Environmental Statement Appendix 16 .B (6.3.16.2) · STEP Scour Time Evolution Predictor TfL Transport for London . Silvertown Tunnel ... Woolwich was created using MIKE21 Flow Model

Environmental Statement Appendix 16 .B (6.3.16.2)

Hydrodynamic Modelling

April 2016

Silvertown Tunnel

Appendix 16.B Hydrodynamic Modelling

Document Reference: 6.3.16.2

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Contents

List of Abbreviations ................................................................................................ 8

Glossary of Terms .................................................................................................... 9

Summary…………………………………………………………………...………………11

1. INTRODUCTION ......................................................................................... 13

1.1 Background ................................................................................................. 13

1.2 Study site .................................................................................................... 13

1.3 Scope of work ............................................................................................. 15

2. MODEL DATA ............................................................................................. 17

2.1 Bathymetry .................................................................................................. 17

2.2 Hydrodynamics ........................................................................................... 17

3. MODEL MESH MODELLING ...................................................................... 19

3.2 Silvertown Temporary jetty .......................................................................... 20

4. HYDRODYNAMIC SIMULATIONS ............................................................. 21

4.1 Simulations background .............................................................................. 21

4.2 Model validation .......................................................................................... 21

5. SEDIMENT SPILL SIMULATIONS ............................................................. 25

5.2 River bed surface sediment sampling ......................................................... 25

5.3 Vibracore Sediment Survey ........................................................................ 28

5.4 Dredging Instrument .................................................................................... 29

5.5 Dredging Log ............................................................................................... 29

5.6 Plume Modelling .......................................................................................... 31

6. MODEL RESULTS ...................................................................................... 35

6.1 Baseline hydrodynamics ............................................................................. 35

6.2 Current speed difference ............................................................................. 40

6.3 Sediment plume modelling .......................................................................... 44

6.4 Temporary jetty pile scour ........................................................................... 54

7. PROPELLER SCOUR ................................................................................. 61

8. CONCLUSION ............................................................................................ 63

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Appendix A. Sediment particle size analysis .................................................. 65

Appendix B. Scour depth evolution ................................................................. 75

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List of Tables

Table 4-1 Simulation matrix showing possible configurations for temporary jetty

structures, tidal conditions and river flow rates ......................................................... 21

Table 5-1 Cefas PSA statistics of the vibracore sediment sampling ........................ 29

Table 5-2 Sediment plume modelling scenarios ....................................................... 34

List of Figures

Figure 1-1 Silvertown temporary jetty design shown as the black outline with black

circular markers representing the temporary jetty piles ............................................ 14

Figure 3-1 Silvertown model mesh with bathymetry in mCD .................................... 19

Figure 3-2 Silvertown model mesh with bathymetry in mCD .................................... 20

Figure 4-1 Mean neap tide with mean river flow ....................................................... 22

Figure 4-2 Mean neap tide with high river flow conditions ........................................ 23

Figure 4-3 Mean spring tide with mean river flow conditions .................................... 23

Figure 4-4 Mean spring tide with high river flow conditions ...................................... 24

Figure 5-1 Location of intertidal and subtidal sample locations ................................ 26

Figure 5-2 Particle size analysis result of subtidal sample SU3 ............................... 27

Figure 5-3 Indicative location of proposed vibracore locations (1-6) and the location

of the moored barges in the centre of the site .......................................................... 28

Figure 5-4 Grain size and settling velocity ................................................................ 33

Figure 6-1 Baseline hydrodynamic flow conditions for mean neap tides and mean

river flow. .................................................................................................................. 36

Figure 6-2 Baseline hydrodynamic flow conditions for mean neap tides and high river

flow. .......................................................................................................................... 37

Figure 6-3 Baseline hydrodynamic flow conditions for mean spring tides and mean

river flow ................................................................................................................... 38

Figure 6-4 Baseline hydrodynamic flow conditions for mean spring tides and high

river flow. .................................................................................................................. 39

Figure 6-5 Difference in simulated current speed for mean spring and high flow

conditions. ................................................................................................................ 40

Figure 6-6 Current speed with and without the temporary jetty structure under mean

neap tide and mean flow conditions. ........................................................................ 41

http://ukr.hybis.info/Projects/WX/Awarded/UA005651/F_Reports/Silvertown%20Preliminary%20Reports/0012-UA005651-UE31U%20ES/ES_Appendices/Formatted_and_Final_Documents/Preliminary_Formatting/Superseded/ES%20Appendix%2016B%20formatted_GG%20DRA.docx#_Toc449446370

http://ukr.hybis.info/Projects/WX/Awarded/UA005651/F_Reports/Silvertown%20Preliminary%20Reports/0012-UA005651-UE31U%20ES/ES_Appendices/Formatted_and_Final_Documents/Preliminary_Formatting/Superseded/ES%20Appendix%2016B%20formatted_GG%20DRA.docx#_Toc449446375

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neap tide and high flow conditions. .......................................................................... 42


spring tide and mean flow conditions ....................................................................... 43


spring tide and high flow conditions.......................................................................... 44

Figure 6-10 Map of maximum of Incremental SSC in mean neap tide and mean river

flow scenario ............................................................................................................ 46

Figure 6-11 Map of mean of Incremental SSC in mean neap tide and mean river flow

scenario .................................................................................................................... 46

Figure 6-12 Map of total bed mass change in mean neap tide and mean river flow

scenario .................................................................................................................... 47

Figure 6-13 Map of maximum of Incremental SSC in mean neap tide and high river

flow scenario ............................................................................................................ 47

Figure 6-14 Map of mean of Incremental SSC in mean neap tide and high river flow

scenario .................................................................................................................... 48

Figure 6-15 Map of total bed mass change in mean neap tide and high river flow

scenario .................................................................................................................... 48

Figure 6-16 Map of maximum of Incremental SSC in mean spring tide and mean

river flow scenario .................................................................................................... 49

