Example of a medium sized cliff failure
eventSince 1996, Mott MacDonald has been responsible for
monitoring and reviewing past and present cliff recession rates at
Hunstanton for the Borough Council of Kings Lynn and West Norfolk.
This project has involved detailed analyses of cliff failure
information to develop a GIS based cliff behaviour model, providing
both temporal and spatial predictions for future recession rates at
critical structure locations.
The Hunstanton cliffs extend for about 1.3km north from the
northern end of Hunstanton promenade and are internationally
recognised as a geological site of importance. The cliffs are some
18m high at their highest point, in the vicinity of Hunstanton
lighthouse and coastguard station. The cliffs have historically
been subjected to continued erosion by the sea and have retreated
locally by up to 30m since 1885. Individual failure events can
involve large volumes of material and therefore represent a
significant safety concern both to the public and to cliff top
amenities.
An initial study was undertaken which highlighted the failure
mechanisms involved in cliff regression and estimates of future
recession rates. It was following these results that Mott MacDonald
was appointed to carry out further annual monitoring surveys
between 1997 and 2004, which culminated in the production of a
detailed review report in 2005.
Each survey covered the measurement of the line of the cliff top
from a static baseline at point locations, as well as the visual
observations of engineering geologists coupled with production of
an annual photographic montage of the cliffs viewed from the beach.
The use of both techniques meant that failures of all sizes,
including those not resulting in regression of the cliff top, were
recorded.
Typical cliff profile of Chalk, Red Chalk and
Carstone depositsThis data along with cliff top
recession information from historical maps and plans were compiled
onto a computerised geographical information system to support the
spatial and temporal assessment presented in the review report. The
review report was able to achieve the following objectives:
- Presentation of the spatial and temporal distribution of
failure types through cliff behaviour zone mapping and a
reassessment of the regression analyses
- Assessment of the sensitivity of the cliff system in relation
to environmental changes, sediment input/output and the influence
of man
- Development of a revised model of predicted
cliff regression, particularly with reference to elements at
risk
- Assist the Council in understanding, with more certainty, the
timescale within which mitigation measures would have to be
implemented to prevent loss of properties and other assets to the
rear of the cliffs.
A particular success of this project was the collection and
analysis of high resolution cliff behaviour data which enabled
predictions of future recession rates to be developed for
particular locations along the cliffs.
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