Surf Zone Currents Research Articles

MIXING OF THERMAL DISCHARGES IN COASTAL WATERS

Mixing of thermal effluents, being discharged from thermal power plants on coastlines and which head into surface waves was investigated by analyzing extensive field and laboratory data on plume and ocean ambient conditions. Emphasis was given on the effect of waves and surf zone currents on the modifications of plume surface area and vertical temperature profile in the near-field area. The results of this investigation showed that large opposing waves increase the plume surface area, in the vicinity of the outfall, for all cases of tide level and wave direction. Moreover, waves focused cold bottom currents on the discharge outlet and consequently the temperature of the released warm water was decreased at the surface and near the bottom. Wave-induced cross flows decreased the plume cumulative surface area which corresponded to fractional excess temperature ranging between 0.8 and 0.5 normalized values. This decrease was shown to be contingent that there is no interaction between the far-field and near-field plume waters. Gradient of wave momentum flux across surf zone was found to be necessary parameter to characterize the incident wave field.

A QUASI-3D MATHEMATICAL MODEL OF COASTAL MORPHOLOGY

A semi-analytical model of 3D nearshore currents and sediment transport is presented. It describes the tidal motion, the waves, the surfzone currents and the sediment transport in complex coastal areas. The results of a first application, to a well-documented case at the Dutch coast, indicate the potential and the shortcomings of the model. The latter are analysed and suggestions for improvement are given.

Surfzone currents, by D.R. Basco

Discussion d'un art. de Basco paru in Miscellaneous Report, 1982, no 82-7, USAE, Coastal Eng. Res. Center

Surfzone currents

Investigations of surfzone currents including longshore currents, rip currents and nearshore circulations have been conducted worldwide for over sixty years. The motivation is understanding and prediction of sediment transport and coastal pollution processes. This article summarizes a recent state-of-the-art review of theory and experiments conducted since 1967 (Basco, 1982, with Volume II including over 350 annotated bibliographic entries). Emphasis is on the now generally accepted, time-averaged theory based upon radiation stress principles. The newly emerging Boussinesq-type theory, which follows the instantaneous water surface and current variations within each wave period, is also briefly considered. Both approaches assume uniform currents with depth, rely upon empirical wave breaking and surfzone energy empiricism, and suffer from a limited data base for verification. Emphasis throughout is on needed areas for further research.

A SURVEY OF MAN-MADE TIDAL SWIMMING POOLS ALONG THE SOUTH AFRICAN COAST

A large number of man-made tidal swimming pools (two examples are shown in Figure 1) exist along the South African coast. They are usually situated on rocky outcrops in the close vicinity of popular sandy bathing beaches to provide protected bathing conditions in these areas mainly for children and elderly people. Some tidal pools, especially along rocky coast, provide the only safe bathing facilities. Besides affording protection against waves and surfzone currents the pools provide protection from sharks. A large number of tidal pools were built during the early 1950s along the Natal South Coast after the occurrence of a relatively large number of shark attacks on bathers on that coast. The semi-diurnal tide with a range of about 1,5 m along the South African coast makes it possible for pools to be built such that water replenishment can occur during every high-water (approximately every 12 hours) during both neap and spring tide periods. Presently, there exists a great need for more tidal pools as part of the demand for more recreational facilities along the South African coast. However, no information on design criteria could be found in the literature.

COMPARISON OF MODEL AND OBSERVED NEARSHORE CIRCULATION

Results from a two-dimensional numerical model for nearshore circulation induced by waves and wind are compared with observations made during two storms at a beach on Lake Michigan. Model-input data include bathymetry, offshore wave characteristics, wind histories, and local water-level changes. The predicted locations of the breaker zone are in rough accord with those observed during the storms. Data for comparison with model results consist of wave and current observations across the surf zone, especially those acquired by using a towed, instrumented sled. The comparisons show that the model often predicts peak currents near the breaker zone quite well, but underestimates the decay of wave height and the strength of longshore currents across the surf zone. Wave breaking on the bar-trough beach structure prevalent in this study apparently is not well represented by the model. An improved breaking criterion, treatment of breaking waves as traveling bores, and inclusion of horizontal mixing of momentum might add to better simulation of surf-zone currents.