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The present project is part of a comprehensive effort by the PI, his students, and collaborators at the Naval Research Laboratory to increase the robustness and viability of the Delft3D model suite as an operational forecasting tool, and aid its continued transition to Navy forecasting centers. Prior projects have focused on determining the model s response to characteristics and sample sizes of bathymetric information. The present project focuses on determining the effect of boundary errors on model response, and the development of methods to ameliorate these issues. These boundary errors arise when the numerical grid for the forcing condition (wave model) is is insufficiently extended in the lateral (longshore) direction relative to the numerical grid for the current (flow model). The primary objective of this work is to investigate the effect of boundary forcing errors on the model response. This has a direct impact on the use of Neumann lateral boundary conditions, since these errors arise from insufficient extension of the WAVE (SWAN) grid beyond the lateral boundaries of the FLOW grid. There is a balance between accuracy and computational efficiency that must be struck; the computational bottleneck in the Delft3D system is SWAN, the wave model, and the run time increases with lateral extent of the WAVE grid. While point-by-point comparisons between model and data for various WAVE grid extensions can offer a view of the dependence of accuracy on grid extension, it does not yield any information on the spatial characteristics of the solution; poor datamodel comparison could be the result of slight spatial mismatches of highly variable solution fields, or oversmoothed solutions which have little relevance to the physics at hand., The original document contains color images.

The present project is part of a comprehensive effort by the PI, his students, and collaborators at the Naval Research Laboratory to increase the robustness and viability of the Delft3D model suite as an operational forecasting tool, and aid its continued transition to Navy forecasting centers. Prior projects have focused on determining the model s response to characteristics and sample sizes of bathymetric information. The present project focuses on determining the effect of boundary errors on model response, and the development of methods to ameliorate these issues. These boundary errors arise when the numerical grid for the forcing condition (wave model) is is insufficiently extended in the lateral (longshore) direction relative to the numerical grid for the current (flow model). The primary objective of this work is to investigate the effect of boundary forcing errors on the model response. This has a direct impact on the use of Neumann lateral boundary conditions, since these errors arise from insufficient extension of the WAVE (SWAN) grid beyond the lateral boundaries of the FLOW grid. There is a balance between accuracy and computational efficiency that must be struck; the computational bottleneck in the Delft3D system is SWAN, the wave model, and the run time increases with lateral extent of the WAVE grid. While point-by-point comparisons between model and data for various WAVE grid extensions can offer a view of the dependence of accuracy on grid extension, it does not yield any information on the spatial characteristics of the solution; poor datamodel comparison could be the result of slight spatial mismatches of highly variable solution fields, or oversmoothed solutions which have little relevance to the physics at hand.

The present project is part of a comprehensive effort by the PI, his students, and collaborators at the Naval Research Laboratory to increase the robustness and viability of the Delft3D model suite as an operational forecasting tool, and aid its continued transition to Navy forecasting centers. Prior projects have focused on determining the model's response to characteristics and sample sizes of bathymetric information. The present project focuses on determining the effect of boundary errors on model response, and the development of methods to ameliorate these issues. The objectives are: 1) Investigate the effect of boundary forcing errors on the model response. 2) Implement a 1D wave driver on the lateral boundaries. 3) Develop methods for correcting the boundary errors to optimize model accuracy.

De veiligheidsbeoordeling en het ontwerp van de primaire waterkeringen van Nederland zijn gebaseerd op probabilistische instrumenten. Bij de toepassing van deze instrumenten wordt rekening gehouden met zowel de statistische onzekerheden als de modelonzekerheden, die de bepaling van de hydraulische belastingen beïnvloeden. De hydraulische belastingen op de waterkeringen worden aangedreven door de wind en/of rivierafvoeren. Echter, de onzekerheden in de windstatistieken als gevolg van effecten van klimaatverandering worden nog niet meegenomen in de bovengenoemde probabilistische instrumenten. De windstatistieken kunnen worden beïnvloed door veranderingen in stormintensiteit en stormfrequentie, d.w.z. veranderingen in het stormklimaat. Daarom wordt in dit rapport een indicatie gegeven hoe de extreme windstatistieken kunnen veranderen door klimaatverandering. Daarnaast wordt aangegeven hoe ingrijpend deze veranderingen kunnen zijn voor de hydraulische belastingen op de Nederlandse primaire keringen. De analyse is gebaseerd op twee benaderingen: (1) data-analyse van de modellen van het ‘Coupled Model intercomparison project 6’ (CMIP6) en (2) een zogenaamde ‘storyline’ benadering., The safety assessment and design of the primary flood defences of the Netherlands are based on probabilistic tools. In the application of these tools, both the statistical as well as the numerical (wave and hydrodynamic) model uncertainties affecting the determination of the hydraulic loads are taken into account. The hydraulic loads of the flood defences are driven by wind forces and or river discharges. However, the uncertainties in the forcing wind statistics due to climate change effects are not yet considered in the above-mentioned probabilistic tools. The wind statistics might be affected by changes in storm intensity and storm frequency, i.e. changes in the storm climate. Therefore, in this report an indication is given of how the extreme wind statistics might change due to the effects of climate change. Additionally, an indication is given of how significant these changes may be for the hydraulic boundary conditions or loads of the Dutch primary defenses. The analysis is based on two approaches: (1) data analysis of the Coupled Model intercomparison project 6 (CMIP6) models and (2) a storyline approach. For the CMIP6 data analysis the extreme daily-averaged winds and resulting surges over the North Sea have been analysed for 25 CMIP6 models. The models do not show a consistent change for the SSP585 scenario (2071-2100) with respect to the reference period (1991-2020) for wind and surge. We conclude that the CMIP6 models do not indicate a consistent change in extreme winds and surges over the North Sea. In addition, the analysis of the normalized (with relation to the present climate) winds and surges does not indicate a change in the thickness of the tail of the extreme value distribution. For the storyline approach we formulated the hypothesis that the effect of future storm climate change on the Dutch hydraulic boundary conditions is limited in the time horizon until 2100, unless it is proven otherwise. The analysis of the storyline underlines this hypothe

