COASTAL RESEARCH
Plymouth, UK

www.coastres.co.uk

REPORT

Assessment of technical reports submitted to Restormel Borough Council by Ampersand Ltd in support of their January 2005 Planning Application for a revised sea wall design at Carlyon Bay beaches

for

CarlyonBayWatch Ltd
Registered Office
1 Wheal Northey
St Austell
Cornwall
PL25 3EF

For onward submission to Restormel Borough Council Planning Committee

Liaison Director of CarlyonBayWatch Ltd: Mr J. T. Beer

COASTAL RESEARCH reference: RXC52A01-D26

Report Date: February 2005

Contents

For an Executive summary – see Conclusions

1. Introduction

2. Outline of Sedimentary Environment

3. Proposed Beach Recharging

4. Insufficient site testing at Carlyon Bay

5. Conclusions

Figures referred to in text

Appendix 1 - DEFRA extract

Appendix 2 - Glossary of Terms

1. Introduction

COASTAL RESEARCH has been engaged by CarlyonBayWatch Ltd to advise on dynamic aspects of the proposed sea wall and associated beach replenishment at Carlyon Bay, near St Austell, Cornwall. This proposed sea wall is currently the subject of a Planning Application to Restormel Borough Council by development company Ampersand Ltd (application submitted January 2005). The sea wall is considered essential by the development company in order to offer protection to the 511 apartments which form the proposed holiday village to be sited on the beach at Carlyon Bay.

Historically, Ampersand Ltd - the present development company - has inherited earlier planning permissions, dating back to October 1990, to build on the beach. It is assumed that in the light of recent experience and official warnings about building on low lying coastal sites, Ampersand Ltd wishes to enhance an earlier sea wall design, in order to strengthen its commercial undertaking and credibility.

This Assessment Report is a review of the dynamic aspects of the January 2005 proposed sea wall planning application. It is based upon a study of four documents commissioned by Ampersand Ltd., archive data and local surveys by COASTAL RESEARCH. The four documents are:

1. Environmental Impact Assessment of The Beach, Carlyon Bay Proposed Beach Replenishment and Revision to Sea Defences by Wardell Armstrong International, dated January 2005. EIAfeb05

2. Flood Risk Assessment by MLM Consulting Engineers with contributions by HR Wallingford, dated 27 February 2004. FRA27feb04

3. Flood Risk Assessment ADDENDUM by MLM Consulting Engineers with contributions by HR Wallingford, dated 30 June 2004. FRAADD30jun04

4. Beach and Sea Wall Management Manual (Report EX 4993) by HR Wallingford, dated June 2004. MANjun04

Abbreviations for these four documents are shown above in bold type, at the end of each full title. These abbreviations are used in this Assessment Report hereafter. Other words in bold type are explained in the Glossary at the end of this Report.

This Assessment Report focuses upon two main areas:

• Proposed beach recharging during sea wall construction and at intervals thereafter - Section 3
• Insufficient site testing at Carlyon Bay - Section 4

2. Outline of Sedimentary Environment

The three beaches at Carlyon Bay (Crinnis, Shorthorn, Polgaver) are currently composed of large deposits of quarry waste from the nearby China Clay industry. Most records indicate that these deposits began to accumulate after 1842 when the Crinnis or Sandy River was diverted underground to emerge at Shorthorn Beach. The deposits transported to the Bay by the River were predominantly quartz gravels but with irregular quantities of degraded feldspar which includes some kaolin clay mineral.

The natural sands in the shallow coastal waters of St Austell Bay are a mixture of fine quartz and calcareous deposits (broken sea shells). The calcareous material has a significantly shorter lifecycle than the quartz at the beaches surrounding St Austell Bay. This is due to it being softer than quartz and therefore prone to attrition. Today, the three beaches at Carlyon Bay contain varying amounts of calcareous material. Before 1842 they would have had significantly smaller beach deposits than today and composed of the St Austell Bay calcareous sand with some larger material from local cliff collapse.

