4.2.6. es section 6. geology and physical processes · section 6: geology and physical processes...

Kentish Flats Offshore Wind Farm Extension Environmental Statement

Section 6: Geology and Physical Processes

IPC Document Ref: 4.2.6

Version No. Raised by Approved by Date of issue Vattenfall

Review

V0.1 Paolo Pizzolla 02.06.11

V1.0 David Brew 25.07.11 04.08.11

V2.0 Sarah Strong &

Paolo Pizzolla

12.08.11

V3.0 David Brew Nick Cooper 01.09.11 06.09.11

FINAL Paolo Pizzolla Marie Guegan 06.09.11

CONTENTS Page

6 GEOLOGY AND PHYSICAL PROCESSES 1 6.1 Introduction 1 6.2 Guidance and Consultation 1 6.3 Methodology 3 6.4 Existing Environment 4 6.5 Potential Effects on Geology 20 6.6 Potential Physical Process Effects during the

Construction Phase 20 6.7 Potential Physical Process Effects during the

Operational Phase 25 6.8 Potential Physical Process Effects during the

Decommissioning Phase 26 6.9 Effect of the Wind Farm on the Coastline 26 6.10 Inter-relationships 27 6.11 Cumulative effects 27 6.12 References 28

APPENDICES

6.1 Geophysical Survey Report 2010 6.2 Cable Route Extension Area Geophysical Survey Report

2011

Kentish Flats Offshore Wind Farm Extension

Environmental Statement Section 6 – Page 1 October 2011

6 GEOLOGY AND PHYSICAL PROCESSES

6.1 Introduction

6.1.1 This section describes the surface and sub-surface geology, marine physical processes (wind, wave and tidal regimes) and marine sedimentary processes (bathymetry, geomorphology and sediment transport) of the Kentish Flats Offshore Wind Farm Extension (Kentish Flats Extension) and adjacent areas. It will focus primarily on the southern and western extension areas and the export cable corridor.

6.1.2 The section provides a baseline description of the geology and physical processes in these areas. This is followed by an assessment of the magnitude and significance of the effects on geology, physical and sedimentary processes resulting from the construction, operation and decommissioning of the development.

6.2 Guidance and Consultation

6.2.1 The National Policy Statement for Renewable Energy Infrastructure (EN-3) (July, 2011) (DECC, 2011) has the following statements which are of relevance to geology and physical processes.

6.2.2 Paragraph 2.6.192 states that:

“The construction, operation and decommissioning of offshore energy infrastructure can affect the following elements of the physical offshore environment:

• waves and tides - the presence of the wind turbine foundations can cause indirect effects on flood defences, marine ecology and biodiversity, marine archaeology and potentially, coastal recreation activities;

• scour effect - the presence of wind turbine foundations and other infrastructure can result in a change in the water movements within the immediate vicinity of the infrastructure, resulting in scour (localised seabed erosion) around the structures. This can indirectly affect navigation channels for marine vessels and marine archaeology;

• sediment transport - the resultant movement of sediments, such as sand across the seabed or in the water column, can indirectly affect navigation channels for marine vessels; and

• suspended solids - the release of sediment during construction and decommissioning can cause indirect effects on marine ecology and biodiversity.”




“The Environment Agency (EA) regulates emissions to land, air and water out to 3 nm. Where any element of the wind farm or any associated development included in the application to the IPC is located within 3nm of the coast, the EA should be consulted at the pre-application stage on the assessment methodology for impacts on the physical environment.”


“Beyond 3nm, the MMO is the regulator. The applicant should consult the MMO and Centre for Environment, Fisheries & Aquaculture Science (Cefas) on the assessment methodology for impacts on the physical environment at the pre-application stage.”


“The assessment should include predictions of the physical effect that will result from the construction and operation of the required infrastructure and include effects such as the scouring that may result from the proposed development.”

6.2.6 Initial consultation on the project was carried out via the Kentish Flats Extension Scoping Document (Royal Haskoning, 2010), as well as further consultation exercises in 2011. Responses received are presented in the Infrastructure Planning Commission (IPC) Scoping Opinion report (IPC, 2010) and the Consultation Report that accompanies this application (Vattenfall, 2011). Table 6.1 summarises issues that were highlighted by the consultees and indicates which sections of the ES address each issue.

Table 6.1: Issues identified by consultees during the consultation process (2010–2011)

Date Consultee Summary of issues Sections where addressed

Dec 2010 IPC (Scoping Opinion 3.29)

If there are areas within the Extension site not covered by existing geological surveys they should be updated.

