1

Geo Factsheet52www.curriculum-press.co.uk Number 352

The Gower: Waves, Tides, and Coastal DepositionThe Gower in South Wales is a peninsula of approximately 180 km² in size which was designated as an Area of Outstanding Natural Beauty in 1956. This Factsheet will use examples from the Gower coastline to examine the role of waves and tides in the creation and modification of depositional landforms including beaches and spits, as well as sand dune and salt marsh ecosystems.

Figure 1a. The Gower in Wales

GowerSt Govan’sHead Worm’s

Head LavernockPoint

TheSevern

St David’sHead

Major cellboundarySub-cell boundaryNet-drift direction

N

30 km

Wales

This map also shows the sediment cell and sub-cells in which material is transported around the Gower coast.

Figure 1b. The Gower Peninsula

Gower

Worm’sHead

OxwichPoint

OxwichBay

RhossiliBay

BurryHolms

Broughton Bay

WhitefordSands

WhitefordPoint

Burry EstuaryLlanrhidianSands

LandimoreMarsh

Llanrhidian

Marsh

Net dri

ftdir

ection

2 km

MarshDunes

This map shows the location of areas and landforms mentioned throughout this Factsheet.

Coastal depositionCoastal deposition takes place when the accumulation of coastal material such as sand and shingle is greater than its depletion. That is, the coast acts as a store of material as inputs exceed outputs. Depositional coastal landforms include beaches and spits and coastal ecosystems such as sand dunes and salt marshes. The material from which depositional landforms are comprised can range in size from large boulders and cobbles to fine silts and clays. The sediment which accumulates may come from a number of sources: • The breakdown of rock faces such as cliffs and headlands.• The breakdown of existing larger calibre material on beaches or

material moved from beaches elsewhere by currents.• Material brought to the coast from inland by rivers.• Deep-water offshore sediments carried and deposited onshore as

sea levels rose after the last Ice Age ended.The Gower is situated within a storm-wave environment and depositional landforms are fed by material which has been eroded by wave action largely from the sedimentary rock outcrops of Carboniferous Limestone, Old Red Sandstone, and boulder clay which make up the coastline of the peninsula. The valley of the River Loughor was occupied by ice during the last Ice Age and has provided a source of glacial deposits which have been transported from inland to the shoreline over time. During the Flandrian Transgression much material was carried onshore by the rising seas as temperatures rose, bringing offshore material landwards towards the Gower coastline.Transportation and sediment cellsThe transportation of sediment around coastlines tends to take place in sediment cells (or littoral cells) which are discrete areas of coastline where sediment is moved. Cell boundaries are often delimited by where the coastline’s orientation shows a marked change, for example, at large headlands (littoral drift divides), or in sediment sinks, which are commonly tidal inlets or estuaries. Cells are defined based on the idea that the movement of most sediment within one cell does not influence adjacent cells. A number of sediment cells have been identified around the coastline of Great Britain. Figure 1a shows the major sediment cell for the Gower, which runs from the River Severn estuary in the east to St David’s Head in the west. Sediment cells can be divided into sub-cells, marked by smaller, yet still significant changes in the coastline. The Gower peninsula itself provides a sub-cell boundary within the larger cell. Worm’s Head (see Figure 1b) marks this drift divide. The sub-cell to the east extends to Lavernock Point. The adjacent sub-cell extends from Worm’s Head to St Govan’s Head in Pembrokeshire. The littoral drift in both sub-cells is mainly eastwards, but drift is not a significant process overall and is variable in direction within more sheltered embayments. There is, however, a clear net drift towards the northeast along the coast from Burry Holms to Whiteford Point in the north-western part of the peninsula which has helped to form a spit (see Figure 1a and Figure 6). Waves and tidesWaves, generated by wind blowing over the sea’s surface, provide a mechanism by which material can be eroded, transported, and deposited within sediment cells. They play an important part in the formation and ongoing modification of depositional landforms.

2

The Gower: Waves, Tides, and Coastal Deposition Geo Factsheet 352

Constructive waves are associated with deposition. These waves have a stronger swash than backwash thus encouraging a net shoreward movement of material. Once established, depositional landforms can be shaped and modified by both constructive and destructive waves. Tides are the result of the gravitational pull of the Sun and Moon on the Earth’s water surface and cause the mean sea level to oscillate. The British Isles experiences two high and two low tides per day and the tides reach different levels at different points throughout the lunar cycle. • High tide: when the coastal waters reach the highest level of the

day, we often say ‘the tide is in’. As the water advances to submerge the intertidal zone, this can be termed lood tide.

• Low tide: the lowest coastal water level of the day, we often say ‘the tide is out’. Ebb tide is the period when the tide ‘goes out’ to reveal the intertidal zone again. At the point between high and low tide, we may say ‘the tide is turning’, the water becomes quite motionless at this time and this phenomenon can be termed slack tide.

• Tidal range: this is the difference between high and low tide levels. It differs throughout the lunar cycle with spring tides giving the largest tidal ranges and neap tides the smallest. Tidal ranges can be classified as macrotidal (>4m between high and low tide), mesotidal (4-2m), and microtidal (<2m).

• Spring tide: this occurs when the Sun and Moon exert pull in the same direction during full and new moon phases, resulting in the highest high tides and the lowest low tides of the cycle.

• Neap tide: this shows the lowest high tides and highest low tides of the cycle during half-moon phases, as the influence of the gravitational pull is lessened as the sun and moon pull in different directions.

The coastline of the Gower is macrotidal, as is most of the rest of the British Isles.

BeachesBeaches are depositional landforms created by the accumulation of both unconsolidated inorganic and organic material – that is, the products of broken down rocks and biological material (most commonly fragments of shells). Beaches form in the intertidal zone between the lowest spring tide level and the highest level reached by the waves, usually a storm beach (see below). The factors that influence beach profiles, which consist of a series of undulating troughs and ridges, can be summarised as follows: 1. Waves - wave energy, which becomes greater as wind strength,

duration and fetch increase, and whether the waves are constructiveor destructive.

