London Gateway Port: Scheme Refinement - Use of Larger Dredger: Hydraulic studies and assessment of environmental significance

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^j

HRWallingford

Working witli water

London Gateway Port

Scheme Refinement - Use of Larger Dredger

Hydraulic studies and assessment of

environmental significance

Report EX 5779

Release 3.0

May 2008

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L o n d o n Gateway Port, Scheme refinement - Use o f larger dredger

H y d r a u l i c studies and assessment o f environmental significance A i H R W a l l i n g f o r d

Document Information

Project London Gateway Port. Scheme refinement - Use o f a larger dredger

Report title Hydraulic studies and assessment o f environmental significance

Client DP World

Client Representative Sandie Wilson Project No. DDR4203

Report No. E X 5779

Project Manager John Baugh

Project Director Dr Mike Deamaley

Document History

Date Release Prepared Approved Authorised Notes

06/05/08 1.0 JVB M P D M P D D r a f t issue

13/05/08 2.0 JVB M P D M P D Updated after client review 14/05/08 3.0 JVB M P D M P D Final release

Prepared Approved

A i i f - h n i - S c p H

HR Wallingford accepts no liability' for the use by third parlies of results or methods presented in this report. The Company also stresses that various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger

H y d r a u l i c studies and assessment o f environmental significance A J^ H R Wallingford

Summary

London Gateway Port

Scheme Refinement - Use o f Larger Dredger

Hydraulic studies and assessment o f environmental significance Report E X 5779

May 2008

B a c k g r o u n d

DP World received approval in May 2007 to build London Gateway Port and Logistics Park. The applications were submitted in 2002 and supported by an Environmental Statement. The project went to Public Inquiry in 2003.

In relation to the port the following marine works were and still are proposed: • Reclamation

• New Jetty for handling aviation and bitumen fuel vessels • Quay wall for berthing o f deep sea container and cargo ships • Quay wall to enable Ro-Ro ships to berth

• Dredging to deepen the navigation channel

O f particular relevance to the marine works is the Mitigation, Compensation, Monitoring Agreement (August 2003). The M C M was developed to secure the measures that would be incoiporated into the pre-, during and post development plan. A n updated version o f the Environmental Statement was produced to include the updates o f the Public Inquiiy and Mitigation, Compensation and Monitoring Agreement (updated Environmental Statement 2004).

Scheme refinements

The Environmental Statement discussed the likelihood of there being a variety of different sized trailer suction hopper dredgers (TSHD) that could build the project however for the purposes o f assessing the impact o f the project on the environment a 6,400m'' capacity TSHD was used. The cuiTent market suggests that it is very likely that a larger dredger would be used to dredge the channel so the detail o f using a larger dredger has been assessed and compared against the smaller dredger presented in the EIA. It is important that the results show similar, i f not reduced impacts and are within the environmental assessment. A modem 13,700m^ TSHD is considered to be representative o f the type o f dredging plant that could be used.

H y d r a u l i c studies

HR Wallingford were commissioned by London Gateway to advise on marine and coastal processes and in particular are responsible for the modelling o f the impacts o f the marine works to enable an understanding o f impact to be developed. They have been involved in the project since 2001.

The modelling, analysis and interpretation described in this report is based on the methodologies originally employed in the hydraulic studies undertaken to support the EIA.

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London Gateway Port - Technical Assistance

H y d r a u l i c studies f o r scheme refinement - Use o f a larger dredger ^ i^ H R Wallingford

Summary continued

The physical impacts o f the use of a modern larger trailer suction hopper dredger have been assessed and contrasted with the impacts o f the use o f a smaller dredger as originally assessed in the EIA. As a result o f the use o f a larger dredger there would be:

• increased production rates;

• an increase in near-field suspended solids concentrations; • an increased retention o f material in the dredger

• overall reductions in losses of material f r o m ovei"flow;

• a greater amount o f fine material retained in the reclamation site; and • scope for a reduced timescale for constmction and period o f impact.

E n v i r o n m e n t a l Considerations

HR Wallingford have had direct responsibility for assessing the impacts on navigation dredging and flood defence. The assessment as to whether or not there are consequences arising that are different to those addressed in the E I A have been made in consuhation with other experts as follows:

Responsibility Organisation Individual Position

Marine Ecology Marine Ecological Surveys Ltd

Dr Richard Newell Chairman Water and Sediment

Quality

Water Research Centre

Dr Mike Gardner Principal Environmental Chemist

Fish Pisces Consei-vation L t d

Dr Peter Henderson Director

The results o f the SEDPLUME modelling demonstrate that the effects of the larger modem dredger working are comparable to those o f the smaller dredger working. Within the redline zone the larger dredger is predicted to result in higher near-field suspended sediment concentrations (an increase o f 20%). The agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-line, at or below the previously agreed thresholds is unaffected.

The important difference between the two dredging scenarios is that the larger dredger w i l l work for less time and release less fines for a given volume o f material. Thus the period over which impacts w i l l occur for the larger dredger w i l l be reduced and the impacts that occur w i l l be similar to those o f the smaller dredger. Monitoring as outlined in the M C M w i l l provide a means of confirming these predictions. The methods for managing the environmental impacts of the dredging activity outlined in the M C M w i l l equally apply for the larger TSHD.

Accordingly the implications for water quality, intertidal invertebrates and organisms that depend upon them as a food resource (birds and fish) are as originally assessed. It is possible that by using a larger dredger the overall timescales o f the project could be reduced in the region of one year which could have positive environmental benefits.

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L o n d o n Gateway Port - Technical Assistance

H y d r a u l i c studies f o r scheme refinement - Use o f a larger dredger ^ i H R Wallingford

Summary continued

Mitigation, Compensation and Monitoring

The Mitigation, Compensation and Monitoring Agreement ( M C M ) sets out the measures that London Gateway w i l l implement in order to minimise the risk of impact to the marine environment. No further measures are required in the M C M to minimise risk to the marine environment as a result of the potential use o f a modem larger trailer suction hopper dredger. Measures as outlined in the M C M w i l l not only provide a means for managing and mitigating dredging and constmction activities, but monitoring w i l l also provide a means o f confimiing the prediction of physical effects and the assessment o f consequence arising on the marine environment.

