Volume , Annex 2.1 Physical Processes Technical Baseline ...

RWE Innogy UK | Triton Knoll Electrical System, Onshore Crossing Schedule

Volume 4, Annex 2.1 Physical Processes Technical Baseline Report

Triton Knoll Electrical System

Environmental Statement, April 2015.

Application Document 6.2.4.2.1

Pursuant to: APFP Reg. 5(2)(a)

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RWE Innogy UK | Triton Knoll Electrical System, Physical Processes Baseline Report

Triton Knoll Electrical System

Environmental Statement

Volume 4: Annex 2.1 Physical

Processes Technical Baseline

April 2015

Drafted By: RWE Innogy UK Approved By: Kim Gauld-ClarkDate of Approval

28/03/2015

Revision A

Triton Knoll Offshore Wind Farm Ltd Auckland House Great Western Way Swindon Wiltshire, SN5 8ZT

T +44 (0)845 720 090 F +44 (0)845 720 050 I www.rweinnogy.com

www.rweinnogy.com/tritonknoll [email protected]

Triton Knoll Offshore Wind Farm Limited Copyright © 2015 RWE Innogy UK Ltd All pre-existing rights reserved.

Liability In preparation of this document Triton Knoll Offshore Wind Farm Limited (TKOWFL), a joint venture between RWE Innogy UK (RWE) and Statkraft UK, subconsultants working on behalf of TKOWFL, have made reasonable efforts to ensure that the content is accurate, up to date and complete for the purpose for which it was prepared. Neither TKOWFL nor their subcontractors make any warranty as to the accuracy or completeness of material supplied. Other than any liability on TKOWFL or their subcontractors detailed in the contracts between the parties for this work neither TKOWFL or their subcontractors shall have any liability for any loss, damage, injury, claim, expense, cost or other consequence arising as a result of use or reliance upon any information contained in or omitted from this document. Any persons intending to use this document should satisfy themselves as to its applicability for their intended purpose. Where appropriate, the user of this document has the obligation to employ safe working practices for any activities referred to and to adopt specific practices appropriate to local conditions.

Triton Knoll Offshore Wind Farm Limited have been awarded EU TEN-E funding to support the development of the Triton Knoll Offshore Wind Farm Electrical System located in both UK Territorial waters and the UK’s Exclusive Economic Zone. The funding which is to be matched will support a number of surveys, engineering reports, and environmental impact assessment studies for the Triton Knoll Electrical System. The studies will form part of the formal documentation that will accompany the Development Consent Order which will be submitted to the Planning Inspectorate. The sum of €1,159,559 has been granted and the process to reclaim this funding is ongoing.

RWE Innogy UK

Marine Physical Environment: Baseline Assessment (Annex 6-1)

R.2277

September 2014

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RWE Innogy UK


Date: September 2014

Project Ref: R/4105/2

Report No: R.2277

© ABP Marine Environmental Research Ltd

Version Details of Change Date 1.0 First Draft 23/05/2014 2.0 Issue After Client Review 11/09/2014

Document Authorisation Signature Date

Project Manager: Nigel West 11/09/2014

Quality Manager: David Lambkin 11/09/2014

Project Director: Bill Cooper 11/09/2014

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Disclaimer:

▪ Any 'Draft' issue of this report, and any information contained therein, may be subject toupdates and clarifications on the basis of any review comments before 'Final' issue. Allcontent should therefore be considered provisional, and should not be disclosed to thirdparties without seeking prior clarification from ABP Marine Environmental Research Ltd("ABPmer") of the suitability of the information for the intended disclosure and should not berelied upon by the addressee or any other person.

▪ Unless previously agreed between the addressee and ABPmer, in writing, the 'Final' issue ofthis report can be relied on by the addressee only. ABPmer accepts no liability for the use byor reliance on this report or any of the results or methods presented in this report by any partythat is not the addressee of the report. In the event the addressee discloses the report to anythird party, the addressee shall make such third party aware that ABPmer shall not be liableto such third party in relation to the contents of the report and shall indemnify ABPmer in theevent that ABPmer suffers any loss or damage as a result of the addressee failing to makesuch third party so aware.

▪ Sections of this report rely on data supplied by or drawn from third party sources. Unlesspreviously agreed between the addressee and ABPmer, in writing, ABPmer accepts noliability for loss or damage suffered by the addressee or any third party as a result of anyreliance on third party data contained in the report or on any conclusions drawn by ABPmerwhich are based on such third party data.

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Summary

ABP Marine Environmental Research Ltd (ABPmer) has undertaken a baseline assessment of the marine physical environment along the proposed Triton Knoll Offshore Wind Farm export cable corridor. This baseline assessment has involved: (i) consideration of guidelines for the development of offshore renewable energy projects with respect to marine physical process issues; (ii) appreciation of particular concerns regarding marine physical process issues raised by stakeholders during consultation; (iii) collation and review of available data, including that obtained from literature reviews and collected during project-specific survey campaigns; and (iv) the development of a baseline understanding of the coastal system, including the:

▪ Water level regime;

▪ Current regime;

▪ Wind and wave regime;

▪ Sediment regime (including seabed sediment distribution, bedload andsuspended load transport);

▪ Morphological regime; and

▪ Water and sediment quality.

The export cable route occupies a macro tidal setting with tidal range increasing towards the coast. Peak flow speeds are in excess of 0.8 m/s and exceed 1 m/s in places. Seabed sediments along the export cable corridor and surrounding area are dominated by sands and gravels although these surficial sediment units are typically very thin. However, they achieve greater thickness (order of several metres) in areas where bedforms are present. At a regional scale, the distribution of these sediments is strongly influenced by the underlying geology/ bathymetry and in particular, Silver Pit, which runs approximately north to south across this region.

The coastal frontage at Anderby Creek (where the cable makes landfall) is characterised by the presence of a sandy beach backed by vegetated sand dunes. Beach levels along this coastline vary seasonally, with steeper waves during winter transporting sands offshore and less steep waves during summer returning sands to the beach. This section of coast is highly sensitive and has suffered from long-term erosion. To counter this erosion and fulfil the ‘Hold the Line’ shoreline management plan, a major beach renourishment scheme (known as ‘Lincshore’) is in place along the entire coast between Mablethorpe and Skegness.

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Abbreviations

ABPmer ABP Marine Environmental Research Ltd BERR Department for Business, Enterprise and Regulatory Reform BGS British Geological Survey BODC British Oceanographic Data Centre Cefas Centre for Environment, Fisheries & Aquaculture Science COWRIE Collaborative Offshore Wind Research into the Environment CPT Cone Penetration Test cSAC candidate marine Special Areas of Conservation DCO Development Consent Order DECC Department of Energy and Climate Change DNV Det Norske Veritas EIA Environmental Impact Assessment ES Environmental Statement HADA Humber Aggregate Dredging Association HSE Health and Safety Executive HW High Water IEMA Institute of Environmental Management and Assessment IPC Infrastructure Planning Commission IPCC Intergovernmental Panel on Climate Change ISQG Interim Sediment Quality Guidelines JNCC Joint Nature Conservation Committee LAT Lowest Astronomical Tide LOIS Land Ocean Interaction Study LW Low Water MAREA Marine Regional Environmental Assessment MCZ Marine Conservation Zone MMO Marine Management Organisation MODIS Moderate Resolution Imaging Spectroradiometer NPS National Policy Statements NSIP Nationally Significant Infrastructure Projects NTSLF National Tide and Sea Level Facility PSA Particle Size Analysis PSD Particle Size Distribution PAH Polycyclic aromatic hydrocarbon PEL Probable Effect Level SAC Special Area of Conservation SCI Site of Community Importance SMP Shoreline Management Plan SNSSTS Southern North Sea Sediment Transport Study SoS Secretary of State SPA Special Protection Areas SPM Suspended Particulate Matter SSC Suspended sediment concentrations SSSI Site of Special Scientific Interest

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TBC To be Confirmed TEL Threshold effect level TKOWF Triton Knoll Offshore Wind Farm UKCP09 United Kingdom Climate Projections UKHO United Kingdom Hydrographic Office WGS84 World Geodetic System, 1984

