Small Island Developing States (SIDS) are increasingly under threat of coastal flooding, which challenges the safety of their societies and vulnerable economies. The emergency of this issue, exacerbated by climate change, has alarmed international organisations and national governments that have been demanding for robust risk assessments to guide the development of resilient adaptation strategies. In SIDS, the paucity of local data, required to perform such kind of coastal risk analyses, hinders the application of highly detailed models that therefore need to rely on inaccurate and publicly available data, thus introducing uncertainty in the assessment. This thesis aims to investigate the uncertainty in input data and its impact on coastal flood damage estimates. This study examines prominent uncertainty sources in the coastal flood risk modeling chain, namely: the stochastic variability of (i) significant wave height and (ii) storm surge water level, the quality of (iii) bathymetry data and (iv) digital elevation models and (v) the choice of depth-damage function. To account for risk temporal changes, two other inputs are included, specifically (vi) different sea level rise projections and (vii) socioeconomic developments. A methodology is developed to test the afore-mentioned inputs through global sensitivity analysis, using an ensemble of hydrodynamic models (XBeach and SFINCS) coupled with an impact model (Delft-FIAT). The impacts of these sources on the flood damage estimates are evaluated in a case study on the islands of São Tomé and Príncipe. Model results indicate, for the current time horizon, depth-damage functions and digital elevation models as the inputs with the most significant contribution to the overall damage estimation uncertainty, yielding a variation in the output prediction of a factor 16 and 10, respectively. As future climate and socioeconomic development uncertainties are introduced in the system, sea level rise projection becomes, followed by digital elevation models and depth-damage functions, the most relevant input for the year 2100. Neglecting economic growth in the risk analysis leads to an extremely high underestimation of damages. However, given the constrained intrinsic uncertainty for the projected societal trends, its sensitivity on the risk output is limited. The scarcity of accurate input data proves to have an enormous impact on risk assessments in Small Island Developing States, leading to considerable prediction error and affecting the model outcome uncertainty. New emerging data collection techniques, such as unmanned aerial vehicles, could augment the trustworthiness of risk assessments by providing more accurate datasets for bathymetry and topography. Furthermore, research efforts could be directed towards developing knowledge on the physics of damages and their implementation in a risk modeling scheme. The uncertainty framework presented could be applied in projects with the aim to support risk communication to stakeholders by portraying the implications of the various inputs used and assumptions made, but also to guide the allocation of limited economic resources towards the acquisition of the input data that matters the most in terms of reliability of damage estimates. ... Read more

Relative sea level rise and climate change represent long-term threats for the ecosystems and flood defence system in the Wadden Sea. Moreover, the flood-dominant tidal basins in the area lead to sedimentation in the navigational channels used by the ferry services that connect the mainland to the Dutch Wadden islands. This report provides flood safety and coastal zone management solutions for the village of Holwerd, in the province of Friesland, Netherlands. High morphological dynamics and biodiversity characterizes this environment. Within this context, the Holwerd municipality, Wetterskip and Rijkswaterstaat were identified as key players to this study from a stakeholder perspective.

Alternative preliminary designs were presented to increase the resilience of the local sea dike and to recommend solutions for improving the ferry connection between Holwerd and the island of Ameland due to current dredging operations considered as excessive and which caused ferry delays. Careful consideration was given to designs due to the Wadden Sea being a UNESCO World Heritage Site and therefore protected under environmental legislation. Therefore, minimal negative environmental and ecological impacts were strived for in design of both the alternative dike designs and ferry connection alternatives. The impact that these alternatives may have on the socio-economy of Holwerd was also assessed where applicable.

Some alternatives were initially eliminated due to fatal flaws and the remaining options were evaluated by means of a multi-criteria analysis. The main criteria considered were the environmental impact, practical implementation, socio-economic impact, safety risk and reliability. Costs were evaluated through a subsequent cost-value assessment. The Wide Green Dike concept was the selected preferred solution as a reinforcement design to withstand extreme wave heights and water levels with a 1000-year return period, including sea level rise projections for the next century. This was achieved by performing an extreme water level analysis and wave transformation using the software Delft3D Wave. The Wide Green Dike concept would therefore ensure that the Holwerd sea dike still sufficiently meets the Dutch flood safety standards in the future. In relation to the navigability of the ferry, channel bend cuts are considered the feasible short-term solution to reduce the sailing time by 10 minutes. The long-term solution, adjusting vessel fleet sizes, potentially decreases the current maintenance dredging volume by 20%. The beneficial use of the dredged material for dike reinforcement and salt marsh development are suggested as integrated Building with Nature solutions to provide additional ecosystem services. Environmental Impact Assessments were conducted to identify potential risks related to each of the preferred alternatives and provide mitigation measures to ensure appropriate project implementation. ... Read more