2. Tidal range – this influences where waves will break on the shoreand the expanse of beach over which the waves may break.

3. The type of beach material – its size, shape and composition,which affects the beach gradient and extent to which the dissipationof wave energy takes place. This is largely linked to percolation,which is greater on coarser shingle beaches compared to sand.

Material size is significant as it influences the steepness and width of a beach. If a beach is made of shingle, the larger material piles up creating steep, narrow beaches; the smaller particle sizes on sand beaches result in them being wide and flat. Cobbles and pebbles (the large calibre beach material), therefore, form the steepest beaches, and very fine sand the most gently sloping (almost entirely horizontal) beaches. Rhossili Bay in the western part of the Gower is a wide embayment formed by the differential erosion of less resistant Old Red Sandstone and boulder clay flanked by the more resistant limestone forming Worm’s Head to the south and Burry Holms to the north. The bay has a 3-mile long sandy beach made up of small-sized sand particles which help to give rise to its wide expanse and low-gradient (Figure 2).

Figure 2 Shows Rhossili Bay at low tide and also shows evidence of the influence of tides on beaches: • Strand line: a non-permanent marker which shows the position of

the last high tide for a current portion of the lunar cycle. It is usually comprised of material such as driftwood, seaweed and non-natural materials washed onshore and deposited by the waves. Figure 3abelow clearly shows the strandline on Whiteford Sands and 3bthe waves meeting the strandline at high tide under non-stormyconditions on Rhossili Bay.

• Storm beach: This is a distinctive raised mound of large calibrebeach material which has been deposited by high energy stormwaves during the highest spring tides. A storm beach can remainin situ until altered by the next point in time at which the wavesreach a similar magnitude.

Another common beach feature whose positioning is determined by tide is a berm. • Berms are seaward-sloping ridges of sand, shingle or pebbles,

which lie parallel to the storm beach, formed by deposition ofcoarse material at the furthest limit reached by the swash duringprevious high tides.

Figure 3a. The strandline at Whiteford SandsFigure 3b. High tide at Rhossili Bay

Figure 2. Rhossili Bay at low tide

Sorting of sediment on beachesIf beach material has been sorted this means that it has been organised and graded, largely according to its size. This happens as particles of different sizes have different velocities at which they will ‘settle’ and be deposited by the waves. As well as storm beaches being evident on beaches in storm-wave environments such as the Gower, particles may be sorted so that the coarser material tends to be found at the furthest limit reached by the waves and the finest sediments are found on the seaward portion of the beach. This is because if the material is carried up the beach by the strong swash of a constructive wave, its strength enables it to transport the larger, heavier sediments to its furthest limit. The water will then percolate down through the deposited coarser material, which will have large spaces between the particles, and will provide a rough surface which will dissipate the wave’s energy. The strength of the backwash is therefore reduced, only enabling it to return the smaller particles seawards. The upper parts of beaches therefore are covered by larger calibre material and the smaller material will be situated further seawards. This sorting can be seen in the photograph of Whiteford Sands below (Figure 4). Particles may also be sorted by the process of longshore drift (see later), with material becoming

The Gower: Waves, Tides, and Coastal Deposition Geo Factsheet 352

3

progressively smaller, smoother, and more rounded along the length of a beach, or spit as the waves carry it further downdrift.

Figure 4. Sorting of coarser and finer grained sediments on Whiteford Sands

Note that the more seaward sandy portion of the beach to the left of the photograph has a gentle gradient compared to the more landward pebble-covered portion of the beach.

SpitsSpits are long, narrow accumulations of sand and/or shingle which are attached to the mainland at one end and extend seawards, usually into an estuary or bay. Longshore drift (also called littoral drift) is the key process involved in the formation of spits and it occurs when the prevailing wind approaches the shore at an oblique angle and material is moved along the coastline. Driven in the direction of the prevailing wind, swash carries sediment such as sand, shingle, and pebbles, up the beach at an angle, and then backwash moves sediment back down the beach at right angles to the shoreline along the steepest path, under the influence of gravity. Repetition of this process means that material will gradually be transported along the coast in a zig-zag fashion, leading to a net movement of material downdrift. Spits are created when the coastline changes direction relatively abruptly, for example at a river estuary or when differential erosion of the coastline has created an indentation or bay, but the longshore drift continues in the same direction as it did before. The material carried by the longshore drift will be presented with more tranquil waters in the lee of the point at which the coastline has changed direction and deposition by constructive waves is encouraged, creating the spit as material continues to be moved along the coast and eventually builds up above sea level in the relatively shallow waters close to the shoreline. Spits may have a recurved end, owing to changes in the dominant wind direction to the second-most-dominant, forcing the waves to move and deposit the material in a slightly different direction. They are unlikely to extend fully across estuaries as the current will carry the material out to sea, meaning that it will not deposit and also, as the water deepens, the material is less likely to be able to build up to the point where it is raised above sea level.

Coastal sand dunes may form to the rear of sand spits, especially in macro-tidal environments, as onshore winds blow dry sands, revealed at low tide, to the back of the sand spit. Mudflats and saltmarshes can develop behind spits as mud and fine silt are deposited in the low-energy, sheltered environment, protected from the full force of the sea. See Figure 6 below for an example of a spit, Whiteford Point, and its associated sand dunes and salt marsh.

Sand dunes and salt marshesSand dunesDune formation requires relatively persistent and strong onshore winds and a plentiful sediment supply. Sand deposited by longshore drift is moved up the beach by wind via the processes of saltation, creep, or sometimes suspension, to create coastal sand dunes. Sand is trapped by an obstacle of some sort – perhaps a berm or driftwood - and is then colonised by vegetation. Vegetation stabilises the sand and encourages more sand to build up. This initial build-up of sand creates embryo dunes. Dunes migrate inland over time as newer embryo dunes are formed at the shore. The mature dunes further inland can reach heights of 10 metres or more, interspersed by dune slacks which dip down towards the water table. Oxwich Bay, in south Gower, is an extensive sandy beach backed by sand dunes (see Figure 5).

