Short-term beach rotation, wave climate and the North Atlantic Oscillation (NAO)

Article

Short-term beach rotation,wave climate and the NorthAtlantic Oscillation (NAO)

T. ThomasCarmarthenshire County Council, UK

M.R. PhillipsSwansea Metropolitan University, UK

A.T. WilliamsSwansea Metropolitan University, UK

R.E. JenkinsSwansea Metropolitan University, UK

AbstractBeach profile surveys, offshore wave climate and variations in atmospheric conditions have been utilized toassess a short-term beach rotation phenomenon in a headland embayment Tenby, West Wales. Beach rota-tion, expressed by subaerial volumetric change, was shown by a negative phase relationship between beachextremities (r ¼ –0.67), while cross-correlation at a one-month timelag increased statistical significance(r ¼ 0.84). Due to beach aspect, gale wave heights decreased as wave direction rotated to the south(R2¼ 0.4) and west (R2¼ 0.65), while offshore wave direction influenced change at the southern and north-ern extremities (R2¼ 0.52 and 0.34, respectively). Shelter from offshore islands and Giltar Headland contri-butes via wave diffraction to accretive, erosive and rotational patterns, and these are sensitive to variationsaround the predominant wave direction (229�). A southerly shift induces north/south sediment movement,as waves diffract around the offshore islands, while a westerly change results in south/north sediment move-ment (i.e. beach rotation), as diffracted wave domination transfers to the headland. A general gale wave heightreduction occurred when the North Atlantic Oscillation (NAO) was weak or in a transitional phase betweenpositive or negative phases (R2¼ 0.69 and R2¼ 0.72, respectively). Morphological change was also attuned toatmospheric variation where a reversal in beach rotation was influenced by variations in positive and negativeNAO/volume correlations and longshore profile location (R2 ¼ 0.54 and 0.69, respectively). The results ofthis study have wider implications for coastal management; it is suggested that models developed in similarsystems elsewhere will form the basis of human intervention or no active intervention strategies.

Keywordsconceptual models, embayment, morphological change, offshore island, sediment transport

I Introduction

Fifty-one percent of the world’s coastlines are

representative of headland bay morphology

(Short and Masselink, 1999), and headlands have

Corresponding author:T. Thomas, Technical Services Department,Carmarthenshire County Council, Cillefwr SA31 3QZ,Wales, UK.Email: [email protected]

Progress in Physical Geography35(3) 333–352

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a significant influence on planform, sediment

transport and morphodynamics of embayed bea-

ches (Carter, 1988; Short and Masselink, 1999).

Owing to the complexity of headland impact in

altering beach morphodynamic behaviour,

embayed beaches receive less attention than

unconstrained beaches. Therefore, little research

has been undertaken in this important environ-

ment (Klein et al., 2002). Most studies tradition-

ally concentrated on small groups of embayed

beaches, and almost all have been conducted in

locations with microtidal/mesotidal ranges (e.g.

Anthony and Dolique, 2004; Dehouck et al.,

2009; Dolique and Anthony, 2005; Jacob et al.,

2009; Klein et al., 2002; Loureiro et al., 2009;

Ojeda and Guillen, 2008; Ranasinghe et al.,

2004; Sedrati and Anthony, 2007).

On a regional basis, Klein and Menezes (2001)

used several parameters (i.e. wave and sedimen-

tary statistics, fall velocity) in conjunction with

beach profiling and plan view characteristics to

typify 17 differently exposed pocket beaches in

Brazil. Bowman et al. (2009) carried out the larg-

est detailed study along the Catalan coast, studying

72 pocket beaches to determine plan view indexes

based on standard planform parameters. Beach

rotation is a phenomenon in embayed beaches that

are exposed to variable directional wave climate

and short-term beach rotation is sometimes caused

by shoreline realignment in response to a shift in

incident wave direction (Klein et al., 2002). These

shifts are commonly associated with changing

climatic conditions (e.g. Ranasinghe et al., 2004;

Short and Trembanis, 2004). However, other vari-

ables influence rotation, as shown by Anthony

et al. (2002) at Montjoly beach, Cayenne, French

Guiana, where patterns of nearshore mudbank

migration induced rotation.

Rotation manifests itself at various temporal

scales and the majority of research has been

focused at the decadal level. In this environment,

detailed studies have been made by Anthony and

Dolique (2004), Dolique and Anthony (2005),

Ojeda and Guillen (2008), Pinto et al. (2009),

Ranasinghe et al. (2004), Short and Trembanis

(2004) and Short et al. (2000). Studies of

short-term morphological response to dynamic

forcing are limited, particularly when applied

to macrotidal beach environments (Poate et al.,

2009). Patterns of shoreline change have been

linked to major atmospheric variations by,

among others, Ranasinghe et al. (2004), Rooney

and Fletcher (2005), Short and Trembanis

(2004), Short et al. (2000), Thomas et al. (2010)

and Vespremeanu-Stroe et al. (2007). Longer-

term beach rotation phenomena in the Southern

Hemisphere have been linked to changes in

the Southern Oscillation Indices (SOI) by, for

example, Ranasinghe et al. (2004), Short and

Trembanis (2004) and Short et al. (2000). Simi-

larly, Thomas et al. (2010) demonstrated links

between the North Atlantic Oscillation (NAO)

and beach rotation.

Few comparative studies have been carried out

detailing short-term rotational processes applied

to a single embayed beach set in a macrotidal

environment. This study assesses local shoreline

evolutionary patterns between November 2008

and April 2010, for one such embayed beach in

South Pembrokeshire West Wales (GR 212200,

198599; Figure 1). Several forcing variables

related to wind and wave regimes, together with

major climatic variations expressed by the NAO

Index, are utilized to evaluate processes govern-

ing short-term beach rotation phenomena. Signif-

icant research findings are assessed with

relevance to other coastal locations worldwide.

