Charlotte Obhrai

A LABORATORY STUDY OF OVERTOPPING AND BREACHING OF SHINGLE BARRIER BEACHES

Shingle and mixed sand / shingle beaches are widespread in many parts of the UK and Europe. These beaches are highly efficient and practical forms of coastal protection, however a shingle beach in common with any other type of beach, can suffer erosion and subsequent landward retreat of the shoreline. Consequently over a period of time a beach which was originally of satisfactory dimensions may be reduced to such an extent that it no longer constitutes an acceptable ‘line of defence’. Anticipating this state is clearly important if shingle beaches are to be managed effectively, and landward structures are not to be damaged by flooding. The classical dynamic equilibrium shingle beach profile has been described using the parametric model of Powell (1990). In theory, a dynamic equilibrium profile should develop for any given combination of wave conditions assuming that there is sufficient time and sediment available for the profile to form. This limitation means that the model is not valid for the prediction of overwashing and breaching of shingle barrier beaches. However it has been used to estimate profile performance in these circumstances (Buijs et al., 2005). An empirical framework, based on extensive fieldwork and physical model data was developed to predict the threshold for breaching of shingle barrier beaches (Bradbury, 2000). The field and model data used to develop the model related only to the shingle barrier at Hurst Spit. Bradbury et al (2005) found that model did not work so well when applied to other sites and concluded that use of the model outside the valid predictive range would result in the under prediction of overwashing. Further data was therefore required to test and extend the range of validity of the Bradbury model.

Influence of infiltration on suspended sediment under waves

The effect of percolation through a permeable bed on sediment suspension under regular waves was examined in a laboratory wave tank (28 in x I in x 1 in), using acoustic backscatter sensors to make rapid (3 Hz) suspended sand profile measurements (0.005 m vertical resolution). Waves of 1.7 s period and heights ranging from 0.14 to 0.185 m were used over sand with a D-50 of 255 mum. Infiltration velocities of 0-5.0 x 10(-4) m s(-1) were used. With percolation through an initially flat bed, ripple development was suppressed, particularly at lower wave heights; ripples took longer to form and were more three-dimensional. Suspension was also suppressed. The total suspended load was correlated with Shields number (at the 1% significance level) when the Shields number was modified to take account of both the infiltration [Nielsen, P., 1997. Coastal groundwater dynamics. Proceedings of Coastal Dynamics, American Society of Civil Engineers, pp. 546-555] and ripple steepness [Coastal Eng. (1986) 23]. The ripple steepness was found to be the most important factor relating to the reduction in the total suspended loads. The influence of infiltration on time-averaged concentration profiles over equilibrium ripples was investigated by switching the percolation on and off for 5-min periods. The total suspended loads were reduced by up to 50% with percolation on. These results suggest that fluctuation of the water table and drainage within a beach will affect sediment transport and ripple dynamics, and that for sand of 0.25 mm, percolation will tend to reduce suspension and transport

Field and Laboratory Measurement of Wave Impacts

An extensive program of ongoing field, laboratory, and numerical investigations into the characteristics of wave impacts is described and early results presented. Shock pressures are found to be highly localized, both spatially and temporally, able to propagate into cracks and, in freshwater at least, capable of reaching surprising intensities. Conventional scaling of 1:4 hydraulic model data implies that ocean waves approximately 4m high could generate heads in excess of 1000m. Attention is drawn to the probability that, in practice, such extreme pressures may be constrained by the acoustic (water hammer) limit. The parameter map (Allstop & Kortenhaus, 2001) used for predicting impacts is shown not to be entirely reliable and preliminary results of the numerical models are discussed.

FIELD AND LARGE SCALE MODEL TESTS OF WAVE IMPACT PRESSURE PROPAGATION INTO CRACKS

Within a large & full scale study on wave impact induced pressures on coastal structures (BWIMCOST) an investigation of impact pressure propagation into structure cracks and fissures was carried out. The mechanism, which is held responsible for localized damage to existing blockwork breakwaters, had previously been verified in small scale model tests and a numerical model had been developed. The current investigation is the first which describes the effect at full scale, with recorded pressures of up to 199 kPa found within the cracks. The experimental results are related to their possible impact on coastal structural integrity.

Suspension by regular and groupy waves over bedforms in a large wave flume (SISTEX99)

Suspended sand concentrations and bedforms under waves were measured in the controlled environment of a large wave flume. Three suspension conditions are discussed here; those occurring with regular (monochromatic) waves of height 0.55m over anorbital ripples, regular waves 1.0m high over orbital bedforms, and repeating wave groups (with a significant wave height of 0.6m) also over orbital-scale features. In all cases the wave-to-wave variability in suspended load was high (∼30%). Patterns of suspension were dependent on the bedform type and on instrument location relative to the bedform. Regular waves suspended an order of magnitude more sediment than groupy waves with a similar significant wave height illustrating,the importance of sequences of high waves in pumping-up sediment concentration into the water column.

Numerical Simulations of Regular and Irregular Wave Forces on a Horizontal Semi-Submerged Cylinder

Two-dimensional (2D) numerical simulations have been performed using OpenFOAM (an open source CFD software package [1]) and waves2Foam (an OpenFOAM based add-on library for wave generations and absorption [2]) to investigate free surface waves past one fixed horizontally semi-submerged cylinder. The 2-D simulations are carried out by solving NavierStokes equations which are discretized based on finite volume method (FVM). Volume of Fluid (VOF) method is employed to capture the free surface in the numerical wave tank. Validation studies have been performed by comparing the numerical results of Stokes first-order wave past a semi-submerged circular cylinder with the published experimental data at different incident wave properties. The numerical results are in good agreement with the experimental data. Subsequently, regular and irregular waves past semi-submerged cylinder at different wave heights and the wave lengths are computed numerically to investigate the effect of the wave height and wave length on wave-structure interaction. The numerical results for irregular ∗Address all correspondence to this author. waves are compared with those induced by regular waves.