appendix a - National Grid

9.2.4 Crossing
TEP on behalf of National Grid 17 of 21
King’s Lynn 400kV Connection
110488-FRA-R1(1)
A box culvert (2m x 1.25m deep) is proposed to the north of Tower 1 as part of the
development work as discussed with the EA. The culvert is to be installed immediately
south of the private access road to the substation.
Two crossings at the River Nar will be required as part of the construction. Pontoon
crossing has been proposed at these locations with scaffolding erected at either side of
the banks for pedestrian access. Pontoon crossings will have less impact to the
watercourse by having minimal construction in the riverbed. Effect to the river flow from
pontoon crossing is also minimised. Details of the two pontoon crossings at the river
are contained in Appendix I. National Grid has consulted Environment Agency and
Natural England on these proposed crossings.
The location of the box culvert and the pontoon crossings are shown in Figure 8.
9.3 Post-Development Situation
Development will introduce new impermeable areas, as the proposal comprises a new
400kv headline supported by steel pylons. A number of aspects will need to be
considered.
9.3.1 Site Drainage
Generally, the preferred drainage regime would be to maintain the existing drainage
pattern with any additional volume generated as a result of development contained
within the site boundary by using SUDS methodology, to minimise the volume and peak
surface water and thus avoiding pollution. This is to minimise any potential impacts that
the development may have to its river wide catchment area.
However, as development will have minimal increase in surface water runoff due to
limited impermeable surfaces. The additional runoff will likely dissipate into the ground
prior to reaching any nearby watercourses. Therefore, it is not anticipated that
additional drainage system will be required as a result of development.
9.3.2 Flood Compensation
As the site is located in Flood Zone 3 (both tidal and fluvial), like-for-like flood
compensation will be required.
In undefended coastal areas, as development is less likely to impact on the water levels
from tidal sea flooding, compensatory storage may not always be required.
Compensatory flood storage or conveyance will be necessary for most developments
located within the undefended fluvial floodplain areas.
However, as the pylons will have minimal footprint and their structure will allow any
floodwater to flow through freely with minimal obstruction, the development will not
impede overland flow. It is not anticipated that flood compensation will be required as
land take from the proposed pylons will be very limited.