Figure 5. Oxwich Bay (see Figure 1b. for location)

Deposits forming Oxwich Bay were carried shoreward by the rising sea level of the Flandrian Transgression and trapped by the enclosure created by a synclinal dip flanked by the limestone outcrops at Great Tor promontory to the east and Oxwich Point to the west. Oxwich Bay beach, like Rhossili beach, is comprised largely of small-grained, sandy material which helps to create the wide, low-gradient profile. Combined with the large tidal range influencing the Gower, this means that there is potential for a very large expanse of sand to be revealed at low tide, especially low spring tide, and this provides a sustained sediment source for theOxwich and Nicholaston dunes that have developed to the rear of the bay.’

Great Tor(limestone)

Cliff line

Cliff line

Wave-cut platformsurroundsheadland

OxwichPoint(limestoneheadland)

Oxwich Bay

Wide, low-gradient

sandy beachOxwich Burrows

sand dunes

Oxwich

Mars

h

wave refraction = energy concentrationon headlands leading to erosion

wave refraction =energy dissipated in bays deposition

1 km

N

ear of Oxwich Bay

Deposits forming Oxwich Bay were carried shoreward by the rising sea level of the Flandrian Transgression. They were trapped by the enclosure created by a synclinal dip flanked by the limestone outcrops at Great Tor promontory to the east and Oxwich Point to the west. Oxwich Bay beach, like Rhossili beach, is comprised largely of small-grained, sandy material which helps to create the wide, low-gradient profile. Combined with the large tidal range influencing the Gower, this means that there is potential for a very large expanse of sand to be revealed at low tide, especially low spring tide and this provides a sustained sediment source for the Oxwich and Nicholaston dunes that have developed to the rear of the bay.

Oxwich Bay – a wide, flat beach backed with sand dunesdunes

Nicholaston BurrowsNicholaston Pill

toleading

4

The Gower: Waves, Tides, and Coastal Deposition Geo Factsheet 352

The Oxwich dune complex to the rear of Oxwich Bay

Whiteford Point, located in north-west Gower, exhibits the typical properties of a spit. Whiteford Sands beach, attached to Broughton Sands to the south, extends northwards for over three kilometres, eventually extending into the mouth of the River Loughor (the Burry Estuary), thus creating the spit. The spit is oriented from southwest to northeast, showing that the dominant wind is coming from the south-west and thus driving the net littoral drift in a north-easterly direction. The spit has a recurved shingle and sand bank at its distal end and its furthest extent is marked by Whiteford Lighthouse, which is surrounded by a mussel bed and quicksand. The spit was able to build up above sea level using its foundation of glacial till deposits left by ice that extended through the Loughor Valley during the last Ice Age. The beach and dunes are composed of material driven landwards by the rising seas of the Flandrian Transgression. The beach is backed by Whiteford Burrows sand dunes; around 3 km long and between 400m and 600m wide, reaching heights of up to 24m.

The spit provides shelter in its lee for the extensive Landimore Marsh, Llanrhidian Salt Marsh and the Llanrhidian Sands mudflats. The fine sands and silty clays of the mudflats are made up of glacial outwash material and glacial drift which has been broken down by fluvial and marine processes.

Whiteford Point – this image shows the distal end of the spit, largely covered in dunes

• Bird, E. (2008) Coastal Geomorphology: An Introduction Wiley• Bridges, E. M. (1997) Classic Landforms of the Gower Coast The

Geographical Association • Davies, A. (2012) Walking on Gower Cicerone• Hill, M. (2004) Coasts and Coastal Management Hodder and

Stoughton• Masselink, G. Hughes, M. G. & Knight, J. (2011) Introduction to

Coastal Processes and Geomorphology (2nd Edition) Routledge• Motyka, J.M. & Brampton, A.H. (1993) Coastal Management:

mapping of littoral cells H.R. Wallingford (A report for MAFF)http://eprints.hrwallingford.co.uk/748/1/SR328.pdf

Acknowledgements: This Geo Factsheet was researched and written by Kate Cowan, a teacher of Geography at King Edward VI High School for Girls, Birmingham, and published in September 2016 by Curriculum Press. All photographs by K. Cowan. ISSN 1351-5136

BurryEstuary

Whiteford Point

Llanrhidiansands(mudflats)

Llanrhidianmarsh

Landimoremarsh

Whi

tefor

d Sa

nds

Whi

tefor

d Bu

rrows

Note the low-gradient wide beach to the left of the photograph, which would provide, at low spring tide especially, a plentiful source of sediment for the Whiteford Burrows dunes.

Llanrhidian Salt Marsh with salt pans and a network of creeks

Bu

SummaryThe Gower peninsula provides examples of depositional landforms and ecosystems including Rhossili Bay, Oxwich Bay and its dunes, and Whiteford Point and its associated dunes and saltmarsh. These examples illustrate how waves and tides are important influences on coastlines regarding the creation and modification of depositional landforms and ecosystems.

Bibliography and further readingDu

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d by

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re as

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after

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ast I

ce A

ge en

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ands

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ple,

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ides

), or

in se

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reat

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ks th

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ift d

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to th

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t

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ds to

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ck P

oint

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an’s

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in P

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ards

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int

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ester

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ed to

form

a spi

t (se

e Fig

ure 1

a and

Fig

ure 6

).

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es an

d tid

esW

aves

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d by

win

d bl

owin

g ov

er th

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’s su

rface

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the

form

ation

and

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ifica

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of d

epos

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al lan

dfor

ms.

2 The G

ower

- wa

ves,

tides

and

coas

tal d

epos

ition

Geo

Fac

tshee

t 352

Cons

tructi

ve w

aves

are a

ssoc

iated

with

dep

ositi

on. T

hese

wav

es h

ave

a stro

nger

swas

h th

an b

ackw

ash

thus

enco

urag

ing

a net

shor

ewar

d

mov

emen

t of m

ateria

l. On

ce es

tablis

hed,

dep

ositi

onal

landf

orm

s can

be sh

aped

and

mod

ified

by

both

cons

tructi

ve an

d de

struc

tive w

aves

.