The quartz material deposited at Carlyon Bay since 1842 is a relatively new sedimentary deposit, in geological terms. Because quartz is one the hardest common materials found in beach and shallow waters deposits it takes a considerable time to reduce to silt and clay particle sizes which are then subject to transport by wind and tidal streams. Another feature of quartz particles from china clay pits is that the grains are much more angular than typical grains from natural beaches containing quartz sand. Points and edges of the crystalline quartz take a very long time to be smoothed. This means that concrete and steel structures are more at risk from corrasion than is to be expected from natural quartz sand beach materials. The angular quartz material of Carlyon Bay beaches is also accelerating the reduction of calcareous material to clay size particles and its subsequent removal from the Bay by residual tidal streams. The amount of partially fragmented feldspar crystal in the total beach sediment is not accurately known, but the consequences for permeability and stability are highly problematic.

The key summary point from the above is that the beach material at Carlyon beaches today is different in terms of mineral composition, particle shape and particle size to sediments at other beaches in the area.

3. Proposed beach recharging

The situation that beach recharging is required at all should sound alarm bells to those concerned with coastal environmental management. The assertion by the planning application that it is required both during sea wall construction and at intervals thereafter is indicative of unknown results once the sea wall is built. Most of the possible scenarios about the beach dynamics after the proposed sea wall is built are derived from the BEACHPLAN and SHINGLE computer simulations. These simulation software are not specified in detail within the documentation, and because of the unusual and artificial nature of the beach sediment at Carlyon Bay it is highly probable that the software is not appropriate. The authors of FRA27feb04 admit to these problems in Section 5.2.5.

Within the coastal engineering community there is considerable interest in rock armouring techniques mainly because of the relative economy of such structures and the fact that pieces may be recovered and reassembled following storm damage. How these structures operate is still the subject of research (Appendix 1). Rock armouring is still in its infancy and as such many schemes must be seen as experimental. The design assertion that beach sediments will fill the pore spaces of the rock armouring and remain in situ must also be seen as an experiment. The designers have not quoted any other similar beach scheme where this has worked.

EIAfeb05 page 38 (para 6.7.3) states that “there is no evidence of sand banks offshore created from material being lost from the beach”. Such an assertion is essential if it is hoped to prove that beach material will not be transported offshore. However, these sand banks exist now (see Figure 1), and there is other evidence that the sea bed off Carlyon Bay is mobile.

EIAfeb05 page 37 (6.6.1) states that build up of beach sediment is expected at the East end of Carlyon Bay. Such prediction is already confirmed from field measurements in Carlyon Bay and the presence of angular china clay quartz sand on Par Beach (East of Carlyon Bay beaches), the latter being indicative of overflow from Carlyon Bay beaches. It is not known if these observations are the result of sand transport before the construction of the steel piling installed during 2004, but the potential is clearly evident; beach recharging will only exacerbate the situation. Longshore drift from bay to bay is the consequence of excess beach sediment becoming available, leading to the overflow condition.

The advice by engineers to now include a cautious Beach and Seawall Management Manual (MANjun04) is indicative of the potential instability of the proposed development. If the measures advised following any of the projected failures of the sea wall or beach system were not able to be actioned rapidly during a prolonged storm (not unknown in the history of weather events) then the consequences for property and human life would be severe. Because of the increase in public concern over aspects of flooding and the demand for accountability and compensation the Carlyon Bay development is a high risk project. If this was a largely uninhabited development the risks might be acceptable, but because the population in a physically confined area may be in excess of 1000, the risks are high.

3.1 Sea Wall Design

The sea wall design from the 1990 Planning Permission is based upon exposed vertical steel piling. This is not dissimilar to the actual structure that was installed during 2004 and in situ on the beach at Carlyon Bay during the storms of late October 2004. This structure suffered damage which was illustrated in numerous newspaper photographs. Some of this steel piling was drawn back offshore and deposited below the low water mark. Figure 2 shows its recovery during a Spring Low Water in February 2005. The beach sediment was similarly drawn back and has not yet been restored to Autumn 2004 levels. Thus the beach must now be classified as being reflective. It cannot be claimed that the beach will absorb wave energy in the way that the EIAfeb05 asserts on page 38 (6.7.3): “Under normal conditions the wave absorption properties will be the same as at present.”

The sea wall design featured in the current planning application (January 2005) has several amendments. However, all features are dependent upon retaining the beach sediment cover to the rock armouring. There is no evidence that the sand cover will remain in place during a storm (see Appendix 1), hence the wall will be subject to similar forces experienced in October 2004. Beach recharging at this site will be cosmetic only, the beach cover will be lost at the critical time when the sea wall requires wave energy to be reduced or dissipated. The addition of the recurved concrete structure located above the steel piles will also enhance reflection. In lighter conditions it will deflect a certain amount of sea spray and wave overtopping, but its effect in high storm conditions will be minimal.