Figure 6-17 Map of mean of Incremental SSC in mean spring tide and mean river

flow scenario ............................................................................................................ 49

Figure 6-18 Map of total bed mass change in mean spring tide and mean river flow

scenario .................................................................................................................... 50

Figure 6-19 Map of maximum of Incremental SSC in mean spring tide and high river

flow scenario ............................................................................................................ 50

Figure 6-20 Map of mean of Incremental SSC in mean spring tide and high river flow

scenario .................................................................................................................... 51

Figure 6-21 Map of total bed mass change in mean spring tide and high river flow

scenario .................................................................................................................... 51

Figure 6-22 Estimate of dry density of sea bed as a function of sand fraction and

consolidation ............................................................................................................ 52

Figure 6-23 Suspended sediment concentration (SSC) in 2006-2007 in the River

Thames. ................................................................................................................... 54

Figure 6-24 Typical flow structure around a pile structure. ....................................... 55

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Figure 6-25 Location of current speed extraction (yellow markers) for jetty pile scour

depth calculations .................................................................................................... 57

Figure 6-26 Scour depth evolution over time for mean spring tide and high flow

conditions at the approach over two tidal cycles ...................................................... 59

Figure 6-27 Scour depth evolution over time for mean spring tide and high flow

conditions at the jetty head over two tidal cycles ...................................................... 60

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List of Abbreviations

DCO Development Consent Order

ES Environmental Statement

FM Flow Model

kW Kilowatt

MT Mud Transport

PLA Port of London Authority

PSA Particle Size Analysis

SSC Suspended Sediment Concentration

STEP Scour Time Evolution Predictor

TfL Transport for London

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Glossary of Terms

Bathymetry The study of underwater depth of lake

or ocean floors.

Blackwall Tunnel A pair of existing road tunnels under

the Thames at Blackwall in east

London

Dredging Dredging is the removal of sediments

and debris from the bottom of lakes,

rivers, harbours, and other water

bodies.

Hydrogeology Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust.

Silvertown Tunnel Proposed new twin-bore road tunnels

under the River Thames from the

A1020 in Silvertown to the A102 on

Greenwich Peninsula, East London.

Vibracore A technology and a technique for

collecting core samples of underwater

sediments and wetland soils.

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SUMMARY

S.1.1 A hydrodynamic model of the River Thames between Greenwich and

Woolwich was created using MIKE21 Flow Model (FM). This model

includes tidal and river discharge boundary conditions. Four flow

scenarios were simulated: spring and neap tidal conditions with mean and

maximum river discharge. Simulated water elevations and current speeds

were validated against the HR Wallingford Thames Model and show good

agreement. A proposed Silvertown Tunnel jetty design is included in the

hydrodynamic model. Comparisons are made between simulations with

and without the jetty structure to show the impact of the jetty piles on flow

velocities. The construction of the jetty causes a reduction in flow velocity

around the jetty head and a slight increase on the approach jetty towards

the nearshore.

S.1.2 The MIKE21 Mud Transport (MT) module is used to assess the movement

of sediment caused by dredging the area around the jetty head.

Assessments are made of suspended and accumulated sediment as a

result of the dredging process. There is shown to be little impact on

suspended sediment concentration and sedimentation, particularly when

compared with background levels of suspended sediment concentration.

S.1.3 An assessment of scour around the jetty piles is made using the simulated

velocities around the jetty structure. The method of Whitehouse 1 was

applied which defines the scour depth as a function of time. An

adjustment factor is applied to account for cohesive sediments following

the method of HR Wallingford 2.A maximum scour depth of 0.46m is

calculated for the 1.016m diameter piles at the approach jetty and jetty

head.

S.1.4 Scour of the river bed due to propeller wash from ships berthed at the jetty

is also calculated. The depth of scour is directly related to the Froude

number, associated with the propeller flux velocity. The largest vessel to

be moored at the jetty is assumed to have a propeller diameter of 2.5m,

minimum height of propeller axis from the bed of 2.25m and engine power

1 Whitehouse, R. J. S. (1998). Scour at marine structures: A manual for practical applications. Thomas Telford, London, p198.

2 Harris J M., Whitehouse R. J. S., Benson T. (2012). The time evolution of scour around offshore structures – the scour time evolution predictor (STEP) model. HR Wallingford Ltd.

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of 1500kW. These conditions give a maximum equilibrium scour depth

caused by propeller wash of 0.8m.

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1. INTRODUCTION

1.1 Background

1.1.1 A new road tunnel has been proposed by Transport for London (TfL)

linking the areas of Greenwich Peninsula and Silvertown, on the banks of

the River Thames. As a part of this project a potential new temporary jetty

structure has been proposed on the northern bank of the River Thames,

to the east of the mouth of the River Lea, also known as Bow Creek. This

report investigates the impact of this structure on the local hydrodynamics

and determines the extent of any scour around the piles and alongside the

temporary jetty.

1.2 Study site

1.2.1 Figure 1-1 shows the indicative location of the temporary jetty structure on

the north bank of the River Thames, with Bow Creek to the west. The

proposed temporary jetty is a ‘T’ shaped structure, with a jetty head

attached to an approach jetty. Figure 1-1 shows the temporary jetty pile

alignments which consists of a total of 25 piles with a diameter of 1016

mm. The temporary jetty piles are indicated as black circular markers

within the T shaped jetty outline. A dredging area for vessels is shown as

the solid orange line polygon. The temporary jetty design shown in Figure

1-1 should not be viewed as the final design of the structure. The

hydrodynamic assessment present herein will assess the impact of the

structure and will therefore inform the final approval by the Port of London

Authority (PLA). Further to this, there will be a Development Consent

Order (DCO) requirement for the local authorities to approve the structure

design and external appearance. The orange trapezoid indicates an area

to be dredged, showing the area required to achieve a flat river bed for

jetty operations. Notwithstanding this, the Works Plans show a larger

dredge area for the Limits of Deviation of jetty operations which includes

the construction of the temporary jetty and the dredging works.