II Physical and geologicalbackground

In England and Wales, Shoreline Management

Plans set out strategic policies that assist coastal

defence decision-making processes. Here, natural

process behaviour determines sediment cell

coastal boundaries. The south and southwest

Wales coastline is designated cell 8 (Figure 1b)

and is subdivided into four subcells that are

assigned coastal engineering groups, made up

of a number of statutory bodies and stakeholders.

334 Progress in Physical Geography 35(3)

South Sands, Tenby, South Wales (UK National

Grid Reference SN 212200, 198599; Figure 1c),

falls within Carmarthen Bay Coastal Engineering

Group sub cell 8c which is bounded by Saint

Anne’s Head to the west, and Worm’s Head to

the east, and is referenced as management unit

2/1 (PCC, 2006; Figure 1c).

The landscape of Tenby and its environs was

formed by intense folding during the Variscan

Orogeny (Hunter and Easterbrook, 2004) which

resulted in the formation of the Ritec fault

(Toghill, 2000), part of which lies along the

line of the River Ritec, near Tenby town (Posford

Duvivier, 1998; Toghill, 2000). The Ritec estuary

was closed to tidal influence c. 1855, following

construction of a railway embankment. The study

area is developed between two headlands of

Carboniferous Limestone, Giltar Headland to

the south and Tenby to the north (Hillier and

Williams, 2006; Owen, 1973). It is a classic

embayment (Posford Duvivier, 1998) and,

similar to that described by Brown et al. (1999),

has a shallow concave profile, wide sandy interti-

dal zone, and a ridge and runnel structure. This

Figure 1. Locality of the study area. (a) United Kingdom; (b) southwest Wales; (c) study area; (d) representativeprofile locations T1 and T2 (south); T3 and T4 (central); T5 and T6 (north).

Thomas et al. 335

zone gives way to a limestone shingle backshore

overlain by a dune system (PCC, 2006), with

shingle being periodically exposed during

storms and high spring tides (Gibbard, 2005).

The distance between the headlands is approxi-

mately 2 km, and the tides are semi-diurnal and

macrotidal, with a mean spring tidal range of

7.5 m. Depth of closure is estimated to be between

5.6 m (using a limited set of beach profiles) and

13.1 m when tidal range is taken into consider-

ation (Thomas, 2007). This littoral zone presents

different levels of human occupation. Tenby to

the north is a heavily urbanized coastal area,

used mainly for tourism, and is important to

the regional economy. Conversely, the dunes,

marshes and headland promontory to the south

are ecologically important conservation areas.

The subtidal area is characterized by a centrally

positioned ‘shore-attached’ sandbank, known

locally as the ‘White Bank’, which is exposed

during the highest spring tides. Incident waves

usually approach from southerly to westerly

directions with an average wave height recorded

from an offshore position of c. 1.2 m, with associ-

ated mean periods of 5.2 s; however, storm waves

of 5.5 m and periods of 8.2 s are not uncommon

(Thomas et al., 2010). 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. Winds predominate from

the south and southwest (alongshore and offshore),

where Giltar Headland and offshore islands

give shelter. Caldey Sound is approximately

1 km wide, its depth below the lowest astro-

nomical tide is 16 m, and it has spring tidal cur-

rents of 1.3 ms�1 (west to east) during flood and

1.1 ms�1 (east to west) during ebb tides.

III Methodology

To analyse beach rotation, six profiles were

surveyed monthly between November 2008 and

April 2010 inclusive (Figure 1d). These profiles

were representative of the southern (T1 and T2),

centre (T3 and T4) and northern beach areas (T5

and T6; Figure 1d). RTK network GPS (Leica

1200þ) was utilized for survey work and

data were recorded to an accuracy of 5 mm +0.5 mm. RTK network GPS is a one-man-

operated surveying system that consists of a rover

unit, controller, communication link, antenna and

poles while permanent base stations throughout

the UK stream corrections via a central correction

service linked directly to a rover (see, for example,

Euler et al., 2002; Leica, 2008a, 2008b; Weber

et al., 2005). Sectional volumes, i.e. the morpholo-

gical variables, were computed directly from the

Regional Morphology Analysis Package (RMAP),

where volume is determined by extrapolating the

area under the curve for one unit length (m3m�1)

of shoreline (Morang et al., 2009). The subaerial

beach volume was used to characterize beach rota-

tion, using the landward fixed benchmark and

mean high water level (Klein et al., 2002; Figure 2).

In order to assess short-term longshore varia-

tion in subaerial volume, it was necessary to

remove high-frequency cut and fill (cross-shore)

noise. A method similar to that developed by

Short and Trembanis (2004) was implemented.

(1) The volume record was transformed into

the standard normal form (Davis, 2002),

using:

z ¼ x� x�=s ð1Þ

where z¼ normalized value, x¼ data vol-

ume record for each profile, x� ¼ average

value for that profile, s ¼ standard

deviation.

(2) Temporal mean survey volumes were

spatially averaged along the beach (repre-

senting the cut and fill behaviour common

to all profiles).

(3) Spatial average values (b) were subtracted

from the local normalized volume (a) to

reveal a time series where high-frequency

behaviour has been removed.

336 Progress in Physical Geography 35(3)

(4) Residual volumes were converted into

dimensional units using:

x ¼ zsð Þ þ x� ð2Þ

Direct comparisons were made with environ-

mental forcing agents (wind and wave climate),

captured at a point located 9 km southeast of the

study site (latitude 51.6�; longitude –4.58�)derived from archive wave model hind-casts

produced by the Met Office. The virtual model

point, located at a depth of 28 m, records significant

wave conditions at three-hourly intervals, while

wind speed was recorded at 20 m above mean sea

level. The numerical model calculates significant

(H1/3) conditions as a resultant of wind-sea and

swell combined using the expression:

Resultant ¼ ðwind � sea2þswell2Þ1=2

ðUKMO; 2009Þ

Breaking wave heights (hb) were obtained

by applying the Komar and Gaughan (1972)

expression:

Hb¼ 0:39g1=5ðTpH20Þ

2=5

where Ho ¼ offshore wave height and Tp ¼associated period. In this study, storm conditions

were characterized by wind speeds >17 ms�1,

while gale conditions were defined as wind

speeds >11 ms�1 that continued through a mini-

mum of one tidal cycle (12-hr period).

