Cross-shore Sediment Transport Research Articles

SEDIMENT TRANSPORT ON NEARLY-PRISMATIC BEACHES

Recent progress in the quantitative modelling of the undertow has stimulated the modelling of cross-shore sediment transport. More so than before it seems now possible to attempt the dynamic modelling of beach profile development. Also, integration of dynamic cross-shore sediment transport formulations in horizontally two-dimensional models for watermotion and sediment transport has recently been suggested. This seems to be a first step of integrating depth-averaged 2DH-modelling with 2DV-profile- modelling. Here an overview is given of these developments and the understanding gained sofar of the several current systems and the induced sediment transport and morphology that are found in the situation of random waves normally and obliquely incident on beaches which vary not or only slowly alongshore.

ESTIMATES OP GROSS-SHORE BEDLOAD AND BED CHANGES

Four series of tracer studies employing radioisotopic sand, tracked remotely from land, were carried out at the IBW PAN's coastal research station at Lubiatowo. Incipient motion of single grains with velocity Vcr1 has agreed with Komar and Miller's (1974) formula, and has been 5...8 times smaller than voro, the threshold velocity of water inducing bulk motion_of sediment. The ratio of average nearbed water velocity, v, to_the effective speed of tracer sand, in the inshore zone, va, is estimated about 100...1,000. The cross-shore sediment transport rates measured are ten times smaller than those predicted by Pruszak's (1987) energetics formula, the discrepancy being due to the real field phenomena of mixing and overburdening in a thick bed layer. Our wave flume and wave tank studies with movable bed point to the importance of initial slope in shore evolution. The laboratory findings on bed variability are extended by our 1987 field data obtained with an ultrasonic setup. Intensive cyclic depth changes are acoorapanied by varying transport rates computed from continuity equation.

TWO-DIMENSIONAL EMPIRICAL EIGENFUNCTION MODEL FOR THE ANALYSIS AND PREDICTION OF BEACH PROFILE CHANGES

A two-dimensional empirical eigenfunction model is proposed for the analysis and the prediction of beach profile change due to longshore and cross-shore sediment transports. Beach profile data from Redhill coast, Taiwan, measured every two months at 150 meters interval along the detached breakwaters are analyzed and the relative importance from two directions is investigated. Furthermore, by employing the method of Markov process and linear regression, a prediction model is formulated which takes into account the effect of breaking waves, bottom sediment and radiation stress of waves. This 2-D model is shown to be effective in the analysis and the prediction of beach changes near the coastal structures.

A MODEL FOR CROSS-SHORE SEDIMENT TRANSPORT

A model for cross-shore sediment transport due to random waves is described which adopts a vertically integrated transport description for sheetflow situations. The formulation of the transport as a function of the instantaneous velocity field is based on the approach of Bailard (1981). This approach assumes in essence that the instantaneous transport is proportional to some power of the instantaneous near-bottom velocity. Implementation of this transport description in a time-dependent model requires a formulation of the time-mean and some low order moments of the near-bottom velocity field. An initial formulation based on a monochromatic, second order Stokes wave representation is presented. The model is checked on the basis of both field and laboratory data. Some consequences for further study are indicated.

Numerical simulation of time-dependent beach and dune erosion

A computational procedure is developed for predicting the time-dependent, two-dimensional beach and dune erosion during severe storms due to elevated water levels and waves. The model employs the equation of sediment continuity and a dynamic equation governing the cross-shore sediment transport due to a disequilibrium of wave energy dissipation levels. These equations are solved numerically by an implicit, double-sweep procedure to determine the change in position of elevation contours in the profile. Given sufficient time, the profile will evolve to a form where the depth, h, in the surf zone is related to the distance seaward of the waterline by the relationship: h = Ax 2 3 , which is consistent with many natural profiles and in which A depends on sediment characteristics. The model is verified qualitatively and quantitatively through application to several idealized cases and through a preliminary simulation of erosion during Hurricane Eloise. In general, the time scales for shoreline response were found to be quite long relative to natural storm systems and erosion in the early response stages was found to be sensitive to storm surge height, but much less sensitive to wave height. The model response characteristics for simulation of erosion due to time-varying storm conditions show a lag between the maximum storm surge elevation and maximum erosion with the maximum erosion rate occurring at the time of the peak surge. For the simulated erosion due to Hurricane Eloise, reasonable agreement was found between the post-hurricane dune profiles and those calculated. However, the eroded volumes were in better agreement than the profile forms as the steepening of the natural dune profiles was not reproduced in the model.

