Feasibility Study of Tidal Lagoon Power Generation in Severn Estuary

27 March 2015

Garrick Wong

Jack Kerr

1

Contents

Executive Summary .................................................................................................................................... 2

1.0 Literature Review ............................................................................................................................. 3

2.0 Initial Site location ........................................................................................................................... 8

3.0 Tidal Power Generation in the Severn ............................................................................................. 12

4.0 Lagoon Power Generation Modes ................................................................................................... 13

5.0 Modelling & Optimisation ............................................................................................................... 15

6.0 Geotechnical Report ....................................................................................................................... 18

7.0 Lagoon Wall Design ...................................................................................................................... 26

8.0 Turbine Housing Design .................................................................................................................. 35

9.0 Connection Detail .......................................................................................................................... 40

10.0 Construction processes ................................................................................................................... 41

11.0 Environmental Impact Assessment ................................................................................................ 44

12.0 Risk Assessment ............................................................................................................................ 48

References ................................................................................................................................................. 50

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Executive Summary

This report explores the feasibility of utilising Tidal Lagoon Technology within the Severn Estuary. An

original prototype design is proposed in response to the site conditions, wherein economy of materials

& function through form are achieved using an elliptical profile in plan. These design innovations

culminate in resource savings of almost 10% in dredged materials and allows economic use of varying

rock armour grades. Through the use of Power Optimisation Models, Two-way generation was deemed

most efficient with an estimated annual output of 17,183 MWh, with the potential to power 4,120 homes.

A ground model was developed through an iterative process, informed by geological maps, Borehole

Logs & the Geological History of the Severn Estuary. A design process was carried out for the proposed

Lagoon Wall & Turbine housing, taking into consideration; materiality & economy, hydraulic &

geotechnical details, in addition to the constructability and corresponding construction sequence.

Environmental & Risk Assessments were conducted to identify, highlight and mitigate potential hazards

introduced to the ecosystems of the Severn Estuary throughout the lifecycle of the proposed Tidal

Lagoon. The proposal is a pilot project that displays the viability of Tidal Energy in 21st Century man’s

hunt for clean & renewable Energy.

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1.0 Literature Review

Global Warming and the consequent climate change is the biggest challenge

facing modern man in the early 21st century, with a net change in global

climate of 2°C believed to result in ‘catastrophic’ Climate Change. In order to

remain below this benchmark figure there is a global Carbon ‘budget’ of 565

Gigatons of CO2 (Carrington, 2015). However if all oil reserves that have been

identified were to be burned, it would result in a release of 2,795 Gigatons of

CO2 (Carrington, 2015). A proactive response is necessary to find alternative

means of energy and in light of this the UK government has introduced certain

targets; namely an increase from the contribution of renewable energies to

15% of the country’s total energy demand by 2020 & a reduction of 80% in

the UK’s CO2 emissions from the 1990 baseline figure by 2050 (Department

of Energy & Climate Change, 2013). Tidal power represents a clean, safe and reliable renewable energy

source, and with the UK sporting over 11,000 miles of coastline, accommodating some of the largest tidal

ranges in the world, there has been a marked increase in the number of proposed tidal energy

developments. However there is much public concern over the environmental & integration impacts of

these large scale, intrusive schemes, and for this reason research in the last decade has become more

‘lagoon’ orientated, tending to shift away from the more extensive and potentially damaging barrage

developments.

1.1 Tidal Arrays (Marine Current Turbines)

Marine Current Turbines use the reliable tidal flows to produce

energy in a similar method to wind turbines. Tidal arrays offer a

less environmentally detrimental solution, than both lagoons and

barrages, having a much smaller impact on the existing coastal

environment. MCT in collaboration with Siemens and SeaGen

constructed a prototype tidal array in Strangford Lough, Northern

Ireland. The tidal array was completed in November 2008, with a

lease for a 5 year temporary deployment (Siemens, 2012). MCT

funded a 3 million ‘Environmental Monitoring Programme’ to survey the after effects of implementing the

scheme. Upon completion of the programme it was concluded that no major environmental impacts had

been detected (Siemens, 2012).

Figure 2 Proposed Tidal Array (Strangford Lough Tidal Turbine, United Kingdom, 2011)

Figure 1 Tidal Power Potential in the UK (Tidal Lagoon Swansea Bay, 2014)

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The array has an electrical output capacity of 1.2 MW. In

September 2012 the Array reached a milestone of having

produced 5 GWh since its commissioning in November

2008. The entire project had a total investment of

£12million, however this figure includes the highly

detailed EMP and research into original & innovative

technologies, the figure is likely to be considerably reduced for any ‘follow-on’ schemes (Siemens, 2012).

Table 1 Summary of Strangford Lough Tidal Array

The perceived success of the Strangford Lough Tidal Arrays has led to the proposal of further schemes

(Strangford Lough Tidal Turbine, United Kingdom, 2011);

8 MW Kyle-Rhea, Scotland

10 MW Anglesey Skerries Project, Wales

30 MW Lynmouth Tidal Current Array, Wales

A tidal array is not suitable for the proposed site (north of Avonmouth) despite the high tidal currents. The

Turbines require relatively deep water, only present along the shipping channels , and are more suited to

locations further out to sea along the Severn Estuary, were shipping lanes become less congested.

1.2 Tidal Barrages

Tidal Barrages create a permanent barrier between the estuary & the outer sea, producing energy using the

relative Head difference across the estuary & the sea-levels. Offering more predictable electrical outputs

than other types of tidal energy, however tending to have far more detrimental environmental risks. Recent

developments in technology and renewed concern for the environmental impacts have led to a move away

from tidal barrage technology.

Location Capacity

(MW)

Annual Generation

(MWh)

Cost

£ millions Constructed Environmental Impacts

Strangford Lough

Northern Ireland 1.2 circa - 1,300 12 2008 Negligible

Figure 3 Strangford Lough Tidal Array Construction (Siemens, 2012)

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Figure 4 La Rance Tidal Barrage (British Hydropower Association, 2009)

1.2.1 La Rance Tidal barrage

The planning of the La Rance barrage took 18 years and the

construction phase began in 1961 lasting 5 years. It is equipped with

24 10MW rated bulb-units. Its capacity is 240MW with actual

maximum output to be at 96MW (RANCE TIDAL POWER STATION,

FRANCE, 2014), producing 610GWh per year (Pierre, 1993).

The barrage cost €95m (1961 money), approximately €580m (2009) and took 20 years to pay for itself.

However the electricity the barrage generates is completely renewable, sustainable and clean, and has since

produced cheaper electricity than nuclear power. 1.8c/kWh with 2.5c/kWh (Wyre Tidal Energy, 2010). It now

brings in €2.2m in tax revenue and 70,000 visitors per year (British Hydropower Association, 2009) making

the barrage a viable tourist attraction.

The estuary initially suffered substantial impact when it was cut off to construct two dams and drained for

the initial years after completion. In addition Silting inland of the barrage was exacerbated due to the low

river flow and subsequently vegetation and fauna disappeared (British Hydropower Association, 2009).

However 10 years after completion, in 1976 it was observed that the habitat had been restored. Despite the

loss of mud flats the birds adapted to the new environments and the number of species returned to pre-

construction levels. By 1980 there were 100 worm species, 47 crustacean species and 70 fish species. (British

Hydropower Association, 2009).

La Rance barrage states a case for the less detrimental long term environmental impacts of a tidal barrage,

however it is considered highly inappropriate for the Severn estuary where there are diverse habitats &

migratory lands of unique environmental interest.

1.3 Tidal Lagoons

Tidal lagoons work in a similar way to barrages; trapping the tidal changes within an enclosed area and

exploiting the subsequent Head difference to produce energy. Unlike a barrage, the lagoon holds water in a

specific location within estuary, as opposed to holding back the entire estuary. Lagoons localise and

minimise the detrimental environmental effects of the barrage whilst maintain the positives, however at a

cost of significantly reduced power outputs.

Location Capacity

(MW)

Annual Generation

(MWh)

Cost

£ millions Constructed Environmental Impacts

La Rance

France

96

610,000

95

1966

Initial severe adverse effects –

construction phase

Long-term –less severe

Table 2 Summary of La Rance Tidal Barrage

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1.3.1 Swansea Bay Tidal Lagoon

A prototype Tidal Lagoon is currently in the design phase for Swansea Bay, UK. The artificial lagoon is to

impound an area of 5 km2 (Department of Trade and Industry; Welsh Development Agency, 2006), using

twenty-four 2.5MW reversible bulb turbines, with a max electrical output capacity of 60 MW. Using tidal

data gathered in an ABPmer Report and assuming turbine efficiencies of circa 85%, the annual power

generation of around 187,000 MWh was estimated by TEL. This was based on Neap tide variation of 4.1 m

& Spring tide variation of 8.5 m (AEA Energy & Environment, 2007).

The tidal lagoon is to be positioned in Swansea Bay, in a location of minimal disturbance to the existing

environment. Modelling is being carried out to ascertain the effects on the coastal erosion/deposition cycles

due to the construction of the lagoon (WS Atkins , 2014). In addition project brings ecological benefits, with

areas of the lagoon to be dedicated to extensive marine farming, as well as housing an educational centre

in the architectural centre-piece building, the Oyster (Friends of the Earth Cymru, 2004).

