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Transcript of Final HANDIN MACLIRRRR
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
2
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.
3
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)
4
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)
5
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
6
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)
7
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)
8
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)
9
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
10
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)
11
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
12
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
13
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
14
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
15
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)
18
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
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