Jncc coastal Directories Project Region 11 The Western Approaches

The tidal range of the Severn Estuary is the largest in the
UK and the second largest in the world.  Under certain
conditions spring tides cause the formation of a tidal wave
known as the ‘Severn Bore’, which can reach as far upstream
as Gloucester.
2.3.5  Wave exposure and sea state
There is a long ‘fetch’ across the Atlantic Ocean and the
prevailing westerly winds generate large waves on the
coasts of Cornwall and parts of Devon.  In addition the
relatively steep nearshore bathymetric gradient results in
high waves approaching close to the coast.  Significant wave
heights during the winter exceed 4 m for 10% of the time
offshore and at Land’s End, and exceed 3 m for 10% of the
time along much of the Cornish coast (
Map 2.3.6
). Wave
conditions as severe as this are found elsewhere in Britain
only off the coast of north-west Scotland.
The wave height predicted to occur once in fifty years is
20 m along the south-west Cornwall coast, decreasing to
about 18 m at the mouth of the Bristol Channel.  This
compares with a predicted height of over 30 m along the
north-west coast of Scotland.
Wave data from the Seven Stones Lightship west of
Land’s End provide evidence that significant wave heights
have increased progressively between 1962 and 1984,
indicating a gradual increase in storminess in the North
Atlantic during this time. 
2.3.6  Water characteristics
Water temperature
The mean sea surface temperatures for summer and winter
are shown on 
Map 2.3.7
.  The data are for August and
February, which are the months of, respectively, highest and
lowest values.  In February the open waters in the south-
west of the region have a temperature of about 9.0-9.5°C,
compared with 10.5°C for the waters at the edge of the
continental shelf some 200 km to the south-west of Land’s
End.  Water temperatures decrease in the shallower waters
to the north-east, and the mean in the inner Severn Estuary
is less than 6°C.  By August temperatures are at a maximum
for the year, attaining approximately 16°C throughout the
region.
Stratification of the water column occurs when the
effects of the warming of the surface water outweigh those
of mixing by winds and tides (see also 
section 4.3
).  Layers
of water develop whose temperature and density vary with
depth.  Stratification develops in the Celtic Sea by late
spring and spreads eastwards along the north Cornwall
coast.  The turbid waters of the Bristol Channel remain well-
mixed throughout the year.  By early winter the coastal
waters of the region have become fully mixed again, leaving
only an offshore tongue of stratified water in the Celtic Sea
(Pingree 1980). 
Salinity
The waters of the southern part of the region are open to the
influence of the Atlantic Ocean, so their mean surface
salinity, at 35 g/kg or more, is typical of oceanic water (
Map
2.3.8
).  Salinity decreases north-eastwards into the Bristol
Channel as the influence of freshwater input increases.
Variations in the salinity of the Severn Estuary and inner
Bristol Channel follow changes in the riverine flow, with
generally lower salinities in winter (when river flows are
greater) than in summer.  Salinity values in the upper
estuary in winter may be as low as 20 g/kg (Collins &
Williams 1981). 
2.3  Wind and water
29
6
7
8
9
10
11
12
5
4
Region 10
Region 12
5°W
Map 2.3.5
Tidal range (m) at mean spring tides.  Source: Lee &
Ramster (1981).  © Crown copyright.
0.5
1.0
1.5
2.0
2.5
3.0
1.5
1.0
0.5
0.5
0.5
0.5
2.5
2.5
3.0
Region 10
Region 12
5°W
exceeded for 10% of the time
exceeded for 75% of the time
Significant wave height in metres
Map 2.3.6
Significant wave height (m) exceeded for 10% and 75%
of the year.  Source: Draper (1991).
16.0
15.5
16.0
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Region 10
Region 12
5°W
Mean surface temperature (ºC)
Summer
Winter
Map 2.3.7 
Mean surface water temperature in summer and winter
(°C).  Source: Lee & Ramster (1981).  © Crown copyright.