Table 3 Summary of Swansea Bay Tidal Lagoon

Location Capacity

(MW)

Annual Generation

(MWh)

Cost

£ millions Constructed Environmental Impacts

Swansea Bay

Wales 60

187,000 TEL

124,000 DTI/WDA

81.5

255

Planning

Phase

Under-review

Considered

Figure 5 Swansea Bay Tidal Lagoon Environmental study synopsis (AEA Energy & Environment, 2007)

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Figure 6 Swansea Bay sea wall Proposal Renders (WS Atkins , 2014)

The Swansea Tidal Lagoon scheme is still in the planning stage. Figures published by TEL, estimating the

costs and energy potential have been called into question by independent reports carried out by the DTI &

WDA, which have produced significantly different cost predictions, as well as annual generation figures

using the same tide data gathered in an independent study by ABPmer.

Tidal Lagoons are very suitable for the proposed site; the Severn’s significant Tidal Range produces a great

enough Head to make a smaller area viable and in addition the Severn estuary is home to a well-protected

ecological system, meaning environment impacts must be kept to a minimum.

1.4 Tidal Electric Power in the Severn Estuary

Severn Estuary has the second highest tidal range in the world, 13.0 m, (Environmental Agency, 2006) which

has potential to supply 7% of the Wales and England’s electricity demand. (Friends of the Earth Cymru,

2004). As such there have been many proposed tidal energy schemes hoping to harness the natures

potential in the Severn estuary, as yet none have been successful. In recent years the environmental

credentials and subsequent protection of the ecological systems within the estuary have been upgraded,

imposing stricter guidelines on hopeful tenders.

The estimated energy output has become a contentious issue between competing tenders; with the use of

different models and prediction methods creating discrepancies between proposal estimates, making direct

comparison difficult (AEA Energy & Environment, 2007).

Table 4 Estimate figures for Severn Estuary proposals (AEA Energy & Environment, 2007)

Figure 7 Map Locations for Severn Estuary proposals (AEA Energy & Environment, 2007)

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2.0 Initial Site location

The proposed site location places the lagoon on the

boundary of the intertidal mud-flats and the main

channel of the estuary. This location is outside of the

more stringent environmental control regions, in close

vicinity to Seabank Power station and also gives greater

ease of construction in the shallow water. The power

produced will be dependent on the available tidal range

and the lagoons volume of water; as such the turbines

will be placed in the deeper waters of the channel and

the enclosed area within the lagoon dredged.

2.1 Site Selection Process

The site was to be located north of Avonmouth & south of the second Severn Crossing. Initially a

geotechnical overview was carried out, followed by comparing and contrasting design factors for three

viable areas and finally the location within the preferred area was nominated with the aim of minimising

environmental impacts.

2.2 Geotechnical Aspects

The geotechnical aspects fundamentally affect construction methods, scheme design

and the levels of complexity.

Desktop studies failed to find borehole logs from the proposed site, thus a collection

was gathered from the Bristol-side shore and within the channel north of the site. It

was found that the ground on the shore consists largely of marl & sandstone, whereas

to the north of the site it is mainly a layer of siltstone atop sandstone. It was deduced

that the geology of the area varies relatively little within the Avonmouth-Severn

Bridge boundary; thereby not representing a critical factor in the initial site selection

2

3

1

4

Dredging

Channel Side Coast Side

Seabank

Power Station

A’ A

Figure 8 Proposed Site location for Prototype Severn Tidal Lagoon Admiralty Charts 1066, 1076

Figure 9 BGS Borehole positions (British Geological Survey, 2015)

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Having concluded there were no geotechnical critical areas within the

site, three areas were considered;

Site A – Deeper water Site within the channel

Site B – Boundary Site along the boundary of the intertidal mudflats

Site C – Shallow Water Site atop the intertidal mudflats

Various factors were consider for each site, shown in tabulated form

below. It was concluded that Site ‘B’, along the boundary, was the most

suitable.

A note on Turbine Placing

The turbine housing will be placed channel side to exploit the higher tidal

fluctuations and to prevent sedimentation build-up at the turbines. The higher tidal

currents and faster moving water within the channel will avoid turbine

sedimentation issues, which otherwise may necessitate annual dredging.

Sedimentation around the perimeter walls is likely, however this should lead to

reduced hydrostatic loading and increased stability.

Possible

sedimentation

around the

Lagoon perimeter

Figure 10 Viable Location Areas

Table 5 Deep - Shallow water comparison

Figure 11 Sedimentation Issues

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2.3 Environmental Impacts Minimised

The proposed site lies along the boundary site B, in a location of minimal environmental disturbance

Table 6 Environmental Impacts comparison

2.4 Wind, Fetch & Angle of Attack – Function

through Form

The site is subject to a prevailing South-westerly

wind, which combines with the long fetch provided by

the shape of the Severn estuary to create a

predominant direction of wave attack from the SW.

This raises the intriguing prospect of delivering a

more economic and efficient design through the

manipulation of the lagoon form.

The ideal form minimises the area open to frontal

attack, in addition to being relatively aerodynamic, so

as not to create any ‘lee-space’ where increased

deposition could occur. In this case an ellipse was

proposed, with the major axis aligned with the

predominant angle of attack, minimising the frontal

area, whilst allowing water to flow freely around the

Designated

Areas

Key Inshore Special

Area of

Conservation with

Marine

Components (GB)

Special Area of

Conservation

Waders and

Wildfowl Autumn

Mean Peeks (GB)

Inshore Special

Protection Area

with Marine

Components (GB)

RAMSAR sites for

England & Wales

Special Protection

Areas

Sites of Special

Scientific Interest

Habitats

Important Bird Areas

(GB)

Intertidal regions of

the Severn estuary are

protected for their

significance to GB bird

species

Site Issues Proposed site lies with

the designated areas

Proposed site lies

outside of the

designated areas

Proposed site lies

outside of the

designated areas

Proposed site lies on a

small Mud/Shingle

habitat area

Proposed site lies outside

of the designated areas

Proposed

location

Figure 12 Predominant Wind Direction & Wind Rose (Avonmouth Windrose Data 1991-2000, 2001)

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perimeter (no corners). In addition, an ellipse is a highly efficient perimeter length: internal area ratio, saving

further on materials.

2.5 Economy of materials

Preferential Armouring will be employed, with the Lagoon walls more heavily armoured along the South

Westerly faces, allowing lower grade rock armour to be utilised on the North Easterly faces. In addition there

will be a gradual decrease in height from southwest to the northeast of the lagoon, with a maximum height

difference on 1.5m, with a saving of 22% volume of material per metre length for the lowest wall heights

(23.5m) relative to the highest. (25 m). It could be noted from the wind vane that the secondary most

prominent angle of wind attack is in fact from the Northeast, however this corresponds with very shallow

water and negligible fetch.

2.6 Summary

The Lagoon is to situated along the intertidal mudflat boundary in a location of minimal environmental

impact. The turbines are to be located in the deeper channel, where the entire tidal range can be exploited

and sedimentation issues are minimised due to fast flowing water. The lagoon will be an elliptical profile, to

create an optimum economic design, with

preferental armouring regions and a varying

height across its length.

Higher grade Rock Armour &

increased Wall Height

Turbine Housing

Figure 13 Turbine shape definition

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3.0 Tidal Power Generation in the Severn

The Severn Estuary is located in a semi-diurnal tidal region i.e. there are two high tide – low tide cycles

approximately every 25 hours. This gives the potential to generate power twice (Ebb generation) or four

times (Flood & Ebb generation) a day.

Tidal Power is a reliable and easily predicted source when compared to other methods of renewable energy,

such as wind which is notoriously unreliable & difficult to accurately predict for the future. In addition the

design life of tidal lagoons is in the vicinity of 120 years, in comparison to the significantly reduced longevity

of wind power infrastructure, typically 20 years. It is also regarded as one of the safest means of renewable

power generation; with the risk of major accidents occurring when in operation is very low.

3.1 Site Specific Tidal Information

The turbines are placed in the deeper water of the channel; as such the full tidal range is harnessed for energy

production.

3.2 Lagoon Design Considerations

Turbines

The turbines require full submergence to operate. This could result in a loss of tidal

Head, and thus power generation. In order to mitigate these effects, the turbines

will be ‘trenched’, i.e. placed lower than the lowest tidal levels. As the turbines will

be deliberately placed in the deeper water of the channel, the extent of the

trenching depth should be minimal, only enough to keep the turbines submerged

in the 1:100 low tide levels.

3.3 Plan Area

In reality due to the nature of the design of the seawall

not being vertical the plan area is 150,000m2 and

decreases as it becomes shallower. Therefore in order

to simplify the simulation the plan area is taken as a

constant 150,000m2.