Nomenclature

Hs Significant Wave Height LAT Lowest Astronomical Tide mg/l Milligrams per litre m/s Metres per second Tp Peak Wave Period º Degree(s) μm Micrometre(s)

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Contents Page

Summary .................................................................................................................... 5 Abbreviations ............................................................................................................. 6 Nomenclature ............................................................................................................. 7 1. Introduction .................................................................................................... 102. Legislation and Guidance .............................................................................. 11

2.1 Generic Law, Policy, Guidance and Regulation .................................. 11 2.2 National Policy Statements and Marine Plans .................................... 13

3. Consultation................................................................................................... 164. Data Sources ................................................................................................. 18

4.1 TKOWF Site Specific Surveys ............................................................ 18 4.2 Further Data Sources .......................................................................... 20

5. Baseline Environment .................................................................................... 205.1 Water Level Regime ........................................................................... 20

5.1.1 Overview ............................................................................................ 20 5.1.2 Tidal Water Levels ............................................................................. 21 5.1.3 Non-Tidal Water Levels ..................................................................... 21 5.1.4 Future Changes to Baseline Water Levels ........................................ 21

5.2 Current Regime .................................................................................. 22 5.2.1 Overview ............................................................................................ 22 5.2.2 Astronomical Tidal Currents .............................................................. 22 5.2.3 Non-astronomical Tidal Currents ....................................................... 23

5.3 Wave Regime ..................................................................................... 24 5.3.1 Overview ............................................................................................ 24 5.3.2 Wave Climate .................................................................................... 24 5.3.3 Extreme Waves ................................................................................. 25 5.3.4 Future Changes to the Wave Regime ............................................... 25

5.4 Sediment Regime ............................................................................... 26 5.4.1 Overview ............................................................................................ 26 5.4.2 Seabed Sediments: Composition and Distribution ............................ 27 5.4.3 Sediment Sub-strata: Composition and Distribution .......................... 28 5.4.4 Conceptual Understanding of the Sediment Regime ......................... 28

5.5 Morphological Regime ........................................................................ 32 5.5.1 Overview ............................................................................................ 32 5.5.2 Contemporary Seabed Morphology ................................................... 32 5.5.3 Shoreline Characteristics ................................................................... 34

5.6 Water and Sediment Quality ............................................................... 35 5.7 Summary ............................................................................................ 36

6. References .................................................................................................... 51

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Tables

1. Summary of National Policy Statements Guidance in relation tometocean and coastal processes .................................................................. 14

2. Recent geophysical and geotechnical surveys undertaken for the TritonKnoll Offshore Wind Farm export cable route ................................................ 19

3. Summary of observational wave records representative of offshore, midand inshore areas .......................................................................................... 24

4. Estimated potential sediment mobility (due to tidal currents only) at aseries of locations along the export cable corridor ......................................... 29

Figures

1. The Study Area .............................................................................................. 382. Seabed and Coastline with Nature Conservation Designations Within the

Study Area ..................................................................................................... 39 3. Data Source Locations .................................................................................. 404. Mean Spring Tidal Currents for High, Low, Ebb and Flood Tides .................. 415. Residual Tidal Flow Over a Spring-Neap Period Across the Study Area ....... 426. Summary of Wave Observations ................................................................... 437. Regional Description of Seabed Sediments Across the Study Area .............. 448. Depth Below Seabed to Base of Underlying Geological Units Along the

Export Cable Corridor .................................................................................... 45 9. Tidally Induced Bed Shear Stress and Mobility Thresholds for Selected

Locations Along the Export Cable Corridor ................................................... 46 10. Bedload Sediment Transport Indicators Across the Study Area .................... 4711. Suspended Particulate Matter Concentrations (Summer and Winter) ........... 4812. Bedform Features Within and Nearby to the Export Cable Corridor .............. 4913. The Export Cable Landfall ............................................................................. 50

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1. Introduction

1.1 This chapter provides a baseline assessment of marine physical processes for the Triton Knoll Electrical System, referred to hereafter as the Triton Knoll Offshore Wind Farm (TKOWF) export cable corridor (Figure 1). The assessment of potential effects and associated impacts on receptors arising from the proposed development are considered separately within Environmental Statement Chapter 6 ‘Marine Physical Environment’.

1.2 The assessment of baseline marine physical processes has been sub-divided into six categories:

▪ Water level regime;

▪ Current regime;

▪ Wind and wave regime;

▪ Sediment regime (including seabed sediment distribution, bedload andsuspended load transport);

▪ Morphological regime (including form and function of both the coastand offshore). The morpho-dynamic regime is defined as a response toboth the hydrodynamic and sediment regime;

▪ Water and sediment quality.

1.3 The baseline describes the variability of these regimes prior to the installation of the export cable. This provides the reference condition to compare against the various phases of project development. These comparisons offer the means to assess the significance of any changes to the baseline including identified receptors (such as designated areas of seabed (Figure 2)) that are attributable to the proposed development.

1.4 Marine physical process receptors are identified in Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6 and include:

▪ The coast at the landfall;

▪ Designated areas of seabed and coastline (e.g. Inner Dowsing, RaceBank and North Ridge SAC); and

▪ Non designated banks (as these may afford protection to the coast bydissipating wave energy).

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1.5 The baseline environment is not static and will exhibit some degree of change with or without the export cable in place due to naturally occurring cycles and processes. Therefore, when undertaking impact assessments it becomes relevant to place any potential impacts in the context of the envelope of change that might occur naturally over the timescale of the proposed development. (The project licence for TKOWF is 50 years.) The potential effects of climate change are also considered in this section. For instance, it is generally anticipated that climate change will result in global scale effects which will be represented at regional scales by the trends in rising mean sea level and increased storminess (Lowe et al., 2009).

1.6 The baseline for coastal processes has been developed through the analysis of data, a programme of site surveys, pre-existing datasets and available literature sources. The baseline has been considered over two spatial scales (Figure 1). These are:

▪ Far-field. Defined as the coastal and offshore area surrounding theexport cable corridor over which remote effects may occur; and

▪ Near-field. Defined as the footprint of the export corridor.

1.7 Together, these form the study area shown in Figure 1.

1.8 To aid description, the cable corridor itself has been sub-divided into four separate areas (Figure 1):

▪ Nearshore Area (water depths less than ~5 m LAT);▪ Inshore Area (water depths between ~5 and 10 m LAT);▪ Midshore Area (water depths between ~10 and 20 m LAT); and▪ Offshore Area (water depths between ~33 m LAT and 8 m LAT).

2. Legislation and Guidance

2.1 Generic Law, Policy, Guidance and Regulation

2.1 A number of generic law, policy, guidance and regulation documents are available which are of relevance to the assessment of physical processes.

2.2 These include:

‘Guidelines for Environmental Impact Assessment’ (Institute ofEnvironmental Management and Assessment (IEMA), 2004);

Coastal Process Modelling for Offshore Wind farm EnvironmentalImpact Assessment’ (ABPmer et al, 2009) (for COWRIE);

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‘Review of Cabling Techniques and Environmental Effects applicable tothe Offshore Wind farm Industry’ (BERR, 2008);

‘General advice on assessing potential impacts of and mitigation forhuman activities on Marine Conservation Zone (MCZ) features, usingexisting regulation and legislation’ (JNCC and Natural England, 2011);

‘Guidelines for data acquisition to support marine environmentalassessments of offshore renewable energy projects’ (Cefas, 2011);

‘Dynamics of scour pits and scour protection - Synthesis report andrecommendations. (Sed02)’ (HR Wallingford et al., 2007);

‘Further review of sediment monitoring data (ScourSed-09)’ (ABPmeret al., 2010);

‘Potential effects of offshore wind developments on coastal processes’(ABPmer and METOC, 2002);

‘Review of Round 1 Sediment process monitoring data - lessons learnt.(Sed01)’ (ABPmer et al., 2007);

‘Advice Note Seven: Environmental Impact Assessment, screening andscoping’ (The Planning Inspectorate, 2012a);

‘Advice Note Nine: Using the Rochdale Envelope’ (The PlanningInspectorate, 2012b);

‘Advice Note Twelve: Development with significant transboundaryimpacts consultation’ (The Planning Inspectorate, 2012c); and

‘Subsea Power Cables in Shallow Water Renewable EnergyApplications’. (DNV, 2014).