IV Results

1 Storm climate

Throughout the 17-month monitoring period,

storm winds > 17 ms�1 were recorded on six

occasions. On 12 December 2008, during a 12-hr

period that showed average wind speeds of

14.9 ms�1, a wind speed of 17.23 ms�1 emanating

from 191� true, was recorded. This generated

combined offshore and breaking wave heights

and wave periods of 2.7 m, 2.8 m and 6.4 s,

respectively. During a four-day period of strong

winds averaging 11.74 ms�1, wind speeds of

22 ms�1 emanating from 207� true were recorded

(17 January 2009), which generated combined

offshore and breaking wave heights and periods

of 3.8 m, 4.1 m and 7.9 s, respectively. A 27-hour

period between 25 and 26 April 2009 recorded

average wind speeds of 12.35 ms�1 with a maxi-

mum of 17.06 ms�1 emanating from 158� true.

This generated a combined offshore wave,

breaking wave height and period of 2.1 m,

2.2 m and 5.2 s, respectively. November 2009

Figure 2. Sketch showing the subaerial morphological zone from which comparative beach volumes werecomputed and from which rotation has been analysed (shaded grey)

Thomas et al. 337

was the most tempestuous month, dominated

by southwesterly winds (average 213� true),

and when wind speeds dipped below 11 ms�1

on only two occasions. Eight storm days were

recorded where maximum wind speeds ranged

from 17.02 ms�1 to 21.36 ms�1. These winds

generated offshore wave heights of between

2.12 m and 5.5 m, breaking waves heights

of between 2.17 m and 6.17 m, and respective

periods of 8.06 s to 14.73 s. A wind speed

of 17.67 ms�1 emanating from 188� true was

recorded on 7 December 2009. This occurred

during a three-day period of strong winds aver-

aging 13.7 ms�1 that generated offshore and

breaking wave heights and periods of 3.98 m,

4.58 m and 10.94 s, respectively. Finally, a wind

speed of 17.78 ms�1 emanating from 122� true

occurred on 12 January 2010. This happened

over a two-day period averaging wind speeds

of 14.82 ms�1, and generating offshore wave,

breaking wave and associated periods of 2.23 m,

2.28 m and 5.31 s, respectively.

2 Gale climate

A gale was defined as a period where wind

speeds >11 ms�1 occurred over a tidal cycle –

minimum 12-hr period (strong wind 22 knots).

Gale events represent 9% of the wave record and

75% of all gale force winds emanated either from

alongshore (south to southwesterly) or offshore

(southwest to west). In the former conditions, the

islands of Caldey and St Margaret’s give shelter

while, in the latter, Giltar promontory gives shel-

ter. Table 1 shows the intersurvey gale record

between November 2008 and April 2010, when

43 gale events lasting a total of 1422 hr took

place. Most notable was the period between

October and December 2009, when 513 gale hrs

in nine separate events were recorded. Average

wind speed and direction were 14 ms�1 and

227�, respectively, which generated offshore and

breaking wave heights and associated periods of

2.7 m, 3.1 m and 7.95 s, respectively. Onshore

winds dominated over three periods between

January and February 2009 and January and

March 2010 (six gale events). They generated

relatively small offshore waves (1.3–1.8 m),

breaking waves (1.4–2.1 m) and associated peri-

ods (5–6.5 s), influenced by a limited fetch dis-

tance. In addition to October and December

2009, three other periods experienced >100 gale

hours (Table 1). Between November and Decem-

ber 2008 (102 hr), recorded offshore and break-

ing wave heights and associated periods were

1.7 m, 1.9 m and 5.8 s, respectively, while

between April and May 2009 (111 hr), these were

2.7 m, 3.0 m and 7.6 s, respectively. Finally,

between December 2009 and January 2010

(117 hr), offshore and breaking wave height and

periods of 2.4 m, 2.9 m and 7.9 s, respectively,

were generated. Each event emanated from a

south to westerly direction (188–274� true).

3 Average wind/wave climate

Table 1 shows the intersurvey averaged wind/

wave climate between November 2008 and

April 2010. Overall average wind speed (8 ms�1,

s ¼ 47) generated offshore and breaking

waves and associated periods of 0.98+1.1 m,

1.26+1.35 m, and 6.39+1.3 s, respectively.

Intersurvey wind direction fluctuated throughout

the monitoring period averaging 190+70� true.

Wind speeds in excess of 10 ms�1 occurred on

two occasions throughout the monitoring period.

The first, between January and February 2009,

from a southerly direction (191� true) generated

offshore and breaking wave heights and associ-

ated periods of 1.5 m, 1.8 m and 6.6 s, respec-

tively. The second, from a south-southeasterly

direction (146� true), generated offshore and

breaking wave heights and associated periods of

1.3 m, 1.7 m and 6.7 s, respectively.

4 Wave climate and subaerial volumechange

Table 2 shows subaerial volume changes

between November 2008 and April 2010 and

338 Progress in Physical Geography 35(3)

these are represented by Figure 3. This shows a

time series of intersurvey subaerial volume

change (T1 and T6), together with daily aver-

aged breaking wave heights and directions based

on wind speeds >11 ms�1, transposed for direct

comparison using equation 1.

Both southern and northern sectors exhibit

patterns of accretion and erosion throughout,

highlighting reversals in trend between beach

extremities. Initially, between December 2008

and February 2009, the highest waves occurred

when wave direction impact was directly behind

the offshore islands (229� true). A reversal

in morphological trend is triggered by a change

in wave direction (south toward east), which

also resulted in a reduction of general wave

heights. Variable, albeit small, changes took

place during spring/summer 2009, except

between April and May. Here a change in wave

direction appears to trigger an increased

pattern of erosive behaviour in the southern

sector. However, recovery had taken place by

the following month. Between October 2009

and January 2010 increasing wave heights

and directional variations associated with

a prolonged period of gales triggered two

reversals in erosive/accretive patterns. The first

was triggered by a negative change in wave

direction (south toward east) that occurred

during early October and the second, a positive

change in late November, was south toward

west.

