Jncc coastal Directories Project Region 11 The Western Approaches

2.4.1  Introduction
Currents produced by waves and tides are the dominant
mechanism of sediment transport, and both are important in
the coastal zone of the region.  Most of the northern coast of
Cornwall and Devon is open to the prevailing westerly
winds, and the resulting waves are agents of sediment
transport down to greater depths than along most British
coasts.  Within the Bristol Channel tides become the more
important mechanism for sediment transport but waves
remain important in the littoral zone.
The dominant sediment transport direction between
Land’s End and Bridgwater Bay is northwards and then
eastwards, but the pattern of small inlets and headlands that
characterises the coast, coupled with the low volume of
available mobile sediment, limits littoral drift along the
coast.  Motyka & Brampton (1993) suggest that there is only
limited evidence for northward drift along the west
Cornwall and Devon coast, though wave action may cause
strong seasonal onshore/offshore movement of sediment.
South and west of Barnstaple Bay tides are generally weak,
and storm waves are responsible for transport of sea-bed
sediment near the coast. 
Bridgwater Bay and the Severn Estuary (defined here as
being east of a line from Brean Down to Lavernock Point)
have a complex pattern of sediment transport.  Detailed
sediment distributions and bedforms within the inner
Bristol Channel and the Severn Estuary are related to a
series of circulation cells generated by tidal currents.  A
number of methods have been employed to identify these
cells and deduce the direction and volume of transport,
including computer modelling, analysis of the asymmetry of
ripples and sand waves at the sea bed, and tracking changes
in sediment grain sizes.  The results show that mud, sand
and gravel display different transport paths. 
Gravel transport is dominated by the wave climate along
the high water mark, sand is transported as bed load, and
mud is transported as suspended sediment.  Because of
their different modes of transport, sand and mud are
deposited in different areas.  Sand transport is particularly
complex, with each major sand bank (e.g. Culver Sand or
Cardiff Grounds) displaying its own circulation cell.  Ebb-
dominated westward transport tends to occur in the mid-
channel, with flood-dominated eastward transport
restricted to narrow coastal boundary zones (Harris &
Collins 1988).  Over the past few thousand years the net
movement of sand has been eastwards from the Celtic Sea
into the estuary.  The major sand banks infilling the central
estuary and the isolated sandbanks found in the inner
Bristol Channel are probably derived from a western,
marine, rather than an eastern, alluvial, source.
Coastal erosion across the region is generally limited in
scale compared with that taking place along the south and
east coasts of England.  Many of the coastal dune systems
along the west coast of the peninsula are undergoing
erosion.  Cliff erosion is taking place in Mount’s Bay and at
Porthleven and beach erosion is taking place at Perran
Sands.  The hanging valleys and waterfalls of the rocky
coast of Devon and Cornwall are an indication that the cliffs
have been eroded, but the current rate of retreat is generally
so slow as to be difficult to measure.  The Carboniferous
rocks forming the cliffs along the north coast of Devon are
retreating at a negligible rate, and cliffs formed of Devonian
rocks are thought to be retreating at about 4 cm/year
(Williams et al. 1991).  This erosion must also have been
taking place through much of the late Tertiary and
Quaternary.  The cliffs of soft Mesozoic rock along the
Somerset coast are undergoing erosion at a rate of about 
20 cm/year, with higher values locally.  Much of the coast of
the Severn Estuary is protected by man-made structures and
is not retreating, but erosion is taking place on the foreshore,
which is becoming steeper. 
2.4.2  Description
Sediment transport is described within the context of coastal
cells and sub-cells.  These divide the coastline into sections
within which sediment erosion and accretion are inter-
related and largely independent of other cells.  Cells are
separated by either littoral drift divides or sediment sinks.  
Motyka & Brampton (1993) describe the coast of the
region as being part of three coastal cells, which are further
divided into eight coastal sub-cells.  The sub-cells are
described below and are shown on 
Map 2.4.1
.  Note that the
sediment transport shown is of sand and gravel ‘bed load’,
not suspended sediments.  The Isles of Scilly were not
included in Motyka & Brampton’s study.
Sub-cell 6d: Rame Head - Lizard Point
Only the westernmost portion of this sub-cell lies within the
region.  There is virtually no net drift within this sub-cell.
Drowned river valleys such as Carrick Roads and Helford
River interrupt sediment movement.  Beaches are of sand or
pebbles, generally confined within individual coves.  There
is local cliff erosion.  
Sub-cell 6e: Lizard Point - Land’s End
There is very low eastward drift of sand in Mount’s Bay and
seasonal alongshore movements along Loe Bar but no
apparent net drift.  Wave action is dominant and can cause
significant onshore and offshore beach movements.  There is
little evidence of recent accretion.  Cliff and beach erosion is
known to occur in Mount’s Bay.
Sub-cell 7a: Land’s End - Trevose Head
There are extensive stretches of sand along this coast,
separated by areas of rocky foreshore.  There is little
evidence of littoral drift in this sub-cell, but waves cause
strong seasonal onshore and offshore movements.  There is
dune erosion at a number of places and cliff erosion at
Newquay.
2.4  Sediment transport
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2.4  Sediment transport
British Geological Survey

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