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Figure 1-1 Silvertown temporary jetty design shown as the black

outline with black circular markers representing the

temporary jetty piles

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1.3 Scope of work

1.3.1 The assessment of the impact of the temporary jetty structure will

investigate the change in local currents due to the movement of water

around the temporary jetty piles. The MIKE21FM hydrodynamic modelling

software is applied to simulate the tidal hydrodynamics, including the flow

of the River Thames and the River Lea. The model is developed on a

finite element flexible mesh of triangles, which allows for increased grid

resolution around complex areas of coastline and regions of interest, while

a coarser resolution can be applied to other regions, allowing for

increased model efficiency. MIKE21FM simulates the water level variation

and flow in response to a variety of forcing functions in coastal areas as

well as inshore waterbodies. The model can include the effects of

convective and cross momentum plus momentum dispersion, bottom and

surface (wind) shear stresses, coriolis and barometric pressure gradient

forcing, evaporation and precipitation, hydraulic structures and wave-

induced currents.

1.3.2 The simulated hydrodynamics around the temporary jetty piles will allow

for an estimate of the extent of river bed scour to be made. The scour

assessment will be made using the Scour Time Evolution Predictor

(STEP) model developed by HR Wallingford3 . The MIKE21 MT module is

used to assess the impact of sediments released into the water column

due to the dredging processes at the jetty head. The MIKE21 MT module

will account for sediment flocculation associated with cohesive sediments,

sediment settling and also resuspension. Regions of sediment

accumulation will show the deposition of sediments around the dredging

area. Suspended sediment concentrations will show the impact to the

water column due to the dredging works as assessed in Chapter 16 –

Water Quality and Flood Risk of the Environmental Statement (ES)

(Document Reference: 6.1.16).


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2. MODEL DATA

2.1 Bathymetry

2.1.1 Bathymetry data for the study site were supplied by the PLA at 10m

resolution and referenced to Chart Datum. These data were supplied on

29 May 2015. Regions of river bed dredging were manually included in

the model bathymetry in accordance with the indicative temporary jetty

specifications provided in the construction method statement Appendix

4.A (Document Reference: 6.3.4.1).

2.2 Hydrodynamics

2.2.1 Hydrodynamic boundary conditions for the Silvertown Tunnel tidal model,

simulated using MIKE21FM, were extracted from the HR Wallingford

River Thames model. Water level, both components of current velocity (u

and v) and river discharge were supplied at three locations: Greenwich

(538500, 178200), Silvertown (539500, 180400) and Woolwich (542000,

179600). A period of 48 hours was covered to allow for model spin up and

include a complete tidal cycle. Mean spring and neap tidal conditions were

included with mean and high river discharge rates to simulate low,

average and high flow conditions around the Silvertown Tunnel temporary

jetty structure. Discharge values were also supplied for River Lea (Bow

Creek), the river tributary to the immediate west of the temporary jetty

structure.

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3. MODEL MESH MODELLING

3.1.1 The model domain was created to cover the region of the River Thames

between Greenwich and Woolwich. The mesh resolution varies from 50m

at the eastern and western boundaries to 7m around the Silvertown

Tunnel jetty structure. The finite element flexible mesh of triangles used

within the MIKE21 modelling software, allows for an efficient increase in

resolution around areas of interest while allowing for coarser resolution in

other regions of the model domain.

Figure 0-1 Silvertown model mesh with bathymetry in mCD

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3.1.2 The mesh resolution can be seen to increase around the location of the

proposed temporary jetty (Figure 3-1).

3.2 Silvertown Temporary jetty

3.2.1 The temporary jetty structure configuration (Figure 1-1) is included in the

Silvertown model as a structure within MIKE21 FM mesh at a sub-grid

scale, with a diameter of 1016mm. This represents the likely development

that would be carried out, which is subject to detailed design and liaison

with the PLA. Location, width and shape of the piles is specified, for the

purpose of this assessment only, so that the effect on the flow can be

modelled by calculating the current induced drag force on each individual

pile. Jetty piles were included at a sub-grid scale where the mesh

resolution was chosen to be small enough to resolve the temporary jetty

but not excessive to reduce run times.

3.2.2 An area around the jetty head has been designated as a dredge area in

the initial design (Figure 1-1) and included in the model domain (Figure

0-2). This area is dredged to a depth of -5.3mCD to allow for vessels with

a draft of 4.642m.

Figure 0-2 Silvertown model mesh with bathymetry in mCD

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4. HYDRODYNAMIC SIMULATIONS

4.1 Simulations background

4.1.1 The conditions simulated with the Silvertown Tunnel hydrodynamic model

include spring and neap tidal conditions with mean and high river flow

rates. Simulations were made with and without the jetty structure so that

the differences in current velocities due to the temporary jetty may be

examined. For the simulations with the temporary jetty structures the

modification to the bathymetry due to dredging for vessels, as shown in

Figure 0-2 Silvertown model mesh with bathymetry in mCD, is also

included. For the baseline simulations, without the temporary jetty

structures, the existing bathymetry was applied. Table 4-1 shows a

simulation matrix of the various jetty, tide and river flow conditions.

