Cost Estimation For Coastal Protection
Cost Estimation For Coastal Protection
Cost Estimation For Coastal Protection
summary of evidence
Report –SC080039/R7
We are the Environment Agency. We protect and improve the
environment and make it a better place for people and wildlife.
Miranda Kavanagh
Director of Evidence
Table 1.1 Typical elements of the cost of beach control works (CIRIA 2010) 5
Table 1.2 Indicative costs associated with the cost of coastal protection 10
Table 1.3 Key cost considerations for coastal works 11
Table 1.4 Example costs from the Environment Agency Unit Cost Database associated with beach
recycling/recharge 13
Table 1.5 Example costs from the Environment Agency Unit Cost Database associated with coastal walls 14
Table 1.6 Example costs from the Environment Agency Unit Cost Database associated with revetments 15
Table 1.7 Example costs from the Environment Agency Unit Cost Database associated with rock groynes 18
Table 1.8 Estimated groyne construction costs (£/m) 19
Table 1.9 Environment Agency suggested maintenance frequencies 20
Table 1.10 Summary of potential monitoring data requirements 24
Table 1.11 Deterioration rates for different materials in coastal environments 28
Table 1.12 Deterioration rates for revetments in coastal environments 29
1.1.3 Revetments
Revetment is a generic term used when an armouring layer is applied to a sloping
surface of an embankment or shoreline. There are two main types of revetments:
permeable and impermeable revetments.
The function of permeable revetments is to reduce the erosive power of the waves by
means of wave energy dissipation in the interstices of the revetment. Permeable
revetments can be built from rock armour, timber, gabions and concrete armour units.
Where frequent wave attack is anticipated, the revetment may be topped by a vertical
or re-curved wall to reduce overtopping.
The cladding or facing layer of revetments may include rock armour, concrete block
matting, precast concrete units, gabion mattresses, asphalt or in situ concrete
slabbing/steps. Options other than rock are generally used where there are over-riding
aesthetic requirements, health and safety issues, or where rock armour is difficult or
too expensive to obtain.
Impermeable revetments are continuous sloping defence structures of concrete or
stone blockwork or mass concrete, and are used to provide a fixed line of defence for
frontages with high value backshore assets or specific amenity requirements. Intended
to withstand storm wave attack over a life expectancy of 30–50 years, amenity facilities
such as promenades, slipways and beach access steps can be built into this type of
revetment. An example is Blackpool Council’s Central Area coastal defences (2005-
2010).
Wire mesh baskets filled with cobbles or crushed rock, gabions are filled in situ, often
with locally available material and therefore have a relatively low capital cost. Gabion
revetments can provide a short term (5–10 years) alternative to rock armour structures
in areas where large rocks are not available at an acceptable cost, or where long-term
protection is not appropriate. Gabions may also be valid choice of defence in less
exposed locations where the expected design life can be much higher.
Timber revetments have been historically used in the UK for coast protection,
particularly on the south and east coasts, where the costs or impacts of a seawall may
have been unacceptable. Construction flexibility allows timber revetments to serve
1.1.4 Breakwaters
Offshore breakwaters are typically built parallel to the shore, either singly to protect a
specific coastal location (for example, at Rhos-on-Sea in north Wales) or in series to
provide protection to longer frontages (for example, Happisburgh to Winterton on the
Norfolk coast). Breakwaters are usually constructed from rock or precast concrete
units. Rock armour facing may be used to minimise wave reflection. In some instances
composite precast unit and rock structures are used to mimic the behaviour of natural
reefs.
Breakwaters are primarily designed to reduce wave energy and wave heights reaching
the shore, but also benefit from reducing longshore transport and encouraging beach
formation in their lee.
1.1.5 Groynes
Groynes are cross-shore structures designed to reduce longshore transport on open
beaches or to deflect nearshore currents away from the shoreline. On an open beach
they are normally built as a series to influence a long section of shoreline that has been
nourished or is managed by recycling. In an estuary they may be single structures.