2.3 Monitoring evidence compiled during the construction and operation of earlier offshore wind farm developments is also now available. Some of this information is contained within the COWRIE ScourSed-09 publication (ABPmer et al., 2010), whilst a number of monitoring reports and previous offshore wind farm EIAs are hosted on the COWRIE website. ABPmer also maintains an offshore wind farm evidence database containing these and other relevant publications (such as academic journal articles and other wind farm Environmental Statement chapters) pertaining to the effects of offshore wind farms upon the physical environment. This ‘evidence base’ provides a critical input to the assessment phase.

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2.2 National Policy Statements and Marine Plans

2.4 The assessment of potential impacts upon marine physical processes has been made with specific reference to the relevant National Policy Statements (NPS) and Marine Plans. These are the principal decision making documents for Nationally Significant Infrastructure Projects (NSIP). Those relevant to the Project are:

Overarching NPS for Energy (EN-1) (July 2011); NPS for Renewable Energy Infrastructure (EN-3) (July 2011); and ‘East Inshore and East Offshore Marine Plans’ (MMO, 2014).

2.5 The relevance of NPS guidance with regard to marine physical processes and how these have been addressed within this assessment are presented in Table 1.

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Table 1. Summary of National Policy Statements Guidance in relation to metocean and coastal processes

National Policy Statement Guidance

Impacts on the Offshore Physical Environment to be Considered

How the Project has Assessed the Impact in

Relation to NPS Guidance

NPS EN-3 (Para 2.6.194)

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

Predictions of the physical impacts that will result from construction and operation of the Project are presented in Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.

NPS EN-1 (Para 5.5.6).

Where relevant, applicants should undertake coastal geomorphological and sediment transfer modelling to predict and understand impacts and help identify relevant mitigating or compensatory measures

Predictions of the physical impacts that will result from construction and operation of the Project are presented in Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.

NPS EN-1 (Para 5.5.7)

The ES should include an assessment of the effects on the coast. In particular, applicants should assess: The impact of the proposed project on

coastal processes and geomorphology,including by taking account of potentialimpacts from climate change. If thedevelopment will have an impact oncoastal processes the applicant mustdemonstrate how the impacts will bemanaged to minimise adverse impactson other parts of the coast;

Predictions of the physical impacts that will occur at the coast from construction and operation of the Project are presented in Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.

The implications of the proposedproject on strategies for managing thecoast as set out in ShorelineManagement Plans (SMPs)…anyrelevant Marine Plans…and capitalprogrammes for maintaining flood andcoastal defences;

The effects of the proposed project onmarine ecology, biodiversity andprotected sites;

The effects of the proposed project onmaintaining coastal recreation sites andfeatures; and

The vulnerability of the proposeddevelopment to coastal change, takingaccount of climate change, during theproject’s operational life and anydecommissioning period.

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National Policy Statement Guidance

Impacts on the Offshore Physical Environment to be Considered

How the Project has Assessed the Impact in

Relation to NPS Guidance


The applicant should be particularly careful to identify any effects of physical changes on the integrity and special features of MCZs, candidate marine Special Areas of Conservation (cSACs), coastal SACs and candidate coastal SACs, coastal Special Protection Areas (SPAs) and potential Sites of Community Importance (SCIs) and Sites of Special Scientific Interest (SSSI)

Designated nature conservation areas have been identified within the list of physical process receptors presented in the Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.

NPS EN-3 (Para 2.6.113)

Where necessary, assessment of the effects on the subtidal environment should include: Environmental appraisal of inter-array

and cable routes and installationmethods;

Habitat disturbance from constructionvessels’ extendible legs and anchors;

Increased suspended sediment loadsduring construction; and

Predicted rates at which the subtidalzone might recover from temporaryeffects.

Predictions of the physical impacts that will result from construction and operation of the Project are presented in the Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.


An assessment of the effects of installing cable across the intertidal zone should include information, where relevant, about:

Predictions of the physical impacts that will result from construction and operation of the Project are presented in the Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6.

Any alternative landfall sites that havebeen considered by the applicantduring the design phase and anexplanation for the final choice;

Any alternative cable installationmethods that have been considered bythe applicant during the design phaseand an explanation for the final choice;

Potential loss of habitat; Disturbance during cable installation

and removal (decommissioning); Increased suspended sediment loads in

the intertidal zone during installation; Predicted rates at which the intertidal

zone might recover from temporaryeffects.

EN-1 (Section 4.8) The resilience of the project to climate change (such as increased storminess) should be assessed in the ES accompanying an application

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3. Consultation

3.1 A number of consultations with regard to the Triton Knoll Offshore Wind Farm have been undertaken. These are summarised below:

3.2 Scoping Opinion provided by the Secretary of State (SoS) (May, 2014):

Impacts on geology below the maximum burial depth of the cable: TheSoS agrees this can be scoped out from further assessment as there isno pathway for impact and on the assumption the assessment will beundertaken in line with the burial depth parameters of the draft DCO;

Elevations in suspended sediment levels in the water column: The SoSconsiders that this information is important for the assessment ofimpacts on receptors including benthic ecology, fish species andimpacts on nature conservation sites and as such this informationshould be presented in the ES. The SoS also refers to and agrees withMMOs comments … which identifies the need for an assessment of thepredicted levels of suspended sediment concentration and sedimentplume characteristics. The SoS notes jetting is considered to representthe worst case scenario and as such the assessment should beundertaken on this basis;

Alteration of bathymetry due to indentations in the seabed from vessellegs and vessel anchors: The SoS does not agree this can be scopedout from further assessment on the basis the cable route passesthrough the Inner Dowsing, Race Bank and North Ridge SCI and asthe construction vessels to be employed have not yet been determined;

Alteration of bathymetry due to the introduction of cable protection: TheSoS notes that elevation of the seabed would be approximately 1mwhere cable protection is required, however does not agree this can bescoped out from further assessment given that the extent of cableprotection has not been defined in the Scoping Report and as noinformation has been provided on the potential effects to thesurrounding areas;

Remobilisation of contaminated sediments: The Scoping Report statesthat the available evidence on contamination along the cable route issparse, but the likelihood of finding high levels of contamination isconsidered low given that the sediments are coarse and there is anabsence of historic waste disposal sites in proximity to the scopingboundary. The SoS agrees that this can be scoped out from furtherassessment in the EIA;

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Operational impacts on geology: The SoS agrees that anymaintenance works would be local and temporary and that this can bescoped out from further assessment;

Operational impacts on the tidal regime: The SoS does not agree thiscan be scoped out from further assessment given that the extent ofcable protection has not been defined in the Scoping Report;

The SoS does agree that any impacts are likely to be localised and assuch cumulative impacts during operation can be scoped out fromfurther assessment;

Operational cumulative and inter-relationship impacts: Paragraph6.5.24 of the Scoping Report states that potential changes insuspended sediment concentrations associated with cable scour will benegligible although earlier states that scour will be assessed in the EIA(paragraph 6.5.17). As such the SoS does not agree this can bescoped out from further assessment. However, the SoS does agreethat any impacts are likely to be localised and as such cumulativeimpacts during operation can be scoped out from further assessment.

3.3 In April 2014, the MMO provided a scoping response to the proposed ‘Triton Knoll Electrical System for the Triton Knoll Offshore Wind Farm’. The main feedback relevant to the marine physical environment include the following comments and recommendations:

It is essential that the potential impacts of all the remedial measuresare assessed in the ES. The MMO advises that an assessment of thequantity and type of cable protection that may be required along theexport cable routes be provided within the ES. Potential impacts onsediment transport and beach profiles should be considered and givenin the context of the type (rock protection / mattressing etc.) andpositioning of any potential cable protection;

… given the dynamic nature of the environment, monitoring is requiredto verify that buried cables do not become exposed, or even free-spanning. Exposed or free-spanning cables can pose a navigationaland safety hazard to other sea users. Further monitoring or mitigationmay be required for this potential risk and the MMO will advise whatmitigation it considers appropriate once we have reviewed theassessment;

The quality standards and assurance methods should be detailed inthe individual survey reports and also included in the documentationsubmitted for the application process.

