Jncc coastal Directories Project Region 11 The Western Approaches

The offshore part of the region may be divided into two
environments: the main part, off the Cornwall and Devon
coast northwards to about Minehead, and in south Wales a
short stretch of coast north of Nash Point, is open to the
influence of the Atlantic Ocean.  The inner Bristol Channel
and Severn Estuary are more sheltered. 
2.3.1  Wind
The wind rose data (
Figure 2.3.1
) for the Isles of Scilly shows
winds predominantly from a westerly direction, with an
average of 25 days of gales per year, most of which occur in
the winter months (Hydrographic Department 1960).  The
wind rose for Cardiff shows a similar westerly
predominance, but the low percentage of winds from
northerly and southerly directions may be due to local
topographic effects.  Gales at the Cardiff station averaged
three days a year. 
Much of the coast of the region is fully exposed to the
prevailing winds.  
Maps 2.3.1 
and 
2.3.2
show that the
maximum wind speeds occur across western Cornwall and
the Isles of Scilly.  Wind speeds decrease eastwards in the
Bristol Channel.  Meteorological Office data show that the
mean wind speeds along the exposed coasts of this region
are similar to those across the west coast of Wales but
slightly less than those across the west coast of Scotland. 
2.3.2  Water depth
The morphology of the sea bed is influenced by the nature
of its bedrock, the exposure of the area to wave attack and
the supply of mobile sediment.  The sea bed of most of this
region slopes steeply and regularly down to a depth of
about 60 m, levelling out onto the westward-sloping
continental shelf (
Map 2.3.3
).  Much of it is covered in thin,
mobile sediments and thus reflects the shape of the bedrock
surface.  This surface was modelled during the late Tertiary
and Quaternary, and changes to the coast since the last
glaciation 10,000 years ago have probably been minimal,
except for the infilling of some areas in the upper part of the
Bristol Channel.  
Detailed analysis of the bathymetry around the coast of
Cornwall and the Isles of Scilly reveals a number of
submerged cliffs down to a depth of about 70 m.  Locally the
2.3  Wind and water
27
2.3  Wind and water
British Geological Survey
2.5
2.5
3.0
3.5
3.5
3.0
Region 10
Region 12
5°W
Map 2.3.1 
Hourly mean windspeed (in m/s) exceeded for 75% of
the time.  Source: Caton (1976). 
Isles of Scilly
Cardiff
0
10%
Figure 2.3.1 
Wind directions at Cardiff and on the Isles of Scilly
shown as % of observations during the years 1916 -
1950.  Flat calm (% of observations) = Cardiff (13.5%);
Isles of Scilly (18%).  Source: Hydrographic
Department (1960).
20
19
18
17
16
16
17
18
19
Region 10
Region 12
5°W
data from Caton (1976)
estimated values
Map 2.3.2 
Hourly mean windspeed (in m/s) exceeded for 0.1% of
the time.  Source: Caton (1976).

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cliffs are breached by valleys incised during periods of
lower sea level and now infilled with Holocene sediment.
The best-developed incised valley in the region is that of the
Severn: in the upper part of the estuary, sand banks infill
and obliterate the valley, but from about Weston-super-Mare
it is a well-developed bathymetric feature, stretching
westwards for some 100 km.
2.3.3  Tidal currents
Along the coast of Cornwall the maximum tidal currents
exceed 2 knots only near headlands and in the channel
between the Isles of Scilly and the mainland (
Map 2.3.4
).
However, going eastwards into the Bristol Channel, speeds
increase progressively, and within the Severn Estuary
maximum currents locally exceed 7 knots (Lee & Ramster
1981).  Admiralty Charts, which are thought to give a more
accurate reflection of currents in restricted embayments
such as the Severn Estuary, show a maximum surface ebb
current of 4.6 knots off Foreland Point and a maximum
flood current of 4.2 knots off Weston-super-Mare.  
Computer modelling of the tides in the area by Uncles
(1983) has shown that the tidal currents are mostly
orientated parallel to the medial axis of the Bristol Channel
but locally point into bays.  This helps to explain why sand
and mud accumulate in Barnstaple, Bridgwater and
Swansea Bays.
2.3.4  Tidal range
The mean tidal range at spring tides along the coast of the
region shows a steady increase north-eastwards from 5 m at
Land’s End to 12.3 m at Avonmouth (
Map 2.3.5
).  This
increase is due to the amplification of the tidal movement as
it is funnelled eastwards up the Bristol Channel.  Storm
surges may increase this value by more than 1.5 m in the
inner Bristol Channel and Severn Estuary. 
Region 11  Chapter 2  Geology and physical environment
28
60
50
40
20
50
20
60
20
40
80
Eddystone
Rocks
60
Bathymetric contours in metres;
figures on the deep side of the line
limit of data
80
40
60
60
50
40
20
80
60
50
40
20
10
10
40
100
120
10
20
40
Celtic
Deep
Bristol
Channel
Severn
Estuary
Seven
Stones
Map 2.3.3 
Bathymetry.  Source: British Geological Survey (1987).
7
6
5
4
3
2
2
1
0.5
2
2
Foreland
Point
Weston-super-Mare
Region 10
Region 12
5°W
Map 2.3.4  
Maximum tidal current speed (in m/s) at mean spring
tides.  Source: Sager & Sammler (1968).

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