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Coastal cliffs can be unstable due to the combined effect of several factors, discussed in this article along with methods to stabilise them.
Coastal cliffs can be unstable due to the combined effect of several factors, such as:
- Erosion of the foot of the cliff caused by wave action and storm surges
- Sliding or weathering of the slope due to geo-technical instability. The erosion of the foot of the cliff normally initiates geotechnical instability, but the sliding/collapse can be of different nature depending on the geo-technical conditions of the slope. There are basically three different situations:
- If the material is non-cohesive material, the weathering of the cliff ill normally occur simultaneously with the erosion of the foot as a talus formation, which is the collection of fallen material forming a slope at the foot of the cliff.
- If the material is a mixture of clay, silt, sand and boulders, such as in the case of moraine till, the slope can be very steep for a period due to the cohesive forces, but the slope will eventually collapse. Smaller or bigger fractions of the cliff will fall in connection with groundwater pressure, frost impact or general weathering, or by sliding. Sliding will especially occur in connection with groundwater pressure.
- If the material consists of plastic clay or silty clay, the collapse of the cliff will be in the form of slides, which can go far behind the top of the cliff.
- Weathering of the cliff by wind transport of sand. This will be most pronounced if the cliff material is sand; however, also exposed cliffs consisting of other types of material can be eroded by sand blown over the cliff from the beach.
Fore cliff morphology and cliff erosion see also the article Rocky shore morphology.
The basic cause of cliff instability is normally the marine erosion of the foot of the cliff, mitigation of this is covered under the protection method: Revetment. Installing the revetment will exclude further erosion of the foot, but at that stage the slope of the cliff may very well be so steep that weathering and sliding may still occur. This can be counteracted by the following means:
- Artificial smoothing of the slope, if there is enough space at the foot as well as at top of the cliff for this. This will counteract future uncontrolled weathering and sliding.
- Smoothing of the slope by filling with granular material at the foot of the cliff. This requires that there is sufficient space at the foot of the cliff for the filling.
- Establish a vegetation cover on the cliff. This can best be done by following the above-mentioned smoothing of the slope. Good vegetation protects against weathering and groundwater seepage, and thereby to some extent against sliding
- Drainage of groundwater. This can be used if the cliff suffers from sliding due to high groundwater pressure and poor drainage conditions. Horizontal and vertical drains can be used as well as the regulation of the surface runoff.
Cliff slopes are often “protected” by dumping assorted rubbish, such as branches etc., over the cliff. It is a bad “solution” because it does not stop the risk of sliding. On the contrary, it spoils the vegetation and thereby increases the risk of sliding.
Cliff stabilisation presupposes that the foot of the cliff has been stabilised. Stabilisation counteracts the natural behaviour of cliffs to slide and weather. Such an active cliff is part of the dynamic coastal landscape and should therefore in principle be maintained as an integrated part of this landscape.
Cliff stabilisation can be applied at all moderately exposed to exposed coasts; however, in order to preserve the dynamic coastal landscape cliff stabilisation should only be used sparingly. Preserving the active cliff at densely populated coasts is normally not feasible due to the limited space. Consequently, cliff stabilisation is normally only used when there is sufficient space in the backland to allow some smoothing.
Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
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