5 Subaerial volume and beach rotation

Table 3, produced from Table 2, shows a cross-

correlation (r) matrix computed at zero lag.

The high correlation between T1 and T2

(r¼ 0.69) indicates that any variations in volume

at profile T1 also occur at profile T2. Similarly,

Table 1. Intersurvey wind and wave climates for the period between November 2008 and April 2010.Wspd ¼ wind speed; Wdir ¼ wind direction; Hs ¼ significant wave height; Hb ¼ breaking wave height;Tp ¼ wave period.

Date Average climate Gale climate

On OffWspd(m/s)

Wdir(deg)

Hs(m)

Hb(m)

Tp(s)

Events(no)

Totalduration

(hr)

Galedays(no)

Wspd(m/s)

Wdir(deg)

Hs(m)

Hb(m)

Tp(s)

Nov08 Dec08 8.2 253 1 1.2 5.4 4 102 12 12.7 274 1.7 1.9 5.8Dec08 Jan09 7.3 126 0.9 1.2 6.9 2 60 7 13.1 230 2.5 2.7 6.9Jan09 Feb09 10.4 146 1.3 1.7 6.7 2 93 18 12.4 143 1.3 1.4 5Feb09 Mar09 5.8 269 0.8 1.1 7.2 2 69 6 13.1 245 2.3 2.6 7.1Mar09 Apr09 7.3 229 0.8 1 5.5 3 42 6 12.7 286 1.6 1.8 5.2Apr09 May09 7.4 177 1 1.2 5.9 3 111 11 12.9 236 2.7 3 7.6May09 Jun09 6.8 166 0.7 0.9 5.9 1 30 2 12 251 1.8 2 5.5Jun09 Jul09 7.8 243 0.9 1.2 5.4 2 36 3 12.4 273 1.3 1.4 4.7Jul09 Aug09 7.3 224 1 1.2 5.2 2 39 6 12.3 214 2.1 2.3 6.1Aug09 Sep09 7.6 124 0.8 1 5.8 4 81 12 12.1 261 1.8 2 5.8Sep09 Oct09 7 233 0.8 1 6.2 1 21 10 12.7 142 2.4 2.8 7.4Oct09 Dec09 10.5 191 1.5 1.8 6.6 9 513 35 14 227 2.7 3.1 7.9Dec09 Jan10 8.3 141 1 1.5 7.8 3 117 10 13.1 188 2.4 2.9 7.9Jan10 Feb10 8.8 153 1.1 1.4 7 3 66 9 13.8 151 1.8 2.1 6.1Feb10 Mar10 7.5 185 1.1 1.5 7.7 1 12 4 12.4 178 1.7 2 6.5Mar10 Apr10 7.3 177 0.9 1.2 7 1 30 3 13.8 280 2 2.2 6.4

Thomas et al. 339

Figure 3. Comparison of normalized (equation 1) breaking wave heights (dashed line), wave direction(solid line) and subaerial volume change; south ¼ light grey, north ¼ dark grey, for the period betweenNovember 2008 and April 2010

Table 2. Profile volume change (m3 m�1) from the fixed benchmark to mean high water level (subaerialzone), together with intersurvey averaged NAO Index values, for the period between November 2008and April 2010

Date on Date off NAO T1 T2 T3 T4 T5 6

Nov08 Dec08 –0.37 51.2 44.9 78 52.8 54.2 67.7Dec08 Jan09 –0.34 49.6 43.9 71.2 51.2 57.7 66.4Jan09 Feb09 0.25 47.5 42.2 70.5 52.4 59.6 68.7Feb09 Mar09 0.39 48 41.6 72.2 54.2 58 74.4Mar09 Apr09 0.01 49.5 42.7 74.2 56.2 59.6 72.2Apr09 May09 0.64 48.2 42.4 75 56.3 64 71.6May09 Jun09 –0.42 45.9 39.9 74.9 53.2 63.4 72.7Jun09 Jul09 –0.94 48.1 39.6 70.8 51.8 61.7 72.8Jul09 Aug09 –0.41 49.2 42.7 67.5 53.1 61.7 72.8Aug09 Sep09 0.27 49.7 45.4 70.8 54.7 59.9 72Sep09 Oct09 –0.06 46.8 46.1 73.5 56.7 60 74.5Oct09 Dec09 –0.05 50.1 43.3 79.1 54.1 57.8 65.9Dec09 Jan10 –1.29 46.2 43 77.8 53 56.1 71.9Jan10 Feb10 –0.62 45.1 42.6 68.7 50.9 62.4 74.6Feb10 Mar10 –0.84 47.3 41 70.4 48.6 65.7 75.1Mar10 Apr10 –0.3 48.6 42.4 71.2 50.6 62.6 72.6

340 Progress in Physical Geography 35(3)

the high positive correlation between T5 and

T6 (r ¼ 0.58) also indicates that any variation

in T5 profile volume also occurs at profile T6.

However, it is the high negative correlation

(r¼ –0.67) between profiles T1 (extreme south)

and T6 (extreme north) that is of most interest, as

this indicates a negative phase relationship

between accretion/erosion patterns between

southern and northern ends of the beach

(i.e. beach rotation). The change from positive

to negative correlations between profiles T3 and

T4 indicate that the centre of the beach acts as

the axis of rotation, and correlates with long-

term rotation patterns demonstrated by Thomas

et al. (2010). The values of Table 2, normalized

using equation 1, and Figure 4a, produced from

transposed data, show a time series of normal-

ized volumes comparing south with north extre-

mities. Four periods of rotation throughout

the monitoring period existed, not necessarily

linked to seasonal change but demonstrating

cyclic sediment exchanges between headlands.

To investigate stronger potential correlations

between south and north beach extremities,

timelagged cross-correlations of volume changes

between profiles T1 and T6 (Table 3) were calcu-

lated and represented graphically (Figure 4b).