Tide

s are

the r

esul

t of t

he g

ravi

tatio

nal p

ull o

f the

Sun

and

Moo

n on

the E

arth

’s wa

ter su

rface

and

caus

e the

mea

n se

a lev

el to

osc

illate

. The

Briti

sh Is

les ex

perie

nces

two

high

and

two

low

tides

per

day

and

the

tides

reac

h di

ffere

nt le

vels

at di

ffere

nt p

oint

s thr

ough

out t

he lu

nar c

ycle.

• Hig

h tid

e: wh

en th

e coa

stal w

aters

reac

h th

e hig

hest

level

of th

e

day,

we o

ften

say

‘the t

ide i

s in’

. As t

he w

ater a

dvan

ces t

o su

bmer

ge

the i

nter

tidal

zone

, thi

s can

be t

erm

ed fl

ood

tide.

• Low

tide

: the

lowe

st co

astal

wate

r lev

el of

the d

ay, w

e ofte

n sa

y

‘the t

ide i

s out

’. Eb

b tid

e is t

he p

erio

d wh

en th

e tid

e ‘go

es o

ut’

to re

veal

the i

nter

tidal

zone

again

. At t

he p

oint

betw

een

high

and

low

tide,

we m

ay sa

y ‘th

e tid

e is t

urni

ng’,

the w

ater b

ecom

es

quite

mot

ionl

ess a

t thi

s tim

e and

this

phen

omen

on ca

n be

term

ed

slack

tide

.• T

idal

rang

e: th

is is

the d

iffer

ence

betw

een

high

and

low

tide

levels

. It d

iffer

s thr

ough

out t

he lu

nar c

ycle

with

sprin

g tid

es g

ivin

g

the l

arge

st tid

al ra

nges

and

neap

tide

s the

small

est.

Tida

l ran

ges

can

be cl

assif

ied as

mac

rotid

al (>

4m b

etwee

n hi

gh an

d lo

w tid

e);

mes

otid

al (4

-2m

) and

micr

otid

al (<

2m).

• Spr

ing

tide:

this

occu

rs wh

en th

e Sun

and

Moo

n ex

ert p

ull i

n th

e

sam

e dire

ction

dur

ing

full

and

new

moo

n ph

ases

, res

ultin

g in

the

high

est h

igh

tides

and

the l

owes

t low

tide

s of t

he cy

cle.

• Nea

p tid

e: th

is sh

ows t

he lo

west

high

tide

s and

hig

hest

low

tides

of th

e cyc

le du

ring

half-

moo

n ph

ases

, as t

he in

fluen

ce o

f the

grav

itatio

nal p

ull i

s les

sene

d as

the s

un an

d m

oon

pull

in d

iffer

ent

dire

ction

s.Th

e coa

stlin

e of t

he G

ower

is m

acro

tidal,

as is

mos

t of t

he re

st of

the

Briti

sh Is

les.

Beac

hes

Beac

hes a

re d

epos

ition

al lan

dfor

ms c

reate

d by

the a

ccum

ulati

on o

f bot

h

unco

nsol

idate

d in

orga

nic a

nd o

rgan

ic m

ateria

l – th

at is,

the p

rodu

cts o

f

brok

en d

own

rock

s and

bio

logi

cal m

ateria

l (m

ost c

omm

only

frag

men

ts

of sh

ells).

Bea

ches

form

in th

e int

ertid

al zo

ne b

etwee

n th

e low

est

sprin

g tid

e lev

el an

d th

e hig

hest

level

reac

hed

by th

e wav

es; u

suall

y

a sto

rm b

each

(see

belo

w). T

he fa

ctors

that

influ

ence

bea

ch p

rofil

es,

which

cons

ist o

f a se

ries o

f und

ulati

ng tr

ough

s and

ridg

es, c

an b

e

sum

mar

ised

as fo

llows

:

1. W

aves

- wa

ve en

ergy

, whi

ch b

ecom

es g

reate

r as w

ind

stren

gth,

dura

tion

and

fetch

incr

ease

, and

whe

ther

the w

aves

are c

onstr

uctiv

e

or d

estru

ctive

.2.

Tid

al ra

nge –

this

influ

ence

s whe

re w

aves

will

bre

ak o

n th

e sho

re

and

the e

xpan

se o

f bea

ch o

ver w

hich

the w

aves

may

bre

ak.

3. T

he ty

pe o

f bea

ch m

ateria

l – it

s size

, sha

pe an

d co

mpo

sitio

n,

which

affe

cts th

e bea

ch g

radi

ent a

nd ex

tent t

o wh

ich th

e diss

ipati

on

of w

ave e

nerg

y tak

es p

lace.

This

is lar

gely

link

ed to

per

colat

ion,

which

is g

reate

r on

coar

ser s

hing

le be

ache

s com

pare

d to

sand

.

Mate

rial s

ize is

sign

ifica

nt as

it in

fluen

ces t

he st

eepn

ess a

nd w

idth

of a

beac

h. If

a be

ach

is m

ade o

f shi

ngle,

the l

arge

r mate

rial p

iles u

p cr

eatin

g

steep

, nar

row

beac

hes;

the s

mall

er p

artic

le siz

es o

n sa

nd b

each

es re

sult

in th

em b

eing

wide

and

flat.

Cobb

les an

d pe

bbles

(the

larg

e cali

bre

beac

h m

ateria

l) th

eref

ore f

orm

the s

teepe

st be

ache

s and

ver

y fin

e san

d

the m

ost g

ently

slop

ing

(alm

ost e

ntire

ly h

orizo

ntal)

bea

ches

. Rho

ssili

Bay

in th

e wes

tern

part

of th

e Gow

er is

a wi

de em

baym

ent f

orm

ed b

y

the d

iffer

entia

l ero

sion

of le

ss re

sistan

t Old

Red

San

dsto

ne an

d bo

ulde

r

clay

flank

ed b

y th

e mor

e res

istan

t lim

esto

ne fo

rmin

g W

orm

’s He

ad to

the s

outh

and

Burry

Hol

ms t

o th

e nor

th. T

he b

ay h

as a

3 m

ile lo

ng

sand

y be

ach

mad

e up

of sm

all-si

zed

sand

par

ticles

whi

ch h

elp to

giv

e

rise t

o its

wid

e exp

anse

and

low-

grad

ient (

Figu

re 2

).