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Water turbidity
The waters off the South-West Peninsula are derived from
the Atlantic Ocean and contain virtually no terrestrially-
derived suspended sediments.  Nearshore, the rivers carry
suspended sediment into the sea, especially after high
rainfall.  Such plumes of suspended sediment are quickly
mixed in by waves and tides. 
The situation is very different in the inner Bristol
Channel and Severn Estuary, where the water is highly
turbid.  This has been determined using satellite imagery
(Collins 1983) as well as by analysis of the water column.
Estimates have been made that the estuary carries 10 million
tonnes of suspended sediments at spring tides (Buck 1993).
The area of highest turbidity is in water depths of less than
30 m, but the boundary moves seaward during spring tides
as strong currents cause resuspension of mud from the sea
bed.  Higher turbidity water tends to move west along the
northern shores of the inner Bristol Channel, while lower
turbidity water is found along the southern shores.  The
observed pattern is of an overall westerly transport of
suspended sediment within the inner Bristol Channel.  The
average turbidity within the Severn Estuary is highest
between Avonmouth and the outer part of Bridgwater Bay,
where the surface water contains 0.5-1.0 g/l of suspended
solids, while at the sea bed the concentration may be three
to six times this value. 
2.3.7  Further sources of information
A.  References cited
British Geological Survey.  1987.  Map of seabed sediment around the
United Kingdom.  1:1,000,000 scale.  Keyworth, British Geological
Survey.
Buck, A.L.  1993.  An inventory of UK estuaries.  Volume 2.  South-west
Britain.  Peterborough, Joint Nature Conservation Committee.
Caton, P.G.  1976.  Maps of hourly wind speed over the UK 1965-73.
Bracknell, Meteorological Office.  (Climatological
Memorandum No. 79.)
Collins, M.  1983.  Supply, distribution and transport of suspended
sediment in a macrotidal environment: Bristol Channel, UK.
Canadian Journal of Fisheries and Aquatic Sciences, 40(1): 44-59. 
Collins, N.R., & Williams, R.  1981.  Zooplankton of the Bristol
Channel and Severn Estuary.  The distribution of four copepods
in relation to salinity.  Marine Biology, 64: 273-283. 
Draper, L.  1991.  Wave climate atlas of the British Isles.  London,
HMSO. 
Hydrographic Department.  1960.  West coast of England pilot.
London, Hydrographic Department. 
Lee, A.J., & Ramster, J.W.  1981.  Atlas of the seas around the British
Isles.  Lowestoft, Ministry of Agriculture, Fisheries and Food.
Pingree, R.D.  1980.  Physical oceanography of the Celtic Sea and
English Channel.  In: The north-west European shelf seas: the sea
bed and the sea in motion.  Vol. 2.  Physical and chemical
oceanography and physical resources, ed. by F.T. Banner, M.B.
Collins & K.S. Massie, 415-465.  Amsterdam, Elsevier.
(Oceanography Series.)
Sager, G., & Sammler, R.  1968.  Atlas der Gezeitenströme für die
Nordsee, den Kanal und die Irische See.  Rostock,
Seehydrographischer Dienst der DDR.
Uncles, R.  1983.  Hydrodynamics of the Bristol Channel.  Marine
Pollution Bulletin, 15(2): 47-53.
B.  Further reading
Banner, F.T., Collins, M.B, & Massie, K.S., eds.  1980.  The north-west
European shelf seas: the sea bed and the sea in motion.  Vol. 2.
Physical and chemical oceanography, and physical resources.
Amsterdam, Elsevier.  (Oceanography Series.)
Barne, J., Davidson, N.C., Hill, T.O., & Jones, M.  1994.  Coastal and
marine UKDMAP datasets: a user manual.  Peterborough, Joint
Nature Conservation Committee.
British Geological Survey.  1994.  Quaternary geology around the
United Kingdom (North and South Sheets).  1:1,000,000 scale.
Keyworth, British Geological Survey.
British Oceanographic Data Centre.  1992.  United Kingdom digital
marine atlas user guide.  Version 2.  Birkenhead, Natural
Environment Research Council.
Department of the Environment.  1992.  Digest of environmental
protection and water statistics.  London, Government Statistical
Service, HMSO.
Dyer, K.R.  1984.  Sedimentation processes in the Bristol
Channel/Severn Estuary.  Marine Pollution Bulletin, 15(2): 53-57. 
Graff, J.  1981.  An investigation of the frequency distributions of
annual sea level maxima at ports around Great Britain.
Estuarine, Coastal and Shelf Science, 12: 389-449. 
Hydrographic Department.  1970.  Underwater handbook, western
approaches to the British Isles.  London, Hydrographic Office.
Hydrographic Department.  1984.  Channel Pilot.  Isles of Scilly and
south coast of England, from Cape Cornwall to Bognor Regis.  NP 27.
London, Hydrographic Office.
Hydrographic Office.  1993.  Admiralty Tide Tables, Vol. 1: European
waters.  London, Hydrographic Office.
C.  Contact names and addresses
Type of information
Contact address and telephone no.
UKDMAP (United Kingdom  *BODC, Birkenhead, 
digital marine atlas) 
tel: [phone]
Version 2.  
Oceanographic maps
Monthly, seasonal and  
J. Hammond, Meteorological 
annual windroses
Office Marine Enquiry Service,
Johnstone House, London Road,
Bracknell  RG12 2SY,
tel: [phone]
*Starred contact addresses are given in full in the Appendix.
Region 11  Chapter 2  Geology and physical environment
30
34.75
35.00
35.00
35.20
34.50
34.75
35.00
35.10
35.20
35.30
Region 10
Region 12
5°W
Mean surface salinity (g/kg)
Summer
Winter
Map 2.3.8  
Mean surface salinity of seawater in summer and winter
in g/kg of total dissolved salt.  Source: Lee & Ramster
(1981).  © Crown copyright.

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