A multi‐century record of linked nearshore and coastal change

Abstract

Using geographic information system (GIS) and field measurements the nearshore morphological variability of a headland bay beach at Tenby, West Wales (51·66 N; -4·71 W) was assessed over historical timeframes . Three areas chosen for detailed analysis were the area between mean low water (MLW) and lowest astronomical tide (LAT) contours; LAT and one fathom contours; and one and two fathom contours. Estuary closure c. 1855 has been suggested as the genesis for long-term beach evolution and did have an initial effect, with northward dune migration and reduced flushing effects. However, this research suggests nearshore bank migration and retrogradation associated with spit collapse took place prior to closure and continued throughout the assessed timeframe. Historical data revealed patterns of shoreward migration demonstrated by changes in orientation, Giltar headland acting as a pivot. Variations in sandbank position correlated with areal reduction of both Giltar spit and White Bank. Temporal offshore areal loss was contrasted against variable gains inshore as offshore banks welded to the beachface. Annual volumetric change analyses represented by profiles that extend 1 km offshore, confirmed Giltar spit and White Bank erosion rates of 91 m 3 yr -1 and 458 m 3 yr -1 respectively, and 220 m 3 yr -1 beachface accretion. Diminution of sediment supply observed over historical timescales was supported by decadal evidence. Here, profile analysis revealed a trend of decreasing volumes both updrift and within the study area, and increasing downdrift volumes. This explained why proximal detachment and sediment redistribution had occurred. Distinct reversal's in shoreline trend (rotation) corresponded to nearshore change; therefore, variations in seabed configuration triggered shoreface dynamic change over century timescales. Five-year cumulative average changes in North Atlantic Oscillation were further correlated to this reversal. As comparable scenarios are likely to exist at other worldwide coastal locations, similar analyses should be incorporated into shoreline monitoring programmes. Consequently, these assessments would inform shoreline trends and support coastal management decisions.

Medium timescale beach rotation; gale climate and offshore island influences

“…Other variables also influence rotation, as shown by Anthony et al (2002) at Montjoly Beach, Cayenne, French Guiana, where patterns of nearshore mudbank migration induced rotation. Similarly, Thomas et al (2011b) linked beach diminution and migration, associated with spit collapse, to multi-century shoreline rotation within the current area of study.…”

“…Longshore sediment movement from south to north is influenced by heavily refracted Atlantic swell waves which undergo diffraction as they encounter the south Pembrokeshire coast and offshore Caldey and St Margaret's Islands. Gibbard (2005) estimated intertidal sediment loss of circa 7000 m 3 yr −1 , while Thomas et al (2011b) demonstrated that there was a reduction in updrift sediment input and downdrift leakage out of the system. The dune system (920 × 10 3 m 2 ) compensates for some of this loss through dune scarping during episodic events but, in general, extensive vegetation retards sediment movement from the dune field to the intertidal zone.…”

“…This may be influenced by the resolution of the data set. Thomas et al (2011aThomas et al ( , 2011bThomas et al ( , 2011c showed that southern volume changes lag behind northern change by 1 month, and when trend reversal occurred, northern variations lagged behind southern change by 2 months. The results from the intertidal zone (Fig.…”

Coasts and climate

“…On the Atlantic coast of Portugal, Dodet et al (2010) showed winter-averaged wave direction and the NAOw to be significantly correlated for the period 1953-2009. Thomas et al (2011aThomas et al ( , 2011b) also demonstrate linkages between NAOw, wind speed and direction, and the rotation of bay beaches in south Wales, UK. While variation in wave height is a clearly significant influence on beach morphology, beach rotation is more closely linked to oscillations in the dominant wind and wave direction between southwest and southeast quadrants.…”

Mesoscale Morphological Change, Beach Rotation and Storm Climate Influences along a Macrotidal Embayed Beach

“…Morphological responses of embayed beaches to storm and gale forcing have also been studied in the Northern [17][18][19] and Southern Hemispheres by amongst others [10,14]. The underlying causes of wave directional change have also been linked to subtidal mud bank and sandbank migration [20][21][22]. Unlike the macrotidal beach work carried out in this research, most rotation studies utilize variations in the location or volume of the subaerial zone to identify shoreline response; this was because almost all were studied in locations with microtidal or mesotidal ranges see for example [10,20,[23][24][25][26][27].…”