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F.E. Roelvink

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Coral Restoration for Coastal Hazard Risk Reduction

The effect of coral restoration on wave transformation over various reef morphologies and the resulting runup

Master thesis - F.E. Roelvink, M.F.S. Tissier, S.G. Pearson, Curt D. Storlazzi, Ap Van Dongeren, A.J.H.M. Reniers

Coral reefs are degrading at an alarming rate, affecting not only the precarious coral ecosystem but also human habitat. The combination of coral degradation, sea level rise and its exacerbated effect in the tropics, and the possible storm intensification increases the flood vuln ...

Colciencias

Student report - V. van der Bilt, T. Olthoff, T.E. Horst, F.E. Roelvink, R.R. Komen, A.H. Tempel, None None

Colombia’s economy has been growing steady in the past decades, partly because of the increase in tourism. However, this tourism also has a downside. Hotels and restaurants are constructed one after another, preferably along the coast. This, combined with factors such as climate change and land subsidence and the fact that people tend to prefer living close to the coast, leads to a vulnerable and erosive coast, especially in the departments of La Guajira and Magdalena. Therefore, Project Colciencias was initiated by three local universities and two companies, a large scale study to improve and implement coastal projects in this area. Focus is on areas that experience critical erosion; erosion that puts local economy, environment and national heritage at risk (Oceanus, 2013). Ten objectives are formulated by the five different parties, e.g. characterization of the coast, development of pilot projects and socialisation of coastal projects. The goal of this multidisciplinary research conducted by TU Delft students in cooperation with Oceanus International is to improve the Colciencias project by combining Dutch and Colombian expertise, technology, efficiency and theory. After analysing the political, cultural, social, hydraulic and environmental situation, and after consulting various stakeholders three research objectives were stated and elaborated. The first goal is to create a framework to ensure a consistent and efficient implementation of the intended pilot projects. Not only does it cover the hydraulic aspects of a project, it also takes into account the socio-economic, environmental and legal factors. To support the part of the framework that comprehends the hydraulic analysis and the development of alternatives for coastal protection, a detailed document on numerical models is written. This is objective number two. Next to information on numerical models in general, the three main types of coastal models are discussed; coastline, regional - and local morphodynamic models. Eight specific models are then further highlighted, resulting in a decision tree that helps in choosing the best model for your specific engineering application. To test the framework, a case study was executed in one of the critical erosion zones, the village of Ciénaga. The framework was followed concisely to evaluate its effectiveness and efficiency. Among other things, a field trip was organised to carry out experiments and execute a survey to explore the socio-economic values of the region. Next to that, interviews were carried out with officials and local parties to obtain information on past projects and the local legal framework. From the case study it was concluded that the framework functions, but that it should be noted that the case study is only a test of the framework, and that much further research needs to be conducted for the actual design and implementation of the coastal plan for Ciénaga. Lastly, our goal was to start a dialogue about the impact of integrated coastal zone management and coastal erosion problems. In practice this resulted in many interviews, a survey, attendance of local community meetings, presentations at the Universidad del Norte for stakeholders of the project and an informal gathering of students at the Universidad de Cartagena. Furthermore, the framework shall be distributed to possible stakeholders as well as an informative presentation along with an introductory movie.

Contributed

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Efficient and accurate modeling of wave-driven flooding on coral reef-lined coasts

On the interpolation of parameterized boundary conditions

Master thesis - V. Bertoncelj, M.F.S. Tissier, Ap Van Dongeren, T.W.B. Leijnse, S.G. Pearson, J.D. Bricker, F.E. Roelvink

Many coral reef islands are low-lying, which in combination with population growth, sea level rise and possibly more frequent extreme weather events is likely to result in increased coastal risk (e.g. Storlazzi et al., 2015). On smaller scales of O(10 km) wave-driven coastal inundation can be accurately predicted with advanced models such as XBeach (Roelvink et al., 2009), at already high computational costs. For larger scales, larger number of islands, for scenario modelling, and for implementation in early warning systems, computationally faster methods are needed. Reduced physics models, which neglect some of the processes (e.g. non-hydrostatic pressure gradient term and viscosity), are a potential solution. However, their accuracy and the best method to force them has not been established. In this research we propose a new methodology to model wave-driven flooding on coral reef-lined coasts. A look-up-table (LUT), composed of XBeach model runs, is combined with a reduced-physics model, SFINCS (Leijnse et al., 2021), to achieve high accuracy predictions at limited computational expense. The LUT consists of pre-run 1D XBeach simulations for several reef profiles from Scott et al. (2020), forced with different offshore wave and water level conditions. Wave conditions close to the shore as predicted by the LUT are used to force SFINCS which then simulates the wave runup, overtopping and flooding. These are forced in SFINCS using random wave timeseries from an interpolated parameterized wave spectrum following Athif (2020). The accuracy of the method is investigated for 6 distinctive cross-shore profiles from Scott et al. (2020), for two wave scenarios (gentle swell and stormy conditions). Results of complete XBeach simulations are compared to LUT-SFINCS simulations with different boundary forcing locations. The sensitivity analysis shows that the most optimal boundary location to initialize the SFINCS model is at a water depth of 0.5 m, shore-ward of the reef edge. Interpolation of the forcing conditions at the boundary is investigated with 9 different interpolation methods. Results reveal that the most accurate method to interpolate spectral parameters (the amount of high frequency wave energy, the amount of low frequency wave energy and the frequency peak of high frequency part of the spectrum) and wave setup at the boundary is the Inverse Distance Weighting method with a power of -2. Errors introduced by the interpolated parameterized boundary conditions lead to runup estimation errors of 10-20% on average with the maximal errors up to 60%. Simulating wave runup with XBeach LUT – SFINCS couple leads to about 50-times higher computational speed compared to the XBeach model simulating hydrodynamic processes along the entire reef profile.