Rock is now often favoured as the construction material; in the past timber was
favoured but is now considered less sustainable. Alternative construction materials
include sheet piles, concrete, open stone asphalt, cribwork and plastic.
Groynes can be used in combination with revetments to provide a high level of erosion
protection.
Groynes can be permeable or impermeable. Impermeable groynes are solid and are
designed to intercept all material arriving on the updrift side. Permeable groynes allow
some sediment to pass through.
Rock groynes have the advantages of simple construction, long-term durability and
ability to absorb some wave energy due to their semi-permeable nature. Wooden
groynes can be less durable (dependant on the specific conditions and beach sediment
they are retaining) and tend to reflect, rather than absorb, energy. Gabions can be
useful as temporary groynes but have a short life expectancy.
Dune thatching
Thatching of exposed dunes faces or blowouts using waste cuttings from forestry
management, or other low cost materials, is a traditional way of stabilising sand,
reducing trampling and protecting vegetation. Materials are low-cost if locally available
and no machinery or skilled labour is required to achieve success, but continual
maintenance is important. The approach is normally carried out with dune grass
planting to encourage dune stability.
Dune fencing
Construction of semi-permeable fences along the seaward face of dunes will
encourage the deposition of wind-blown sand, reduce trampling by people and
livestock, and protect existing or transplanted vegetation. A variety of fencing materials
can be used successfully to enhance natural recovery. Fencing can also be used in
conjunction with other management schemes to encourage dune stabilisation and
reduce environmental impacts.
Access control
Dune erosion can be exacerbated by uncontrolled access that destroys vegetation
growth and makes the sand more susceptible to aeolian forces. The provision of
boardwalks in association with fencing controls access and provides improved
conditions for dune stabilisation.
Table 1.1 Typical elements of the cost of beach control works (CIRIA 2010)
Subject Costs to be included
Preliminaries • Project coordination, management and administration
Planning and design • Survey, data collecting and observations
• Model studies
• Design and contract preparation
• Statutory procedures and licences
• Economic appraisal
• Environmental impact assessment and licences
• Safety planning supervision
Construction • Contract payments including adjustments, claims and so on
• Supervision (including safety) and administration costs
• Ancillary works for environmental improvement, amenity of
services
Land or property • Purchase or lease of land either as part of the works or for
construction
• Compensation payments to affected owners
Operation and • Operational activities
maintenance
• Monitoring and maintenance including replacement of
elements having a shorter life than the overall scheme
• Repairs
Costs of preliminaries
The costs incurred for preliminary works is difficult to estimate as they depend on many
variables including:
• size of partnership
• objectives
• funding arrangement
• roles and responsibilities
• staff involved in negotiations
Only through experience can preliminary costs be estimated accurately. The case
studies from the FD2635 research give examples of costs and he timescales required
for effective partnership working and consultation.
1.3.2 Approvals
The process of obtaining the approvals and consents necessary to implement a flood
and coastal defence scheme is a further factor which must be considered when
calculating whole life scheme costs.
Statutory approvals
There are a number of statutory licensing approvals which may be required for coastal
protection schemes. Experience based on FD2635 research indicates that the
preparation and approvals of these can take time. Where statutory nature conservation
approvals are required English Nature can be a valuable project partner.
Consents for all coastal protection schemes may be require the following:
• consents under Section 5 (approval to carry out works) and Section 34
(navigation requirements) of the Coast Protection Act 1949 (CPA)
• Food and Environmental Protection (FEPA) licences – required for the
deposits of substances or articles in the sea or under the seabed including
disposal at sea of dredged material, or where construction work which
involves deposition of materials below the high water mean spring tide
• land drainage consent if applicable under the Land Drainage Act 1991
amended by Flood and Water Management Act 2010
Until recently FEPA licences had to be obtained before construction could commence.
The process took a statutory 12 weeks to turn around with a full submission being
required if an extension of time was needed. FEPA licences were granted by non-
technical staff members, which caused problems, particularly when communicating
engineering matters.
