Empirical model of morphodynamic beachface behaviour for low-energy mesotidal environments

Four years of monthly monitoring were carried out on a South Atlantic beach in Spain, in a low-energy mesotidal environment where beaches change slowly, from reflective to dissipative states, following a typical seasonal behaviour. In this work a two-dimensional study of beach morphological Variations has been made, related to the incident wave variations. After applying several morphodynamic parameters to the field data, poor or null representative results were obtained. It was then necessary to design other type of characterisation of the incident wave energy. In this sense, the intertidal normalised beach slope was compared with the erosive potential of the incident waves, expressed as a combination of the dimensionless grain fall velocity parameter (Omega) and the energy density of waves. Median grain size did not vary significantly during the surveys. For this reason, a new parameter, named wave erosivity factor, was introduced by considering the fall velocity of grains as a constant in the Omega parameter. The resulting ratio between normalised beach slope and wave erosivity expresses the equilibrium state of the beach for any given energy level. The departure from the equilibrium curve is largest in the intermediate situations, while at the extremes the points are better adjusted to asymptotic tendencies towards equilibrium: on reflective states, small increases in the wave erosivity will produce important beach changes; on dissipative beaches, important increases in the wave erosivity will not produce significant morphological modifications. The resulting equilibrium curve is presented as a function of the natural range of morphological variation of this beach. (C) 2000 Elsevier Science B.V. All rights reserved.