Robert Jak McCarroll, Gerd Masselink, Nieves G. Valiente, Erin V. King, Timothy Scott, Christopher Stokes, Mark Wiggins

Accurate knowledge of the sediment budget of a coastal cell is necessary for coastal management and predicting long-term coastal change. An important component in the sediment budget of many wave-dominated embayed coastlines is the amount of sediment that bypasses rocky headlands, which present partial barriers to alongshore transport. Despite a recent surge in research interest in headland bypassing, there is as yet no universal method for estimating bypass rates. Here we show, for the first time, that wave-forced bypassing of an isolated headland can be estimated using a parameterisation of wave conditions, shoreface slope, sediment size and headland cross-shore extent. XBeach was used to simulate instantaneous rates of alongshore flux off the apex of an idealised and isolated headland, with >1100 simulations, testing various combinations of wave forcing and morphological factors. The ratio of headland cross-shore extent to surf zone width 〖(X〗_H/X_S) was found to be the critical control on bypass rates. Bypass is predicted as: Q_bypass= Q_0 f(X_H/X_S ); where Q_0 is based on an existing alongshore transport expression (Van Rijn, 2014) and f(X_H/X_S) is a negative exponential function. This formulation was found to accurately replicate bypass rates modelled by XBeach, generally to within a factor of two. Headland shape is a lower-order control on bypass and can be excluded from the parameterisation. This approach was then demonstrated to succesfully estimate bypass rates for a field site, and can be adapted to accommodate time-varying wave conditions and water levels. This new expression can be used to provide a first-pass estimate of wave-forced bypassing rates for a wide-range of headland types. This will be of use to coastal managers, scientists and engineers working on rocky and embayed coastlines.