The FEPA and CPA licensing has now been replaced by the New Marine Licensing
System, introduced under the Marine and Coastal Access Act 2009, which came into
force in April 2011. Further information on the new system can be found in:
• Factsheet – New Marine Licensing System (Defra 2010)
• guidance from the Marine Management Organisation (MMO 2011)
Funding approvals
The majority of funding for flood and coastal erosion schemes is derived from Defra
Flood Defence Grant in Aid (FDGiA), an approval process which takes around 6–9
months.
However, there is evidence that non-FDGiA (funding or contributions) can be secured.
Examples include:
• local authority maintenance budgets such as highways departments to
protect local infrastructure
• Regional Development Agencies – such as the ‘Civic Pride’ initiative
• European Union funding programmes such as the European Regional
Development Fund or Interreg
• Commission for Architecture and the Built Environment
• Heritage Lottery Fund
Further detail on funding, including information on external contributions, is given
below. Additional information can be found in Defra and Environment Agency (2011).
Notes: The Scottish Natural Heritage (SNH) costs relate to a 2000 cost base and
the Environment Agency costs relate to a 2007 cost base. An allowance for
inflation using a suitable index is required to update these values to present
day costs.
Detailed costs at the detailed design stage will need to be developed using specialist
advice, standard rates and a bill of quantities. Guidance on detailed costs is provided in
standard price estimating books such as SPONS (Davis Langdon 2011) and the
Institution of Chemical Engineer’s CESMM price database (ICE 2012).
The sections below provide indicative costs and guidance on whole life cost estimation
for coastal flood protection measures. The information given is suitable for early stage
appraisals, national level assessments or outline design stage only. While the unit rates
provided are suitable at the outline design stage, a bill of quantities will be more
appropriate at the detailed design stage.
Estimated capital costs for various coastal elements are available in the Flood Risk
Management Estimating Guide (Environment Agency 2010) – also known as the Unit
Cost Database (UCD). The costs available are based on out-turn costs from a large
number of projects to install defences or coastal erosion for the purposes of flood risk
management in England and Wales. The costs include all associated works, temporary
works and any contractor variations, compensation events/delay costs.
The UCD costs are broken down into the key asset types including walls, revetments
and beach recycling/recharging/nourishment. Other cost information for groynes has
been collated from a number of other sources.
Table 1.3 Example costs from the Environment Agency Unit Cost Database
associated with beach recycling/recharge
Scheme Description Length Volume Total Cost Cost
(m) (m3) cost (£/m) (£/m3)
(£k)
West Clacton – Beach 3,250 689,000 6,276 1,931 9
Jaywick Sea nourishment,
Defences (1999) 500,000 m3
sand from
Long Sands,
plus 100,000
tonnes of
rock
Happisburgh to Beach 4,300 8,888 2,067
Winterton Sea recharge
Defences
Intermediate Works
– Phase 3 (2000)
Happisburgh to Beach 2,000 438,000 5,889 2,945 13
Winterton Sea recharge
Defences – Phase 3
(2002)
Shoreham and Beach 28,000 891 32
Lancing Beach recharge
Defences – Phase 1
(2002-2003)
Seaford Bulk Shingle 360,000 451 1
Recycling Scheme recycling,
(2002-2003) maximum
2 km haul
Pett Sea Defences Beach 50,000 745 15
(2005-2006) recharge
1.5.2 Walls
In addition to physical size (length, depth) of the walls, the most important issues that
will affect the cost of the completed structure are as follows:
• access constraints – distance to work site, ease of movement along site
length, need for temporary access and so on
• weather – winter working will have an influence on productivity and
therefore likely higher costs than working during the summer
• quality of materials for building, facing and finishing the structures (such as
coping stones)
• economies of scale – whether the wall is a short, isolated section with high
mobilisations costs or a long length of uniform construction type
• precast or in situ construction
The Unit Cost Database (2010 version) provides unit costs for a range of wall types,
typically used in the fluvial environment (retaining and with cut-off and piled
foundations). These costs are included in the fluvial flood defence evidence summary.
