The use of simulation models to assess dam failure consequences has progressively advanced in Australia over the past few years. For example, it is now common for HEC-LifeSim to be used to estimate potential loss of life from the failure of large dams with large populations at risk downstream. Since its introduction to Australia, numerous presentations and papers have been provided by USACE and industry professionals that highlight the benefits of using HEC-LifeSim for a range of different case studies.
Whilst the majority of the literature published to date have focused on the benefits of simulation modelling, this paper identifies some of the technical challenges that can arise, particularly in the evacuation modelling component of HEC-LifeSim. The techniques that have been used to overcome these challenges are also discussed using three case studies.
The first case study demonstrates the sensitivity of the life loss to changes in cell size and the output interval of the gridded hydraulic data. This is done by comparing the differences in life loss between high-resolution and low-resolution models for three dambreak models. The second case study illustrates the importance of the road network representation in HEC-LifeSim because the resolution of the road network is important to achieve plausible estimates of the fatalities along roads, and logical animations of the mobilisation. The final case study demonstrates the implications of coincident flow modelling on the life loss, and therefore the importance of understanding the hydrology of the target and neighbouring catchments.
This paper provides a checklist that prompts practitioners to consider some of the lessons learnt over the last few years and is envisaged to be a working document that improves the defensibility and robustness of HEC-LifeSim estimates throughout the industry.
Sedimentation of reservoirs is acknowledged as a global issue and likely impacts water storage capacity in Australia. This major challenge to our future water supply is a highly complex process with deposition leading to infilling of the reservoir of course sediments in headwaters following major inflows, progressively to finer fractions towards dam walls. Wave action and catchment inflows during drawdown conditions will further transport and redistribute sediments into the main body of the reservoir.
Managing reservoir sedimentation requires an understanding of the sediment types and deposition patterns across the reservoir. Once the location and type of sediment is known, strategies to mitigate the effects on the reservoir can be determined. Methods typically used for determining sedimentation of a reservoir are empirical or modeling techniques that rely on detailed data from inflow events, suspended solids loads and flow rates. In the absence of this data, more direct measurements to quantify the amount of sediment present can be used. Direct measurements are more robust than modelling approaches that utilise rating curves that can result in over estimations of the sediment present. This study combined several measurement techniques to produce high spatial coverage of the reservoir floor. Detailed validation of this approach was undertaken in one representative reservoir prior to adopting this approach across multiple reservoirs.
New Zealand’s economy is heavily dependent on export revenues generated by primary industries such as dairy, meat, agriculture, horticulture and viticulture. For these sectors, securing water for irrigation has been a key factor for growth. New Zealand has a temperate climate with generally wet winters and dry summers. The availability of water in the dry summer period is very important for these sectors to maximise production. A considerable amount of investment has already been made in the construction and operation of reservoirs for irrigation purposes. However, because climate change effects (more frequent occurrences of extreme events such as droughts and flash floods) have been observed around the world and the need for restrictions imposed on the use of water resources by regulators for environmental reasons, the need for developing water storage reservoirs has become more essential than ever. Climate change effects are already being factored into current practice. Drawing on the author’s experience, this paper discusses the potential impacts of climate change, with an emphasis on the effects of drought, on the design, construction and operation of water storage facilities with changes necessary to improve the resilience of new dams in
response to climate change. The paper also aims at raising awareness among the farming community so they can appreciate the associated risks and issues with climate change and be more cautious about planning and budgeting for their future investments in dam and irrigation projects.
Otago Regional Council (ORC) own and operate the Lower Taieri, Lower Clutha, and Alexandra Flood Protection Schemes. Collectively the schemes comprise over 220 km of earthfill levees, together with numerous appurtenant structures, such as major spillways, flood gates and pumping stations. The schemes provide flood protection to significant and varied communities and infrastructure adjacent to the Clutha and Taieri rivers, for example Dunedin Airport, and towns such as Balclutha and Outram. The works were constructed at various times since the 19th century to a range of standards, and assets are at various lifecycle stages.
Regular and systematic condition and structural integrity assessment is a key aspect of operating flood protection schemes for resilient communities. This can be challenging due to the large spatial extent of multiple schemes. Efficient and effective on-the-ground visual inspection of the entire network is key. A field assessment methodology was developed which combined on-the-ground visual assessment with innovative use of GIS technology, for field data capture, recording, analysis and presentation.
The structural assessment methodology used LiDAR-derived digital elevation models (DEMs) integrated with the field data to screen the levee networks based on geometry condition, to identify critical locations for analysis. Levee susceptibility to hazards such as overtopping scour, piping, seismic performance and slope instability was assessed utilising a semi-quantitative multi criteria analysis. Subsequent efforts were focused on critical locations enabling analysis which would not be efficient on a scheme-wide scale. An outcome included a GIS database to enable rapid future review of asset information and condition.
The assessment coincided with the July 2017 Taieri River flood – the largest event in almost forty years, and a timely reminder of the importance of flood protection infrastructure for community resilience. This event also highlighted the importance of making use of such events to field-truth assessment results and test assumptions about scheme performance and vulnerable locations.
Australasian and global need and demand for water resilience often changes reservoir use from single purpose to multipurpose. These changes are affecting existing dam and reservoir structures and operations, as well as those planned or under construction. The International Commission on Large Dams (ICOLD) recognised this issue and established a working group to investigate and prepare Bulletin 171 titled Multipurpose Water Storage “Essential Elements and Emerging Trends”, which is now and available on the ICOLD website.
The Bulletin’s scope was to provide a global view on the dynamics of multipurpose water schemes (MPWS) by presenting essential elements and emerging trends for planning and managing reservoir and dam infrastructure, with source data collected from 52 global case studies including five from New Zealand and two from Australia.
Water storage design and implementation has evolved significantly in recent decades, and further
development is expected as innovative approaches emerge in search of optimal sustainable solutions. The focus of Bulletin 171 is therefore not on what should be done, but rather what is being done, how, and by whom. Essential elements represent a recommended checklist for implementing MPWS storage, while emerging trends is a snapshot of the current state-of-the-art for MPWS projects.
This paper presents a summary of Bulletin 171 and its findings, and a brief overview of the new and
complementary ICOLD Committee ‘T’ which is assessing emerging challenges and needs for dams in the 21st century.
Many quantified risk assessments finish the failure mode event tree at the estimated occurrence of an embankment breach leading to dam failure outflows and downstream consequences. In some situations, for dams with multiple embankments with potentially different consequences downstream of each embankment, the possibility for further breaches may be pertinent if there may potentially be higher consequences for a multiple breach scenario. The location of an initial breach and sequence of subsequent breaches could also result in different contributions to total risk.
This paper discusses a method applied to investigate the conditional probability of flood overtopping breaches for multiple earth-fill embankments with grass covered downstream slopes.
For the subject dam, preliminary modelling identified that for a flood overtopping breach of an embankment the breach’s development may not be sufficient to reduce the lake level and sustained overtopping flow over the remaining embankment crests could lead to further embankment breaches.
A Monte Carlo dam breach simulation modelling approach was used with a large number of flood events. The simulation modelling considered erosion initiation for a grass slope due to the combination of velocity and duration of flow, and erosion continuing to breach based on duration of flow after erosion initiation. Potential uncertainty of erosion initiation and erosion continuing to breach were represented with probability distributions in the Monte Carlo modelling.
The results from the large number of dam breach simulations were then analysed with post processing to derive conditional probabilities for single or multiple breaches and breach sequence.