Applicability of parametric beach morphodynamic state classification on embayed beaches

Use of parametric classification of beach morphodynamic state has been steadily increasing in coastal research, despite identification of several shortcomings of their representativeness as elementary beach descriptors. In this paper, we analyse the parametric classification of beach morphodynamic state in a set of six embayed beaches in southwestern Portugal, exposed to diverse settings, from high-energy (dissipative to intermediate) to low-energy (intermediate to reflective) conditions. Applicability of parametric approaches considered within the dimensionless space defined by Ω (dimensionless fall velocity) and RTR (relative tidal range) according to averaged wave, tide and sediment characteristics of beaches, was assessed in association with a probabilistic analysis, implemented through a Bayesian network model, that considered the full range of wave, tide and sediment conditions at each site. Both parametric approaches were compared to field-based beach state classification implemented using a novel hierarchical framework for beach state analysis. The classification obtained within a probabilistic framework provided an innovative approach for analysis of beach state and mobility, extending the insights on beach morphodynamic behaviour obtained from averaged environmental parameters. Reflective to lower-intermediate beach conditions showed better agreement with parametric approaches, while poor beach state differentiation was achieved for intermediate beach types. Limitations in the application of beach classification models result from (i) an inadequacy of existing beach state types in differentiating beaches, particularly within the intermediate domain and for geologically controlled embayed beaches, and (ii) shortcomings involved in the parametric approaches. The beach state models disregard the major role of geological control in embayed beach morphodynamic behaviour (in modulating beach shape and segmentation, influencing sediment size and availability, causing alongshore variations in the wave field and creating topographically induced nearshore circulations). Existing beach state models are unable to adequately represent the widely variable conditions observed in embayed beaches.