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3.4 In September 2010 the Infrastructure Planning Commission (IPC) provided a scoping opinion on the TKOWF scoping report (including the original cable export route). Additionally, the MMO provided a response to the “Proposed Triton Knoll wind farm export cable off the Lincolnshire coast” survey scoping paper that was submitted in July 2012. Although the opinions focussed on the wind farm itself and an earlier cable route, or scoping for marine surveys, they are still relevant to the current study.

3.5 The principal feedback relevant to the marine physical environment from these consultations include the following comments and recommendations ensuring that:

….surveys are all relevant and up to date. Previous data cannot berelied upon. Great care should be taken to ensure the assessment isundertaken against a consistent baseline;

IPC draws attention to the potential impacts on the coast and foreshorewhich should be assessed;

Cross-reference should be made to the coastal processes section ofthe ES and the consideration of sediment transport;

Potential impact of the proposed development on the proposed SAC(Inner Dowsing, Race Bank and North Ridge);

….scour and its associated impacts around export cable …. that havethe potential to become uncovered due to changes in seabedmorphology should be fully explored in the ES;

… evidence of sediment type that the route traverses; and

An assessment of the predicted levels of suspended sedimentconcentrations (SSC) associated with the export cable route works arerequired along with an assessment of the predicted sediment plumecharacteristics.

4. Data Sources

4.1 TKOWF Site Specific Surveys

4.1 In order to support the baseline characterisation and the subsequent impact assessment of the project, from installation through to decommissioning, a number of site specific surveys have taken place over the proposed export cable corridor route. These are detailed in Table 2.

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Table 2. Recent geophysical and geotechnical surveys undertaken for the Triton Knoll Offshore Wind Farm export cable route

Type of Survey

Year of Survey Company Location Survey Methods

Geophysical 2009 Gardline Geosurvey Ltd

Initial export cable route and partial fan area

Single and multi-beam echo sounder, side scan sonar, sub bottom profiler (boomer) and magnetometer.

Geophysical 2012 Osiris Projects Proposed Triton Knoll export cable route and partial fan area

Single and multi-beam echo sounder, side scan sonar, sub-bottom profiler (pinger and boomer) and magnetometer.

Geotechnical 2012 Osiris Projects Proposed Triton Knoll export cable route and fan area.

CPT’s at 58 locations and vibrocores at 18 locations.

Benthic 2008, 2010 RPS Initial export cable route and partial fan area

Grab sample Particle Size Analysis (PSA) data.

4.2 The recovered vibrocores from the Osiris 2012 geotechnical survey were subject to PSD classification and strength testing laboratory analysis (Osiris, 2013b). Six geotechnical locations from this survey were located in the fan area (which was subject to geophysical survey by Gardline in 2009). The extent of the Gardline and Osiris geophysical surveys, along with the locations of the CPT and vibrocores from the Osiris geotechnical survey, are shown in Figure 3.

4.3 The TKOWF site itself has been subject to geophysical and geotechnical surveys by Osiris in 2008 and 2009 respectively. Metocean data was also collected from the site by Emu in 2009 (Emu, 2009).

4.4 A marine ecological survey was carried out at the TKOWF site in 2008 and across the originally proposed export cable route and intertidal zone in 2010. These included collection of particle size data and sediment chemistry (RPS, 2008).

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4.2 Further Data Sources

4.5 Additional information has also been obtained from other sources to complement that obtained from the geophysical, geotechnical, benthic and metocean surveys described above. This additional data acquisition has included:

British Geological Survey (BGS) grab samples and vibrocore data;

Southern North Sea Sediment Transport Study (SNSSTS, 2002);

UKHO admiralty publications;

Humber Regional Environmental Characterisation (Tappin et al., 2011);

Humber Marine Regional Environmental Assessment (MAREA)(HADA, 2012); and

The Environment Agency beach monitoring datasets.

4.6 The Humber Regional REC published in 2011, (Tappin et al. 2011) is a multidisciplinary marine study of an area of 11,000 km2 off the east coast of England. The area covers the proposed export cable route and provides regional scale information on the geology, biology and archaeology of the marine environment (Figure 3)

4.7 The Environment Agency has undertaken regular strategic coastal monitoring of the Lincolnshire coast. The rationale behind the programme is to assist the implementation of appropriate and sustainable works on the coast. Analysis of these datasets enables understanding of past changes to the coast in this region.

5. Baseline Environment

5.1 Water Level Regime

5.1.1 Overview

5.1 Marine water level measurements typically contain both a predictable astronomical tidal signal (i.e. that caused by the sun and moon) and a more random non-tidal signal, typically related to meteorological influences and referred to as the tidal residual. These distinct components of the water level regime are described separately below, using information from:

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The TKOWF site-specific metocean surveys;

UKHO tidal statistics for Skegness and Inner Dowsing Light;

The National Tide and Sea Level Facility (NTSLF); and

The Atlas of UK Marine Renewable Energy Resources (ABPmer et al.,2008).

5.1.2 Tidal Water Levels

5.2 A tidal amphidrome centred off the west coast of Denmark governs the tidal phase conditions in this part of the southern North Sea, with the tidal wave rotating anticlockwise, flooding to the south and ebbing to the north twice a day (a semi-diurnal tide). The cable route occupies a macro tidal setting with tidal range increasing towards the coast (Figure 1). At the eastern end of the cable corridor (in the Offshore Area), the mean spring tidal range is approximately 4.9 m, increasing to 5.8 m at the Nearshore Area adjacent to the landfall. By comparison, the mean neap range varies between approximately 2.4 and 2.9 m along the export cable corridor, between the Offshore Area and Nearshore Area.

5.1.3 Non-Tidal Water Levels

5.3 In addition to water level variation induced by tidal influences, change may also occur as a result of non-tidal meteorological influences. For example, high pressure systems will cause the water level to be depressed (negative surge) whilst low pressure systems will enable water levels to rise (positive surge). Persistent winds associated with these low pressure systems may generate storm surges.

5.4 The geometry and location of the North Sea Basin makes it particularly susceptible to large surge events (Environment Agency, 2011; Flather and Williams 2000). The most recent large surge event occurred on the 5 December 2013 with water levels around 2 m above the predicted level within the study area.

5.1.4 Future Changes to Baseline Water Levels

5.5 Mean sea level along the export cable corridor is likely to alter during the 21st Century as a consequence of climate change. A rise in sea level may allow larger waves, and therefore more energy, to reach the coast in certain conditions, and consequently result in increased erosion (Balson et al., 2001).

5.6 Information on the rate and magnitude of anticipated relative sea level change at the landfall during the 21st century is available from United Kingdom Climate Projections (UKCP09) (Lowe et al., 2009). These findings suggest

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that by 2050, relative sea level will have risen by approximately 0.25 m above 2014 levels with rates of change increasing over time. Evidence presented in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (IPCC, 2013) suggests that the UKCP09 estimates may lack conservatism. However, the predictions remain valid (Horsburgh and Lowe, 2013): moreover, the projected rise in sea level during the 21st century will be minor in comparison to the existing tidal variation in water level and is unlikely to directly impact the area under consideration.

5.2 Current Regime

5.2.1 Overview

5.7 The current regime within the study area comprises (i) astronomically driven tidal currents; and (ii) non-tidal currents associated with meteorological forcing. In this region, storm surge currents are the most significant non-tidal currents and may cause an increase in the locally observed current speed, additional to that expected from astronomical forcing alone. Orbital currents associated with the passage of waves are also superimposed upon the time mean flow. These components of the current regime have been described below, using:

Metocean survey records from the TKOWF site (full vertical profile); British Oceanographic Data Centre records (single depths only); UKHO TotalTide records; and Depth averaged output from numerical tidal models.

