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A comprehensive introduction to coastal storms and their associated impacts. Coastal Storms offers students and professionals in the field a comprehensive overview and groundbreaking text that is specifically devoted to the analysis of coastal storms. Based on the most recent knowledge and contributions from leading researchers, the text examines coastal storms' processes and characteristics, the main hazards (such as overwash, inundation and flooding, erosion, structures overtopping), and how to monitor and model storms. The authors include information on the most advanced innovations in forecasting, prediction, and early warning, which serves as a foundation for accurate risk evaluation and developing adequate coastal indicators and management options. In addition, structural overtopping and damage are explained, taking into account the involved hydrodynamic and morphodynamic processes. The monitoring methods of coastal storms are analyzed based on recent results from research projects in Europe and the United States. Methods for vulnerability and risk evaluation are detailed, storm impact indicators are suggested for different hazards and coastal management procedures analyzed. This important resource includes: Comprehensive coverage of storms and associated impacts, including meteorological coastal storm definitions and related potential consequences; A state-of-the-art reference for advanced students, professionals and researchers in the field; Chapters on monitoring methods of coastal storms, their prediction, early warning systems, and modeling of consequences; Explorations of methods for vulnerability and risk evaluation and suggestions for storm impact indicators for different hazards and coastal management procedures. Coastal Storms is a compilation of scientific and policy-related knowledge related to climate-related extreme events. The authors are internationally recognized experts and their work reflects the most recent science and policy advances in the field.

Groundwater flows have been recognized as an important factor controlling bank collapse in both fluvial and tidal environments. These flows are strongly related to water‐level fluctuations in the channel and contribute to seepage erosion and variations in bank soil properties. Despite the more frequent, periodic water level changes in tidal settings, seepage‐driven bank collapses of tidal channels have gained less attention as compared to their fluvial counterparts, leaving a remarkable knowledge gap in understanding the differences between these two systems. Here, we refine an established numerical model for bank collapse by including seepage dynamics and validate the model using novel data from laboratory experiments and field observations. From a mechanistic perspective, seepage‐driven bank collapse plays an equivalent role in the widening of river and tidal channels. At the same time, periodic variations in water level occur more regularly in tidal channels than in rivers, making seepage‐driven bank collapse more likely to affect tidal channels. Plain Language Summary Bank collapse is a key process in the evolution of both fluvial and tidal channels, threatening farmlands and infrastructures. Recent studies have shown that the impact of groundwater flow on bank collapse is as important as surface water flow. Until now, most studies looked at rivers and tidal channels separately because they behave differently. However, our research found that the underlying processes causing collapse might be quite similar in both environments, showing the need to compare them. By updating an existing scientific model to better account for the effects of underground water, and testing it with novel laboratory experiments and field observations, we have concluded that tidal channels are more prone to bank collapse. This happens because the bank soil properties in tidal environments weaken more frequently due to the regular rise and fall of tides as compared to the recurrence of flood waves in fluvial counterpart. Our work helps to explain why bank collapses happen and could help the design of suitable countermeasures for preventing the damage of nearby land and structures. Key Points Seepage processes are likely to promote shear failure when water level is low The collapse‐induced upper retreat distance follows a dimensionless exponential relation for both fluvial and tidal cases Compared to fluvial counterpart, tidal channels experience periodic weakening of soil properties leading to more frequent bank collapses

Changes in upstream land-use have significantly transformed downstream coastal ecosystems around the globe. Restoration of coastal ecosystems often focuses on local-scale processes, thereby overlooking landscape-scale interactions that can ultimately determine restoration outcomes. Here we use an idealized bio-morphodynamic model, based on estuaries in New Zealand, to investigate the effects of both increased sediment inputs caused by upstream deforestation following European settlement and mangrove removal on estuarine morphology. Our results show that coastal mangrove removal initiatives, guided by knowledge on local-scale bio-morphodynamic feedbacks, cannot mitigate estuarine mud-infilling and restore antecedent sandy ecosystems. Unexpectedly, removal of mangroves enhances estuary-scale sediment trapping due to altered sedimentation patterns. Only reductions in upstream sediment supply can limit estuarine muddification. Our study demonstrates that bio-morphodynamic feedbacks can have contrasting effects at local and estuary scales. Consequently, human interventions like vegetation removal can lead to counterintuitive responses in estuarine landscape behavior that impede restoration efforts, highlighting that more holistic management approaches are needed. Upstream land-use changes are transforming coastal environments around the globe. Mangrove removal aims at restoring estuarine ecosystems but counterintuitively enhances sediment trapping. More holistic management approaches are needed.

