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A CH2M Hill Company
Options Appraisal Report
Document: DCSSPA-WBS301/001
Scarborough Spa Coast Protection Scheme
Birse Coastal for Scarborough Borough Council
November 2011
A CH2M Hill Company
Halcrow Group Limited
Burderop Park, Swindon, Wiltshire SN4 0QD
tel 01793 812479 fax 01793 812089
halcrow.com
Halcrow Group Limited has prepared this report in accordance with
the instructions of the client, Birse Coastal for the client’s sole and specific use.
Any other persons who use any information contained herein do so at their own risk.
© Halcrow Group Limited 2011
Halcrow is a CH2M HILL company
Options Appraisal Report
Scarborough Spa Coast Protection Scheme
Birse Coastal for Scarborough Borough Council
November 2011
Document history
Options Appraisal Report
Scarborough Spa Coast Protection Scheme
Birse Coastal for Scarborough Borough Council
This document has been issued and amended as follows:
Version Date Description Created by Verified by Approved by
1.0 14.11.11 Draft for Birse review J Young
S Trinder*
J Young
R Moore*
J Young
1.1 15.11.11 Draft for SBC submission J Young
S Trinder*
M
Glennerster
R Moore*
J Young
1.2 30.11.11 Final Submission J Young
S Trinder*
M
Glennerster
R Moore*
J Young
*Cliff sections
Contents
1 Introduction 3 1.1 Site Location 3
1.2 Background to the Scheme 3
1.3 Purpose of this Report 3
1.4 Structure of this Report 3
2 Site Description 3 2.1 Coastal Defences and Slope Instability 3
2.2 Geological Setting 3
2.3 History of Slope Instability 3
2.4 Nature of Slope Instability at the Spa Cliffs 3
2.5 Hydrogeology and groundwater 3
2.6 Monitoring 3
2.7 Ground Models and Stability Analysis 3
2.7.1 Ground models 3
2.7.2 Results of analysis 3
2.8 Priority Areas 3
2.9 Foreshore problems 3
2.10 Interdependency of cliffs and foreshore 3
3 Existing Engineering Measures 3 3.1 Foreshore Coast Protection Measures 3
3.2 Existing Slope Stability Measures 3
3.2.1 Low height masonry retaining walls 3
3.2.2 Ground anchors 3
3.2.3 Retaining wall, rock revetment and mesh and anchors 3
4 Information Collected for Appraisal 3 4.1 Summary of Survey Work 3
4.1.1 Topographic surveys 3
4.1.2 Bathymetric survey 3
4.1.3 Geomorphological mapping survey of the cliffs 3
4.1.4 Extended Phase 1 Habitats survey 3
4.1.5 Baseline noise survey 3
4.1.6 Rapid marine ecology review 3
4.1.7 Cultural Heritage 3
4.1.8 Foreshore ground investigation 3
4.2 Environmental Scoping Consultation 3
4.3 Project Development Workshop 3
5 Options Considered - Foreshore Works 3 5.1 Appraisal Period & Design Standard 3
5.2 Climate Change 3
5.3 History of Options Appraisal for the Spa Frontage 3
5.4 Short Listed Options 3
5.5 Option Costs 3
5.6 Carbon Calculator 3
6 Options Considered – Cliff Stabilisation Works3 6.1 Introduction 3
6.2 Solutions identified for the Spa cliffs (Priority Area 1) 3
6.3 Cliff Stabilisation Options 3
6.4 Outline Costs of Options 3
7 Option Selection 3 7.1 Introduction 3
7.2 Appraisal Summary Tables 3
7.3 Scoring and Weighting 3
7.4 Economic Indicators 3
7.5 Option Selection Discussion 3
7.6 Summary of Scheme Preferred Option 3
8 What Happens Next 3
9 References 3
Appendices
A Summary Consultation Feedback
B Selected Stability Analysis Results
C Drawings
D Cost Breakdowns
E Appraisal Summary Tables
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1 Introduction
Halcrow Group Ltd has been commissioned by Birse Coastal as Designer for the
Early Contractor Involvement (ECI) stage of the Scarborough Spa Coast Protection
Scheme, for Scarborough Borough Council.
1.1 Site Location
The Spa Management Unit (22A/4 to 22B/2) covers approximately 450m of coastal
defences along the undercliff at the southern end of South Bay. The defences are
backed by a cliff system which, for the purposes of this report, covers a c.1km stretch
of coastline between the Cliff Bridge to the north and the former location of the South
Bay Pool to the south (see Figure 1).
The cliffs vary in height from approximately 50m above Ordnance Datum (AOD)
near the Spa Chalet to approx. 60mAOD above the old South Bay Pool. The cliff top
forms a gently undulating plateau upon which the Esplanade runs parallel to the cliff
line. Many large private residences and hotels are situated on the cliff top, set back
between some 30m and 100m from the cliff edge.
The cliff slopes have been extensively landscaped over the last hundred years or so
and feature a dense network of footpaths and steps, some formal terrace gardens and
a Cliff Lift in the vicinity of the Spa Complex. The foreshore comprises a wide sandy
beach overlying a rock platform which surfaces in places. A seawall and promenade
protects the base of the cliffs as far as the rock revetment beneath the former Holbeck
Hall Hotel.
1.2 Background to the Scheme
The Scarborough Spa Coast Protection Scheme is the first scheme to be promoted
from the ‘Holbeck to Scalby Mills Strategy Review’ (Halcrow 2008).
The existing coastal defences are in a poor condition and are reaching the end of their
serviceable life, wave overtopping can be excessive posing a risk to pedestrians and
the cliffs to the rear of the Spa show strong evidence of instability. (A full discussion
of the problem is provided in Section 2).
During the Strategy a range of generic solutions were considered at a strategic level
to address the problems at this section of coast. Those that were considered to be
technically and economically viable were evaluated further by economic and
environmental appraisal. At a strategic level, a preferred solution to hold the line by
constructing a rock revetment in front of the existing seawall, a new wave return wall
along with slope stabilisation works was identified as the most acceptable solution,
considering technical, economic and environmental issues.
This solution is undergoing development as part of the current project which will
result in the preparation of a Business Case which will be submitted to the
Environment Agency in order to obtain funding for construction of the works. The
project is classified as a ‘Supported Change’ project under Flood & Coastal Erosion
Risk Management – Appraisal Guidance (FCERM-AG, Environment Agency, 2010).
The Business Case is being produced jointly by Royal Haskoning (as Employer’s
Agent) and the Birse Coastal/Halcrow Design & Build project team.
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1.3 Purpose of this Report
This report documents the outcome of the Options Appraisal stage of the project. The
solution identified by the strategy has been considered further, including a short-list
of options to deliver the strategic solution.
1.4 Structure of this Report
Section 2 provides an overview of the site, with Section 3 discussing the engineering
measures currently in place on site. Section 4 summarises surveys, consultation and
workshops undertaken to support the development of the scheme and inform the
options appraisal.
Sections 5 and 6 discuss the options considered for the foreshore element of the
scheme and the cliff works respectively. Although the options are presented within
separate sections, the foreshore and cliff works are interdependent. Section 7
discusses the option selection process and identifies the preferred scheme option.
Section 8 explains the next stages of the project and Section 9 documents the
references used within the report.
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Figure 1 Location Plan
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2 Site Description
2.1 Coastal Defences and Slope Instability
The existing coastal defences at the Spa, along with the majority of the coastal defence
systems in Scarborough, are ageing, in poor condition and subject to an aggressive
wave climate. The areas behind the defences are also currently subject to to wave
overtopping rates that far exceed recognised safe levels and which are predicted to
increase due to the effects of climate change and sea level rise. Furthermore, the
defences at the Spa are backed by steep coastal slopes which show evidence of
ongoing ground movement and instability. These factors, together with
environmental considerations of predicted climate change scenarios and sea level
rise, demonstrate the requirement for proactive coastal defence and cliff stabilisation
works to minimise any potential risks to the public and coastal assets in the Spa area.
The Scarborough Coastal Defence Strategy Review: Holbeck to Scalby Mills
(Halcrow, 2008) identified the northern section of the Spa Cliffs as the “degraded
remnants of a large pre-existing landslide”, and the southern section as “over
steepened and formed of weak glacial materials prone to cliff failure”. The strategy
identified the risk of cliff instability to the coastal defence scheme proposals including
the potential for major reactivation of the “pre-existing” landslides caused by extreme
groundwater levels and slope drainage failure, and renewed cliff foot erosion should
the seawalls fail; for the over steepened cliff section there is concern that a major
“first-time” landslide could occur caused by the expansion of shallow landslides,
extreme groundwater levels and slope drainage failure, and renewed cliff foot
erosion should the seawall fail.
2.2 Geological Setting
The geology of the Spa Cliffs consists of a variable thickness of glacial sediments (tills
and sand and gravel lenses) which cap the Middle Jurassic Scalby Formation
(mudstone and sandstone) or the underlying Scarborough Formation (limestone and
mudstone) (Rawson and Wright, 2000). This stratigraphy has been confirmed by
ground investigations undertaken on the cliffs of South Bay (Norwest Holst, 1996,
1998) following the large failure at the Holbeck Hall Hotel in 1993.
The till was deposited during the Late Devensian glaciation around 18,000 years ago
by an ice sheet flowing towards the south to southwest from the North Sea basin. The
thickness, elevation and composition of the till are known to vary considerably
within South Bay. At the northern end of the study area near the Spa Chalet, the till is
approximately 60m thick, and extends down to typically -13mAOD. In the southern
part of the study area, the till is considerably thinner at approximately 27m,
extending down to 24mAOD. In the north, the borehole logs show greater thicknesses
of sands/gravels, typically between 20mAOD and -7mAOD. Further south, localised
lenses of sand are present. The sand and gravel lenses may represent laterally
discontinuous glacial meltwater channels and will be more permeable than the
surrounding materials, acting to transport water and permit build-up of porewater
pressures within the till materials.
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The Scalby Formation is present below the till and forms part of the Ravenscar Group
of Middle Jurassic age. It comprises interbedded sedimentary strata including
mudstones, siltstones and sandstones which have been subsequently weathered.
The elevation of rockhead and thickness of the till varies considerably across South
Bay and this determines whether bedrock or till crops out at sea-level. Rockhead rises
to the south from -13mAOD in the vicinity of the Spa Chalet, where till is exposed at
sea-level; behind the Spa complex rockhead has been found at 7m AOD where the
majority of the cliff is formed in till; and in the south at the Italian Gardens, only the
upper half of the cliff is formed in till and rockhead is exposed at approximately
27mAOD (Norwest Holst, 1998).
