An XBeach derived parametric expression for headland bypassing

Abstract

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. As yet, there is no universal method for estimating bypass rates. Here we show that wave-forced bypassing of an isolated headland can be estimated using a parameterisation of wave conditions, shoreface slope, sediment size, headland toe-depth and 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 wave and morphological factors. Key variables were headland cross-shore extent as the ratio of toe-depth to bed slope (XH = htoe/tanβ) and surfzone extent as a function breaking depth and slope (Xs = hb/tanβ). The critical ratio controlling bypassing was found to be XH/XS (equivalent to htoe/hb, for an idealised headland). Bypass is predicted as: Qbypass = Q0 f(XH/XS); where Q0, the alongshore transport rate immediately updrift of the headland, is based on an existing expression (Van Rijn, 2014) and f(XH/XS) is a negative exponential function. This formulation accurately replicated bypass rates modelled by XBeach, generally to within a factor of two. Headland shape was found to be a lower-order control on bypass and was excluded from the parameterisation. Preliminary analysis suggests the expression may also be applied to groynes, though more specific testing is required. Once adapted to time-varying wave and water levels, the expression succesfully predicted bypass rates for three observed headlands over a single event. Best results were obtained using measured toe-depth to parameterise headland morphometry. Further field data are required for robust calibration, in particular for large and complex headlands that vary substantially from the idealised model used for development. The bypass expression can be used to provide a first-pass estimate of wave-forced bypassing for a wide-range of headland types; this will be of use to coastal managers, scientists and engineers working on rocky and embayed coastlines.