Figu

re 2

Sho

ws R

hoss

ili B

ay at

low

tide a

nd al

so sh

ows e

vide

nce

of th

e inf

luen

ce o

f tid

es o

n be

ache

s:

• Stra

nd li

ne: a

non

-per

man

ent m

arke

r whi

ch sh

ows t

he p

ositi

on o

f

the l

ast h

igh

tide f

or a

curre

nt p

ortio

n of

the l

unar

cycle

. It i

s usu

ally

com

prise

d of

mate

rial s

uch

as d

riftw

ood,

seaw

eed

and

non-

natu

ral

mate

rials

wash

ed o

nsho

re an

d de

posit

ed b

y th

e wav

es. F

igur

e 3a

belo

w cle

arly

show

s the

stra

ndlin

e on

Whi

tefor

d Sa

nds a

nd 3

b

the w

aves

mee

ting

the s

trand

line a

t hig

h tid

e und

er n

on-st

orm

y

cond

ition

s on

Rhos

sili B

ay.

• Sto

rm b

each

: Thi

s is a

dist

incti

ve ra

ised

mou

nd o

f lar

ge ca

libre

beac

h m

ateria

l whi

ch h

as b

een

depo

sited

by

high

ener

gy st

orm

wave

s dur

ing

the h

ighe

st sp

ring

tides

. A st

orm

bea

ch ca

n re

main

in si

tu u

ntil

alter

ed b

y th

e nex

t poi

nt in

tim

e at w

hich

the w

aves

reac

h a s

imila

r mag

nitu

de.

Anot

her c

omm

on b

each

featu

re w

hose

pos

ition

ing

is de

term

ined

by

tides

is a

berm

.• B

erm

s are

seaw

ard-

slopi

ng ri

dges

of s

and,

shin

gle o

r peb

bles

,

which

lie p

arall

el to

the s

torm

bea

ch, f

orm

ed b

y de

posit

ion

of

coar

se m

ateria

l at t

he fu

rthes

t lim

it re

ache

d by

the s

wash

dur

ing

prev

ious

hig

h tid

es.

Figu

re 3

a. Th

e stra

ndlin

e at W

hitef

ord

Sand

s

Figu

re 3

b. H

igh

tide a

t Rho

ssili

Bay

Figu

re 2

. Rho

ssili

Bay

at lo

w tid

e

Sorti

ng o

f sed

imen

t on

beac

hes

If be

ach

mate

rial h

as b

een

sorte

d th

is m

eans

that

it ha

s bee

n or

gani

sed

and

grad

ed, l

arge

ly ac

cord

ing

to it

s size

. Thi

s hap

pens

as p

artic

les o

f

diffe

rent

size

s hav

e diff

eren

t velo

cities

at w

hich

they

will

‘sett

le’ an

d

be d

epos

ited

by th

e wav

es. A

s well

as st

orm

bea

ches

bein

g ev

iden

t on

beac

hes i

n sto

rm-w

ave e

nviro

nmen

ts su

ch as

the G

ower

, par

ticles

may

be so

rted

so th

at th

e coa

rser m

ateria

l ten

ds to

be f

ound

at th

e fur

thes

t

limit

reac

hed

by th

e wav

es an

d th

e fin

est s

edim

ents

are f

ound

on

the

seaw

ard

porti

on o

f the

bea

ch. T

his i

s bec

ause

if m

ateria

l is c

arrie

d

up th

e bea

ch b

y th

e stro

ng sw

ash

of a

cons

tructi

ve w

ave,

its st

reng

th

enab

les it

to tr

ansp

ort t

he la

rger

, hea

vier

sedi

men

ts to

its f

urth

est l

imit.

The w

ater w

ill th

en p

erco

late d

own

thro

ugh

the d

epos

ited

coar

ser

mate

rial,

which

will

hav

e lar

ge sp

aces

betw

een

the p

artic

les an

d wi

ll

prov

ide a

roug

h su

rface

whi

ch w

ill d

issip

ate th

e wav

e’s e

nerg

y. T

he

stren

gth

of th

e bac

kwas

h is

ther

efor

e red

uced

, onl

y en

ablin

g it

to re

turn

the s

mall

er p

artic

les se

awar

ds. T

he u

pper

par

ts of

bea

ches

ther

efor

e

are c

over

ed b

y lar

ger c

alibr

e mate

rial a

nd th

e sm

aller

mate

rial w

ill b

e

situa

ted fu

rther

seaw

ards

. Thi

s sor

ting

can

be se

en in

the p

hoto

grap

h

of W

hitef

ord

Sand

s belo

w (F

igur

e 4).

Parti

cles m

ay al

so b

e sor

ted

by th

e pro

cess

of l

ongs

hore

drif

t (se

e late

r), w

ith m

ateria

l bec

omin

g

The G

ower

- wa

ves,

tides

and

coas

tal d

epos

ition

Geo

Fac

tshee

t 352

3 prog

ress

ively

small

er, s

moo

ther

and

mor

e rou

nded

alon

g th

e len

gth

of

a bea

ch o

r spi

t as t

he w

aves

carry

it fu

rther

dow

ndrif

t.

Figu

re 4

. Sor

ting

of co

arse

r and

fine

r gra

ined

sedi

men

ts

on W

hitef

ord

Sand

s

Note

that

the m

ore s

eawa

rd sa

ndy

porti

on o

f the

bea

ch to

the l

eft o

f

the p

hoto

grap

h ha

s a g

entle

gra

dien

t com

pare

d to

the m

ore l

andw

ard

pebb

le-co

vere

d po

rtion

of t

he b

each

.