The severity of weathering of rockhead also varies across the study area. In the south,
below the Italian Gardens, the top 4m of the Scalby Formation is described as a
‘completely weathered’ siltstone. In the vicinity of the Cliff Lift in the centre of the
site, the top 3m of the Scalby Formation is reported to be ‘moderately weathered’
sandstone (Norwest Holst 1998). It is thought that the base of the 1993 Holbeck Hall
Hotel landslide (located approx. 250m south of the study area) was located within a
heavily weathered sandy siltstone layer of the Scalby Formation somewhere below
25m AOD (Moore, 1996; Geotechnical Engineering, 1985).
2.3 History of Slope Instability
A history of instability has been recorded within the Spa Cliffs frontage at
Scarborough since 1737. Numerous investigations and previous studies have been
undertaken at the site. For further details the reader is referred to Halcrow’s July 2011
Spa Cliff Stabilisation Data Review & Survey Report (Halcrow, 2011).
2.4 Nature of Slope Instability at the Spa Cliffs
Halcrow Group Ltd undertook geomorphological mapping of the study area in May
2011. This is reported in depth in Halcrow’s July 2011 report. Essentially the mapping
showed that the nature of the cliff failures/instability evident differs in the north of
the study area compared to the south, roughly along a line some 50m south of the
Cliff Lift. The northern geomorphological map (covering The Spa area) is reproduced
as Figure 2.
Evidence of previous coastal landslides indicates that to the north of the line cliff
instability features are generally shallower and more localised, within the glacial till
materials, except for an area just north of the Spa complex where a wide area of back-
tilted ground and the presence of a ridge/central plateau suggests the existence of a
deep-seated landslide system.
South of the line, there is evidence from the mapping of more extensive deep-seated
coastal landslide systems, with movement occurring within the Scalby Formation.
The shape of the large, deep relict rotational slides is partly masked by further slips
which have occurred at the lateral margins of the larger slips.
Cliff instability and ground movement damage noted during the mapping is
widespread across the The Spa area as shown on Figure 3. It should be noted that
there were constraints with respect to access for the damage survey, for example the
area above the Spa complex which could not be directly inspected due to footpath
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closures. Also the foreshore road and the Spa complex buildings themselves were not
inspected for damage. Further details about the damage survey, including the
classification system, are given in Halcrow’s July 2011 report. The damage at the site
was generally classified as Slight, with a few incidences of Moderate damage in the
vicinity of the Cliff Lift and several notable areas of concern and Serious damage
above the Spa complex. A feature classed as Negligible damage is shown in the north
of the site above the steep slopes directly above the foreshore road.
Slope stability analyses were conducted by Mouchel and found that the lowest
resistance shear planes were located within a weak mudstone layer at the interface
between the glacial till and underlying bedrock. They suggest that rainfall is likely to
cause excess porewater pressures within this layer promoting slope failure. The
analysis also showed that high piezometric levels near the cliff toe reduced the
overall cliff stability significantly (Mouchel, 2009).
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Figure 2: Geomorphological Map (North)
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Figure 3: Damage Map (North)
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2.5 Hydrogeology and groundwater
Groundwater strikes recorded in boreholes located in South Bay were reviewed by
Mouchel during their geotechnical monitoring programme.
Mouchel noted the following main points in their series of monitoring reports,
summarised in Halcrow’s 2011 report:
• In general, groundwater strikes were recorded as having slow to slight seepage
after 20 minutes, suggesting that they relate to perched water tables within the
tills which have little hydrostatic pressure.
• At the till-bedrock interface large variations in piezometric pressures were
noted. Mouchel concluded that it is likely that during high rainfall events,
porewater pressures rise sufficiently to trigger slope instability.
• Groundwater monitoring carried out by Mouchel appears to largely reflect
seasonal rainfall patterns, especially at shallow depths. Some exceptions
indicate increased groundwater levels over the summer months, which are
attributed to blocked drainage or other external influences. There is no overall
pattern of change in groundwater levels observed within South Bay cliffs area
over the monitoring period.
It is noted that Mouchel has included groundwater levels monitored in inclinometer
tubing in their assessment. Results should be reviewed at a future date excluding this
data, because inclinometer installations are fully grouted and may permit water
ingress from the surface, thus making the data unlikely to be representative of
surrounding groundwater levels.
Springs and seepages at the ground surface were recorded by Halcrow during the
geomorphological mapping survey in May 2011. Seepages recorded were
concentrated in the south of the site in the area of cliffs above the former South Bay
Pool and some seepages were noted from within the rock outcrops at lower levels in
the cliffs south of the pool. One seepage was noted above the Spa complex in the
lower part of the slope. These seepages could be natural or result from drainage
originating from development and services.
2.6 Monitoring
Scarborough Borough Council has commissioned Mouchel to carry out regular
monitoring of a number of its coastal sites. South Bay is included within the
monitoring regime. Results of the groundwater monitoring are discussed under
Section 2.5 above. Results of inclinometer monitoring are summarised below. Further
details are provided in Halcrow’s July 2011 report.
A series of ground investigations carried out in the late 1990s saw the installation of
12 inclinometers and 22 piezometers within the cliffs of South Bay. Further
inclinometers have been installed in late 2010/early 2011 to replace those which were
no longer functioning.
Monitoring to 2006 showed deep movement in the north, centre and south of the site
(BHI1 – 20mm movement at 17m depth in the cliff top at northern end of Esplanade,
St Nicholas Cliff, BHH6 – 25mm at 14m depth in the cliff top directly behind the Spa
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complex and Bh2 – 7.5mm at 30m depth at the south eastern extent of the Putting
Green in the south of the site).
More recent monitoring under the Mouchel contract has revealed movement
attributed to surface creep or shallow instability up to approx. 6.5m depth behind the
Spa, just north of the Cliff Lift, mid-slope and immediately above the Clock Café, mid
slope below the Italian Gardens and in the Putting Green area in the south of the site.
Monitoring of the replacement instruments has begun but is too early to show any
significant movement.
2.7 Ground Models and Stability Analysis
2.7.1 Ground models
Preliminary ground models were prepared for five section lines through the South
Bay Cliffs, from Line 1 in the north of the study area, Lines 4 and 5 in the vicinity of
the Spa complex, Line 6 close to the Clock Café and Line 11 through the cliffs above
the former South Bay Pool. The particular section lines were chosen for analysis to
characterise the relative stability of the various cliff behaviour units as identified from
previous studies. The locations of the section lines are shown on Figure 4. This
analysis is reported in full in the ‘Cliff Stability Stage 1 Part 2 Report – Ground
Modelling and Options Identification’ (Halcrow, 2011). The key results for the Spa
frontage are summarised below.
2.7.2 Results of analysis
Results of the analyses undertaken are expressed in the form of an overall or global
factor of safety. For new design, a global factor of safety of greater than 1.3 would
normally be desirable. In assessing stability of existing slopes a lower factor of safety
of 1.2 may be considered acceptable. A global factor of safety is calculated as the sum
of forces resisting movement/sum of disturbing forces for the slope.
The slopes have been analysed with post-peak parameters, modelling long term
conditions. Published data shows that as the age of slopes increases, the intact
strength of clays (in particular fissured clays) reduces with time to a strength less
than the peak strength. This strength is known as the post peak effective shear
strength (or fully softened effective shear strength) and has been modelled for the
tills, mudstones and siltstones at the Scarborough South Bay with c’ = 0kPa and peak
values of ø’ (angle of internal friction). Refinement of the c’ value would be required
to model current conditions and this has not been undertaken. Therefore the models
show that with time (and given the assumptions used for example for groundwater
levels) the slopes will become less stable and in due course the slopes modelled will
fail, where a factor of safety of less than 1 resulted.
The slopes have also been modelled with residual parameters used in particular
cases, representing the scenario where an ancient failure surface could be reactivated,
in particular where inclinometers or geomorphological features provide evidence that
movement has already occurred at a particular depth.
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Line 4
Line 4 is located above the Spa complex. Borehole logs indicate that the upper part of
the slope is formed from cohesive Glacial Till deposits, with Scalby Formation
sandstone outcropping at beach level. There is uncertainty as to the founding strata of
the Spa complex, whether the buildings are underlain directly by bedrock, or whether
Glacial Till is present beneath. Further ground investigation is recommended beneath
or beyond the Spa complex to confirm the ground conditions for this section.
Enquiries to Scarborough Borough Council regarding the construction of the Spa
complex or the presence of cellars would also help in determining the ground
conditions beneath the building.
The first model presented in Appendix B for Line 4 has a minimum factor of safety of
1.4 for the slope above the Spa complex, modelled with assumed groundwater levels
in the upper part of the cliff and post peak parameters in the mudstone extending
beneath the Spa complex. Given there is evidence of movement in the area, this
model needs to be refined as it is not reflecting the anticipated long term situation of
the cliffs at this location.
Further analysis was carried out, using residual soil parameters for the Scalby
Formation Mudstones and modelling a deeper slip surface passing beneath the Spa
complex as instability has been noted on site in this area, suggesting movement has
already been occurring in this slope. A factor of safety of 0.9 was recorded for this
slip, showing that with the ground conditions as modelled, the slope in this area will
not be stable in the long term and suggesting that residual parameters may be acting
over parts of the slip surface in the current situation. Note this model assumes that
the mudstone extends beneath the Spa complex, which would need to be confirmed
by ground investigation (planned for the detailed design stage).
The results are presented in Appendix B.
Line 5
Line 5 is also located behind the Spa complex, however the Glacial Till slope is
slightly steeper than section Line 4.
Borehole logs indicate that the upper part of the slope comprises cohesive Glacial Till
deposits. A significant thickness of glacial sands and gravels was encountered in
BHG1 (SBC Report 63) with Scalby Formation sandstone outcropping at beach level.
As for Line 4, there is uncertainty whether the Spa complex is directly underlain by
bedrock, or whether Glacial Till is present beneath the building. Further ground
investigation is recommended beneath or beyond the Spa complex to confirm the
ground conditions for this section.
A minimum factor of safety of 0.65 was calculated for the slope above the Spa
complex using post-peak parameters, modelling a slip which daylights above the Spa
complex. This low factor of safety is considered to be due to the steepness of the
Glacial Till slope and the high groundwater level modelled within the slope based on
data from monitoring at BHG3. A better understanding of the groundwater
conditions within this slope is required in order to improve the accuracy of this
model. Again, significant cohesion must be operating within the slope for the slope to
remain at such a high angle.
