Shoreline retreat and free overhang in overdeveloped tombolo behind detached breakwaters: Empirical, semi-analytical and numerical approach

Multiple detached breakwaters with narrow gaps have been constructed to dissipate wave energy to a beach and to promote salient or tombolo in the lee for shore protection since the 1960s. Although geometric parameters (breakwater length and distance offshore) are operative for the preliminary design of these structures, neither shoreline retreat from its initial position in the middle of each compartment was assessed, empirically or semi-analytically, nor was the tombolo's overhang length along the structure quantified, if it's overdeveloped. In this study, prototype data are collected to derive empirical relationship for a salient and tombolo geometry in static equilibrium. A semi-analytical approach is then used to derive algebraic equation for estimating shoreline retreat from an initial position, for three stages of shoreline planform, from a salient to initial tombolo and to an over-developed tombolo with free overhang. The results of this new approach are verified using empirical data and an approximate equation for shoreline retreat derived from the numerical results extended from the results of XBeach model. This also enables the backtracking to the existing shoreline, where extra hard protection is constructed in the middle of a compartment between consecutive DBWs, to its initial position. To mitigate the scenario of erosion, a preferred alternative combining nourishment with parabolic model is demonstrated to produce a stable beach with adequate beach buffer.