Spits

Spits

are l

ong,

nar

row

accu

mul

ation

s of s

and

and/

or sh

ingl

e whi

ch

are a

ttach

ed to

the m

ainlan

d at

one e

nd an

d ex

tend

seaw

ards

, usu

ally

into

an es

tuar

y or

bay

. Lon

gsho

re d

rift (

also

calle

d lit

tora

l drif

t) is

the k

ey p

roce

ss in

volv

ed in

the f

orm

ation

of s

pits

and

it oc

curs

when

the p

reva

iling

win

d ap

proa

ches

the s

hore

at an

obl

ique

angl

e and

mate

rial i

s mov

ed al

ong

the c

oastl

ine.

Driv

en in

the d

irecti

on o

f the

prev

ailin

g wi

nd, s

wash

carri

es se

dim

ent s

uch

as sa

nd, s

hing

le an

d

pebb

les u

p th

e bea

ch at

an an

gle a

nd th

en b

ackw

ash

mov

es se

dim

ent

back

dow

n th

e bea

ch at

righ

t ang

les to

the s

hore

line a

long

the s

teepe

st

path

, und

er th

e inf

luen

ce o

f gra

vity

. Rep

etitio

n of

this

proc

ess m

eans

that

mate

rial w

ill g

radu

ally

be tr

ansp

orted

alon

g th

e coa

st in

a zig

-zag

fash

ion,

lead

ing

to a

net m

ovem

ent o

f mate

rial d

ownd

rift.

Spits

are

crea

ted w

hen

the c

oastl

ine c

hang

es d

irecti

on re

lativ

ely ab

rupt

ly, f

or

exam

ple a

t a ri

ver e

stuar

y or

whe

n di

ffere

ntial

eros

ion

of th

e coa

stlin

e

has c

reate

d an

inde

ntati

on o

r bay

, but

the l

ongs

hore

drif

t con

tinue

s

in th

e sam

e dire

ction

as it

did

bef

ore.

The m

ateria

l car

ried

by th

e

long

shor

e drif

t will

be p

rese

nted

with

mor

e tra

nqui

l wate

rs in

the l

ee

of th

e poi

nt at

whi

ch th

e coa

stlin

e has

chan

ged

dire

ction

and

depo

sitio

n

by co

nstru

ctive

wav

es is

enco

urag

ed, c

reati

ng th

e spi

t as m

ateria

l

cont

inue

s to

be m

oved

alon

g th

e coa

st an

d ev

entu

ally

build

s up

abov

e

sea l

evel

in th

e rela

tively

shall

ow w

aters

close

to th

e sho

relin

e. Sp

its

may

hav

e a re

curv

ed en

d ow

ing

to ch

ange

s in

the d

omin

ant w

ind

dire

ction

to th

e sec

ond-

mos

t-dom

inan

t, fo

rcin

g th

e wav

es to

mov

e and

depo

sit th

e mate

rial i

n a s

light

ly d

iffer

ent d

irecti

on. T

hey

are u

nlik

ely

to ex

tend

fully

acro

ss es

tuar

ies as

the c

urre

nt w

ill ca

rry th

e mate

rial o

ut

to se

a, m

eani

ng th

at it

will

not d

epos

it an

d als

o, as

the w

ater d

eepe

ns,

the m

ateria

l is l

ess l

ikely

to b

e abl

e to

build

up

to th

e poi

nt w

here

it is

raise

d ab

ove s

ea le

vel.

Coas

tal sa

nd d

unes

may

form

to th

e rea

r of s

and

spits

, esp

ecial

ly in

mac

ro-ti

dal e

nviro

nmen

ts, as

ons

hore

win

ds b

low

dry

sand

s, re

veale

d

at lo

w tid

e, to

the b

ack

of th

e san

d sp

it. M

udfla

ts an

d sa

ltmar

shes

can

deve

lop

behi

nd sp

its as

mud

and

fine s

ilt is

dep

osite

d in

the l

ow-e

nerg

y,

shelt

ered

envi

ronm

ent,

prot

ected

from

the f

ull f

orce

of t

he se

a. Se

e

Figu

re 6

belo

w fo

r an

exam

ple o

f a sp

it, W

hitef

ord

Poin

t, an

d its

asso

ciated

sand

dun

es an

d sa

lt m

arsh

.

Sand

dun

es an

d sa

lt m

arsh

es

Sand

dun

esDu

ne fo

rmati

on re

quire

s rela

tively

per

sisten

t and

stro

ng o

nsho

re w

inds

and

a plen

tiful

sedi

men

t sup

ply.

San

d de

posit

ed b

y lo

ngsh

ore d

rift i

s

mov

ed u

p th

e bea

ch b

y wi

nd v

ia th

e pro

cess

es o

f salt

ation

, cre

ep o

r

som

etim

es su

spen

sion

to cr

eate

coas

tal sa

nd d

unes

. San

d is

trapp

ed b

y

an o

bstac

le of

som

e sor

t – p

erha

ps a

berm

or d

riftw

ood

- and

is th

en

colo

nise

d by

veg

etatio

n. V

egeta

tion

stabi

lises

the s

and

and

enco

urag

es

mor

e san

d to

bui

ld u

p. T

his i

nitia

l bui

ld-u

p of

sand

crea

tes em

bryo

dune

s. Du

nes m

igra

te in

land

over

tim

e as n

ewer

embr

yo d

unes

are

form

ed at

the s

hore

. The

matu

re d

unes

furth

er in

land

can

reac

h he

ight

s of

10 m

etres

or m

ore,

inter

sper

sed

by d

une s

lacks

whi

ch d

ip d

own

towa

rds t

he w

ater t

able.

Oxw

ich B

ay, i

n so

uth

Gowe

r, is

an ex

tensiv

e

sand

y be

ach

back

ed b

y sa

nd d

unes

(see

Fig

ure 5

).

Figu

re 5

. Oxw

ich B

ay (s

ee F

igur

e 1b.

for l

ocati

on)

Depo

sits f

orm

ing

Oxwi

ch B

ay w

ere c

arrie

d sh

orew

ard

by th

e

risin

g se

a lev

el of

the F

landr

ian T

rans

gres

sion

and

trapp

ed b

y

the e

nclo

sure

crea

ted b

y a s

yncli

nal d

ip fl

anke

d by

the l

imes

tone

outcr

ops a

t Gre

at To

r pro

mon

tory

to th

e eas

t and

Oxw

ich P

oint

to th

e wes

t. Ox

wich

Bay

bea

ch, l

ike R

hoss

ili b

each

, is c

ompr

ised

large

ly o

f sm

all-g

rain

ed, s

andy

mate

rial w

hich

help

s to

crea

te th

e

wide

, low

-gra

dien

t pro

file.