Tide Type High (m) Low (m)

Spring 13.64 1.21

Neap 9.81 5.04

Mean 12.01 2.23 Table 7 Tid Levels Avonmouth Data (January 2012)

Figure 14 Proposed trenching system

Figure 15 Dredging details

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4.0 Lagoon Power Generation Modes

There two possible methods of power generation from a tidal lagoon; Ebb Generation & Two-way

Generation. Both utilise low Head (maximum 14m), as such reaction turbines are the only viable option for

this specific site. There

4.1 Ebb Generation

Ebb generation entails generating power from the outgoing tide. It works by allowing the lagoon to flood

with the incoming tide then holding back the water at high tide, then subsequently releasing it at an

optimised time to make best use of the Head difference between the retained Head and the low tide Head

of the estuary. Ebb generation produces energy approximately twice a day. The process is illustrated below:

1. Lagoon fills up as tide level rises

2. Water is contained in the lagoon while the tide level of the estuary decreases

3. Release lagoon water at optimised times to maximise power output

4. Water levels in both the lagoon and estuary

5. Rising tide repeats the cycle

The graph displays the water levels in the lagoon relative to the estuary. The draining of the lagoon is

delayed to allow the build-up of a Head difference, thus maximising the flow rate and power output.

1 3 4 5 2

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4.2 Flood & Ebb Generation

Flood & Ebb generation allows the generation of power from both the incoming and outgoing tide. It works

by holding the water levels at both the higher tide & lower tide times, using the subsequent head difference

to generate energy from both phases of the tidal cycle, thus energy is produced approximately four times a

day.

The advantage of this is a steady, continuous supply of electricity directly to the national grid, which is

particularly suitable as a primary source of electricity. Whilst it may appear obvious two-way generation is

superior one-way, current turbine technologies can only achieve approximately 50% efficiency (compared

85% of one-way flow).The whole process is illustrated below:

1. Tide level rises in the estuary, the lagoon is held constant

2. Power is generated as the lagoon fills up

3. Tide level in estuary drops, the lagoon is held constant

4. Power is generated as the lagoon drains

5. Both estuary and the lagoon return to low tide level

1 2 3 4 5

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5.0 Modelling & Optimisation

In order to simulate the power generation with respect to the change in tidal levels a realistic model was

required. This was achieved as follows;

5.1 Modelling the Tidal Range

The tidal fluctuations were modelled as a sinusoidal curve, with the amplitude and datum values retrieved

from the Avonmouth Tide data over a

period of several years. The Curve has

a period of 750 minutes,

corresponding to 12.5 hour cycle.

Therefore there are slightly less than

two cycles a day, this was taken into

consideration when calculating the

annual output figures.

Generation Strategy Power Output Turbine Efficiency Recreational Use

Ebb Generation Two peaks – not ideal for a

primary energy source

The turbines tend to be more

efficient in this mode of

operation, with η=0.85

The lagoon water levels

fluctuate less frequently

Flood & Ebb

Generation

Four peaks, spread across the

day – more suitable to primary

energy adaption

The turbines tend to be less

efficient in this mode, with

η=0.5

The lagoon water levels

fluctuate more frequently

Table 8 Comparison of power output of different modes

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200 1400 1600Est

ua

ry W

ate

r Le

ve

ls

Minutes

Sinusoidal Curve of Mean Tidal

Range

Figure 16 Optimisation logical sequence

16

0

5000

10000

15000

20000

25000

30000

0 2 4 6 8 10 12

Po

we

r O

utp

ut

KW

h/3

7.5

hrs

Number of Turbines

FLOOD & EBB Flow Power Optimisation

1.5m Turbine Diameter 2m Diameter Turbine 2.5m Diameter Turbine 3m Diameter Turbine

5.2 Optimisation Study

The power output for both ebb & two-way generation were optimised for the mean tidal variation, the

variables being turbine size, turbine diameter and the extent of the holding period. It was considered

improbable that an array of turbines of varying diameters would be employed, as this would instigate higher

costs, and as such this option was not investigated.

EBB Flow

This graph represents the power output vs Number of turbines for the various turbine diameter options. For

each case the holding time was optimised for maximum power generation. The graph above represents the

ebb flow generation, below the Flood-ebb generation.

Ebb & Flood Generation

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 2 4 6 8 10 12

Po

we

r O

utp

ut

KW

h/2

5h

rs

Number of Turbines

EBB Flow Power Optimisation

1.5m diameter turbine 2m diameter turbines

17

5.3 Output & Analysis

Following the optimisation study, for both ebb & two-way flow three 3m diameter turbines were deemed

most suitable. It should be noted that the study was carried out using mean tide levels and as the efficiency

is not linear, this may not represent a fully accurate output prediction (much lower outputs at neap & much

higher at spring shown in table 10). However for the purposes of this report, mean tide data was deemed

sufficient. The annual Power output was estimated for both ebb & two-way flow using three 3m diameter

turbines, the results are shown in the table below.

Ebb Generation Flood & Ebb Generation

Annual Power Output

Estimation

11,242 MWh 17,183 MWh

Number of Turbines 3 3

Turbine Diameters 3 m 3 m

Table 9 Power output and turbine sizing comparison

5.4 Conclusion

Flood & Ebb flow Generation produces 53% more power annually in comparison to Ebb flow generation.

Therefore this study concludes that for the proposed Severn Estuary Tidal Lagoon Flood & Ebb generation

is advised. Table 10 shows the expected power output.

No. of

Turbines

Turbine

Diameters

Mean Tide

Output/day

Neap Tide

output/day

Spring Tide

Output/day

Monthly

Generation

Estimated annual

Generation

3 3m 47.1 MWh 23.4 MWh 51.723 MWh 1431 MWh 17,183 MWh

Table 10 Optimisation of Flood&Ebb flow Generation

The average adjusted electricity consumption per household in 2013 was 4,170 kWh once a temperature

factor has been applied to the data ( (Department of Energy & Cliamte Change, 2014). As such the planned

Severn Tidal Lagoon has the potential to supply electrical energy to 4,120 homes. The table below gives an

indication of the environmental cost of producing the predicted annual generation (17,183 MWh) using fossil

fuel alternatives.

Table 11 Carbon emission comparison with fossil fuels

Assuming a rough estimate each tree is locking up 0.546 kg of carbon per year – equivalent to 2 kg of

carbon dioxide. (The Forestry Commission, 2014)

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6.0 Geotechnical Report

This section overlooks the geotechnical feasibility of the scheme, highlighting the main areas of concern

and possible solutions. In the first section a ground model is developed through an informed iterative

process, involving updating and refinement. Having defined and set parameters on the assumed ground

model, certain design problems are explored and checked in the Lagoon Wall Design (section 7.0) & Turbine

Housing Design (section 8.0).

6.1 Geological History of the Severn Estuary

Geological maps of the

Severn Estuary illustrate that

the bedrock is comprised of

an upper layer of relatively

soft Jurassic (150 – 200 Ma)

and Triassic (200 – 251 Ma)

rocks overlying harder

Carboniferous (290 – 360

Ma) and Mid-Devonian (360

– 410 Ma) rocks (Crowther,

Dickson, & Truscoe, 2008).

Figure 17 shows a simplified

map of the ‘surface’ bedrock

for the Severn Estuary area. Folding (anticline & syncline formations) has led to the older Carboniferous &

Devonian rocks becoming exposed. Proposed to the south-east of the map are highly simplified lines of

folding, however further north towards the intended site, the geological situation becomes more complex;

with several folding lines converging It can however be interpreted that the site lies in something of a

syncline, the surface bedrock being Triassic Mercia Mudstone Group, overlying Carboniferous Limestone

atop Devonian red Sandstone.

6.2 Triassic Rock Formations in the Severn

In Britain the Triassic period rocks are the Sherwood Sandstone Group

(formed in earliest periods of the Triassic Era), the Mercia Mudstone

Group/Keuper Marl and the Penarth Group (formed in the late Triassic)

(Hobbs, Hallam, Forster, Entwisle, & Jones, 2002). The layers are

stratified with the youngest overlying the elder, Figure 18. In the Severn

Estuary the Mercia Mudstones are interbedded with halite, gypsum &

Anticline Syncline

Anticline

Figure 17 Simplified geology of the Severn Estuary (Crowther, Dickson, & Truscoe, 2008)

Figure 18 Triassic Rock Formations (Hobbs, Hallam, Forster, Entwisle, & Jones, 2002)

19

anhydrite deposits in addition to, sometimes thick, bands of sandstone (Hobbs, Hallam, Forster, Entwisle,

& Jones, 2002). Figure 17, shows a strata of Arden Sandstone; in the Severn Estuary the equivalent layer is

named Butcombe/North curry Sandstone, and was formed in the same short-lived episode of deltaic

deposition (Hobbs, Hallam, Forster, Entwisle, & Jones, 2002), both members consisting of grey-green

siltstone, mudstone & thick sandstone beds.

6.3 British Geological Survey Maps

Information from the ‘Bristish Geological Viewer’ coincided

with earlier findings, identifying the surface bedrock

sorrounding the site as Mercia Mudstone Group – Sandstone

& Mudstone dating back to the Triassic Period (200 – 251

miilion years ago). Mercia Mudstone groups or Keuper Marl,

is a formation of mudstones, siltstones, sandstone & halites.

The Marl is dominantly red, less commonly green-grey,

mudstones and subordinate siltstones with strata’s of

sandstone present. Physical properties of the layer range

from strong clays to weak rocks, with strength tending to

increase with depth and being dependent on the extent of

weathering.

6.4 Superficial Deposits – Surface Geology

Information from the ‘Bristish Geological Viewer’ identified the

surface geology sorrounding the site as Tidal Flat deposits

consisting of Clay, Sand, Silt & Gravel; commonplace in shoreline

areas, dating back to the Quaternary Period (2 million years ago).

Normally a consolidated soft silty clay, with layers of sand and

gravel.