6.3 (Osiris geophysical survey was undertaken)


Information from adjacent sites should not be used as a proxy for the site characterisation or the baseline. Therefore, the Commission welcomes the inclusion of new and specific geophysical surveys of the Extension.

6.3 (Osiris geophysical survey was undertaken)

Dec 2010 EA (Scoping Opinion 3.31)

The ES should demonstrate the accuracy of assumptions made in relation to the results of the previous surveys undertaken for the KFOWF e.g. localised impacts. Any differences which may alter the assumptions made, such as a different foundation type or method

6.6, 6.7, 6.8



Date Consultee Summary of issues Sections where addressed

of seabed preparation, should be clearly identified in the ES.


The ES to provide confidence in the accuracy of the results.

6.6, 6.7, 6.8


No modelling to be undertaken − need to provide confidence in using KFOWF assumptions.

6.6, 6.7, 6.8

June 2011

MMO (s42 consultation response)

MMO would advise you to consider an assessment of the compression marks observed in the bathymetry from the spuds of the commissioning vessels. Are there areas within the wind farm where the clay layer is deeper, thinner or shallower?

6.6.7

June 2011

MMO (s42 consultation response)

With regard to Section 6.4.15, the interaction of the monopoles and the sand waves should be analysed in further detail.

6.7.6

6.3 Methodology

6.3.1 The geology of the Kentish Flats Extension is described in geological sequence from the oldest to the youngest strata. The geology is described to a depth of 50m sub-seabed across the extension areas and to 10m sub-seabed along the cable route corridor. The Osiris Report (2010) has been used to describe the geological conditions as it is a site specific study completed for Kentish Flats Extension1. Other papers and reports have been referenced as needed.

6.3.2 The methods adopted to understand changes to the physical and sedimentary processes are different to those adopted for other sections of this Environmental Statement (ES). This is because the development of the Kentish Flats Extension will have effects on the hydrodynamic and sedimentary process regimes, but these effects in themselves are not considered to be impacts; the impacts will be to other resources such as marine ecology. Hence, the commentary in this section focuses on describing the changes/effects rather than defining the impact. Potential impacts on marine ecology resources caused by changes in geomorphology and sedimentary processes are described in Section 10 (Benthic and Intertidal Ecology). Four main information sources have been reviewed to establish the baseline physical and sedimentary processes:

1 Osiris have conducted more work in 2011 surveying the eastern cable corridor which was

not covered in the 2010 survey.



• HR Wallingford. 2002. Kentish Flats Offshore Wind Farm. HR Wallingford Report EX4510, April 2002;

• HR Wallingford. 2003. Kentish Flats Offshore Wind Farm Metocean Study. HR Wallingford Report EX4725, March 2003; and

• GREP. 2002. Kentish Flats Environmental Statement, August 2002.

• Osiris. 2010. Proposed Extension to Kentish Flats 2 Offshore Wind Farm Geophysical Survey. Draft Report – Volume 2, November 2010.

6.3.3 Consideration is given in this section to the items listed in Section 3.3 of the Offshore Wind Farms: Guidance Note for Environmental Impact Assessment in Respect of FEPA and CPA Requirements (MCEU, 2004):

• Scour around wind turbine foundations and cables;

• Effect of cable laying on suspended sediment concentrations;

• Effect of the wind farm on wave and tidal patterns, seabed forms and sediment pathways;

• Effect of the wind farm on the coastline;

• Cumulative (other wind farms) and in-combination (other activities) effects; and

• Influence of climate change.

6.4 Existing Environment

Tertiary London Clay and Surface Sediment Veneer

6.4.1 The cable route corridor and extension areas comprise London Clay (Osiris, 2010) (Figure 6.1). In the Thames region the London Clay is approximately 150m thick and comprises firm to hard clayey silts, silty clays and silts (Cameron et al., 1992).

Quaternary Channel Fills

6.4.2 Across the southern extension area and the northern part of the cable route corridor the London Clay is overlain by sediments that fill shallow depressions or channels in the London Clay surface (Osiris, 2010). Of particular note is a northeast-southwest oriented channel fill that crosses the centre of the southern extension. The channel fill is approximately 1,500m wide, has a maximum thickness of 9m (Figures 6.1 and 6.2) and comprises sand and clay. This channel is believed to be part of the palaeo-Swale River System identified by GREP (2002) across the main Kentish Flats site. Elsewhere along the cable route and southern extension area, several smaller channel fills reach thicknesses up to 7m.