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L o n d o n Gateway Port - Technical Assistance

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger H y d r a u l i c smdies and assessment o f environmental significance

Title page i

Document Information ii

Summaiy Hi

Contents vii

1. Introduction 1 1.1 Background 1 1.2 Scheme refinements and Environmental Statement 1

1.3 Environmental Assessment 1 1.4 Mitigation, Compensation & monitoring Agreement 2

1.5 Objectives 2 1.6 Approach 3 1.7 Structure o f the report 3

2. Use o f a larger dredger - D R L modelling of production rates 4

2.1 Introduction 4 2.2 Selection o f Soils type for modelling 4

2.3 Assumptions for D R L TSHD production Modelling 4

2.4 D R L production modelling Test results 5 3. Use o f a larger dredger - SEDPLUME dispersion modelling 7

3.1 Approach to the modelling 7 3.2 Results o f SEDPLUME modelling - m i d and far-field impacts 7

3.3 Resuhs o f SEDPLUME modelling - near-field impacts 8

3.4 Impacts of dredging with the larger dredger 9 4. Discussion of the significance o f scheme refmements 10

4.1 O v e m e w 10 4.2 Upstream o f Coalhouse Point 10

4.3 Lower Hope Reach 10 4.4 Mucking Flats 10 4.5 North shore berths in Sea Reach 11

4.6 Holehaven Creek 11 4.7 Blyth Sands 11 4.8 Chapman Sands, Southend and Leigh Flats 11

4.9 Maplin Sands and Foulness 11 4.10 Berthing and manouevering area 11

4.10.1 Maintaining the estuary cross-section 11

4.10.2 Reclamation Losses 12 4.10.3 Duration o f impacts 12 4.10.4 Water quality risk management 12

4.11 Navigation Channel 13 4.11.1 Duration o f impacts 13

4.11.2 Water quality risk management 13

4.12 Offshore channels and banks 14 4.13 Managed realignment sites A and X 14

5. Conclusions 15 6. References 17

1-771

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H y d r a u l i c smdies and assessment o f environmental significance ^ HR Wallingford

Contents continued

Tables

Table 1 6,400 TSHD, 4km haul Table 2 6,400 m^ TSHD, 131an haul Tables 13,700 m^ TSHD, 4km haul Table 4 13,700 m^ TSHD, 13km haul

Table 5 Summary of results of TSHD production model for dredging Outer Fine Silty Sands

Figures

Figure 1 Location o f proposed red-line zone and boundaiy

Figure 2 Rate o f release o f fine material f r o m consecutive loads for smaller (green) and larger (red) TSHDs operating continuously so as to achieve a total tonnage to be placed in the reclamation o f 140,000 dry tonnes

Figure 3 Predicted peak depth averaged suspended sediment concentration increases above background levels (upper figure) and time series (lower figures) o f concentration increases at Locations 1 and 2. Spring tide simulation, dredging 4km f r o m the port

Figure 4 Predicted peak depth averaged suspended sediment concentrafion increases above background levels (upper figure) and time series (lower figures) o f concentration increases at Locations 3 and 4. Spring tide simulation, dredging 4km f r o m the port

Figure 5 Predicted peak depth averaged suspended sediment concentration increases above background levels (upper figure) and time series (lower figures) of concentration increases at Locations A and B. Spring tide simulation, dredging 41an f r o m the port

Figure 6 Central figure showing predicted peak depth averaged suspended sediment concentrations around the dredging activity, 6,400m^ TSFID working at 131an. Six side figures show time series o f depth averaged and near bed concentrations at six locations in close proximity to the centre line o f dredging

Figure 7 Central figure showing predicted peak depth averaged suspended sediment concentrations around the dredging activity, 13,700m'' TSHD working at 13km. Six side figures show time series o f depth averaged and near bed concentrations at six locations in close proximity to the centre line o f dredging

Figure 8 Illustration of the relationship between the dredging footprint (the hatched zone) and the red-line monitoring position for the scenario o f dredging with the smaller TSHD (same simulation as shown in Figure 6)

Figure 9 Illustration of the relationship between the dredging footprint (the hatched zone) and the red-line monitoring position for the scenario o f dredging with the larger TSHD (Same simulation as shown in Figure 7)

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H y d r a u l i c studies and assessment o f environmental significance ^ J ^ H R Wallingford

1. Introduction

1.1 B A C K G R O U N D

DP World received approval in May 2007 to build London Gateway Port and Logistics Park. The appUcations were submitted in 2002 and supported by an Envu-onmental Statement. The project went to Public Inquiry in 2003.

In relation to the port the following maiine works were and still are proposed: • Reclamation

• New Jetty for handling aviation and bitumen fuel vessels • Quay wall for berthing o f deep sea container and cargo ships • Quay wall to enable Ro-Ro ships to berth

• Dredging to deepen the navigation channel

O f particular relevance to the maiine works is the Mitigation, Compensation, Monitoring Agreement (August 2003). The M C M was developed to secure the measures that would be incoiporated into the pre-, duiing and post development plan. A n updated version o f the Environmental Statement was produced to include the updates o f the PubUc Inquiry and Mitigation, Compensation and Monitoring Agreement (updated Environmental Statement 2004).

1.2 S C H E M E REFINEMENTS A N D ENVIRONMENTAL S T A T E M E N T

The Environmental Statement discussed the likelihood o f there being a variety of different sized trailer suction hopper dredgers (TSHD) that could build the project however for the purposes o f assessing the impact o f the project on the environment a 6,400m' dredger was used.

The current market suggests that it is very likely that a larger dredger would be used to di-edge the channel so the detail o f using a larger dredger has been assessed and compared against the smaller dredger presented in the EIA. It is important that the results show similar, i f not reduced impacts and are within the environmental assessment. A modem 13,700m^ TSHD is considered to be representative of the type o f dredging plant that could be used.

1.3 ENVIRONMENTAL A S S E S S M E N T

HR Wallingford were commissioned by London Gateway to advise on marine and coastal processes and in particular were responsible for the modelling of the impacts o f the marine works to enable an understanding o f impact to be developed.

Water Research Centre, were appointed to advise on water quality impacts. Marine Ecological Surveys on marine ecology impacts and Pisces Consei-vation on the fish resource.

The hydraulic studies play an important part in enabling the ecological team to assess the impacts on the environment. HR Wallingford have therefore led in modelling the impact o f the use of a larger di-edger and making a comparison of physical effects as previously presented in the EIA. Based on those resuhs the specialist team have reviewed their parts

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger H y d r a u l i c studies and assessment o f environmental significance

of the environmental assessment to confum that the impacts are within the envelope of the original environmental assessment. Their assessment is built into Section 4 of this report. The approach used in the original envii-onmental assessment has been used in comparing the scenarios to ensure continuity and a robust assessment that meets the standards of the regulatory requirements.

