The primary objective of the study was to develop a sustainable coastal defence scheme for managing coastal erosion and flooding risks along the Eastoke Point frontage.
This coastline has a long-term history of retreat as a result of rising sea levels, variations in the wave climate and changes in the position and alignment of both the deep channel through the entrance to Chichester Harbour and the banks of the ebb shoal delta that form on either side of that channel. This study includes a review of all these effects as an essential background to the consideration of the future management of the Eastoke Point coastline.
The overall coastal defence policy for this frontage was established in 1996 in the East Solent Shoreline Management Plan and reconfirmed during the North Solent SMP in 2010 as “Hold the Line”. Subsequently, in 2006, a detailed Coastal Defence Strategy Study for the whole Eastoke peninsula (The Eastoke Sectoral Strategy Study) was carried out and considered some options for suitable coastal defence schemes for Eastoke Point that would provide an adequate standard of protection to these low-lying residential areas. The Eastoke Sectoral Strategy Study supported the Hold the Line policy for the frontage and recommended that the defences are improved to give a standard of protection of 0.5%, i.e. protecting the assets at risk from storm events with a return period of 200 years.
Flood and Erosion Risks
In 1992 a breach occurred in the defences around Eastoke Point and emergency works were undertaken. These works involved the construction of 150 metres of rock revetment and rock stub groynes. At present, beach levels along much of the southern coastline of the Eastoke peninsula are being managed to provide an adequate standard of defence against coastal flooding and erosion. In addition, defences have recently been improved along the northern shoreline of the peninsula, reducing risk of tidal inundation from that coastline. Around Eastoke Point, however, the present standard of protection against coastal flooding and erosion is considered to be low.
A wide range of defence options for Eastoke Point have been assessed during this the study. An initial long-list of techniques used around the UK and overseas were assessed for their suitability for the Eastoke Point frontage, based on specialist judgement, and were reduced in number to a short-list of potentially suitable methods. This Initial screening of coastal defence ‘elements’ and of beach management methods identified a number of possible approaches for managing flood & erosion risks at Eastoke Point. However, it was concluded that no single element was sufficient on its own, mainly because of the very variable nature of the hydrodynamic and sediment transport processes and thus of the beaches themselves. Any successful scheme would need to be capable of adaptation in the light of changes to wave conditions, tidal levels and currents which are all affected by unpredictable changes to nearshore seabed. As a result a number of combinations of coastal defence elements and management methods were short-listed as follows:
Option 0 – Do Nothing
Option 1 – Do minimum
Option 2 - Beach Recharge & recycling (alone)
Option 3 - Timber groynes, recharge & recycling
Option 4 - Rock groynes, recharge & recycling
Option 5 - Rock revetment with groynes, recharge and recycling
Option 6 - Seawall with groynes, recharge and recycling
The short-listed options were compared using a multi-criteria analysis that allowed consideration of their technical feasibility, their likely environmental acceptability including sustainability and their economic benefits.
Conclusions and Recommendations
The study has identified Option 5 as the preferred coastal defence approach for Eastoke Point. The proposed scheme comprised of the following principal coastal defence elements:
- Groyne replacement or improvement
- Rock revetment
- Improvement or installation of splash wall
- Beach recharge
- Groyne maintenance
- Revetment maintenance
- Maintain splash wall
- Beach recycling and management