Quantifying coastline change uncertainty using a multi-model aggregation approach

Quantifying coastline change uncertainty using a multi-model aggregation approach

Dagalaki, Vasiliki (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)

Aarninkhof, Stefan (mentor)
de Boer, Wiebe (graduation committee)
Scheel, F (graduation committee)
Kroon, Anna (graduation committee)

Delft University of Technology

Civil Engineering

2018-12-12

Evolution of coastline position under the influence of natural and anthropogenic processes is directly linked to the development of seaside societies. In the context of coastal zone management, process-based morphodynamic models are often used topredict coastline evolution and support the decision-making process for adaptation/mitigation strategies. Frequently, the processes driving themorphodynamic evolution transcend the applicability limits of a single model. In those cases, model ensembles can be used to estimate coastline change under the joint effect of the relevant processes. However, model output and in extent the aggregated result are characterised by uncertainty originating among others from forcing variability and parameter imprecision. The increasing exposure ofc oastal societies to coastal recession risks and emergence of risk-informed coastal zone management create the need for aggregated coastal recessionestimates with quantified uncertainty. This study investigates different statistical methods for forcing and parameter uncertainty quantification around coastline change estimates from process-based morphodynamic models. Subsequently a numerical convolution approach for the aggregation of the probabilistic coastal recession estimates from multiple models was formulated to account for the combined uncertain effect of processes acting on different timescales. The methods of this study were applied on Anmok beach, South Korea, a coastal stretch experiencing erosion caused by long, intermediate and short timescale processes. Available UNIBEST-CL+ and Delft3D model schematizations from theCoMIDAS research program, capable of simulating the relevant processes, were utilised. Following a literature review, two methods were considered applicable for process-based morphodynamic models: Standard Monte Carlo (SMC) and Latin Hypercube Sampling (LHS). The application of both methods on the UNIBEST-CL+model schematisation enabled the evaluation of their relative performance based on the precision of the different coastline change estimates achieved for the different sample sizes. Only LHS was applied on the Delft3D model schematisation due to computational demands limitations. Subsequently,the scenario-based approach currently used for the aggregation of multi-modelcoastline change outputs was extended to explicitly account for the uncertainties quantified in the individual model outputs. A numerical convolution approach, using Monte Carlo sampling, was suggested for linear super position of the contributing probability distribution functions. The advantages of this approach include speed, ease of implementation,comprehensibility and high resolution even at the tails of the aggregated distributions. Utilising this approach, the effect of alternative interventions (combinations of various breakwater designs with a small-scale nourishment) on the coastline change probabilities was quantified. The results showed that both methods (i.e., SMC and LHS) with adequate sampling can produce probability distribution outputs for coastline change when applied to the process-based models. SMC remains the most suitable method for coastline change uncertainty quantification for models with small simulation durations. The method gives quantified estimates of the precision, enabling the achievement specific target precisions, with the respective computational cost. For the smaller sample sizes used, LHS gave better precision results, proving more suitable for models with longer computational time. On the downside, without extra iterations of the procedure only upper estimates of the achieved precision for a specific sample size can be obtained. The probabilistic aggregation framework presented in this thesis has several advantages compared to the scenario-based approach currently used. It allowsfor quantified coastline change uncertainty estimates with the respective precision estimates and provides the distribution of the uncertainty across its range, information that could not be derived using the scenario-based approach. Different coastline change percentile estimates or confidence intervals with practical use to decision makers and the likelihood of any coastline change realisation of interest can be evaluated. The probabilistic uncertainty quantification and aggregation framework is believed to be useful for intervention assessment and comparison. It allows for the assessment of uncertainty around the morphological response under the combined effect of the processes acting on the coast with/without the intervention and thus the evaluation of the probabilistic intervention impact. Different interventions can be compared in terms of the probability of inducing desired/undesired morphodynamic realisations as well as in terms of the uncertainty range in the coastline change estimates.

uncertainty
uncertainty propagation
coastline position
Monte Carlo
Latin Hypercube
convolution
aggregation
precision
uncertainty quantification
coastline change

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Student theses

master thesis

© 2018 Vasiliki Dagalaki