University of Twente Student Theses


Risk Assessment Methodology for Quantifying the Impact of Scour : a serious threat to river crossings

Huurne, Ramon ter (2014) Risk Assessment Methodology for Quantifying the Impact of Scour : a serious threat to river crossings.

Abstract:Transport systems are exposed to many risks. Damages done to transport systems may cause a lot of problems, not only in economic matters, but even in the form of casualties and fatalities. Therefore, risk assessment of these risks is very important. One of these risks is scour which can cause damage to river crossings. In this report, the main question is how the impact of scour on river crossings can be quantified. In other words, how vulnerable is a river crossing to scour. Scour is the removal of streambed or bank material from the river crossings foundation due to the flowing water. The removal of this material may lead to unstable foundations, which can eventually result in the collapse of a river crossing. How serious the problem is, illustrates the fact that in the United states, scour is the most common cause of highway bridge failure (Kattel & Eriksson, 1998). Because of the serious threat of the scour to river crossings, risk assessment is a very important part of the risk management process. In the risk assessment procedure, we can identify three phases: risk identification, risk analysis and risk evaluation. In this thesis the identified risk for river crossings is scour. Scour can occur in many different ways. This all depends on the circumstances that are present on a certain location. To get a clear insight which variables contribute to scour, scour quantification models were analysed. The most important parameters from these models are flow, soil and structure characteristics. Besides, it showed that river crossings are vulnerable to scour because of the decrease of the bearing capacity of the soil and the exceedence of the ductility limit of the structure. With the information gathered during the risk identification process, risk analysis is possible. For the determination of the impact of scour to river crossings, fragility curves and a risk model using Bayesian Belief Network (BBN) are considered as a good methodology. A fragility curve shows the probability of failure, a form of vulnerability, given a certain loading or intensity measure. In the case of scour, scour depth has been chosen as an intensity measure. BBN risk models provide a network with all the variables and relations between the variables, combined with the so called Bayesian probabilities. These probabilities show the probability of occurrence of each variable based on expert knowledge and „belief‟. Besides Bayesian probabilities, classical probabilities, which are based on historical data about events or simulations models, may be blend with Bayesian probabilities to try to get the model as accurate as possible. The fragility curve and BBN risk models are developed concerning the risk of scour to river crossings. For fragility curves, different damage states are possible. These damage states shows how vulnerable a structure is depending on a certain degree of the intensity measure. These damage states are often slight, moderate, severe and complete damage. Damage states also represent a degree of serviceability. For the damage states these are respectively fully serviceable, serviceable but impaired, not serviceable and collapsed. For example, a certain amount of scour may cause an exceedence of the slight damage state, this means the river crossing is no longer fully serviceable. The information that should be gathered to develop the fragility curve for scour is the information needed to calculate the scour depth, the probability of failure and the limit states of each damage state. For the development of the BBN risk model, it is very important to analyse the process of how scour occurs. The scour quantification models provide very useful information about the process of scour. They show the variables that are contributing to scour and the relationship between them. With this information, it is possible to set up a network with all the variables and the relations between them. However, as the quantification of the impact of scour is the main question, the network itself is not enough. Therefore, BBN risk models show the probabilities of each variable, and in the end, what the impact of scour is. This impact is regulated the same as with the fragility curves, who‟s damage states are integrated into the BBN risk model. This means that as an output for the BBN risk model, the damage states are given. The calculations of the probabilities for each variable can be done by using historic data or simulation models. However, each river crossing has a different set of variables and parameters, why it is impossible to set up a general BBN risk model what can be applied to all the scour events and river crossings. Therefore, only the probabilities of heavy rainfall that cause floods are determined though these only can be applied on river crossings in the Netherlands, as the data is obtained from a Dutch institute. The risk analysis of scour shows how scour may occur and how this can be quantified in such a way, that it is clear whether or not the river crossing is still serviceable or not. Therefore, fragility curves and BBN risk models provide an excellent insight in the impact that scour may have on river crossings. They quantify how vulnerable a river crossing is, and based on this information, measures can be taken or not. Although the report does not provide calculated examples of a fragility curve of BBN model for scour, they are widely adopted in other risk assessment projects. For this reason, as a risk evaluation and the last step of the risk assessment process, the methodologies shown in this report considering fragility curves and BBN risk models are a perfect way to quantify the impact of scour on river crossings. The outcomes of a fragility curve and BBN risk model can be used for further research in the areas of risk management models and risk mitigation strategies. It is clear that scour is a big problem and measures has to be taken and scour management and scour mitigation is needed.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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