Due to accelerating climate change, which entails more extreme storm conditions and exponential sea level rise, the protective function of our dikes and levees is under growing pressure. The presence of so-called forelands (e.g., beach, mudflat, and tidal marsh) are promising Nature-based Solutions to reduce the probability of dike failure and to mitigate the consequences after a dike breach. However, the positive effects of forelands depend, for instance, on sediment composition, geometry, and the presence of vegetation. These effects have not been studied in detail and have not been incorporated into reliable physical and validated mathematical models yet.

This report describes an experimental study on the stability of different foreland types. Particularly, the following aspects were considered: investigating whether surface and headcut erosion are indeed the dominant foreland erosion mechanisms, plus providing experimental data for validation purposes for models such as the extended BRES model.

Twelve experiments were performed in the sediment flume facility of the Hydraulic Laboratory of the faculty of Civil Engineering and Geosciences at the Delft University of Technology. A foreland during breaching conditions (i.e. high flow velocities) was simulated by an experimental setup, a so-called broad-crested weir. This weir was developed, constructed, and built within the facility to simulate and study erosion processes of two foreland sediment types, sand and clay. Geotechnical properties were obtained by performing sediment analyses in the Geoscience and Engineering Laboratory. In order to try to exclude the influence of cracks, a special adjustment of the facility was made by covering the sediment package with plywood panels; these experiments are referred to as ‘Tompouce’. For collecting data, the facility was fully instrumented with hydraulic sensors (discharge, flow velocities, and water levels) and synchronized cameras. The erosion process was analyzed and quantified from the visual data.

The experiments enabled us to answer the main research question, “Which erosion mechanisms are important to consider for forelands under dike breach conditions?”. For non-cohesive sediments, surface erosion should be considered the dominant erosion mechanism. In the case of cohesive sediments, however, headcut block erosion should be considered the dominant erosion mechanism. Here, when cracks are present, the erosion rate increases considerably. Especially the latter effect is a new finding; this mechanism should be included in foreland-dike breach development modeling. More research is needed into the quantitative effect of cracks on the stability of a vegetated foreland when it is subject to (sudden) high-water events.

Animal-induced anomalies on a levee surface negatively affect the erosion-resistance of a levee, leading to rapid failure of the inner slope in case of overtopping and/or overflow during high water. Where active prevention techniques of burrows from large animals, such as muskrats, are applied in and around levees in the Netherlands, the identification of smaller burrow holes made by voles and moles and the evaluation of the severity of the damage rely solely on the assessment and experience of the levee inspector.
The Polder2C’s project aims to quantitively assess the impact of animal burrows on the levee surface by conducting experiments at the Living Lab Hedwige and Prosper Polder (LHPP) and this thesis aims to contribute in this by analyzing the experiment results and developing a model for levee inspection that includes uncertainties of the geometrical characteristics of vole and mole burrow systems.
This thesis distinguishes two phases, the first of which focuses on the question: ‘How do animal burrows influence levee performance?’, and it aims to form a foundation for the second phase of the report by focusing on the behavior of burrowing animals and the geometrical characteristics of their burrows by conducting a literature review on existing research on this topic. Guidelines were set up that contribute to the identification of mole and vole burrow system by investigating the main differences between them, with the aim to easily identify separate burrow hole groups in the experiment data. These conclusions could then be used to identify different vole and mole burrow hole groups in data that was collected from several experiments at the Living Lab Hedwige and Prosper Polder (LLHPP) between October 2021 and February 2022 and answer the research question for the second phase, which was: ‘Based on data from levee inspection experiments, which probabilistic model can assess the inspection success rate of animal burrows?’.

Under the current climate change projections, droughts and extreme rainfall events are becoming more common and intense. During sustained droughts, soil cracks can develop in the topsoil of a dike. These soil cracks can enhance the infiltration and drainage of the dike, affecting the dike stability. This study’s main objective is to better understand the potential influence of cracks on the hydrology and dike stability of canal dikes. A finite element model (FEM) was built to study the hydrological response and stability of a canal dike due to rainfall infiltration through a uniformly distributed cracked top layer. This model included the fast hydrological response of the fractures and their exchange with the soil. A surrogate model (a simplified data-driven machine learning model) was built using training data generated by the FEM to reduce the computational burden and allow for a sensitivity and stochastic reliability assessment. An evaluation of important model features, such as the connectivity among cracks and soil swelling, revealed that the cracks have a very localized influence on the infiltration and drainage of water through the dike. Water drainage in the toe of the dike is restricted by soil swelling resulting in a larger decline in dike stability. The sensitivity analysis found that the cracks facilitate more infiltration than drainage before the crack parameters, such as the crack aperture and the amount of cracking, reached a threshold. Above this threshold, drainage provided by the cracks is greater than the infiltration. The failure probability was calculated for a variety of different rainfall patterns. A pattern where the rainfall intensity incrementally increases over time results in the highest failure probability. The main conclusion from this study is that soil cracks influence the stability of the dike by enhancing infiltration and drainage under rainfall. Under low-intensity rainfall, the cracks in the toe facilitate more water to drain out of the dike than rainfall to infiltrate, stabilizing the dike. However, in the absence of cracks in the toe, the cracks provide more infiltration than drainage resulting in a lower dike stability. Under high-intensity rainfall, cracks in the crest and the inner slope of a dike advance the wetting front resulting in a larger decline in dike stability than a dike without soil cracks. Rainfall infiltration and drainage in a cracked dike depend on both rainfall characteristics, such as the average rainfall intensity, duration, and pattern, and the crack features, such as the crack width, amount of cracking, and the connectivity of the cracks. Since this study was not validated with observations, it is recommended that further studies aim to validate the observed influence of the cracks by conducting field experiments.