6.4.3 The western extension area is crossed by two large channel features (Osiris, 2010). Across the central part of the extension area a channel is approximately 1,500m wide and filled with up to 18m of sediment (Note: that this estimate may include some infill sediment) (Figures 6.1 and 6.3). The fill comprises a two-part sequence, with clays in its lower part and sand in its upper part. Across the northern end of the western extension a channel is at least 2,000m wide and contains up to 28m of sediment (note that this estimate may include some infill sediment) (Figures 6.1 and 6.3). The features within this channel cannot be resolved easily because of acoustic ‘blanking’ which occurred during the geophysical programme. This phenomenon is caused by loss of the seismic signal in the channel related to pockets of gas derived from organic material within the fill sediments or by coarser sediments present within and at the base of channel.



Bathymetry

6.4.4 The bathymetry of the cable route corridor gradually increases to the north from the coast with a maximum water depth of -5m CD (Osiris, 2010) (Figure 6.4). Across the southern extension the seabed elevation is consistently between -3.5m CD and -4.5m CD apart from several linear north-south oriented shallow ridges (-2.5m CD to -3.0m CD) created by sand-gravel waves (Figure 6.5). The seabed elevation across the western extension varies from -4.0m CD to -4.5m CD in areas without significant sand and up to -1.0m CD along the crests of several sand banks that cross the site from west-southwest to east-northeast.

Wind Climate

6.4.5 HR Wallingford (2003) investigated the wind conditions at Kentish Flats using UK Meteorological Office data from near North Foreland between October 1986 and March 2002. They showed that the prevailing winds are from westerly and southwesterly directions, with the strongest winds from the same directions.

Wave Climate

6.4.6 The wave climate at Kentish Flats was modelled at two points by HR Wallingford (2003). The points were AP1 at the northern extremity of the main site and AP2 at the southern extremity of the main site which is adjacent to both the western and southern extensions (Figure 6.6). Wave conditions were modelled for several tide levels including mean high water springs and mean low water springs and for different return periods (1, 10, 50 and 100 years). The results show that the largest waves approach from the 15-45oN sector. The highest significant wave heights (average height of the highest one third of waves) from this sector at AP2 for return periods of 1, 10, 50 and 100 years at mean high water spring are 1.9, 2.2, 2.4 and 2.5m, respectively (HR Wallingford, 2003) (Figure 6.6). The corresponding maximum wave heights are 3.4, 3.9, 3.8 and 3.8m, respectively.

Astronomical Tidal Range

6.4.7 The mean high water spring and mean low water spring elevations at Herne Bay, from Admiralty Tide Tables (2010) are 5.4m CD and 0.5m CD, respectively. The mean spring tide range is therefore 4.9m. With seabed elevations between -5m CD and -1m CD, the maximum water depths under normal conditions are between approximately 10.5m and 5.5m. A list of tidal datums at Herne Bay is provided in Table 6.2.



Table 6.2 Predicted tidal elevations at Herne Bay (Admiralty Tide Tables, 2010)

Elevation2

m CD m OD

Highest Astronomical Tide 5.7 3.0

Mean High Water Spring 5.4 2.7

Mean High Water Neap 4.3 1.6

Mean Sea Level 2.7 0.0

Mean Low Water Neap 1.5 -1.2

Mean Low Water Spring 0.5 -2.2

Lowest Astronomical Tide 0.1 -2.6

Extreme Water Levels

6.4.8 The astronomical tidal elevations can be raised significantly by interaction with surge events influenced by global weather systems. Positive surges cause higher tidal elevations in the southern North Sea. Predicted extreme water levels for Herne Bay, elevated by surges progressing across the southern North Sea, are presented in Table 6.3 (Dixon and Tawn, 1997).

Table 6.3 Estimated extreme still water levels for Herne Bay (Dixon and Tawn, 1997)

Extreme Water Levels Return Period

m CD m OD

1 Year 6.1 3.4

10 Years 6.6 3.9

50 Years 6.9 4.2

250 Years 7.4 4.7

Climate Change

6.4.9 Kentish Flats Extension will have a lease period of 50 years, during which time site conditions may vary due to the effects of global climate change, particularly sea-level rise.

2 A chart datum is the level of water that charted depths displayed on a nautical chart are

measured from. The United Kingdom Hydrographic Office use the Lowest Astronomical Tide

(LAT) to define chart datums. LAT is the lowest levels which can be predicted to occur under

average meteorological conditions.

Ordnance Datum or OD is a vertical datum used by an ordnance survey as the basis for deriving

altitudes on maps. Usually mean sea level (MSL) is used for the datum.