FIELD INVESTIGATION OF LONGSHORE TRANSPORT DISTRIBUTION

Following a change in wave direction, the active contours in an idealized pocket beach respond by rotating such that they approach a perpendicular orientation relative to the incoming wave rays. Assuming that cross-shore sediment transport does not contribute to this contour rotation, and that the contours are in the early stages of this equilibration process, the amount of contour rotation can be interpreted as the cross-shore distribution of the longshore sediment transport. As part of the Nearshore Sediment Transport Study, detailed nearshore profile measurements were conducted at Santa Barbara, California. Twenty-two of these profile lines were located on Leadbetter Beach, which is a quasi-pocket beach. To explore the concept described above, two of the nine intersurvey periods were selected due to their strong indications of wave direction change. Analysis of these data sets yielded two estimates of cross-shore distribution of longshore sediment transport which were compared with those presented by Komar, Fulford and Tsuchiya. Although these three distributions differ significantly, the effect of the tidal variations is to "smear" the differences in the inferred distributions as evident in the contour displacements. It was found that none of the relationships for longshore transport distribution predicted the amount of transport inferred in water depths greater than one meter. It is possible, especially for one of the intersurvey periods that the changes in contour locations were so extreme that substantial crossshore sediment transport was induced and would be interpreted as longshore transport occurring in water depths greater than had actually occurred. The method introduced here should be useful in other field and laboratory programs to investigate the cross-shore distribution of longshore sediment transport.

A Coastal Morphodynamic Model For Cross-shore Sediment Transport

A numerical model to represent the mechanisms of cross-shore sediment transport induced by wave and storm events has been established. The model consists of three sub-models (wave transformation, morphodynamic movement, and sedimentation) capable of representing the transport and redistribution of four grain sizes over a coastal area. It is designed to simulate the morphological evolution caused by crossshore transport. This model can thus be superimposed on existing long-shore sediment transport models. An application to a well-documented South Australian coastline suggests that the model is able to reproduce erosion and accretion trends observed in nature. The short computer run-time permits efficient model calibration and verification.

On iron, and some improvements in its manufacture

The coastal dynamics along the sandy beaches in the Algiers bay is strongly influenced by the hydrodynamic agents, morphological factors, human activities, societal infrastructure and presence of hard structures, which disrupts their hydro-sedimentary balance. The present work contributes to the knowledge of hydrodynamic and morpho-sedimentary forcing mechanisms at sandy beaches by focusing on the impact of extreme sea-weather events of the two most contrasting periods of the year (winter and summer) on coastal processes. Two beaches in the eastern part of Algiers bay were studied by combining field measurements of currents and sediment flux with numerical models (MIKE 21/3 Coupled Model FM). The model validation was done based on the measured nearshore current and sediment transport during low to moderate energy swell conditions with current meter ‘Global water Fp101’ and ‘Krauss sediment traps’. Time series of wind and wave established by Infoplaza Marine weather database were used as input at the open boundaries of the model domain. The results showed a reversal of the dominant direction of the longshore currents, and a very high prevalence of cross-shore sediment transport over longshore sediment transport (LST) due to the coarse grain size of the sediment (very coarse sand). The results indicated that, waves coming from the west prevail in winter have an average wave height varies from 1.5 to 2.1 m, driven a longshore current of 0.17–0.27 m/s, with an estimated seasonal cross-shore sediment transport between 1530 and 5540 m3/year/m. During the summer season, waves approaching from the northeast dominate with an average of Hs that does not exceed 1 m, these waves generate a rip current equal to 0.24 m/s and this current can generate a cross-shore sediment flux in the order of 4000 m3/year/m. The results also reveal the development of morphological landforms represented in the submerged longshore bars at the shallow water depending largely on the hydrodynamic conditions and the cross-shore flux between the land/sea interfaces.