Home Database Adaptation options Cliff strengthening and stabilisation
Adaptation option

Cliff strengthening and stabilisation

Coastal cliffs can be differentiated according to their morphology and structure: cliffs can be loose – sand, silt, clay, marl and chalk – or hard, made of limestone, sandstone, granite and other rocks. Loose cliffs are more prone to erosion and landslide than rocky cliffs, which  are more characterised by rock or block fall. Cliff erosion in coastal areas usually regards the erosion of the cliff foot, caused by wave and storm surges action, resulting in a gradual retreat of the coastline. Climate change aggravates coastal erosion even more: sea level rise, increased frequency and intensity of storminess, changes in prevalent wind directions and higher waves put Europe's coast under additional pressure.  

Coastal cliffs provide habitat to a rich range of flora and fauna. The cycle of cliff fall followed by cliff stabilisation creates a mix of vegetation and bare rock which constitutes a suitable but precarious habitat for rare and threatened plants and animals. Many cliff species are listed in the annexes of the EU Habitat Directive as well as some cliff habitats, requiring close monitoring and management. Moreover, coastal cliffs support the tourist offer both as viewpoints at the top and bathing areas below. As long as there are enough space and buffer zones, erosion processes do not significantly threat people or assets. The challenges arise when coastal zones are highly urbanised and urbanisation draws nearer to cliffs and shorelines, making the buildings and residents more exposed to possible damages from erosion. Casualties can occur with people falling from the cliff top, slipping along the paths or being hit by falling stones while sunbathing on the underlying beaches.  

Cliff strengthening and cliff stabilisation techniques aim at increasing the strength and overall stability of the cliff slope as well as protecting the foot of the cliff against erosion.  

Cliff strengthening techniques are “grey” measures and include the following: 

  • Cliff reshaping/profiling: changing the slope angle, and/or reducing cliff heights by removing unstable blocks. In some cases, terraces can be created. The angle at which the cliff becomes stable depends on the rock type, geological structure and water content. This technique enhances the overall stability of the cliff as it reduces mass movements on the cliff. This measure is not suited for rocky cliffs or high and strongly sloped cliffs. 
  • Cliff drainage: eliminating surface runoff and infiltration on the slope. This can be done by creating ditches at the top and/or on the slope of the cliff. Reducing pore pressure can also be achieved by piping water out of the cliff. This method is suited for limited runoff and infiltration, and is applied to rocky cliffs. Drainage can sometimes be applied to groundwater level, when groundwater plays an important role in rock deterioration. This is achieved by drilling and inserting draining tubes or perforated metallic tubes on the slope surface. 
  • Rock bolting/pinning: this technique involves securing unstable rocks to increase cohesion and stability and prevent slippage, using metallic bolts, tie-rods, steel soil nails driven horizontally into the cliff. It prevents mass movements such as rockslides and collapses and thus reduces net erosion rates. 
  • Concrete buttress and riprap strips: reinforced concrete support on undercut rocky zone on the cliff or the foot; ripraps (strip of stones and concrete) are placed at the foot of the cliff to prevent marine erosion. This technique is suited for small and medium rocky compartments. 
  • Reinforced geogrid and pinned net: stabilising the slope by using a reinforced polymer grid, attached to the side with anchors, or wrapping unstable blocks by pinning nets or grids to the side of the cliff to prevent rockslide. Geogrids are suitable for soft cliffs with limited heights to avoid landslides. Nets are suitable for rocky cliffs with limited volume instability. 

Cliff stabilisation techniques are instead “green” measures and encompass:  

  • Littoral strip reloading: placing sand or pebbles at the foot of the cliff to compensate littoral imbalance caused by marine erosion. It is similar to beach nourishment and generally suited for areas with insufficient longshore sediment transport. 
  • Re-vegetation: managing existing vegetation to regain damaged areas or establishing a vegetation cover on the slope to limit the risk of instabilities. This can be applied by creating forested berms or water draining ditches. The nature of the vegetation planted varies according to the level of instability of the slope. On very mobile slopes, fast growing and deep rooted species are preferred as they grasp the soil and prevent movement. On more stable slopes, a plant ground-cover can be effective as it acts like a protective skin. This technique is particularly suitable to loose rocky cliffs and sandy cliffs. 

Decisions on the methods to be applied are based on the natural characteristics of the cliff (nature of the cliff, cliff geometry, hydraulic behaviour and mechanical forces), the type of instability, socio-economic stakes and access conditions. In practice, the two approaches are often combined as revegetation alone is only a short term solution, which does not completely stop the erosion of the coastline. If structural erosion is not countered it will eventually lead to the steepening of the cliff and negate the effects of stabilisation measures.  

