Ground investigation and monitoring works:Barton-on-SeaNew Forest District Council is undertaking a series of ground investigations along the cliff at Barton-on-Sea. A number of boreholes will be installed in order to sample, test and monitor the subsurface geology with the intention of gaining a better understanding of the geological properties, stability and role of groundwater within the cliffs. Weather permitting, the works are likely to be undertaken during autumn/winter 2012 and will last for approximately six weeks.

Coastal Geology Management and EngineeringThe geology of the cliffs at Barton-on-Sea are comprised of weak, interbedded sands and clays overlain by unconformable plateau gravel. The frontage lies within an active landslide complex, which is driven by groundwater and cliff toe erosion.

Since the late 1960s the engineering response to coastal instability has focussed 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. Additionally a groundwater drainage system, consisting of a sheet pile 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 pile 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.

Poole and Christchurch Bays Shoreline Management Plan (SMP)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. Visit twobays.net for further details.

Site of Special Scientific Interest (SSSI)The site lies within the Highcliffe to Milford-on-Sea SSSI, which is notified because of the geological interest afforded by the fossil rich strata as well as the contemporary invertebrate habitats. Formal notice of these works (to Natural England) is required under Section 28H of the Wildlife and Countryside Act 1981.

The Cliff Instability Study In 2010, the council secured central government funding of £300,000 to carry out a series of ground investigations at Barton-on-Sea.

The aims of this study are to:

• undertakegroundinvestigationworksandmonitoringto enable a better understanding of the site

• gainbetterinformationoncurrentlandslidestability,cliffrecession processes and groundwater behaviour of the site

• developviableengineeringandmanagementoptions

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

Monitoring• Geotechnicalmonitoringinstrumentswillbefittedinside

the boreholes such as:- inclinometers (to detect sub-surface movement) - piezometers (to measure groundwater levels)

• Thecouncilwillregularlydownloadthedataoveraperiod of at least 18 months

• Thecouncilwillalsocontinuemonthlyin-houseslopemovementmonitoring and rainfall data collection programmes

Final Report:At the end of the monitoring period, a feasibility report will be produced by the consultant that will detail a range of engineering and management options for groundwater control and slope stabilisation measures at Barton-on-Sea.

For more information contact Pete Ferguson or Steve Cook (Coastal Protection Group Engineers) on 023 8028 5588 or visit

newforest.gov.uk/coastal

Barton-on-Sea cliff instability:Ground investigation and monitoring worksKEY:

BH01 Borehole number and location

Site of Special Scientific Interest (SSSI)

• Itisproposedthatatotalof18boreholeswillbeinstalledalongthe frontage [orange circles]. Ten of the boreholes will be situated along the cliff-top with a further eight along the under-cliff.

• Boreholeswillbeinstalledtoadepthofbetween20mand35mandit will take approximately five days to install each hole.

• Duringtheworksperioditislikelythatnoiselevelswillincrease,especiallyduring borehole installation. The council apologises in advance for the inconvenience this may cause.

• Aseparatesubsurfacegeologicalsurveywillalsobeundertakenalongthecliff-top.

Inset: Example of a drilling rig which will be used to drill the boreholes

N

BH01

BH02

BH04BH03

BH09

BH13

BH08BH05

BH06

BH07

BH11

BH12

BH15

BH16

BH17

BH18

BH14

BH10

100100 50 0 metres

Site access:Fisherman’s walk

Hoskin’s Gap

Car park and site compound

Site access:Sea Road

MARINE DRIVE WEST

MARINE DRIVE

MARINE DRIVE EAST

2008 Aerial photograph courtesy of Channel Coastal Observatory / New Forest District Council