Sandy beaches comprise approximately 31% of the world's ice-free coasts. Sandy coastlines around the world are continuously adjusting in response to changing waves and water levels at both short (storm) and long (climate-driven, from El-Nino Southern Oscillation to sea level rise) timescales. Managing this critical zone requires robust, advanced tools that represent our best understanding of how to abstract and integrate coastal processes. However, this has been hindered by (1) a lack of long-term, large-scale coastal monitoring of sandy beaches and (2) a robust understanding of the key physical processes that drive shoreline change over multiple timescales. This perspectives article aims to summarize the current state of shoreline modeling at the sub-century timescale and provides an outlook on future challenges and opportunities ahead.

Beaches around the world continuously adjust to daily and seasonal changes in wave and tide conditions, which are themselves changing over longer time-scales. Different approaches to predict multi-year shoreline evolution have been implemented; however, robust and reliable predictions of shoreline evolution are still problematic even in short-term scenarios (shorter than decadal). Here we show results of a modelling competition, where 19 numerical models (a mix of established shoreline models and machine learning techniques) were tested using data collected for Tairua beach, New Zealand with 18 years of daily averaged alongshore shoreline position and beach rotation (orientation) data obtained from a camera system. In general, traditional shoreline models and machine learning techniques were able to reproduce shoreline changes during the calibration period (1999-2014) for normal conditions but some of the model struggled to predict extreme and fast oscillations. During the forecast period (unseen data, 2014-2017), both approaches showed a decrease in models' capability to predict the shoreline position. This was more evident for some of the machine learning algorithms. A model ensemble performed better than individual models and enables assessment of uncertainties in model architecture. Research-coordinated approaches (e.g., modelling competitions) can fuel advances in predictive capabilities and provide a forum for the discussion about the advantages/disadvantages of available models.

Video measurements of wave runup were collected during extreme storm conditions characterized by energetic long swells (peak period of 16.4 s and offshore height up to 6.4 m) impinging on steep foreshore beach slopes (0.05–0.08). These conditions induced highly dissipative and saturated conditions over the low‐sloping surf zone while the swash zone was associated with moderately reflective conditions (Iribarren parameters up to 0.87). Our data support previous observations on highly dissipative beaches showing that runup elevation (estimated from the variance of the energy spectrum) can be scaled using offshore wave height alone. The data is consistent with the hypothesis of runup saturation at low frequencies (down to 0.035 Hz) and a hyperbolic‐tangent fit provides the best statistical predictor of runup elevations. Key Points The best statistical predictor of runup is an hyperbolic‐tangent fit It provides evidence of saturation at infragravity frequencies Accounting for wave period is critical in explaining runup variability

Parallel tidal channel systems, characterized by commonly cross-shore orientation and regular spacing, represent a distinct class of tidal channel networks in coastal environments worldwide. Intriguingly, these cross-shore oriented channel systems can develop in environments dominated by alongshore tidal currents, for which the mechanisms remain elusive. Here, we combine remote sensing imagery analysis and morphodynamic simulations to demonstrate that the deflection of alongshore tidal currents at transitions in bed elevation determines the characteristic orientation of the parallel tidal channels. Numerical results reveal that sharp changes in bed elevation lead to nearly 90-degree intersection angles, while smoother transitions in bed profiles result in less perpendicular channel alignments. These findings shed light on the potential manipulation of tidal channel patterns in coastal wetlands, thus equipping coastal managers with a broader range of strategies for the sustainable management of these vital ecosystems in the face of climate change and sea level rise. Cross-shore channels can counterintuitively form at coasts dominated by alongshore currents. Sharp changes in bed elevation around the mean sea level lead to nearly 90-degree intersection angles between the channel and the shoreline.