Results show improved correlation (r ¼ –0.84)

at a one-month lag, which indicates that

southern volume variations lag behind northern

variations by one month. When a reversal in trend

occurs, northern variations lag behind southern

change by two months.

6 Climatic variation and beach rotation

NAO is dominant in variations of North Atlantic

winter climate from central North America to

Europe, including Northern Asia, and is the

Table 3. Pearson correlation coefficients set at azero timelag compare longshore volumetric varia-tion (Table 2). The levels of statistical significance areindicated in brackets; italic bold numbers¼ p < 0.05.

T2 0.69(0.00)

T3 0.25 0.20(0.18) (0.23)

T4 0.46 0.41 0.40(0.04) (0.06) (0.06)

T5 –0.43 –0.53 –0.53 –0.25(0.05) (0.02) (0.02) (0.18)

T6 –0.67 –0.60 –0.47 –0.31 0.58(0.00) (0.01) (0.03) (0.12) (0.01)

T1 T2 T3 T4 T5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

Sep-08 Dec-08 Mar-09 Jul-09 Oct-09 Jan-10 May-10Duration (months)

Nor

mal

ized

val

ues

T1 T6

(a)

−1−0.8−0.6−0.4−0.2

00.20.40.60.8

−6 −4 −2 0 2 4 6

Lag (months)

Cor

rela

tion

(b)

Toward north Toward south

Figure 4. A comparison of profile volume variation between north and south beach extremities for theperiod November 2008 to April 2010, graphically represented by (a) time series and (b) cross-correlationset at five-month timelags

Thomas et al. 341

greatest source of interannual variability in wave

climate variation across UK shores (Bell and

Visbeck, 2009; Woolf and Coll, 2006; Woolf

et al., 2002). Here, irregular oscillations through-

out the instrumental record were observed, with

an unusually large trend of increase in the NAO

Index in the later 20th century (Woolf et al.,

2002). Therefore, to understand changes in wind

and wave regimes, Bacon and Carter (1993)

suggested consideration be given to atmo-

spheric pressure gradients. Phillips and Crisp

(2010) showed that higher numerical NAO

Index values result in larger tidal ranges, irre-

spective of phase. It is reasonable to deduce

that sediment exchange and, therefore, beach

rotation phenomena should also be linked to

separate positive/negative values.

In order to test this assumption, Table 2 was

utilized to produce a cross-correlation (r) matrix

computed at zero lag (Table 4), which compares

NAO Index (with outlier removed) with subaerial

volumes. Variable statistically insignificant

correlation ( p > 0.05) is shown between NAO

Index and all profiles with the exception of T4

(r ¼ 0.72). In contrast, a statistically significant

correlation ( p < 0.05) exists when negative

NAO Index values were considered (r ¼0.60, 0.72, 0.70 and –0.75, respectively), with

the exception of profiles T3 and T5 (r ¼0.53 and –0.53, respectively); nevertheless,

both results fall within a 90% confidence level

( p < 0.1). Positive correlations at the southern

end (T1 and T2) indicate that accretion is

expected during a negative index phase,

whereas erosion is expected at the northern end

(T5 and T6), as demonstrated by negative cor-

relations (r ¼ –0.52 and –0.75, respectively).

Statistically insignificant correlations existed

when positive NAO Index values were consid-

ered (r ¼ –0.46, –0.19, 0.27, 0.13 and 0.11,

respectively), with the exception of profile T5 (r

¼ 0.64). Negative correlations at the southern

end (T1 and T2) indicate erosion during a pos-

itive index phase, whereas accretion is

expected at the northern end (T5 and T6) demon-

strated by positive correlations. These results find

agreement with Thomas et al. (2010) who identi-

fied long-term southern end erosion during peri-

ods that were dominated by positive index values.

It is unrealistic to expect immediate beach

response to changes in NAO Index. Therefore,

correlations between subaerial volumes and

NAO Index values were computed to investigate

the existence of stronger cross-correlations at a

timelag between both south and north beach

extremities. Figures 5a and 5b, produced from

Table 2, compare NAO Index values, with

profile volumes T1 and T6, respectively. Lags

associated with peak values indicated that varia-

tions in beach volume to the south lag behind

NAO variation by one month with statistically

significant correlation (r ¼ 0.57, p < 0.05).

A similar scenario exists at the northern end;

however, statistically, the correlation is insignif-

icant (r ¼ –0.33, p >0.05). Both results showed

improved correlation in relation to previous

results (Table 4). Figures 5c and 5d, produced

from Table 2, compare negative NAO Index

Table 4. Pearson correlation coefficients set at a zero timelag compare NAO and volumetric change(Table 2). The levels of statistical significance are indicated in brackets; italic bold numbers ¼ p < 0.05.

NAO 0.24 0.36 0.28 0.72 –0.24 –0.22(0.19) (0.09) (0.15) (0.00) (0.20) (0.22)

Negative NAO 0.60 0.72 0.53 0.70 –0.52 –0.75(0.03) (0.01) (0.06) (0.01) (0.06) (0.01)

Positive NAO –0.46 –0.19 0.27 0.13 0.64 0.11(0.22) (0.38) (0.33) (0.42) (0.12) (0.43)

T1 T2 T3 T4 T5 T6

342 Progress in Physical Geography 35(3)

values with profile volumes T1 and T6,

respectively. Lags associated with peak values

indicate that variations in beach volume at

the southern end again lag behind negative

NAO variation by one month, and improve the

already statistically significant results presented

above (r ¼ 0.84, p < 0.01). In contrast, the peak

value associated with the northern end suggests

that there is little or no timelag between negative

NAO and volume change, possibly influenced

by the strong, statistically significant negative

correlation shown in previous results (r ¼ –

0.75, p < 0.01; Table 4). Figures 5e and 5f, again

produced from Table 2, compare positive NAO

values with profile volumes T1 and T6, respec-

tively. Lags associated with peak values indi-

cate that variations in the southern and

northern ends lag behind NAO variation by one

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

−6 −4 −2 0 2 4 6

Lag

Cor

rela

tion

(a)

