Two-dimensional Beach Profile Research Articles

A Cross-Shore Beach Profile Evolution Model

Developing an accurate and reliable time-averaged beach profile evolution model under storm and nonstorm conditions is a challenging task. Over the last few decades, a number of beach deformation models have been developed under limited experimental conditions and uncertainties, and sometimes they required a long computation time. It is quite evident that a large amount of wave, current, sediment and beach profile data is available today. The present study leads to the development of a simple two-dimensional beach profile evolution model with on-offshore sand bar formation under nonstorm and storm conditions based on the time-averaged suspended sediment concentration models of Jayaratne and Shibayama [Jayaratne, M. P. R. and Shibayama, T. [2007] “Suspended sediment concentration on beaches under three different mechanisms,” Coastal Eng. J., JSCE 49(4), 357–392.] and Jayaratne et al. [Jayaratne, M. P. R., Sritharan, S. and Shibayama, T. [2011] “Examination of the suspended sediment concentration formulae using full-scale rippled bed and sheet flow data,” Coastal Eng. J., JSCE 53(4), 451–489.]. These models were formulated for computing sediment concentration in and outside the surf zone under three different mechanisms: (1) suspension due to turbulent motion over sand ripples, (2) suspension from sheet flow layer and (3) suspension due to turbulent motion under breaking waves. The suspended load is calculated by the product of time-averaged sediment concentration and undertow velocity from edge of the wave boundary layer to wave trough and mass transport velocity from wave trough to crest (bore-like wave region). Sediment transport in wave boundary layer is computed from the modified Watanabe [Watanabe, A. [1982] “Numerical model of nearshore currents and beach deformation model,” Coastal Eng. Jpn., JSCE 25, 147–161.] model. Rattanapitikon and Shibayama [Rattanapitikon, W. and Shibayama, T. [1998] “Energy dissipation model for regular and irregular breaking waves,” Coastal Eng. J., JSCE 40(4), 327–346.] wave model is used to calculate the average rate of energy dissipation due to wave breaking. The beach deformation is calculated from the conservation of sediment mass while the avalanching concept of Larson and Kraus [Larson, M. and Kraus, N. C. [1989] SBEACH: Numerical model for simulating storm induced beach change, Report 1, Technical Report CERC-89-9, US Army Eng. Water. Exp. Station.] is used to re-distribute the sediment mass in neighboring grids for a steady solution. Published field-scale experimental and natural beach profiles from five high-quality data sources from 1983–2009 [Kajima et al., 1983; Kraus and Larson, 1988; Port and Airport Research Institute, Japan, 2005, 2009; Hasan and Takewaka, 2007, 2009; Ruessink et al., 2007] are used to verify the performance of the proposed numerical model. The key feature in this process-based model is that it takes about a couple of minutes to simulate beach profiles of a 2–3 days storm qualitatively at a fairly satisfactory level using a standard personal computer. It is found that the present numerical predictions are not better than the null hypothesis as the model is in a stage of ongoing development. Therefore, it is believed that the final model is often of more value to a practical coastal engineer than a very detailed study of hydrodynamics and sediment transport study, however an incorporation of swash dynamics, more precise evaluation of offshore sand bar formation and continuation to a longer time scale with precise beach deformation are recommended as the next stage of the model.

Field Verification of a Computational Fluid Dynamics Model for Wave Transformation and Breaking in the Surf Zone

The commercial computational fluid dynamics model FLOW-3D (Flow Science, Inc., Santa Fe, N.M.) is used to simulate two-dimensional wave transformation and breaking across a naturally barred beach profile. Fine scale pressures and velocities are computed for a 35.5 min period over a two-dimensional beach profile measured during the 1990 Delilah field experiment. The model is driven by observed wave spectra obtained in 8 m water depth, and results compared with a cross-shore array of pressure sensors and current meters extending from near the shoreline to beyond the surf zone and the spatial distribution of wave breaking patterns obtained from video data. In the calculations, wave breaking is a natural consequence of the fluid dynamics and does not require the use of empirical formulations or breaking criteria. Good agreement between modeled and observed wave height transformation, mean cross-shore flow, and wave breaking variability suggests that the model can be used as a numerical laboratory to study the wave breaking and dissipation process in detail, and perhaps lead to improved parametrizations for more computationally efficient numerical models.