Sea-Level Rise

6.4.10 The Intergovernmental Panel on Climate Change (IPCC) (2007) estimated a global average sea-level rise over the 20th century of between 1.2mmyr-1 and 2.2mmyr-1 with an average value of 1.7mmyr-1. Between 1961 and 2003, the

rate was estimated at 1.8mmyr-1 (1.3−2.3mmyr-1) rising to 3.1mmyr-1

(2.4−3.8mmyr-1) between 1993 and 2003. As climate change takes effect and the earth warms, sea level will continue to rise. Central estimates of projected future relative sea-level rise for London up to 2050 reported by Lowe et al (2009) ranged from 0.18m for the lowest emissions scenarios to 0.26m for their higher emissions scenarios (relative to a 1980-1999 baseline). Since these potential changes in sea level will occur over the expected life time of the wind farm, it is necessary to anticipate greater water depths.

Storm Surges

6.4.11 The occurrence of storm surge events may be altered in the future by changes in storminess (the number, location or strength of storms). Lowe et al P (2009) indicated that the projected future trends in 50-year storm surge are less than 40mm above current average storm surge levels by 2050, not including sea-level rise. This magnitude of change is within what might be expected through existing natural variation.

Tidal Currents

6.4.12 Tidal currents in the Thames Estuary generally flow from east to west on the flood tide and west to east on the ebb tide. Tidal currents at locations close to the extension site are measured at three Admiralty tidal diamonds, which show that higher velocity currents occur on the ebb tide compared to the flood tide. At these diamonds, maximum spring flood tidal velocities range from 0.45 to 0.7ms-1 and maximum spring ebb tidal velocities range from 0.6 to 0.9ms-1.

6.4.13 HR Wallingford (2003) modelled peak flood currents under mean spring conditions that flow from east to west across Kentish Flats extension and cable corridor (Figure 6.7). Flows diminish in velocity towards the north Kent coast. The reverse occurs on the ebb tide, with peak currents flowing from approximately west-southwest to east-northeast (Figure 6.8). Predicted maximum depth-mean flow velocity across the Kentish Flats site is approximately 1.2ms-1 (on an ebb tide).

6.4.14 Tidal current flows were also modelled under more extreme conditions, where wind effects result in increased flow velocities. The modelling considered wind effects under a range of return periods approaching from the 270oN sector (HR Wallingford, 2003). The resulting depth-mean current velocities for 1-, 10-, 50- and 100-year wind conditions are shown as roses for three points in Figure 6.9. Maximum depth-mean current velocities of approximately 1.3ms-1, 1.35ms-1 and 1.4ms-1 were modelled for return



periods of 10, 50 and 100 years, respectively. These represent approximately 5%, 9% and 11% increases over the 1-year maximum tidal current velocity.



Geomorphology

Sand Banks and Bedforms

6.4.15 The western extension is crossed by four tidal sand banks of various sizes, forming part of the East Middle Sand – East Spaniard bank complex, with the surfaces of three of these banks sculpted into fields of megaripples (Osiris, 2010) (Figure 6.4). The geometrical definition of ripples, megaripples and sand waves are provided in Table 6.4. The outlines of these banks, which are up to 350m wide and oriented west-southwest to east-northeast is approximately defined by the 4m CD contour with their crests reaching maximum heights of 0.5m CD. The megaripples are up to 0.5m high with wavelengths between 3.0 and 23.0m and crests oriented approximately north to south across the axis of the banks (Figure 6.10). There are also several isolated sand wave features crossing the eastern part of the southern extension in approximately a north-south direction (Osiris, 2010) (Figure 6.5).

Table 6.4 Definitions of bedform dimensions used by Osiris (2010)

Definition Height (m) Wavelength (m)

Ripple <0.1 <1.0

Megaripple 0.1-3.0 1.0-30

Sand waves 3.0-15.0 30-500

Surface Sediment Veneer

6.4.16 Across the Kentish Flats Extension, isolated patches of London Clay outcrop at the seabed. Where it is close to the seabed (mainly along the cable route and the southern extension area) it is overlain by a thin veneer (less than 0.5m) of shelly sandy gravel. This veneer probably originated as a lag deposit formed by the scouring action of tidal currents over the London Clay removing finer particles to leave a layer of relatively coarse sediment. Megaripples, sculpted by tidal currents, measuring less than 0.5m high and

2.0−5.0m in wavelength are present in the coarse veneer near the southern extension and the northern end of the cable route corridor.

