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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




November 2011

Document history




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

Doc no: Version: Date: 30 November 2011 Filename: DCSSPA-WBS301-001_V1-2.doc

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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.


21

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.


22

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


23

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


24

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)


25

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


26

Photo 3.2b Photo 3.2c Photo 3.2d

Photo 3.2e Photo 3.2f (Mouchel, 2009)


27

Figure 6: Existing stability measures in the vicinity of The Spa frontage


28

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).


29

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


30

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


31

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


32

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.


33

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.


34

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.


35

Figure 7 History of options appraisal to date


36

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.


a medium height wave wall




level.


a low wave wall




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.


37

Figure 8 Delineation of frontage into northern and southern sections for Options 3 to 5


38

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.


39

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)


40

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).


41

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


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


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


42

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


12,015 10,376

4b Improve – rock revetment, medium


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.


43

Figure 9 Carbon footprint estimates for capital works

Option selection is discussed in Section 7, following a review of the cliff stabilisation

measures.


44

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.


45

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


46

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.


47

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.


48

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


49

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


Cliff stabilisation via piled array,

horizontal drainage and walls/soil

nailing

3b/C3 Improve – rock revetment, high




nailing

4a/C3 Improve – rock revetment,

medium height wave wall,

constant rock crest level



nailing

4b/C3 Improve – rock revetment,

medium height wave wall, varied

rock crest level



nailing

5a/C3 Improve – rock revetment, low




nailing

5b/C3 Improve – rock revetment, low




nailing

6/C3 Improve – concrete stepped

revetment, high wave wall



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.


50

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.


51

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


52

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).


Table 7.3 Economic Appraisal Summary


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).


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.


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.


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.


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

Japan Landslide Society 21-1.

Martin PL and Warren CD, 1991. The design and performance of drainage measures

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


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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