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A deeper slip surface was also modelled, passing beneath the Spa complex within the
Scalby Formation Mudstone. A factor of safety of 1.2 was recorded for this slip using
post-peak parameters. This is a higher factor of safety than resulted from analysis of a
similar slip surface in Line 4, because the mudstone layer is overlain by higher
strength sandstone in Line 5, whereas in Line 4 the mudstone was encountered at the
top of the Scalby Formation. As explained above, additional ground investigation in
the area of the Spa complex will aid understanding of the ground conditions at the
toe of the slope.
The results for the Line 5 analysis are presented in Appendix B.
Figure 4 Location of analysis section lines
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2.8 Priority Areas
Various site walkovers, damage mapping and stability analysis have resulted in four
Priority Areas being identified within the South Cliffs Area:
• Area 1: The full height of the Spa cliffs, directly above and behind the Spa
complex
• Area 2: The central area of the Clock Café cliffs, south-west of the Clock Café
• Area 3: The central area of the Old South Bay Pool cliffs, directly above the
location of the Old South Bay Pool
• Area 4: Area of rock outcrops presenting a potential rockfall hazard in the
lower part of the cliffs between Old South Bay Pool and the Clock Café.
The priority areas were identified on the basis of the following criteria:
• Evidence of recent and ongoing movement (Areas 1, 3 and 4)
• Presence of seepages (Areas 1, 2 and 3)
• Presence of oversteep till slopes (Areas 1, 2 and 3)
• Potential for injury to people by direct rockfall (Area 4)
• Potential for damage to significant properties/businesses (Areas 1 and 2)
• Potential for landslip/falls affecting amenity areas (Areas 1, 2, 3 and 4)
Priority Area 1 falls within the The Spa Management Unit and is illustrated and
described below (the other Priority Areas are described in full in the ‘Cliff Stability
Stage 1 Part 2 Report – Ground Modelling and Options Identification’ (Halcrow,
2011)). These areas fall outside the Spa management unit and will be considered
separately as the assessment of the other management units progresses.
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Figure 5 Priority Area 1
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Priority Area 1 is considered to be the highest risk area, with receptors at risk
including the cliff top properties and the Spa complex. Priority Area 1 (PA1) defines
a developing incipient landslide within a larger cliff behaviour unit at the Spa, which
covers the full extent of the shoreline management unit 22A/4 to 22B/2, and a small
section of cliffs to the north. PA1 is characterised by steep Glacial Till slopes (up to
48°) overlying mudstones, sandstones and siltstones of the Scalby Formation at levels
close to or below beach level. Ground investigation has shown the till to comprise a
considerable thickness of sands and gravels beneath firm to stiff clays, with thinner
beds of sand and gravels and clays below (BHG1 and G2).
Groundwater monitoring of deep and shallow installations in BHG1 indicates a wide
variation in groundwater levels and the presence of perched water in the glacial
materials. A response zone at the top of the sands and gravels indicates water is
confined in this layer.
Monitoring of the inclinometer in BHG2 has shown possible surface creep recorded
in the top 7m with possible movement at depth in the glacial clays (Mouchel, 2011).
Further monitoring is needed to confirm the depths of these movements. In the
interpretation of future data, the exact location of the instruments should be
considered. From the September 2011 Halcrow site visit, it appears that the
replacement borehole BH14 (SBC Report 188) has been installed outside the zone of
movement of the landslide system.
More compelling evidence is apparent by linking surface damage due to ground
movement with the cliff geomorphology, including: cracking to the steps south of the
cliff lift; displacement of the keystone in the arch beneath the cliff lift; significant
damage to the path and wall north of the cliff lift towards the top of the slope;
cracking and depression along the road behind the masonry retaining wall at the top
of the slope; cracking of the masonry wall at the top of the slopes; movement in the
footpath on the slopes above the area of the recent soil nailing; and signs of distress to
the footpaths in this area. There is evidence that relatively recent repairs/ additional
support has been needed to the slopes in this area: the presence of the newly soil-
nailed slopes above the main spa complex and the anchored wall some 50m north of
the cliff lift.
Photo 2.8a Displacement of keystone beneath cliff
lift
Photo 2.8b Recurring
damage to path and wall
north of Cliff Lift
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Photo 2.8c Depression behind retaining wall above
Spa slopes
Photo 2.8d Movement
evident in path directly
above soil nailed area
Photo 2.8e Recently soil nailed slopes above Spa
complex.
Some of the till slopes in this area are very steep, typically 40° to 42° and one area up
to 48°. A significant amount of cohesion must be acting for these slopes to be standing
at such steep angles. As the effective cohesion reduces with time, the slopes will
become less stable. Stability analyses show inherently unstable slopes for such steep
angles.
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2.9 Foreshore problems
The seawall along the toe of the cliff is more than 100 years old and is in a poor
condition with block movement and loss of pointing visible across its surface. The
sea wall has suffered damage in the past which has been repaired by Scarborough
Borough Council.
The seawall suffers from regular wave overtopping, as illustrated in Photos 2.9a to
2.9c.
Photo 2.9a Overtopping at the Spa frontage, 8th April 2005
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Photo 2.9b Overtopping at Spa, 20th March 2007 (approx 1 hour before high water)
Photo 2.9c Overtopping at Spa, 23rd November 2007
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Overtopping discharge occurs because of waves running up the face of a seawall. If
wave run-up levels are high enough water will reach and pass over the crest of the
wall. In cases where the structure is vertical (like the Spa seawall), the wave may
impact against the wall and send up a vertical plume of water over the crest.
Although overtopping discharges are quoted in terms of litres per second per metre,
in reality there is no continuous discharge over the crest of a structure during
overtopping. The process of wave overtopping is very random in time and volume
and this contributes towards the safety risk for pedestrians along the promenade.
The random nature of the hazard means that it can take pedestrians by surprise.
The main hazards on or close to coastal structures are of death, injury, property
damage or disruption from direct wave impact, or death by drowning. On average,
approximately two to five people are killed each year in the UK through wave action,
chiefly on seawalls and similar structures (although this rose to 11 in the UK during
2005) (EurOtop, 2007). Damage to property (buildings and cars) is also a
consideration at the Spa frontage.
The Holbeck to Scalby Mills Strategy Review (Halcrow 2008) reports that foreshore
lowering is also a problem within the strategy area. Topographic beach profiles for
the period 2008-2011 for the South Bay Area have been downloaded from the North
East Coastal Observatory website and imported into Halcrow’s SANDS software
(Shoreline And Nearshore Data System). The profiles show an expected seasonal
trend of erosion over the winter period and accretion over the summer. However,
the results of a cumulative assessment across the beach profiles indicate an overall
loss of sediment across the southern part of South Bay over that period.
Foreshore lowering (erosion in the vertical plane) results in deeper water at coastal
structures, which can result in higher wave forces acting upon the defences. Higher
waves may also increase wave overtopping discharge. Foreshore lowering can also
result in structures being undermined, leading to failure and potential marine erosion
of the toe of the cliffs, if not repaired promptly. Foreshore lowering will also mean a
reduction in the weight of material acting at the toe of the cliff, which could
contribute to stabilisation issues.
2.10 Interdependency of cliffs and foreshore
New guidance on the funding and assessment of coastal erosion, cliff instability and
coastal landslides in England and Wales was published in October 2010, and
published on the EA’s website in March 2011. The guidance recognises that in all cliff
systems there are many interrelated factors which may contribute to coastal
instability and erosion in addition to the action of the sea (such as, groundwater). It is
recognised that coastal defences may be at risk from landsliding occurring behind the
defences itself in situations where a holistic approach is not adopted. Grant in aid
under the Coast Protection Act 1949 may be provided to assist with investigations of
coastal erosion risk problems.
Fundamental to establishing a compelling case for funding is the requirement for
adequate demonstration of the following:
• Cliff recession processes must have been properly investigated and be adequately
understood to inform management options.
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• A fundamental link with the sea must be established to allow a project to be
considered for grant in aid under the Coast Protection Act 1949.
• ‘Cliff recession’, ‘coastal erosion’ and ‘erosion by the sea’ includes processes
which are affected by the action of the sea such as coastal landslides.
Preliminary work recently completed for this Spa Cliffs pre-PAR stage has identified
the full extent, mechanisms and causes of cliff instability and erosion that threatens
existing coastal defences and amenity infrastructure and proposed coastal defence
improvements.
The Spa Cliffs were formed by coastal erosion processes operating over the past 3,000
years when sea level reached current-day levels. Over the historical period coastal
erosion has been prevented through the introduction of coastal defences in Victorian
times. However, these defences have not prevented the ongoing degradation and
instability of the steep cliffs above, albeit these cliff instability processes have been far
less active than they would have been in their natural state.
Nonetheless, over the historical period the Spa Cliffs have been subject to ongoing
ground movement and occasional landslide events which have had an adverse
impact on the infrastructure and coastal defences at the Spa. The site surveys and cliff
stability analysis completed for the pre-PAR demonstrate widespread evidence of
progressive deep-seated failure of the Spa Cliffs, superimposed by localised ‘shallow’
failures of the over-steepened slopes forming the headscarp. These cliff failure
mechanisms result from the removal of toe support and slope over-steepening in the
past by coastal erosion; progressive failure of the cliffs is governed by the effective
stresses in the cliff materials which fluctuate in response to weathering processes and
seasonal changes in groundwater. The frequency and magnitude of ground
movement and landslide events can be exacerbated by development activities and are
particularly sensitive to uncontrolled drainage and leakage of water.
Of most concern at the Spa is the evidence of damage to infrastructure around the
headscarp and sidescarps, including displacement of the high gravity retaining wall
below Esplanade Road, the Cliff Lift arch bridge, and the many walls and paths
constructed on the cliffs. Together, the nature of this evidence reveals ongoing
incipient deep-seated failure of the Spa Cliffs which if fully mobilised would have a
profound impact on the Spa buildings, cliff access and gardens, coastal defences and
Esplanade Road. In this context, it is worth remembering the events that unfolded at
the Holbeck Hall Hotel in South Bay in 1993 when failure of the cliffs developed
rapidly over a period of hours and days culminating in 60m or more recession of the
cliff top, run-out of a large debris lobe over the sea-wall and across the foreshore, and
destruction of the hotel, cliff paths and amenity. Intervention in the form of a
combined coastal defence and cliff stabilisation scheme will mitigate such potential
outcome.
