A number of reports examining the quality of the Island’s sea defences have been produced in recent years:
http://www.gov.gg/coastaldefence see pdf reports at bottom of page
In 1998 Posford Duvivier was commissioned by the Board of Administration to undertake an Island-wide survey of all the built defences.
Subsequently, Posford Duvivier produced a substantial report (PD 1999) setting out detailed analyses of all the defences of Guernsey and Herm.
In 2006 Haskoning UK Limited provided an update on the work of Posford Duvivier (Haskoning 2007). This contained more detailed recommendations for the upkeep of sea defences in Guernsey and Herm, taking account of forecasts for climate change and the outputs of computer modelled tidal movements.
Haskoning identified a number of areas of the island at risk of flooding from the sea and focused on six areas where coastal inundation is likely to have the most far-reaching effects. This was based on a very simplistic review of areas below the 6.0m OD contour.
The Environment Department then turned its attention to the potential for tidal flooding. Haskoning UK Limited were commissioned to undertake a further study (Haskoning 2012) to report on the options for mitigating flooding in the areas identified in Haskoning 2007 as being most at risk from sea level rise associated with climate change and refine the areas affected using a more analytical approach.
The Coastal Defence Project Group (CDPG) was consequently formed, comprising representatives from the Environment Department, States Property Services and other parts of the States; it included two Board Members. The CDPG set about creating a priority order for tackling the identified flood risks using a weighted evaluation method, as detailed in Billet D’Etat XV July 2013 and endorsed by the States of Deliberation in September 2013.
Phase 1 – St Sampson
The evaluation methodology detailed in the States report confirmed the two highest priority areas are The Bridge, St Sampson and Belle Greve bay.
The first phase of the Coastal Defence Flood Risk Strategy is focussed on the area at greatest risk, St Sampson’s.
The Haskoning 2012 report Volume II – Local Area reports, provides a detailed discussion of the St Sampson Harbour area (Coastal Unit 18) with respect to potential flood risk. This report focuses on management around the Harbour but includes an assessment of the potential flood risk to the area from adjacent sections of the coastline, in particular the area around Le Grande Havre (Coastal Unit 12).
The town centre surrounds the inner harbour on the east and south face, with industrial developments including the shipyard, various heavy industry firms and the Guernsey Power Station located predominately on the north side.
Land reclamation works have been undertaken south of the harbour entrance, and along the perimeter of Bulwer Avenue. Other industrial infrastructure is protected by the more recent rock revetment and breakwater structures, which also provide a safe mooring haven for approximately 140 smaller vessels.
The main coastal road runs through St Sampson across the ridge of high ground to the back of the Harbour, The Bridge. This is a key transport route between the north and south of the island. Other principal transport routes between the north and south run across the valley behind St Sampson.
St Sampson and its Harbour provide an essential commercial and industrial area of the island and there is significant commercial development of the low lying land within the hinterland. The main residential development is set back to either of side of the Harbour and to the higher ground either side of the main valley. The main shopping centre of St Sampson lies around and to the back of the harbour and there are areas of residential property both within this and across the valley floor further within the hinterland.
The local area and the main valley behind is, therefore, very important in terms of sustaining the economic value of Guernsey, in terms of the important transport links and in terms of residential development.
The 2007 Strategy document summarised the defence at St Sampson as follows:
The boat yard (formerly the coal yard) is defended by a sloping masonry wall. Rock pillars exist at random spacing along the wall, presumably to reduce wave energy and subsequent overtopping. A secondary wall exists behind the defences.
St Sampson Harbour is protected by vertical masonry breakwaters, which are known to be highly permeable resulting in saline intrusion into the foul sewerage system on high tides. Reclamation of the Longue Hougue commenced in the early 1990s but the Harbour itself has not changed shape since 1880. Much of the land to the south of Saint Sampson Harbour is reclaimed and is protected by a rock revetment.