5.2.2 Astronomical Tidal Currents

5.8 The current time-series data listed above have been used to calibrate and validate the numerical tidal model employed in this investigation. The validated model provides a more extensive spatial and temporal description of astronomical tidal processes. Tidal current predictions from the tidal model have been plotted to show patterns of mid-flood and mid-ebb currents during spring and neap tides (Figure 4), as well as residual flow over a spring-neap cycle (Figure 5). Together, these figures show:

Tidal currents in this region generally follow the orientation of thecoastline and flow in a southerly direction during the flood tide and in anortherly direction during the ebb tide;

With the exception of the Nearshore Area immediately adjacent to thelandfall, at all locations along the export cable corridor peak springflows exceed 0.8 m/s and in places exceed 1.0 m/s;

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The highest flows are encountered at the southern end of Silver Pit (inthe Offshore Area) and to the west of Inner Dowsing, approximately 8km offshore from the landfall in the Inshore Area;

The weakest tidal currents are encountered in the Nearshore Area,close to the landfall. Here, maximum spring current speeds are around0.6 m/s;

Patterns of residual flow (which are important for the net transport ofsuspended sediments – Section 5.4.4)) are known to be complex inthis region (ABPmer, 2010). However, along much of the cable corridorresidual flow is generally in a northerly direction. The exception to thisgeneral pattern is close to the coast within the Inshore and NearshoreAreas, where residual flow is to the south; and

Across the littoral zone (i.e. within the Nearshore Area), residual tidalflow will act in conjunction with the wave driven currents, drivinglongshore sediment transport.

5.2.3 Non-astronomical Tidal Currents

5.2.3.1 Surge currents

5.9 As stated in Section 5.1, the geometry and location of the North Sea Basin makes it particularly susceptible to storm surges (e.g. Suthons, 1963). Surge currents are superimposed upon astronomical tidal currents and when combined, these have the potential to enforce a short-term control upon the sediment regime. This is most pronounced in the shallow nearshore environment (SNSSTS, 2002). In this region, peak surge currents are typically directed to the southeast and are in the range 0.6 to 0.8 m/s for a 1:50 year surge event (HSE, 2002).

5.2.3.2 Wave induced orbital currents

5.10 Individual waves are associated with circular or elliptical water movements beneath them. If this motion extends to the seabed, as would be the case for intermediate and shallow waters, an oscillatory (to-and-fro) near-bed current will result. Wave induced currents oscillate at wave period timescales (order of seconds), typically with a symmetrical near-sinusoidal pattern, unless in particularly shallow water (e.g. <5 m LAT) such as that found within the Nearshore Area. Although wave induced orbital currents may have the capacity to mobilise sediment at the bed, it is only in very shallow water (where non-symmetrical patterns of orbital flow occur) that they may contribute to net (longshore) sediment transport (Section 5.4.4).

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5.3 Wave Regime

5.3.1 Overview

5.11 The wave regime frequently plays an important role in the erosion, transport and deposition of sediments. In deeper (i.e. >20 m LAT) water (i.e. in some sections of the Offshore Area) the wind dominates the character of the waves. However, as waves travel into shallower water (i.e. within the Mid, Inshore and Nearshore Areas) interaction with the seabed causes shoaling, refraction, and eventually breaking. These processes act to realign the wave crests with the bed contours. These processes generate complex patterns of wave-induced currents which can influence the transport of sediments (Section 5.4.4). These currents can act independently or in conjunction with the tidal and wind induced currents.

5.12 The wave climate within the study area has primarily been described using wave records sourced from WaveNet, as well as from the TKOWF metocean survey (Emu, 2009).

5.3.2 Wave Climate

5.13 A frequency analysis of wave heights and direction is presented as a series of wave roses in Figure 6 and summarised in Table 3. It is important to note that these records are of differing temporal duration and are not all from coincident time intervals. Care must therefore be exercised when making direct comparisons.

Table 3. Summary of observational wave records representative of offshore, mid and inshore areas

Buoy/ Deployment

Approx. Water Depth

(m LAT)

Data Period

Analysed

Most Frequent

Wave Direction

Most Frequent

Significant Wave

Height (m) and

Percentage of Record

Maximum Observed Significant

Wave Height (m)

and Associated Direction

Sector

Most Frequent

Mean Wave

Period (s) and

Percentage of Record

Peak Observed Average

Wave Period (s)

and Associated Direction

Sector Offshore

West Silver Pit 20 (06/10/2006

to 28/10/2009)

NNE 0.5 to 1.0 (36%)

6.0 (NNE)

3 to 4 (36%)

11.0 (NE)

Dowsing 22 (06/10/2003

to 16/05/2014)

N 0.5 to 1.0 (32%)

6.0 (NNW)

4 to 5 (39%)

18.5 (ESE)

Midshore/ Inshore

Chapel Point 12 (04/09/2012

to 16/05/2014)

NNE 0.5 to 1.0 (49%)

3.7 (NNE)

3 to 4 (42%)

8.5 (NNE)

Theddlethorpe 10 (06/08/2006

to 29/10/2009)

NE 0.5 to 1.0 (40%)

3.0 (NE)

2 to 3 (40%)

7.6 (ENE)

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5.14 The main characteristics of the wave regime within the study area are as follows:

At the eastern end of the export cable corridor, the prevailing wavesare from the north/ north-northeast and coincide with the longest fetchdistances which extend out into the northern North Sea;

Closer in to the coast the wave climate is complex, with both refractionand sheltering effects (associated with sand banks and the coast)causing a focusing effect upon incoming waves;

The available records show that the largest waves are observed indeep offshore waters, decreasing in a landwards direction. Indeed, inthe Offshore Area (in the vicinity of the cable fan area), maximumobserved significant wave heights are around 6 m whilst equivalentvalues for locations within the Inshore Area are less than 4 m;

The most frequently observed wave periods are typically between 3and 4 seconds; these are indicative of locally generated wind waves;and

Long period (i.e. >8 second) swell waves are also observed in therecord, albeit relatively infrequently. These long period waves havegreater capability to interact with the seabed, temporarily mobilisingsurficial seabed sediments in shallow (i.e. <20 m LAT) water such asthat found within Mid, Inshore and Nearshore Areas (Section 5.4.4).

5.3.3 Extreme Waves

5.15 The UK Met Office maintains a wave model which includes the North Sea and has been in operation for some 20 years. Data from this model enables the characterisation of the low-frequency but high energy events in the vicinity of the TKOWF site. (This information is therefore only applicable to the eastern end of the export cable corridor.) The largest waves approach from the north, with a wave height and period of 4.0 m / 6 to 8 seconds for a 1:1 year wave event and 6.0 m / 7 to 10 seconds for a 1:10 year wave event.

5.3.4 Future Changes to the Wave Regime

5.16 Changes in wave climate over the 21st Century may involve changes in mean wind speed and direction and these changes could impact on the present day wave regime. Modelling as part of UKCP09 (Lowe et al, 2009) currently gives the best projection of the likely future wave climate across the study area. These analyses suggest that seasonal mean and extreme waves will experience little change in this area of the southern North Sea. Indeed, changes in the winter mean wave height are projected to be ~0.2 m whilst changes in the annual maxima are projected to be <0.1 m.

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5.4 Sediment Regime

5.4.1 Overview

5.17 The surficial seabed sediments present on the UK Continental Shelf vary spatially in character (e.g. grain size distribution) and thickness. The potential for the transport of these sediments is locally controlled by the net action of tidal and surge currents and waves in variable proportions; the relative contribution and dominance of these different driving factors is both spatially and temporally variable (e.g. Kenyon & Cooper, 2005).

5.18 Mobilisation of sediments will occur when the shear stress imposed on the seabed by these hydrodynamic forces exceeds a certain threshold relevant to the specific material type found there. The main bedload transport events will occur during major storms when wave induced orbital currents combine with tidal currents. This can lead to the erosion and transportation (either as bedload or suspended load) of sediments. Spatial gradients in the properties and availability of sediment and the erosive forcing applied to them leads to the natural formation of net sediment transport pathways and areas of net erosion (i.e. a sediment source) or deposition (i.e. a sediment sink). Over longer time-scales, the behavioural changes in the sediment regime will determine the net morphological evolution of the seabed.