Clay & silt deposits in coastal estuarine deposits exhibit pronounced

horizontal stratification resulting in a marked anisotropic

permeabilbity, with horizontal permability being orders of

magnitude gretaer than the vertical (Fell, MacGregor, & Stapledon,

1992). in lagoon design foundation seepage & permeability are

critical to design, and as such the behaviour of the soils should be understood.

= Mercia Mudstone

Group – Mudstone

= Mercia Mudstone

Group - Sandstone

= Tidal Flat deposits consisting of Clay, Sand, Silt & Gravel

Figure 19 Severn Estuary Bedrock BGS

Figure 20 Severn Estuary Surface Geology

20

Figure 22 Simplified Borehole Logs

6.5 Borehole logs

A collection of borehole logs have been retrieved from the BGS website to

further inform the ground profile of the site. In summary the boreholes agree

strongly with earlier findings. Boreholes taken for the construction of the

second Severn crossing display ground profiles with Keuper Marl Sandstone

& Mudstone surface geology. The ground profile along the Severn Crossing is

mainly of Sandstone & Siltstone. Little or no argillaceous tidal flat deposits

are evident along the Severn Crossing which is thought to be the main reason

for its site selection.

Three existing borehole logs were then checked on the East coast of the estuary, parallel to the proposed

site location. The boreholes were the closest onshore boreholes to the proposed site, were aligned linearly

in line with the site and each were in excess of thirty metres deep.

These boreholes were used in conjuction with other logs from along the shore & across on the west shore

tto compile an ‘average’ borehole that summed up the geotechnical situation. This borehole is shown in

table 12.

& Marl

& Marl & Marl

& Marl

Figure 21 Borehole Locations

21

Table 12

ALLUVIUM Clay-rich Tidal Flat deposits of clay, silt, sand & gravel ( 2 - 10 m)

The boreholes exhibited an uppermost layer of clay-rich tidal

flat deposits consisting of bands of Clay, Silt, Sand & Gravel

(alluvium). This was underlayn in some causes by a thin band

of gravel, however this feature was not commonly observed

throughout the local boreholes

KEUPER MARL Marl interbedded with sandstone bands (35- 40 m)

In the majority of bores the upper tidal deposit layer was

overlaying Keuper Marl; in this case predominantly red marl

interbedded with fine pale green sandstone bands. The layer

was generally described as ‘hard’ with some reports

commenting on the a three and a half hour delay due to

neccesity of having to chisel through the deeper marl &

sandstones strata. This strata of Marl bedded with sandstone

continued to an approximate depth of 45m on the east shore

BUTCOMBE SANDSTONE Pennant-like Sandstone (20 – 25 m)

A thick purple stained, grey hard fine-grained pennant-like

sandstone with coal measures was reported. This layer was

found to be substantial, continuing to an approximate depth

of 70m. It was considered likely that this strata may represent

the Butcombe/North curry Sandstone

KEUPER MARL Mudstone with coal present (40 m)

A thick strata of grey, slightly silty mudstone, getting darker

with depth and with bands of coal present. This layer

continued to approximate depth of 105m

SHERWOOD SANDSTONE/ UPPER DRYBROOK SANDSTONE (unknown)

A thick layer of sandstone was situated, thought to be the

Sherwood Sandstone Group or perhaps the Upper Drybrook

Sandstone of the Late Carboniferous Period explainng the

coal measures present in the strata above, a hypothesis

agreeing with the second Severn Bridge Crossing ground

profile

Table 12 Borehole Stratification Analysis

6.6 Second Severn Crossing Ground Profile

Figure 23 depicts the ground-model used in the construction of the second Severn Crossing. The model

agrees strongly with many of the findings stated earlier. In addition this information highlights the existence

of geological faults in close proximity to the site. Further investigation will be required to ascertain the full

affect these features will have on the proposed Tidal Lagoon, e.g. off-shore boreholes extracted at the site

and along the major axis, to find inconsistences across the site. The East side of the model corresponds

strongly to the gathered data.

Figure 23 Severn Estuary Ground Build-up

22

6.7 Design Ground Model

6.8 Specific Site Ground Model

i) The depth of the alluvial soils ranges from 2-6m across the site, post dreging constant 2m. As

such the alluvial soils in design are stiffer than the normally consolidated superficial strata.

ii) The extent of the weathered Mudstone layer is typically between 10-15 m, however it may be as

deep as 30m (Hobbs, Hallam, Forster, Entwisle, & Jones, 2002). For the site ground model the

depth of weathered Mudstone has been assumed as 10m.

6.9 Design Parameters

In order to proceed with the geotechnical design, mechanical properties have to be assigned to the ground

model. Mercia Mudstone groups are classed as a problematic soil, due to variable mechanical properties

10m

0m

10m

20m

30m

40m

50m

60m

70m 725m

Proposed

Site

10m

2m

25m

Assumed

Bedrock

Alluvial Soils

Weathered Mudstone

UnWeathered Mudstone

Butcombe Sandstone

Alluvial Clays

Weathered Marl & Sandstone

Unweathered Marl & Sandstone

Butcombe Sandstone

Keuper Marl Mudrock

West East

Figure 24 Design Ground Model

Figure 25 Specific ground model

23

depending on the rocks stage of development & extent of weathering. For the subject of this report design

properties from similar case studies will be employed, however it is heavily advised that thorough ground

investigations are carried out.

6.9.1 Alluvial Soils

Alluvial soil consist of bands of gravel, sand, silt &

clay. The layer may be highly variable across the site,

with a thorough geotechnical study highly

recommended to confidently and accurately design

the lagoon. The uppermost layers tend to represent

recent deposists which are normally consolidated,

highly compressible, agrillaceius and exhibit far increased horizontal permeabilities (orders of magnitude)

due to the mechanical prcess of deposition. The lower layers represent tidal deposition from the last 2

million years, exhibiting similar properties with respect to permeability and clay-rich consistancy, tending

to be stiffer than the upper layers, with correspondingly higher undrained shear strengths 40 – 80 kN/m2

(Hawkins, 1984). In the ground model suggested, the overlying strata of alluvial soil is representing this

higher strength band. The recent, normally consolidated band is likely to have been largely eroded towards

the faster flowing channel and is to be dredged in the mudflat regions.

6.9.2 Mudstone Classifications

Mercia Mudstone groups are subdivided into zones, depending on the extent of weathering and thus

mechanical behaviour. Figure 19 shows the classification system proposed by Skempton & Davis and used

throughout this report (Skempton & Davis, 1966)

Figure 26 Anisotropic Permeability (Fell, MacGregor, & Stapledon, 1992)

Figure 27 Mercia Mudstone group zonal classifications (Skempton & Davis, 1966)

24

25

Figure 28 Summary of geotechnical tests (British Geological Survey, 2002)

6.10 Recommended Geotechnical Testing

*Using Wire-line double tube core barrel with continuous core liner & estuary

A summary of geotechnical testing methods is outlined in Fig.28. These tests are essential to establish soil

conditions reliably.

26

Figure 30 Breakwater schematic

Figure 29 Lagoon wall dimension

7.0 Lagoon Wall Design

7.1 Lagoon Wall Build-up

Precast concrete caissons were considered for the lagoon wall, but due to the length of the wall (1624 m)

and the elliptical shape (difficult to precast curves – not constant dimensions) a rock mound wall was

considered more appropriate (using Geotubes & pumped dredged material). In addition the rock mound

wall uses lower energy and recycled onsite materials.

Turbine Housing

252 m

756 m

60 m

Dredged filled Geotubes

Toe berm protection

Heavy Geotextile & Rock Underlayer

2.5-6 t Armour Rock Layer

Dredge filled Argillaceous Core

Scour Protection

0t – 0.3t rear face protection

27

The core will comprise of a compacted dredged argillaceous fill, packed within a permanent Geotube

falsework structure laid out on the estuary bed. The Geotubes will be pumped with onsite dredged alluvial

soils, akin to those used in the fill. Dredging from the site will produce 525,000 m3 of reusable material, with

the overall required volume estimated at just over 1 million cubic metres (1,023,000 m3). The remaining

necessary fill will be sourced locally from licensed dredging areas within the Severn estuary (see Dredging

Section 10.1).

Immediately on top of Geotubes & fill core there will be a sand gravel layer, protected by a Heavy Geotextile

& Rock Underlayer. Upon the seaward face will be rock armour grading from 2.5 – 6t. On the rear ward face

there will some protection against overtopping, with rock armour ranging from 0-0.3 t (note there will be no

rearward rock armour to the northeast of the lagoon. The structure will be topped with a concrete cap (wave

wall superstructure).

7.2 Design Dimensions

The lagoon wall is 1,624m in length around the circumference of the ellipse. The Height varies from 25 –

23.5 m along its length, and the width varies from 96.4 – 90.4 m, resulting in 7% saving in dredged

materials (117,000 m3). The rock armour is 6 tonnes along the South westerly face, reducing to a minimum

of 2.5 tonnes along the North easterly face.

