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Introduction

Background

The coastal town of Barton on Sea lies towards the centre of Christchurch Bay.  The beach below the town faces approximately due south and is located adjacent to cliffs which are weak and very susceptible to coastal erosion and cliff top recession.

cliff wedges The cliffs are comprised of layers (beds) of clay and sand with a capping of gravel and dip gently from east to west.  The process of erosion at Barton is due to both the sea attacking the base of the cliff (toe) and the presence of water (groundwater). The rock structures along the toe of the cliff provide protection from the sea, without these the sea would remove clay and sand, thus leading to instability, slumping and recession of the cliff.  Groundwater moving through the sands and gravels to the impermeable clay strata can also cause instability, by exerting pressure at the interface between the different layers, which can result in landslides.

Although the Barton frontage is currently protected from erosion from the sea with a rock revetment along the toe, there is potential for erosion to occur to the west along the frontage at Naish.

Since the late 1960s the engineering response to coastal instability has focused on maintaining the stability of the cliff.  This has been achieved using rock armour revetments and rock groynes to protect the cliff toe from wave attack and to add toe weighting to reduce the landsliding risk.  Additionally, a groundwater drainage system, consisting of a sheet piled wall and drainage pipes removes groundwater from the cliff.

Recently, a series of deep seated landslides and shallow mudslides has caused significant sections of the sheet piled wall and drainage system west of Hoskin's Gap to fail.  There are concerns that further cliff instability could reduce the effectiveness of the toe protection structures.

What are we doing?

In 2010 the council secured central government funding of £300,000 to carry out ground investigations at Barton on Sea.

A number of boreholes will be installed in order to sample, test and monitor the subsurface geology.  A total of 18 boreholes will be installed, ten of these will be situated along the cliff top with a further eight along the undercliff.  A layout plan detailing the proposed locations of these boreholes can be found on the adobe icon Borehole Locations Map [2Mb].

Each borehole will be installed to a depth of between 20 and 35m and it is expected that it will take approximately 5 days to install each hole.  A separate subsurface geological survey will also be undertaken along the cliff top.

A consultant is being employed by the council to undertake the study and will be on site to supervise the ground investigation work.

Why are we doing it?

Following the SMP Review in January 2010 the adopted policy for the next 100 years is managed realignment.  The intention of this policy is to maintain a degree of control over cliff recession through the maintenance and adaptation of existing defences and to investigate ways to improve drainage and reduce the erosion rate.  This study is the first stage in helping to deliver this policy.

The aims of this study are to:

  • Enable a better understanding of the site to be gained
  • Gain better information on current landslide stability, cliff recession processes and groundwater behaviour of the site
  • Develop viable engineering and management options

When are we doing it?

Weather permitting, the works are likely to be undertaken during winter 2012/13 and will last for approximately six weeks.

What monitoring will be done?

Geotechnical monitoring instruments will be fitted inside the boreholes, such as: inclinometers to detect subsurface movement and piezometers to measure groundwater levels.

To enable sufficient data to be gathered from the monitoring instruments there will be a need to collect data over a 12 to 18 month period.  The council will regularly download the data and will also continue monthly slope movement monitoring and rainfall data collection programmes.

Will a report be produced?

At the end of the monitoring period, a feasibility report will be produced by the consultant that will detail options for the future management of Barton on Sea.

Updated: 2 Dec 2014
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