A number of more relevant coastal projects are also provided, although the number
and type of projects are limited and therefore only provide example historical data and
reference projects to assist appraisers in cost estimation.
Table 1.5 summarises the unit costs for these coastal assets. It highlights the wide
range in costs and emphasises the need for site-specific consideration for more
detailed design estimates.
Table 1.4 Example costs from the Environment Agency Unit Cost Database
associated with coastal walls
Type Description Length Height Total Cost Cost
(m) (m) cost (£k) (£/m) (£/m²)
Raising Raise and modify 3,000 1,704 568
existing wall
Concrete retaining wall, 100 1.0 149 1,490 1,490
clad both sides with
stone, tie to existing
Wave/ Sea defence wall – 300 0.5 395 1,317 2,633
retaining brick clad, granite
wall coping
Seawall 450 0.4 861 1,913 5,467
Sea defences 1,190 3.8 2,456 2,064 543
Reinforced concrete 75 2.0 472 6,293 3,147
wave return wall
Tidal sea defence – 370 1.5 1,488 4,022 2,681
seawall, stone clad
Wave return wall 822 1.8 1,237 1,505 836
Wave return wall and 515 442 858
crest slab
Quay Reinforced concrete 300 6.5 3,277 10,92 1,681
wall quay wall – masonry 3
facing, including water
cavity to rear, some
piled, counter walls
Piling Piling to quay wall 30 14.0 95 3,167 226
Piling to slipway 70 48 687
Table 1.5 Example costs from the Environment Agency Unit Cost Database
associated with revetments
Scheme Description Length Volume Total Cost Cost
(m) (m3) cost (£/m) (£/m3)
(£k)
Happisburgh to Rock armour 35 2,180 1,019 29,114 467
Winterton Sea – land and
Defences sea
placement
South Felixstowe Rock armour 24,117 1,730 72
Flood Alleviation on beach
Scheme
Minsmere Tidal Rock/stone 25 57 2,280
Sluice Outfall revetment
Improvement works
Works
Happisburgh to Revetment 1,700 16,320 311 183 19
Winterton Sea protection
Defences – works
Phase 1
Intermediate
1
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Scheme Description Length Volume Total Cost Cost
(m) (m3) cost (£/m) (£/m3)
(£k)
Works
Tendring and 3-5t Rock 650 24,375 1,017 1,564 42
Hollane Tidal armour
Defences
West Clacton – Rock armour 1,000 257,000 2,380 2,380 9
Jaywick Sea
Defences
High Knocke to Rock armour 1,256 54,501 5,840 4,649 107
Dymchurch Sea
Defences
Alkborough Tidal Rock armour 83 8,330 1,457 17,554 175
Defence Scheme erosion
protection
There is currently very little information available for impermeable or other types of
revetments other than that provided by the Environment Agency Unit Cost Database in
Table 1.6. Additional research or collation of this information is recommended to
ensure these data are made available and reviewed in the future.
1.5.4 Breakwaters
Offshore breakwaters generally have a length similar to their distance offshore, which
is typically 200–300 m. Structures may be placed either singly to protect a specific
coastal location, or as a series to provide protection to longer frontages. Breakwaters
may be constructed as a standalone element or as part of a wider coastal management
strategy, for example, in combination with groynes or as part of a beach recharge
scheme. The design may therefore consider a certain applicable deterioration or
damage to the breakwater structure that will not result in catastrophic failure or failure
to the overall scheme purpose.
Appraisers should determine the issues most likely to influence costs and consider
these with specialists during the design phase. The most important aspects influencing
offshore breakwaters are fully described in the report by Crossman et al. (2003) and
summarised below.
• Geometry of the breakwater. Costs will depend on the sizing, slope,
complexity and design of the structures.
• Materials type and source. Costs will depend on the cost of the rock,
accessibility of the site, delivery methods and distance to the quarry or local
source material. Design of the structure can be adapted to a particular or
local source material. Costs may also depend on the grading and
proportion of the rock that can be used and the degree of excess material.
Armour sizing will also influence plant requirements and costs.
