CURRENT PROJECTSCliff Monitoring and Stabilisation Works
Canford Cliffs, 2017-2020
Phase 2 of works to address the landslip at Canford Cliffs in February 2017 involves a 12-month programme of stabilisation works.
2nd June 2020
The 2017 landslip at Canford Cliffs
In February 2017, at Canford Cliffs, a section of cliff approximately 10m wide x 0.5m deep eroded and slipped, resulting in the closure of two blocks of beach huts directly below it and postponement of a planned investment in seafront infrastructure.
Following completion of the 2017-2019 cliff face monitoring project (full details below) the Council agreed a 12-month programme of stabilisation works at a cost of approximately £3.1 million.
Vegetation clearance January 2020
Vegetation was cut back to enable structural engineers to assess the condition of the cliff face and monitor its stability. This will help make the final preparations for engineering works to stabilise the cliff.
By cutting the vegetation close to ground level we can complete wildlife surveys to ensure all protected reptiles and nesting birds are excluded from the proposed working area. Reptiles are being moved to a prepared ‘friendly’ habitat outside of the exclusion zone and once the stabilisation works have been completed they will be able to colonise the whole area again.
Borehole Drilling, April 2019
On the promenade and beach hut terraces around block numbers 1–11 (borehole location map, pdf). This further ground investigation is required to provide the necessary information to complete the designs for cliff stabilisation, the new beach hut foundations and the retaining wall located behind the beach huts.
The area of cliff affected by the landslip is part of the Poole Cliffs SNCI (Site of Nature Conservation Interest) which is managed by the Council 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.
During the period of cliff stability monitoring it was necessary to clear cliff vegetation to enable structural engineers to assess the condition of the cliff face. At the same time reptiles were encouraged to gradually move away from the ‘exclusion zone’ to pre-prepared reptile-friendly habitats beyond it. Surveys were carried out until winter hibernation to ensure the zone was clear of reptiles in anticipation of cliff stabilisation works.
The Council’s biodiversity officer has been working, and continues to work throughout this project to protect both habitats and wildlife.
Monitoring Cliff Stability
Monitoring movement of the cliff face at Canford Cliffs following the landslip in February 2017.
Borough of Poole
24 April 2017
05 February 2019
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, has been postponed as a result of the landslip and ongoing investigations.
Project Progress (latest first)
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.
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.
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
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.