A new approach for handling complex morphologies in hybrid shoreline evolution models

Abstract

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 (BrierSkillScore=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 (BrierSkillScore=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.