EXPERIMENTAL STUDY ON ON-OFFSHORE SEDIMENT TRANSPORT OF ACCRETIVE BEACH

In treating coastal proceses, sediment transport is usually divided into along-shore and on-offshore components. It is believed that the on-offshore component has a prominant connection with short-term profile changes, observed during storm wave climates. Obviously its shift of sand plays a very vital role in shoreline migration. In other words, the beach profile has great bearing on coastal phenomena related to on-offshore sediment transport. As we know, there have been many studies on this kind of sediment transport rate, and considerable amount of knowledge on this problem has been accumulated so far. Yet it seems that we are still far from a reliable formulus to estimate the beach profile changes. The reason why is due to the complexity of mechanics of sediment transport. Therefore, the aim of this study is to examine experimentally the mechanism between onoffshore sediment transport and the deformation processes of twodimensional beach profile. Then , a predictive model of the temporal and spatial distribution of net on-offshore sediment transport based on two-dimensional beach profiles and an equation of continuity of sediment transport is proposed. Various parameters of net on-offshore sediment transport in this model are discussed also.

LABORATORY STUDY ON TWO-DIMENSIONAL BEACH TRANSFORMATION DUE TO IRREGULAR WAVES

In the present study, effects of irregular waves on two-dimensional beach transformation and related phenomena were investigated through a series of laboratory experiments. Attempts were made to determine a representative wave of irregular wave trains which controlled individual phenomenon related to the two-dimensional beach profile change. It was found that the representative wave is different for each phenomenon. For the macroscopic beach profile change, it is the mean wave which represents whole incident waves. On the other hand, some of microscopic phenomena, such as initiation of sand movement and sand ripple formation, are controlled by larger waves in the wave train selectively, of which representative wave is the significant wave.

EXPERIMENTS ON BEACH PROFILE CHANGE WITH A LARGE WAVE FLUME

Two-dimensional beach profile changes were investigated with a newly constructed prototype-scale wave flume. The flume is 205 m long, 3.4 m wide and 6 m deep. Sand of two grain sizes was used in the experiments. Analysis of the results was made through use of the parameter C, introduced by Sunamura and Horikawa (1974) to classify beaches as either erosional and accretionary. Beach profile changes obtained in the flume were similar to those in the prototype (field). Net sand transport rate distributions were classified into five types, two of which do not seem to have been observed in laboratory (smallscale) experiments. A simple model describing the five types was developed for evaluating two-dimensional beach profile changes.

SEDIMENT TRANSPORT AND BEACH TRANSFORMATION

Laboratory and field investigations were performed in order to formulate a predictive model of two-dimensional beach profile change. The observed transport was classified into six types, and transport formulas were deduced for each type based on a microscale description of sediment movement caused by wave action. A numerical model of two-dimensional beach transformation was then developed. Beach profile changes calculated with the model were then compared with the laboratory results. The model was found to give reasonable results except in the vicinity of the wave plunging point. The sediment transport calculation is based on a sinusoidal velocity profile. The model appeares to give good results as long as the wave motion can be reasonably approximated by linear wave theory.

AN HEURISTIC MODEL OF WAVE HEIGHT DISTRIBUTION IN SURF ZONE

Until now, almost every study on coastal processes has considered the basic type of two-dimensional beach profile as being of constant slope. However, as our knowledge on this problem advances, we realize the importance of the influences of the bottom configuration on the hydrodynamic phenomena in a given area. Figure 1 shows a recent experimental result on the longshore current profile on a step type beach. ( Here the step type beaches are defined as those which have a step in the bottom profile, whether the beaches are of accretion type or not.)