Vietnam has many embankment dams to supply water to the agricultural sector. Most of these dams were built between 1970 and 2000 but have degraded significantly since their construction due to a number of different reasons. Identifying the main potential failure modes for these dams aims to improve their dam safety management systems as well as help to target dam safety rehabilitation works. The research was conducted by analysing 207 Dam Safety Reports and Feasibility Studies published by the Vietnamese Ministry for Agricultural and Rural Development between 2017 and 2019. The priority level of rehabilitation required to these dams was assessed by analysing whether overtopping, seepage and slope stability related potential failure modes were likely to occur. The results revealed the main potential failure modes of embankment dams in Vietnam and the possible reasons for these are discussed. The approaches to rehabilitate the dams that are outlined in the Feasibility Studies were also analysed and are discussed in general terms. The results provide valuable insight into commonly encountered dam safety issues with embankment dams in Vietnam.
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Dams and levees within the U.S. Army Corps of Engineers (USACE) inventory were constructed for a variety of purposes including flood control, navigation, hydropower, recreation, and fish and wildlife conservation. USACE transitioned to using life safety risk as a key input to all dam and levee safety decisions in 2006. This was implemented for many reasons, paramount among them is forming a consistent basis to evaluate the safety of dams and levees and prioritize the implementation of risk reduction measures in a consistent manner across the agency to best utilize available resources. This requires knowledge of what constitutes unacceptable risks that would require risk reduction actions. The Tolerable Risk Guidelines (TRG) were developed for this purpose, and to form a common basis for dam and levee safety evaluations and decisions. Protection of life is paramount, and there are four TRG related to (1) understanding the risks surrounding dams and levees, (2) building risk awareness, (3) fulfilling daily responsibilities, and (4) continually considering actions to reduce risks. The USACE policies have evolved over time, but the fundamental principles that underpin the TRG have been fairly consistent for the past 10 years. The evolution of the TRG have come as a result of the experiences using these principles to support more than 2,500 safety decisions. This paper describes the rationale behind the selection of the TRG.
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.
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.
K.A. Crawford-Flett, J.J.Eldridge, E.T. Bowman, C. Gordon
This paper provides an interpretation of factors governing the manifestation of internal erosion in a New Zealand canal that was constructed during the 1970s. Liner and subgrade soils were sampled during de- watering of Tekapo Canal in 2013, following the surveillance of erosion events over the preceding decades. This paper focuses on the interpretation of erosion susceptibility of liner and subgrade soil gradations sampled at four locations. Of the four locations, Sites 2, 3, and 4 were associated with internal erosion defects. A single location (Site 1) was selected to provide benchmark “intact” (un-eroded) samples.
Interpretation of susceptibility of the widely-graded soils to internal erosion mechanisms was achieved through the application of established empirical techniques for internal stability, filter compatibility, and segregation. Analysis of gradations, which are believed representative of some – but likely not all – canal soils, showed that Sites associated with erosion defects had liner-subgrade interfaces that permitted “some erosion” (NE < D15F < EE), while the Site showing no sign of erosion possessed an interface that met modern filter retention criteria for No Erosion. Based on gradation analysis, internal instability is considered a possibility for subgrade materials in particular. It is possible that subgrade materials that fail No Erosion criteria for liner retention may not represent as-built material and may instead have lost finer fractions in situ due to seepage-induced instability, leaving a coarser-than-placed and filter-incompatible subgrade.
This case study demonstrates the use of gradation-based empirical methods as initial screening tools to assess the susceptibility of soils to internal instability, filter compatibility, and segregation. The relationship between the internal stability of a filter and the filter’s particle retention performance (compatibility) is emphasised. As well as gradation susceptibility, the assessment of other factors such as segregation and hydraulic loads must be considered in order to better-understand susceptibility to erosion mechanisms.
Computational Fluid Dynamics (CFD) is the science of predicting momentum, mass and heat transport, and can aid in design and safety issues for dam resilience in modern settings. Applications of CFD have historically been in the aerospace, automotive and chemical process industries with limited application in the hydraulic engineering field; possibly due to the associated computational intensity that is typically required. However, over the past two decades the cost of computing power has decreased substantially while the processing speed has increased exponentially. These developments have now made the application of CFD in the commercial environment feasible. CFD is particularly valuable in complex flow situations where the outputs required cannot be provided by a traditional hydraulic assessment approach and where there are stakeholder drivers such as service life, insurance cover and safety implications of infrastructure. The need for CFD when these drivers and complex flow situations arise, are demonstrated by means of a case study.
In the case study, CFD was used to investigate the flow patterns and the predicted performance of the outlet pipework from Massingir Dam in Mozambique. Three flow scenarios with appropriate pressure and flow boundary conditions were analysed for the outlet pipework, which included bifurcations for power generation from the main discharge conduits. Specific concerns addressed were, firstly, the possible excessive negative pressure in the region of the offtake for power generation and the potential for cavitation effects and, secondly, unacceptable velocity gradients in the power offtake pipework. Results showed that although some negative pressures were possible in one flow scenario, mitigation measures based on the CFD outputs could be considered and designed before construction.
The implementation of CFD in the above case study displays how risk in design can be reduced to ensure safety issues are addressed effectively.