−0.4

−0.2

0

0.2

0.4

0.6

0.8

Lag (months)

Cor

rela

tion

(b)

−1−0.8−0.6−0.4−0.2

00.20.40.60.8

1

Lag (months)

Cor

rela

tion

(c)

−1−0.8−0.6−0.4−0.2

00.20.40.60.8

1

Lag (months)

Cor

rela

tion

(d)

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

1.2

Lag (months)

Cor

rela

tion

(e)

−0.4

−0.2

0

0.2

0.4

0.6

0.8

Lag (months)

Cor

rela

tion

(f)

−6 −4 −2 0 2 4 6

−6 −4 −2 0 2 4 6 −6 −4 −2 0 2 4 6

−6 −4 −2 0 2 4 6 −6 −4 −2 0 2 4 6

Figure 5. Cross-correlation analyses for the period November 2008 to April 2010 comparing: NAO Indexand beach volume variation, (a) south (T2), and (b) north (T12); negative NAO Index and volume variation,(c) south and (d) north; positive NAO Index and volume variation, (e) south and (f) north

Thomas et al. 343

month. The lag associated with the southern end

highlights a change from the negative correlation

of previous results (Table 4) to a positive correla-

tion, whereas an improved, albeit statistically

insignificant, correlation is shown to the north

(r¼ –0.33, p > 0.05).

7 Wave climate and beach rotation

Notwithstanding the aforementioned qualitative

observations (Figure 3, Table 1), no significant

correlations were found between subaerial

volume variation, wind speed, and wave height

climates throughout the 17-month monitoring

period. However, other research has suggested

that rotational response is often caused by shifts

in incident wave direction (Klein et al., 2002;

Ojeda and Guillen, 2008; Short and Masselink,

1999). Consequently, subaerial volume and

offshore wave directional components were

transposed for direct comparison using equation

1 and to produce Table 5.

Between November 2008 and April 2010,

the offshore average wave directional

component emanated from the south-southwest

with relative consistency throughout (229�+20�

true; s ¼ 11�), ranging between 209� and 249�

true. Similarly, gale waves also emanated from

the south-southwest (229�+53�; s¼ 26�), with

far more variability ranging from 177� to 283�.Figure 6a, produced from Table 5, shows time

series normalized variations in beach volume,

together with average wave directions and a

pattern of change emerges, even though wave

direction only fluctuates by a few degrees.

As the wave direction rotates through zero

(i.e. 229�), rotational changes also take place. In

contrast, Figure 6b, produced from Table 5, con-

cerning gale wave direction, shows no obvious

patterns. With the exception of July 2009, when

the wave direction was at its most westerly, and

January 2010 when it was southerly, both condi-

tions prompted a reversal in volumetric

distribution.

The beach extremities showed varying sta-

tistical results. Pearson correlation coefficients

set at zero lag between southern end volumes

Table 5. Normalized intersurvey volumes based on the mean high water and wave directional componentsfor the period between November 2008 and April 2010

Date Normalized values

On Off Average wave dir Gale wave dir T1 T6 Average wave dir Gale wave dir

Nov08 Dec08 255 253 1.5 –1.1 2.3 0.9Dec08 Jan09 214 231 0.6 –1.5 –1.5 0Jan09 Feb09 220 208 –0.6 –0.8 –1 –0.9Feb09 Mar09 243 243 –0.3 1.1 1.2 0.5Mar09 Apr09 237 264 0.6 0.4 0.6 1.3Apr09 May09 224 223 –0.1 0.2 –0.5 –0.3May09 Jun09 225 237 –1.4 0.5 –0.5 0.3Jun09 Jul09 234 285 –0.2 0.5 0.3 2.1Jul09 Aug09 235 221 0.4 0.5 0.4 –0.4Aug09 Sep09 240 236 0.7 0.3 –1.1 0.2Sep09 Oct09 241 227 1.7 –1.5 1 –0.1Oct09 Dec09 226 228 0.9 –1.7 –0.4 –0.1Dec09 Jan10 218 208 –1.2 0.3 –1.2 –0.9Jan10 Feb10 225 178 –1.9 1.1 –0.5 –2Feb10 Mar10 230 199 –0.7 1.3 0 –1.2Mar10 Apr10 223 246 0.1 0.5 –0.7 0.6

344 Progress in Physical Geography 35(3)

and average and gale wave direction correlate

significantly (r ¼ 0.45 and 0.47, respectively;

p < 0.05). In contrast, the north showed insigni-

ficant correlations (r ¼ –0.02 and –0.11,

respectively; p > 0.05). To investigate stronger

cross-correlations, timelags between average

and gale wave directions and south/north beach

extremity responses were computed. Figures

6c and 6d refer to average wave conditions.

Timelags associated with peak values indicate

that volume variation in the southern end (Figure

6c) lags behind wave directional change by

approximately one week (by interpolation), with

only a slight increase in significance. However,

Figure 6. Normalized time series for the period November 2008 and April 2010 showing: beach volumechange (T1 and T6) and (a) average wave direction, (b) gale wave direction; cross-correlations between aver-age wave direction and (c) southern end volume and (d) northern end volume; cross-correlations betweengale wave direction and (e) southern end volume and (f) northern end volume

Thomas et al. 345

the northern end (Figure 6d) lags behind by one

month, and shows a much improved correlation

(–0.35; p < 0.1), albeit statistically insignificant

at the 95% confidence level. Figures 6e and 6f

refer to gale wave conditions. The timelag asso-

ciated with peak values show a similar scenario

to previous results, a southerly one-week lag and

a northerly one-month lag, together with similar

improvements in correlation.

V Discussion

On sandy coastlines, the understanding of storm/

gale effects on beach morphodynamics is crucial

for beach management (Jacob et al., 2009).

Based on results, the most common regional

storm and gale occurrences have varying effects

on South Sands morphology. Nine percent of the

wind/wave record represents gale/storm condi-

tions, which have no seasonality (i.e. summer

fair weather swell dominated/winter steep storm

wave dominated). South to westerly directional

winds dominated (75%; Table 1) and all offshore

gale waves emanated from south to west,

irrespective of wind direction (Table 5). This is

particularly noteworthy in January 2009, where

wind direction emanated from 143� true, yet

resultant waves emanate from 208� true.

