Does a more sophisticated storm erosion model improve probabilistic erosion estimates?

The dependency between the accuracy/uncertainty of storm erosion exceedance estimates obtained via a probabilistic model and the level of sophistication of the structural function (storm erosion model) embedded in the probabilistic model is assessed via the application of Callaghan et al.'s (2008) Joint Probability Model (JPM) at Narrabeen beach, Australia with three different structural functions: (a) Kriebel and Dean (1993) (analytical); (b) SBEACH (semiempirical); and (c) XBeach (fully process based). Results indicate that the accuracy is greatest for JPM-SBEACH and lowest for JPM-XBeach. The most uncertain results are given by JPM-XBeach while the most robust results are given by JPM-SBEACH. Thus, it appears that increasing the level of sophistication of the structural function beyond the semi-empirical SBEACH model, may not always lead to better results and may even be counter-productive.Hydraulic EngineeringCivil Engineering and Geoscience

The mean suspended sediment concentration profile of silty sediments under wave-dominant conditions

Suspended sediment concentration (SSC) is one of the fundamental topics in sediment study. The parameterization of the SSC profile of silty sediments is still under-researched. This study focuses on the mean SSC profile for silty sediments under non-breaking wave-dominant conditions. First, inspired by a 1DV model, different types of the distribution of mean eddy viscosity were proposed, i.e., a toe-type distribution over flat bed and a constant-toe type distribution over rippled bed. Then, the time-averaged diffusion equation for suspended sediment transport was analytically solved, and expressions for the mean SSC profiles were derived. The expressions involve several basic physical processes, including the effects of bed forms, stratification, hindered settling and mobile bed. Verification using a number of experimental datasets showed that the proposed expressions can properly calculate the mean SSC for silt and are applicable for sand as well. In conclusion, this research provides an approach to estimate the mean SSC for silty sediments under wave-dominant conditions, which is expected to be applicable for engineering practice and numerical modelling.Accepted Author ManuscriptCoastal Engineerin

Relationship between three-dimensional radiation stress and Vortex-Force representations

In numerical ocean models, the effect of waves on currents is usually expressed by either vortex-force or radiation stress representations. In this paper, the differences and similarities between those two representations are investigated in detail in conditions of both conservative and nonconservative waves. In addition, comparisons between different sets of equations of mean motion that apply different representations of wave-induced forcing terms are included. The comparisons are useful for selecting a suitable numerical ocean model to simulate the mean current in conditions of waves combined with currents.Coastal EngineeringEnvironmental Fluid Mechanic

Development and Validation of Quasi-Eulerian Mean Three-Dimensional Equations of Motion Using the Generalized Lagrangian Mean Method

This study aims at developing a new set of equations of mean motion in the presence of surface waves, which is practically applicable from deep water to the coastal zone, estuaries, and outflow areas. The generalized Lagrangian mean (GLM) method is employed to derive a set of quasi-Eulerian mean three-dimensional equations of motion, where effects of the waves are included through source terms. The obtained equations are expressed to the second-order of wave amplitude. Whereas the classical Eulerian-mean equations of motion are only applicable below the wave trough, the new equations are valid until the mean water surface even in the presence of finite-amplitude surface waves. A two-dimensional numerical model (2DV model) is developed to validate the new set of equations of motion. The 2DV model passes the test of steady monochromatic waves propagating over a slope without dissipation (adiabatic condition). This is a primary test for equations of mean motion with a known analytical solution. In addition to this, experimental data for the interaction between random waves and a mean current in both non-breaking and breaking waves are employed to validate the 2DV model. As shown by this successful implementation and validation, the implementation of these equations in any 3D model code is straightforward and may be expected to provide consistent results from deep water to the surf zone, under both weak and strong ambient currents.Coastal Engineerin

A probabilistic approach to investigate the effect of wave chorology on process-based morphological modelling

This paper demonstrates the sensitivity of morphological process-based models to the chronology of input wave conditions. In this research the effect of an emerged offshore breakwater on the morphology of the beach is investigated. A 30 day long morphological simulation with real time history of the wave (brute force - base case) is compared with 150 different simulations of the same case with schematized wave conditions comprising different chronologies. The comparison between each case and the base case is quantified via the Brier Skill Score. This research shows that the skill score of a simulation largely affected by the sequence of the input wave conditions, and the best result is obtained by averaging all 150 simulations.Hydraulic EngineeringCivil Engineering and Geoscience

Contribution of infragravity waves to run-up and overwash in the pertuis breton embayment (France)

Wave run-up and dune overwash are typically assessed using empirical models developed for a specific range of often-simplistic conditions. Field experiments are essential in extending these formulae; yet obtaining comprehensive field data under extreme conditions is often challenging. Here, we use XBeach Surfbeat (XB-SB)-a shortwave-averaged but wave-group resolving numerical model-to complement a field campaign, with two main objectives: i) to assess the contribution of infragravity (IG) waves to washover development in a partially-sheltered area, with a highly complex bathymetry; and ii) to evaluate the unconventional nested-modeling approach that was applied. The analysis shows that gravity waves rapidly decrease across the embayment while IG waves are enhanced. Despite its exclusion of gravity-band swash, XB-SB is able to accurately reproduce both the large-scale hydrodynamics-wave heights and mean water levels across the 30 × 10 km embayment; and the local morphodynamics-steep post-storm dune profile and washover deposit. These findings show that the contribution of IG waves to dune overwash along the bay is significant and highlight the need for any method or model to consider IG waves when applied to similar environments. As many phase-averaged numerical models that are typically used for large-scale coastal applications exclude IG waves, XB-SB may prove to be a suitable alternative.Hydraulic Structures and Flood Ris

