure
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Chriselyn Kavanagh, Simon Lang, Andrew Northfield, Peter Hill

The U.S. Army Corps of Engineers have recently releasedHEC-LifeSim1.0, a dynamic simulation model for estimating life loss from severe flooding (Fields, 2016). In contrast to the empirical models that are often used to estimate life loss from dam failure, HEC-LifeSim explicitly models the warning and mobilisation of the population at risk, and predicts the spatial distribution of fatalities across the structures and transport networks expected to be inundated. This capability provides additional insights to dam owners that can be used to better understand and reduce the life safety risks posed by large dams. In this paper, we demonstrate the use of HEC-LifeSim to model the potential loss of life from failure of five large Australian dams. Particular attention is paid to how the predicted life loss varies with warning time, in a manner that depends on human response and the transport network’s capacity for mass evacuations, and the modelled severity of flooding. We also examine how the HEC-LifeSim estimates of life loss compare with those from the empirical Reclamation Consequence Estimating Methodology (RCEM).
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2017 Papers

2017 – Tailoring Consequence Assessments for Retarding Basins
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Andrew Northfield, Simon Lang, Peter Hill

Melbourne Water currently manages more than230retarding basins (RBs). A large portion of these are less than 4 metres high, and traditionally structures of this size have not been subject to intermediate or detailed ANCOLD Consequence Assessments. However, the need to understand the failure consequences for smaller structures has increased over time, as risk based approaches to managing safety have expanded from large dams to other water retaining assets.

Undertaking detailed consequence assessments for all Melbourne Water’s RBs would not be practical, given the costs and time involved. Therefore, this paper describes a method for assessing the level of ANCOLD Consequence Assessment that is justified, based on the structure’s attributes. It also presents an equation that was used to estimate peak outflows from RB failure. The peak outflow estimates can be used to model approximate failure inundation extents downstream of small dams and RBs.

The paper draws on work that HARC have recently undertaken for Melbourne Water to assess the failure consequences for 88 RBs. The outcomes are relevant to other organisations that own or manage significant numbers of small water dams or RBs.
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2018 Papers

2018 – Using HEC-LifeSim to Better Understand and Reduce Dam Failure Consequences for Three Case Studies Around Australia
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Hench Wang, Andrew Northfield, Peter Hill

HEC-LifeSim modelling has been emerging in the industry over the last few years and is increasingly becoming the preferred method for detailed consequence and failure impact assessments. The increased adoption rate of HEC-LifeSim modelling is a result of advancements to computation power and hydraulic modelling techniques and allows dam owners to obtain more robust and consistent estimates of the potential loss of life (PLL) compared to the traditional Graham (1999) and RCEM (USBR, 2014) approaches.

This paper will demonstrate, through the use of three examples, how the inputs and outputs from HEC- LifeSim have been used to identify potential ways to better understand the consequences as a result of dambreak.
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2019 Papers

2019 – Modelling Reservoirs with a 2D Hydraulic Model
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Andrew Northfield, David Stephens, Tim Craig, Mitchell Smith

In recent times two dimensional (2D) hydraulic modelling has become the most common type of modelling for undertaking dambreak assessments. Direct map outputs such as depth and depth-velocity product are very useful in assessing risk across a floodplain. The temporal output from 2D models also enables the tracking of flow across a floodplain, helping practitioners and dam owners alike make informed decisions on warning time and evacuation routes. These outputs form essential input to packages such as HEC-LifeSim an agent-based simulation model for estimating life loss by simulating population redistribution during an evacuation.

A number of investigations have shown the hydraulic model, TUFLOW, is able to simulate the hydraulic conditions expected in a dambreak flood wave, giving confidence in the model’s ability to correctly capture the flood wave propagation. Notwithstanding this ability, there remains uncertainty over the best methodology to adopt when assigning a breach hydrograph to the model and in turn the impact this choice has on assessing downstream populations at risk.

A commonplace method of assigning dam breach hydrographs is to model the reservoir and dam structure with a 1D model or spreadsheet, where the storage is represented with a stage storage relationship and outflow through a time-varying breach is calculated using level-pool routing. The resulting hydrograph is then applied directly to a 2D model immediately downstream of the dam to model the propagation of flow downstream.

An alternative approach consists of representing the entire reservoir, dam and downstream floodplain in the 2D model. This allows for the dynamic effects of bathymetric constrictions in the reservoir to be accounted for which could greatly impact on the timing and shape of the dam breach hydrograph. However, this comes at a cost, as representing the reservoir in 2D requires bathymetry data which can be expensive to capture and also may require a major extension of the model domain.

In this paper the ‘Fully 2D’ and ‘Stage storage relationship 1D/Spreadsheet’ approaches are compared for a number of case studies.
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2020 Papers

2020 – Estimation of consequences for WaterNSW’s diverse portfolio of Greater Sydney dams
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Andrew Northfield, Peter Hill, Muhammad Hameed, Hench Wang, Sam Banzi

In 2018 WaterNSW undertook a Portfolio Risk Assessment (PRA) for 20 dams across the greater Sydney area.

This paper describes the estimation of consequences for this large and diverse portfolio of dams. For some dams the population at risk were greater than 100,000 people whereas for others there were no permanent PAR which required the careful consideration of itinerants. This diversity of the dams required that the approach for estimating the consequences be tailored to the specific characteristics. For example, the approaches for estimating the potential loss of life (PLL) varied from a detailed simulation model (HECLifeSim) to a simpler empirical approach (Reclamation Consequence Estimation Methodology (USBR, 2014) to bespoke consideration of itinerant campers and users of walking tracks. For some dams the economic costs were driven by direct infrastructure costs whereas for other the indirect costs dominated the total economic cost for failure.
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Search Results: “” — Page 1

Hench Wang, Andrew Northfield, Peter Hill

Andrew Northfield, David Stephens, Tim Craig, Mitchell Smith