7.2 Lagoon Wall Height

The height of the design height of the lagoon wall has an impact on the required width, the volume of

material necessary and the thus the economy of design. As such the height should preferable be kept to an

absolute minimum, whilst also performing to standard. The design height of the lagoon wall is dependent

on four factors;

i) The depth of the water + the maximum tidal range

ii) The forecast increased sea levels due to climate change for the intended design life (120 years)

A A’

B

B’

25 m 23.5 m

96.4 m 90.4 m

28

iii) The future significant wave height (mean of the top third waves) dependent upon design life and

the acceptable probability of overtopping

iv) The Wave run-up.

i) Tidal Range & Water Depth

The intended site has maximum depth of 2m and a maximum tidal range of 14m, coresponding to a

required height of 16m for the first factor.

ii) Climate Change – Sea Level Rise

The effects of global warming and climate change have

particular importance to coastal infrastructure. Sea level-

rises are forecast to be approximately one metre globally by

2100 (Scambos & Abraham, 2014). This should be taken into

consideration for long-term coastal infrastructure design,

such as this proposed Avonmouth Severn Lagoon. The

structure should be appropriately designed, so as to be fit

for purpose throughout its entire lifespan.

In addition to this global sea-level rise, the melting and

collapsing of Ice sheets (namely the WAIS west Antarctic ice

sheet) is likely to cause an amplified effect on the Northern

Hemisphere sea-level rises, with a ‘redistribution of water in

the changes gravitational field (due to loss of mass at the

Antarctic) resulting in a greater than average sea-level rise in the Northern Hemisphere (Scambos &

Abraham, 2014). In response to these findings, there will be a 1.1 m proposed additional height added to

negate the effects of climate change sea level rises.

Increased Hs

Climate Change Sea Level Rise

Tidal Range +

Safety Factor

Height of

Sea Wall

Wave Run-up

Figure 32 Sea level rise due to global warming (Scambos & Abraham, 2014)

Figure 31 Total height of breakwater

29

iii) Predicting Significant Wave Height

The Significant Wave Height for a year is equal to the mean of the top third wave heights recorded that year.

Using wave data collected at Avonmouth over a 20 year period, between 1980 & 1999, the future Hs can be

extrapolated using mathematical functions, in this case logarithmically using the Gumbel Distribution.

The Gumbel distribution

was used to predict the Hs

up until a 225 year return

period. The next step was

choosing the appropriate

return period for our

structure.

The Lagoon is to be designed for a 120 year design life, typical for coastal structures. As the structure will be

off-shore with little human footfall (bar maintenance personnel) and there will be some minimal rear

armouring; a 50% chance of overtopping throughout its design life was deemed acceptable.

𝑃(𝑜𝑣𝑒𝑟𝑡𝑜𝑝𝑝𝑖𝑛𝑔) =1

𝑇

𝑃(𝑛𝑜 𝑜𝑣𝑒𝑟𝑡𝑜𝑝𝑝𝑖𝑛𝑔) = 1 −1

𝑇

𝑃(𝑛𝑜 𝑜𝑣𝑒𝑟𝑡𝑜𝑝𝑝𝑖𝑛𝑔 𝑖𝑛 120 𝑦𝑒𝑎𝑟𝑠) = (1 −1

𝑇)

120

𝑃(𝑛𝑜 𝑜𝑣𝑒𝑟𝑡𝑜𝑝𝑝𝑖𝑛𝑔, ℎ𝑎𝑝𝑝𝑒𝑛𝑖𝑛𝑔 𝑖𝑛 120 𝑦𝑒𝑎𝑟𝑠) = 1−(1 −1

𝑇)120

𝑇 = 174 𝑦𝑒𝑎𝑟𝑠

Summary For a 50% chance of overtopping in a 120 year design life, had a corresponding return

period of 174 years, thus producing a Hs of approximately 3 metres. The 3m figure is

measured from the trough to the crest of the wave, thus a 3m Hs results in an additional

height of 1.5m.

iv) Wave Run-up

Wave run-up was estimated using the approximate equation

Which was found to be 5.4 m for the lagoon wall – representing a critical consideration.

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250

De

sig

n W

ave

He

igh

t

Return Period

Gumbel Distribution

30

7.3 Design Height Conclusion

Therefore summing the contributions from all the factor the required design height of the wall was 24m.

One extra metre was added by means of a safety factor, making the design height of the wall 25m. As

previously mentioned the wall height will vary from 25m down to 23m in the north east corner, 23.5m

corresponds to the halving of Hs, due to the shape of the lagoon.

7.4 Rock Armour – Van der Meers Formula

The Van der Meer equation was used for preliminary sizing of the rock armour units. Unlike the Hudson

formula, the Van der Meer equation can be applied to impermeable and over-topped structures, as well as

giving an indication of damage levels and taking into account wave period and storm duration.

The sizing of the rock armour is conducted through Van Der Meer formulae. The diameter of the rock armour

is determined to be 2.39m, which amounts to 6.0 Tonnes for each rock. It is to be applied to the estuary side

of the lagoon as the primary defence.

Overtopping has been identified to be taken into consideration thus protection on the lagoon side is deemed

necessary. Since large waves are not expected inside the lagoon and the impact from overtopping is low,

only minimum grade of rock armour is applied to the lagoon side.

7.5 Lagoon Wall Failure Modes

Toe Erosion

Liquefaction of Subsoils

Slope Failure

Overtopping

Crest Erosion

Leeside Damage

Hydraulic damage

Internal Erosion &

Erosion of Subsoil

Cap Movement

Figure 33 Possible failure modes

31

7.6 Lagoon Wall Geotechnical Design

In this section the geotechnical design of the lagoon wall & turbine housing are considered.

Permeability Checks…

In order for the lagoon wall to operate properly,

seapage must be be kept to a minimum. There are

two types of seepage;

i) through the wall

ii) through the foundation beneath the wall.

Option 1 is prevented through the use of geotubes

filled with compacted dredged material. Option 2

is slightly harder to prevent, but first calculations

were carried out in order to check the need for

action…

The relative permeabilities of the alluvial layer (5 x

10-6 ) and the weathered mudrock layer (3 x 10-6 ),

were for simplicity considered equal. Thus creating

a permeable strata 12m deep.

Seepage Flow net solution

In order to account for the anisotropic behaviour of

flow through the layer, a simple scaling factor was

used; √𝑘𝑣𝑘ℎ

⁄

Initial Sizing – Sliding & Overturning Checks…

𝜌𝑔𝐻𝑤2

2−

𝜌𝑔𝐻𝑙2

2= 𝑊∗𝛼𝑡𝑎𝑛𝜙′

Uplift pressure, resulting from the Head difference

either side of the lagoon wall, produces a

considerable eccentric uplift force. This acts against

the weight of the wall, reducing the resisting

frictional force on the base. Using a prudent safety

factor the required Width to prevent sliding was

found to be 96.4 m.

Overturning was found to be non-critical; this was

expected due to inherently stable triangular profile

of the lagoon wall.

In order to resist sliding a relatively large width is

required, resulting in a considerable volume of

materials required for the lagoon wall (1.23 x 106 m3)

of which 525,000m3 will be recycled on-site material.

The further materials will be sourced locally (Section

10).

An angle of 27° correlates with the effective angle

of friction for the dredged material within the

geotubes. Resulting in a stable lagoon wall, unlikely

to fail in slump.

25m

96.4m

27.4°

14m

Impermeable unweathered Mudrock

5 x 10-6 8 x 10-8

32

Lagoon Wall Foundations…

Terzaghi’s Bearing Capacity Check

The lagoon wall at 1684m in length and 96.4m in

width is considered a strip foundation and as such

Terzaghi’s bearing capacity equation gives the

exact solution (satisfies upper & lower bound

thereoms).

For an undrained soil this simplifies to…

It could be considered that during operation water

depth within & without of the lagoon would never

be less than 2m, thus qo may be taken a 20 kN/m2.

τu = 40 kN/m2

qf = 225.6 kN/m2 (no safety Factors applied)

For the lagoon wall qf = (0.5*W*H*γb)/W = 225 kN/m2

This equates to an extra capacity of 0.6 kN/m2

without FOS, this is unacceptable. However it should

be considered that the soil below the foundation is

assumed to fail in plastic equilibrium with active &

passive zones separated by a fan of radial shear

zones; the overlaying alluvial layers are only 2m in

depth, meaning these ‘zones’ lie within the

weathered mudstone & sandstone strata.

Which has a corresponding bearing capacity;

qf > 500 - 2000kN/m2

Therefore Bearing capacity was considered

sufficient for shallow foundations.

Nf = 5.8 Nd = 7 H = 14m k =8x10-8

q =k H (Nf/Nd) = 9.28 x 10-7 m2/s

This is a very low value, equating to a flow through

the entire length of the lagoon wall of 5.65 m3/hr.

Over an area of 150,000m2 this equates to negligible

Head loss.

Darcy’s Law

Seepage through the foundation was shown to be

negligible. However there are less favourable

conditions; the option below considers the case

when some form of borrowing marine life has

effectively removed the vertical permeability

barrier, in addition to this the alluvium layer has an

undetected 1m deep band of gravel…

i = dH/dL = 14 / 96.4 = 0.145

Flow/m length = k.i.(depth of layer)

= 7.26 x 10-3 m2/s

Should this gravel layer and vertical disturbance

coincide across only a third of the site, this would

equate to a volume loss of 22,006 m3/hr and 0.19m

of head within the lagoon. Assuming that the Head

held at maximum for one hour is equivalent to the

changing head over the holding period of four

hours, this equates to an annual power loss of 0.379

GWh.

