Andrew Chadwick

A review and assessment of longshore sediment transport equations for coarse-grained beaches

Previous assessments of analytical longshore sediment transport formulae have been heavily biased towards sand-sized sediment. All have noted the shortage of high quality field data from coarse-grained beaches against which to test predictions of longshore transport rates. In this paper, 12 existing formulae were identified as being potentially applicable for coarse-grained sediments and predictions from these formulae are compared using a measured annual transport rate from a shingle beach and a concurrent hindcast wave climate. Two new empirical equations are also derived, one from a numerical model calibrated against the same data set, the other derived from field experiments on coarse grained beaches. Energetics-based equations are found to give reasonable predictions of the shingle transport, despite being derived for sand beaches. In contrast, those dimensional analysis type equations which had been validated using laboratory data, grossly over-predicted the measured transport rates. The most accurate predictions were from formulae previously validated at sites similar to that used for this comparison and therefore require further testing against field data from dissimilar sites.

Velocity predictions for shoaling and breaking waves with a Boussinesq-type model

Abstract In this paper, the performance of a 1-D Boussinesq model is evaluated against laboratory data for its ability to predict surf zone velocity moments. Wave evolution over a plane beach and a complex bathymetry both extending into the surf-zone is examined for six cases. For the plane beach, these comprise two cases, a spilling and a plunging cnoidal wave. For the complex bathymetry, these comprise four cases of longer and short wavelengths (spilling and plunging breakers), with regular and irregular periodicity. The model evaluation places emphasis on parameters of the wave field that could be used for the prediction of sediment transport; orbital velocity, undertow, velocity skewness, kurtosis and asymmetry. It is found that, despite an overestimation of the depth-averaged horizontal velocity in the regular waves cases, the predicted higher order velocity moments and undertow are in good agreement with the laboratory data. A bispectral analysis demonstrates that the nonlinear transfers of energy amongst the low order harmonics are well reproduced, but energy exchanges with the higher harmonics are less well predicted. As a result, the model handles velocity moments better in the shorter wave tests than in the long wave cases where triad interactions are stronger. Of the four parameters describing wave breaking, the model behaviour is most sensitive to the critical wave front slope φ B , especially with regard to velocity skewness and kurtosis predictions. It is also found that increasing the thickness of the surface roller for the case of plunging breakers improves the models performance.

STRAND: A Model for Longshore Sediment Transport in the Swash Zone

In this paper we report on the development and performance of an engineering model, STRAND which has the aim of predicting longshore movement of coarse sediment above the still water line of steep beaches. The model assumes that this transport is driven by swash run-up at the edge of an unsaturated inner surfzone and uses Nielsens (1992) formulation for sediment transport rate. The hydrodynamic sub-model is shown to agree well with field measurements of swash run-up and swash period. We argue that consideration of interactions between subsequent swash events implies that a monotonic relationship between transport rate and incident wave period is inappropriate. Bulk longshore transport rates are shown to compare reasonably with previous estimates from field studies in the UK and accounts for up to 50% of the net longshore flux. Agreement of this simplified model with one of the best available laboratory data sets, Kamphuis (1991a,b), is very good indeed. However, new laboratory and field data are required before stronger conclusions can be drawn.

An evaluation of directional analysis techniques for multidirectional, partially reflected waves Part 1: numerical investigations

Recent studies of advanced directional analysis techniques have mainly centred on incident wave fields. In the study of coastal structures, however, partially reflective wave fields are commonly present.In the near structure field, phase locked methods can be successfully applied. In the far field, non-phased locked methods are more appropriate. In this paper, the accuracy of two non-phased locked methods of directional analysis, the maximum likelihood method (MLM) and the Bayesian directional method (BDM) have been quantitatively evaluated using numerical simulations for the case of multidirectional waves with partial reflections. It is shown that the results are influenced by the ratio of distance from the reflector (L) to the length of the time series (S) used in the spectral analysis. Both methods are found to be capable of determining the incident and reflective wave fields when LIS > 0.5.The BDM provides, in most cases, more accurate estimates of incident significant wave height, average reflection coefficients and directional spreading.