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3 Existing Engineering Measures
3.1 Foreshore Coast Protection Measures
The foreshore comprises a wide sandy beach overlying a rock platform which
surfaces in places. A seawall and promenade protects the base of the cliffs from
marine erosion as far as the rock revetment beneath the former Holbeck Hall Hotel to
the south of the scheme area.
The seawall protects the toe of the cliff from marine erosion and provides some
degree of protection to the buildings on the under cliff (The Spa Complex) from wave
action and wave overtopping.
The seawall fronting the Spa is reaching the end of its serviceable life. The 2005
strategy review assessed that the existing defences along this frontage were
inadequate, based on their anticipated structural performance over the next 50 years.
The residual life for this frontage was estimated in the Holbeck to Scalby Mills
Strategy Review as being two to five years, with a likely failure mechanism of wall
cracking, undermining and toe erosion.
Wave overtopping poses a significant risk along this frontage to people, vehicles, the
promenade and associated infrastructure. The predicted mean wave overtopping
rate for a present day event with an Annual Exceedance Probability (AEP) of 10% (1
in 10 year event) is almost 20 l/s/m. The current overtopping rate exceeds
recommended overtopping limits for pedestrians in EurOtop (EA/ENW/KFKI, 2007)
– a mean discharge of 0.1 l/s/m is recommended for pedestrian safety1. Wave
overtopping rates will increase with the predicted effects of climate change, as sea
levels rise.
Drawing DCSSPA/WBS-301-001 in Appendix C provides an overview of the different
types of seawall construction which are summarised as follows:
Chainage -0 to 30m – Northern tie in
At the northern boundary of the scheme is a set of access steps from the promenade
to the beach. The current ‘spaw’ feature is present within the recessed archway
towards the bottom of the steps.
1 Mean discharge rate of 0.1 l/s/m is recommended for aware pedestrians, clear view
of the sea, not easily upset or frightened, able to tolerate getting wet, wider walkway.
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Photo 3.1a Steps at the northern tie-in
Chainage 30 to 235m – Northern wall section
Masonry blockwork seawall with a splash wall along the crest. The wall dates back
to the mid 19th century, with the current alignment at the Sun Court being
constructed in the early 20th century. The wall is fronted by a concrete and steel sheet
piled apron along an 80m stretch.
Photo 3.1b Northern section, looking south, apron visible in front of wall
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Chainage 235 to 295m – Closed colonnade section
There are two sections of infill panels within the closed colonnade section. The first
consists of three panels of grey concrete, which have been finished to look like blocks.
The second consists of five blue/grey panels constructed of jointed blockwork.
The first section (most southerly) was in-filled with a 1m thick reinforced concrete
wall, and finished to look like blockwork. This location was an extension of the open
colonnade section and was walled off due to the poor condition of the colonnades.
This former section of colonnade has been blocked off at the southern end (where it
abuts the currently open colonnade section) in addition to the wall on the beach
frontage.
The second section (most northerly) has been walled off with concrete blocks of
unknown thickness. It is assumed that the void space behind them is similar to the
colonnade area behind the first section, although this is not known.
This area was blocked off in the late 1970’s or early 1980’s due to the extended
building works carried out to the frontage of the Ocean Room. There is a dividing
wall between the two sections of closed colonnade.
On the outside face of the seawall, there is a set of access steps from the upper
promenade to the beach at the interface between the closed and open colonnade
sections to the south.
Photo 3.1c Closed colonnade section
Chainage 295 to 355m – Open colonnade section
The open colonnade section features three access steps from the beach into the
colonnade area. There is a further set of access steps from within the colonnade to the
upper deck (promenade) level.
Northern closed section
Southern closed section
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Photo 3.1d Open colonnade section
Chainage 350 to 380m – Southern tie-in
At the southern end of the scheme there is a short stretch of wall featuring access
steps to the beach. The wall curves round to the foot of a cobbled slipway.
Photo 3.1e Southern tie-in (southern extent of scheme, slipway to left of photograph)
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3.2 Existing Slope Stability Measures
Numerous engineering measures have been employed in the past throughout the
Scarborough South Bay Spa Cliffs to improve the stability of the cliffs. Figure 5 shows
the location of some of the measures identified during the geomorphological
mapping surveys in 2011.
3.2.1 Low height masonry retaining walls
Low height masonry walls are a feature of the site, located above and below the
network of paths which criss-cross the cliffs (Photo 3.2a). The paths and gardens tend
to follow the geomorphological features of the site and the low retaining walls allow
local steps in elevation to be achieved. The walls provide support to shallow soils,
permitting the formation of shallower slopes behind the vertical walls or allowing
construction of paths on steep slopes. Such walls are easily undermined by deeper
slips and there is evidence on site of distortion and cracking suffered by the walls,
particularly where mortar joints have reduced drainage through the walls (Photos
3.2b and c). Photo 3.2d shows a localised failure of oversteep till slopes above a
masonry wall. Walls have also degraded and crumbled away in areas of the site.
Photo 3.2e shows a failure beneath and through a masonry wall in the slopes above
the Spa complex taken in 2011, with Photo 3.2f (Mouchel, 2009) showing the same
masonry wall/slope failure in 2009.
Photo 3.2a
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Photo 3.2b Photo 3.2c Photo 3.2d
Photo 3.2e Photo 3.2f (Mouchel, 2009)
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Figure 6: Existing stability measures in the vicinity of The Spa frontage
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3.2.2 Ground anchors
Ground anchors have been employed towards the top of the slope in the centre of the
site at Prince of Wales Cliffs, supporting a path and the upper part of the cliffs
(Photos 3.2g and 3.2h). The anchors act in tension, supporting the slope and unstable
materials by anchoring them to deeper soils/rocks using a grouted bar or tendon,
tensioned at the surface with an anchor head. Grouts used may be cement based or
resin based. A reinforced concrete structural element has been formed on the surface
of the soils to seat the anchor heads.
Photo 3.2g
Photo 3.2h (Mouchel, 2009)
The road above the area of ground anchors is constructed above a masonry-faced
retaining wall (Photo 3.2g).
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Soil nailing is known to have been installed in the mid to upper slopes above the
Ocean Room of the Spa complex in February 2003 (SBC Report 152), but no further
details or plans are provided.
Soil nailing was used in 2011 to repair a small slip above the Spa complex. Further
details are provided in Section 2.8.
3.2.3 Retaining wall, rock revetment and mesh and anchors
At the toe of the cliffs above the foreshore road approaching the Spa complex from
the north, engineering works have been undertaken to stabilise a section of cliff some
100m long. SBC Report 152 suggests the works comprised construction of a concrete
counterfort gravity wall, deep drainage and the installation of MacMat R slope-
surface stabilisation mattresses. No details or plans are provided in the report. On
site it is unclear exactly the location or extent of the different measures, but where
vegetation is not obscuring the slope, it is possible to see some of the works. Furthest
north stands an approx 3m high retaining wall, faced in masonry with drainage holes
evident towards the base of the wall (Photo 3.3a). South of this is an inclined section
of rock revetment (Photo 3.3a) and beyond this the slope appears to have been faced
in mesh (presumably the MacMat mattress described in the report) (Photo 3.3c). This
is generally a solution used for rock slopes (mesh and rock anchors). Anchors were
not seen within the meshed area during the geomorphological walkover, however the
consistent slope angles in this area (Photo 3.3b) looking along the road suggest that
engineering measures are present along its entire length. SBC Report 152 suggests the
works were “recent” at the time of writing in 2003 and had been carried out following
a slope inspection study undertaken in 2001.
Photo 3.2i Retaining wall on foreshore approach road
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Photo 3.2j Steep engineered slopes to the foreshore approach road
Photo 3.2k Mesh used as a stability measure adjacent to the foreshore approach road
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4 Information Collected for Appraisal
4.1 Summary of Survey Work
A number of surveys have been undertaken at the site to support the scheme’s
development since spring 2011. These are summarised below.
4.1.1 Topographic surveys
A topographic survey of the seawall and promenade was undertaken, recording
levels along the toe and crest of the wall along with other features such as drainage
outlets, pattress plates, and street furniture. Additionally, eleven profiles through the
cliff area were surveyed to provide information for the cliff stability assessment.
4.1.2 Bathymetric survey
The nearshore seabed area was surveyed to provide information for the numerical
wave modelling. This information will also be useful for planning deliveries to site
during construction.
4.1.3 Geomorphological mapping survey of the cliffs
Fieldwork was undertaken to produce an accurate record of the slope morphology,
identify key geological features, record evidence of damage due to ground movement
and to inform an interpretation of the geomorphology and mechanisms of cliff
instability.
4.1.4 Extended Phase 1 Habitats survey
A desk study was undertaken to gather information on protected sites and species
within 2km of the site. This was followed by a site survey to identify and map all
habitats within and up to 30m from the site, identifying the potential for/presence of
protected or invasive species and habitats within the site boundary.
There is suitable habitat within the site boundary to support breeding birds and bats.
Recommendations were made for further survey and/or mitigation where considered
necessary in relation to the proposed works. Additionally, a large stand of the
invasive introduced species Japanese knotweed is present within the site, on the cliffs.
If works are proposed within 10m of a known stand, then a mitigation/eradication
strategy will be required to ensure compliance with current legislation.
4.1.5 Baseline noise survey
Noise measurements of existing ambient noise conditions were taken at locations
representative of the closest noise sensitive receptors to the proposed works location.
The noise measurements were taken in the daytime, evening and night-time periods.
This baseline noise measurement data will be used to help monitor and manage noise
levels during construction.
4.1.6 Rapid marine ecology review
An initial literature search was followed by a site walkover where the seawall and
intertidal area were inspected, looking for the presence/absence of key coastal
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terrestrial ecological elements, such as sand tolerant plants. The seawall biota was
examined and photographed in order to determine if there were any species of
particular note. Sampling of areas of both rocky shore and sandy shore was
undertaken, assessing the density of macrofauna present. The beach and intertidal
surveys revealed no species of conservation or commercial importance and reflected
the dynamic nature of the shore due to the relatively wave exposed nature of the
shore.
4.1.7 Cultural Heritage
A desk study and site walkover was undertaken to document the known cultural
heritage (archaeology, historic buildings and designated areas) at the site in order to
inform the design of the scheme and provide an early warning of heritage-related
constraints to the scheme. The study concluded that there were three designations
that would be affected by the proposed scheme: the Scarborough Conservation Area,
the Grade II* listed Spa Complex and the Historic Park and Garden of the South Cliff
Gardens. The site (cliff and foreshore) was considered to have low archaeological
potential.