Seepage Solutions

In the event the geological study reveals bands of

high permeability materials within the alluvial layer,

a trench filled with bentonite concrete (C) is

recommended. In addition to reducing

permeability this structure would also decrease the

uplift forces. Sheet pile (E) alternatives do not have

sufficient design life and extended berms (F) are

costly and may possibly prove ineffective.

5 x 10-2

(Fell, MacGregor, & Stapledon, 1992)

96.4m

33

Settlement

There are three components of foundation

settlement…

i) Immediate Elastic

ii) Consolidation

iii) Creep Settlement (not considered)

Average Immediate Displacement

Assuming the Butcombe Sandstone strata acts as a

rigid bedrock & using an influence factor

determined from the Christian & Carrier charts, the

immediate elastic settlement is estimated…

I = μ1μ0

Alluvial Strata

E = 10 MPa H = 2m B = 96.4m μ1 = 0.02

μ0 = 1.0

δ = 0.04338 m

Weathered Mudrock & Sandstone Strata

--

E = 96 MPa μ1 = 0.06

μ0 = 1.0 μ1=0.02

δ = 0.00904 m

Unweathered Mudrock & Sandstone

--

E = 328MPa μ1 = 0.15

μ0 = 1.0 μ1 = 0.06

δ = 0.00595 m

Total Elastic Settlement = 0.0584 m

Consolidation Settlement

The co-efficient of volume compressibility, mv can

be used to determine the consolidation settlement.

Mv is obtained from oedometer testing test data, for

a range of load increments (mv not constant).

σ'1 = Final effective pressure H = layer thickness

σ'0 = Initial effective pressure

Initial effective stress was found using a graph similar

to the one shown above. Change in stress was

found using Giroud’s Chart, 1970.

Alluvial Strata

z = 1m σ’1 = 235 kN/m2 σ’0 = 10 kN/m2

mv = 0.4 δ = 0.18m

Weathered Mudrock & Sandstone Strata

z = 7 m σ’1 = 309 kN/m2 σ’0 = 84 kN/m2

mv = 0.085 δ = 0.19m

Unweathered Mudrock & Sandstone

z = 14.5 m σ’1 = 386 kN/m2 σ’0=179kN/m2

mv = 0.06 δ = 0.3105m

Total consolidation Settlement = 0.6805m

Total Settlement = 0.7389m

qf = 225 kN/m2

12m

2m

37m 12m

842m

48.2m

z1

z2

z3

34

7.6 Consolidation Time

The total amount of settlement predicted for the lagoon

wall equates to 0.7389m, a considerable amount.

Sequential Construction of the wall will be used to pre-

consolidate the lagoon foundations. Using Terzaghi’s

one dimensional consolidation theorem the time

required for consolidation can be estimated;

After 2 months consolidation, 28% of total settlement will have occurred. This is the recommended

consolidation time in which will dictate the construction processes discussed in section 10.0.

Alluvial Layer Weathered Mudrock Unweathered Mudrock

1 month consolidation 50% - 0.09m <10% - 0.010 m -

2 month consolidation 70% - .126m 14% - 0.027 m -

50% consolidated 1 month 2.9 years 27.4 years Table 13 Final consolidation estimation

It can clearly be seen that the time

for consolidation is highly

dependent on drainage length.

Figure 34 Cv (m2/year) for mudrocks across the UK (Hobbs, Hallam, Forster, Entwisle, & Jones, 2002)

35

7

6

2

1

3

5

4

Figure 36 Turbine Housing Detail

8.0 Turbine Housing Design

The turbine housing is located to the southwest ‘corner’ of the lagoon in the deeper water of the estuary

channel, allowing the full tidal range to be exploited for power generation. Additionally the turbine housing

will be ‘trenched’ to a depth of 2m to ensure the turbines are fully submerged during operation.

8.1. Turbine Caisson details

Turbine Housing

252 m

756 m

60 m

Figure 35 Turbine housing dimension

36

The turbine housing, shown in Fig.36 & 37, is a precast reinforced

concrete caisson, constructed off-site and floated in by barges.

Voids in the concrete are filled with dredged materials effectively

as ballast to add stability to the structure against the

hydrodynamic & static forces. A trench of 2m deep will be

dredged, effectively entirely removing the alluvium strata.

8.2 Standing wave propagation

The vertical nature of the caisson combined with the reinforced concretes impermeable characteristics (zero

wave transmission), results in the high wave reflectance. Standing waves are produced through the

interference, resulting in increased hydrodynamic forces and toe scour.

The most detrimental impacts standing waves pose on the turbine caisson is scouring at the base. Adequate

protection is essential to protect the integrity of the caisson and the performance of the turbines. The

solution to scour is to increase the length and durability of the toe berm, in a similar manner top that shown

in Fig. 38.

The increased wave amplitude from the standing wave (as much as doubled) produces an increase of 1.5m

in water levels (Hs/2). However this case is less onerous than run-up caused by the lagoon wall, as such 25m

height is adequate.

8.3 Permeability

The concrete structure provides an excellent impermeable barrier, the use of the concrete ensuring water

tightness of the housing which is essential to the integrity of the structure, as well as the effectiveness of

the power generation. The trenching of the turbines effectively removes the variable alluvium layer beneath

the turbine housing, leaving an effectively impermeable marl & sandstone strata.

Figure 38 Toe berm protection

60 m

40 m

25 m

Figure 37 Turbine housing dimension

37

8.4 Turbine Housing Geotechnical Design

The mass of the caisson was estimated at 720,000 kN. Over an area of 24000 m2, this equates to a q0 of 300

kN/m2. Using Terzaghi’s equation the bearing capacity of the underlying weathered Marl & Sandstone strata

varied from best case >2000 kN/m2 to worst case 719.7 kN.m2.

However it transpired that bearing capacity was not the critical case, instead sliding & overturning induced

by the static & dynamic water pressures, uplift water pressure & the often forgotten hydrodynamic drag

forces exerted on the turbines during operation (most prominent at initial water level release, when the

turbine is initially stationary – coinciding with maximum water pressures). Pile foundations were therefore

deemed necessary.

8.5 `Pile Design

The use of 25 m length ϕ=600 mm piles is proposed for design.

Qs = 2πrdατu α=0.7 for weathered, 0.5 for unweathered

Qb = τuNcπr2 Nc taken as 9.0 (clay figure)

Qu = min (Qu/2 or Qb/3 + Qs/1) = 11,243 kN

Therefore using an unfactored total foundation load of 720,000 kN, 66 piles are required. The piles will be

arranged as shown below.

14m

10m

15m

25m

τu = 440 kPa

τu = 1 MPa

Figure 39 Pile arrangement

2.5 m

5m

5m

15m

15m

60 m

38

8.6 Sliding & Overturning Forces

Sliding and overturning forces are resisted through the moments

created by the composite action of the piles and shear. With a

spacing of 15 m between piles in the width direction, large

resisting moments can be induced with little changes in pile

bearing pressures. CFA piles are capable of carrying uplift,

however the piles are more likely to experience load reduction as

opposed to actual uplift forces (uplift capacity equal to Qu – Qb).

8.7 Pile Specifications

Displacement/preformed piles are not suitable for this scheme, due to the high ‘locked in’ horizontal stresses

exhibited in Mudrocks. Continuous flight Auger CFA piles are more appropriate; the auger is drilled to a

specified depth, then slowly extracted as concrete is pumped through the hollow stem. CFA piles do not

leave open boreholes and as such collapse due to the high horizontal stresses are mitigated. In addition, the

ground profile consists of high cohesion soils, meaning initial forces exerted on the piling rig are reduced.

Furthermore, such an arrangement would minimise the environmental impacts to the estuary such as

lowering the risk of disrupting potentially contaminated soils.

8.8 Ground Beam

In order for the piles to act compositely,

producing the moment resistance required, they

must be connected using high strength & stiffness

ground beams. The 22 ground beams run parallel

to the direction of width and act to distribute the

loads across three piles. Each pile group is bound

by a ground beam as shown in Fig. 41. It has been

found that insitu ground beam is the optimal

construction method, to tie the beams with the

piles. In-situ stitch are then cast for the beam – precast caisson connection.

Sum of Forces

Figure 40 Schematic of equilibrium of forces

Figure 41 Pile - beam arrangements

39

8.9 Pile Load Testing

Pile load testing should always be carried out, due to the high levels of uncertainty in the analysis of pile load

capacities. The check gives an indication of pile capacity, pile settlement & the accuracy of the proposed pile

design. There are various testing currently being employed in the industry, a brief summary is produced in

Table 14 (Federation of Piling Specialists, 2006).

Test type Advantages Disadvantages

Static Maintained Load

Suitable for all soil conditions and

pile types

Tension and lateral testing are

possible

Reaction piles and frames

are required and

relatively expensive

The test requires working

at height

Long duration required

for reliable results

Dynamic Load Test

Fast and relatively cheap

Suitable for CFA piles

Correlation with static tests on

bored piles are good

Results may be

unrepresentative

Time related effects and

length of pile must be

taken into consideration

Rapid Load Test Suitable for CFA piles and pile

groups

Issues with presence of

chalk

Only applicable to piles

<40m

Table 14 Comparison of various pile load tests

40

1

3

4

5

2

9.0 Connection Detail

Previously established are an impermeable lagoon wall & turbine Housing designs.

