Pieter van Gelder

Stochastic simulation of episodic soft coastal cliff recession

Probabilistic methods provide a means of demonstrating the potential variability in predictions of coastal cliff recession. They form the basis for risk-based land use planning, cliff management and engineering decision-making. A range of probabilistic methods for predicting soft coastal cliff recession has now been developed, including statistical techniques, process-based simulation and structured use of expert judgement. A new episodic stochastic simulation model is introduced, which models the duration between cliff falls as a gamma process and fall size as a log-normal distribution. The method is applied to cliff recession data from a coastal site in the UK using maximum likelihood and Bayesian parameter estimation techniques. The Bayesian parameter estimation method enables expert geomorphological assessment of the local landslide characteristics and measurements of individual cliff falls to be combined with sparse historic records of cliff recession. An episodic simulation model is often preferable to conventional regression models, which are based on assumptions that are seldom consistent with the physical process of cliff recession.

Optimal maintenance decisions for berm breakwaters

Abstract To prevent coastal lines of defence from being affected by severe hydraulic loads from the sea, berm breakwaters can be used. Although berm breakwaters are dynamically stable in the sense that they allow for some rock displacement, they can fail due to severe longshore rock transport. To avoid this type of failure, berm breakwaters have to be inspected and, if necessary, have to be repaired. A decision model is presented enabling cost-optimal maintenance decisions to be determined while taking account of the (possibly large) uncertainties in: (i) the limiting average rate of occurrence of breaches in the armour layer and (ii) given a breach has occurred, the limiting average rate of longshore rock transport. The stochastic process of rock displacement is modelled by a modified generalised gamma process, enabling the uncertainty in these limiting averages to be explicitly taken into account.

The Effect of the 18.6-Year Lunar Nodal Cycle on Regional Sea-Level Rise Estimates

Abstract BAART, F.; VAN GELDER, P.H.A.J.M.; DE RONDE, J.; VAN KONINGSVELD, M., and WOUTERS, B., 2012. The effect of the 18.6-year lunar nodal cycle on regional sea-level rise estimates. Sea-level rise rates have become important drivers for policy makers dealing with the long-term protection of coastal populations. Scenario studies suggest that an acceleration in sea-level rise is imminent. The anticipated acceleration is hard to detect because of spatial and temporal variability, which consequently, have become important research topics. A known decadal-scale variation is the 18.6-year nodal cycle. Here, we show how failing to account for the nodal cycle resulted in an overestimation of Dutch sea-level rise. The nodal cycle is present across the globe with a varying phase and a median amplitude of 2.2 cm. Accounting for the nodal cycle increases the probability of detecting acceleration in the rate of sea-level rise. In an analysis of the Dutch coast, however, still no significant acceleration was found. The nodal cycle causes sea level to drop or to rise at an increased rate; therefore, accounting for it is crucial to accurately estimate regional sea-level rise.

Quantitative fault tree analysis for urban water infrastructure flooding

Flooding in urban areas can be caused by heavy rainfall, improper planning or component failures. Few studies have addressed quantitative contributions of different causes to urban flood probability. In this article, we apply probabilistic fault tree analysis for the first time to assess the probability of urban flooding as a result of a range of causes. We rank the causes according to their relative contributions. To quantify the occurrence of flood incidents for individual causes we use data from municipal call centres complemented with rainfall data and hydrodynamic model simulations. Results show that component failures and human errors contribute more to flood probability than sewer overloading by heavy rainfall. This applies not only to flooding in public areas but also to flooding in buildings. Fault tree analysis has proved useful in identifying relative contributions of failure mechanisms and providing quantitative data for risk management.

A framework for risk criteria for critical infrastructures: fundamentals and case studies in the Netherlands

Critical infrastructures are complex societal systems. For that reason, risk criteria for critical infrastructures are also ‘part’ of the risk criteria for complex societal systems. The questions to be resolved are: (1) are the societal risk criteria of complex systems defined and quantifiable; and (2) is it known how the risk criteria of critical infrastructures relate to the risk criteria of the complex system as a whole. In other words, what certainty is there that the risk criteria of critical infrastructures meet the risk criteria of complex systems as a whole. A complex system in this respect may be a society as a whole, e.g. a nation.