The increased height of the sea wall between the 1990 and 2005 planning applications presents an additional problem. The design heights available at this present time indicate that the proposed housing development will be at a lower level than the promenade located between the primary and secondary sea walls (EIAfeb05 Figure 4). The exact height difference has yet to be determined but based upon provisional information it may be of the order of 2 metres. If this is not the proposed case then a huge amount of land fill will be required across large sections of the site. If it is accepted that the sea wall could remain unbreached during a storm then the resulting wave overtopping and any precipitation could be contained behind the sea walls and lead to flooding of the residential area. The sea wall would then become a dam trapping water in the housing complex. There is insufficient data on the existing beach sediments to guarantee that such flooding could be drained through the ground (see Section 4).

The siting of the sea wall is the main problem on this beach. It is too far seaward, there is insufficient active beach width to allow the beach to be dissipative. The wall will become dangerously reflective in storm conditions; hence it risks its own destruction by being undermined. The fact that it is not connected to any bedrock at its foot merely adds to the potential risk.

4. Insufficient site testing at Carlyon Bay

In order to have any confidence in numerical modelling studies it is essential to load the simulation software application with the best available data on the known environment. In the case of the hydrography of St Austell Bay area, the density and dates of bathymetric soundings has not been declared. This has consequences for the precision of modelling results from the BEACHPLAN and SHINGLE simulations.

The permeability of the existing beach material is critical for the drainage of any wave over topping activity. It is partly influenced by the non-quartz component of the total beach sediment (see Section 2 in this Report). This is discussed in FRA27feb04 section 2.4.3 but there is a reference to the intention to carry out further permeability tests. These data are critical for the objective analysis of the planning application because of the risk of flooding from a combination of rain and wave overtopping in what is proposed to become a basin enclosed by the sea wall (see Section 3.1 of this Report). The sediment structure throughout the site is known to be variable.

5. Conclusions

The Beach has become, since installation of the steel piles, a much more reflective beach. Hence reliance on the proposed high angle beach slope to protect the structure of the sea wall is flawed.

The existing quantity of sediment at three beaches of Carlyon Bay means that it is close to a sediment overflow condition for losses through longshore drift to the East. Adding more sediment beyond the proposed sea wall will only lead to accelerated losses to the East. Hence recharging will be an endless but fruitless solution to protecting the base of the proposed sea wall.

Insufficient attention has been given to the unique properties of the sediment deposit in Carlyon Bay. The consequence of this is that the proposed sea wall is an entirely experimental structure.

Figures referred to in text

Figure 1. Bar visible at Spring Low Water – Friday 11 February 2005

Figure 2. Recovery of Steel Piles – Friday 11 February 2005

Appendix 1

Extract from DEFRA Flood and Coastal Erosion Risk Management, Issue No. 6, June 2004, page 2:

Appendix 2 - Glossary of Terms

attrition – the denudation, or degradation, of adjacent particles by reciprocal wear and tear

corrasion – the denudation, or degradation, of a fixed or static structure by the action of particles being blown or hurled at its outer surface

dissipative beach – a beach where almost all incoming wave energy is absorbed by friction, causing waves to break several surf lines from the shore; the surf lines become smaller and weaker as they approach the beach. Low angle surf beaches are typical examples.

longshore drift – the result of waves approaching a beach at an angle, causing sediments to be thrown up the beach at the same angle. When wave water runs back down the beach (backwash) it tends to drag sediment straight down the beach (by gravity selecting the steepest route). Thus, after one wave cycle a quantity of sediment at the shoreline has moved along the beach by a several tens of centimetres, or several metres with bigger waves.

reflective beach – a beach where wave energy is not wholly absorbed by frictional wave breaking or interparticle friction within agitated beach sediments. As such the incoming wave energy is reflected seaward, or sideways, causing sediment to be transported. This can be responsible for moving beach sediments offshore, well beyond low water mark.

Michael J. Fennessy MA PhD FRIN              21 February 2005
COASTAL RESEARCH
Tamarisks
Waresfoot Drive
Crediton
EX17 2DG