There are many dams in Australia with appurtenant features such as spillway gates, large capacity outlet works, power stations and transfer tunnels. These features can play a significant role in how these dams are operated during flood events and allow for additional flexibility to implement flood mitigation activities such as pre-releases and surcharge depending on authorised operating procedures for the dam.
Typical practice in many dam flood hydrology studies has been to significantly simplify or even ignore the impacts of these features on the dam water level frequency curve. For example, it may have been assumed that spillway gates were either fully open or changed from fully closed to fully open in a uniform manner regardless of inflow rate. Whilst this approach significantly simplifies routing of floods through these storages, it may produce results which are inconsistent with the expected flood probability of the dam given its current operating procedures, especially for relatively frequent flood events. This is particularly critical for risk assessment where definition of the flood loading probabilities requires robust estimates of water level AEPs for all events.
In a number of recent studies, greater emphasis has been placed on detailed modelling of the effects of spillway gates and other outlet works on dam flood hydrology. This has required site-specific algorithms to be developed which incorporate the characteristics of the spillway gates or other features at each structure, as well as the flood operations procedures for the dam. This paper presents a number of case studies where explicit simulation of dam flood operations has had a significant impact on the resulting flood frequency curve and downstream flow rates and discusses the implications of that on dambreak modelling and risk assessment for those dams.
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New technology and outputs from flood forecasting systems can raise issues for dam safety managers in how they use uncertain information to make critical dam safety decisions. In particular, making operational decisions around pre-releases based on forecast inflow presents challenges. In this case dam safety risk needs to be weighed up with other risks such as increasing downstream flooding, or being able to supply water into the future. The process of developing a flood forecasting system should be a close collaboration between the developers and the users. This ensures that outputs provide meaningful information that can be used to support operational decision-making in a flood or emergency response situation.
This paper describes taking the data from the transducer recording of dynamic fluctuations at 300 Hz in the physical hydraulic model of the stilling basin of Fairbairn dam and analysing the response of the proposed design solution to these loads. The analysis not only looked at the direct time history loading, but reviewed the response of the anchoring system to the inertial and damping loads. A further extension of the analysis allowing for the stiffness of water has come up with some findings that verify what has intuitively been believed about the design of spillway stilling basin slabs.
While structures such as a dam walls, pipelines, gas storage tanks, and nuclear facilities are vulnerable to the shaking from earthquakes, they are even more susceptible to differential movement on faults passing beneath their foundations.
In the past, the probability of surface rupture of a fault was calculated by making some simplistic assumptions about the distribution of earthquake magnitudes. Improved databases of earthquake ground faulting now allow the probability of surface rupture to be estimated in a more realistic fashion. Computing software that uses a Monte Carlo approach has been developed to allow the effect of various scenario choices on rupture probability to be investigated.
Using this software, it is found that the most significant influence on rupture probability is the long-term fault slip-rate. Other assumptions about the faulting style, maximum magnitude and conversion parameters have only a moderate influence on the results.
There have been several instances in recent history in Australia of surface faulting due to earthquakes, but there has been only limited damage to infrastructure due to the remoteness of these earthquakes. The software that has been developed will allow a considered assessment and comparison of the hazard and risk due to both ground shaking from earthquakes and from surface rupture.
The U.S. Army Corps of Engineers (USACE) Risk Management Center (RMC) developed the Reservoir Frequency Analysis software (RMC-RFA) to facilitate, enhance, and expedite flood hazard assessments within the USACE Dam Safety Program. RMC-RFA is a stochastic flood modeling software that employs advanced statistical and computing techniques, allowing a user to perform a screening-level stage-frequency analysis on a desktop PC with runtimes on the order of seconds to a few minutes. RMC-RFA utilizes an inflow volume-based stochastic simulation framework that treats the seasonal occurrence of the flood event, the antecedent reservoir stage, inflow volume, and the inflow flood hydrograph shape as uncertain variables rather than fixed values. In order to construct uncertainty bounds for reservoir stage-frequency estimates, RMC-RFA employs a two looped, nested Monte Carlo methodology. The natural variability of the reservoir stage is simulated in the inner loop defined as a realization, which comprises many thousands of events, while the knowledge uncertainty in the inflow volume-frequency distribution is simulated in the outer loop, which comprises many realizations.
Stage-frequency curves derived with RMC-RFA are compared to those derived with more complex, precipitation-based simulation frameworks, such as the Monte Carlo Reservoir Analysis Model (MCRAM), the Stochastic Event Flood Model (SEFM), and the Watershed Analysis Tool (HEC-WAT). The inflow volume-based framework employed by RMC-RFA produces stage-frequency curves that strongly agree with the more complex, precipitation-based methods. Furthermore, the results from the alternative methods fall within the RMC-RFA uncertainty bounds, demonstrating its robustness. In this sense, the RMC-RFA simulation framework lends itself to a value of information approach to risk management, where knowledge uncertainty can be efficiently quantified at a screening-level assessment, and then the value of performing more complex and sophisticated studies to reduce uncertainty can be considered.
Following the catastrophic failure of the bottom outlet conduits of the Massingir Dam, a rehabilitation project was launched involving the installation of steel liners and the rehabilitation of the hydromechanical equipment. This paper describes the testing of an emergency gates for possible use as a control gate to maintain supply to downstream water users. It further describes the innovative use of alternative access for concreting and other services, the use and benefits of self-compacting concrete for infill concreting between the steel liner and existing concrete and the programme and cost benefits of pressurising the steel conduit prior to concrete encasement.