4.1.8 Foreshore ground investigation
A number of trial pits were excavated on the foreshore area in order to investigate
where the rock head lies in relation to the beach surface. Ten trial pits were planned
and executed, with rock (sandstone) being found at 0.25m to 1.2m below beach level
except one trial pit (TP8) where clay was encountered. Six further trial pits were
excavated in the vicinity of TP8’s position to investigate the extent of the clay and to
try to locate bedrock in this position. The clay was found in each of the six additional
trial pits and rock level could not be located in this area due to the instability of the
excavations.
4.2 Environmental Scoping Consultation
The scheme will be subject to the Environmental Impact Assessment (EIA)
Regulations through both the Town and Country Planning Act and the Marine Works
(EIA) Regulations 2007. The Scoping Stage of the EIA has been undertaken between
April and July 2011. Many of the surveys discussed in Section 3.1 fed into the
Scoping Stage.
An Environmental Scoping Consultation Document (ESCD) was prepared, presenting
the preferred option from the strategy. This was followed by a discussion of the
baseline conditions at the site under categories such as cultural heritage, population,
land use, traffic and transportation, air quality, climate change, biodiversity and
ecology.
The ESCD was issued to statutory consultees and other stakeholders for review and
comment. Embedded within the ESCG were a number of questions seeking further
information on key issues related to the Spa frontage. The ESCD was also available
on the internet from the Scarborough Borough Council website and members of the
public were invited to comment on the document and provide information and views
to help develop the scheme. The consultation period ended on the 5th July and a
Summary Feedback Document was compiled based on the results of responses
received. A copy of this is provided in Appendix A1.
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Subsequent to this consultation, an additional option was added to the shortlist – a
concrete stepped revetment with a wave wall, to provide an alternative material to
the rock armour. A Scoping Consultation Addendum was issued to the same
consultees as the original consultation exercise and comments were invited on the
option. The responses to this consultation are summarised in Appendix A2.
4.3 Project Development Workshop
During the consultation period for the ESCD, a Project Development Workshop was
held at the Spa. This was attended by members of the project team and a range of
stakeholders, both internal within Scarborough Borough Council and external parties
such as English Heritage and the Environment Agency. Details of the proposed
foreshore works were presented and discussed.
Key issues that were raised at the workshop included:
• The potential increase in the height of the wall in relation to the promenade
and how this could impede views from the promenade out to sea.
• The potential reduction in beach area caused by the footprint of the rock
structure.
• The visual impact of the scheme, obscuring the stone wall and how it would
impact on views to and from the Spa.
• Potential impacts upon the historical setting of the Spa, including the South
Cliff Gardens.
• Noise during construction and the effects of congestion through Scarborough
caused by construction traffic/deliveries.
A number of opportunities were also identified, including:
• Making more of the historical ‘spa waters’ aspects, e.g. the feature at the steps
to the north of the scheme and the buried pump room.
• The potential to link the works to the refurbishment of the Children’s Corner
Building.
• Improvements to the promenade so that it is better suited to
buggies/wheelchair users.
• Replanting the gardens with stabilising species.
• Habitat creation and enhancing biodiversity.
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5 Options Considered - Foreshore Works
5.1 Appraisal Period & Design Standard
Following the guidance in FCERM-AG, the appraisal period for the business case is
100 years. The options within this stage of the appraisal have been designed with a
design standard of 1% AEP for rock stability (1 in 100 years) and with an overtopping
limit of 0.1 l/s/m in a 10% AEP event (1 in 10 years). This standard was identified
within the Holbeck to Scalby Mills Strategy Review (Halcrow, 2008).
5.2 Climate Change
An adaptive approach to climate change was considered for the scheme, e.g. build in
a proportion of climate change allowances now (say 50 years), with a future phase of
capital works to implement the latter component of climate change allowances.
However, this approach was not considered appropriate for the rock armour
structure as it was generally agreed that if future works were required to the toe, then
it would be difficult to carry these out without destabilising the existing construction.
It was also recognised that it would be more efficient to construct the rock in one
phase. Where an adaptive approach has been considered for rock armour on another
site, it was found that it required additional rock armour in order to retrofit the initial
construction to the required end design, resulting in an increase in materials and cost.
At this stage, the wall has been sized using the full climate change allowance of 0.88m
over 100 years (Defra 2006). Following identification of the preferred option, the
approach to climate change will be determined through the outline design stage,
where an adaptive approach will be investigated. This will not affect option selection
at this stage.
5.3 History of Options Appraisal for the Spa Frontage
Figure 7 illustrates the history of options appraisal for addressing the problems at
The Spa frontage, leading to the options short-listed for appraisal within this current
phase of the project.
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Figure 7 History of options appraisal to date
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5.4 Short Listed Options
As illustrated above, the initial options short list for the foreshore works was
developed following the Project Development Workshop (Section 4.3). The concrete
stepped revetment option was subsequently added to the short list as an alternative
material to rock armour. The short list of options for the foreshore works is presented
in Table 5.1.
Table 5.1 Short listed options for foreshore works
Option no.
Name Description
1 Do Nothing
No further work will be undertaken. The condition of
the defences would deteriorate over time, resulting in
failure.
2 Do Minimum
Maintain defences, repairing as necessary. Defences are
not improved and wave overtopping will increase
overtime. Costs increase overtime as climate change
effects
3 Improve – rock revetment with
a high wave wall
Improve by constructing a rock revetment in front of the
existing seawall. A new wave wall would be
constructed, with a height of 1.4m above the walkway
level.
4 Improve – rock revetment with
a medium height wave wall
Improve by constructing a rock revetment in front of the
existing seawall. A new wave wall would be
constructed, with a height of 1.1m above the walkway
level.
5 Improve – rock revetment with
a low wave wall
Improve by constructing a rock revetment in front of the
existing seawall. A new wave wall would be
constructed, with a height of 0.6m above the walkway
level. The wall would be topped by a handrail to take
the barrier to the minimum requirement of 1.1m
(Building Regulations, ODPM, 2000).
6
Improve – concrete stepped
revetment with a wave wall
1.4m in height*
Improve by constructing a concrete stepped revetment
in front of the existing seawall, with a 1.4m wave wall at
the crest of the revetment.
*Only one wave wall height was considered for the concrete stepped revetment
structure. This form of structures is less hydraulically efficient than the rock armour
revetment and it was recognised early in its development that the structure footprint
would become prohibitively large with a lower wave wall.
Feedback from the Project Development Workshop highlighted a desire to reduce the
visual impact of the rock armour (as seen from the beach) by leaving a greater height
of the existing masonry wall exposed. To investigate this, two sub-options have been
considered within Options 3 to 5 for the northern section of the frontage, whereby a
lower rock crest level is considered. The extents of the northern and southern
frontages are illustrated in Figure 8.
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Figure 8 Delineation of frontage into northern and southern sections for Options 3 to 5
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Drawings for Options 3 to 6 are provided in Appendix C, whilst Table 5.2 summaries
the key dimensional differences across the ‘Improve’ options.
Table 5.2 Key dimensions for Options 3 to 6*
Option No.
Wave wall height (m) / level (mODN)
Crest width (m)
Crest level (mODN)
Footprint width (m)**
3a – north
3a -south 1.4m / +7.9mODN 10.5m
+6.0mODN
+6.0mODN
22.7m
21.5m
3b – north
3b - south 1.4m / +7.9mODN 10.5m
+5.0mODN
+6.0mODN
20.7m
21.5m
4a – north
4a -south 1.1m / +7.6mODN 12.0m
+6.0mODN
+6.0mODN
24.2m
23.0m
4b – north
4b -south 1.1m / +7.6mODN 12.0m
+5.0mODN
+6.0mODN
22.2m
23.0m
5a – north
5a -south 0.6m / +7.1mODN*** 13.5m
+6.0mODN
+6.0mODN
25.7m
24.5m
5b – north
5b -south 0.6m / +7.1mODN*** 13.5m
+5.0mODN
+6.0mODN
23.7m
24.5m
6 - north
6 - south 1.4m / +7.9mODN 10.0m
+6.0mODN
+6.0mODN
37.0m
32.0m
*Dimensions are based on appraisal stage designs and are subject to
refinement/change at future design stages. ** Typical footprint width; will vary due
to bed level. *** With additional 0.5m high railing to provide 1.1m barrier height.
5.5 Option Costs
There are no costs associated with Option 1Do Nothing. Option 2 Do Minimum is a
repair/maintain option. The costs for this option was derived during the Holbeck to
Scalby Mills Coastal Defence Strategy and is based on historical spend on both the
foreshore (seawall) and the cliffs. The Do Minimum costs allow for annual
maintenance to be carried out to both the seawall and the cliffs. There is also a
provision for ongoing repairs to the seawall to address local failures that are expected
to occur as a result of a Do Minimum approach. Both the maintenance allowance and
the repairs costs increase over time as the condition of the frontage deteriorates.
Capital works costs have been derived for Options 3 to 6 by Birse Coastal. Detailed
breakdowns are included in Appendix D with a summary in Table 5.3. The costs in
Table 5.3 allow for a grey concrete with a plain finish to the in-situ concrete and
assume rock deliveries during daytime tide only. Deliveries and rock placement are
assumed to be between 0700 and 1900 hours.
The costs include Contractor’s fees and prelims but exclude project development fees
such as site investigations, detailed design, EIA, planning application and site
supervision/contract administration. The fee for these activities is of the order of £1
million.
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Table 5.3 Capital works costs (excluding Optimism Bias)
Option No
Option name Capital costs (£k)
3a Improve – rock revetment, high wave wall, constant
rock crest level
6,918
3b Improve – rock revetment, high wave wall, varied
rock crest level
6,269
4a Improve – rock revetment, medium height wave
wall, constant rock crest level
7,180
4b Improve – rock revetment, medium height wave
wall, varied rock crest level
6,550
5a Improve – rock revetment, low wave wall, constant
rock crest level
7,157
5b Improve – rock revetment, low wave wall, varied
rock crest level
6,612
6 Improve – concrete stepped revetment, high wave
wall
7,498
Capital costs include preliminaries at 15% and contractor’s fee at 8.4%.
The capital costs in Table 5.3 have been further developed to consider enhanced
concrete finishes and/or restrictions on rock deliveries. . These are based on recent
similar Birse Coastal schemes and/or budget estimate advice from suppliers and
could be subject to significant variation. These are presented in Table 5.4.
