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 slowl y, from reflective to dissipative states, following a typical seasonal beha viour. In this work a two-dimensional study of beach morphological Variatio ns has been made, related to the incident wave variations. After applying s everal morphodynamic parameters to the field data, poor or null representat ive results were obtained. It was then necessary to design other type of ch aracterisation of the incident wave energy. In this sense, the intertidal n ormalised beach slope was compared with the erosive potential of the incide nt waves, expressed as a combination of the dimensionless grain fall veloci ty parameter (Omega) and the energy density of waves. Median grain size did not vary significantly during the surveys. For this reason, a new paramete r, named wave erosivity factor, was introduced by considering the fall velo city of grains as a constant in the Omega parameter. The resulting ratio be tween normalised beach slope and wave erosivity expresses the equilibrium s tate of the beach for any given energy level. The departure from the equili brium curve is largest in the intermediate situations, while at the extreme s the points are better adjusted to asymptotic tendencies towards equilibri um: on reflective states, small increases in the wave erosivity will produc e important beach changes; on dissipative beaches, important increases in t he 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.