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A numerical tool integrating hydrodynamics, vegetation growth, stochastic selection, and Monte‐Carlo simulation is developed to predict channel network evolution in a newly inundated back‐barrier wetland. Focusing on the formation and evolution of tidal channels, model simulations suggest a clear tendency towards specific creek topologies with high probability despite creek evolution being subject to random perturbations. These creek topologies are primarily determined by wetland topography while the influence of vegetation is secondary. Ensemble results of Monte‐Carlo simulations identify threshold points leading to bimodal separation of the emergent creek networks characterized by different complexity and efficiency. This, in turn, assists the recognition of necessary human interventions to encourage the formation of complex and far‐reaching creek networks that can promote exchange of materials between estuaries and salt marshes and, hence, maintain salt marsh ecosystem structure and function. Plain Language Summary: A large part of the world's low‐lying areas is heavily managed to protect lives, assets, and landscapes from marine‐derived natural hazards. However, climate change is putting unprecedented pressure on these managed environments, forcing the adoption of "no active intervention" or "managed realignment" strategies in areas where "hold the line" options cannot be justified due to financial constraints. This leads to the very likely possibility that unmaintained coastal defenses will begin to fail, raising questions as to the potential impacts on low‐lying regions as well as the options available to mitigate impacts through targeted interventions. Of particular interest is the question of whether it is possible to facilitate the shift of managed coastal wetlands towards intertidal environments in which tidal creeks form and evolve to support salt marshes. Combining the use of a numerical model and multiple model runs, we find that although the evolution of tidal creeks is subject to random disturbances, creek networks are likely to form with a clear tendency towards specific types. Grouped results of multiple, random simulations are useful in identifying interventions that are necessary for developing far‐reaching tidal creek networks. Influence of vegetation is found to be secondary compared to the wetland topography in terms of influencing tidal creek evolution. Key Points: A reduced complexity model is developed based on simple rules for projecting tidal creek network developmentThe influence of vegetation in determining probabilities of creek formation is secondary compared to wetland topographyEnsemble prediction of tidal creek evolution is effective for identifying threshold points and targeted interventions [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]

The hydrodynamic and residual transport patterns arising from oscillating tidal motion have important consequences for the transport of sediments, and for the evolution of the shoreline, especially under macro- and meso-tidal conditions. For many locations there are significant uncertainties about residual currents and sediment transport characteristics, and their possible influence on the morphological evolution of the coastline and on the character of the bed. Herein we use the coastline of SE England as a test case to investigate possible changes in residual currents, and residual transport patterns from neap to spring tide, the reciprocal interaction between residuals and the character of the bed, and the morphological evolution of the coastline at a century timescale. We found that in the long term the morphology of the system evolves toward a dynamic equilibrium configuration characterized by smaller, and spatially constant residual transport patterns. While the spatial distribution of residual currents maintains a similar trend during both neap and spring tide, during spring tide and for large areas residual currents switch between northerly and southerly directions, and their magnitude is doubled. Residual eddies develop in regions characterized by the presence of sand bars due to the interaction of the tide with the varying topography. Residual transport patterns are also computed for various sediment fractions, and based on the hydrodynamics and sediment availability at the bottom. We found that the distribution of sediments on the bed is significantly correlated with the intensity of residuals. Finally, the majority of long-term morphological changes tend to develop or augment sand banks features, with an increase in elevation and steepening of the bank contours. [ABSTRACT FROM AUTHOR]

The hydrodynamics of distributary channels has tremendous impact on nutrient and dissolved oxygen circulation, transport of sediments, and delta formation and evolution; yet many processes acting at the river–marine interface of a delta are poorly understood. This paper investigates the combined effect of river hydrograph and micro-tides on the hydrodynamics of a delta distributary. As the ratio between river flow to tidal flow increases, tidal flood duration at the distributary mouth decreases, up to the point when flow reversal is absent. Field measurements in a distributary of the Apalachicola Delta, Florida, USA, reveal that, once the flow becomes unidirectional, high-discharge events magnify tidal velocity amplitudes. On the contrary, while the flow is bidirectional, increasing fluvial discharge decreases tidal velocity amplitudes down to a minimum value, reached at the limit between bidirectional and unidirectional flow. Due to the different response of the system to tides, the transition from a bidirectional to a unidirectional flow triggers a change in phase lag between high water and high water slack. In the presence of high riverine flow, tidal dynamics also promote seaward directed Eulerian residual currents. During discharge peaks, these residual currents almost double mean velocity values. Our results show that, even in micro-tidal environments, tides strongly impact distributary hydrodynamics during both high and low fluvial discharge regimes. [ABSTRACT FROM AUTHOR]