Bed Load Sediment Transport

6.4.17 Bed load sediment transport across Kentish Flats is driven by tidal currents which can be explained by the present of sand banks and their megarippled surfaces. The geometry of the megaripples does not show any definitive transport direction and their shape may change as the tide turns.

6.4.18 HR Wallingford (2002) indicated that residual tidal currents are relatively weak and rotational, lending support to the observed long-term stability of the banks in this area (Burningham and French, 2008, 2009). GREP (2002) also



assessed the historical stability of East Middle Sand and East Spaniard bank. They showed that between 1844 and 1995, East Spaniard bank reduced in size and East Middle Sand extended and realigned to the east. They argued that the sand banks in the Kentish Flats area are relatively stable compared to other banks within the Outer Thames Estuary.

Suspended Sediment Concentrations

6.4.19 As part of the turbidity monitoring for the Kentish Flats main wind farm, background suspended sediment concentrations were measured at a point approximately 5km west of the array and export cable corridor (EMU, 2005c). An optical backscatter sensor was deployed approximately 1m above the seabed between October 2004 and January 2005 and readings taken every 10 minutes. The modal suspended concentration derived from the data was between 50mgl-1 and 55mgl-1 (Figure 6.11). The data indicate that the background suspended sediment values rarely exceeded 120mgl-1.

6.5 Potential Effects on Geology

6.5.1 The London Clay and channel fills are static geological units that will not be affected by the construction, operation and decommissioning of the wind turbines and export cable.

6.6 Potential Physical Process Effects during the Construction Phase

6.6.1 This section focuses upon those elements of the construction stage that would have the greatest potential to affect physical and sedimentary processes. These are construction activities (plant and vessels), installation of wind turbine foundations and laying of the export and inter-array cables.

Changes to Wave Climate

6.6.2 Potential changes to wave climate during construction relate principally to the legs of jack-ups used to install the wind turbines. Given the amount of time jack-ups are deployed at each wind turbine location (1-2 days) and the very small size of the legs compared to the wavelength of typical waves, the effects are considered negligible.



Changes to Tidal Currents

6.6.3 Potential changes to the tidal current regime during construction also relate to the interruption caused by the jack-up legs during wind turbine installation. The effects on tidal currents are considered negligible for similar reasons to those provided for wave climate effects (above).

Changes to Sediment Transport and Morphology

6.6.4 Potential changes to the sediment transport regime and seabed morphology are related to three construction elements; driving monopiles into the seabed, the physical presence of plant on the seabed, and laying cable by ploughing the seabed. Changes to transport and morphology will depend on the level of disturbance created by the construction methods and the type of sediment disturbed at the surface and in the sub-surface.

Monopile Installation

6.6.5 Piles are likely to be driven into the seabed by pile-driving using a hydraulic hammer. Piles will first be up-ended to the vertical position and held by a pile ‘gripper’ before being driven into the seabed by the pile hammer. Piling times for each of the foundations installed at Kentish Flats main wind farm were between 1 and 2 hours (Vattenfall, 2009) and similar durations are expected during installation for the extension. A limited amount of site preparation work may be required to remove any obstructions at the surface (such as boulders) which may release very limited amounts of fine sediment locally into the water column. The pile-driving operation itself may also suspend sediment. However, the volumes released into the water column are considered to be very small for both these operations, and the effects will be negligible.

Presence of Plant

6.6.6 Potential changes to seabed morphology could take place at base of the construction jack-up rig legs used to install the wind turbines. Here, depressions in the seabed could be caused by the rig sinking into the seabed under its own weight. EMU (2005a) observed small depressions (0.5 to 2.0m deep) in the seabed of Kentish Flats main wind farm where the jack-ups had been located. These were evident as clusters of six regularly spaced depressions adjacent to each of the wind turbine locations (Vattenfall, 2009).

6.6.7 The size of the depression created by the jack-ups will depend largely on the substrate on which they are founded. Three main substrates are likely to be encountered; London Clay with gravel veneer, sand bank and channel fill. London Clay will be strongest substrate material and able to withstand the weight of the jack-ups without too much disturbance. The sand bank sediment would be more compressible and also more mobile and hence a depression would fill in rapidly after the jack-up was removed. The lithology of



the channel fills is variable and may be clay, sand, interlayered sand and clay, or homogeneous mixtures of the two. The channel fills are also variable in thickness. The channel fills will respond differently to the pressure of the jack-ups depending on their sediment strength (related to type) and thickness.

