Medium time-scale behaviour of adjacent embayed beaches: Influence of low energy external forcing

Abstract

Medium term (decadal) beach profile response to external forcing was assessed on two adjacent embayed beaches (North and South Sands) in Pembrokeshire, West Wales. Both have contrasting physical and geological characteristics: a headland bay backed by dunes and a constrained embayment backed by geological promontories, promenades, walls and rock revetments. Paired t-tests showed significant changes at all cross-shore profile locations (95% confidence), with south and north shores respectively exhibiting falling and rising beach levels. South to north sediment transport was revealed by volumetric variation ( r = 0.83), and longshore sediment distribution ( r = 0.91). North Beach level variations lagged behind South Beach by one-year ( r = 0.85). A reduction in high wind speed frequency, coincidental to spring tides, was correlated with falling South Beach levels ( r = 0.87) and rising North Beach levels ( r = −0.92). Heavily refracted Atlantic swells also have contrasting effects on these systems. Multiple regression models applied to beach level change for both systems showed high correlation ( R 2 = 0.93 and 0.85 respectively), suggesting that swell wave period and strong north-westerly winds that occur during high spring tides had cumulative effects. Furthermore, variations in the North Atlantic Oscillation (NAO) index were correlated to cumulative changes in mean sea level, wind wave height and south-westerly winds ( R 2 = 0.75). While NAO variations were linked to variations in South Beach morphology, it was insignificant on North Beach ( r = −0.72 and −0.14 respectively). Cross-correlation analyses showed that North Beach morphological changes lagged behind NAO Index variation by one-year with much improved correlation ( r = 0.77). A similar scenario existed when comparing Mean Sea level and beach level differences ( r = −0.54 and −0.32 respectively). Similar behaviour should be exhibited at other worldwide embayments, and it is suggested that this work is repeated to establish specific responses, to will underpin intervention or no active intervention strategies.