Table 5.4 Cost variations summary
Cost variation £k
3a 3b 4a 4b 5a 5b 6
As Table 5.3 with
patterned
concrete finish to
wave wall
6,960
(+41)
6,319
(+49)
7,219
(+38)
6,594
(+44)
7,184
(+26)
6,646
(+34)
7,539
(+41)
As Table 5.3 with
stone cladding
and stone coping
to wave wall
8,139
(+1,221)
7,707
(+1,438)
8,342
(+1,162)
7,876
(+1,325)
7,893
(+736)
7,674
(+1,062)
8,719
(+1,221)
As Table 5.3 with
no restrictions on
rock deliveries
6,218
(-700)
5,720
(-549)
6,416
(-764)
5,947
(-603)
6,329
(-828)
5,954
(-657)
7,498
(0)
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Cost variation £k
3a 3b 4a 4b 5a 5b 6
As Table 5.3 with
patterned
concrete finish to
wave wall and
no restrictions on
rock deliveries
6,260
(-659)
5,769
(-500)
6,455
(-726)
5,991
(-559)
6,356
(-802)
5,989
(-623)
7,539
(+41)
As Table 5.3 with
stone cladding
and stone coping
to wave wall and
no restrictions on
rock deliveries
7,439
(+421)
7,158
(+888)
7,578
(+398)
7,273
(+722)
7,065
(-92)
7,016
(+405)
8,719
(+1,221)
Whole life costs over the 100 year appraisal period have been derived to allow a
comparison of the Present Value cost of each option. This includes design fees leading
up to construction, site supervision fees and future maintenance costs. The costs for
Option 2 Do Minimum have been adopted from the Holbeck to Scalby Mills Strategy,
updated to the current base date (2011 Q3).
In accordance with FCERM-AG, an Optimism Bias (OB) has been applied to all
option costs. The recommended OB allowance at PAR stage is 30%, although this can
be varied across options if there is a difference in the risk profile. In the case of the
options considered for the Scarborough Spa foreshore, there is a difference in the risk
profiles for the rock armour revetment options (Options 2 to 5) and the concrete
stepped revetment option (Option 6). Taking the standard 30% starting allowance at
PAR, the risk profile has been adjusted using the categories within the FCERM-AG
supplementary guidance on Optimism Bias (Defra, March 2003). The variations in
risk profile are presented in Table 5.5.
The whole life cost (cash and discounted Present Value) are presented in Table 5.6.
Present value costs have been calculated using the discount rates recommended by
FCERM-AG (3.5% to Year 30, 3.0% from Year 31 to 75 and 2.5% thereafter).
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Table 5.5 Adjusted Optimism Bias adjustment for Improve Options
*Average % for FCERM projects, table adapted from FCDPAG3 Supplementary Guidance Note March 2003.
Ave.%* 3a 3b 4a 4b 5a 5b 6 Notes on adjustments
Late contractor involvement in design 1 0 0 0 0 0 0 0 Design & build procurement route teams contractor and design together
at an early stage, reducing the potential for conflict in later stages.
Dispute and claims incurred 11 11 11 11 11 11 11 11 No change
Procurement
Other 1 1 1 1 1 1 1 1 No change
Degree of complexity 4 4 4 4 4 4 4 8 Option 6 is a more complex option than the rock options
Degree of innovation 4 4 4 4 4 4 4 4 No change
Environmental Impact 13 13 13 13 13 13 13 16 Option 6 has a greater environmental impact upon the SSSI and will
require additional activities to mitigate.
Project
specific
Other 9 9 9 9 9 9 9 9 No change
Inadequacy of business case 23 23 23 23 23 23 23 23 No change
Funding availability 2 2 2 2 2 2 2 4 Concrete revetment is likely to require a greater contribution to secure
funding.
Project management team 1 1 1 1 1 1 1 1 No change
Client
specific
Poor project intelligence 8 8 8 8 8 8 8 8 No change
Public relations 5 8 8 8 8 8 8 12 Options 3-5 and 6 have visual impacts upon the site which are likely to
raise public relations issues. Impact of Option 6 is considered greater due
to larger footprint and recent publicity.
Environment
Site characteristics 4 5 5 5 5 5 5 8 Uncertainty over depth of bedrock in northern frontage carries a higher
risk for Option 6 due to subsequent complexity of toe detail. Small
increase in risk for Options 3-5.
Economic 5 5 5 5 5 5 5 5 No change
Legislation / regulations 4 4 4 4 4 4 4 4 No change
Technology 4 4 4 4 4 4 4 4 No change
External
influences
Other 1 1 1 1 1 1 1 1 No change
Sum 100 103 103 103 103 103 103 119
Adjusted OB Factor at PAR stage - % 30 31.2 31.2 31.2 31.2 31.2 31.2 35.7
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Table 5.6 Whole life costs including Optimism Bias (baseline costs, no adjustments for enhanced concrete finishes or removal of rock delivery restrictions)
Option No.
Option Name Cash cost (£k)
Present Value cost
(£k)
2 Do Minimum* 25,492 5,574
3a Improve – rock revetment, high
wave wall, constant rock crest level
11,672 10,056
3b Improve – rock revetment, high
wave wall, varied rock crest level
10,871 9,276
4a Improve – rock revetment, medium
wave wall, constant rock crest level
12,015 10,376
4b Improve – rock revetment, medium
wave wall, varied rock crest level
11,239 9,619
5a Improve – rock revetment, low wave
wall, constant rock crest level
12,256 10,410
5b Improve – rock revetment, low wave
wall, varied rock crest level
11,581 9,755
6 Improve – concrete stepped
revetment, high wave wall
19,712 12,559
*Includes cliff maintenance costs
5.6 Carbon Calculator
An initial assessment of the potential carbon footprint of Options 3 to 6 has been
made using the Environment Agency’s Carbon Calculator Tool (Version 3.1.2,
Environment Agency, 2010). This is based predominantly on the materials used in
the capital works and the transport involved in bringing them to site. The assessment
is based on the following assumptions regarding transportation/source of the
materials:
• Quarried materials transported by boat from Larvik, Norway
• Pre cast concrete units transported by boat from Belfast, Northern Ireland (road
transportation from landing port not quantified)
• In situ concrete transported by road from a batching plant within a 30 mile
radius of Scarborough
• Other misc materials (hand railing) transported by road from within a 50 mile
radius of Scarborough
The results are presented in Figure 9. The rock armour revetment options (Options 3
to 5) have a similar footprint, around 2,200 tonnes fossil CO2. The footprint of the
concrete stepped revetment is more than double that of the rock armour options, at
4,800 tonnes fossil CO2.
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Figure 9 Carbon footprint estimates for capital works
Option selection is discussed in Section 7, following a review of the cliff stabilisation
measures.
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6 Options Considered – Cliff Stabilisation Works
6.1 Introduction
The anticipated nature of ground movement and cliff instability at the Spa Cliffs has
been summarised in Section 2 and it is key that any stabilisation solutions taken
forward for the Spa Cliffs address the nature of any anticipated or existing instability,
including deep-seated and or shallower/ superficial failure mechanisms.
The landscape and environment of the Spa Cliffs are important considerations in the
choice of stabilisation methods. The nature of the network of paths, low-height
masonry retaining walls and gardens will need to be respected in the solutions
adopted to avoid adverse impacts on the existing landscape and environment of the
cliffs. It is possible for solutions to blend with the landscape and environment. For
example, drainage measures will be largely invisible, below ground level. Concrete
walls if employed may be faced with masonry to reflect the existing walls. Careful
planting may be used within a garden environment to conceal as far as possible soil
nails or retaining structures. If reprofiling is needed then it can be designed with
curves rather than straight lines, reflecting the landscape and aesthetics of the
surrounding area.
The main methods of slope stabilisation used in Britain, which may be used singly or
in combination are (after Hutchinson, 1984):
• Drainage – both sub-surface and surface water
• Modification of the slope profile by excavation and/or filling
• Restraining and other structures including soil nails and anchors
6.2 Solutions identified for the Spa cliffs (Priority Area 1)
The nature of the instability to be addressed at Priority Area 1 is shallow instability in
the oversteep till slopes in the upper part of the slopes and deeper seated instability
affecting the entire slope, and distress of the masonry wall at the top of the slope.
The deeper seated instability may be addressed by a combination of two measures:
• Drainage to reduce the confined water pressures in the sand and gravels. Near-
horizontal bored drains in the thick sand and gravel horizons identified at both
cross-sections in Area 1 (near the cliff lift and further north at Line 4) will act to
reduce porewater pressures in the sands and gravels. If further ground
investigation identifies high porewater pressures in the lower sands and gravels,
drainage wells may be required too.
• Construction of a pile array towards the toe of the slopes above the buildings/
developments at the toe. The array of piles would be designed to key into the
sandstones of the Scalby Formation beneath the slopes, with the arching effect of
the soils between the piles combined with the key into the stronger materials
beneath providing resistance to movement of the slope above. This structural
solution would be discreet, located almost entirely beneath the ground surface.
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Alternative solutions which may be used to address deep seated instability such as
reducing driving forces by removing materials from an upper slope and increasing
resistance to movement by adding load at the toe of a slope are not compatible at this
location because of the need to maintain the form/nature of the existing landscape.
Due to the oversteep (in the long-term) nature of the upper slopes, two main
approaches to increasing stability are structural support and regrading or
anchoring/nailing and meshing, each combined with drainage measures as necessary
as described below:
• Regrading to reduce oversteep soil angles would need structural support, to
allow the slopes to be slackened between the supports, such as piled walls. Such
walls could be masonry faced/clad to blend in with the surrounding area, walls
may need to be anchored to provide a more efficient design. The construction of
the piled walls to an adequate depth would mean that small height increases up
the slope between the piled walls could be achieved by use of gravity masonry
walls, similar to the existing walls. Existing walls could be assessed and reutilised
where possible, with additional drainage provided where necessary. This
approach would require some rationalisation of the paths behind the Spa
complex, although the nature of the area would be respected, with masonry-clad
walls supporting slopes at a safer angle.
• The existing profile could be mostly maintained if a solution involving soil
nailing and or ground anchors is adopted. A facing would be needed between the
nails such as mesh or geogrid; care would need to be taken to ensure the finished
slopes were aesthetically acceptable. An area of slope recently repaired using
such techniques (see Photo 5.1.1e) does not blend in with the surroundings in its
current condition. Possibly future re-vegetation may help. A similar approach
using slope stabilisation mattresses has revegetated well over a period of some 8
to 9 years since installation (see Photos 3.3a, b and c).