This shows that approaching waves generated

in the Bristol Channel are heavily refracted prior

to reaching the offshore Islands, Caldey and St

Margaret’s. The sheltering and shadowing

effects given by these islands, along with Giltar

Headland itself protect South Sands. Such pro-

tection decreases as wave direction rotates about

a point directly behind the Islands, an area

open to North Atlantic swell waves (229� true);

coincidently, the highest waves throughout the

studied period also emanate from this direction

and decrease in height with directional rotation

irrespective of direction.

The statistical significance of these effects is

shown by comparing wave direction (Table 5)

and offshore gale wave heights (Table 1).

Southern rotation (229� toward 180� true) given

by yghs ¼ 0.018xwadir – 1.862 (R2 ¼ 0.40) and

is a function of limited fetch within the

Bristol Channel. Western rotation, as a result

of increased shelter from the Pembrokeshire

coastline (229� toward 270� true) is given by

yghs ¼ –0.017xwadir þ 6.194 (R2 ¼ 0.63), where

yghs¼ wave height and xwadir¼ wave direction.

From data presented here, it is reasonable to

deduce that wave diffraction has a considerable

influence on behavioural patterns as wave

direction rotates. Moreover, southerly shifts

(from 229� true) give rise to dominant diffrac-

tion around the easternmost point of Caldey

Island; in contrast, westerly shifts give rise to

dominant diffraction around Giltar Headland.

This change is important for beach rotation as

shown by Gibbard (2005), who assessed beach

behaviour using the numerical model SWAN

(Simulating WAves Nearshore) to simulate

transformation of selected offshore wave condi-

tions (1:1 mo, 1:1 yr and 1:50 yr). Results

showed that southeasterly waves under both

1:1 mo and 1:1 yr simulations cause drift direc-

tions to the south albeit weakly at the southerly

end, and that under the 1:50 yr simulation a drift

convergence is likely near the central sector.

Under southwesterly waves, drift is generally

to the north under all conditions; however, under

the 1:50 yr simulation erosion is likely to extend

to the central sector. With waves from a south-

erly direction, erosion is likely within the central

sector, but under the 1:50 yr simulation this is

likely to extend farther south.

Beach rotation expressed as changes in

volume show that four reversals in trend took

place during the monitoring period, with no sea-

sonal or cyclic occurrence (Figure 7a). A negative

phase relationship was established between south

and north sectors (beach rotation) (r ¼ –0.67,

p < 0.05; Table 3). A significant correlation

between T1 and T2 (southern) showed that when

changes took place in one profile similar changes

were likely in the adjacent profile (r ¼ 0.69,

p < 0.05; Table 3). A similar scenario existed

between northern profiles (r ¼ 0.58, p < 0.05;

346 Progress in Physical Geography 35(3)

Figure 7. (a) Comparison of wave direction and volume change, applied beach extremities. (b) Morphologicalmodel showing the general trend of sediment erosion/accretion, through rotation of wave direction. (c) Con-ceptual model of expected sediment movement, based on wave direction 180–235�. (d) Conceptual model ofexpected sediment movement, based on wave direction 235–260�(grey arrows indicate lowering lowering sub-aerial volume, white arrows increasing subaerial volume). Approximate wave alignments taken from Gibbard(2005) based on the SWAN model. (e) Comparison of negative/positive NAO/beach volume correlation coef-ficients, with profile location alongshore. (f) Conceptual morphological modal of beach response to variationsin NAO Index.

Thomas et al. 347

Table 3). Centrally the region of rotation is

between T3 and T4, as distinct negative/positive

change took place (Table 3), which concurs with

the findings of Ranasinghe et al. (2004). Varia-

tions in beach volume in the south lag behind

associated variations in the north by one month.

When a reversal in trend is encountered, north

lags behind south by two months (Figure 4b).

Lag disparity may be associated with diffracted

wave direction. A number of researchers

(i.e. Klein et al., 2002; Ojeda and Guillen, 2008;

Short and Masselink, 1999) suggested that beach

rotation is caused by localized retreat or advance

of the shoreline, that is, not necessarily long-term

loss or gain of sediment. The beach often returns

to pre-existing conditions, in response to a shift

in incident wave direction. Therefore, wave

direction has a significant effect on the beach

morphology of headland bays, particularly

rotational phenomena (Klein et al., 2002;

Ranasinghe et al., 2004; Short et al., 2000).

Figure 7a was produced from offshore wave

directional components, and volume changes

(Table 2), incorporating a timelag of one month.

Correlation was found between volume changes

at the beach extremities (T1 and T6) and

offshore wave direction, given by the regression

equations y¼ 0.116xþ 21.30 (R2¼ 0.521), and y

¼ –0.161x þ 108.54 (R2 ¼ 0.344), respectively.

Clearly, the R2 value representing c. 50% and

30% of data variation suggests that observed

morphology changes cannot be related solely

to wave direction. Other variables have influ-

ence, but the results are statistically significant

at the 95% confidence level ( p < 0.05). Not-

withstanding the above, it is the negative/

positive correlation between south and north

that is of most interest, as this indicates that

variations in wave direction have opposite

effects on beach morphology which concurs with

the findings of Gibbard (2005). That is, south to

southwesterly wave regimes would cause erosion

in the south and accretion in the north; conver-

sely, wave regimes that increasingly emanate

from south through southeast would have the

opposite effect and influence rotational patterns.