Com

bine

d wi

th th

e lar

ge ti

dal r

ange

influ

encin

g th

e Gow

er, t

his m

eans

that

ther

e is p

oten

tial f

or a

very

large

expa

nse o

f san

d to

be r

evea

led at

low

tide,

espe

cially

low

sprin

g tid

e, an

d th

is pr

ovid

es a

susta

ined

sedi

men

t sou

rce f

or th

e

Oxwi

ch an

d Ni

chol

asto

n du

nes t

hat h

ave d

evelo

ped

to th

e rea

r of

the b

ay.’

Oxwi

ch B

ay –

a wi

de, f

lat b

each

bac

ked

with

sand

dun

es

Grea

t Tor

(lim

esto

ne)

Cliff

line

Cliff

line

Wav

e-c

ut p

latfo

rmsu

rroun

dshe

adlan

dOx

wich

Poin

t(li

mes

tone

head

land)

Oxwi

ch B

ayW

ide,

low-

grad

ient

sand

y be

ach

Oxwi

ch B

urro

wssa

nd d

unes

Oxwi

ch M

arsh

wave

refra

ction

=en

ergy

conc

entra

tion

on h

eadl

ands

eros

ion

wave

refra

ction

=en

ergy

diss

ipate

d in

bay

s

depo

sitio

n1

kmN Th

e Dep

osits

form

ing

Oxwi

ch B

ay w

ere c

arrie

d sh

orew

ard

by th

e

risin

g se

a lev

el of

the F

landr

ian T

rans

gres

sion.

The

y we

re tr

appe

d

by th

e enc

losu

re cr

eated

by

a syn

clina

l dip

flan

ked

by th

e lim

esto

ne

outcr

ops a

t Gre

at To

r pro

mon

tory

to th

e eas

t and

Oxw

ich P

oint

to

the w

est.

Oxwi

ch B

ay b

each

, lik

e Rho

ssili

bea

ch, i

s com

prise

d

large

ly o

f sm

all-g

rain

ed, s

andy

mate

rial w

hich

help

s to

crea

te th

e

wide

, low

-gra

dien

t pro

file.

Com

bine

d wi

th th

e lar

ge ti

dal r

ange

influ

encin

g th

e Gow

er, t

his m

eans

that

ther

e is p

oten

tial f

or a

very

larg

e exp

anse

of s

and

to b

e rev

ealed

at lo

w tid

e, es

pecia

lly

low

sprin

g tid

e, an

d th

is pr

ovid

es a

susta

ined

sedi

men

t sou

rce f

or

the O

xwich

and

Nich

olas

ton

dune

s tha

t hav

e dev

elope

d to

the r

ear o

f

the b

ay.'

Oxwi

ch B

ay –

a wi

de, f

lat b

each

bac

ked

with

sand

dune

s4 Th

e Gow

er -

wave

s, tid

es an

d co

astal

dep

ositi

on G

eo F

actsh

eet 3

52

The O

xwich

dun

e com

plex

to th

e rea

r of O

xwich

Bay

Whi

tefor

d Po

int,

loca

ted in

nor

th-w

est G

ower

, exh

ibits

the t

ypica

l

prop

ertie

s of a

spit.

Whi

tefor

d Sa

nds b

each

, atta

ched

to B

roug

hton

Sand

s to

the s

outh

, ext

ends

nor

thwa

rds f

or o

ver t

hree

kilo

metr

es,

even

tuall

y ex

tendi

ng in

to th

e mou

th o

f the

Riv

er L

ough

or (t

he B

urry

Estu

ary)

, thu

s cre

ating

the s

pit.

The s

pit i

s orie

nted

from

sout

hwes

t

to n

orth

east,

show

ing

that

the d

omin

ant w

ind

is co

min

g fro

m th

e

sout

h-we

st an

d th

us d

rivin

g th

e net

litto

ral d

rift i

n a n

orth

-eas

terly

dire

ction

. The

spit

has a

recu

rved

shin

gle a

nd sa

nd b

ank

at its

dist

al

end

and

its fu

rthes

t ext

ent i

s mar

ked

by W

hitef

ord

Ligh

thou

se, w

hich

is su

rroun

ded

by a

mus

sel b

ed an

d qu

icksa

nd. T

he sp

it wa

s abl

e to

build

up

abov

e sea

leve

l usin

g its

foun

datio

n of

glac

ial ti

ll de

posit

s

left b

y ice

that

exten

ded

thro

ugh

the L

ough

or V

alley

dur

ing

the l

ast

Ice A

ge. T

he b

each

and

dune

s are

com

pose

d of

mate

rial d

riven

landw

ards

by

the r

ising

seas

of t

he F

landr

ian T

rans

gres

sion.

The

beac

h is

back

ed b

y W

hitef

ord

Burro

ws sa

nd d

unes

; aro

und

3 km

long

and

betw

een

400m

and

600m

wid

e, re

achi

ng h

eight

s of u

p to

24m

.

Note

the l

ow-g

radi

ent w

ide b

each

to th

e lef

t of t

he p

hoto

grap

h,

which

wou

ld p

rovi

de, a

t low

sprin

g tid

e esp

ecial

ly, a

plen

tiful

sour

ce o

f sed

imen

t for

the W

hitef

ord

Burro

ws d

unes

.

Llan

rhid

ian S

alt M

arsh

with

salt

pans

and

a netw

ork

of cr

eeks

The s

pit p

rovi

des s

helte

r in

its le

e for

the e

xten

sive L

andi

mor

e Mar

sh,

Llan

rhid

ian S

alt M

arsh

and

the L

lanrh

idian

San

ds m

udfla

ts. T

he fi

ne

sand

s and

silty

clay

s of t

he m

udfla

ts ar

e mad

e up

of g

lacial

out

wash

mate

rial a

nd g

lacial

drif

t whi

ch h

as b

een

brok

en d

own

by fl

uvial

and

mar

ine p

roce

sses

.

