Sea Defences
The sea defences and Sandy River form the detailed element of the planning application and have been designed by internationally recognised consultants HR Wallingford and MLM.
The objectives include the essential aim to achieve adequate flood protection which will not involve beach recharge and provide a 1:200 year level of flood protection to meet the PPS25 Government guidelines.
We were also asked to differentiate the sea defences on Crinnis from those on Shorthorn. As part of the lighter touch on Shorthorn, we have proposed a softer defence – a reinforced sand dune – instead of the original concrete sea wall.
The setting back of the sea defences out of the inter-tidal zone and further landward creates a larger beach area of eight acres in comparison with the extant scheme and the 2006 sea defence scheme and satisfies the Council’s requirement for no beach recharge.
The detailed analysis is set out fully in the Environmental Statement’s section on Water Resources and the Flood Risk Assessment (FRA) which includes beach modeling, 2D and 3D model testing of the sea defences and a detailed appraisal of the proposals.
The FRA demonstrates that, subject to detailed design, the critical requirements set out in Government Guidance (PPS25) are met and there will be no unacceptable risk of flooding to the proposed development within the site, or an increase in flood risk elsewhere.
The proposed sea defences have been designed to avoid, minimise and mitigate the flood risk for the site, and provide a 1:200 year level of flood protection. They have also been designed to take into account the effect of climate change, sea level changes and more extreme conditions over the lifetime of the project and to ensure that safe access/egress from all properties is maintained.
The sea defence on Crinnis takes a hard engineering approach. It will comprise a concrete recurve seawall – the primary wall – with a promenade incorporating secondary and tertiary defences.
An alternative, less heavily engineered, form of defence is proposed for Shorthorn, designed in the form of two bunds each built of beach material on the seaward and landward faces, with an engineered core. This is in keeping with the lower intensity of development proposed for this part of the site.
No sea defences are proposed for Polgaver.
Plans of the sea defences can be downloaded from the following links:
1. Plan of Sea Defences 1 of 2
2. Plan of Sea Defences 2 of 2
The route of the Sandy River has been designed to accommodate an increased flow of water from its catchment, via the adits, should these be improved in the future. The design also takes into account the effects of high tides. A circular stilling basin immediately landward of the sea wall will dissipate waves that propagate up the river channel under extreme conditions. The stilling basin is to be lined with rock and contain a series of rock features that interrupt the path of incoming waves and promote dissipation of waves.
In order to maintain a high quality beach, management of the beach including raking, litter collection and localised re-profiling will be undertaken, for amenity purposes.
Plan view of part of the proposed sea defences on Crinnis
View A3 PDF
A storm warning system will be in operation in order to identify trigger levels for closure of the parts of the development, i.e. the promenade, under certain extreme storm conditions. An evacuation plan for the public and residents is to be produced and agreed.
Overall conclusion:
The objectives can be achieved
Beach
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1.
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Beach is stable
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2.
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Sea defences behind the natural beach line
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3.
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Beach will continue to ‘act’ naturally
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4.
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No maintenance or intervention except for amenity reasons, as with many Cornish beaches
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Sea Defences
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1.
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Designs on Crinnis and Shorthorn have been extensively tested and use proven techniques
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2.
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Designs protect against extreme event – allowing for climate change /
sea level rise
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3.
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Safe access / egress from all properties – including Shorthorn and at the promontory
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Sandy River
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1.
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Design allows for climate change and for others to double the flow
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2.
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River channel lined to prevent undercutting
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3.
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Outfall designed to control movement
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4.
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Access allowed for maintenance
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The research process involved 4 steps:
Step 1: Review the beach data
Step 2: Beach modelling
Step 3: 2D model testing
Step 4: 3D model testing
Survey results gathered during step 1 show the beach is stable with the beach material retained within the bay:
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Year
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Site / beach area m²
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1933
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209,470
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Stent/ material still deposited, via Sandy River
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1970 / 1
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200,920
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Beach reaches equilibrium – it is stable
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Sept 2002
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199,470
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Oct 2003
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201,630
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April 2004
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201,330
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Nov 2004
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197,520
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major storm – beach area recovered by March 2005
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March 2005
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201,320
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Oct 2006
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203,320
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Feb 2007
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198,390
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May 2007
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202,030
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Aug 2008
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201,160
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Beach remains at its stable equilibrium
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Source of information: A number of independent surveys including data from the Plymouth Coastal Observatory.
The data shows that the temporary wall, installed in 2004, has not affected coastal processes.
The 3D testing approach and performance requirements have been agreed with the Environment Agency and a Flood Risk Assessment has been submitted.