Coastal Engineering Journal

This paper investigates the wave-induced flow field and water particle movement inside and around a vertical permeable structure, which is affected by the interaction between water waves and porous media. A three-dimensional fully nonlinear numerical model is developed to study the interaction between water waves and permeable structures. Modified resistance terms inside the equations of motion are proposed, which are able to consider the size of porous material directly inside the governing equations. It is understood that the wave-induced flow field and water particle movement inside and around the permeable structure are strongly influenced by the wave period or relative width of the permeable structure (structure width to wavelength ratio). Other parameters such as porous material size, porosity, and resistance coefficients affect the flow field. However, their degree of influence differs depending on the wave period.

A new breaker height formula is developed based on a re-analysis of available laboratory data. The analysis shows that the breaking wave steepness is mainly governed by its deepwater wave steepness. The power form could be used to fit the relationship between breaking wave steepness and its deepwater wave steepness. This finding is new. Although the breaker height has been a subject of study for a century, no researcher has tried to relate the breaking wave steepness to the deepwater wave steepness. Bottom slope effect is included explicitly into the present formula. The present formula and existing formulas are examined and compared with a large number and wide range of published laboratory data (695 cases collected from 26 sources). Overall, the present formula gives very good predictions over a wide range of experimental conditions and is better than existing formulas.

An approach by which the stochastic properties of the bed shear stresses can be derived from the irregular nonlinear wave motion outside the laminar bottom boundary layer is presented. It is demonstrated how bed friction formulas valid for regular second order Stokes waves can be used to find the cumulative distribution function of individual shear stress maxima for nonlinear irregular waves. The friction factor for nonlinear random waves is also determined. An example is given, and an extension to nonlinear random waves plus current flow is also suggested.

The effects on the sediment transport rate due to inclusion of seepage process in the mass of the beach sand in the swash zone are investigated numerically. A higher order Boussinesq model for breaking and non-breaking waves is extended in the swash zone and is coupled with a porous flow model which is based on the shallow water equations. To take into account of the infliltration-exfiltration processes in the sediment transport, a modified Shields parameter is used to account the effects of stabilization or destabilization of the surface layers and boundary layer thickening or thinning. The numerical results indicated that for fine sediments, the time-averaged onshore transport is decreased, while the offshore transport is increased. For coarser sediments the time-averaged onshore transport is increased, while the offshore transport is decreased. The above results are in agreement with field measurements as well as with experimental data. The exact value of the critical grain size changeover point is no straightforward to estimate. Present numerical experiments indicate that the value of the critical grain size changeover point appears to lie somewhere between 0.4 and 0.6 mm. Relatively small changes in the estimation of the friction factor might change the direction of the apparent influence of infiltration-exfiltration.

An assumption of collinear straight coastlines in the far field, traditionally used in wave models for harbors, is a limitation to wave simulations in the domain extending to infinity, since this assumption is invalid for most real coastlines. By alleviating this limitation based on the geometric-optics approximation, functions of these coastal wave-deformation models are extended to be able to predict wave patterns around the semi-infinite breakwater and convex and concave coasts, such as bulkheads with discontinuous alignment. Mapped infinite elements are also formulated for exterior wave problems. A basic concept in developing is that the true decaying property of scattered waves, i.e. with the modes of r^-1/2, r^-3/2,…, where r is the radial distance, must be represented directly in infinite elements. To do so, a complementary element is introduced to the infinite element. Infinite element integrals can be expressed explicitly because of analytical integrability due to weak singularity in the infinite mapping. Three test problems for coastal wave-deformation models are solved combined with the cubic infinite element with r^-1/2 and r^-3/2 decays; this combination technique is found to be effective and accurate for problems of wave scattering.

This paper presents an application of the neural network (NN) model for forecasting and filtering problems. Instead of using the gradient descent method as used by the standard back-propagation network, the present study adopted the Levenberg-Marquardt algorithm. It was shown that the Levenberg-Marquardt algorithm tends to reach termination criteria more quickly making the searches employing the full model less time consuming. In the first part of forecasting problem, multilayer feedforward (MLFF) network was constructed by trials to forecast the tidal-level variations at the Chao Phraya river mouth in Thailand. Unlike the well-known conventional harmonic analysis, the neural network model uses a set of previous data for learning and then forecasting directly the time series of tidal levels. It was found that lead time of 1 to 24 hourly tidal levels can be successfully predicted using only a short-time hourly learning data.

In the second part of filtering problem, the MLFF network was also used to establish a stage-discharge relationship for tidal river. Comparisons were made among conventional methods, i.e. by using multivariate functions (Linear and power models) and the NN model. The NN model was found to be able to model the transfer function too much higher accuracy than multiple regression analysis. The root mean square errors (RMSE) and the mean relative error (MRE) for the NN model are about 8 cm and 6.4% whereas the same errors for multiple regression analysis are 16 cm and 12%, respectively. The efficiency index for the linear, power, and NN models is 0.4, 0.7, and 0.9, respectively. In addition, the obtained stage-discharge relationship by the NN model can indicate reasonably important behavior of the tidal influences, i.e. there are stronger influences of tide during the dry period than the flood period.

The most popular coastal diffusion modelling technique is the traditional particle tracking method which can only model Fickian diffusion. However, in reality, non-Fickian diffusion dispersion in coastal surface waters has been observed by some researchers. A new Particle Tracking technique is introduced in this article that generates non-Fickian diffusion by employing fractional Brownian motion (fBm). The authors found that for coastal surface waters, the Hurst exponent H is not restricted to the range 0 ~ 1. A newly developed Particle Tracking method that generates non-Fickian diffusion by employing accelerated fractional Brownian motion (AfBm) has been first studied and introduced. Numerical techniques to model the non-Fickian spreading observed in the real coastal data have been successfully developed and the simulation results are encouraging when compared with the observed data and the results of conventional dispersion modelling.