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Our coasts are facing a growing threat from rising sea levels and detrimental storm events. Nature-based solutions are gaining interest for their potential to provide multiple ecosystem services, including coastal protection. Seagrass can influence coastal sediment transport and wave propagation, however, whether seagrass can be used as an effective intervention for coastal protection is unclear. Using the Delft3D model, this study looks at how seagrass affects coastal hydrodynamics in a macrotidal bay, under idealised scenarios that simulate a rise in sea level and storm wave heights, emulating projected sea conditions under climate change. Through the use of a habitat suitability model, a seagrass patch in Morecambe Bay was simulated, based on a location within the bay that is most suitable for seagrass species Zostera marina. Hydrodynamic simulations were run for a number of scenarios for a domain with and without a seagrass patch. Scenarios included variable boundary wave heights, sea levels and vegetation parameters, to test the influence of seagrass in future climates. Results show that there is a reduction in mean and maximum wave height inside and behind the seagrass patch, with the largest changes in wave dissipation observed during higher boundary wave conditions. In these conditions the maximum wave height reduced by over a half within the patch. Simulations examining the influence of seagrass density and canopy height found that a lower density seagrass patch reduced maximum wave height more substantially than higher density patches. Although alternative hard engineered solutions are able to attenuate wave energy more effectively than seagrass, these results demonstrate seagrass could play an important role in conjunction with salt marsh restoration or existing hard engineering solutions, helping to mitigate the risk of flooding and erosion, whilst providing additional ecosystem services such as carbon sequestration and habitat provision. • Habitat suitability model locates site to test seagrass as a nature-based solution. • Simulations find seagrass to influence a reduction in significant wave height. • Potential for seagrass to be integrated into managed realignment plans. [ABSTRACT FROM AUTHOR]

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide‐surge‐wave conditions, Delft3D‐FLOW‐WAVE in a series of eight model simulations for four historic storm events. Plain Language Summary: Coastal zones worldwide are subject to combined effects of astronomical tides, meteorological storms surges, waves, and wind during storms and hurricanes, which can lead to flooding, property damage, and casualties. Coastal communities and critical infrastructure rely on accurate water level and wave forecasts to mitigate these combined hazards. Forecasts utilize hydrodynamic numerical models, which need to accurately represent these hazards and how they interact with, and feedback to, each other. This study uses a model, Delft3D‐FLOW‐WAVE, to calculate how tides and waves from four historic storm events combine to contribute to water level, wave height, and hazard proxy (water level + 1/2 wave height) in the Severn Estuary, southwest England. Additional simulations are run to show how local winds can further contribute to the hazard. Results show that including locally generating winds in simulations of water level, wave height, and hazard proxy is most important for accurate representation of physical processes that contribute to coastal hazards. Excluding locally generated winds from numerical model predictions could mean that flood alerts, warnings, and evacuation orders are missed, or coastal protection schemes are underdesigned, potentially leading to more flooding. Key Points: Delft3D‐FLOW‐WAVE is used to quantify uncertainty in water level and waves in a hypertidal estuary due to coupling and forcing processesExclusion of locally generated winds underestimates significant wave height at high water by up to 90.1% in the outer estuaryLocal atmospheric forcing is vital in coastal hazard predictions for timely flood warnings and to avoid underdesigning protection schemes [ABSTRACT FROM AUTHOR]

The probability of pressurized flow conditions occurring in existing bridges is forecast to increase due to possible changes in extreme precipitation, storm surges, and flooding predicted under climate change scenarios. The presence of a pressure flow is generally associated with scouring processes in proximity to the bridge. Scouring can also occur around bridge piers, possibly causing infrastructure failure. Although there is a vast literature on bridge pier scour and pressure flow scour, only a few studies have investigated their combined effect. This study will provide a new overview of the main features of bridge pier scour under pressurized flow conditions, based on laboratory experiences. Special focus is placed on the analysis of the flow features under pressure and free surface conditions and to the temporal evolution of the scour. A comparison with existing literature data is also conducted. The results highlight the nonlinear nature of scour processes and the need to consider pressurized flow conditions during structural design, as the interaction between pressure flow and the bridge pier strongly influences scour features and leads to scour depths much greater than the sum of the individual scours created only by pressure flow or pier presence. [ABSTRACT FROM AUTHOR]