• Additional ground investigation proposed in the tills will allow further
identification of perched water tables in the till to be targeted with localised
drainage measures, reducing the porewater pressure at these locations and hence
reducing the incidence of shallow slips in the upper slopes. (Newly installed
BH14 at the top of the slope was carried out using open borehole rotary drilling
techniques, therefore provides limited data on the ground conditions because
alternating bands of sand and gravel with clay Glacial Till were not logged in
detail.)
Additional work will also be required to the existing masonry and concrete wall at
the top of the slope. This is exhibiting signs of distress and movement, including
cracking through the wall, slight movement forward down the slope shown by the
depression in the pavement behind the wall and movement within the wall, exhibited
by the cracking evident between the capping beam and the front face of the wall. The
wall should be inspected by a structural engineer and evidence of all distress mapped
in detail. Additional information should be sought from the Council in relation to any
construction or repair drawings that may be available in their archives. An
assessment can then be made of the nature of the movement and appropriate
remedial measures designed. Such measures may include anchoring or a piled toe
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wall in front of existing wall to support the existing wall and effectively isolate it
from movement of slopes beneath.
6.3 Cliff Stabilisation Options
In accordance with FCERM-AG, all options considered are assessed against the
baseline of Option 1 Do Nothing. A Do Minimum option (Option 2) is also
considered.
The Do Minimum option has been adopted from the Holbeck to Scalby Mills Strategy
Review (Halcrow 2008). It allows for ongoing maintenance to the cliffs with the costs
being based on historic expenditure on the South Bay cliffs by Scarborough Borough
Council.
The package of measures discussed in Section 6.2 has been derived on the basis of the
existing information but without the benefit of the specialised ground investigation
within the cliffs area (currently programmed for the post-PAR detailed design stage).
As such, there are continued uncertainties regarding the cliffs (including limit of
loading during construction etc) and it is not possible (at this time) to fully identify
the defined package of works for the upper slopes. Therefore we have developed an
Improve option for the cliffs which provides flexibility in the ‘above ground’
solutions (shallow slope solutions), depending upon the results of the ground
investigation.
In summary, Option C3 Improve for the cliffs comprises:
• Piled array at the rear of the Spa complex, extending beyond the cliff lift, in
conjunction with near horizontal drainage to address the deep seated
instability.
• Construction of piled/masonry walls and regrading at locations and/or soil
nailing, to address the oversteep slopes in the glacial till material (shallow
failure mechanism).
• Works to rear masonry wall at the crest of the slope – this asset (a highways
retaining wall) is understood to be owned / maintained by North Yorkshire
County Council (NYCC).
• Other peripheral works including repairs in the vicinity of the cliff lift,
reinstatement of footpaths, repairs to existing masonry walls, landscaping and
planting.
Two concept sketches that illustrate how this option could be implemented are
provided in Appendix C.
6.4 Outline Costs of Options
There are no capital works associated with Option 1 and Option 2. A capital works
budget estimate of £3.8 million has been derived for Option C3 by Birse Coastal in
conjunction with their suppliers. The costs are purely a budget estimate at this stage
and cannot be confirmed until a full GI survey is carried out and the exact ground
conditions are known. Prices are based upon the available information and report
and involved a walkover visit with the piling / drilling contractors.
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We have assumed that the cliff works are carried out concurrent with the foreshore
works and therefore have not included for any additional Contractor preliminaries
other than those specific to the cliff works. (Capital works value of £3.8 million
includes preliminaries and Contractor’s fee).
It should be noted that the solutions identified in this assessment are different to
those identified by the previous strategy reviews (High Point Rendall, 1998, 2005)
which were adopted for the Holbeck to Scalby Mills Strategy Review (Halcrow 2008).
The High Point Rendall solution used horizontal drainage to address the deep seated
instability. Based on our recent work, we feel that this does not provide a suitable
solution on its own. Whilst drainage would be beneficial it would not provide the
support to the upper slope required to improve the current factor of safety. This
support is provided by the proposed pile array.
A detailed cost breakdown is included in Appendix D.
As with the foreshore works, we have reviewed the Optimism Bias allowance for the
cliff stabilisation works, as detailed in Table 6.1.
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Table 6.1 Adjusted Optimism Bias adjustment for Improve Options
Ave.%* C3 Notes on adjustments
Late contractor involvement
in design
1 1 No reduction as option is not
progressed sufficiently to warrant
reduction
Dispute and claims incurred 11 11 No change
Procurement
Other 1 5 Low number of specialist contractors
may result in competitiveness of market
not materialising
Degree of complexity 4 12 Complex package of solutions
proposed.
Degree of innovation 4 4 No change
Environmental Impact 13 13 No change
Project
specific
Other 9 9 No change
Inadequacy of business case 23 23 No change
Funding availability 2 2 No change
Project management team 1 1 No change
Client
specific
Poor project intelligence 8 8 No change
Public relations 5 8 No change Environment
Site characteristics 4 10 GI to take place at a later point, so
uncertainty regarding ground
conditions.
Economic 5 5 No change
Legislation / regulations 4 4 No change
Technology 4 4 No change
External
influences
Other 1 1 No change
Sum 100 118
Adjusted OB Factor at PAR
stage - %
30 35.4
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7 Option Selection
7.1 Introduction
This section of the report brings together the options discussed in Sections 5 and 6 to
identify the preferred option for the Scarborough Spa Coast Protection Scheme.
As discussed in Section 2.10, it is important to consider both the cliff stabilisation
works and foreshore works in conjunction to address the coast protection issues at
the site. Table 7.1 presents how the options considered in Sections 5 and 6 has been
paired to provide combined options for both cliff and foreshore.
Table 7.1 Combination of Options considered
Option No.
Foreshore Option Cliff Option
1 Do Nothing Do Nothing
2 Do Minimum Do Minimum
3a/C3 Improve – rock revetment, high
wave wall, constant rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
3b/C3 Improve – rock revetment, high
wave wall, varied rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
4a/C3 Improve – rock revetment,
medium height wave wall,
constant rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
4b/C3 Improve – rock revetment,
medium height wave wall, varied
rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
5a/C3 Improve – rock revetment, low
wave wall, constant rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
5b/C3 Improve – rock revetment, low
wave wall, varied rock crest level
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
6/C3 Improve – concrete stepped
revetment, high wave wall
Cliff stabilisation via piled array,
horizontal drainage and walls/soil
nailing
7.2 Appraisal Summary Tables
The information in Sections 5 and 6 has been detailed along with key environmental
issues from the ESCD into Appraisal Summary Tables. These can be found in
Appendix E. A high level comparative appraisal is presented in Table 7.2.
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Table 7.2 High level comparative summary of options
1 2 3A / C3 3B / C3 4A / C3 4B / C3 5A / C3 5B / C3 6 / C3 Comments
Brief description –
foreshore works
Rock, 1.4m
high wall,
constant
rock crest
Rock, 1.4m
high wall,
variable rock
crest
Rock, 1.1m
high wall,
constant
rock crest
Rock, 1.1m
high wall,
variable rock
crest
Rock, 0.6m
high wall,
constant
rock crest
Rock, 0.6m
high wall,
variable rock
crest
Concrete,
1.4m high
wave wall
Brief description – cliff
stabilisation works
Do Nothing Do
Minimum
Pile array to rear of Spa complex, upper slope stabilisation (low level retaining walls and/or soil nailing), landscaping
and repairs to gravity wall at crest of cliff (highway retaining structure – inclusion in scheme subject to agreement
between SBC and NYCC)g
Costs – capital (excl.
risk) NIL £11,709k £11,059k £11,970k £11,341k £11,947k £11,402k £12,372k
Costs – capital (incl.
risk) NIL £15,497k £14,647k £15,840k £15,015k £15,809k £15,095k £16,777k
Costs – Present Value
inc OB
NIL
£2,282k £14,837k £14,057k £15,158k £14,401k £15,191k £14,537k £17,341k
Refer to Tables 5.5 and 6.1 for risk allowances included within
options
Economic impacts
Property
Infrastructure
Transport
Land use
Indirect effects upon
businesses
Options 1 and 2 will result in damages occurring through failure
of seawall/cliffs.
Options 3 to 6 all significantly reduce likelihood of damages
occurring by stabilising cliffs and protecting seawall/toe of cliff.
Options 3 to 6 may have indirect effects upon businesses in the
short-term, during construction. However, no longer term
effects anticipated.
Environmental impacts
Designated sites – SSSI 1= 1= 3= 3= 5= 5= 7 Options 3 to 7 ranked on relative proximity/encroachment to
SSSI.
Coastal processes
1= 1= 1= 1= 1= 1= 7 Options 1 and 2 will
Rock options broadly similar, whilst concrete revetment is more
reflective and could result in greater scour than rock revetment
option.
Marine biodiversity
2 1 4 3 6 5 Option 6 has worst impact as combination of largest footprint,
combined with regular cleaning to prevent slippery surfaces
firming. Options 3 to 5 ranked on footprint area.
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1 2 3A / C3 3B / C3 4A / C3 4B / C3 5A / C3 5B / C3 6 / C3 Comments
Terrestrial biodiversity
Options 3 to 6 will impact upon the terrestrial habitats in the
short term during construction. However, longer term effects
are expected to be positive due to long term stabilisation of cliffs
and anticipated improvements included as part of the scheme
(landscaping etc).
Historic environment
1 5= 2= 5= 2= 5= 2= 8 Option 2 is likely to retain historic features for the longest time,
although condition will deteriorate over time. Options 3 to 6
ranked depending upon degree of exposure of masonry wall
post-construction.
Landscape
1= 1= 1= 1= 1= 1= 7 Options 1 and 2 likely to result in cliff failures, adversely
impacting the landscape. Options 3 to 6 significantly reduce the
risk of cliff failure but rock/concrete options will have an impact
upon the landscape.
WFD status/objectives
Management of cliff erosion )Options 3 to 6) will reduce the
potential for release of natural and man-made materials into the
coastal waterbody.
Noise
Carbon calculator - - 5 1 6 2 3 4 7
Social impacts
Recreation / amenity 3 2 5 4 7 6 1 Options 3 to 6 ranked on permanent loss of beach area to public
use. Option 6 ranked 1 as steps can be used as seats.