With rapid urbanization, Low Impact Development (LID) is promoted as an alternative to Conventional Drainage (CD), seeking a natural solution for current urban water problems. The positive effects of LID were the main theme of recent LID researches, but this project aims to deeply explore the hidden troubles about the extreme peak runoffs influenced by LID city on catchment scale.
In this research, the SUPERFLEX conceptual model was adapted to a rural-urban semi-distributed model to simulate the current rainfall-runoff relationship of the catchment where San Antonio city is located in. Besides, the model expressions of 4 representative LID practices (bioretention cells, vegetated swales, green roofs, and permeable pavements) were devised under SUPERFLEX framework. To deal with the prediction uncertainty, three urban development scenarios in 2040 and five LID implementation scenarios were designed for San Antonio city. And their influences on the basin peak runoffs will be quantitively studied.
Research result shows that, the urban runoff tended to swing between extreme flood and extreme drought in the reference situation; And next, the infill urban development strategy, which means developing the vacant or undeveloped land within an existing community, was more helpful on peak runoff and total runoff volume control than sprawl urban development strategy with the same population growth; Thirdly, the bioretention cells, vegetated swales, and permeable pavements had similar good performance on peak runoff reduction, which can be mainly ascribe to the stormwater infiltration process. As for the retention of total runoff volume, the bioretention cells, permeable pavements, and green roofs perform better than vegetated swales since the rapid water transportation character of vegetated swales decrease the water residence time for infiltration;
The runoff reduction function of LID practices performs effective on the large peaks in dry and normal seasons, but it will be restrained significantly in flood season. According to model result, the rural peak runoffs happened 6.5 to 15.5 hours after the urban peaks. And for 4 LID implementation scenarios in which 15% of urban grey areas is covered by LID practices, the urban peaks are delayed between 0.5 and 2.5 hours. And for the scenario with the LID cover areas as 50% of the urban grey areas, the time lag of urban peaks varies from 0.5 to 6.5 hours. For this scenario, since the obvious time delay of urban peaks, more stack of urban and rural peaks is caused by the time approaching of urban and rural peaks, which causes the increases of two total basin peaks in flood season from 3.57 to 3.65 mm/d and from 6.35 to 6.47 mm/d respectively.
In conclusion we may say that the stacking effect of LID implementation on total basin runoff is limited in the case of San Antonio basin, partly due to the fact that only a small part of this basin is urbanized.

Rapid urbanisation and globalisation are bringing increasingly complex issues to the forefront. Improper planning of human activities and over exploitation of the surrounding natural resources has successfully damaged the biodiversity and the natural processes. Today humanity is at a stage where these ecosystem services are essential for our existence but the resources have been exploited beyond their capacity. In addition, climate change adds additional long-term threats due to erratic weather patterns and extreme natural events. Coastal Lagoons are one such geographical feature where such complexities are very visible. Given the high fertility of the surrounding land and the biodiversity hosted by the lagoons, they are rich resource banks for settlements to thrive on. This has led to issues like water pollution, loss of biodiversity and urban encroachment. Despite protection from the international communities like the Ramsar Convention, most wetlands are degrading everyday. The need of the hour is to find innovative middle ground solutions, where the services can be availed without degrading the environment. Further, to plan these services in a way that they are instrumental in reviving and enriching the lost ecosystem. This project attempts to present on such design and strategy for the Muni-Pomadze Lagoon (MPL) in Ghana. Considering the complexity of the issues, the project chose a interdisciplinary and collaborative approach to produce a holistic solution for the site. Further, it uses the principles of Nature-Based Design and 4-P framework (People, Planet, Prosperity and Project) to guide and reflect on the design. (van Dorst & Duijvestein 2004) This report attempts to contribute to the research on interdisciplinary design processes. Further, it aims to be a starting point and guideline for the Forestry Commission and Municipal body of Winneba, for better conservation of the Muni Lagoon.