Empirical modelling of beach evolution: implementation of coupled cross-shore and longshore approaches

Understanding shoreline evolution, and in particular, the consequences of shoreline erosion is a
major societal concern that threatens to become even more important in the future with the impacts
of climate change. Thus, it is necessary to improve both knowledge of the dominant physical processes
controlling medium to long-term shoreline evolution and the capabilities of morphological evolution
models to simulate beach changes at these spatial and temporal scales.
Empirical models may be an ideal choice for modelling complex and dynamic environments such as
sandy beaches at large spatial (beach) and long temporal (years to decades) scales. They reproduce
the effects of the main morphodynamical processes with low computational cost and relatively high
accuracy, in particular when high quality, long-term data are available for calibration.
Here, to broaden its range of application, a cross-shore equilibrium model, which has demon-
strated its accuracy and efficiency in reproducing shoreline and intertidal beach profile changes at
several micro and macrotidal beaches, is extended to couple it with a longshore beach evolution
modelling approach. The selection of a particular longshore model (based on a one-line approach),
and its implementation and validation with benchmark test cases of shoreline evolution caused by
the effects of diffusion, high angle wave instabilities, and coastal structures are presented.
The new hybrid model is applied at Narrabeen beach to reproduce the long-term evolution of
beach contours near the shoreline. The model is calibrated and tested using the 40-year timeseries of
monthly subaerial beach profile surveys conducted along 5 cross-shore profiles along the 3.6km-long
Narrabeen-Collaroy embayment. The novelty of the current work is to focus on reproducing changes
at different altitudes, with the objective of assessing the cross-shore variability of the longshore
sediment flux, which is assumed constant in most one-line longshore transport models. The coupled
model performance is discussed, and the results are compared to existing studies that have simulated
shoreline evolution at Narrabeen using other morphological change models.