1.4 MITIGATION, C O M P E N S A T I O N & MONITORING A G R E E M E N T

The Mitigation, Compensation and Monitoring Agi'eement sets out the measui-es that London Gateway w i l l implement in order to minimise the risk of impact to the marine environment. The measures include the provision o f a detailed dredge plan, mitigation during constnaction, management of dredging activities, monitoring during construction and the provision o f Sites A and X and the loinning o f an Ecological Advisoiy Group (EAG). The measures are designed to protect wildlife, navigation and flood defence. One key element o f the M C M is the concept o f a red-line zone within which the constmction activities take place. The zone extends 3,000m away from any constmction activity in the direction of tidal flow and 200m across the tidal flow (see Figure 1). Around the peiimeter o f this zone continuous monitoring of suspended sohds concentrations, dissolved oxygen, temperature and salinity w i l l be undertaken with data transmitted in near real-time. "Caution" and "Stop" thresholds for suspended solids concentrations and dissolved oxygen were estabhshed by the parties to the M C M Agi'eement'. These thresholds have been set at levels which i f not exceeded, based on the resuhs o f numerical modelling of plume dispersion, would result in increases in suspended sediment concentration and deposition over intertidal areas which were considered acceptable within the EIA. Exceedence of a threshold would require modification or cessation of constmction activity.

Inside the red-line zone, which represents a mixing zone within which the constmction activities would take place, a programme of characterisation monitoring is to be undertaken to enable verification of the EIA assessment.

In addition to the real-time and characterisation monitoring a programme of sensitive receiver monitoring is also to be undertaken which w i l l establish further understanding o f baseline conditions over the intertidal areas and provide further basis for validating the plume dispersion modelling and EIA.

In considering the significance o f scheme refinements it is therefore essential that as well as considering the environmental assessment it is important to consider measui-es set out in the Mitigation, Compensation and Monhoring Agreement and check that these measures still apply.

1.5 OBJECTIVES

The objectives o f this note are to:

• present and compare the hydraulic studies for the original scheme against the scheme refinements - in this case the use o f a larger TSHD dredger

' The parties to the MCM Agreement are the Peninsular and Oriental Steam navigation Company, London Gateway Port Limited, the Port of London Authority, the Environment Agency, and Natural England (then English Nature).

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L o n d o n Gateway Poit. Scheme refinement - Use o f larger dredger

H y d r a u l i c smdies and assessment o f environmental significance

AiHR

Wallingford

• consider tlie implications of the hydraulic modelling resuhs f r o m the scheme refinements on the marine environment specifically navigation dredging, flood defence, ecology and fisheries

• assess the implications for water quality, intertidal invertebrates and organisms that depend upon them as a food resource (birds and fish) and detemine whether: a) there is a need for further assessment relating to these disciplines

b) there is a need to identify further measures not set out in the Mitigation, Compensation & Monitoring Agreement

c) ultimately the assessment is within the envelope o f the original London Gateway Environmental Impact Assessment

1.6 A P P R O A C H

The modelling, analysis and interpretation described in this report is based on the methodologies originally employed in the hydraulic studies undertaken to support the EIA and has been undertaken by HR Wallingford.

H R WaUingford have had direct responsibility for assessing the impacts on navigation dredging and flood defence. The conclusions regarding whether or not there are consequences arising that are different to those addressed in the E I A have been made in consuhation with other experts as follows:

Marine Ecology Marine Ecological Sui'veys L t d

Dr Richard Newell Chairman Water and Sediment

Quality

Fish Pisces Conservation L t d

1.7 S T R U C T U R E OF T H E REPORT

This report comprises five sections as follows: Section 1 (this section) Introduction

Section 2 D R L TSHD production modelling - smaller versus larger dredger Section 3 SEDPLUME dispersion modelling - smaller versus larger dredger Section 4 Discussion o f the key conclusions o f both the E I A and assessment o f

the scheme refinements and the significance o f these refinements i n relation to geographical areas o f the Thames.

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Hydraulic studies and assessment o f environmental significance I HR Wallingford

2 .

Use of a larger dredger - DRL modelling of

production rates

2.1 INTRODUCTION

In November, 2001, Dredging Research L t d (DRL) carried out analysis and produced a report for the Port o f London Authority entiüed "Preliminary Estimates o f Ovei-flow Losses and Material Recovery". This report was passed to HR Wallingford to enable them to deteiTnine appropriate source terms for the turbidity modelling for the London Gateway Port dredging activities.

In order to consider the consequences o f a larger dredger being used HR Wallingford requested that D R L carry out additional analysis o f overflow rates and cycle times for a modem larger dredger for a limited number o f ahemative scenarios.

2.2 SELECTION OF SOILS TYPE FOR MODELLING

Production rates for dredging in a variety o f soils types were investigated by D R L in 2001. The dredging scenario considered as a precautionaiy basis for informing the E I A was the dredging o f a soil denoted in the E I A as "Outer Fine Silty Sands" or "Sih I " . This material was assessed as having a fines content o f 28% and it was considered in the E I A that the simulation o f the dredging o f this type o f material was a reasonable precautionary approach to considering the issues associated with the dredging operation.

2.3 A S S U M P T I O N S FOR DRL TSHD PRODUCTION MODELLING

The D R L TSHD production model used in the assessment o f the smaller dredger was again used for the cun^ent simulations o f the larger dredger. Two dredger sizes were examined. These were selected as 6,400 m^ (as originally modeUed in 2001) and 13,700 m'', the latter dredger having only one loading pipe (typical o f a modem dredger o f this type). The dredgers have been simulated for n o m a l loading, for haul distances o f 4 k m and 13 Ian. Loading and overflowing is continued to the point at which the maximum loading rate per cycle is achieved. In the E I A dredging impacts were assessed by considering typical dredging operations centred at 4km, 13km and 25km f r o m the upstream limits o f the works. This ensured that the scale o f the dredging works was considered. During the EIA dredging at 4km f r o m the upstream limit o f the works was found to be the most sensitive location in temis o f potential for impact on adjacent intertidal areas.

In the simulation the following assumptions are made:

• The model is based on a TSHD dredging at the speed o f 2 knots over a 2,000 metre trail length.

• The trailer sails laden and unladen without speed restrictions. • The trailer spends 5 minutes tuming at the end o f each mn.

• The overflowed material w i l l exit through the bottom o f the hull, which w i l l be immersed at a draft o f 8.00 metres below the water surface for the smaller dredger and 11 metres for the larger vessel.

e Pumping ashore takes 60 minutes thi'ough an 800 m m diameter pipeline for the smaller dredger and 130 minutes for the larger through a 900 mm pipe. The velocity o f the mixture in this pipeline would be around 5.5 m/s.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger H y d r a u l i c shtdies and assessment o f environmental significance

• During tiie pumping asiiore some 6,500 dry tonnes o f solids would be transfeiTed to the reclamation area by the smaller dredger and 14,000 dry tonnes for the larger vessel.