EVALUATION OF BEACH MODELLING TECHNIQUES BEHIND DETACHED BREAKWATERS

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

REFLECTION OF OBLIQUELY INCIDENT WAVES AT LOW-CRESTED STRUCTURES

The paper presents an experimental investigation of obliquely incident wave reflection at low-crested structures within the EU-funded project DELOS. The physical model tests were carried out with incident wave attack angles varying over a wide range from 0° to 60° at a multidirectional wave basin. Data comparisons suggest that long-crested and short-crested waves give similar reflection from low-crested structures. The reflection coefficient is noticeably dependent on the wave angle of incidence. On the basis of the test data, two wave reflection formulae including the influence of incident angles are proposed for low-crested rubble mound structures and smooth structures respectively. Wave spectra analyses show that the reflected peak period is quite close to the incident peak period, and the reflected directional waves more widely spread than the incident waves.

The evolution of an equilibrium bay

This paper reports on laboratory morphological studies of a shore-parallel porous breakwater system. A physical model study of the Elmer breakwater scheme, West Sussex, was conducted in the UK Coastal Research Facility (UKCRF) at HR Wallingford as part of an EPSRC-funded composite model evaluation (LUPY project). Mobile bed experiments, using both sand and anthracite as model sediments are described and discussed. Subsequent analysis showed that the evolution of equilibrium bays and their final width depend not only on the breakwater length and gap width, but also on the properties of the chosen model sediment and the permeability of the structure. It was also found that the 3D morphological changes influence the hydrodynamics, which in turn influences the evolution of the equilibrium morphological features.

Numerical Simulation of Wave Overtopping Using an Incompressible SPH Method

In this paper an incompressible Smoothed Particle Hydrodynamics (SPH) method combined with a Large Eddy Simulation (LES) model is used to investigate wave overtopping. SPH is a pure Lagrangian approach which can handle large deformations of the free surface with high accuracy. Two different cases are used to test the model, namely storm wave overtopping of a fixed horizontal deck and random wave overtopping of a sloping seawall. The computations are validated against experimental and numerical data in the literature and good agreement is found.

An evaluation of directional analysis techniques for multidirectional, partially reflected waves Part 2: application to field data

Based on the findings of a numerical investigation, presented in the Part 1 companion paper, two methods of directional analysis, the maximum likelihood method (MLM) and the Bayesian directional method (BDM) are applied to over 80 field data sets. These cover a wide range of environmental conditions, for which multidirectional, partially reflective sea states exist. The results show that trends similar to those found using the numerical simulations are observed in the field estimates of relative predictions of incident significant wave height, average reflection coefficients, main directions and directional spreading. It is concluded that overall the BDM produces the more accurate results when applied to real sea waves.

APPRAISING SPIT DYNAMICS AND ESTUARY RESPONSES: A COASTAL MANAGEMENT STUDY FROM THE EXE ESTUARY, UK

The paper describes geomorphological analysis and morphodynamic modelling of the double spit enclosed Exe Estuary, UK. The long-term morphodynamic behaviour of the Exe Estuary system and the surrounding shoreline is studied. The geomorphological analysis was based on available historic data on the estuary. The morphodynamic modelling methodology is two-fold: A systems model based on a Boolean approach is used to predict and investigate future morphodynamic response of the estuary to changes in external forcing. A 1-line shoreline model is used to investigate morphodynamics of Dawlish Warren spit (the only active spit at the mouth of the estuary) and its future morphologies. It was found that over long term time scales, the estuary will reach a stable morphological state or evolve cyclically between two morphological states, depending on future changes to environmental forcing such as waves and tides. It was also found that littoral transport control has a significant effect on the long term sustainability of the Dawlish Warren sand spit and the estuary as a whole.