5.19 Along most of the export cable corridor, the potential for sediment transport is predominantly under the control of tidal forcing, with the net direction of transport controlled by tidal asymmetry. However within the shallow Nearshore Area, wave driven currents will combine with tidal currents to drive longshore sediment transport. (This is an important determinant of beach morphology in this area (Section 5.5.3)).

5.20 During high energy storm events, sediment entrainment may also occur as a combination of the directional tidal and storm surge currents combined with the non-directional wave induced currents. Although these events are less frequent than peak tides alone they are significant in terms of sediment transport due to their greater combined current speed and because the sediment transport rate is proportional to the cube function of the current speed.

5.21 Primary datasets used to support the analysis include:

Geophysical surveys undertaken by Gardline (2009) and Osiris(2013a);

Geotechnical survey undertaken by Osiris (2013b);

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Benthic survey undertaken by RPS (2008) and Gardline (2009); and

Benthic survey undertaken to support the Humber REC (Tappin et al.2011).

5.4.2 Seabed Sediments: Composition and Distribution

5.22 The main characteristics of the surficial seabed sediments across the near-field and far-field are described below:

The available sidescan-sonar and grab sample data describes theseabed along the export cable corridor and surrounding area to bedominated by sands and gravels (Figure 7);

At a regional scale, the distribution of these sediments is stronglyinfluenced by the underlying geology/ bathymetry and in particular,Silver Pit, which runs approximately north to south across the studyarea;

Along the export cable corridor, surficial sediment units are typicallyvery thin (<0.5 m) although achieve greater thickness (order of severalmetres) where bedforms are present (Section 5.5.2.1) (Osiris,2013a,b);

The distribution of seabed sediments across the Nearshore andMidshore Areas of the export cable corridor is complex, with sands,gravels and clays present (Osiris, 2013a). These clays are likely to beexposures of the underlying Bolders Bank Formation;

The distribution of sediments across the Offshore Area of the exportcable corridor is more homogenous with silty gravelly sand and gravellysand dominating. However, subtle localised variations in sediment sizeand composition are expected between sand wave crests and troughs(RPS, 2008; Osiris, 2013a);

PSA data is available from Triton Knoll bank (which is located withinthe cable fan area (RPS, 2008)). This reveals the presence of poorlysorted gravelly sand with a modal grain size of ~300 µm (mediumsand); and

A large number of boulders are present along the export cable corridor,the largest of these having dimensions greater than 1 m (Gardline2009; Osiris, 2013a).

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5.4.3 Sediment Sub-strata: Composition and Distribution

5.23 The main characteristics of the sub-strata and geology across the near-field and far-field are shown in Figure 8 and outlined below:

Along the majority of the route, Quaternary glacial till depositsbelonging to the Bolders Bank Formation are present beneath thesurficial (Holocene) sediments. The Bolders Bank Formation is a stiff tovery stiff gravelly sandy clay, with occasional sandy horizons andoccasional boulders (Osiris, 2013a,b);

The Bolders Bank Formation is associated with a large number ofburied channel deposits, termed ‘intra-Bolders Bank channels’. Someof these channels may contain coarser grained infill sediments,although the majority are likely to contain finer grained sedimentsincluding soft clays, sands and possible peat beds (Osiris, 2013a,b);and

In the Offshore Area at a distance of approximately 13 km from theTKOWF site, Cretaceous chalk is present at or extremely close (i.e.less than 0.5 m) from the seabed surface. This chalk is present for adistance of approximately 3.5 km and is extremely weak to very weakand structureless (Osiris, 2013a,b).

5.4.4 Conceptual Understanding of the Sediment Regime

5.4.4.1 Sediment mobility

5.24 Coastal and shelf regions are energetic environments and are often subject to frequent mobilisation and re-suspension events due to the action of waves and currents (Van Raaphorst et al., 1998). In this section, an assessment has been undertaken regarding sediment mobility along the export cable corridor. This has been achieved by calculating the threshold bed shear stresses required to initiate transport of sediment present along the route, using standard methods described in Soulsby, (1997).

5.25 Tidal current time-series have been extracted from the numerical tidal model at a series of locations along the export cable corridor. These have subsequently been used to calculate an equivalent bed shear stress time-series (due to currents only) for a 16 day period (encompassing a spring-neap cycle). Calculated bed shear stress values for selected locations along the export cable corridor are plotted in Figure 9 alongside the threshold values for mobility of a selection of sediment grain sizes. (These are based on a d50 value for the seabed of 750 µm). The proportion of the time-series during which these sediment fractions are potentially mobilised is also summarised in Table 4.

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Table 4. Estimated potential sediment mobility (due to tidal currents only) at a series of locations along the export cable corridor

Location

Sediment Fraction

Silt (33.5μm)

Fine Sand (187.5 μm)

Medium Sand

(375 μm)

Coarse Sand

(750 μm)

Very Coarse Sand (1,500

μm)

Granule Gravel

(2,000 μm)

Location 1 (Inshore Area)

Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps

Only mobile during highest springs

Mobility % time 84% 75% 69% 55% 24% 8%

Location 2 (Midshore Area)

Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps


Not mobile

Mobility % time 83% 71% 64% 46% 13% 0%

Location 3 (Midshore Area)

Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps

Mobile during springs only


Mobility % time 84% 70% 62% 45% 14% 2%

Location 4 (Offshore Area)

Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps

Mobile during springs only


Mobility % time 81% 67% 59% 43% 16% 3%


Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps


Not mobile

Mobility % time 77% 61% 52% 35% 7% 0%


Mobility Summary

Mobile during

springs and neaps

Mobile during

springs and neaps

Mobile during

springs and neaps

Not mobile during

weakest neaps


Not mobile

Mobility % time 78% 60% 50% 33% 7% 0%

Based on a d50 value for the seabed of 750 µm

5.26 On the basis of Figure 9 and Table 4, the following generalised observations are made:

Silt and sand (fine, medium and coarse grained) is expected to bemobile during both spring and neap tides along the export cablecorridor;

Very coarse sand is generally only expected to be mobile during springtides, whilst gravel sized material will be either immobile or only foundto be mobilised during the highest spring tides;

The spatial variation in the extent to which very coarse sand and gravelis mobile reflects the variation in peak flow speeds shown in Figure 4;and,

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It is important to note that the calculated bed shear stress is sensitiveto the ‘roughness’ of the seabed with coarser grained and / or morerippled surfaces inducing greater flow turbulence and hence bed shearstress than a fine grained and / or flat surface for the same flow speed.In terms of both grain size and the potential for the development ofripple bedforms, there is known to be some variability along the exportcable corridor (Figure 7). This variation is likely to result in a degree ofspatial variability in the inferred bed shear stress and resultingsediment mobility.

5.27 During high energy storm events, sediment entrainment may occur as a combination of tidal and wave induced currents (Draper, 1967). For example, were a 1:1 year wave event (Section 5.3.3) to coincide with spring tides, at Location 5 (water depth ~17 m LAT) pebble sized gravel (10,000 µm) could become mobile for short periods of time (i.e. order of seconds)1. In comparison, under tidal currents alone, granule gravel sized material (2,000 µm) would be immobile.

5.28 As previously stated the peak offshore tidal currents are the main control on net sediment movement. With the exception of shallow areas close to the shore, waves will stir material, rather than contribute to net bedload transport. However, it is worth noting that material entering into suspension as a consequence of waves stirring the bed may be transported by tidal flow.

5.4.4.2 Sediment transport pathways

5.29 The dynamics of sediment transport in the southern North Sea have been considered in some detail by a number of authors over several decades. Attempts at mapping transport directions have been made through consideration of the morphology of bedform features (e.g. Kenyon & Cooper, 2005; Kenyon et al, 1981) and through modelling analyses (e.g. SNSSTS, 2002) and these studies have played an important role in understanding past morphological change at the sea bed (e.g. Cooper et al., 2008). At a regional scale, these studies provide an insight into the broad scale sediment transport pathways across the sea bed. However, local scale variations in bed morphology and hydrodynamic conditions can give rise to complex and varied sediment transport patterns which may differ from the regional-scale trends.