However the connection detail must be equally impermeable to prevent seepage

& subsequent head & power losses. A flitch-plate-like connection is suggested,

wherein a section of the precast caisson ‘slots’ into and sits within the lagoon wall;

resulting in an impermeable connection, plan section details at 2/3 wall height are

depicted below.

Turbine housing – breakwater connection detail

1 Turbine Housing

2 Turbine housing extension

3 Rock armour

4 Dredged Alluvial Argillaceous Fill

5 Geotubes

Figure 42 Flitch Plate Concept

41

10.0 Construction processes

The construction sequence for the lagoon are three fold; i) Dredging ii) Lagoon wall construction iii)

turbine housing construction

10.1 Dredging Plan

The Avonmouth and Royal Portbury Docks is a dredging site,

operated by The Bristol Port Company. They are licensed to

dispose of 3,224,000 tonnes of dredged materials but only

279,888 of the allowance were used in 2012. The site is expected

to yield approximately 525,000 m3, this accounts for 50% of the

required material, subsequent extra dredged materials will be

obtained from the Avonmouth Docks.

A Cutter suction dredger will be employed. This particular type of dredger allows the simultaneous

construction of Geotubes, reducing construction time. It has been determined that the lagoon would not be

able to provide sufficient sediments to construct all the Geotubes required. The extra materials will come

from the various licensed dredging sites as shown in Figure 43.

In order to increase the rate of compaction of the fill in the

breakwater a combination of drained reclamation and

sequential construction is employed.

Drains are placed inside the breakwater prior to the laying of the

fill materials. As the fill builds up it also exerts pressure to

consolidate, and extract water from within the layers.

It has also been identified that adequate time

management in the breakwater construction it is

possible to achieve 28% total settlement in two

months. Therefore each Geotubes is to be

constructed continuously as a whole around the

perimeter of the lagoon to optimise the compaction

and construction time.

The construction sequence of the breakwater is illustrated in the following page.

Figure 44 Licensed dredging site (Severn Estuary Partnership, 2011)

Figure 45 Drained reclamation (HKAA, 2011)

Figure 43 Suction dredger (Velde, 2011)

42

Geotubes act as permanent falsework

for the core

First layer is left for initial settlement for 2

months to achieve 28% of total settlement

Topping out of the breakwater

Overlaying concrete as binding agent for the

Geotubes, also provides a platform for the rock armour to rest

on

Rock armour applied on the outer most

layer as primary sea defence

43

10.2 Turbine Housing Construction

1) Pile Loading Test

2) Trench Dredged

3) CFA pile construction (sixty-six 300mm diameter )

4) Insitu Ground Beams

5) Floated Caisson, connected with an insitu stitch

6) Dredged Ballast Added

7) Turbines Installed

2

3

4

5

7

44

11.0 Environmental Impact Assessment

Environmental impacts have proven to be the most important issue encountered in the development, thus

an Environmental Impact Assessment is conducted to identify, categorise and mitigate the impacts. 7 key

areas have been identified outlined below.

11.1 Coastal Processes

The lagoon is thought to pose moderate impacts on the coastal processes, such as sedimentation, erosion

and water circulation.

The natural sedimentation processes are most extensive during the 2 tides each day. The lagoon is situated

in the tidal mudflats which may alter the sedimentation pattern for the surrounding mudflats. It is thought

that the mudflats to the East of the lagoon may suffer a reduced replenishment of fresh sediments which is

potentially damaging to the marine life. However, the tidal flow at the site is thought to be high and with

the dominant wave direction from the south west, it is believed that the east of the lagoon will only suffer

moderate sedimentation losses. On the other hand the north and south face of the lagoon is thought to

suffer high sedimentation gain which is deemed beneficial as the wall becomes more resistant to wave

loading.

Water currents are thought to be altered at the location of the turbines which poses risks of scouring around

that area. However, the turbines are housed in concrete housing which has high resistance to scouring. Risks

of scouring to the surrounding area are low.

The shape and orientation of the lagoon align with the direction of flows thus the disruption to the flow of

currents is minimal. At the face of the turbine housing may observe a degree of obstruction to the flow but

is thought to cause minor damage.

11.2 Marine Ecology

The construction will inevitably affect the migratory animals such as birds and fish during their spawning

periods. The main migratory bird species that reside in the estuary are: Bewick’s Swan, European Whilte

Fronted geese, Shelduck and Redshank; the main fish species are Salmon, Sea Trout, River & Sea Lamprey,

Twaite and Allis Shad. These fish are valued with international importance thus any impact to the ecosystem

must be minimal and temporary.

Construction is carried out in phases with the most disruptive works being to be completed during non-

spawning periods, thus minimising the impacts to the marine ecology at the immediate and neighbouring

areas.

45

11.3 Marine traffic routes assessment

The proposed site lies beyond major cargo routes with the main Avonmouth port lying roughly 6km to the

south and subsequent marine traffic to the north is represented mainly by small fishing and leisure vessels.

Most of the lagoon is in the mudflats thus normal commercial and leisure marine traffic are not expected to

be present in the vicinity. Licensed dredging sites lie within a 6 km radius of the lagoon, but are not expected

to be interrupted by the development (in fact local dredging sites will be utilised for locally sourced fill

materials). Final disruption to shipping routes is of low to insignificant impact and provision of adequate

signage, radio warning messages and other measures will be implemented to minimise the risk of collision

with the lagoon.

11.4 Noise and Vibration

Airborne and waterborne noises are expected to affect receptors in the immediate vicinity. A baseline study

to compare the current noise level and the predicted noise generated during and after the construction is to

be conducted to identify the severity of the noise and vibration impact on different regions at and around

the development.

Due to the varying tide levels throughout the day and the offshore nature of the site, it is expected that

construction works will be carried out 24hours a day. Therefore the noise mitigating measures should be

employed throughout the construction of the development at all times.

Noise generated during the construction stage can be categorised into 2 types: Traffic and construction.

Traffic noise and vibration are believed to pose minimum impact on the local residents as the majority of

the construction materials will be brought in by sea, and the site is roughly 1km away from the nearest

residential area.

Noise travels faster in water hence the impacts are believed to be most detrimental to the marine receptors.

The bird and fish species identified in 11.2 are of international importance thus adequate mitigating

measures will be employed to minimise the impacts on them.

Most works should be undertaken during the day to reduce disruption to the animals’ resting periods. The

most disruptive phase of the development is the construction of the turbine housing. CFA Deep Cement

Mixing will be employed as the piling method thus minimising the noises as well as disruption to potentially

contaminated soil discussed in section 11.1.

11.5 Air Pollution

Air pollution is not regarded as an issue in this development since there is no onshore traffic thus no dusts

or aggregates will be brought in/transported by vehicles. The main materials used are either dredged from

the seabed and pumped directly into Geotubes or brought in by sea and lowered into the estuary.

46

11.6 Water Quality

The proposed site lies outside any of the protected areas thus minimises the effects to the local wildlife, and

it is believed that no protected species reside or spawn in this region (Tidal Lagoon Swansea Bay, 2012). The

dredging will be conducted in phases thus minimising the suspended seabed sediments, which should

reduce the deterioration of water quality.

A detailed water quality impact analysis will be conducted with accordance to Water Framework Directive

(WFD), Bathing Water Directive (BWD) and the Shellfish Water Directive (SWD).

Should the seabed sediments be deemed contaminated adequate measures will be implemented to

minimise the disruption of the soil. Environmental dredging will be employed at locations with

contaminated soils and deep cement mixing is an option for the foundation for the turbine caissons to

reduce the disruption to the soil.

Lubricants used for the turbine may lead to water pollution. Thorough examination on the toxicity of the

chemicals is mandatory to minimise the release of toxic chemicals to the water.

11.7 Decommissioning considerations

During the decommissioning of the lagoon several factors have to be taken into consideration:

Disruption to the new marine ecosystem

Suspension of small construction materials in the water deteriorating water quality

Noise and landscape pollution which may be unwelcoming to migratory birds and fish

It is thought that should the lagoon require decommissioning by the end of its design life it would cause

major disruption to the newly formed ecosystem. It was observed that in La Rance Tidal Barrage the

diversity of marine animals had increased despite severe destruction during the construction stage. It is

therefore important to minimise the disruption to the new ecosystem both during the construction and

decommissioning stage.

The breakwater is ideally designed to be 100% recyclable and to release minimum particles to the estuary

during the decommissioning, thus maintaining the water quality. The fill compacted within the Geotubes

structure is dredged out before the disintegration of the breakwater, which is believed to significantly

reduce the amount of suspended particles.

It is worth noting that some marine organisms may take shelter on the rock armour and form a new

ecosystem, which should be taken into consideration during the decommissioning.