The impact of tide-induced morphological changes and water level variations on the sediment transport in a tidally dominated system has been investigated using the numerical model Delft3D and South-East England as a test case. The goal of this manuscript is to explore the long-term changes in morphology due to sea level rise and the large-scale morphodynamic equilibrium of the South-East England. Our results suggest that the long term (century scale) tidally-induced morphological evolution of the seabed slows down in time and promotes a vanishing net transport across the large scale system. Century-scale morphologically updated simulations show that both morphological changes and net transport values tend to decrease in time as the system attains a dynamic equilibrium configuration. Results further suggest that the presence of a gradual increase in mean sea level accelerates the initial morphological evolution of the system whose morphological rate of change gradually attains, however, same plateau values as in the absence of sea level rise. Given the same base morphology, increasing water levels enhance residual currents and the net transport near the coastline; and vice-versa, decreasing sea levels minimize both residuals and net transport near the coastline. The areas that are more affected by, water level and morphological changes, are the ones where the net transport is the highest. This manuscript explores and allows extending the idea of morphodynamic equilibrium at a regional scale, larger than the one for which this concept has been generally explored i.e., estuarine scale. [ABSTRACT FROM AUTHOR]

The energetic, macrotidal shelf off South West England was used to investigate and classify the impact of different tide and wave conditions and their interactions on sand transport at a regional scale using a coupled hydrodynamic, wave and sediment transport model (Delft3D). Residual currents and sediment transport patterns are important for the transport and distribution of littoral and shelf sea sediments, the morphological evolution of the coastal and inner continental shelf zones and coastal planning.Sand transport magnitude and direction across this macrotidal environment are heavily influenced by waves. Median (50% exceedance) and extreme (1% exceedance) wave conditions were simulated during neap and spring tides. Effects such as areas shifting from flood- to ebb-dominant sand transport, reversals in the dominant sand transport path and activation of sand transport at depths of 120m were predicted under extreme forcing relative to tide-only and median wave conditions. Wave-tide interactions cause a significant non-linear enhancement of sand transport magnitude, as determined by the difference between sand transport magnitude in a coupled model and its wave-only and tide-only components. A new continental shelf classification scheme was created based on sand transport magnitude due to wave-forcing, tide-forcing and non-linear wave-tide interactions. Non-linear interactions are the dominant forcing mechanism for sand transport across most of this macrotidal environment during extreme waves at springs. At neaps, non-linear interactions drive a significant proportion of sand transport under median and extreme waves despite negligible tide-only sand transport. This emphasises the critical need to consider wave-tide interactions when considering sand transport in energetic environments globally. The above classification method requires a computationally expensive coupled numerical model, and the next step is to make it more generally applicable based on readily available tide and wave characteristics. A weighted K-nearest neighbour classifier was used to predict wave, tide or non-linear interaction dominance of net sand transport based on readily available tide and wave parameters. Of the parameters tested, tide range, mean significant wave height, mean peak period, mean wave direction, water depth and maximum current speed and direction were identified as predictors. Classifications were predicted with an overall accuracy of 81.3% using all identified predictors and 74% using a subset of tide range, mean significant wave height and water depth, based on 5-fold cross validation. A simplified classification scheme was predicted with 93.1% accuracy for all predictors and 88.9% for the subset. This classification scheme may be used to identify the primary forcing mechanisms and relative magnitudes of net sand transport across the shelf across a range of wave-tide regimes, using only readily available wave and tide data as inputs, with benefits to coastal researchers and planners working in energetic, sandy environments worldwide. [ABSTRACT FROM AUTHOR]