Longer-term evolution of constant erosion within

the south sector and out of phase accretion within

the north sector, as demonstrated by Thomas

et al. (2010), caused by consistent decadal

south-southwest wind regimes, is reinforced by

this research. Furthermore, relatively small incre-

mental changes in wind regime can have major

influences on beach morphology. Centrally, only a

weak correlation was shown between volume

change and wave direction throughout the

17-month monitoring period, possibly influenced

by the presence of a subtidal bank, which affords

a degree of protection. Figure 7b, produced by

comparing normalized volumes (Figure 6a,

Table 2), provides a model of morphological

change based on variation in wave directional

component. Waves that emanate from a southerly

direction (180� true) incur erosive tendency in the

south that decreases as wave trains rotate through

to 229� true, from which an accretion tendency

exists through to the west (270� true). The reverse

is true for the northerly shore. Figures 7c and 7d

depict conceptual models of expected morpho-

logical change in response to variations in wave

direction. Waves that approach from a south-

westerly towards south direction diffract

around the easternmost point of Caldey Island

and enter South Bay (the area of water

fronting South Sands). Southerly waves would

obviously induce greater diffraction driving

sediment alongshore from north to south

(Figure 7c), and with less severity as wave direc-

tion rotates toward southwest, as the Island

itself affords greater protection. As the waves

rotate from southwesterly toward west, the

Caldey Island diffraction becomes less important

with wave approach concentrated between the

offshore Island and the mainland (through

Caldey Sound). The headland promontory

becomes the focal point for diffraction, which

reverses the trend (Figure 7d).

Short-term variation in volume and variation

in NAO Index showed statistically insignificant

correlation (Table 4). However, cross-correlation

348 Progress in Physical Geography 35(3)

improvements (Figures 5a and 5b) at a one-month

timelag do highlight an inverse relationship of

rising volumes in the south (r ¼ 0.57, p < 0.05;

Figure 5b), and lowering volumes in the north

(r ¼ –0.33, p > 0.05; Figure 5a). Comparison

of gale wave heights (Table 1) and NAO Index

values (Table 2), highlight a toning down of

gale wave height when the NAO is weak, or

in transitional stage between positive and

negative phases. This is given by the regression

equations yghs¼ 2.05xPNAOþ 1.301 (R2¼ 0.69)

and yghs ¼ 1.24xNNAO þ 2.541 (R2 ¼ 0.72),

respectively, where yhs ¼ gale wave height,

xNNAO ¼ negative NAO Index values and

xPNAO ¼ positive NAO Index values. Results

agree with both Woolf et al.’s (2002) findings

regarding the influence of waves and NAO

variation, and Phillips and Crisp’s (2010) work

on sea-level variations with NAO Index and

phase. Figure 7e, produced from Table 4, depicts

Pearson correlation values at zero lag by pro-

file, for both negative and positive NAO values.

Positive correlation is shown when considering

positive NAO values. In contrast, a negative

correlation is shown when negative values are

considered given by the regression equations

yPNAO ¼ 0.148xPr – 0.436 (R2 ¼ 0.538) and

yNNAO ¼ –0.294xPr þ 1.243 (R2 ¼ 0.686),

respectively, where yPNAO ¼ positive NAO

Index values, yNNAO ¼ negative NAO Index

values and xPr ¼ profile location alongshore.

These results are of most interest as they

indicate that lowering volumes are expected

in the south and rising volumes in the north

during positive NAO phases. Conversely,

rising volumes are expected in the south and

lowering volumes in the north during negative

NAO phases, and these are depicted by a mor-

phological model in Figure 7f. Conceptually,

during positive NAO, sediment movement

would be similar to that shown in Figure 7d and

during negative NAO to that in Figure 7c.

The relative importance of cross-shore pro-

cesses can also translate into short-term profile

realignment, where onshore/offshore movement

could also result in subaerial volume adjustments,

i.e. non-rotational responses. The cut and fill

behaviour was subsequently removed to better

reflect subaerial volumes associated purely with

longshore rotation. However, the hiatus of the

tide across the entire intertidal zone (i.e. shoal-

ing), surf and swash zones are important, and are

subject to ongoing research in this embayment.

Short-term morphological change was dependent

on incident wave direction and diffraction, which

promotes longshore transport and an apparent

rotational response. Furthermore, the two pro-

montories acted independently to promote trend

reversals, i.e. beach rotation. Climatic variability

also plays an important role, especially with

reductions in incident wave height caused by

NAO Index changes. It is likely that similar

cycles of beach erosion/accretion and rotation are

a widespread phenomenon on headland beaches,

in response to changing atmospheric conditions

that influence wave height and direction.

VI Conclusions

Increasing physical, environmental and socio-

economic pressures are being placed upon the

world’s coastlines and, therefore, a compre-

hensive understanding of coastal processes

is important. Based on the results presented

in the above analysis of beach data from

South Sands, Tenby, large-scale atmospheric

pressure gradients should also be considered

in the development of new beach manage-

ment responses, particularly in the context

of timelag influences. These provide associ-

ated linkages with changes in wind/wave pat-

terns and subsequent shoreline morphological

change. Analyses resulted in the establishment

of timelag-associated links between morphologi-

cal change, climatic variation, and beach rota-

tion. Variations in incident wave direction,

and NAO were established as key drivers that

control short-term morphological change. This

knowledge enabled two models of short-term

beach rotation to be proposed. The first

Thomas et al. 349

considered incident wave direction, the second

climatic variability.

Notwithstanding the above, it is accepted that

further analyses are required to separate cut

and fill behaviour across the whole macrotidal

profile. Findings presented here form part of

continuous ongoing research into evolutionary

patterns of this headland embayment. However,

it is likely that similar geomorphologic settings

exist, where cycles of beach erosion/accretion

associated with beach rotation phenomena are

influenced by the presence of offshore islands,

in conjunction with more typical headland pro-

montories. The assessment of short-term

responses to changing atmospheric conditions

that influence wave height and their respective

directional components have implications for

coastal zone management, and careful analysis

is required of these phenomena over both short

and longer timescales. This is particularly

important at locations where coastal engineers

and planners consider hard engineering solu-

tions. The assessments and developed concep-

tual models are therefore important and should

be repeated on a wider scale, especially where

coastal areas are deemed to be at risk, so that

suitable coastal management policies can be

developed. These would underpin intervention

or no active intervention decisions, thereby

enabling more effective use of limited resources.

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