151. The Holocene sands vary in thickness from several tens of metres beneath tidal sand ridges and sand waves to a thinner veneer in deeper areas. The sand is marine and predominantly fine to medium grained with local laminae of mud.
7.5.4 Water Levels 152. Marine water levels are predominantly governed by astronomical effects but can also be significantly influenced (elevated or depressed) by meteorological influences and surge effects.
7.5.4.1 Astronomical Tidal Levels 153. The East Anglia THREE site is located only 10km (at its closest point) to the northwest of an amphidromic point1 that is positioned just outside the central, eastern boundary of the East Anglia Zone (Figure 7.3).
154. Due to this, the tidal range across the windfarm area is relatively low. At the amphidromic point, the tidal range is near zero. Tidal range then increases with radial distance from this point.
155. The tidal range increases towards shore along the offshore cable corridor. At the shore it reaches a value of 3.6m on mean spring tides at Harwich (located approximately 7km to the southwest of the cable landfall).
7.5.4.2 Non-tidal Water Levels 156. The North Sea is particularly susceptible to storm surges and water levels can become elevated between 1.5 and 1.7m above astronomical tidal levels during a 1 in 1 year return period surge event, and between 2.3 and 2.5m during a 1 in 100 year return period surge event (GL Noble Denton 2011).
7.5.4.3 Climate Change 157. Due to global climate change and local land level changes, mean sea level at the shore is expected to be between 19 and 27cm higher by 2050 than 1990 values (Lowe et al. 2009).
158. Climate change is projected to have an insignificant effect on the height of storm surges over the lifetime of the proposed project (Lowe et al. 2009), although there is generally expected to be an increase in their frequency of occurrence.
1 Amphidromic points are locations at which the tidal rise and fall is zero; patterns of high and low tides rotate around these points.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 34
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