Scenario 2: Trenching and ducting 103. Crossing points and crossing methodologies (e.g. HDD at main rivers) have been selected to minimise impacts and preserve valuable features as far as possible. HDD will be used to cross all main rivers including the River Deben, Kirton Creek, Martlesham Creek and the River Gipping, avoiding impacts on major watercourses and those of high sensitivity and international value. During the drilling process fluid, as stated above in section 21.6.1.3.1, impact from loss or spillage of fluid will be minimal and unlikely to be detrimental to water quality. The impact of drilling fluid loss is considered to be temporary and of low magnitude.
104. All remaining (non HDD) watercourses are considered to be of local value and to have a medium or low sensitivity. The crossing of these watercourses would be carried out using an open-cut technique. The exact methodology used to achieve an open trench across each stream and the temporary bridge arrangements required would be decided by the main works contractor in conjunction with the statutory authority. Method statements would be agreed with the statutory authority prior to any watercourse crossings taking place.
105. The open-cut technique of damming and over pumping will minimise the sediment loading into the watercourse, although there will be a temporary impact upon reinstatement of the watercourse. Dewatering of excavations when rainfall or surface water runoff has to be removed or shallow groundwater is encountered may also lead to sediment laden run off. The stockpiling of spoil from trenching activities would be away from the edge of any watercourses, in accordance with the embedded mitigation. The impact of sediment loading is considered to be temporary and of low magnitude.
106. Excavation and disturbance of soils has the potential to temporarily open the soil structure creating pathways for the mobilisation and transmission of contaminants. Chapter 19 Soils, Geology and Ground Condition identified a number of potentially contaminated sites and landfills within 250m of the onshore electrical transmission works. Where these sites are crossed and are near to watercourses, detriment to the water quality may occur. The majority of potentially contaminated sites relate to historical activities such as the placement of artificial ground or former railway lines. Two sites were identified as active landfills (Bamford Dairy and Blood Hill). Bamford Dairy is recorded to lie within the works footprint. Two historical landfills (Sandpit Tuddenham St Martin and Culpho Hall) are also recorded to cross the cable route. The creation of a pathway along the line of cables, ducting and granular surround is considered to be permanent and in the areas where this enables leachates or gases
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 21 Water Resource and Flood Risk Page 38
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