5.30 There are two mechanisms of sediment transport:

Bed-load transport. This process refers to all sedimentary grains thatmove, roll or bounce (saltation) along the sea bed as they aretransported by currents. This mode of transport is principally related tocoarser material (sands and gravels); and

1 Calculated using standard methods described in Soulsby (1997)

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Suspended-load transport. This process refers to particles of sedimentthat are carried above the sea bed by currents and are supported in thewater without recourse to saltation. These two mechanisms of transportcould be controlled by different processes and hence require separateconsideration.

5.31 Bedload Transport: Information pertaining to sediment transport in the southern North Sea is available from a number of scientific publications, specifically Kenyon & Cooper, (2005), Cooper et al, (2008) and Tappin et al., (2011). A schematic map outlining the broad scale sediment transport pathways identified in these studies is presented in Figure 10 and is summarised below.

Offshore from the Lincolnshire coast in the vicinity of the TKOWF siteand cable fan area, sediment transport is predominantly to the north-northwest (Stride, 1974; 1988; Collins et al, 1995). This pattern is inbroad agreement with modelling results of maximum bottom stress forthe area (Pingree & Griffiths, 1979);

Where flow is diverted (such as around the margins of Triton Knoll andInner Dowsing sand banks) localised changes to the broad scalesediment transport paths occur (Houbolt, 1968; Caston, 1972; Stride,1988; Collins et al, 1995); and

In the Nearshore Area along the Lincolnshire coast sediment transportis to the south, converging on the Wash.

5.32 It is worth noting that over much of the study area, and in particular where there are accumulations of gravel, the seabed sediments are largely immobile during all but the most extreme wave and surge conditions (Section 5.4.4.1).

5.33 Suspended Sediment Transport: A major source of suspended sediment in this region is erosion of the Holderness cliffs. The majority of the eroded volume (~60%) is composed of silts and clays, which are transported in suspension in a general southerly direction by both wave induced littoral drift and offshore tidal currents.

5.34 Suspended sediment concentrations (SSC) have been inferred from acoustic backscatter data collected during the TKOWF site metocean surveys. These data show that during spring tides (when current speeds are greatest), an increase in SSC is observed. During peak spring flow, observed SSC’s reach up to ~15 mg/l at measurement location SW (which is located ~2 km from the export cable corridor). During neap tides, background values are more typically in the order of 0 to 5 mg/l. The available evidence suggests that when current speed exceeds approximately 1m/s at this location, a threshold

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value of bed shear stress is exceeded since SSC values are significantly increased at this time (ABPmer, 2010).

5.35 A series of monthly/seasonal turbidity maps are also available from Dolphin et al. (2011). These maps were compiled from several sources including research cruise databases, a numerical model and the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite database. Of the sources used in the Dolphin et al. investigation, the MODIS observations provide the most comprehensive data due to their high spatial and temporal resolution. On the basis of this data, it is found that during winter months surface Suspended Particulate Matter (SPM) concentrations are typically around 10 mg/l in the Offshore Area of the cable corridor, increasing to ~60 mg/l in the Inshore/ Nearshore Area (Figure 11). During summer months, SPM concentrations along the corridor are typically in the range 5 to 10 mg/l. It is noted that throughout the year there is a strong east to west gradient in SPM, with the greatest concentrations observed at the coast in Inshore/ Nearshore Areas.

5.36 Continuous (near surface) turbidity measurements were also collected from the southern North Sea as part of the LOIS project (LOIS, 1998). Overall, these show good agreement with the spatial variation in turbidity shown by the MODIS satellite data, with locations HW-9 and HW-12 recording an average turbidity of 46 and 27 mg/l respectively (~20 month analysis period). The data also show quite pronounced variation about the mean value, with maximum turbidity readings at HW-9 (located in the Midshore Area) and HW-12 of 152 and 174 mg/l respectively.

5.5 Morphological Regime

5.5.1 Overview

5.37 Morphology refers to formations on the seabed (such as channels, banks and sand waves) which may endure for varying timescales. The contemporary morphology of the export cable corridor, along with the coastal characteristics of the coastline at and close by to the landfall is described in this section. Seabed morphology is considered alongside knowledge of sediment transport to develop a regional-scale conceptual understanding and to assess the degree to which areas of the seabed may be active or relict and changing in form or level over time.

5.5.2 Contemporary Seabed Morphology

5.38 The bathymetry of the route corridor has been determined from the geophysical surveys undertaken in 2009 and 2012. This is displayed in Figure 1 and is summarised below:

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Water depths along the cable route corridor are generally less than20 m LAT although reach a maximum depth of ~33 m LAT in theOffshore Area of the export cable corridor (Gardline, 2009; Osiris,2013a);

With the exception of the Nearshore and Inshore Areas, the shallowestwater depths are encountered within the cable fan area. Here waterdepths of ~8 m LAT are observed (Gardline, 2009; Osiris, 2013a);

Seabed gradients are generally less than 5° although some localisedslopes are reported by Osiris to be up to 25° in the Midshore Area;these slopes are associated with localised seabed features (Gardline,2009; Osiris, 2013a); and

Seabed gradients across the Nearshore Area are typically <0.5°,increasing up to ~4° in places across the intertidal area (Osiris, 2013a).

5.5.2.1 Active bedforms

5.39 The cable fan area overlaps with the western margin of Triton Knoll sand bank, an open shelf sinuous bank (Kenyon and Cooper, 2004) located to the south of the TKOWF site. This active bank is primarily controlled by the action of tidal currents and is part of the Race Bank – North Ridge – Dudgeon Shoal bank system (Figure 1).

5.40 Analysis of historic UKHO admiralty charts undertaken by ABPmer (2010) provides tentative evidence for localised small-scale changes in sea bed morphology across and in the vicinity of the bank. Similar bathymetric changes to those encountered along the margins of Triton Knoll sand bank have also been observed at other sand banks located off the north Norfolk coast (D’Olier, pers. comm.). A number of these banks fluctuate sinuously along and around a relatively permanent axial trend and this causes local changes to the position of the margins of the bank, to height variations along its more crestal sections and to the position of the ends of the banks.

5.41 Several large sand waves are present along and nearby to the export cable corridor (Osiris, 2013a). These are especially prevalent in the Offshore Area and are shown in Figure 12, alongside the location of areas of sand waves mapped from the Humber REC (Tappin et al., 2011).

5.42 The largest sand waves are observed across the eastern and southern side of the fan area and are associated with Triton Knoll sand bank. The sand waves in this area have heights up to 7 m and wavelengths between 40 m and 400 m. These features are asymmetric in profile with the steeper southern slopesup to 11°, indicating northerly sediment transport along the central and eastern flank and southerly transport along the western flank (Figure 10) (Osiris, 2013a).

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5.43 Outside the cable fan area the sand waves are observed for a further ~4 km although they are much smaller, with heights of 2 m to 3 m. Occasional isolated sand waves have been reported along the rest of the survey corridor with heights of up to 4 m and localised slopes up to 25° (Osiris, 2013a).

5.5.2.2 Relict bedforms

5.44 The Offshore Area of the export cable corridor lies just to the south of the deepest part of Silver Pit channel. Silver Pit is a broad north-south channel feature that extends, approximately, 90 km northwards from the mouth of The Wash with a maximum depth of about 80 m (Figure 12).

5.45 The origin of the channel has been the subject of debate. Dyer and Huntley (1999) suggested this feature to be Holocene in age and possibly formed by tidal scour and the trenches in the mouths of tidal deltas that retreated across the shelf as sea level rose. However, it is now generally understood that it formed by subglacial erosion and sediment backfill beneath the outer margins of a receding ice sheet (Praeg, 2003; Pearce et al., 2012). It is suggested that the cable route itself lies within the glaciofluvial outwash fan (Tappin et al., 2011).

5.5.3 Shoreline Characteristics

5.46 The proposed landfall location for the export cable route corridor, is situated close to Anderby Creek Figure 13. The frontage at Anderby Creek is characterised by the presence of a sandy beach backed vegetated sand dunes (providing a natural sea defence). The presence of small vegetated dunes indicates that the upper beach at least has remained stable over time. Overall, the stretch of coast between Anderby Creek and Chapel St Leonards (to the south of the landfall) shows less variability than that to the north although the general trend is erosional, as a consequence of rising sea level and falling sediment supply (Blott, 2001; Environment Agency, 2010 and 2012).

