<p>The frequent urban floods in Jakarta and Bandung, Indonesia affect the lives and livelihoods of millions of people. Floods cause damage and casualties, while climate change, unchecked development and land subsidence are worsening the problem. One factor contributing to these floods is floating debris clogging the city's drainage structures. A major proportion of floating debris consists of macro plastics which are extremely persistent in the environment. Trash racks that are clogged due to continuous accumulation of plastics in front of them can block the water flow in the river, leading to an increase in upstream water level and causing floods. </p><p>The understanding of transport and accumulation of the macro plastics in the river systems is limited as the field surveys are difficult to perform and the variety of properties of plastic debris is enormous. However, understanding of the origin, fate and pathway of plastic waste is required in order to come up with an optimal solution for plastic collection and prevention of harmful accumulation in front of the hydraulic structures. With this urge in mind field observations will be conducted on the selected river sections in Bandung and Jakarta during the monsoon season in 2020. Field observations will include the measurements of bathymetry, velocity profiles, concentrations and the characterization of floating debris, as well as identifying the accumulation hot spots of floating debris. Furthermore, experimental and numerical modelling will be performed based on the data collected during the field campaign in order to couple different debris classes to a range of riverine situations and understand the differences in their driving mechanisms.</p><p>Using a combination of field measurements, experimental modelling and empirical relations we aim to investigate the driving mechanisms of riverine plastic transport and changes in hydraulic properties due to local disturbances of the current. We will therefore link the type of hydraulic structures and the extend of obstructions due to accumulation of plastic debris to the changes in the upstream water level. This will lead to a better understanding of plastic transport in the river systems in Bandung and Jakarta, to formulate design criteria for structures in trash-laden streams and devise ways to pass trash during floods.</p>

Implementing adequate defences for low-lying coastal area against coastal flooding requires thorough knowledge of all potential influences leading to increased sea levels, including low-frequency sea level oscillations. We present and describe several methods applicable for the analysis of low-frequency sea level oscillations in the Mediterranean Sea: wavelet analysis, spectral analysis, moving-periodogram analysis, and rotary spectral analysis. These methods were applied for characterisation of subinertial sea level oscillations with periods greater of the period of inertial oscillation (18 hours in the Northern Adriatic Sea) on measured sea surface elevations and current velocities in the Mediterranean Sea. Preliminary analysis was performed on observations of a storm event in the Adriatic Sea at the end of January and the beginning of February 2014, revealing a peak in the frequency spectrum in the frequency band between 0.3−0.4 day−1. Further analysis was done on long-term tide gauge measurements available for 62 stations in the Mediterranean basin. The application of the selected methods provided a preliminary set of seasonal occurrences and durations of subinertial oscillation. This sets the ground for further investigation into the propagation of low-frequency sea level oscillations throughout the Mediterranean basin and for characterisation of the mechanisms triggering the process, including with regard to climate change.

<p>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.</p><p>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).</p><p>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 preferred boundary location to initialize the SFINCS model is at a water depth between 0.5 m and 2.5 m, preferably shoreward of the reef edge. Errors introduced by the generated parameterized spectra lead to runup estimation errors of up to around 40% depending on reef geometry. The developed methodology will be applied to a case study of Majuro Island, the Republic of Marshall Islands, as proof of concept.</p><p> </p><p><strong>References</strong></p><p>Athif, A. A. (2020). Computationally efficient modelling of wave driven flooding in Atoll Islands: Investigation on the use of a reduced-physics model solver SFINCS. Master’s thesis, IHE, the Netherlands.</p><p>Leijnse, T., van Ormondt, M., Nederhoff, K., and van Dongeren, A. (2021). Modeling compound flooding in coastal systems using a computationally efficient reduced-physics solver: Including fluvial, pluvial, tidal, wind-and wave- driven processes. <em>Coastal Engineering</em>, 163:103796.</p><p>Roelvink, D., Reniers, A., Van Dongeren, A. P., De Vries, J. V. T., McCall, R., and Lescinski, J. (2009). Modelling storm impacts on beaches, dunes and barrier islands. <em>Coastal engineering</em>, 56(11-12), 1133-1152.</p><p>Scott, F., Antolinez, J. A. A., Mccall, R., Storlazzi, C., Reniers, A., and Pearson, S. (2020). Hydro-Morphological Characterization of Coral Reefs for Wave Runup Prediction. <em>Frontiers in Marine Science</em>, 7(May):1–20.</p><p>Storlazzi, C. D., Elias, E. P., and Berkowitz, P. (2015). Many atolls may be uninhabitable within decades due to climate change. <em>Scientific reports</em>, 5:14546.</p>

<p>The state of coral reef ecosystems is highly dependent on the availability and ratio of essential resources such as oxygen, minerals and nutrients, and the presence of pollutants, pathogens and other possible stressors. The distribution of these inputs is dynamic and depends on many factors, including the nearshore hydrodynamic processes. These are unique processes, consisting of tidal pumping, nearshore circulation, and wave action. Furthermore, these processes are highly influenced by complex reef bathymetry and the physical roughness of the reef. The latter has a crucial role in the boundary layer characteristics, which influences uptake by reef organisms at smaller spatial scales.</p><p>The understanding of distribution and transport of particulate and dissolved substances is challenging as field surveys are difficult to perform and there is a large variety of coral shapes. However, assessing the hydrodynamic processes is a necessary first step in order to link the sources and sinks of substances with the coral health and growth. Within the interdisciplinary research program SEALINK, we aim to assess the distribution and pathways of substances around the island of Curaçao. Field observations on selected sites along the coast of Curaçao include current and wave measurements with Acoustic Doppler Current Profilers and flow visualization with fluorescent dye.</p><p>We will present preliminary results from the field campaign showing velocity fields and wave transformation on different stations along the cross-shore transects on the reef platform. Using a combination of field observations and 3D non-hydrostatic Computational Fluid Dynamics models, we investigate the mixing mechanisms and local energy balance at scales of O(10 m) on the selected reef quadrants. This serves as a basis for a further analysis with Lagrangian Particle Tracking methods to track the selected substances identified with other field campaigns within the SEALINK program.</p>