Journal Journalof ofCoastal CoastalResearch Research
SI 64
pg -- pg 180 184
ICS2011 ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Morphodynamic classification of sandy beaches in enclosed basins: the case study of Alimini (Italy) I. Lisi †§, M.G. Molfetta ‡, M.F. Bruno‡, M. Di Risio § and L. Damiani ‡ †Italian National Institute for Environmental Protection and Research, Rome, 00166, Italy
[email protected]
‡Polytechnic of Bari, Bari, 70125,
[email protected],
[email protected],
[email protected]
§ DISAT-LIAM, University of L’Aquila, L’Aquila, 67040, Italy marcello.dirisio@ univaq
ABSTRACT Lisi, I., Molfetta, M.G., Bruno, M.F., Di Risio, M., and Damiani, L., 2011. Morphodynamic classification of sandy beaches in enclosed basins: the case study of Alimini (Italy). Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208 Morphodynamic state of sandy beaches is considered an important issue for a proper coastal management and planning. The Wright and Short (1984) method is frequently used to classify and to predict sandy beach morphodynamic states. However, it has been recognized that the Dean parameter, developed for the Australian coast for breaking conditions, could not provide accurate estimates when it is applied in semi-enclosed basins, such as the Mediterranean Sea. This paper aims at assessing the validity of the Wright and Short method to the study site of Alimini. Alimini is a sandy beach located at the Southern part of the Adriatic Sea (Italy, Apulian Coast). Morphodynamic features from a video monitoring station were used to validate the Wright and Short method based on wave data analysis. On the basis of the case study and past researches a new parameter is proposed. The new parameter seems to be able to successfully describe the morphodynamic state, at least for beach systems similar to the Alimini’s beach. ADDITIONAL INDEX WORDS: morphodynamic state, video monitoring, semi-enclosed basins
INTRODUCTION The study of coastal systems evolution is an important issue for a proper coastal management and planning of sandy beaches. For this reason, it is often recommended the use of morphodynamic classification of the beach system in order to forecast changes in beach morphology, when dealing with coastal management. The classification should be based on the mutual interaction of all hydrodynamic and morphological parameters (i.e. wave climate, mean grain size, beach shape) in order to provide a complete description of the physical processes occurring in coastal areas and affecting the beach response (Van Rijn, 1998; Stive et al., 2002). For natural beaches, the assumption of constant wave forcing is not satisfied, as accretion and erosion conditions alternate over time (Quartel et al., 2008) and the beach response to wave forcing is not instantaneous (Wright et al., 1985). The analysis of the natural morphodynamic responses under different environmental conditions is a crucial point for a proper shore protection and restoration planning. Indeed, a beach is a system that dynamically responds to a wide range of morphologic and hydrodynamic processes (Benedet et al., 2004). If the equilibrium of natural beach is perturbed, the equilibrium will tend to be re-established due to the action of both cross- and long-shore sediment transports. The beach equilibrium state is strongly related to the mean wave climate, whereas storm events induce perturbations mainly of the cross-shore profile (e.g. bars and troughs). As far as the cross-shore evolution is concerned, Dean (1973) has shown that the dimensionless sediment fall velocity (often referred to as the Dean Number) play a key parameter to predict the barred or non-barred nature of the cross-shore equilibrium
profile. However it has been shown (e.g. Chiaia et al., 1990; Kraus et al.,1991; Di Risio et al., 2010) that the Dean Number does not suffice for cross-shore equilibrium profile definition, but also wave steepness has to be taken into account. As far as the morphodynamic state of a beach system is concerned, involving both cross- and long-shore features, past researches proposed different criteria for beach states classification. Usually these classifications are based on the beach morphodynamic features. Indeed, these classifications are referred to a restricted number of beach types (Van Rijn, 1998; Short, 1999), based on the detection of different bars systems in the surf zone (Greenwood and Davidson-Arnott, 1979). The most recent and complete classifications are based on the mutual interactions between the main physical parameters affecting the morphodynamic state of the beach evolution (Wright and Short, 1984; Lipman and Holman, 1991, Klein and Menezes, 2001) related to sedimentology and geomorphology (i.e. sediment types and grain size, seabed slope, presence of bars and dunes) and hydrodynamics (i.e. wave parameters, wave energy, currents). One of the most used method to predict the morphodynamic states of sandy beaches is that proposed by Wright and Short (1984), hereinafter referred to as WS method, based on the Dean Number evaluated for breaking waves:
Ω=
Hb ωT p
(1)
where Hb is the significant breaking wave height, ω is the settling velocity and Tp the incident wave period. The main result of this method, based on field observations for the Australian coasts, is related to the morphodynamic classification in dissipative
Journal of Coastal Research, Special Issue 64, 2011 180
Morphodynamic classification of sandy beaches in enclosed basins.
(Ω>6), intermediate (1