• Construction methods. These influence plant equipment and access
requirements. Requirements for foundation, excavation and toe protection
works will often require different and more costly marine plant to be used.
1.5.5 Groynes
Important factors to consider when costing groyne schemes include:
• functional design –length, spacing, height, depth of groynes
• materials to be used to construct the groynes
• length of beach requiring protection
• beach exposure conditions
• tidal range, geomorphology of the beach deposits and the depth of the
bedrock if piles are to be driven
The beach material is also important as groyne’s design life depends on abrasion rates
under different exposure and sediment size conditions (more abrasion and a reduced
design life have been recorded for shingle beaches).
Timber groynes
Design guidance on timber groynes is provided by Crossman and Simm (2004). A
number of case studies and examples of timber groyne costs are provided below to
illustrate the range of typical costs.
• Within the Environment Agency asset deterioration guidance document
(Environment Agency 2009), Bournemouth Borough Council cited costs of
around £200,000 per timber groyne in 2004.
• A case study of coastal defences in Norfolk from the Geocases website
suggests costs for rocks of £40–50/m3 with a rock groyne typically costing
£125,000. 2
• Bournemouth Borough Council’s rolling groyne reconstruction programme
cited costs of £200,000 per timber groyne (Crossman and Simm 2004).
However, this high cost was in part due to the small tidal range and length
of the groynes necessitating considerable temporary works and hard
underlying strata which required pre-boring for the piles with high-pressure
water lances.
• Worthing Borough Council reported within the Environment Agency asset
deterioration guidance document (Environment Agency 2009) that typical
costs for a 70 m long softwood timber groyne were now £100,000.
2
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• The Norfolk Coastal Defences case study (Environment Agency 2009)
suggests costs of £1,000/m for timber groynes. Assuming a typical groyne
length of 100 m, that is £100,000 per groyne.
• Eastbourne’s coastal protection scheme included 94 timber groynes (65–
110 m in length) at a total cost of £30 million (£320,000 per groyne) (Suffolk
Coastal District Council 2009).
• Waveney District Council developed a £7 million scheme at Southwold to
construct both rock and timber groynes. The timber groynes (45 m in length
with a 110 m spacing) were constructed at a cost of approximately
£105,000 per groyne (Suffolk Coastal District Council 2009).
• A cost benefit analysis in 2004 estimated timber groyne costs of £1,330/m
for the groynes in Swanage (Suffolk Coastal District Council 2009).
Rock groynes
The construction costs of rock groynes will depend on the scope and complexity of the
site and design of the groynes. Site accessibility may also influence the design and
ability to construct or top up rock groyne structures.
The Environment Agency’s Unit Cost Database (2010 version) provides cost data on a
number of rock groyne schemes (Table 1.7). These costs are based on out-turn costs
of completed projects in the Unit Cost Database.
Table 1.6 Example costs from the Environment Agency Unit Cost Database
associated with rock groynes
Scheme Description Length Volume Total Cost
(m) (m3) cost (£k) (£/m3)
Felixstowe Ferry Groynes – Unknown 2,290 220 96
Sea Defences rock
South Felixstowe Groynes – Unknown 27,344 1,713 63
Flood Alleviation rock
Scheme
A number of case studies and examples of timber groyne costs are provided below to
illustrate the range of typical costs.
• Waveney District Council developed a £7 million scheme at Southwold to
construct a field of rock groynes (45 m in length with a 70–80 m spacing) at
a cost of approximately £200,000 per groyne (Suffolk Coastal District
Council 2009).
• A cost benefit analysis in 2004 for the Swanage frontage estimated rock
groyne costs of £2,410/m for the 30–50 m long groynes and £3,930/m for
175 m long groynes (Suffolk Coastal District Council 2009).
• An example of the use of rock groynes is provided in the CIRIA Rock
Manual (CIRIA 2010b). A 4 km length of shoreline was protected using a
combination of 33 (70 m long) rock groynes and breach recharge. The total
cost was £12 million.
