Compound floods, which can be attributed to different drivers (pluvial, fluvial, surge, tide, and waves), generate a larger flood hazard when drivers co-occur than when they occur in isolation of each other. Current compound flood risk assessments are affected by a curse of dimensionality, where a larger number of events need to be numerically simulated to understand the response of risk to drivers. This research aims to create a methodology that improves the quantification of compound flood risk by using a Treed Gaussian Process (TGP) for the case study of Charleston. A TGP can actively learn from the response of damages to drivers to reduce the number of events that need to be simulated. By comparing this approach with a state-of-the-art approach, the research shows a reduction of the computational cost by a factor of 4, an improvement in the root mean square error by a factor of 8, and an improvement in the estimate of Expected Annual Damages (EAD) by a factor of 20. This reduction in computational cost allows for the inclusion of random variables that are normally assumed constant such as the duration and time lag of drivers. A sensitivity analysis demonstrates these variables produce a statistically significant difference in the estimate of EAD, which increases its value from 172 to 219 Million USD. The research also shows the combination of events caused by drivers leading to extreme damage changes when including these additional random variables, although surge is always found to be dominant. By applying the TGP to multiple outputs, the research demonstrates the TGP is not only applicable to the case study, which shows a TGP can be implemented in current flood risk assessments. ... Read more

Nature-based initiatives have emerged as potential solutions to problems caused by saltwater intrusion in deltas found globally, but their successful implementation is enabled partly by a multi-stakeholder approach. The latter involves managing several parallel stakeholder objectives, which usually requires quantitative knowledge to understand possible collisions of interests. On that account, the present work developed a systematic approach (referred to as the `comparison tool') to quantitatively compare the objectives of multiple stakeholders interplaying in a delta. As a first approach, the comparison tool is intended to support decision-making to deal with potential conflicts between freshwater supply and port logistics interests. In particular, the present juncture in the Rhine-Meuse Delta in the Netherlands was used to investigate potential trade-offs generated by the nature-based shallowing (or river bed heightening) of the Rotterdam Waterways. The comparison tool is founded on the objective-based assessment of Building with Nature (BwN) solutions. The effects of a BwN solution are assessed separately for each functional requirement and then are related in a combined assessment. The impact assessment for the shallowing of the Rotterdam Waterways required two numerical modelling studies. First, effects on the hydrodynamics and salt transport in a partially-mixed estuary were modelled with the Operationeel Stromingsmodel Rotterdam (OSR), developed by the Port of Rotterdam Authority. Secondly, changes in meso-scale traffic flows over the port network were modelled with the OpenTNSim developed by TU Delft. Afterwards, the effects on freshwater supply and port performance (capacity and efficiency) were quantified separately and then compared. This research delivered a systematic procedure and demonstrated how a combined assessment could be performed in the context of nature-based solutions to mitigate salt intrusion. The most important outcome entails quantitative trade-offs between port efficiency and freshwater supply over a range of bed level increase from 0.0 m to 3.9 m. In general, results showed that the improvement towards the objective of port logistics always goes to the detriment of the freshwater supply objective while increasing bed level. Also, this study found that a collision of interests between the two types of end-users might worsen for a bed level increase over 2 m. Additional results showed that shallowing could be associated with benefits for freshwater supply through a decrease in the duration of water shortages due to a retreat in salt intrusion. The latter holds for specific environmental conditions of low river discharge and mild wind. Also, this study concluded that shallowing could negatively affect port efficiency due to heavier vessel traffic and more burdensome tidal window restrictions, which can result in an exponential growth of vessels average waiting times if the bed level increases over 2 m. Since the freshwater and port sub-systems were simplified, uncertainty in the results was unavoidably accepted. Despite these simplifications, this research demonstrated the main principles in implementing the comparison tool and lays the groundwork for more comprehensive models. In addition, several recommendations for policy-making are proposed, setting a basis for later discussions between freshwater supply and port-related stakeholders in the Rhine-Meuse Delta. ... Read more

Nature-based Solutions (NbS) have recently gained more interest in hydraulic engineering. It is based on the concept of using forces of nature rather than working against them. In addition, it focuses on using natural processes to fulfil co-benefits for the parties involved.
One of the parties involved is nature. However, a quantitative analysis of ecological development is necessary to determine possible co-benefits for nature. This is still found challenging due to the dependency on many variables, the difference in spatial and temporal scales, the limitations in available information, and the non-linearity.

Ecological development can be expressed with ecotopes, linking geomorphological and hydrological characteristics to abiotic characteristics. A Dutch Ecotope System for Coastal Waters (ZES.1) is a classification system of Rijkswaterstaat. It is a hierarchical system based on abiotic characteristics that classify ecotopes based on thresholds that are determined by ecological differences.

In this thesis, the Ecotope Map Maker based on Abiotic characteristics (EMMA), is developed. It uses data from a validated hydrodynamic model as input and subsequently maps ecotopes based on the ZES.1. Ecotope labels are composed by combining labels that are given to values of salinity, inundation, flow velocity and substrate composition.

The thresholds are calibrated using an ecotope-map of the Western Scheldt of RWS. This map is based on aerial photographs, laser altimetry, soundings, field measurements, and several models. The performance increased from 63 % to 84.6 % after the calibration. This increase is mainly due to (1) differences in the underlying data and (2) the application of deviating thresholds in the ecotope-map of RWS compared to the ZES.1.

EMMA is developed for the preliminary design stage. How EMMA can be implemented is demonstrated by applying EMMA on an idealised estuary. Different ecotopes and varying acreages of ecotopes are found when the depth of the estuary is modified.

In conclusion, EMMA creates many possibilities for ecotope-maps since it no longer depends on aerial photographs and other real-time data. A translation can be conducted between ecotopes and ecosystem services when a monetary value is preferred, or ecotopes can be broken down into eco-elements, which can subsequently be linked to biodiversity. With EMMA it is possible to predict ecological development, which contributes to the design of NbS. ... Read more