2.4 DRL PRODUCTION MODELLING TEST RESULTS

The test results f r o m the D R L production model are presented at the back o f this report in Tables 1 to 4. They are summarised in Table 5 below.

Table 5 Summary of results of T S H D production model for dredging Outer Fine Silty Sands

6,400m^ T S H D 13,700m^ T S H D

Overflow period 132 minutes 139 minutes Average Overflow 1146kg/s 1060kg/s % of fines in overflow 45% 57% Average oveiTlow o f fines 510kg/s 607kg/s Dry mass o f cargo in hopper 6,500 Tonnes 14,000

Tonnes Mass o f fines lost for winning a single 4,047 Tonnes 5,074 Tonnes cargo

Mass o f sands lost for winning a single 4,992 3,801 cargo Tonnes Tonnes Mass o f fines lost for winning 100,000 62,000 36,000 Tonnes o f material Tonnes Tonnes Mass o f sand lost for winning 100,000 77,000 27,000 Tonnes o f material Tonnes Tonnes % of fines in reclamation material 9.0% 12.7%

In the E I A it was assessed that a total o f 7.17 M m o f material would be lost f r o m overflow f r o m the dredging by the 6,400m^ TSHD. This volume comprised some 4.55 M m ' o f fine material (<60 microns) and 2.62 Mm^ o f fine sand. Note that the overall loss o f sand in the original E I A assumed some re-dredging o f sands arising f r o m overflow. The larger dredger production model has only been lam for the "Outer Fine Silty Sands as in the E I A this was the most sensitive material type. From Table 5 i t can be seen that the larger dredger releases significantly less material (fines and sands) through overflow when dredging the "Outer Fine Silty Sand" than the smaller dredger. In the E I A it was previously assumed that throughout the whole dredge that about 1.0 M m ' o f fines out of the total o f 4.55 Mm^ would be lost f r o m ovei-flow f r o m dredging i n the inner part o f the approach channel between 0 and 13 k m f r o m the London Gateway Port. This fine material could contribute to siltation on intertidal areas and the existing riverside berths. Based on the results o f the production modelling for the larger TSHD this figure w i l l reduce. A reduction in the release o f flnes in this area w i l l lead to a reduction in supply o f fines to the most senshive intertidal areas and the existing riverside berths during the construction period.

In the E I A it was assumed that up to 1.2Mm' o f fine material might be lost during the reclamation process. Fine material f r o m this source could contribute to siltation on intertidal areas and the existing riverside berths. The assumption in the E I A that the losses f r o m the reclamation activity would all be o f fines was precautionary because a proportion o f the losses w i l l be o f coarser material.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger Hydraulic smdies and assessment o f environmental significance

A consequence o f reduced losses f r o m the dredging activity is that the total volume o f material recovered w i l l increase. The average fines content o f the placed material w i l l increase slightly. There w i l l remain the need to manage fines within the fill material during the engineering works.

It remains the case that losses o f fines during reclamation w i l l be no more than the precautionary volume of 1.2Mm^ used in the E I A . Therefore the analysis in the E I A regarding the impacts o f losses o f fines remains unchanged.

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3. Use of a larger dredger - SEDPL UME

dispersion modelling

3.1 A P P R O A C H TO T H E MODELLING

The resuhs o f the TSHD production modelling summarised in Section 2.4 were used as input to the SEDPLUME plume dispersion model to simulate the fate o f fine material released by the dredging activity.

The SEDPLUME model has previously been used extensively within the EIA to examine the fate o f overflow f r o m the 6,400m^ TSHD. The resuhs have been published in various o f the documents supporting the E I A . A key summary document is HR Wallingford 2003 Report EX 4750 "Summary o f predicted intertidal sedimentary processes" which demonstrated the effect o f dredging activity in tenns o f elevated concentrations and deposition on the intertidals.

The scenarios modelled represented:

• a 15 tide period o f spring tides with dredging o f silty sand at a location 4km f r o m the port - this was considered to be the most sensitive scenario in terms o f potential for impact on intertidal areas based on the E I A studies.

continuous dredging with the TSHD to w i n a mass o f about 140,000 diy tonnes to be placed into the reclamation.

The dredging period simulated in the E I A with the 6,400m^ TSHD was 22 loads over ten tides dredging at a location centred at 4 k m f r o m London Gateway Port.

The dredging period simulated with the 13,700m^ TSHD in this study was 10 loads over flve tides dredging at a location centred at 41an f r o m London Gateway Port.

• a 3 tide period o f spring tides with the dredging o f silty sand at a location 13km f r o m the port - this was the location used previously in the E I A to demonstrate the detail o f the increases in near-field suspended sediment concentrations above background levels.

Figure 2 shows a graphical representation o f the rate o f mass released f o r the two dredging scenarios over the number o f cargos required to achieve a mass o f about 140,000 diy tonnes to be placed into the reclamation. The red line represents the cumulative mass released f r o m the larger dredger and the green line that f r o m the smaller dredger. The red dots represent the cumulative mass o f material being placed into the reclamation f r o m the larger TSHD and the green dots the mass f r o m the smaller TSHD. The rates o f release o f mass are broadly similar over the first 5 tides for the two dredgers but by the tune the same mass has been placed into the reclamation the smaller dredger has released about 80% more fines than the larger one.

3.2 RESULTS OF S E D P L U M E MODELLING - MID A N D FAR-FIELD

IMPACTS

The results of the plume modelling for dredging at 41an f r o m the port are shown in Figures 3, 4 and 5. The upper graphic in each figure shows the predicted peak depth

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H y d r a u l i c studies and assessment o f environmental significance ^ i H R Wallingford

average concentration^ predicted to occur over tlie fifteen tide simulation for the larger TSHD for different areas o f the lower Thames Estuary. The figures are consistent in style with those shown in HR Wallingford (2003). The lower gi-aphic o f the figures shows a time series at the same two locations identified in the upper figure. The red line shows the time series o f predicted concentrations arising f r o m dredging with the larger TSHD whilst the green hne shows that f r o m the smaller TSHD (i.e. the original result used to support the EIA).

It can be seen that at the sites which are affected most rapidly by the dredging activity (Sites 1, 2, 3, 4 and A ) the red and green lines are similar for the first 5 tides and then, because the dredging activity with the larger TSHD has ceased the red line falls below the green line. A t the far-field site (Site B ) , the impacts o f the two operations are similar with a lag o f about 7 tides occumng before any effects f r o m the dredging activity occur this far seaward. Points 1 to 4 were selected during the E I A process to represent near and mid-field locations illustrative o f the intertidal areas. Points A and B were selected in the E I A as these are locations where the Environment Agency regularly obtain water samples as part o f their monitoring o f the Outer Thames Shellfish Waters.

