Observation and Prediction of Three-Dimensional

8 Three-dimensional beach features such as crescentic sandbars and rip channels influence beach 9 response to, and recovery from, storm waves, as well as significantly affecting the safety and 10 amenity provided by the surf-zone for beach water-users. In this contribution temporal variations in 11 subtidal and intertidal beach three-dimensionality are observed at a high-energy macrotidal beach, 12 and a simple equilibrium model is developed to predict the changes over multi-year timescales. A 13 dataset of 5.5 years of quasi-weekly bar measurements, and quasi-monthly intertidal surveys from 14 Perranporth beach (Cornwall, UK) were used to quantify seasonal to inter-annual changes in three15 dimensionality. The three-dimensionality of the outer bar displayed significant annual periodicity, 16 with annual minima and maxima occurring in winter and spring, respectively. The lower intertidal 17 beach displayed a similar periodicity, but developed three-dimensionality 1-4 months before the 18 outer bar. The model predicts increases or decreases in the scale of three-dimensional features by 19 examining the disparity between instantaneous wave conditions and a temporally varying 20 equilibrium wave condition. A tidally-modulated wave power term determines the rate of 21 morphological change. Negative feedback was found to be an important process governing the 22 changes in three-dimensionality; while free morphological behaviour may drive three-dimensional 23 growth, negative feedback exerts stability in the system, making it inherently predictable using a 24 temporally varying equilibrium value. The model explained 42% and 61% of the overall variability in 25 outer bar and lower beach three-dimensionality, respectively. It skilfully predicted changes outside 26 the training data range, during the most energetic 8-week period of waves measured in the last 65 27 years off SW England, in winter 2013/14. The model outperformed a simple baseline model (a linear 28 fit), as well as a comparable linearized feedback model from the literature, providing the first long29 term (multi-year) predictions of seasonal to inter-annual beach three-dimensionality for a macrotidal 30