Ventnor Coastal Protection and Slope Stabilisation Scheme Information Page

7 December

The esplanade between Wheelers Bay and Bonchurch will remain open until Mid-January.

Core samples

Concrete samples, taken from Wheelers Bay esplanade, are undergoing laboratory testing. Additionally, CCTV and laser scans undertaken beneath the esplanade will inform where and how we can go forward in the next phase of works.

Trail pits and Boreholes

Our first consideration must always be to minimise the danger to all, especially when the investigations will be undertaken in a constrained and exposed location over the winter period.

Therefore a decision has been taken to  reduce the number, extent and scope of the boreholes due the significant risk associated with undertaking mitigation works ahead of drilling individual boreholes. The risk management works, in the form of stripping back the esplanade and slowly digging down five metres in each proposed borehole location, was deemed necessary due to the presence of a Southern sewerage main and potentially other unmarked services embedded within the esplanade.   

Intrusive investigation of the defences, fronting the Bonchurch Cliffs,  will potentially take place later on the scheme development, dependent upon the results derived from the trial dewatering scheme, being undertaken above Ventnor’s Western Cliffs.  

We will now focus our efforts in the vicinity of Wheelers Bay, where the coastal defences are in poorer condition.

Trial dewatering borehole

Phase 1: Above Western Cliffs 

Four of the seven monitoring boreholes have now been successfully drilled. However, the rate of drilling has been significantly slower than forecast due to the presence of sizeable boulders. Ground water samples,  taken from the constrained aquifer thirty-five meters below sea level, have shown slightly above average iron content and more siltation than anticipated.  

Additionally,  the initial results indicate the aquifer is heavily fractured, thereby limiting the area of effectiveness of an individual extraction wells. These finding will be used to inform the design and equipment to be used during Phase 2, the trial ground water extraction tests. Ground water levels and landslide movement, within the immediate area of the trial dewatering borehole, will be monitored throughout during this second phase.

Whilst the ground investigations are taking place, we will update our website on a weekly basis so that you are always aware of our most up to date programme. The dates may well change, either because of the information collected or because of adverse weather.

The next update will be published here on 14th December 2023.

Ground penetrating radar surveys (GPR Surveys)

GPR surveys are a non-intrusive investigation approach to understand the sea defence dimensions, voiding and location of services within the defences. This is useful information that will inform the next phase of investigative works which is broadly known as Ground Investigations. Unfortunately, the GPR results were inconclusive, as is often the case when surveying reinforced concrete due to the presence of steel which disrupts the radar.

Visual Asset Inspection

To assess the quality of the Ventnor Coastal defences and how they will react to sea rise, a specialist has observed and assessed them. This information will now be used to prioritise works and where to target further investigations.

The asset inspection looked for signs of deterioration, assigning a grading from 1 (very good) to 5 (very poor/failed). The grading estimates the assets remaining ‘useful’ life. For example,  how long until refurbishment and/or replacement of the asset is required. As part of this assessment, climate change projections such as sea level rise and increased storminess and rainfall totals are all considered. The information will help target further investigations and inform the sequence in which works are undertaken.  

Topographic Survey of the land and coastal defences between Steephill Cove and Monks Bay

Topographic Survey of the land and coastal defences between Steephill Cove and Monks Bay.

A topographic survey is an accurate depiction of a site that shows the natural and man-made features and their levels. It is done by land surveyors using specialised equipment and skills. 

The aim of the topographic survey is to gain a better insight into how the landscape will interact with any investigative works and identify problems before works commence.

Property Level Threshold Survey

In addition to the topography survey, property level threshold surveys have been completed. This is a process that assesses the level of flood risk to a property or site based on the critical water level entry points of the property. It is required for some developments located within flood zones.

The pictures below show Property Level Thresholds being assessed in Ventnor