6.6.8 Through monitoring it was shown (EMU, 2006a, b, 2007, 2008) that post-

construction the depressions filled in at a rate of 0.2−0.5m every six months. Hence, construction effects on seabed morphology due to plant are considered to be localised and transient in nature and therefore negligible.

Cable Installation

6.6.9 From the wind turbine array, one or two export cables will be laid southwards

to the cable landfall point at Herne Bay, a distance of 8−10km. Each of these export cables and the inter-array cables may be installed using a cable plough (or other methods) and burial is expected to reach a target depth of at least 1m (Vattenfall, 2009). The sediment transport regime could potentially be changed by the seabed ploughing or jetting; the seabed will be disturbed and there is the potential to release additional suspended sediment into the water column. The volume of sediment released and its ultimate destination will depend on the ploughing method, the type of sediment that is disturbed (particularly the particle size), and the direction, strength and persistence of tidal currents. Sand and gravel will be moved a relatively short distance, whereas silt and clay will be suspended into a plume and dispersed over a wider area.

6.6.10 EMU (2005c) collected suspended sediment concentration data during installation of the three export cables for the main wind farm array. Monitoring during cable laying was conducted from a mobile vessel using a hand-held turbidity meter deployed every 10 minutes. Readings were taken at 1m above the seabed and at 1m intervals to the sea surface. The monitoring vessel was located no less than 500m down-tide from the cable installation vessel.

6.6.11 The results show that the modal concentration during the first cable laying period was the same as the baseline concentration. The modal values during laying of the second and third cables were between 55mgl-1 and 60mgl-1 representing an approximate 10% increase over baseline conditions. The suspended sediment concentrations generated during the cable laying process consistently fell below the threshold values agreed with Cefas. Given that the surface geology of any new cable routes will be similar to the three original cables, it is anticipated that additional suspended sediment released by further cable laying will be low level and not exceed the stated thresholds. Hence, the effect will be negligible.



6.7 Potential Physical Process Effects during the Operational Phase

6.7.1 This section focuses upon those elements of the wind farm operation that would have the greatest potential to affect physical and sedimentary processes. The potential effects are related to the operation of the wind turbines only as there will be no operational effects related to the cables because they will be buried beneath the seabed.

Changes to Wave Climate

6.7.2 Waves will be affected due to sheltering, diffraction and refraction around the wind turbine foundation, but only in the immediate vicinity of each proposed wind turbine. The diameter of each foundation (up to a maximum of 6m) will be narrow compared to typical wavelengths and so the potential effect is considered to be small and local to each wind turbine. Elsewhere, where the influence of the wind turbines is not felt, there will be no changes to the wave climate. Also, ABPmer (2003) indicated that wind turbines scatter wave energy individually with no augmentation of wave height due to interaction. Hence, the effect will be negligible

Changes to Tidal Currents

6.7.3 Changes to tidal current velocities will be restricted to small ‘wakes’ around each of the proposed wind turbine foundations. The piles would break up the otherwise rectilinear tidal flow pattern, causing bifurcation of flow around them. This would involve acceleration in flow around the edges of each pile (leading to scour, paragraph 6.7.4), with reduction (shadow) in flow directly in the lee of each pile. Other than in the immediate vicinity of the wind turbines, modification of tidal currents will not take place. Given the small size of the structures and the local scale of the effect compared to the large spatial scale of the tidal current processes operational in the Outer Thames Estuary, the effects are considered to be minimal. Also, ABPmer (2003) indicated that changes to tidal currents local to individual wind turbines are very small. Hence, the effect will be negligible.

Changes to Sediment Transport and Morphology

6.7.4 The local acceleration of tidal current flow around each monopile (in combination with wave effects) will tend to scour sediment from the base of the foundation. The depth of scour will depend on the physical conditions and the cohesiveness of the substrate. Sediment may be transported away as bed load or in suspension. GREP (2002) predicted that the theoretical maximum scour that could take place in non-cohesive sand would be 2,000 to 2,250m3 per wind turbine based on scour depth being equal to 2.25 times the diameter of the foundation (Whitehouse, 1998).