Table 4-1 Simulation matrix showing possible configurations for

temporary jetty structures, tidal conditions and river flow rates

Temporary jetty Option

Tide River flow

Without Neap Mean

Without Neap High

Without Spring Mean

Without Spring High

With Neap Mean

With Neap High

With Spring Mean

With Spring High

4.2 Model validation

4.2.1 The simulated hydrodynamic conditions listed in Table 4-1 Simulation

matrix showing possible configurations for temporary jetty structures, tidal

conditions and river flow rates are compared against the HR Wallingford

River Thames model results at a point close to the Silvertown Tunnel

temporary jetty (539500, 180400), using the baseline simulation without a

temporary jetty structure present. The HR Wallingford River Thames

model is validated against an estuary wide survey undertaken in 2004 as

a part of the Environment Agency TE2100 study, with further validation in

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20094 . The Mean Absolute Errors for the validation of discharge rates are

between 5-10%, indicating a good performance in the HR Wallingford

Thames model5. Figure 4-1 to Figure 4-4 shows the comparisons between

the Silvertown model and the HR Wallingford model simulated surface

elevation and both components of velocity (u and v). The comparisons in

Figure 4-1 to Figure 4-4 show very good agreement between simulated

hydrodynamics, with occasional differences for peak ood and peak ebb

velocity components. However, this is seen as an acceptable level of

model comparison and within the limits of the calibration.

Figure 4-1 Mean neap tide with mean river flow

4 HR Wallingford. (2015). LRS Central London Pier Extensions, hydrodynamic and scour assessment. HR Wallingford Ltd.

5 Baugh J.V., Littlewood M.A., “Development of a cohesive sediment transport model of the Thames Estuary”. Proceedings of the 9th International Conference on Estuarine and Coastal Modelling, 2005.

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Figure 4-2 Mean neap tide with high river flow conditions

Figure 4-3 Mean spring tide with mean river flow conditions

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Figure 4-4 Mean spring tide with high river flow conditions

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5. SEDIMENT SPILL SIMULATIONS

5.1.1 The proposed temporary jetty development in Silvertown may require

dredging to maintain a minimum depth of -5.3 CD to manoeuvre the

vessels. To establish the possible sediment plume impact due to the

proposed dredging, sediment samples were collected. The results of

sediment sample analysis have provided an insight in to the local

sediment properties and also provided the data inputs for sediment plume

model.

5.2 River bed surface sediment sampling

5.2.1 Surface sediment samples were collected by ABP Marine Environmental

Research Ltd at representative sample locations agreed with the Marine

Management Organisation in December 2015 in the vicinity of the

temporary jetty and dredge pocket and are shown in Figure 5-1. The

sediment samples were collected in both the intertidal and sub tidal

regime. The samples were analysed to obtain the particle size distribution

in the sediments to be dredged. Due to operational limitations only one

sub tidal sample result is available in the proposed dredging area i.e. SU3

(Figure 5-2). The sample proved to be composed of coarse-grained

material (gravel) on the river bed limiting the amount of sediment which

could be collected in the grab sampling. However, the results from all the

intertidal samples are presented in Appendix A. The sediment sample

analysis shows that, in the current dominated area (i.e. in the dredging

area and mid channel) the fraction of fine sediments is low, whereas in the

intertidal area, being a weak current area the fine material fractions are

high.

5.2.2 Since the current speed in the channel is in the order of 1-1.5 m/s the

deposition of fine material was ruled out. The sediment sample SU3

shows a silt content of only 1.5%. Considering the tidal current strength

and non-availability of the results of other subtidal sample, the sediment

sample SU3 is chosen as the representative sediment sample for the

dredging area. Hence it is established that percentage contribution of

fines in sediments to be dredged is 1.5-2%. The intertidal sediment

samples show a very high percentage of silt and clay. The high

concentration of fine sediments in intertidal sediment suggests that spilled

fine sediments mobilised due to the dredging would be deposited in the

intertidal area. The sediment sample results obtained from ABP Marine

Environmental Research Ltd can be found in Appendix A.

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Figure 5-1 Location of intertidal and subtidal sample locations

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.

Figure 5-2 Particle size analysis result of subtidal sample SU3

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5.3 Vibracore Sediment Survey

5.3.1 An additional sediment sampling survey was conducted within the

dredging area by Seastar Survey Ltd on the 28 January 2016. This was to

determine the level of contaminants present in the sediment. A vibracore

sampling survey collected sediment samples at 1, 2 and 3m at 4-6 sites

within the dredge area.

Figure 5-3 Indicative location of proposed vibracore locations (1-6)

and the location of the moored barges in the centre of the site

5.3.2 During the survey operation it was noted that sediment conditions

prevented the collection of cores down to 3m depth at any of the sampling

locations. The area was characterised by very dense brown clay beneath

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a surface of more mobile surface sediment, predominantly gravelly sand.

The clay plugged the vibracore barrel and prevented further sediment

from being collected. This limited the sub-sampling that was possible at

each site and reduced the number of samples for analysis. Particle size

analysis (PSA) conducted by Cefas (Table 5-1) shows the percentage of

clay for all samples to be within 71-89%. .

Table 5-1 Cefas PSA statistics of the vibracore sediment sampling

5.4 Dredging Instrument

5.4.1 The particle size analysis of the sediment sample in the dredging area

suggests that a mechanical dredger with a small draft would be required

to carry out the dredging work. A range of dredger types would be

available to the Contractor, with a backhoe dredger the most likely option

and probably represents the option with least headspill. As other dredging

options may be considered, this assessment assumes a grab dredger with

a capacity of 6m3. This represents intermediate worst case spill condition

of the dredging options available.

5.5 Dredging Log

5.5.1 The following dredge log calculations were completed using the sediment

sample, choice of dredging equipment and dredging volume. Total dredge

volume = 54,700m3.

5.5.2 It is a general practice to include over-dredge volume in the dredge log

calculation. It is required as the cut by grab dredger is not smooth, so, to

achieve the desired dredged depth, over-dredge is required. In this case

we assume the over dredged volume to be 10% of the total estimated

dredging volume.

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5.5.3 Total volume to be dredged including over dredge volume is therefore

60,170 m3.

5.5.4 Bucket capacity of the grab dredger is 6 m3.

5.5.5 In general, there is slurry formation during the dredging process and lifting

the dredged material through the water column. Therefore each grab is

less than the bucket capacity. The Dredging handbook6 suggest a

reduction factor of 0.72 to be applied in case of sediments with silt and

sand content. The effective volume of sediment in each grab will be:

5.5.6 6 m3 * 0.72 = 4.32 m3.