Tide-surge interaction (TSI) is a critical factor in assessing flooding in shallow coastal systems, particularly in estuaries and harbours. Non-linear interactions between tides and surges can occur due to the water depth and bed friction. Global investigations have been conducted to examine TSI, but its occurrence and impact on water levels in Aotearoa New Zealand (NZ) have not been extensively studied. Water level observations from 36 tide gauges across the diverse coast of NZ were analysed to determine the occurrence and location of TSI. Statistical analysis and numerical modelling were conducted on data from both inside and outside estuaries, focusing on one estuary (Manukau Harbour) to determine the impact of TSI and estuarine morphology on the co-occurrence rate of extreme events. TSI was found to occur at most sites in NZ and primarily affects the timing of the largest surges relative to high tide. There were no regional patterns associated with the tide, non-tidal residual, or skew-surge regimes. The strongest TSI occurred in inner estuarine locations and was correlated with the intertidal area. The magnitude of the TSI varied depending on the method used, ranging from -16 cm to +27 cm. Co-occurrence rates of extreme water levels outside and inside the same estuary varied from 20% to 84%, with TSI modulating the rate by affecting tidal amplification. The results highlight the importance of investing in a more extensive tide gauge network to provide longer observations in highly populated estuarine coastlines. The incorporation of TSI in flooding hazard projections would benefit from more accurate and detailed observations, particularly in estuaries with high morphological complexity. TSI occurs in most sites along the coast of NZ and has a significant impact on water levels in inner estuarine locations. TSI modulates the co-occurrence rate of extreme water levels in estuaries of NZ by affecting tidal amplification. Therefore, further investment in the tide gauge network is needed to provide more accurate observations to incorporate TSI in flooding hazard projections.

The Pearl River Estuary, home to one of the world's densest human populations, exemplifies the delicate balance between natural dynamics and anthropogenic pressures. Our study explores how human activities and environmental changes interact to reshape this vital system, potentially driving it toward a tipping point. By integrating historical data, a Bayesian Network model, and a process‐based morphodynamic model, we quantify the relative contributions of sediment supply, land reclamation, dredging, sand mining, and sea‐level rise to estuarine evolution. Sediment supply remains the dominant driver, but human interventions and rising sea levels significantly disrupt the system's dynamics potentially leading to a tipping point in the estuarine morphodynamics of the Pearl River Estaury. We identify three distinct phases of estuarine evolution, revealing how cumulative pressures driven by anthropogenic pressure could force the estuary into different states with ecological, economic and societal consequences. These findings provide a general and transferable framework for detecting human impacts on estuarine systems and inform climate adaptation strategies. Plain Language Summary This study investigates the evolution of the Pearl River Estuary, focusing on how human activities and environmental changes interact to reshape this vital system. By integrating historical data, a Bayesian Network model, and a process‐based morphodynamic model, the research quantifies the contributions of sediment supply, land reclamation (LR), dredging, sand mining, and sea level rise (SLR) to estuarine changes. Sediment supply remains the dominant driver, but human interventions and rising sea levels significantly disrupt the system, potentially pushing it toward a tipping point. The study identifies three phases of estuarine evolution, highlighting how cumulative pressures from human activities can force the estuary into different states with ecological, economic, and societal consequences. Key findings include the relative impacts of sediment supply (30.9% in Upper Lingding Bay, 36.1% in Lingding Bay), LR (26.0%, 21.0%), and SLR (10.6%, 13.2%). The research underscores the importance of considering multiple factors in estuarine management to balance human needs and environmental resilience, offering insights for sustainable coastal ecosystem management. Key Points Sediment supply drives estuarine evolution, but human activities and Sea‐level Rise (SLR) disrupt dynamics, risking tipping points The estuary transits from rapid infilling (low human impact) to slow infilling and an uncertain future (increased human impact) Balancing sediment supply, human activities, & SLR is key to sustainable estuarine management & coastal resilience

IntroductionSaltmarsh introduction has been widely implemented to restore ecosystem services and promote sedimentation in tidal mudflats, yet its effects on tidal network dynamics remain hard to predict. The interplay between saltmarsh extent and sediment availability in shaping long-term mudflat morphodynamics is not fully understood.MethodsWe develop a two-dimensional biomorphodynamic model to examine the individual and combined influences of saltmarsh presence and sediment availability on the evolution of tidal-flat channels.Results and discussionOur results demonstrate that sediment availability controls the long-term morphological change of mudflats, while the presence of saltmarshes exerts substantial short-term alterations in mudflat evolution. During the initial phase of saltmarsh introduction, vegetation promotes the development of tidal networks, characterized by channel elongation, narrowing and deepening. However, under higher sediment supply, saltmarshes restrict sediment deposition on landward and central mudflats compared to that on unvegetated flats. Furthermore, sediment availability primarily facilitates the extension of pre-existing channels, while saltmarshes play a dual role in both generating new channels and elongating existing ones. This distinction highlights the competing mechanisms driving channel network development.