6.7.5 Post-construction bathymetric surveys around four monopiles within the Kentish Flats main wind farm array were undertaken by EMU (2004, 2005a, b, 2006a, b, 2007, 2008). The results showed that scour reached depths



between 1.2 and 2.3m late in 2005 approximately one year after foundation construction (2004). A period of partial backfill then occurred and the scour depths then stabilised over the next 1.5 years between 1.5 and 1.9m over a diameter of 5-10m. These data indicate that scour will take place quickly before conditions equilibrate (approximately one year) and no more sediment is released. In addition, the scour volume predicted by GREP (2002) of approximately 2,000m3 per wind turbine is an over estimate. Assuming a diameter of 10m and a scour depth of 1.9m, the maximum volume of sediment released is potentially an order of magnitude less at approximately 150m3 per wind turbine. The large difference between predicted scour (paragraph 6.7.4) and observed scour is probably due to the actual substrate being far more cohesive (i.e. Quaternary channel fills) than the non-cohesive sand used in the prediction. Hence, modifications to the sediment transport through scour at Kentish Flats Extension will be localised, small volume and short-lived. Hence, the effect will be negligible.

6.7.6 The physical obstruction of a wind turbine will alter the pattern of megaripple and sand wave migration as they pass the foundation. Given that each turbine is only up to 6m in diameter and Osiris (2010) show that megaripple crest lengths are several hundreds of metres long the megaripple will simply migrate 'through' each wind turbine with some local interruption to the process at the turbine itself. There is no effect on tidal current processes away from each turbine foundation and the megaripples will migrate as they did prior to installation. Away from the wind turbines variations in bathymetry are within the range of natural variation associated with the mobility of megaripples across the area (±0.2m).

6.8 Potential Physical Process Effects during the Decommissioning Phase

6.8.1 The removal of the foundations, down to 1m below the seabed, has the potential to affect seabed conditions and the prevailing physical processes (export and inter-array cables will be left in place). Any effects arising from decommissioning will be of no greater magnitude than those described for the installation and operational phases and are not therefore regarded as significant.

6.9 Effect of the Wind Farm on the Coastline

6.9.1 Wave and tidal current effects are considered to be limited to the vicinity of the wind turbine monopile foundations, with no significant interactions between structures, and therefore no cumulative influence. The local changes to waves and tidal currents at the monopiles will therefore not cause any change to the wave and tidal current processes along the north Kent shoreline. This conclusion is supported by industry research that concluded far-field effects due to influences on the wave climate and tidal current regime will be negligible for a situation similar to the Kentish Flats layout (ABPmer, 2003; Cefas, 2005). Natural variations in the extent and elevation of the numerous sand banks and sand waves in the Outer Thames Estuary will



have a much greater impact on physical processes at the coast than any effects caused by the monopile foundations.

6.9.2 Given that wave and tidal currents at the coast will not be affected by the development, then sediment transport will also not be affected. Also, increases in suspended sediment loads due to cable ploughing, monopile installation and scour after construction have been shown to be very low level. Hence, Kentish Flats Extension will have negligible impact upon the general bedload and suspended sediment transport regime at the coast.

6.10 Inter-relationships

6.10.1 There are no inter-relationships between water quality and other parameters considered within the ES. However, several parameters covered in the ES relate back to this section.

6.11 Cumulative effects

Other Wind Farms

6.11.1 No cumulative effects on the physical processes as a result of interactions with other offshore wind farms in the vicinity, such as London Array and the Greater Gabbard / Galloper Offshore Wind Farm are anticipated. Given the distances between these other projects and the Kentish Flats Extension (Table 6.5) the potential cumulative morphological effects are negligible.

Table 6.5 Distance from Kentish Flats Extension to other Outer Thames Estuary /

Southern North Sea wind farms Estimated extreme still water levels for

Herne Bay (Dixon and Tawn, 1997)

Name Distance (km)

London Array 24.8

Gunfleet Sands I 29.3

Gunfleet Sands II 28.7

Thanet 29.8

Galloper 61.3

Greater Gabbard 63.9

Norfolk R3 zone 91.8

Other Activities

6.11.2 Capital dredging of Princes Channel approximately 3.4km north of the Kentish Flats Extension is undertaken by the Port of London Authority. Deepening through dredging of approximately 8m has taken place between



2006 and 2008. The capital dredging was subject to an Environmental Impact Assessment which concluded that the dredging works would not change tidal current velocity or direction, sediment transport or erosional processes and would have little effect on the overall wave climate (PLA, 2004). Given that potential effects on physical processes of the Kentish Flats Extension will be restricted to near-field change only, in-combination effects are unlikely to occur. Elsewhere, aggregate dredging occurs approximately 40km from the site; as a result cumulative effects on physical processes are considered highly unlikely to occur.

6.12 References

ABPmer (2003). Assessment of potential impact of Round 2 offshore wind farm developments on sediment transport. Prepared by ABPmer for the DTI.

Burningham, H. and French, J. (2008). Historical changes in the seabed of the greater Thames Estuary. The Crown Estate, 54pp.