5.5.7 The cycle time for each grab is assumed to be 210 seconds. The cycle

time includes the time required to lower the grab to the river bed, time

taken for dredging and lifting the dredged material through the water

column.

5.5.8 The grab dredger can work continuously, therefore continuous 24 hour

dredging operation is considered. This will lower the overall dredging cost

and it will also shorten the number of days required for the dredging.

5.5.9 The dredging rate is estimated to be 0.02m3⁄s.

5.5.10 The total time required to dredge the sediments is estimated as 34 days at

a dredging rate of 0.02m3⁄s. The total fine percentage in the dredged

sediment will be 2% (taken from subtidal sediment sample SU3). Since

the percentage of silt and clay in the sediment to be dredged is only 1.5%,

the dry density of the bed sediment is assumed to be 2,650 kg⁄m3

whereas the bulk density will be in the order of 2,000 kg⁄m3. The

percentage of fines spilled during the operation is assumed to be 7%.

Considering the above information, the spill flux is estimated to be 0.0576

kg/s. The formula used for spill flux is given below:

Spill volume (m3) = Volume of Dredged material (m3)*%fines*%spill of

fines

Spill flux= (Spill volume*Density of sediment)/Spill duration

6 R. N. Bray, J. M. Land, A. D. Bates. (1997). Dredging: A handbook for Engineers. Second Edition.

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Volume of dredged material per grab= 4.32 𝑚3

Spill duration = Cycle time of per grab = 210 seconds

Percentage fines in dredged materials= 2%

Percentage spillage of fines during one dredge cycle= 7%

Wet density of the dredged materials= 2000 𝑘𝑔

𝑚3⁄

Spill volume per grab (𝑚3) = 4.32 (𝑚3) *2%*7% = 0.006 𝑚3

Spill flux = (0.006( 𝑚3)*2000(𝑘𝑔

𝑚3⁄ )/210 (s) = 0.0576 kg/s

5.6 Plume Modelling

5.6.1 The main objective of the dredging plume simulations is to identify areas

which are potentially exposed to higher concentrations of suspended

sediments and to siltation of dredged sediments. By knowing the extent

and magnitude of the dredging plumes, the construction method and

schedule can be optimised to reduce the environmental impact on

sensitive areas, if necessary. This is assessed in volume 1, Chapter 10 –

Marine Ecology of the Environmental Statement (Document Reference:

6.1.10).

5.6.2 The sediment transport/dredge plumes have been modelled by the MIKE

21 Mud Transport module (MIKE 21 MT). The MIKE 21 MT module

describes erosion, transport and deposition of mud or sand/mud mixtures

under the action of currents.

5.6.3 All dredging activities have been modelled by using the spill flux

calculated in the previous section. Based on the particle size analysis

result of sediment sample SU3, two fractions were considered for

suspended sediment. One fraction corresponds to the silt whereas the

other one is a representative of fine sand. No initial suspended sediment

concentrations are present in the model. Also the bed layer is defined as

hard rock without lose sediments, only settled sediment from the dredging

activities can re-suspend into the water column. The dispersion

coefficients are set proportional to the current. The water flowing through

the boundaries has no suspended sediment concentration. Applying a

value for the background would elevate suspended sediment levels

concentrations found in the receptor areas.

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5.6.4 The settling velocity is dependent on the specific weight of the material,

kinematic viscosity and grain diameter. The settling velocities were

chosen from the chart given by USGS (Figure 5-4).

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Figure 5-4 Grain size and settling velocity7

5.6.5 The settling velocity of silt fraction is chosen as 0.0005m/s whereas for

the fine sand the settling velocity is chosen as 0.0026m/s. The critical

7 U.S. Geological Survey Open file report 00-358

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erosion shear stress is set to 0.15N/m2 whereas critical shear stress of

deposition is set to 0.07N/m2. The bed roughness is set to a Nikuradse

roughness of 0.001m.

5.6.6 The sediment plume modelling was carried out for four scenarios, similar

to the hydrodynamic modelling. The sediment plume modelling was

carried out for the baseline case which is does not contain the temporary

jetty structure. The scenarios considered for sediment plume modelling is

in Table 5-2. The simulations for each scenario were carried out for two

calendar days.

Table 5-2 Sediment plume modelling scenarios

Temporary jetty Option

Tide River flow

Without Neap Mean

Without Neap High

Without Spring Mean

Without Spring High

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6. MODEL RESULTS

6.1 Baseline hydrodynamics

6.1.1 Figure 4-1 to Figure 4-4 shows the simulated U and V velocity

components at the Silvertown temporary jetty site, with a comparison

against the HR Wallingford Thames model. This shows the baseline

hydrodynamic conditions at the site of the proposed temporary jetty

structure. Figure 6-1 to Figure 6-4 shows simulated current speeds for

baseline model conditions, without the temporary jetty or dredge area, at

selected times of flood and ebb tides. Figure 4-4 and Figure 6-4 show that

under the worst case hydrodynamic conditions, mean spring tide with high

river flow, simulated current speeds are between 1-1.5m/s.

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Figure 6-1 Baseline hydrodynamic flow conditions for mean neap

tides and mean river flow.

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Figure 6-2 Baseline hydrodynamic flow conditions for mean neap

tides and high river flow.

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Figure 6-3 Baseline hydrodynamic flow conditions for mean spring

tides and mean river flow

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Figure 6-4 Baseline hydrodynamic flow conditions for mean spring

tides and high river flow.