5.47 The wider frontage in the vicinity of the landfall is backed by a variety of ‘hard’ defences (armoured revetments) and dunes which together with the beach provide protection. A short distance behind the coastal defences, the terrain is typically at a lower level than the crest level of the coastal defences and this low lying land sometimes extends several kilometres inland. This means large areas are at risk of flooding should the coastal defences be severely overtopped or breached (Scott Wilson, 2010).

5.48 The beaches along this frontage were formerly a thin sand veneer over a glacial till foundation. The sand veneer comes from fine and medium grained sands (originating from the eroding Holderness cliffs and offshore banks) moving southwards by longshore drift. However, modelling studies indicate

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that almost the entire annual sand input from Holderness is accreted in the Haile Sands/Donna Nook/Saltfleetby inter-tidal zone which means that minimal sand is now available for transport south of Mablethorpe. Sediment eroded from the underlying till of the southern Lincolnshire coast contributes only minor volumes of sand to the beaches (Scott Wilson, 2010).

5.49 Historically, during storms, the thin sand cover moved seaward and the underlying till was exposed and eroded, with fine material carried offshore in suspension. Although the sand veneer returned during calm periods due to the longshore transport of sediment from the north (Section 5.4.4; Figure 10), the volume of material was generally insufficient to maintain the pre-storm crest levels. To counter this erosion (which is thought to have commenced in the 1970s), the Environment Agency has undertaken a major beach renourishment scheme (known as ‘Lincshore’) along the entire coast between Mablethorpe and Skegness. This programme of beach renourishment started in 1994 and is ongoing.

5.50 As a consequence of the large storm surge event which took place in December 2013 (Section 5.1.3), the dune systems along this coast were subjected to significant wave erosion. Accordingly, an enhanced programme of beach renourishment has been scheduled for 2014 to repair both the dunes and adjacent beaches. This will involve circa 520,000 m³ cubic metres of sand to be pumped from licensed off-shore dredging sites onto 20 km of beach, including Boygrift, Trusthorpe, Sutton on Sea, Chapel Six Marshes, Ingoldmells, Trunch Lane, Huttoft and Moggs Eye (Environment Agency, 2014) (Figure 1). In comparison, The Lincshore scheme renourishment volume placed between Mablethorpe to Skegness in 2008 was approximately 400,000 m3.

5.51 The current shoreline management policy for this stretch of coastline is ‘hold the line’, continuing the present day protection against a 1:200 year flood event (Scott Wilson, 2010). The current Lincshore scheme has a recommended strategy for the next 50 years which would extend until approximately 2055 (Halcrow, 2004). However, foreshore steepening continues making continuing beach nourishment increasingly difficult to sustain.

5.6 Water and Sediment Quality

5.52 No evidence of contaminated sediments was found in the desktop review of the TKOWF site (Triton Knoll Offshore Wind Farm ES, Chapter 2 Physical Processes). In addition, all the metals analysed in the marine ecological survey in 2008 were considerably below the Canadian Interim Sediment Quality Guidelines (ISQG) Probable Effect Level (PEL), the level at which toxicity effect would be evident, or Cefas AL2, the level at which dredged material is considered unfit for sea disposal. Similarly, all the PAHs analysed were considerably below the Canadian ISQG PEL, the level at which toxic

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effect would be evident. A number of PAHs were above the Canadian ISQG Threshold Effect level (TEL) and the Cefas generic trigger level. However, none were found to be at levels that are considered to be of concern to the marine environment during cable trenching and laying operations (RPS, 2008).

5.53 In addition to the sediment quality data, it is noted that the cable corridor is generally characterised by coarse sediments which are less likely to contain contaminants than muds. This, coupled with the absence of historic waste disposal sites in close proximity to the cable corridor, means that sediments in the export cable route are not expected to have an elevated level of contaminants relative to background levels. Accordingly, no further assessment of the potential for contamination to arise from seabed disturbance has been undertaken.

5.7 Summary

5.54 This annex draws together an evidence base which contains data obtained from literature reviews as well as that collected during the TKOWF site survey campaigns. From this evidence base, a baseline review is provided of the local hydrodynamic, sedimentological and morphological regime along the proposed export cable route. A summary of the key findings which are of relevance to the impact assessment are provided below.

Water depths are variable along the cable corridor and reach amaximum of 33.5 m LAT at the southern end of Silver Pit;

The export cable route occupies a macro tidal setting although thespring tidal range is found to be larger with increased proximity to thecoast;

Tidal currents generally follow the orientation of the coastline and flowin a southerly direction during the flood tide and in a northerly directionduring the ebb tide. Peak flow speeds are found to be in excess of 0.8m/s and exceed 1 m/s in places. The characteristics of the tidal regime(including residual flow) are of direct relevance to understanding thedispersion of fine grained sediments released into the water columnduring the construction phase. (This information is important for theassessment of impacts on receptors including benthic ecology and fishspecies; see Triton Knoll Electrical System Environmental StatementVolume 1; Chapter 8 and 9);

Sidescan sonar data and grab samples have been collected along thecable route and these show that the superficial sediments primarilycomprise Holocene sands and gravels. These sediments are oftenarranged into mobile bedforms and in places, large (up to 7 m high)sand waves are found. Frequent boulders measuring up to 0.5 m are

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also encountered. This information on the surface and sub-surface geology is directly inputted to numerical models used to consider the characteristics of sediment plumes associated with the construction phase (see Triton Knoll Electrical System Environmental Statement Volume 1; Chapter 6);

Occasionally clay is found in grab samples and it is possible that thisbelongs to the underlying Bolders Bank Formation indicating locallythin Holocene cover. Sediment mobility is likely to be variable along thecable route and be strongly influenced by variations in sea bedsubstrate and bed morphology. However, given that sand waves arefrequently encountered, it would appear that in places the bed is highlymobile. This has important implications for cable burial depth and theextent to which alternative cable protection measures such as rockarmouring are required;

The Lincolnshire coastline south of the Humber Estuary, is a highlysensitive stretch of coast that has suffered from long-term erosion. Tocounter this erosion and fulfil the ‘Hold the Line’ shoreline managementplan, a major beach renourishment scheme (known as ‘Lincshore’) is inplace along the entire coast between Mablethorpe and Skegness;

The coastal frontage at Anderby Creek (where the cable makeslandfall) is characterised by the presence of a sandy beach backedvegetated sand dunes. Wave exposure characteristics, the shape ofthe coastline and the offshore geology determine the sedimentaryresponse of this coastline; and

Beach levels along this coastline vary seasonally, with steeper wavesduring winter transporting sands offshore and less steep waves duringsummer returning sands to the beach. The extent of these seasonal aswell as longer term (i.e. decadal) scale changes is of relevance todetermining appropriate design measures at the landfall. These includecable burial depth and set back distances for jointing bay infrastructure.

5.55 The information contained within this Annex has been used to identify marine physical environment receptors within the study area and help inform the assessment of potential impacts. These are presented in Volume 1; Chapter 6 of the Triton Knoll Electrical System Environmental Statement.

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Figure 1. The Study Area

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Figure 2. Seabed and Coastline with Nature Conservation Designations Within the Study Area

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Figure 3. Data Source Locations

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Figure 4. Mean Spring Tidal Currents for High, Low, Ebb and Flood Tides

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Figure 5. Residual Tidal Flow Over a Spring-Neap Period Across the Study Area

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Figure 6. Summary of Wave Observations

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Figure 7. Regional Description of Seabed Sediments Across the Study Area

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Figure 8. Depth Below Seabed to Base of Underlying Geological Units Along the Export Cable Corridor

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Figure 9. Tidally Induced Bed Shear Stress and Mobility Thresholds for Selected Locations Along the Export Cable Corridor

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Figure 10. Bedload Sediment Transport Indicators Across the Study Area

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Figure 11. Suspended Particulate Matter Concentrations (Summer and Winter)

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Figure 12. Bedform Features Within and Nearby to the Export Cable Corridor

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Figure 13. The Export Cable Landfall

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