Scenario 1: Pre-installed ducts 91.
The only intrusive works would comprise the excavation of jointing pits, as ducts would already be in place for the cable installation. The impact of the excavation works from landfall to Great Bealings is considered to have a low magnitude of impact, given that any changes to groundwater quality would only have a minor short term impact. As such, the significance of impact is considered to be minor adverse.
92.
From Great Bealings to the converter station, despite the presence of SPZs, the level of activity means that any changes to groundwater quality would be of negligible magnitude. Therefore the significance of the impact is considered to be a minor adverse impact.
Scenario 2: Trenching and ducting 93.
There is unlikely to be significant dewatering required as part of the trenching and ducting works. All trenches are planned to be excavated to a maximum depth of 1.5mbelow ground level and it is considered unlikely that the groundwater table will be encountered within this depth. Further details relating to the likely depth of the groundwater table will be included in the final ES. If dewatering is required, it is likely to comprise pumping of perched water or surface water runoff from within the trench. As such, the magnitude of impact to groundwater levels is considered to be negligible and therefore potential impact to the groundwater resources is considered to be negligible.
94. HDD would be carried out at sensitive crossing points such as rivers including the River Deben, Mill River (Kirton Creek), Martlesham Creek and the River Gipping. Drilling fluid will be used 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. The significance of the impact is minor adverse.
95.
From Great Bealings to the converter station, the level of magnitude of potential impact from the construction works is considered to be medium, given that there is the potential to have a slight impact on resource use (based on an impact to groundwater quality). There is increased risk, especially in the areas of the Gipping Valley, where there are exposed chalk outcrops, and areas where the onshore electrical transmission works are within the SPZs. Therefore the significance of the impact is considered to be a moderate adverse impact.
96.
Excavation and trenching works will disturb soils and temporarily open the soil structure creating pathways for the mobilisation and transmission of contaminants.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 21 Water Resource and Flood Risk Page 35
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