University of Twente Student Theses


Interdependency of Stochastic Variables Determining Normative Water Levels in the Alblasserwaard

Roosjen, W. (2019) Interdependency of Stochastic Variables Determining Normative Water Levels in the Alblasserwaard.

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Abstract:The Netherlands has always been vulnerable to floods due to its low elevation. To reduce the probability of floods in regional water systems to an acceptable level of safety, norms have been introduced. These norms are based on the probability of occurrence of these floods. The regional water systems are generally assessed on the level of safety using a hydraulic model of the water system to determine the frequency of occurrence of certain water levels for each location, the so-called normative water levels. This could be used to determine the required top level of regional flood defences. There are currently two main methods which are used to determine these normative water levels, the time series method and the stochastic method. The first method uses measured time series of for example precipitation and wind as input for the model. Then, the frequencies of occurrence of water levels are determined based on the resulting water levels in the model. As these measured time series are generally not sufficient in length for the return periods of the norms (e.g. once in 100 years), extrapolation based on the available observations takes place to obtain the water levels for longer return periods. The stochastic method extrapolates on the input variables or also-called stochastic variables (e.g. precipitation and wind) to generate model events with a certain probability. An important advantage of this second method is that extreme situations are tested in the water system. However, a disadvantage compared to the time series method is that it is often assumed that these input variables are independent of each other in order to avoid (complex) multivariate probability models. This assumption potentially affects the probabilities of occurrence of certain situations (e.g. large precipitation volumes with high wind speeds and high external water levels), as interdependencies might be present in reality. To assess whether there are any interdependencies between these input variables, and what the effects of these interdependencies on the joint probabilities and normative water levels are, a case study on the Alblasserwaard is performed (location in Figure 1). To determine the joint probabilities of the input events, the method ‘copula’ is used. This method is used to construct joint probability distributions based on the dependence structure between the variables and the probability distributions of each variable. Important advantages over other methods are that each probability distribution can take any form and is independently chosen of the dependence structure, while extensions to higher dimensions are also possible using the theory of vine copula. Summing up, the research goal of this study is: “To evaluate to what extent including interdependence of stochastic variables affects normative water levels in comparison to assuming independence between these variables, by using a copula approach in a case study of the Alblasserwaard”
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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