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• A new hybrid 2D/one-line shoreline modelling approach is proposed. • The new hybrid approach accounts for alongshore variations in closure depth. • The new hybrid approach improves shoreline predictions in a complex morphology. • Closure depth specifications significantly affect shoreline prediction accuracy. Hybrid shoreline evolution models are being increasingly used to inform the management of sandy coastal systems. These models generally apply a two-dimensional physics-driven approach to calculate littoral drift and the one-line theory to update the shoreline morphology. As per the one-line theory, the calculated littoral drift is uniformly distributed over the active coastal profile. A key challenge facing the application of hybrid models is that they fail to consider complex morphologies when updating the shoreline morphology. Complex morphologies are defined herein by non-parallel depth contours to the shoreline, a characteristic feature of many vulnerable sandy coastal systems. This study illustrates the deficiency of the current hybrid 2D/one-line approach when applied to hindcast shoreline change from 2014 to 2016 along a sandy coast with fringing reefs in Puerto Rico. Results show that the hybrid approach is unable to predict observed shoreline change (B r i e r S k i l l S c o r e = 0) as a result of the one-line theory assumption of a spatially constant closure depth, which defines the offshore extent of significant cross-shore sediment transport. To address this, a new hybrid approach is developed for application in complex morphologies that accounts for alongshore variations in the closure depth. In the new hybrid approach, the coast is divided into segments according to the alongshore distribution of fringing reef substrate with each segment having a different closure depth specified based on their underlying bed morphology. Results show that this new hybrid approach enables a more realistic simulation (B r i e r S k i l l S c o r e = 0.4) of observed shoreline change. This finding explicitly demonstrates that the closure depth is an important variable in shoreline evolution models. It also implicitly indicates that we are likely to better simulate shoreline evolution in complex morphologies over timescales of concern in coastal management by allowing the closure depth to vary alongshore in hybrid models. [Display omitted] [ABSTRACT FROM AUTHOR]

Hybrid 2D/one-line shoreline models are becoming increasingly applied over the mesoscale (101–102 years; 101–102 km) to inform coastal management. These models typically apply the one-line theory to simulate changes in shoreline morphology based on littoral drift gradients calculated from a 2DH coupled wave, flow, and sediment transport model. However, the key boundary conditions needed to effectively apply hybrid 2D/one-line models and their applicability beyond simple planform morphologies are uncertain, which can potentially comprise coastal management decisions. To address these uncertainties, an extensive numerical modelling campaign is carried out to both assess the sensitivity and calibrate an advanced hybrid 2D/one-line model (MIKE21) against six variables in three different sandy coastal system morphologies: (a) a simple planform morphology with a gentle sloping profile, (b) a simple planform morphology with a steep sloping profile, and (c) a complex planform morphology. The six variables considered include nearshore discretisation, bathymetry, bed friction, sand grain diameter, sand porosity, sediment grading, and the weir coefficient of hard defence structures. Five key conclusions are derived from the sensitivity testing and calibration results. First , the optimal boundary conditions for modelling shoreline evolution vary according to coastal geomorphology and processes. Second , specifying boundary conditions within physically realistic ranges does not guarantee reliable shoreline evolution predictions. Third , nearshore discretisation should be treated as a typical calibration parameter as (a) the finest discretisation does not guarantee the most accurate predictions, and (b) defining a discretisation based on process length scales also does not guarantee reliable predictions. Fourth , hybrid 2D/one-line models are not valid for application in complex planform morphologies plausibly because of the one-line theory assumption of a spatially invariable closure depth. Fifth , hybrid 2D/one-line models have limited applicability in simple planform morphologies where the active beach profile is subject to direct human modification, plausibly due to the one-line theory assumption of a constant time-averaged coastal profile form. These findings provide key theoretical insights into the drivers of shoreline evolution in sandy coastal systems, which have practical implications for refining the continued application of shoreline evolution models. • The hybrid 2D/one-line shoreline modelling approach is tested in three coastal morphologies. • Optimal boundary conditions correspond to observed coastal morphology. • The nearshore discretisation in shoreline models should be treated as a calibration parameter. • The hybrid 2D/one-line shoreline modelling approach is limited to simple planform morphologies. [ABSTRACT FROM AUTHOR]