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ABSTRACT The long term evolution of the Molise coast (South-East Italy) is analyzed using the Littoral Drift Rose (LDR) concept, coupled to the GENESIS numerical model, which is based on the one line contour equation. LDR has been used to define a single, climate-equivalent, time-invariant sea state, which has been supposed to entirely rule the shoreline changes. Particular attention has been drawn to a 5 km long reach of coast, just South to the mouth of the Trigno river. Results of the analysis indicate the adopted procedure, even if extremely simplified, can explain nearly 90% of the observed shoreline trend.
NATURE OF THE PROBLEM Analysis and prediction of long-term evolution of shorelines, are challenges engineers and scientists have recurrently to face, especially when structural or non structural protection measures exist or are to be planned. (e.g., Hanson and Kraus, 1990, Wamsley and Ahrens, 2003). This kind of problems are usually addresses via the so called one contour line numerical models (OCL, e.g. Hanson and Kraus 1989), which assume the submerged beach to follow rigidly the shoreline movements, and account in a simplified manner the main wave-structure interaction processes, including wave diffraction (Penny and Price, 1952, Goda etal., 1978), wave transmission (Van der Meer et al., 2005, Buccino et al., 2018) and, more rarely, wave reflection (Zanuttigh et al., 2008, Buccino et al., 2018).
Wave climate is indeed a leading input to OCL; however, it is often reduced to a single sea-state (or, at most, a restricted set of sea-states), which is somehow supposed to rule the entire shoreline evolution. It is worth highlighting that this equivalent wave approach (EWA) is widely employed in the engineering practice, although some literature studies (e.g. Silvester, 1984) seem to indicate it may be not completely appropriate.
Now, in view of the above discussion, two issues deserve to be investigated; one is the most convenient way to select the climate-equivalent sea state, given the lack of widely accepted procedures. The other is assessing at which extent EWA could explain the long term evolution of shorelines, when used to force a one contour line model. This paper attempts responding the questions above by examining the case of Molise coast, South-East Italy; fronting the Adriatic Sea, the latter is in fact inherently subject to a bimodal wave climate, which may render the equivalent wave approach questionable in se.
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