3.3 RESULTS OF SEDPLUME MODELLING - NEAR-FIELD IMPACTS

To consider the detail of the impacts on increased suspended sediment concentrations immediately around the dredging activity the results of simulations o f a shorter (36 hour) period o f dredging activity at 13km f r o m London Gateway Port have previously been used in the EIA. Figure 6 shows the predicted time series o f suspended sediment concentrations adjacent to the navigation channel over a 3 tide period o f dredging activhy for the smaller TSHD. Time series o f depth averaged and near bed ^suspended sediment concentrations are shown around a central figure which shows the peak depth averaged concentration^ above background levels associated with the plume. The dredging path is shown on the central figure along with three pairs o f points. Each pair of points is separated by about 400m. The central pair o f points is about 200m either side o f the dredging path''. The southerly point o f the westernmost pair o f points is approximately on the streamline f r o m the dredging path and thus exhibits the highest instantaneous concentrations. Figure 7 shows the same f o m a t o f results f o r the simulation o f 3 tides o f continuous dredging with the larger TSHD.

It can be seen that the magnitude and f o r m o f the predicted near-field suspended sediment increases are similar f o r the two dredgers simulated. The effect o f the higher discharge rates for the larger TSHD can be seen within the red-line zone through the plot of peak depth averaged concentrations which shows larger areas o f elevated concentrations immediately around the dredger path. However, when these effects are advected and dispersed across the flow to the channel edges, and outside o f the redline zone, the effects are less apparent between the two dredgers.

Figures 8 and 9 schematically illustrate aspects o f the approach that is proposed for management o f the dredging using thresholds for Zones and real time monitors as set out i n the Mitigation, Compensation, Monitoring Agreement (2003). The dredging

^ The peak depth average concentration is the highest predicted value over the simulation period of 15 tides. The SEDPLUME model results for these simulations are presented in depth average form only with no representation of vertical structure in the plume.

' For the shorter period simulations the model resuhs have been processed differently so that the vertical structure of the plume can be illustrated.

Peak near bed concentrations within 200m of the dredging activity were used as the basis for establishing the thresholds for controlling the dredging activity.

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footprint (navigation cliannel) is illustrated by the hatched zone in the figures. The location o f the dredging path illustrated in Figures 6 and 7 has been superimposed on this area. A red-line has been added to the figure which illustrates the red-line boundaiy shown in Figure 1 for this section o f the channel. The boundary is 200m away from the dredging footprint. Six locations on the red-line have been identified and the figures around the edges o f the main figure show the predicted time series o f suspended sediment concentrations at mid depth and near bed for the scenarios illustrated respectively in Figui-es 6 and 7.

It can be seen that on the red-line the predicted depth average suspended sediment concentrations m the bottom metre o f the water column are typically less than 800mg/l for the scenario considered. Note that the dredging is simulated as repeatedly taking place to the south o f the centre-line o f the navigation channel. Highest concentrations are predicted to occur on the red-line in this instance with the smaller dredger.

3.4 IMPACTS OF DREDGING WITH T H E LARGER DREDGER

The predicted impacts o f the smaller dredger worlcing were used as input to the development of thresholds f o r real-time control o f the construction activities within the M C M to provide a robust means o f ensuring that the impacts f r o m the dredging would not exceed acceptable levels. The same levels w i l l also be applicable for control o f the larger dredger.

The results o f the SEDPLUME modelling demonstrate that the effects o f the larger dredger working are comparable to those o f the smaller dredger working. Within the redline zone the larger dredger is predicted to resuh in higher near-field suspended sediment concentrations (an increase o f 20%). However, the agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-line, at or below the previously agreed thi-esholds is unaffected (as illustrated by comparisons of Figures 8 and 9).

The important difference between the two dredging scenarios is that the larger dredger w i l l work for less time and release less fines for a given volume o f material. There is also the potential to reduce the overall timescale o f the project, potentially in the region of one year.

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4. Discussion of the significance of scfieme

refinements

4.1 O V E R V I E W

The resuhs o f the SEDPLUME modeUing demonstrate that the effects o f the larger modem dredger working are comparable to those o f the smaller dredger working. Within the red-line zone the larger dredger is predicted to resuh in higher near-field suspended sediment concentrations (an increase of 20%). However, the agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-line, at or below the previously agi'eed thresholds is unaffected. The important difference between the two dredging scenarios is that the larger dredger w i l l work for less time and release less fines for a given volume o f material. Thus the period over which impacts w i l l occur for the larger dredger w i l l be reduced and the impacts that occur whilst the dredger is working w i l l be similar to those o f the smaller dredger. Monhoring as outlined in the M C M w i l l provide a means o f confiiming these predictions. The methods for managing the environmental impacts o f the dredging activity outlined in the M C M w i l l equally apply for the larger TSHD.

Accordingly the implications for water quality, intertidal invertebrates and organisms that depend upon them as a food resource (birds and fish) are as originally assessed. This is discussed below in relation to geographical areas.

4.2 U P S T R E A M OF COALHOUSE POINT

No significant changes arise f r o m the use o f a larger TSHD. Monitoring as outlined in the M C M w i l l provide a means o f confirming this prediction.

4.3 LOWER HOPE REACH

No significant changes arise f r o m the use o f a larger TSHD although the period o f constmction would be reduced. The methods to control the dredging activity outlined in the M C M w i l l apply. Monitoring as outlined in the M C M w i l l provide a means of confinning this prediction.

4.4 MUCKING FLATS

The assessment o f the original scheme predicted that large natural fluxes o f sediment on and o f f the mudflats w i l l lead to changes to the morphology o f Mucking Flats after the constmction o f the westem bund o f the second stage o f reclamation works. Increases in sediment supply f r o m reclamation and dredging losses were assessed to contribute to the evolution o f the moiphological change on Mucking Flats.

No impacts over and above those predicted for the smaller TSHD are predicted to occur i f the larger dredger is used. The E I A included highly precautionaiy assumptions about the loss o f fines f r o m the reclamation activity. The impact assessment for the original scheme on ecology o f both invertebrates and birds on Mucking Flats is therefore the same as for the refinements discussed in this report. Monitoring as outlined in the M C M w i l l provide a means o f confinning this prediction.

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4.5 NORTH SHORE BERTHS IN SEA REACH

In the E I A it was demonstrated that during the constmction period siltation rates at the existing berths were predicted to increase above background levels. This increase arises f r o m overflow losses and f r o m reclamation losses. Total overflow losses are predicted to be less for the larger TSHD and the losses f r o m the reclamation activity were precautionaiy. Thus i f the timescales for the development were unchanged and a larger dredger was used the predicted impacts f r o m constmction activities on navigation dredging at these berths would be no greater than that presented in the E I A . W i t h a reduced timescale for constmction it is possible that at times sihation rates might be greater than that previously assessed in the E I A albeit the overall volumes o f sihation would be within that previously assessed. Monitoring, as described in the M C M , w i l l be required to manage safe operations at the berths.