1.6.3 Revetments
No records of any readily available cost information associated with the intermittent
costs for revetment protection measures were available at the time of writing. Further
information or records from local authorities or the Environment Agency should be
sought to provide guidance for this protection measure.
1.6.4 Breakwaters
The Rock Manual (CIRIA 2007) suggests the following range of repair and upgrading
requirements for breakwaters:
• simple maintenance that does not require removal and handling of a
substantial volume of material
• repair involving heavy work and even reconstruction of one of more parts of
the structure (carried out when the structure is at risk of further deterioration
or has suffered damage that diminishes the performance of the structure)
• preventative rehabilitation and reconstruction of a significant part of the
structure (topping up breakwaters with additional rock may require
considerable dismantling work to ensure interlocking of individual rocks can
be achieved)
• reconstruction or replacement of the entire length of a breakwater
The design and evaluation procedure for breakwaters typically attempts to minimise
whole life costs by the selection of design conditions that balance the initial capital
costs and any longer term O&M costs. This process aims to determine an accepted
level of damage for which the structure is designed that will ultimately influence the
frequency of maintenance works and the whole life costs associated with these
structures.
1.6.5 Groynes
Typically, timber elements have relatively short design lives and structures have
significant monitoring and maintenance obligations. This may not be practical where
access is difficult or dangerous and this is often cited as a disadvantage. Rock groynes
have less maintenance requirements and lower long-term maintenance costs. Rock
groynes are unlikely to require annual maintenance costs, but may require intermittent
maintenance as for other rock structures.
The need to maintain or replace individual structures may provide opportunities for
modification or adaptation during the scheme life and, in some circumstances such as
a large groyne field, the individual structures can be replaced on a rolling programme of
approximately the same duration as the structure life. This enables expenditure to be
maintained at a relatively steady level while facilitating a long-term relationship with
external contractors, evolution of design and construction practices, and continuity of
knowledge and experience.
Timber groyne maintenance activities are recorded in Crossman and Simm (2004) and
may include the following specific activities:
• replacement of damaged or degraded elements
• adjustments to structure profile or configuration
• repairs to ensure public safety
• preventative action to avoid further damage – may include replacement of
pile protection or replacement of loose planking
Available costs of timber groyne maintenance are limited, although some specific
information from the Defra/Environment Agency asset deterioration report
(Environment Agency 2009) is provided below.
• Bournemouth Borough Council cited maintenance costs of around £500 per
groyne per year.
• Canterbury Borough Council reported annual inspection and plant
replacement costs of £3,000–5,000 to maintain about 430 groynes
(excludes cost of timber).
The costs of timber groyne maintenance recorded by Suffolk Coastal District Council
(Suffolk Coastal District Council 2009) suggested the following examples:
• Dover coastal defence maintenance cost per groyne is approximately £700
per groyne
• estimated repair work to Great Yarmouth timber groynes is approximately
£1,000–2,000 per year per timber groyne
• Waveney District Council estimated maintenance costs for timber groynes
is £1,500 per year per groyne for the first 10 years and then reducing
As part of the Southern Coastal Group, New Forest District Council has developed
some innovative solutions to timber groyne management so as to reduce costs,
simplify the maintenance, provide a more sustainable solution and improve the
condition and performance of the groynes at two sites – Calshot and Milford-on-Sea.
Due to the coarse beach material at Milford-on-Sea, shingle abrasion was a major
issue with lifespan of piles of typically 3–5 years. Poor quality mild steel fixings had
also been used and were severely corroded creating weak connections and beach
material was being lost as some piles were too short and also unstable. The solutions
developed included the use of timber pile protection which can be replaced when worn
and costs four times less than pile replacement (an annual saving of £13,000 has been
made). Use of stainless steel fixings which do not corrode and can be reused has
created further savings. The depth and length of the groynes have been increased
which, combined with beach recharge, has helped to reduce future large-scale losses
(Southern Coastal Group 2010).
General costs associated with coastal monitoring will vary depending on the type of
monitoring works undertaken. Average costs suggested by the strategic regional
coastal monitoring programmes (2011-2016) indicated that the proposed funding of
ongoing national monitoring could be approximately £1,558 per km per year, though
there is regional variability (£960–1,955 per km per year) that reflects local risks and
assets (Southern Coastal Group 2010a).
