COMPLETED PROJECTSMonitoring Cliff Stability, Canford Cliffs
Monitoring movement of the cliff face at Canford Cliffs following the landslip in February 2017.
Borough of Poole
24 April 2017
05 February 2019
5th March 2019
March 2019 – cliff stabilisation works to go ahead
Geology reports having been reviewed and options considered, the Council has agreed a recommendation for a 12 month programme to stabilise the whole cliff face at a cost of approximately £3.1 million. The works will allow the re-commencement of an extensive development scheme that was postponed by the landslide. Under the scheme, existing beach huts will be rebuilt, and additional huts installed along with a new café & public toilets [scheme details at https://seafront.pooleprojects.net/canford-cliffs]
In February 2017, at Canford Cliffs, a section of cliff eroded and slipped; it is now resting half way down the cliff face. Since the original slip more material has moved with it. There is some concern the eroded section could continue to move downwards to promenade level, especially in the event of heavy rain.
The slip, at approximately 10m wide x 0.5m deep, is large enough to warrant further investigation in order to establish how active it is, and whether there is any other movement in the area. Stabilisation measures can then be decided.
A short stretch of footpath on Cliff Drive was undermined by the slip and is temporarily closed.
The planned investment in new beach huts and seafront infrastructure, due to start in 2017, was postponed as a result of the landslip and during the period of cliff monitoring.
- December 2018 we cleared more vegetation extending eastwards towards the zig-zag footway and Holm Oak (which remains), enabling structural engineers to assess the condition of the cliff face and monitor its stability. This work reveals the full extent of the required cliff stabilisation work. Spraying and vegetation clearance continued throughout the winter months to allow wildlife mitigation. By cutting the vegetation close to ground level we provide light for the reptiles, areas for nesting birds and give the native heathers a chance to re-establish.
- During July 2018 reptile fencing was used to create an exclusion zone, preventing reptiles from moving into the potential cliff stabilisation working area; those already in the area were moved to pre-prepared ‘reptile-friendly’ habitats outside the exclusion zone. Surveys continued until winter hibernation to ensure they had all been removed.
- 29th May 2018 Spraying cliff vegetation to allow reptile surveys; a reduction in thick vegetation will encourage protected reptiles to gradually move away from sprayed areas in preparation for the stabilisation works later in the year. Further spraying & cutting throughout the growing season as required.
- 19th March 2018 Drilling for soil samples up to a depth of 30m, over a period of 3-4 weeks. This work is part of the continuing ground investigation to help understand the geology of the site; the information collected will help inform what cliff stabilisation works will be needed in the future.
- 8th January 2018 Further cliff movement requires a safety zone to be set up on the promenade at the base of the area.
- 18th September 2017 Work began to remove vegetation cover from the site; bare ground allowed engineers to gain a better understanding of the condition of the cliff face during the monitoring period. This work was carried out in two stages to give the opportunity to move any emerging protected reptiles to the reptile-friendly landscape prepared for them on either side of the cleared area.
- 12th June 2017 We assessed the cliff stabilisation schemes that were implemented in the 1970s either side of the current slip before deciding what scheme is needed (and if we need to re-do the 70s work)
- 4th May 2017 Channel Coastal Observatory started their monitoring.
- 2nd May 2017 all the targets (45 in total, 3 rows of 15) are now in place.
- 24th April 2017 work commenced on the installation of steel targets at regular intervals along and down a 300m section of cliff at Canford Cliffs. The monitoring of these targets will identify whether the cliff is moving or stable, and help establish what future cliff stabilisation works may be needed.
Once a fortnight, over an initial three month monitoring period, a laser scanner will read these targets and accurately measure their position, within a +/-2mm tolerance, to see if there is movement. The targets are needed because the laser cannot see through the vegetation to read the cliff face itself. The regional Channel Coastal Observatory will conduct the monitoring using the same laser scanner they already use to measure the level of Poole’s beaches for coast protection purposes.
March 2018: Drilling for soil samples
Diagram showing cross section of works at Cliff Drive (not to scale)
June 2017: Assessing previous cliff stabilisation work
Exposing the previous stabilising system either side of the current cliff slip ready for close observation.
May 2017: Monitoring methods
1. Small markers (pins) are inserted into areas of hardstanding, such as the pavements / promenade to act as reference points
2. Scanner targets are set on the markers each time a new scan is undertaken, enabling the scan data to be coordinated to Ordnance Survey grid
3. During a survey, the scanner emits laser light which reflects off hard objects back to the scanner; enabling it to record data points in relation to itself up do a distance of 300m.
5. The data from each survey will then be compared to see if there is any movement on the box targets in the cliff, and hence the cliff itself
6. The laser scanning equipment used to do the survey can record information to within a few millimetres of accuracy
7. The scanner also has an in-built camera which takes photographic images of the cliff to add colour to the 3D dataset
April 2017: The slip and installing the targets
The area of cliff where the slip has occurred is part of the Poole Cliffs SNCI (Site of Nature Conservation Interest) which is managed by the Borough of Poole for it’s nature conservation interest. It provides an ideal habitat for the nationally rare Sand Lizard and Dartford Warbler which are both protected species, and other birds that might be nesting in the gorse bushes.
Over the next few months key species will be monitored to establish numbers and locations of birds nests and areas used by reptiles. This will ensure any disturbance will be kept to a minimum during future cliff stabilisation works.
If cliff monitoring shows that a cliff stabilisation programme of works is necessary, vegetation will be cleared from the affected areas of cliff slope in the autumn when the birds are no longer nesting. This will also enable a more detailed reptile survey to be commissioned, if necessary, in the spring of 2018.
The Borough of Poole’s biodiversity officer will work throughout this project to protect the nationally rare habitat of maritime heath on the cliffs while the works are undertaken.
Historical cliff slips at Canford Cliffs
Over the past 40 years there have been three significant cliff slips at Canford Cliffs Chine. The first two happened during the 1970s and one of these resulted in a block of beach huts being demolished (these were only reinstated in 2014). The third significant slip took place in 1993.
Each of the slips at Canford Cliffs required stabilisation schemes including a system of transverse steel beams positioned down the cliff face and a series of soil nails to pin and hold a top section of the cliff.