4.6 HOLEHAVEN CREEK

The original E I A demonstrated that losses o f fines during constmction may influence the intertidal area o f the Creek and the subtidal accretion in the mouth o f the Creek. This is unchanged by the use o f a larger TSHD albeh that changes may occur more rapidly under an accelerated programme. Monitoring as outlined in the M C M w i l l provide a means of confirming this prediction.

4.7 BLYTH SANDS

No significant changes arise f r o m the use o f a larger TSHD although the period o f constmction would be reduced. The methods to control the dredging activity outlined in the M C M w i l l apply. Monhoring as outlined in the M C M will provide a means o f confiiTning this prediction.

4.8 C H A P M A N SANDS, SOUTHEND A N D LEIGH FLATS

Short term impacts on Chapman Sands, Southend and Leigh Flats during the constmction period are predicted to be o f the same magnitude for both scenarios. Less fines are expected to be released into the system over the complete dredge period and it is expected that the impacts will be less. W i t h a reduced constmction programme the period when such impacts may occur w i l l be reduced. No significant changes to the moiphology are predicted here. The methods to control the dredging activity outlined in the M C M will apply. Monitoring as outlined in the M C M w i l l provide a means o f confirming this prediction.

4.9 MAPLIN SANDS A N D FOULNESS

No significant changes arise f r o m the use o f a larger TSHD although the period o f constmction would be reduced. The methods to control the dredging activity outlined i n the M C M w i l l apply. Monhoring as outlined in the M C M will provide a means o f confirming this prediction.

4.10 BERTHING A N D MANOUEVERING A R E A

4.10.1 Maintaining the estuary cross-section

It is intended that the constmction impact on flows in Sea Reach wiU be managed through broadly maintaining the cross-sectional area o f the estuary as the constmction proceeds. This is a matter o f phasing reclamation, constmction o f the bunds and

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L o n d o n Gateway Port, Scheme refinement - Use o f larger dredger Hydraulic studies and assessment o f environmental significance

dredging o f tlie adjacent cross-section. This was a condition o f the original submission and would not be changed by the use o f a larger dredger. Monitoring as outlined in the M C M w i l l provide a means o f confirming this prediction.

4.10.2 Reclamation Losses

In the E I A it was assumed that up to 1.2Mm^ o f fine material might be lost during the reclamation process. The assumption that the losses f r o m the reclamation activity would all be o f fines was precautionaiy because a proportion o f the losses w i l l be of coarser material.

A consequence o f reduced losses f r o m the dredging activity is that the total volume o f material recovered w i l l increase. Thus placement into the reclamation site w i l l increase, and the average fines content o f the placed material w i l l increase slightly. There w i l l remain the need to manage fines within the fill material during the engineering works. It remains the case that losses o f fines during reclamation w i l l be no more than the precautionary volume o f 1.2Mm^ used in the EIA. Therefore the analysis in the E I A regarding the impacts o f losses o f fines remains unchanged.

The mechanism for controlling runoff and losses f r o m dredging activity is in the f o i m of real time monitors on the red line and application o f thresholds to Zones 1 to 3. The Zone 1 thresholds are relevant to the control o f the reclamation activities. Weir boxes built into the reclamation bunds are the main mechanism for controlling runoff f r o m the reclamation. The ability to control nanoff f r o m the reclamation has not changed f r o m the original scenario and the same conditions set out i n the M C M w i l l apply. Monitoring as outiined in the M C M w i l l provide a means o f confinning these predictions.

4.10.3 Duration of impacts

Within the navigation channel ("red-line boundaiy") temporaiy increases in suspended sediment concentrations and temporaiy deposits are likely to be slightly higher, on average, than for the smaller TSHD dredging scenario because o f the increased production rates with the larger TSHD. The duration that such impacts occur for w i l l be reduced because o f the increased production rates with the larger TSHD and the overall reduced time required for the dredging. Characterisation monitoring as outiined in the M C M w i l l provide a means o f confirming this prediction.

4.10.4 Water quality risk management

The results o f the SEDPLUME modelling demonstrate that the effects o f the larger dredger working are comparable to those o f the smaller dredger working. Within the redline zone the larger dredger is predicted to result in higher near-field suspended sediment concentrations (an increase of 20%). However, the agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-lme, at or below the previously agreed thresholds is unaffected.

The E I A identified that during dredging there was a potential for elevated concentrations o f TBT in the sediments associated with the dredging o f the berthing and manoeuvring area leading to an increase i n total T B T concentrations above the Environmental Quality Standard (EQS).

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A targeted approach to dredging in the berth approaches w i l l be adopted to minimise the potential release of T B T contaminated sediment mto the water column in accordance with conditions set out in the M C M . Monitoring is described in the M C M that w i l l characterise potential contaminant levels in the near-field mixing zone around the dredging operations (within the red-line zone) in the berth approaches. Programmes o f monitoring are also described in the M C M that w i l l verify T B T concentrations in the wider estuary.

The measures proposed in the M C M would equally apply for both a smaller and larger dredger.

4.11 NAVIGATION CHANNEL

4.11.1 Duration of impacts

Within the navigation channel ("red-line boundaiy") temporary increases in suspended sediment concentrations and temporary deposits are likely to be slightly higher, on average, than for the smaller TSHD dredging scenario because o f the increased production rates with the larger TSHD. The duration that such knpacts occur for a given volume o f material w i l l be reduced because of the increased production rates with the larger TSHD. There is also the potential to reduce the overall timescale o f the project, potentially i n the region o f one year. Characterisation monitoring as outlined in the M C M w i l l provide a means o f confiiming this prediction.

A key parameter detemiining the impact o f any disturbance on animal and plant populations is the length o f time over which the disturbance w i l l occur. Because most species are at their most vuhierable during reproduction and early life, an important consideration in any assessment is the number o f reproductive seasons over which the disturbance w i l l last. Specific examples o f vulnerable life stages relevant to the London Gateway Port proposal include the spawning and reproductive migrations o f fish. Reducing the time span o f the constmction and thus the number o f reproductive periods that would be affected would have a clear advantage to recruitment to these populations. I f a larger dredger were deployed for a limited period then this would have insignificantly greater effect on the biota than the effects of the smaller dredger considered in the EIA. Further, i f the larger dredger were deployed for sufficient time to reduce the entire timescale o f the project by at least one year then the time span o f the constmction activity brings forward the date by which the system would recover. I n part, this is caused by the earlier cessation o f activity, but is also linked to the reduction in the number o f recmitment periods that would be disturbed.