Storm surge is often the greatest threat to life and critical infrastructures during hurricanes and violent storms. Millions of people living in low-lying coastal zones and critical infrastructure within this zone rely on accurate storm surge forecast for disaster prevention and flood hazard mitigation. However, variability in residual sea level up-estuary, defined here as observed sea level minus predicted tide, can enhance total water levels; variability in the surge thus needs to be captured accurately to reduce uncertainty in site specific hazard assessment. Delft3D-FLOW is used to investigate surge variability, and the influence of storm surge timing on barotropic tide-surge propagation in a tide-dominant estuary using the Severn Estuary, south-west England, as an example. Model results show maximum surge elevation increases exponentially up-estuary and, for a range of surge timings consistently occurs on the flood tide. In the Severn Estuary, over a distance of 40 km from the most upstream tide gauge at Oldbury, the maximum surge elevation increases by 255%. Up-estuary locations experience short duration, high magnitude surge elevations and greater variability due to shallow-water effects and channel convergence. The results show that surge predictions from forecasting systems at tide gauge locations could under-predict the magnitude and duration of surge contribution to up-estuary water levels. Due to the large tidal range and dynamic nature of hyper-tidal estuaries, local forecasting systems should consider changes in surge elevation and shape with distance up-estuary from nearby tide gauge sites to minimize uncertainties in flood hazard assessment. [ABSTRACT FROM AUTHOR]

Astronomical high tides and meteorological storm surges present a combined flood hazard to communities and infrastructure. There is a need to incorporate the impact of tide-surge interaction and the spatial and temporal variability of the combined flood hazard in flood risk assessments, especially in hyper-tidal estuaries where the consequences of tide and storm surge concurrence can be catastrophic. Delft3D-FLOW is used to assess up-estuary variability in extreme water levels for a range of historical events of different severity within the Severn Estuary, southwest England as an example. The influence of the following on flood hazard is investigated: (i) event severity, (ii) timing of the peak of a storm surge relative to tidal high water and (iii) the temporal distribution of the storm surge component (here in termed the surge skewness). Results show when modelling a local area event severity is most important control on flood hazard. Tide-surge concurrence increases flood hazard throughout the estuary. Positive surge skewness can result in a greater variability of extreme water levels and residual surge component, the effects of which are magnified up-estuary by estuarine geometry to exacerbate flood hazard. The concepts and methodology shown here can be applied to other estuaries worldwide. [ABSTRACT FROM AUTHOR]

Salt marshes are important ecosystems but their resilience to sea-level rise and possible increases in storm intensity is largely uncertain. The current paradigm is that a positive sediment budget supports the survival and accretion of salt marshes while sediment deprivation causes marsh degradation. However, few studies have investigated the combined impact of sea-level rise and increased storm intensity on the sediment budget of a salt marsh. This study investigates marsh resilience under the combined impact of various storm surge (0 m, 0.25 m, 0.5 m, 1.0 m, 2.0 m, 3.0 m and 4.0 m) and sea-level (+0 m, +0.3 m, +0.5 m, +0.8 m and +1.0 m) scenarios by using a sediment budget approach and the hydrodynamic model Delft3D. The Ribble Estuary, North-West England, whose salt marshes have been anthropogenically restored and have a high economic and environmental value, has been chosen as test case. We conclude that storm surges can positively contribute to the resilience of the salt marsh and estuarine system by promoting flood dominance and by triggering a net import of sediment. Conversely, sea-level rise can threaten the stability of the marsh by promoting ebb dominance and triggering a net export of sediment. Our results suggest that storm surges have a general tendency to counteract the decrease in sediment budget caused by sea-level rise. The timing of the storm surge relative to high or low tide, the duration of the surge, the change in tidal range and vegetation presence can also cause minor changes in the sediment budget. • Storm surges can positively contribute to the resilience of salt marshes. • Sea-level rise can threaten the stability of salt marshes. • Intense storms can counteract the negative impact of sea-level rise. [ABSTRACT FROM AUTHOR]

Abstract Wave overtopping and subsequent coastal flood hazard is strongly controlled by wind and water levels, and is especially critical in hyper-tidal estuaries where even small changes in wave heights can be catastrophic if they are concurrent with high spring tide. Wave hazard in estuaries is largely attributed to high amplitude shorter period, locally generated wind waves; while low amplitude longer period waves rarely impact low-lying coastal zones up-estuary. Here, the effect of wind and wave properties on up-estuary wave propagation and the sensitivity of significant wave height are investigated numerically along the shoreline of the Severn Estuary, southwest England, as an example. Representative values for wind speed and direction, wave height, period and direction are used to identify key combinations of factors that define the wave hazard generation. High amplitude, short period wind waves are sensitive to opposing winds, with a steepening effect that varies along the estuary shoreline, highlighting the effect of estuarine geometry on wave hazard. Low amplitude, long period wind waves respond with maximum variability in significant wave height to strong winds resulting in their propagation further up-estuary. Our results advance current understanding of the compound interaction between wind and waves, and identify critical conditions maximizing the hazard and hazard variability along the shoreline. The outcomes from this research can help to avoid economic losses from operational downtime in ports and harbors, inform sustainable coastal sea defense design and understand how wave hazard may vary under future climate due to changing storm tracks. Results can also be applied to the design of coastal infrastructure and facilitation of emergency response planning. Highlights • Key combinations of wind wave factors simulated to define wave hazard. • Opposing winds steepen high amplitude waves to increase overtopping hazard. • Stronger winds amplify low amplitude waves and facilitate propagation up-estuary. • Results can inform sea defense design and reduce losses from operational downtime. [ABSTRACT FROM AUTHOR]