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6.2 Current speed difference

6.2.1 To determine the effect of the temporary jetty structure on the

hydrodynamic flow, comparisons of simulated current speed with and

without the temporary jetty are calculated for all flow conditions. An initial

spatial comparison is made where the simulations without the temporary

jetty is subtracted from the simulations with the temporary jetty, so that

any increase in current speed is shown by positive values while negative

values represent a decrease in current speed. The difference between the

simulations is calculated at every time step, accounting for the initial

model spin up time of 1 hour. The statistical mean is then calculated.

Figure 6-5 shows the difference in current speed under spring tide and

high river flow conditions.

Figure 6-5 Difference in simulated current speed for mean spring and

high flow conditions.

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Figure 6-6 Current speed with and without the temporary jetty

structure under mean neap tide and mean flow conditions.

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structure under mean neap tide and high flow conditions.

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structure under mean spring tide and mean flow conditions

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structure under mean spring tide and high flow conditions

6.2.2 Figure 6-5 shows that the temporary jetty would have a greatest impact on

the current speeds in the local area around the temporary jetty head. The

reduction in current speed around the temporary jetty head of the new

temporary jetty design is likely due to the drag influence caused by the

jetty head. Figure 6-5 also shows a slight increase in current speed

inshore of the temporary jetty due to the constriction of flow in this area

caused by the frictional influence of the jetty head and the constriction of

flow between the piles of the approach temporary jetty. Figure 6-6 to

Figure 6-9 shows the simulated current speeds with and without the

temporary jetty structure at the jetty head. A reduction in current speed

can be seen in all flow conditions and most pronounced around peak tidal

speeds. The reduction in current speed at peak ebb and flood tide is

within the range of 0.05m/s to 0.1m/s.

6.3 Sediment plume modelling

6.3.1 The results of the various plume modelling scenarios can be seen in

Figure 6-10 to Figure 6-21. The results of each scenario are presented as

a map of maximum and mean incremental increase in suspended

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sediment concentration (SSC) and total bed mass change over the

simulation period (i.e. 2 calendar days). Figure 6-10 to Figure 6-21 shows

that the maximum concentration of suspended sediments over the

simulation influences the entire length of the model domain, primarily on

the northern river bank, with greatest concentrations around the dredge

site. The mean concentration of suspended sediments is much lower and

found in the dredge location. This suggests that, due to the relatively high

current velocities, suspended sediments from the dredging operation

would be transported away from the dredge site. Relatively low

concentrations of suspended sediments will remain around the dredge

and temporary jetty location. The faster flow conditions of spring tides and

high river flows shows lower mean concentrations of suspended

sediments around the dredge location, this shows how localised effects of

the dredging operation would change with the local flow conditions. It

should be noted that the concentration of suspended sediments simulated

from the dredging operation will be considerably lower than the

background level of suspended sediment concentration. Therefore, there

would be a negligible impact on the surrounding environment.

6.3.2 The total bed mass change represents accumulated sediments released

during the dredge operation. The simulated results show that there is not

significant accumulation of sediments around the temporary jetty

structure. Higher flow speeds associated with spring tides and high river

flows would transport the suspended sediments to further reaches in the

model domain.

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Scenario 1: Neap tide with mean river flow

Figure 6-10 Map of maximum of Incremental SSC in mean neap tide

and mean river flow scenario

Figure 6-11 Map of mean of Incremental SSC in mean neap tide and

mean river flow scenario

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Figure 6-12 Map of total bed mass change in mean neap tide and


Scenario 2: Neap tide with high river flow

Figure 6-13 Map of maximum of Incremental SSC in mean neap tide

and high river flow scenario

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Figure 6-14 Map of mean of Incremental SSC in mean neap tide and

high river flow scenario

Figure 6-15 Map of total bed mass change in mean neap tide and


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Scenario 3: Spring tide with mean river flow

Figure 6-16 Map of maximum of Incremental SSC in mean spring tide

and mean river flow scenario

Figure 6-17 Map of mean of Incremental SSC in mean spring tide and


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Figure 6-18 Map of total bed mass change in mean spring tide and


Scenario 4: Spring tide with high river flow

Figure 6-19 Map of maximum of Incremental SSC in mean spring tide

and high river flow scenario

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Figure 6-20 Map of mean of Incremental SSC in mean spring tide and


Figure 6-21 Map of total bed mass change in mean spring tide and


6.3.3 The deposition of sediment is expressed as bed mass change which gives

the accumulated mass of sediment deposited per metre square area. The

simulation result shows that the deposition will be negligible except at

some locations. This is attributed to the fact that the current magnitude in

the domain is high, so deposition will be minimum and sediment will be

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advected to distant sites. However, at a small number of locations, a

theoretical maximum deposition is seen in the domain where there is eddy

formation. In these locations the deposition is in the order of 1-4 kg/m2.

6.3.4 In order to convert the siltation rate into increase in depth it is required to

know the dry density of the sea bed. The dry density depends on the sand

fraction and degree of consolidation of the fine sediments.

6.3.5 An estimate of the dry density can be found by referring Figure 6-22. For

half consolidated sediment with a sand fraction between 0.2 to 0.4 the dry

density is in the range from 800 kg/m2 to 1000 kg/m2.

Figure 6-22 Estimate of dry density of sea bed as a function of sand

fraction and consolidation

6.3.6 𝑋 ((1 − 𝑛)𝑌)⁄ During the two day simulation period, the theoretical

deposition at a small number of sites is 1-4 kg/m2 at some locations in the

model domain as seen in the plots shown above. The increase in

sediment depth is calculated as:

6.3.7 Where X is the accumulation of sediment mass, n is the porosity (0.4) and

Y is the dry density of sediment. In these small “pockets” the theoretical

deposition of sediment may be in the order of 2x10-3m to 6x10-3m (2-

6mm) considering the dry density of 800kg/m3 over the complete dredge

duration. However, it should be noted that these areas of accumulation

are not in the immediate region of the temporary jetty structure.

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6.3.8 Clearly this is a theoretic calculation at a small number of predicted sites

in the model domain. This has to be compared with the known natural

suspended sediment load which varies over the spring to neap tidal cycle

and on a seasonal basis with river flow and equinoctial tidal variation.

