Robert Jak McCarroll, Gerd Masselink, Nieves G. Valiente, Timothy Scott, Mark Wiggins, Josie-Alice Kirby, Mark Davidson

Predicting change to global shorelines presents an increasing challenge as sea-level rise (SLR) accelerates. Many shoreline prediction models use variations of the ‘Bruun-rule’, failing to account for relevant processes and morphologic complexity. To address this, we introduce a simple rules-based model (ShoreTrans) designed for complex, real-world profiles that predicts change across a wide variety of sand, gravel, rock and engineered coasts at a temporal scale of 10–100 years, accounting for shoreline trends as well as variability. The model translates 2D cross-sections of the shoreface, using the surveyed profile, then integrates these cross-shore shoreface changes across multiple alongshore profiles to assess a simplified 3D sediment budget. Uncertainty is accounted for using a probabilistic distribution for model inputs and Monte Carlo simulations. The model accounts for: (1) dune encroachment/accretion; (2) barrier rollback; (3) non-erodible layers; (4) seawalls; (5) lower shoreface transport; (6) cross-shore storm erosion; (7) alongshore rotation; and (8) other sources and sinks. We apply the model to two macrotidal UK embayments: Perranporth (sandy, dissipative, cross-shore dominant transport) and Start Bay (gravel, reflective, bi-directional alongshore dominant), then use idealised models to investigate the relative importance of forcing controls on shoreline recession and beach width. For the dissipative sandy site, the primary modes of coastal change are predicted to be short-term storm erosion and SLR translation. By contrast, for the reflective gravel site, the primary mode is multi-decadal longshore sediment flux, while short-term alongshore rotation and SLR translation are secondary. Relative to the new model, the Bruun-rule under-predicts shoreline recession in front of cliffs, seawalls and for low barriers that rollback, and over-predicts where large erodible dunes are present. Beaches in front of seawalls and cliffs are predicted to shrink, such that narrow beaches (<50 m width) may disappear under 1-m SLR. ShoreTrans is easily transferable to many coastal environments and will provide a useful tool for coastal practitioners to make rapid estimates of future coastal change to complex shorelines.