Morphodynamic Resilience of Intertidal Mudflats on a Seasonal Time Scale

Intertidal mudflats are morphodynamic features present in many estuaries worldwide. Often located between vegetated shores and deep channels they comprise valuable ecosystems and serve to protect the hinterland by attenuating waves. Although mudflats are persistently present on yearly to decadal time scales, little is known on their morphodynamic adaptation to short-term variations in forcing such as storms, spring-neap tidal cycles, and sediment supply. This study aims to explore the morphodynamic resilience of mudflats to seasonal variations in forcing. First, we compare transects observed in South Bay, California, at 3- to 6-monthly intervals. Second, we present the results of a process-based, morphodynamic profile model (Mflat). Mflat is an open source, Matlab code that describes both cross-shore and alongshore tidal hydrodynamics as well as a stationary wave model. An advection-diffusion equation solves sediment transport while bed level changes occur by the divergence of the sediment transport field. Mflat reproduces the observed South San Francisco Bay profile in equilibrium with significant skill. Short-term variations in hydrodynamic forcing and sediment characteristics disturb the profile mainly at the channel-shoal edge. The modeled profile disturbance is consistent with observations. The modeled profile is remarkably resilient since it recovers to the equilibrium profile within weeks to months. The model results suggest that 3-monthly observation intervals are probably too long to discriminate processes responsible for the profile disturbance. These processes may include variations in sediment supply, mudflat erodibility, and wave action as well as the spring-neap tidal cycle.Coastal Engineerin

Development and field validation of a 2DH curvilinear storm impact model

The formulations of the 2DH process-based, nearshore morphological model XBeach were extended to allow for curvilinear grids using a finite volume approach. The formulations were tested for schematic cases such as a circular island and a field validation study was carried out for the case of storm erosion during a construction phase of the soft sea defense of the Maasvlakte-2 extension of Rotterdam port. It was found that the 2D curvilinear version performs significantly better than individual 1D runs. The overall skill of the 2D model for this case can be classified as 'reasonable' while the prediction of the erosion can be classified as 'good'.Hydraulic EngineeringCivil Engineering and Geoscience

Improving predictions of swash dynamics in XBeach: The role of groupiness and incident-band runup

In predicting storm impacts on sandy coasts, possibly with structures, accurate runup and overtopping simulation is an important aspect. Recent investigations (Stockdon et al., 2014; Palmsten and Splinter, 2016) show that despite accurate predictions of the morphodynamics of dissipative sandy beaches, the XBeach model (Roelvink et al., 2009) does not correctly simulate the individual contributions of set-up, and infragravity and incident-band swash to the wave run-up. In this paper we describe an improved numerical scheme and a different way of simulating the propagation of directionally-spread short wave groups in XBeach to better predict the groupiness of the short waves and the resulting infragravity waves. The new approach is tested against field measurements from the DELILAH campaign at Duck, NC, and against video-derived runup measurements at Praia de Faro, a relatively steep sandy beach. Compared to the empirical fit by Vousdoukas et al. (2012) the XBeach model performs much better for more extreme wave conditions, which are severely underestimated by existing empirical formulations.For relatively steep beaches incident-band swash cannot be neglected and a wave-resolving simulation mode is required. Therefore in this paper we also test the non-hydrostatic, wave-resolving model within XBeach for runup and overtopping against three datasets. Results for a high-quality flume test show non-hydrostatic XBeach predicts the run-up height with good accuracy (maximum deviation 15%). A case with a very shallow foreshore typical for the Belgian coast at Wenduine was compared against detailed measurements. Overall the model shows correct behavior for this case. Finally, the model is tested against a large number (551) of physical model tests of overtopping from the CLASH database. For relatively high overtopping discharges the non-hydrostatic XBeach performs quite well, with increasing accuracy for increasing overtopping rates. However, for relatively low overtopping rates of less than 10-20 l/m/s, the model systematically underestimates measured overtopping rates.Coastal Engineerin

Efficient Modeling of Complex Sandy Coastal Evolution at Monthly to Century Time Scales

With large-scale human interventions and climate change unfolding as they are now, coastal changes at decadal timescales are not limited to incremental modifications of systems that are fixed in their general geometry, but often show significant changes in layout that may be catastrophic for populations living in previously safe areas. This poses severe challenges that are difficult to meet for existing models. A new free-form coastline model, ShorelineS, is presented that is able to describe large coastal transformations based on relatively simple principles of alongshore transport gradient driven changes as a result of coastline curvature, including under highly obliquely incident waves, and consideration of splitting and merging of coastlines, and longshore transport disturbance by hard structures. An arbitrary number of coast sections is supported, which can be open or closed and can interact with each other through relatively straightforward merging and splitting mechanisms. Rocky parts or structures may block wave energy and/or longshore sediment transport. These features allow for a rich behavior including shoreline undulations and formation of spits, migrating islands, merging of coastal shapes, salients and tombolos. The main formulations of the (open-source) model, which is freely available at www.shorelines.nl, are presented. Test cases show the capabilities of the flexible, vector-based model approach, while field validation cases for a large-scale sand nourishment (the Sand Engine; 21 million m3) and an accreting groin scheme at Al-Gamil (Egypt) show the model’s capability of computing realistic rates of coastline change as well as a good representation of the shoreline shape for real situations.Coastal Engineerin