Jetty Non Technical Summary - EDF Hinkley Point

cliff at the location of the proposed jetty development contains evidence to indicate that
the inter-bedded limestones and shales of the Blue Lias cliff top have retreated at around
0.13m/yr since 1888, and mean high water mark by around 0.04m/yr.
8.1.3 Sediment transport within the inter-tidal area comprises mud moving as suspended
sediment, sand moving in suspension and as bed load, and gravel and coarser particles
moving on the upper part of the rock platform as bedload. The most obvious evidence of
coastal sediment transport at Hinkley Point (coarse grained sediment) is shown by the
ridge of shingle and coarser particles at the crest of the beach immediately west of the
existing seawall protecting the current Hinkley Point Power Station Complex. The
seawall projects out approximately 10 to 20m from the cliff line further west and it is likely
that, due to the longshore drift of this coarse beach sediment, the accumulation of a
wider shingle beach here compared to elsewhere has occurred. The existing seawall at
Hinkley Point is not considered to be acting as a barrier to hinder shingle transport
around Hinkley Point.
8.1.4 Further offshore in Bridgwater Bay, there is a large variation in the type and distribution of
seabed sediment along the coastline in front of Hinkley Point. Sediments generally range
from finer sediment in the east (around Bridgwater Bay) to coarser material in the west.
The seabed sediments immediately offshore of Hinkley Point are described as a thin
muddy veneer overlaying the bedrock.
8.1.5 The suspended sediment concentrations in the Severn Estuary result from the strong
tidal currents and are generally exceptionally high, creating a turbid environment. A field
campaign by Ravensrodd Consultants found suspended sediment concentrations within
the Bristol Channel within the range of less than 100mg/l to approaching 200,000mg/l
(fluid mud). This range of suspended sediment concentrations was also reported around
Hinkley Point following four monitoring campaigns over the winter of 2008/2009. This
monitoring recorded a maximum suspended sediment concentration of 1,795mg/l, a
mean concentration of 264mg/l, and a minimum concentration of 33mg/l. Although some
increase with depth was observed, the water column was well mixed. Both the values
and distribution of suspended sediment concentrations show differences between spring
and neap tides.
8.1.6 Monitoring found tidal currents of Hinkley Point to be generally orientated along the main
axis of the estuary (i.e. shore-parallel), with currents reaching a maximum velocity of
1.7m/s on spring tides and 1.4m/s on neap tides at an offshore monitoring location. The
inshore monitoring site showed peak currents to attain only 1.5m/s on spring tides and
1.0m/s on neap tides. At all locations ebb currents were stronger than flood currents.
Main Predicted Impacts
8.1.7 During the jetty development’s construction, piling could disturb sediment so the chosen
piling technique to be employed (specific methodologies would be confirmed by
contractors) would minimise the degree of sediment disturbance, through the choice of
access method and type of piling.
8.1.8 In addition, the dredged berthing pocket could be subject to sediment deposition.
However, the proposed design of the berthing pocket (i.e. its specific alignment relative to
the tidal flows) has been chosen in order to encourage hydrodynamic conditions that
promote self-scouring of this dredged area and, therefore, to reduce the requirement for
frequent maintenance dredging during the jetty development’s operational phase.
Hinkley Point C Preliminary Works
Non Technical Summary
Temporary Jetty Development 45 November 2010