Shoreface mesoscale morphodynamics: A review

At seasonal to century timescales (mesoscale), the shoreface is a critical zone seaward of the surf zone and/or beachface, in which waves interact with the mobile seafloor to cause morphological change. This has important (and often unacknowledged) implications for adjacent shoreline form and behaviour both now and in the nearfuture. The shoreface has been relatively little studied from a mesoscale morphodynamic (morphological change over time) perspective and various definitions exist regarding its extent and morphodynamic subdivisions. To overcome the diversity and ambiguity of existing definitions we propose a standard terminology involving the external limits and subordinate zones of the shoreface. In our definition, the landward limit of the shoreface coincides with the seaward limit of the fair weather surf zone, and where no surf zone is present, the base of the beachface. The shoreface itself is subdivided into upper and lower shorefaces, separated by the depth of closure (DoC) as defined by Hallermeier (1981). The seaward limit of the lower shoreface is defined by the limit of significant sediment transport, indicated by bed shear stress according to Valiente et al. (2019 All boundaries are temporally variable according to wave characteristics and timescale of study. The upper shoreface is dynamic at seasonal to annual timescales and interacts with the adjacent surfzone via wave transformation and two-way sediment exchange. The lower shoreface is dynamic at decadal to millennial timescales and it interacts with the adjacent upper shoreface and inner shelf. The upper shoreface is strongly influenced by wave hydrodynamics whereas the lower shoreface is less dynamic and its shape is more heavily influenced by geological factors (nature and/or abundance of sediment, depth and erodibility of rock outcrop, etc.). Sediment exchange both within the shoreface and between shoreface and adjacent environments is strongly event-driven. Longshore, onshore and offshore transport mechanisms have been documented. The shoreface profile influences, and is influenced by, wave transformation, however, the widely adopted shoreface equilibrium profile is not universally applicable. Instead, a diversity of shoreface morphologies exists in two and three dimensions. These are likely related to sediment supply and accommodation and we propose a spectrum of shoreface types based on these variables. Recent studies have shown that large-scale 3-D forms (e.g. shoreface-connected ridges and sorted bedforms) strongly influence shoreline behaviour, however, the dynamics of these shoreface bedforms requires further investigation. Each type of shoreface likely exhibits distinctive behaviour at the mesoscale (time scale of 10(1) to 10(2) years and a spatial scale of 10(1) to 10(2)km). This is proposed as a unifying model with which to integrate studies of shoreface dynamics at different spatial and temporal scales.