Burningham, H. and French, J. (2009). Seabed mobility in the greater Thames Estuary. The Crown Estate, 62pp.

Cameron, T.D.J., Crosby, A., Balson, P.S., Jeffrey, D.H., Lott, G.K., Bulat, J. and Harrison, D.J. (1992). United Kingdom offshore regional report: the geology of the southern North Sea. London: HMSO for the British Geological Survey.

Cefas (Centre for Environment, Fisheries & Aquaculture Science) (2005). Assessment of the significance of changes to the inshore wave regime from an offshore wind array. Cefas Report AE1227.

Department of Energy and Climate Change (DECC) (2011). National Policy Statement for Renewable Energy Infrastructure (EN-3) July 2011.

Dixon, M. J. and J. A. Tawn (1997). Spatial analyses for the UK coast. Proudman Oceanographic Laboratory, Internal Document, No 112, 217pp.

EMU (2004). Kentish Flats Offshore Wind Farm Pre-construction Swath Survey. Report to NEG Micon UK Ltd, Report Ref 04/J/1/02/0708/0457.

EMU (2005a). Kentish Flats Offshore Wind Farm Post-construction Swath Survey. Report to NEG Micon UK Ltd, Report Ref 05/J/1/02/0758/0477.

EMU (2005b). Kentish Flats Offshore Wind Farm Post-construction Swath Survey 2. Report to Kentish Flats Ltd, Report Ref 05/J/1/02/0869/0564.

EMU (2005c). Kentish Flats Monitoring Programme. Turbidity Monitoring, April 2005. Report No. 05/J/1/01/0733/0500.

EMU (2006a). Kentish Flats Offshore Wind Farm Post-construction Swath Survey 3. Report to Kentish Flats Ltd, Report Ref 06/J/1/02/0942/0590.



EMU (2006b). Kentish Flats Offshore Wind Farm Post-construction Swath Survey 4. Report to Kentish Flats Ltd, Report Ref 05/J/1/02/1019/0655.

EMU (2007). Kentish Flats Offshore Wind Farm Post-construction Swath Survey 5. Report to Kentish Flats Ltd, Report Ref 05/J/1/02/1057/0677.

EMU (2008). Kentish Flats Offshore Wind Farm Post-construction Swath Survey 6. Report to Kentish Flats Ltd, Report Ref 07/J/1/02/1157/0729.

GREP (2002). Kentish Flats Environmental Statement, August 2002.

HR Wallingford (2002). Kentish Flats Offshore Wind Farm. HR Wallingford Report EX4510, April 2002.

HR Wallingford (2003). Kentish Flats Offshore Wind Farm Metocean Study. HR Wallingford Report EX4725, March 2003.

Infrastructure Planning Commission (IPC) (2010). Kentish Flats Scoping Opinion Available from URL: http://infrastructure.independent.gov.uk/wp-content/uploads/2010/12/101207_EN0100036_Kentish_Flats_scoping_opinion_Web_Version.pdf Accessed on 16/12/10

Intergovernmental Panel on Climate Change (IPCC) (2007). Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Core Writing Team, Pachauri, R. K., & Reisinger, A., (eds.)). IPCC, Geneva, Switzerland.

Lowe, J. A., Howard, T. P., Pardaens, A., Tinker, J., Holt, J., Wakelin, S., Milne, G., Leake, J., Wolf, J., Horsburgh, K., Reeder, T., Jenkins, G., Ridley, J., Dye, S., Bradley, S. (2009). UK Climate Projections Science Report: Marine and Coastal Projections. Met Office Hadley Centre, Exeter, UK.

MCEU (Marine Consents and Environment Unit) (2004). Offshore Wind Farms: Guidance Note for Environmental Impact Assessment in Respect of FEPA and CPA Requirements published by the Marine Consents and Environment Unit (Version 2).

Osiris (2010). Proposed Extension to Kentish Flats 2 Offshore Wind Farm Geophysical Survey. Draft Report – Volume 2, November 2010.

PLA (Port of London Authority) (2004). Princes Channel Development: Phase II Dredging.

Royal Haskoning (2010). Kentish Flats Extension Environmental Scoping Study.

Vattenfall (2009). Kentish Flats Offshore Wind Farm. FEPA Monitoring Summary Report, March 2009.



Vattenfall (2011) Kentish Flats Extension Consultation Report. IPC document number 3.1.

Whitehouse, R.J.S. (1998). Scour at Marine Structures. Thomas Telford, 216pp.

4.2.6. es section 6. geology and physical processes · section 6: geology and physical processes...

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