Many small island developing states (SIDS) are among the most vulnerable to climate change (e.g. sea level rise) and seasonal to inter-annual climate variability, and subsequently experience flooding due to swell waves and wind waves, coastal erosion and salinisation of freshwater lenses. To counteract this, reef flat mining for sand and aggregate offers a possible solution to the material demand for coastal protection infrastructure and other engineering projects. However, the knowledge on the effects of these pits is limited to only two studies at a single reef: near-shore wave transformation based on measurements (Ford et al., 2013) and a numerical modelling study (Yao et al., 2016). This MSc thesis provides insights on the effects of pits, related to hydrodynamics and wave runup, on a large variety of fringing reefs, by using a 1D and 2DH process-based wave-resolving hydrodynamic model (XBeach non-hydrostatic+, “XBnh+”). Model results indicate that excavation pits cause a decrease in amplitude for waves in the infra-gravity (IG) frequency band. For the majority of the modelled reefs, this was mainly due to a modification of the longest natural frequency of the reef caused by the pit, resulting in a decrease in resonant amplification. Both pit width and cross-shore location have a strong influence on this mechanism. The observed changes in variance in the high frequency (HF) band can be partly explained by a decrease in wave dissipation, as well as a decrease in wave-wave (triad) interaction, both associated with (locally) increased water depth, which causes an increase of the HF peak and a decrease of the HF tail. Of all modelled reefs, there is a 15% chance of an increase in wave runup due to the presence of a pit. This probability is lowest for pits with narrow width and/or located close to the reef crest. The change in runup ranges from +10% to -20%. An increase in runup is caused by a combination of increased wave energy at peak frequencies, as well as small, NUS-TUD Double MSc Degree Programme, Civil Engineering | Hydraulic Engineering, Water Management

Many small island developing states (SIDS) are among the most vulnerable to climate change (e.g. sea level rise) and seasonal to inter-annual climate variability, and subsequently experience flooding due to swell waves and wind waves, coastal erosion and salinisation of freshwater lenses. To counteract this, reef flat mining for sand and aggregate offers a possible solution to the material demand for coastal protection infrastructure and other engineering projects. However, the knowledge on the effects of these pits is limited to only two studies at a single reef: near-shore wave transformation based on measurements (Ford et al., 2013) and a numerical modelling study (Yao et al., 2016). This MSc thesis provides insights on the effects of pits, related to hydrodynamics and wave runup, on a large variety of fringing reefs, by using a 1D and 2DH process-based wave-resolving hydrodynamic model (XBeach non-hydrostatic+, “XBnh+”). Model results indicate that excavation pits cause a decrease in amplitude for waves in the infra-gravity (IG) frequency band. For the majority of the modelled reefs, this was mainly due to a modification of the longest natural frequency of the reef caused by the pit, resulting in a decrease in resonant amplification. Both pit width and cross-shore location have a strong influence on this mechanism. The observed changes in variance in the high frequency (HF) band can be partly explained by a decrease in wave dissipation, as well as a decrease in wave-wave (triad) interaction, both associated with (locally) increased water depth, which causes an increase of the HF peak and a decrease of the HF tail. Of all modelled reefs, there is a 15% chance of an increase in wave runup due to the presence of a pit. This probability is lowest for pits with narrow width and/or located close to the reef crest. The change in runup ranges from +10% to -20%. An increase in runup is caused by a combination of increased wave energy at peak frequencies, as well as small, NUS-TUD Double MSc Degree Programme, Civil Engineering | Hydraulic Engineering, Water Management

Many low-elevation, coral reef-lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave-induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave-induced flooding a challenge. A process-based wave-resolving hydrodynamic model (XBeach Non-Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef-lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef-lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave-induced flooding on coral reef-lined coasts may change due to climate change., Coastal Engineering, Environmental Fluid Mechanics

Many low-elevation, coral reef-lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave-induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave-induced flooding a challenge. A process-based wave-resolving hydrodynamic model (XBeach Non-Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef-lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef-lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave-induced flooding on coral reef-lined coasts may change due to climate change., Coastal Engineering, Environmental Fluid Mechanics