6.3.9 Previous studies of SSC in the River Thames have shown a seasonal

change with typical tide average values of 100 mg/l in summer, generally

low flow, and 50mg/l in winter, high flow, periods8. A study analysed two

years (2006-2007) of turbidity data to understand the turbidity maximum in

the Thames9 (Figure 6-23).The SSC sampling results for the 2006-2007

study show that for the Silvertown temporary jetty site (approximately

11km below London Bridge), concentration levels are within the range of

50-100 mg/l. Therefore the levels of SSC simulated from the dredge spill

modelling (Figure 6-10 to Figure 6-21) are negligible when compared with

the background levels.

8 Thames Tideway Tunnel. Application for Development Consent, Environmental Statement. Volume 3: Project-wide effects assessment appendices. Doc Red: 6.2.03 9 Mitchell S., Akesson L., Uncles R. (2012). Observations of turbidity in the Thames Estuary, United Kingdom. Water and Environment Journal.

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Figure 6-23 Suspended sediment concentration (SSC) in 2006-2007

in the River Thames.

6.4 Temporary jetty pile scour

6.4.1 Scour depth evolution around the temporary jetty piles was calculated

using the simulated depth averaged velocity. Understanding scour is

important as it may erode existing habitats and disperse river bed

sediments. Figure 6-24 shows the typical flow pattern around a temporary

jetty pile and the development of scour around it.

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Figure 6-24 Typical flow structure around a pile structure.

6.4.2 The sediment surveys have shown that the river bed is characterised by a

top layer of mobile sediment underlain by consolidated clay which is more

resistant to erosion. The method of Whitehouse10 was applied, which

defines the scour depth S(t) as a function of time using the following

equation:

𝑆(𝑡) = 𝑆𝑒 [1 − 𝑒𝑥𝑝 (−

𝑡

𝑇𝑠

)𝑛

] (1)

6.4.3 Where Ts is the time scale of the scour process given by equation 2, Se is

the equilibrium scour depth given by equation 4 and n is a power normally

assumed to be 1.

𝑇∗ =

[𝑔(𝑠 − 1)𝑑503 ]

12⁄

𝐷2𝑇𝑠

(2)


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6.4.4 Where d50 is the median grain size (m), g is the gravitational acceleration

(ms-2), D is the diameter of the pile (m) and T* is the dimensionless time

scale for currents given as:

𝑇∗ =

𝛿𝜃−2.2

2000𝐷

(3)

6.4.5 Where δ is the boundary layer thickness, assumed to be depth of flow for

tidal conditions and θ is the Shields parameter. The equation for the

equilibrium scour depth Se is given by:

𝑆𝑒 = 1.5𝐾1𝐾2𝐾3𝐾4𝐷 tanh (

ℎ

𝐷)

(4)

6.4.6 Where K1 is the correction factor for pile nose shape, K2 is the correction

factor for the angle of approach of the flow and K3 is the correction factor

for bed conditions, varying between 0 and 1 depending on flow conditions,

so that:

𝐾3 = 0 If 𝑈

𝑈𝑐𝑟< 0.5

𝐾3 = 2 (𝑈

𝑈𝑐𝑟

) − 1 If 0.5 ≤𝑈

𝑈𝑐𝑟< 1 (5)

𝐾3 = 1 If 𝑈

𝑈𝑐𝑟≥ 1

6.4.7 The parameter K4 is the correction factor for size of bed material, U is the

depth averaged current speed and Ucr is the threshold depth averaged

current speed. The scour depth methodology detailed so far has assumed

a gravely sand, non-cohesive sediment type. Previous studies have

shown that as the clay content of the sediment increases, the scour depth

ratio (St/D) decreases. The sediment clay content can be represented by

the use of a reduction factor multiplier on the scour depth. HR

Wallingford11 proposed a reduction factor Kcc, which represents the

fractional clay content C, with the expression:


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𝐾𝑐𝑐 =1

(1 + 𝐶)2

(6)

6.4.8 This reduction factor reduces scour depth in a mixed sand-clay sediment

type by approximately 70%, assuming an 80% clay content, as seen in

the vibracore sediment samples. Simulated current speeds were extracted

from the Silvertown model at two locations on the temporary jetty

structure, at the jetty head and on the approach jetty (Figure 6-25).

Figure 6-25 Location of current speed extraction (yellow markers) for

jetty pile scour depth calculations

6.4.9 Figure 6-26 and Figure 6-27 shows the scour depth evolution over time for

mean spring tide and high river flow conditions at the jetty head and

approach pontoons. Scour depth evolution for all tidal and river flow

conditions can be found in Appendix B. This shows a maximum scour

depth of 0.46m around the temporary jetty piles, which is approximately

0.45 times the diameter of the pile (0.45D). This is less than the 1.3D

scour depth value of Harris et al.12, however this could be due to the

inclusion of the reduction factor due to a 80% clay sediment type.


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6.4.10 The lateral extent of the scour around the temporary jetty piles is a

function of scour depth and the angle of repose of the sediment. A

laboratory study of tidal scour showed that lateral extents of 3.5 times the

predicted scour depth can be expected13. This method predicts a scour

width 1.61m scour width for the 1.016m piles. The slope of the upstream

and side edges of the scour hole will tend to be steeper than the

downstream edge (Figure 6-26). Therefore the extent of the lateral scour

will be greatest at the downstream edge of the temporary jetty structure,

and will therefore be less, parallel to the jetty head. The downstream edge

will also alternate with the flooding and ebbing tide. The lateral extent of

the scour will taper from the scour depth to the river bed. For cohesive

sediments, steeper slopes may be possible when compared with con-

cohesive sediments. Therefore the prediction of the lateral extent of scour

should be seen as a worst case scenario.