Health / wellbeing
Key
positive impact 1, 2, 3, ETC
moderate positive impact 1, 2, 3, ETC
uncertainty/neutral impact 1, 2, 3, ETC
moderate negative impact 1, 2, 3, ETC
negative impact 1, 2, 3, ETC
Ranking of the impacts of options where impacts are
relatively similar. Rank 1 = best
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7.3 Scoring and Weighting
Following a review the guidance relating to scoring and weighting in FCERM-AG, it
was not considered worthwhile applying scoring and weighting to the options
considered. The monetised impacts were considered unlikely to be greater than £12m
(10% of the PV damages) and the difference in environmental impacts were not
identified as significant. The differences in impacts between the very similar options
(foreshore Options 3 to 6) are generally driven by different structure dimensions and
can therefore be considered as tangible (but non-monetised) within the option
appraisal.
7.4 Economic Indicators
Within FCERM-AG, the identification of the preferred option primarily hinges on the
economic appraisal, where the whole life costs and benefits of each option are
compared.
In accordance with the Streamlining procedures, the benefits assessment undertaken
for the Holbeck to Scalby Mills Strategy Review (Halcrow2008) has been adopted,
with updates to the market value of assets to reflect the current market and to
provide benefits values at the same base date as the project costs (2011 Q3). The
benefits update was undertaken by Royal Haskoning working in conjunction with
John Chatterton (personal communication John Chatterton to Royal Haskoning, 1st
April 2011.)
The results of the economic appraisal are presented in Table 7.3. As there is no
increase in design standard across Options 3 to 6 (they all reduce erosion to the same
residual probability), the incremental analysis is undertaken using Do Minimum as
the base incremental option.
Option 2 Do Minimum has the highest benefit cost ratio; however, this option does
not address the problems at the Spa frontage.
The Benefit Cost Ratios (BCRs) of the Improve options range from 6.6 to 8.1, with
Option 3B having the highest BCR. Options 3 to 6 deliver the same benefits in terms
of avoidance of eroded assets, so the difference in BCR is driven by differences in cost
– there is a Present Value cost (PVc) difference of £2.1 million across the options
(capital cost difference of £1.2 million).
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Table 7.3 Economic Appraisal Summary
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7.5 Option Selection Discussion
This section weighs up the results of the AST and the economic appraisal, leading to
the identification of the preferred option.
Option 1 Do Nothing would result in cliff failure with impacts upon cliff top
properties, the Spa complex and the promenade and beach. The cliff instability also
puts the coastal defences at risk. There is clear evidence of a developing incipient
landslide, identified as “Priority Area 1 (PA1)”.
Option 2 Do Minimum, features ongoing repairs/maintenance to the cliffs and
seawall; however repairs under a 'do minimum' approach would not address the
problems at the Spa cliffs, namely the over steepened upper slopes and the deep
seated instability, and will have little effect upon the long term stability of the cliffs.
Wave overtopping would increase over time, as the result of the predicted effects of
climate change.
For the cliff area, the pile array and near horizontal drainage within Option C3 is
aimed at addressing the deep seated instability and will provide support to the upper
slopes which will in turn be further stabilised by a combination of low level piled
retaining walls and regrading of upper slopes and/or soil nailing, coupled drainage
works. Other works within Option C3 include rehabilitation of the retaining wall
along the crest of the slope, repairs in the vicinity of the cliff railway,
landscaping/replanting and relaying of paths.
All of the improve options considered for the foreshore will prevent marine erosion
of the base of the cliff, protect The Spa Complex and promenade and will reduce
wave overtopping to an acceptable level.
Table 7.3 presents the benefit cost ratios for the Improve options.
Table 7.3 Summary of Benefit Cost Ratios for Improve Options
Option BCR Rank Other considerations
Option 3A/C3 7.7 4
Option 3B/C3 8.1 1
Highest wave wall of the options, future potential
increases in climate change allowances will increase
visual impact.
Of the two options, higher risk carried by Option 3B
due to greater wave forces upon wall.
Smallest footprint on the beach (approx 1ha).
Option 4A/C3 7.5 5
Option 4B/C3 7.9 2
Moderate height wave wall, with ability to
accommodate a further increase (to 1.4m) should
climate change predictions worsen.
Of the two options, higher risk carried by Option 4B
due to greater wave forces upon wall.
Moderate footprint on the beach (approx 1.05ha).
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Option BCR Rank Other considerations
Option 5A/C3 7.5 6
Option 5B/C3 7.9 3
Lowest wall offers greatest flexibility in terms of
future potential increase in climate change
allowances. However, this option has the greatest
footprint of the rock revetment options (approx
1.1ha) and will incur greater carbon costs due to
additional replacement of hand railing (compared to
Options 3 and 4).
Option 6/C3 6.6 7
Worst option economically and environmentally.
Largest overall footprint on beach (approx. 1.4ha).
Rejected first in option selection.
Option 3B has the highest benefit cost ratio (BCR=8.1) and is the economically
preferred option, on the basis that it is the lowest cost Improve option and all
Improve options deliver the same value of monetary benefits. However, other factors
from the appraisal along with technical considerations also contribute to the selection
of the overall preferred option.
Option 3A/B has the highest wave wall of the options considered and this will have
the greatest visual impact on receptors both landward and seaward of the defence
structure. The wave wall is at the highest level considered acceptable – if climate
change predictions worsen, then any future adaptations required would significantly
increase this impact.
The option with the next highest benefit cost ratio is Option 4B (BCR=7.9); this has a
wall height of 1.1m. If future climate change allowances increased, then there is
scope for raising the wall to the highest considered within this appraisal (1.4m).
Option 4B has the varied rock crest level (lower in the northern end) to expose more
of the existing masonry seawall. However, there is a greater risk associated with this
option as wave forces upon the wall will increase with a greater exposed face height.
Due to the lack on information available on the wall and deferment of the structural
survey to the detailed design phase for budget reasons, there is a risk that significant
strengthening work would be required to provide this area of exposed face.
The Improve Option with the third highest benefit cost ratio is Option 5B (BCR=7.9).
Option 5A/B has the lowest wave wall; however they have the greatest footprint of
the rock options considered. Options 5A/B also requires handrail replacements at
regular intervals in the future, which carries a future carbon cost not incurred in to
the same degree in Options 3 and 4.
Given the relatively small differences in cost between the options and the degree of
uncertainty surrounding the wall condition at this stage, it is recommended that
Option 4A be selected as the preferred option for the foreshore works. This should be
implemented in conjunction with Option C3 for the cliff stabilisation measures.
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7.6 Summary of Scheme Preferred Option
The preferred option for the Scarborough Spa Coast Protection Scheme is Option
4A/C3 comprising:
• rock revetment of constant crest level +6.0mODN with a 1.1m high wave
wall.
• pile array along the rear of the Spa complex with near horizontal drainage to
address the deep seated failure mechanism.
• combination of low level piled retaining walls and regrading of upper slopes
and/or soil nailing to address the shallow seated instabilities, coupled with
drainage measures.
• works to rehabilitate the retaining wall along the crest of the promenade
(subject to agreement between SBC and NYCC).
• other peripheral works including repairs in the vicinity of the cliff railway.
• relaying of paths and landscaping/replanting.
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8 What Happens Next
The recommended scheme option will be presented to the Scarborough Borough
Council Cabinet Meeting in December 2011. Following approval of the preferred
option, the project team will continue with the scheme as follows:
• Outline design of Option 4a for the foreshore works.
• Review and update of capital costs based on outline design.
• Scheme risk workshop to develop scheme specific risk register, leading to
calculation of 50%ile and 95%ile risk allowances for final scheme economics
and funding application.
• Preparation of Project Appraisal Report (PAR) business case, with supporting
appendices for submission to the Environment Agency’s Large Projects Review
Group (LPRG).
• Following approval by LPRG, the detailed design of the project will start,
including additional ground investigation, further numerical modelling (long
shore transport) and an Environmental Impact Assessment (EIA). Planning
permission for the scheme will be sought and an exhibition will be held to
inform the public and stakeholders about the scheme.
Activities covered in the final bullet point are subject to a funding approval from the
Environment Agency.
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9 References
Department of the Environment, 1996. Landslide investigation and management in
Great Britain: a guide for planners and developers. HMSO: London.
Defra, 2003. FCDPAG3 Supplementary Note to Operating Authorities – Revisions to
Economic Appraisal Procedures Arising from the New HM Treasury “Green Book”
Defra, 2006. FCDPAG3 Supplementary Note to Operating Authorities – Climate
Change Impacts.
EA/ENW/KFKI, 2007. EurOtop. Wave Overtopping of Sea Defences and Related
Structures: Assessment Manual.
Eastwood and Partners (1989) Structural Inspection of the South Bay Pool
Environment Agency, 2010. Carbon Calculator Tool (Ref 300_10_SD18).
Environment Agency, 2010. Flood & Coastal Erosion Risk Management – Appraisal
Guidance (FCERM-AG).
Geotechnical Engineering Ltd, 1985. Ground Investigation Holbeck, Scarborough.
Remedial works for landslip.
Halcrow, 2008. Scarborough Coastal Defence Strategy Review - Holbeck to Scalby
Mills.
Halcrow, 2011. Scarborough Spa Coast Protection Scheme: Spa Cliff Stabilisation –
Data Review and Survey Report.
Hutchinson JN, 1984. Landslides in Britain and their countermeasures. Journal of
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installed for the stabilisation of Taren Landslide, South Wales, UK. Landslides, Bell
(ed.) Balkema, Rotterdam.
Moore R, 1996. The Holbeck Hotel, Scarborough: now you see it, now you don’t!
Presentation to the ICE (Municipal Group) 20.11.96
Mouchel, 2009. Analysis and Interpretation of Coastal Monitoring Data –
Geotechnical Interpretative Report. Ref. 721228/001/GR/01/FINAL February 2009.
Norwest Holst, 1996. Ground Investigation Holbeck Gardens, Esplanade Crescent,
Scarborough.
Norwest Holst, 1998. GI Reports for Areas D to I, Holbeck to Scalby Mills Coastal
Defence Strategy
Office of the Deputy Prime Minister, 2000. Building Regulations Part K – Protection
from falling collision and impact.
Rendel Geotechnics (1995) Coastal Planning and Management. Applied Earth Science
Mapping: Filey to Scarborough, North Yorkshire
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Rawson PF and Wright JK, 2000. The Yorkshire Coast. Geologists’ Association Guide
No. 34.
For details of your nearest Halcrow office, visit our website halcrow.com
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