Regional Frequency Analysis of Extreme Wave Heights: Trading Space for Time

In this paper a regional frequency analysis is suggested in order to obtain more accurate estimations for the tails of wave height distribution functions. A case study with wave height data on the Norts Sea is presented.

Coastal Protection Strategies for the Red River Delta

Abstract The coastal zone in the Red River Delta of Vietnam is under large threat due to fluvial flooding, coastal flooding, and coastal erosion. As an important input for ongoing studies that focus on finding an optimal coastal protection strategy for reducing the vulnerability of the coastal region, this paper aims to describe the current situation of the coastal region and assess its present protective measures. In the region, sea dikes have been used as the predominant countermeasure with two intended functions: (i) protecting low-lying areas from coastal flooding and (ii) reducing the risk to the hinterland caused by erosion. However, the sea dikes seem not to function well and are insufficient to withstand dike breaches at the low frequency they are designed for. To have better insight into the actual situation of the whole system, this paper first investigates the historical development of the coastlines in the delta based on available information. Second, hydrodynamic and morphological processes of the coastal and estuarine systems are reviewed and further analysed. An effectiveness assessment of the present protection strategies is subsequently presented. Finally, further research needed to improve the rehabilitation of coastlines and the safety of the region is discussed based on analysis results.

Distribution functions of extreme sea waves and river discharges

Some of the important elements to be considered by the designer in the field of water defenses and hydraulic structures include the determination of the maximum environmental loads such as maximum wave height, maximum water level and maximum river discharge at locations of interest. These maxima can be estimated by various statistical methods based on observations. The main point of interest is the behaviour of each method for predicting these extrema and its deviation from the true value. An overview of statistical methods is given to determine the extreme values of river and sea related variables. The method of Regional Frequency Analysis (RFA) is proposed to predict the occurrence probabilities of these extrema. The RFA intends to extend the dataset by pooling the data of neighboring locations and establishes the regional growth curve of a so-called “homogenous region”. An important aspect is the formation of the homogeneous regions because only for those regions data can be pooled. Statistical methods will also be presented to derive homogeneous clusters. Applications include the prediction of extreme river discharges in Northwest and Central Europe and extreme waves along the Dutch North Sea coasts.

Some issues about the generalization of neural networks for time series prediction

Some issues about the generalization of ANN training are investigated through experiments with several synthetic time series and real world time series. One commonly accepted view is that when the ratio of the training sample size to the number of weights is larger than 30, the overfitting will not occur. However, it is found that even with the ratio higher than 30, overfitting still exists. In cross-validated early stopping, the ratio of cross-validation data size to training data size has no significant impact on the testing error. For stationary time series, 10% may be a practical choice. Both Bayesian regularization method and the cross-validated early stopping method are helpful when the ratio of training sample size to the number of weights is less than 20. However, the performance of early stopping is highly variable. Bayesian method outperforms the early stopping method in most cases, and in some cases even outperforms no-stop training when the training data set is large.

Integrated Safety and Security Risk Assessment Methods: A Survey of Key Characteristics and Applications

Over the last years, we have seen several security incidents that compromised system safety, of which some caused physical harm to people. Meanwhile, various risk assessment methods have been developed that integrate safety and security, and these could help to address the corresponding threats by implementing suitable risk treatment plans. However, an overarching overview of these methods, systematizing the characteristics of such methods, is missing. In this paper, we conduct a systematic literature review, and identify 7 integrated safety and security risk assessment methods. We analyze these methods based on 5 different criteria, and identify key characteristics and applications. A key outcome is the distinction between sequential and non-sequential integration of safety and security, related to the order in which safety and security risks are assessed. This study provides a basis for developing more effective integrated safety and security risk assessment methods in the future.