13 Escarameia M and May RWP (1999). Scour around structures in tidal flows. HR Wallingford report SR521

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Figure 6-26 Scour depth evolution over time for mean spring tide and

high flow conditions at the approach over two tidal cycles

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Figure 6-27 Scour depth evolution over time for mean spring tide and

high flow conditions at the jetty head over two tidal cycles

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7. PROPELLER SCOUR

7.1.1 Ship propeller wash has the potential to cause erosion or scour of the sea

bed and therefore induce instability near to the jetty structure. Previous

studies have shown that the Froude F0 number influences scour depth,

given by:

𝐹0 =𝑈0

√(𝑔𝑑50 𝛿𝜌 𝜌⁄ )

(7)

7.1.2 Where U0 is the flux velocity of the propeller, g is acceleration due to

gravity, d50 is median grain size, δρ is the difference in density of

sediment and fluid and ρ is the density of the fluid. For berthing vessels

using propellers, maximum jet velocity generally occurs when the vessel

is stationary or slow moving (Hawkswood et al., 2014). The maximum jet

velocity U0 is given by:

𝑈0 = 1.48 (

𝑃𝑏

𝜌𝐷𝑝2

)

13⁄

(8)

7.1.3 Where Pb is the engine power and Dp is the propeller diameter. The

maximum equilibrium scour depth is then calculated using the non-

dimensional formula of Hong et al. (2012):

𝑑𝑠,𝑡

𝐷𝑝= 1.171𝐹0

0.872 (𝑦0

𝐷𝑝)

−0.761

(𝑑50

𝐷𝑝)

0.34

(9)

7.1.4 Where y0 is the height of the propeller axis from the bed. The largest

vessel proposed to be moored at the Silvertown jetty is the Dolphin HAV,

with a gross tonnage of 2075t, length of 88.3m and draft of 4.64m. The

propeller diameter is assumed to be 2.5m, the minimum height of the

propeller axis from the bed is 2.25m and the engine power is taken as

1500kW. It should be noted that this method so far is calculating propeller

scour depth for a sediment type of gravely sand. However, vibracore

sediment samples have shown a mobile layer of non-cohesive sediment

overlying consolidated clay. This clay layer will be harder to erode and will

therefore reduce scour depths. Applying a similar scour reduction factor to

that applied to the time evolving scour around the temporary jetty piles

(Eq 6) will allow for an estimate of the reduction in scour depth due to the

presence of clay. This reduction factor reduces the scour depth due to

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propeller wash to 0.8m. It should be noted that this scour would be very

localised to the immediate area under the moored vessel’s propeller. This

section of the Thames is also subject to regular vessel movement e.g.

from the Thames clipper which docs just opposite the proposed

Silvertown temporary jetty.

7.1.5 The equation for maximum bottom velocity due to propeller wash is:

𝑉𝑏𝑚𝑎𝑥 = 𝐶1𝑈0𝐷𝑝 𝐻𝑝⁄ (12)

7.1.6 This generates a maximum bottom velocity Vbmax of 0.88m/s.

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8. CONCLUSION

8.1.1 The hydrodynamic modelling for the Silvertown Temporary jetty model

has shown that the inclusion of the temporary jetty structure would reduce

velocities around the jetty head. The reduction in current speeds is due to

the increased drag forces generated by the series of temporary jetty piles

aligned with the direction of flow. The reduction in current speed is within

the range of 0.05-0.1 m/s and is most prominent at peak ebb and flood

velocities. The sediment plume modelling results for all scenarios

considered shows that there would be negligible incremental impact due

to the proposed dredging work. The current speed in the river is high (on

the order of 1-1.5 m/s) and the proposed dredging rate is too low to have

any significant increase in SSC or sedimentation. Simulated SSC levels

from the dredging works, of approximately 5mg/l, are seen to be much

lower than the naturally occurring background levels of SSC, typically in

the range of 50-100mg/l.

8.1.2 Scour depths for the temporary jetty piles were calculated using the

simulated current speeds under all tidal and river flow conditions. These

show that for the indicative 1016mm diameter D piles a maximum scour

depth of 0.46m occurs at the approach jetty and jetty head. This is

approximately 0.45D, which is less than the 1.3D scour depth value of

Harris et al.14 , potentially caused by the inclusion of the clay reduction

factor. The lateral extent of the scour around the temporary jetty piles was

found to be 1.61m. It should be noted that these scour calculations

represent a worst case scenario, as sediment surveys show that the area

consists of a mobile layer of non-cohesive sandy gravel which overlays

consolidated clay.

8.1.3 Scour depth was also calculated as a result of the propeller wash of

vessels moored at the Silvertown temporary jetty. Worst case scenario

conditions were applied, with water depths of 1m and the largest potential

moored vessel used. Some assumptions were made for vessel

specifications, with propeller diameter set to 2.5m and engine power at

1500kW. A maximum scour depth due to propeller wash with 1m

underkeel clearance would be in the region of 0.8m, caused by a


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maximum near bed velocity of 0.89m/s. It should be noted that this scour

would be confined to the immediate area beneath the propeller.

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Appendix A. Sediment particle size analysis

Figure A-1 Particle size analysis result of intertidal sample IN1a

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Figure A-2 Particle size analysis result of intertidal sample IN2x

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Figure A-4 Particle size analysis result of intertidal sample IN5

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Appendix B. Scour depth evolution

Figure B-10 Scour depth evolution over time for mean neap tide and

high flow conditions at the jetty head over two tidal cycles

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high flow conditions at the approach jetty over two tidal cycles

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mean flow conditions at the jetty head over two tidal cycles

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mean flow conditions at the approach jetty orientation over two tidal

cycles

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Figure B-14 Scour depth evolution over time for mean spring tide

and mean flow conditions at the jetty head over two tidal cycles

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Figure B-15 Scour depth evolution over time for mean spring tide

and mean flow conditions at the approach jetty over two tidal cycles