Given that the dredging, even on the reduced time scale, w i l l continue to meet environmental standards as outlined in the M C M there are clear population ecological advantages i n completing the work to a reduced programme. Monitoring as outlined i n the M C M w i l l continue to provide a means of confirming this assessment.

4.11.2 Water quality risk management

Impacts outside o f the channel ("red-line boundary") are expected to be similar to those for the smaller TSHD scenario considered in the EIA. The dredge management regime outlined i n the M C M , which includes the use of thresholds for Zones 1 to 3 and monitors placed along a 200m buffer outside the dredge footprint w i l l apply.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger Hydraulic studies and assessment o f environmental significance

The resuhs o f the SEDPLUME modehing demonstrate that the effects o f the larger dredger worlcing are comparable to those of the smaller dredger working. Within the redline zone the larger dredger is predicted to result in higher near-field suspended sediment concentrations (an increase o f 20%). However, the agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-line, at or below the previously agreed thresholds is unaffected.

The E I A identified that during dredging there was a potential for elevated concentrations of TBT i n the sediments associated with the dredging of the berthing and manoeuvring area leading to an increase in total T B T concentrations above the Environmental Quality Standard (EQS).

Levels o f TBT in the sediments o f the navigation channel are comparable to those at the intertidal receptor sites. Monitoring is described in the M C M that w i l l characterise potential contaminant levels in the near-field mixing zone around the dredging operations (within the red-line zone) in the navigation channel. Programmes of monitoring are also described in the M C M that w i l l verify T B T concentrations in the wider estuary.

The measures proposed in the M C M would equally apply for both a smaller and larger dredger.

4.12 OFFSHORE CHANNELS A N D BANKS

The use o f a larger TSHD to undertake the dredging in this area w i l l not alter the overall impact o f the dredging in this area. A larger dredger could complete the dredging in this area more quickly than the smaller dredger. Characterisation monitoring, as outlined in the M C M , w i l l be important to apply to the dredging in this area,

4.13 MANAGED REALIGNMENT SITES A AND X

The M C M states in Section 11.4 that the managed realignment site North o f Mucking Flats (She A ) w i l l be completed no later than the time o f commencement o f the westem bund for Phase 2 of the reclamation. Similarly the M C M states that the North Kent realignment site (Site X ) w i l l be completed no later than 48 months f r o m the commencement o f the constmction o f the westem bund.

There is also the potential to reduce the overall timescale o f the project, potentially in the region o f one year. This does not change the environmental assessment or requirements o f the M C M .

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H y d r a u l i c studies and assessment o f environmental significance A ^ H R Wallingford

5. Conclusions

The modelhng, analysis and interpretation described in this report is based on the methodologies originally employed in the hydraulic studies undertaken to support the EIA and has been undertaken by HR Wallingford.

The physical impacts o f the use o f a modem larger trailer suction hopper dredger have been assessed and contrasted with the impacts o f the use o f a smaller dredger as originally assessed i n the EIA. As a resuh o f the use o f a larger dredger there would be: • increased production rates;

• an increase in near-field suspended solids concentrations within the footprint o f the dredging hself;

• an increased retention o f material in the dredger

• overall reductions in losses of fine material f r o m overflow; • a greater amount o f material retained in the reclamation site; and • scope for a reduced timescale for constmction and period o f impact.

HR Wallingford have direct responsibility for assessing the impacts on navigation and flood defence. The assessment as to whether or not there are consequences arising that are different to those addressed in the E I A have been made in consuhation with other experts as follows:

Marine Ecology Marine Ecological Surveys L t d

Dr Richard Newell Chaiman

Water and Sediment Quality

Fish Pisces

Conservation L t d

The resuhs o f the SEDPLUME modelling demonstrate that the effects o f the larger modem dredger working are comparable to those o f the smaller dredger working. Within the redline zone the larger dredger is predicted to resuh in higher near-field suspended sediment concentrations (an increase o f 20%). However, the agreed approach to maintaining suspended sediment concentrations in close proximity to the dredger on the red-line, at or below the previously agreed thi-esholds is unaffected. The important difference between the two dredging scenarios is that the larger dredger w i l l work for less time and release less fines for a given volume o f material. Thus the period over which impacts w i l l occur for the larger dredger w i l l be reduced and the impacts that occur w i l l be similar to those of the smaller dredger. Monitoring as outlined in the M C M w i l l provide a means o f confirming these predictions. The methods for managing the environmental unpacts o f the dredging activity outlined i n the M C M w i l l equally apply for the larger TSHD.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger Hydraulic studies and assessment o f environmental significance

Accordingly the implications for water quality, intertidal invertebrates and organisms that depend upon them as a food resource (birds and fish) are as originally assessed. It is possible that by using a larger dredger the overall timescales o f the project could be reduced in the region o f one year which could have positive environmental benefits. The Mitigation, Compensation and Monitoring Agreement ( M C M ) sets out the measures that London Gateway w i l l implement in order to minimise the risk o f impact to the marine enviromnent. No further measures are required in the M C M to minimise risk to the marine environment as a resuh o f the potential use o f a modem larger trailer suction hopper dredger.

Measures as outlined in the M C M w i l l not only provide a means for managing and mitigating dredging and constmction activities, but w i l l also provide a means o f confirming the prediction o f physical effects and the assessment o f consequence arising on the marine environment.

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H y d r a u l i c smdies and assessment o f environmental significance iii HR Wallingford

6. References

Dredging Researcli Limited (2001) London Gateway, Preliminaiy Estimate o f overflow losses and material recovery, November 2001

HR Wallingford (2003) London Gateway Port, Summary o f predicted intertidal sedimentaiy processes. Report E X 4750, March 2003.

London Gateway Port (2003) Mitigation, Compensation and Monitoring Agreement, August 2003.