1.7.1 Walls
Following structure completion, there should be regular monitoring to ensure the
structure continues to perform satisfactorily. Environmental monitoring should take
place such as:
• beach/seabed levels adjacent to the structure
• wave, wind and tidal climate at the site
Regular monitoring is important to plan for maintenance. Generally the frequency
should be immediately after construction, after extreme storm events, annually and
every five years for submerged elements. Monitoring methods for modified seawalls
include (CIRIA 2010a):
• visual inspection at low tide
• general, fixed aspect and aerial photography
• profile surveys of structure and foreshore
• inspection of voids
Erosion of the toe is a common problem and is the mechanism most likely to cause
structural failure. Monitoring of the toe is therefore vital.
1.7.3 Groynes
Post-project monitoring should be carried out at least bi-annually to assess the beach–
dune evolution and the success of the scheme relative to its objectives. Monitoring
must include adjacent shorelines as well as those immediately within the groyne
scheme.
Groyne heights, lengths and profiles can be modified if monitoring indicates that the
initial layout is not achieving the required objectives. Modification is easier to achieve
with timber rock structures than with rock.
Monitoring methods for groynes typically include:
• visual inspection at low tide
• general, fixed aspect and aerial photography
1.8.1 Walls
Deterioration rates for walls within the coastal environment depend in part on their
construction materials and on the degree to which they are exposed. Table 1.11
provides best estimates for deterioration (in years) from condition Grade 1 (very good)
to each consecutive grade for different materials. For example, it takes 15 years for a
Grade 1 brick and masonry defence to deteriorate to a Grade 2 (good) condition and
75 years to deteriorate to a Grade 4 condition (poor).
1.8.3 Groynes
The design life of a timber groyne will depend on a number of factors but particularly:
• biological attack – fungal decay, marine borers and insect attack
• abrasion – which is linked to sediment characteristics and can lead to faster
deterioration of timbers and consequent reduction in life expectancy)
• wave climate – more aggressive wave climate will reduce life expectancy
Within the second edition of the Beach Management Manual (CIRIA 2010a), Case
Study 15.1 Coastal Defences at Whitstable Kent contains detailed information on the
design issues affecting the cost of this timber groyne and beach recharge project. With
respect to whole life costing the local authority client required a design life of 80 years
(before total renewal) with an allowance for major maintenance after 40 years.
Following tests it was concluded that, for coastal defence structures, tropical
hardwoods were the only type of timber that met this requirement.
Further research on the performance of different types of timber has been carried out
by Bournemouth Borough Council. Greenheart, Purple Heart and Ekki have been
tested at a demonstration site and to date Ekki has been most successful. The Purple
Heart timber planks were found to be heavily infested with gribble (marine borers) after
a very short period of time.
Timber groynes have a typical life expectancy of 10–25 years but life expectancy
depends on the species of timber used. In tests, Canterbury Council found that non-
tropical hardwoods (oak and Douglas fir) lasted between 5 and 10 years before decay
affected the structure. The types of tropical hardwoods used in groynes (Greenheart
and Ekki) have strengths twice that of non-tropical European hardwoods such as oak.
Timbers of lesser strengths would have to be increased in section to meet the design
stresses, with consequential increases in cost and timber usage (CIRIA 2010a).
In schemes requiring large quantities of tropical hardwoods, evidence of a legal and
sustainably managed source will usually be required. The Whitstable scheme was able
to make a substantial cost saving by removing purchase of the timber from the main
contract and purchasing on a shipment basis direct from the country of origin (CIRIA
2010a).
The average design life of rock groynes is anticipated to be higher – in the region of 50
years – but will depend on the local conditions and design of the structures (as
discussed above).
Gabion groynes are generally not recommended due to the short design life associated
with these structures. The design life may be in the region of 1–5 years, but will depend
heavily on the local conditions and shoreline exposure.
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