Variability in sandbank behaviour at decadal and annual time-scales and implications for adjacent beaches

Sandbanks are an important coastal resource, both in terms of the activities that occur on them (e.g., dredging, offshore wind farms) as well as the direct and indirect effects they can have on adjacent beaches. Due to the large temporal and spatial domains that sandbanks occupy, there have been relatively few investigations that examine bank behaviour and impacts on other sedimentary bodies (beaches and other banks). Navigation charts near historical ports can yield long datasets for sandbank analysis; however large time intervals and gaps in the data make it difficult to distinguish trends from cycles and gradual from episodic behaviour. In this paper we assemble a new 182-year historical dataset that has significant improvements in data density and temporal coverage (i.e., fewer gaps and smaller time intervals) and permits a more detailed investigation of patterns in bank behaviour. The sandbank exhibits 70-80 year cyclical behaviour in its movement, volume and elevation and holds two distinct and associated morphological states (deltaic and elongate). Annual bathymetry shows gradual bank migration is common to both morphologies but bank reconfiguration (elongate?deltaic) occurs rapidly (relative to the cycle period), on a bank-wide scale and may be considered to be episodic. The switch from deltaic to elongate morphology is hypothesised to result from upstream morphology-tide interactions induced by changes in the shape or position of other banks. The cyclical nature of bank elevation, extent, proximity to shore and shape has important implications for adjacent beaches as variation in sandbank characteristics alters the transmission and alongshore distribution of wave energy (through refraction and dissipation of wave energy by breaking). The latter controls net longshore sediment transport and therefore erosion/accretion trends. Bank-beach interactions are shown to be complex; high bank elevations, which can provide the greatest coastal protection by reducing wave energy incident to the shoreline, are actually associated with the most severe erosion in the historical record. To further elucidate bank-beach relations, current field and numerical modelling efforts examine the direct (sediment exchange) and indirect (refraction, dissipation) linkages between bank and beach behaviour