Jetty Non Technical Summary - EDF Hinkley Point

high tidal range, there is a large dilution capacity available to buffer other suspended
solids and other potential water quality impacts during high tide periods and the overall
water quality status sensitivity is low. The sensitivity of the inter-tidal water quality status
is increased during low tide periods when water is retained in pools and foreshore
channels that support marine ecology. During these periods there is reduced dilution
capacity to potential impacts from foreshore discharges and an increased sensitivity.
8.4.4 The sub-tidal areas off Hinkley Point are characterised by high suspended solids
concentrations due to sediment mobilisation under high tidal flows. The tidal disturbance
of the sediments would also lead to mobilisation of any associated sediment
contaminants into the well-mixed water column. It has been shown that disturbance of
the superficial sediments in the area around Hinkley Point has a low influence on
chemical water quality status.
8.4.5 In terms of sediment quality, survey data indicate the presence of elevated metal and
hydrocarbon concentrations, but very few of the metal concentrations recorded would be
considered to be significantly contaminated. It is suggested that elevated metal
contamination in surface sediment may be representative of wider baseline
concentrations and the result of contaminant redistribution from elsewhere in the Bristol
Channel. Concentrations of other organic substances were found to be generally present
at low concentrations.
Main Predicted Impacts
8.4.6 The impact assessment found that there is the potential for water quality status to be
adversely affected as a result of a range of activities associated with the jetty
development. The impacts that have been identified during the construction of the jetty
may be broadly divided into:
• disturbance of marine sediment and mobilisation of associated contaminants into the
Severn Estuary (e.g. due to dredging of the berthing pocket or scouring around the
jetty’s piles);
• discharge of water containing sediment, hydrocarbons and/or concrete leachates into
the Hinkley Point C Drainage Ditch and the Severn Estuary (e.g. surface water run-off
from the onshore storage area);
• discharge of freshwater onto the foreshore where Corallina is present in drainage
channels; and
• accidents and incidents.
8.4.7 During construction of the jetty no significant impacts are predicted to arise in relation to
water quality due to the measures that would be put in place to control the quality and
volume of discharges into the surface drainage (i.e. the Hinkley Point C Drainage Ditch)
and onto the foreshore and the waters of the Severn Estuary. In addition, the sensitivity
of the local marine water quality conditions to increases in suspended solids is low given
the existing high background concentrations.
8.4.8 The impacts associated with the operation of the jetty are likely to arise from
maintenance dredging of the berthing pocket, scouring of sediment from the seabed
around the jetty’s infrastructure (e.g. piles and berthing pocket), and freshwater
discharges into the marine environment from the onshore storage area. However, the
impacts assessed would be of negligible or minor significance.
Hinkley Point C Preliminary Works
Non Technical Summary
Temporary Jetty Development 53 November 2010