Whi

tefor

d Po

int –

this

imag

e sho

ws th

e dist

al en

d of

the

spit,

larg

ely co

vere

d in

dun

es

Sum

mar

yTh

e Gow

er p

enin

sula

prov

ides

exam

ples

of d

epos

ition

al lan

dfor

ms

and

ecos

ystem

s inc

ludi

ng R

hoss

ili B

ay, O

xwich

Bay

and

its d

unes

and

Whi

tefor

d Po

int a

nd it

s ass

ociat

ed d

unes

and

saltm

arsh

. The

se

exam

ples

illu

strate

how

wav

es an

d tid

es ar

e im

porta

nt in

fluen

ces o

n

coas

tline

s reg

ardi

ng th

e cre

ation

and

mod

ifica

tion

of d

epos

ition

al

landf

orm

s and

ecos

ystem

s.

Bibl

iogr

aphy

and

furth

er re

adin

g

• Bird

, E. (

2008

) Coa

stal G

eom

orph

olog

y: A

n In

trodu

ction

Wile

y

• Brid

ges,

E. M

. (19

97) C

lassic

Lan

dfor

ms o

f the

Gow

er C

oast

The G

eogr

aphi

cal A

ssoc

iatio

n

• Dav

ies, A

. (20

12) W

alkin

g on

Gow

er C

icero

ne

• Hill

, M. (

2004

) Coa

sts an

d Co

astal

Man

agem

ent H

odde

r and

Stou

ghto

n• M

asse

link,

G. H

ughe

s, M

. G. &

Kni

ght,

J. (2

011)

Intro

ducti

on to

Coas

tal P

roce

sses

and

Geom

orph

olog

y (2

nd E

ditio

n) R

outle

dge

• Mot

yka,

J.M. &

Bra

mpt

on, A

.H. (

1993

) Coa

stal M

anag

emen

t:

map

ping

of l

ittor

al ce

lls H

.R. W

allin

gfor

d (A

repo

rt fo

r MAF

F)

http

://ep

rints.

hrwa

lling

ford

.co.u

k/74

8/1/

SR32

8.pd

f

Ackn

owled

gem

ents;

This

Geo

Facts

heet

was r

esea

rche

d an

d wr

itten

by

Kate

Cowa

n, a

teach

er

of G

eogr

aphy

at K

ing

Edwa

rd V

I Hig

h Sc

hool

for G

irls,

Birm

ingh

am. A

ll

phot

ogra

phs b

y K.

Cow

an. I

SSN

1351

-513

6

Burry

Estu

ary

Whi

tefor

d Po

int

Llan

rhid

iansa

nds

(mud

flats)

Llan

rhid

ianm

arsh

Land

imor

em

arsh

Whi

tefor

d Sa

nds

Whi

tefor

d Bu

rroes

Brou

ghto

nBa

ySW Ne

tdrif

tdi

recti

onNE 1 km

Dune

sM

arsh

Shin

gle

Salt

mar

shes

Salt

mar

shes

are c

omm

only

foun

d in

rive

r estu

aries

or b

ehin

d sp

its, i

n

shelt

ered

wate

r (a l

ow en

ergy

envi

ronm

ent).

Mud

flats

deve

lop

in

the s

helte

red

zone

as fi

ne p

artic

les o

f silt

and

mud

are d

epos

ited.

Thes

e mud

flats

can

be co

loni

sed

by sa

lt-to

leran

t (ha

loph

ytic)

vege

tatio

n, in

cludi

ng sp

ecies

such

as S

alico

rnia

and

spar

tina,

which

are a

ble t

o co

pe w

ith re

gular

subm

erge

nce b

y th

e

fluctu

ating

tida

l lev

els. T

he p

lants

trap

mor

e mud

and

silt o

ver t

ime

and

even

tuall

y cr

eate

an ar

ea th

at wi

ll re

main

expo

sed

for

incr

easin

gly

long

er p

erio

ds b

etwee

n tid

es. E

stabl

ished

saltm

arsh

es

can

be sp

lit in

to tw

o zo

nes:

mud

flats,

cove

red

at or

dina

ry h

igh

tides

,

are f

ound

on

the s

eawa

rd si

de o

f the

mar

sh an

d th

e swa

rd zo

ne,

which

is o

nly

cove

red

perio

dica

lly d

urin

g hi

gh sp

ring

tide.

Whe

n th

e

swar

d zo

ne is

floo

ded

by th

e tid

e, th

e salt

y se

awate

r col

lects

in

hollo

ws, w

hich

may

enlar

ge an

d be

com

e inc

reas

ingl

y sa

line,

form

ing

saltp

ans (

a netw

ork

of cr

eeks

whi

ch al

low

the s

eawa

ter to

dra

in

away

cut a

cros

s the

zone

).

Figu

re 6

. Whi

tefor

d po

ints

BroughtonBay

SW

Net d

rift

dire

ct io

nN

E1 km

Dunes

Marsh

Shingle

re hes

ition

eac

s bT m’s

nock

c o mo

or Salt marshesSalt marshes are commonly found in river estuaries or behind spits, in sheltered water (a low energy environment). Mudflats develop in the sheltered zone as fine particles of silt and mud are deposited. These mudflats can be colonised by salt-tolerant (halophytic) vegetation, including species such as Salicornia and spartina, which are able to cope with regular submergence by the fluctuating tidal levels. The plants trap more mud and silt over time and eventually create an area that will remain exposed for increasingly longer periods between tides. Established saltmarshes can be split into two zones: mudflats, covered at ordinary high tides, are found on the seaward side of the marsh, and the sward zone, which is only covered periodically during high spring tide. When the sward zone is flooded by the tide, the salty seawater collects in hollows, which may enlarge and become increasingly saline, forming saltpans. A network of creeks which allow the seawater to drain away cut across the zone. Figure 6. Whiteford point