48

12.0 Risk Assessment

Risks incurred in operation

No. Activity Probability Consequence Risk

classification Mitigation

Final score after mitigation

1 Deterioration of the

connection between seawall and turbine caisson

2 4 8 Routine inspection to the status of the water seal

Repair any damaged observed to the wall structue 4

2 Seepage through the

breakwater 2 4 8

Construction of Geotubes are subjected to strict quality control

Close monitoring the loss of breakwater aggregate

Pressure gauges installed along the perimeter to monitor pressure difference between the estuary and lagoon

6

3 Being hit by loose heavy

objects during maintenance 3 4 12

Deploy sufficient fully trained personnel to carry out maintenance

All loose parts are secured before being lifted

7

4 Drowning during turbine

maintenance 2 4 8

Only qualified personnel allowed in the area

Oxygen supply (tanks or pipes) are checked prior to maintenance

2

5 Power under-production 2 3 6 Routine maintenance to lubricate the turbine rotor

Turbine blades are regularly inspected and repair/replace if damaged

2

6 Power over-generation 1 2 2 In-built emergency brakes are deployed above certain

rotation speed 1

7 Turbines trapped by obstacles 2 3 6

Turbine rotation speed is capped and over-rotation is prevented by the deployment of emergency brakes, thus marine life will not be caught in the turbine

Self-cleaning screening devices are installed on the estuary face, opposite tides will flush the sediments out

3

49

Risks incurred in construction phase

No. Activity Probability Consequence Risk

classification Mitigation

Final score after mitigation

1 Collapse of breakwater 2 4 8

Sufficient time for aggregate settlement prior to further aggregate laying

Constant monitoring on the movement of the breakwater

4

2 Falling from height 3 4 16

Safety belt and non-slip footwear are worn at all times by all personnel at the site

Ensure scaffolding is secured and stable working platform

8

3 Drowning in the estuary from

falling 3 4 12

Safety belt and high visibility vests are worn at all times by all personnel

Provision of life belt at the site

6

4 Differential settlement of the

breakwater 2 2 4

Routine monitoring of settlement at regular intervals along the perimeter of the lagoon

Allow sufficient time for materials to settle in the breakwater

2

5 Disruption of potentially

contaminated tidal flat deposits 3 3 9

Adequate project planning to ensure minimum disruption without compromising progress

Alternative dredging method to localise the impacts

3

6 Collision with delivery vehicles 4 3 12 Speed limit strictly enforced

Live-time Notification and communication of vehicular movements

6

7 Delay of construction 4 3 12

Proper preliminary planning to optimise the construction stages without clashes

Constant communication with suppliers for up-to-date materials availability

6

8 Water contamination from the chemicals used in the turbines

2 3 6 Comprehensive testing to the toxicity of the

chemicals used to ensure no harm is done to the environments

3

50

References

AEA Energy & Environment. (2007, October). Retrieved February 09, 2015, from http://www.sd-

commission.org.uk/data/files/publications/TidalPowerUK4-Severn_non-barrage_options.pdf

Avonmouth Windrose Data 1991-2000. (2001, January). Retrieved from Bristich Atmospheric Data Centre:

https://badc.nerc.ac.uk/data/surface/station_lists/ukstation.wind.html

British Geological Survey. (2002). Engineering geology of British rocks and soils. November. Retrieved from

http://nora.nerc.ac.uk/3664/1/RR01002.pdf

British Geological Survey. (2015, February 14). Borehole Scans. Retrieved from Bristich Geological Survey:

http://mapapps.bgs.ac.uk/geologyofbritain/home.html?mode=boreholes

British Hydropower Association. (2009, October 15). La Rance Tidal Power Plant. Liverpool.

Carrington, D. (2015, January 7). Leave fossil fuels buried to prevent climate change, study urges. Retrieved from The

Guardian: http://www.theguardian.com/environment/2015/jan/07/much-worlds-fossil-fuel-reserve-must-

stay-buried-prevent-climate-change-study-says

Chandler, R., & Davis, A. (1973). Further work on the engineering properties of Keuper Marl.

Cripps, J., & RK, T. (1981). The engineering properties of Mudrocks . Quarterly Journal of Engineering Geology, 325 -

346.

Crompton, D. (2002). Cardiff Bay Barrage. Proceedings of the ICE - Water and Maritime Engineering, 154(2), 81-88.

Crowther, S., Dickson, A., & Truscoe, K. (2008). Severn Estuary Rapid Coastal Zone Assessment Survey. Swindon:

English Heritage.

DEFRA. (2015, February). Magic. Retrieved from Magic: http://magic.defra.gov.uk/

Department of Energy & Cliamte Change. (2014). Energy Consumption in the UK (2014). London: British Government.

Department of Energy & Climate Change. (2012, October 12). Increasing the use of low-carbon technologies. Retrieved

from GOV.UK: https://www.gov.uk/government/policies/increasing-the-use-of-low-carbon-technologies

Department of Energy & Climate Change. (2013, January 22). Reducing the UK’s greenhouse gas emissions by 80% by

2050. Retrieved from GOV.UK: https://www.gov.uk/government/policies/reducing-the-uk-s-greenhouse-

gas-emissions-by-80-by-2050

Department of Trade and Industry; Welsh Development Agency. (2006). Tidal Lagoon Power Generation Scheme in

Swansea Bay. Cardiff: Crown.

Environmental Agency. (2006, May 31). Retrieved February 09, 2015, from Internet Archive:

http://web.archive.org/web/20070930155720/http://www.environment-

agency.gov.uk/commondata/acrobat/severnpositionmay2006_1508223.pd

Federation of Piling Specialists. (2006). Handbook on Pile Load Testing. Beckenham: FPS. Retrieved March 15, 2015,

from http://www.fps.org.uk/fps/guidance/technical/piletesting/06-02-

27%20load%20testing%20handbook%20%282006%29.pdf

Fell, R., MacGregor, P., & Stapledon, D. (1992). Geotechnical Engineering of Embankment Dams. Rotterdam: Balkema.

Friends of the Earth Cymru. (2004, January). Retrieved February 09, 2015, from

http://www.foe.co.uk/sites/default/files/downloads/severn_barrage_lagoons.pdf

Hawkins, A. B. (1984). Depositional characteristics of estuarine alluvium:. Engineering Geology and Hydrogeology,

219-234.

HKAA (Director). (2011). Land reclamation [Motion Picture]. Retrieved from

https://www.youtube.com/watch?v=iaJcX_3Xx84

51

Hobbs, P. R., Hallam, J. R., Forster, A., Entwisle, D. C., & Jones, L. D. (2002). Engineering geology of British - Mudstones

of the Mercia. Nottingham: British Geological Survey.

Madison, J., Chambers, S., Thomas, A., & DB, J. (1996). The determination of the deformation and shear strength

characteristics of Trias and Carboniferous strata from in situ and laboratory testing for the Second Severn

Crossing. In Advance in Site Investigation Practice (pp. 598 - 906). London: Thomas Telford.

Marine Traffic. (2015, February). Retrieved from Marine Traffic: http://www.marinetraffic.com/

Office for National Statistics. (2013). Household Energy Consumption for England & Wales,20 05-11. London: Bristish

Government.

Pierre, J. (1993). TIDAL ENERGY: PROMISING PROJECTS LA RANCE, a successful industrial-scale experiment. Paris:

ELECTRICITE DE FRANCE- D.E.P.T.

Pratt, N. J. (2002). Cardiff Bay Barrage: Construction. Proceedings of the ICE - Water and Maritime Engineering, 154(2),

137 - 148.

RANCE TIDAL POWER STATION, FRANCE. (2014). Retrieved February 08, 2015, from the greenage:

http://www.thegreenage.co.uk/cos/rance-tidal-power/

Scambos, T., & Abraham, J. (2014). Antarctic ice sheet mass loss. ICE.

Severn Estuary Partnership. (2011). Severn Estuary Partnership Maps. Retrieved from Severn Estuary Partnership:

http://www.severnestuary.net/sep/estuary/images/map_SOSER_Dredging.gif

Siemens. (2012, February 17). Siemens strengthens its position in ocean power – Acquisition of Marine Current Turbines.

Retrieved February 07, 2015, from Siemens:

http://www.siemens.co.uk/pool/news_press/news_archive/2012/siemens-marine-current-turbines.pdf

Strangford Lough Tidal Turbine, United Kingdom. (2011). Retrieved from Power-technology: http://www.power-

technology.com/projects/strangford-lough/

Tellam, J., & Lloyd, J. (1981). Hydrpgeology of Bristish onshore non-carbonate Mudrocks. Quaterly Journal of

Engineering Geology, 347 - 355.

Tidal Lagoon Swansea Bay. (2012, October). Environmental Impact Assessment. Retrieved March 15, 2015, from Tidal

Lagoon Swansea Bay:

http://tidallagoon.opendebate.co.uk/files/TidalLagoon/PEIR/Chapter_9_Fish_Recreational_and_Commercia

l_TLSB_PEIR.pdf

Tidal Lagoon Swansea Bay. (2014). Tidal Lagoon Swansea Bay Project Introduction. Retrieved from

http://tidallagoon.opendebate.co.uk/files/TidalLagoon/Project_introduction_to_Tidal_Lagoon_Swansea_Ba

y.pdf

Velde, M. V. (2011, August). What is "STPM" ? Retrieved from The Art of Dredging:

http://www.theartofdredging.com/stpmfordummies.htm

WS Atkins . (2014). Swansea Bay Tidal Lagoon. Retrieved February 7, 2015, from Atkins Global:

http://www.atkinsglobal.com/en-GB/projects/swansea-bay-tidal-lagoon

Wyre Tidal Energy. (2010). La Rance Barrage. Retrieved February 08, 2015, from Wyre Tidal Energy:

http://www.wyretidalenergy.com/tidal-barrage/la-rance-barrage