A MORPHOLOGICAL MODEL OF THE BEACH PROFILE INTEGRATING WAVE AND TIDAL INFLUENCES

Abstract

Meso- and macrotidal beaches constitute a significant proportion of the world's beaches. However, by far, they have been less studied and understood than tideless beaches. Even though in recent times several local studies have addressed this problem, few attempts to model tidal beaches exist. A morphological model capable of predicting the beach profile behaviour under different wave and tide conditions is proposed. It is based on the concept of the two-section equilibrium beach profile, and has been validated with field and laboratory data. This is achieved by means of two parameters: the modal tidal range and the dimensionless fall velocity. Tide is considered a local variable whose principal effect is the lengthening of the intertidal profile. The greater the tidal range, the wider the intertidal profile, here defined as the surf profile. The dimensionless fall velocity defines the transition from dissipative to reflective situations in beaches of any given tidal range. Wave height is the controlling parameter in seasonal beach changes: as the wave height decreases, the beach profile changes from erosive to accumulative. These morphological changes in the surf and shoaling sections of the profile occur in the opposite direction. Whilst in the surf profile the slope of the upper part of this section becomes steeper and the concavity of whole section increases; in the shoaling profile, the upper part flattens resulting in a less concave section. In this transition, the slope break between surf and shoaling profiles, here defined as the discontinuity point, becomes smoother and difficult to identify. The main morphological parameter of the model, xo, describes the length of the surf profile. This parameter is capable of expressing slightly different variability in micro-, meso- and macrotidal beaches. In microtidal beaches, the length of the surf profile decreases proportionally to the dimensionless fall velocity; in meso- and macrotidal beaches xo decreases from the dissipative to the intermediate state, but increases from intermediate to reflective. This results from the flattening of the lower part of the surf profile in the reflective case, where a small change in tidal range generates a high stretching of the surf profile. This beach morphological model is presented as a framework to understand the first-order behaviour of beaches under the action of waves and tides. This becomes a useful and easy-to-apply tool in coastal management and prediction of equilibrium beach profile under diverse conditions.