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger ^ 9 H y d r a u l i c studies and assessment o f environmental significance A r f HR Wallingford

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H y d r a u l i c smdies and assessment o f environmental significance I HR Wallingford

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H y d r a u l i c smdies and assessment o f environmental significance | A ! ^ H R Wallingford

Table 1 6,400 T S H D , 4km haul

Dredger capacity (cubic metres) 6400 Distance to reclamation (km) 4

Soil Type Outer Fine Silty Sands Assumed in situdensity (Mg/cum) 1.9

Assumed fill dry density (Mg/cum) 1.6 Loading method Normal Insitu Output (cu.tn/wk) 325750.85 Fill Output (cu.m/wk) 134925.37 Total cycle dme (mins) 287.21 Overflow period (mins) 132.26 Overflow rate (kg/sec) 1145.97 Particle Size Distributions:

Size Assumed FiU Overflow Microns In SituPSD PSD PSD Less 20 20 4.78 33.19 20-60 8 4.17 11.32 60-80 22 19.02 24.59 80-100 15 16.39 13.80 100-150 25 36.05 15.41 150-200 5 9.03 1.51 200-300 2 4.11 0.17 300-400 2 4.30 0.00 400-600 0.5 1.08 0.00 600-1000 0.5 1.08 0.00 1000-2000 0 0.00 0.00 2000-4000 0 0.00 0.00 4000+ 0 0.00 0.00

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Hydraulic smdies and assessment o f environmental significance A J^ H R Wallingford

Table 2 6,400 T S H D , 13km haul

Assumed fdl dry density (Mg/cum) 1.6 Loading method Normal Insitu Output (cu.m/wk) 271833.92 Fill Output (cu.m/wk) 112593.08 Total cycle time (mins) 344.17 Overflow period (mins) 132.26 Overflow rate (kg/sec) 1145.97 Particle Size Distributions:

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HydrauHc smdies and assessment o f environmental significance HR Wallingford

Table 3 13,700 in' T S H D , 4km haul

Assumed fill dry density (Mg/cum) 1.6 Loading method Normal Insitu Output ( cu.m/wk) 361602.51 Fill Output (cu .m/wk) 211688.06 Total cycle time (mins) 409.42 Overflow period (mins) 139.33 Overflow rate (kg/sec) 1060.55 Particle Size Distributions:

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Table 4 13,700 m' T S H D , 13km haul

Assumed fill dry density (Mg/cum) 1.6 Loading method Normal Insitu Output ( cu.m/wk) 324196.1 Fill Output (cu .m/wk) 189789.73 Total cycle time (mins) 456.66 Overflow period (mins) 139.33 Overflow rate (kg/sec) 1060.55 Particle Size Distributions:

Size Assumed Fill Overflow Microns In SituPSD PSD PSD Less 20 20 7.81 43.29 20-60 8 4.87 13.98 60-80 22 20.31 25.24 80-100 15 16.95 11.27 100-150 25 34.89 6.11 150-200 5 7.56 0.11 200-300 2 3.04 0.00 300-400 2 3.04 0.00 400-600 0.5 0.76 0.00 600-1000 0.5 0.76 0.00 1000-2000 0 0.00 0.00 2000-4000 0 0.00 0.00 4000+ 0 0.00 0.00

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H y d r a u l i c smdies and assessment o f environmental significance A J^ H R Wallingford

Table 5 Summary of results of T S H D production model for dredging Outer Fine Silty Sands

6,400m' 13,700m' Comment (Larger TSELD vs Smaller T S H D T S H D T S H D )

Overflow period 132 minutes 139 minutes Larger TSHD overflows f o r 5% longer Average 1146kg/s 1060kg/s Larger TSHD oveiilows 8% less Oveidlow

% of fines in 45% 57% Higher proportion o f fines in overflow oveiflow f r o m larger TSHD

Average 510kg/s 607kg/s Rate o f loss o f fines for larger TSHD is overflow o f fines about 20% greater

Dry mass o f 6,500 Tonnes 14,000 Hopper capacity o f larger TSHD is 215% cargo in hopper Tonnes greater

Mass o f fines lost 4,047 Tonnes 5,074 Tonnes Total input o f fines per cargo for larger for winning a TSHD is about 25% greater

single cargo

Mass of sands 4,992 3,801 Total input o f sands per cargo f o r larger lost for winning a Tonnes Tonnes TSHD is 25% less

single cargo

Mass o f fines lost 62,000 36,000 Input o f fines f r o m larger TSHD is about for winning Tonnes Tonnes 40% less

100,000 Tonnes o f material

Mass o f sand lost 77,000 27,000 Input o f sands f r o m larger TSHD is about for winning Tonnes Tonnes 65 % less

100,000 Tonnes o f material

% of fines in 9.0% 12.7% For dredging o f Outer Fine Silty Sands the reclamation larger TSHD w i l l place 40% more fines material into the reclamation

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L o n d o n Gateway Port, Scheme refinement - Use o f larger dredger ^ 9 H y d r a u l i c smdies and assessment o f environmental significance K r f HR Wallingford

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H y d r a u l i c smdies and assessment o f environmental significance

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H y d r a u l i c smdies and assessment o f environmental significance iii HR Wallingford

Figure 2 Rate of release of fine material from consecutive loads for smaller (green) and larger (red) T S H D s operating continuously so as to achieve a total tonnage to be placed in the reclamation of 140,000 dry tonnes

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H y d r a u l i c smdies and assessment o f environmental significance AT^HR Wallingford

Figure 3 Predicted peak depth averaged suspended sediment concentration increases above background levels (upper figure) and time series (lower figures) of concentration increases at Locations 1 and 2. Spring tide simulation, dredging 4km from the port

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H y d r a u l i c studies and assessment o f environmental significance Ad HR Wallingford

I

Figure 4 Predicted peak depth averaged suspended sediment concentration increases above background levels (upper figure) and time series (lower figures) of concentration increases at Locations 3 and 4. Spring tide simulation, dredging 4km from the port

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L o n d o n Gateway Port, Scheme refinement - Use o f larger dredger

H y d r a u l i c smdies and assessment o f environmental significance A J^ H R Wallingford

Figure 5 Predicted peak depth averaged suspended sediment concentration increases above background levels (upper figure) and time series (lower figures) of concentration increases at Locations A and B . Spring tide simulation, dredging 4km from the port

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H y d r a u l i c studies and assessment o f environmental significance fc^HR Wallingford

Figure 6 Central figure showing predicted peak depth averaged suspended sediment concentrations around the dredging activity, 6,400m' T S H D working at 13km. Six side figures show time series of depth averaged and near bed concentrations at six locations in close proximity to the centre line of dredging

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H y d r a u l i c smdies and assessment o f environmental significance fc^HR Wallingford

Figure 7 Central figure showing predicted peak depth averaged suspended sediment concentrations around the dredging activity, 13,700m' T S H D working at 13km. Six side figures show time series of depth averaged and near bed concentrations at six locations in close proximity to the centre line of dredging

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Hydraulic studies and assessment o f environmental significance ^ i H R Wallingford

Figure 8 Illustration of the relationship between the dredging footprint (the hatched zone) and the red-line monitoring position for the scenario of dredging with the smaller T S H D (same simulation as shown in Figure 6)

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Figure 9 Illustration of the relationship between the dredging footprint (the hatched zone) and the red-line monitoring position for the scenario of dredging with the larger T S H D (Same simulation as shown in Figure 7)

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L o n d o n Gateway Port. Scheme refinement - Use o f larger dredger ^ j ^ '