In order to enhance the protection of coastal resources in a holistic way, practical measures such as cliff strengthening and stabilisation techniques should be integrated in a broader integrated coastal management zone management (ICZM) plan, involving multiple scales of governance. ICZM includes principles that are also important for coastal erosion management, such as the involvement of all relevant parties and the inclusion of a long term perspective in coastal management. An example of cliff stabilisation techniques adopted within a ICZM can be found in the coastal area of Marche region in Italy. The ICZM plan included the stabilisation of a cliff in Mount Conero. placing large blocks at its base. This mountain is designated as protected side under the EU Natura 2000 network, and the works incorporated the need to maintain site conditions. In the coastal city of Omiš (Croatia) a combination of cliff strengthening techniques was implemented from 2016 to 2018 along 2.5 km of coast around the city: rock fixing with geotechnical anchors; steel clamps to stabilize the unstable rock parts; steel nets for protection; mechanical “tentacles” for the movable parts of the crushed rock; anti-rockslide barriers, etc. 

Additional Details
Reference information

Adaptation Details

IPCC categories

Structural and physical: Ecosystem-based adaptation options, Structural and physical: Engineering and built environment options

Stakeholder participation

Whenever a project is expected to have a significant impact on threatened and valued species and habitats protected under the EU Natura 2000 network, its ‘appropriate assessment’ (see legal aspects, below) could include a public participation process, but this is not mandatory. If the realisation of these techniques falls under an Integrated Coastal Zone Management (ICZM) process, stakeholder involvement will be required and will play a major role. As stressed by the 2002 EU Recommendation on ICM (2002/413/EC) and the 2013 Communication (EC COM(2013) 133), the involvement of all parties and all levels concerned (i.e. national, regional and local administrations, economic operators, local communities etc.) fosters consensus-building on and public acceptance of the proposed measures. The role of stakeholder engagement is strongly remarked also by the EU Maritime Spatial Planning Directive). It highlights the importance of taking into account land-sea interactions (including coastal erosion) when planning for the marine space. Moreover, public participation may be required under national procedures or if a planning permission is needed from a municipal authority (e.g. for the placing of materials on a cliff face). 

Success and Limiting Factors

Most of the cliff strengthening techniques (bock bolting, geogrid, concrete buttress, reshaping, drainage) allow a significant reduction of cliff erosion. However, the specific type of cliff needs to be considered in the choice of the technique. Some of these techniques, such as rock bolting and pinned nets, can only be applied for localised stabilisation and cannot remedy overall instability. Some cliff strengthening techniques (i.e. concrete buttress, rip-raps, pinned nets) can strongly impact the landscape. Cliff reshaping can disturb biodiversity by destroying habitats, Although in some cases the combination with revegetation can even improve local habitats. Moreover, reshaping work can have a strong visual impact depending on the scale of the works and can negatively influence tourism .  

On the other hand, “green” stabilisation measures have little impact on the landscape. Thus, they are usually supported by littoral users and considered as beneficial for recreational purposes. However, revegetation  only stabilises the upper layer of soil and can usually only be applied to small areas. The type of vegetation planted has to be carefully chosen  according to the nature of the soil or the rock surface. Local species should be preferred. If not well managed, the growth of the roots can have the reverse effect of causing instability by causing rock fracturation. In most cases, revegetation alone will only be a short term solution. Structural erosion will significantly diminish its benefits, unless littoral strip loading is simultaneously applied. 

Littoral strip reloading has similar shortcomings as beach nourishment: reload material should match the characteristics of the native one, the source area must be close enough, repeated reloading is usually required as it does not stop ongoing erosion., etc. In general, most of these techniques require regular maintenance and inspections to ensure they remain effective.  

Costs and Benefits

Some of the cliff strengthening techniques have high start-up costs as they require preliminary studies and specialised private contractors to be hired. Rock bolting can be complicated to implement and therefore costly. On the contrary, rip-rap strips are a rather low cost method. The installation of a geogrid can also limit the costs as it can avoid resorting to costly solutions. However, in nearly all cases, specialised civil engineering contractors must be hired. Regular maintenance costs will be needed for cliff reshaping, concrete buttress and rock bolting. This is also valid for techniques aiming at preventing collapses and rock falling such as geogrid and pinned nets. These need regular inspections and surveillance for safety reasons. 

The benefits of cliff strengthening and stabilisation techniques must be balanced with the implementation and maintenance costs. Letting the cliff erode has been considered in some areas as more cost-efficient than stabilisation or reshaping measures (see the adaptation option Retreat from high risk areas). 

    The following key EU Legislation can be relevant in the context of the implementation of cliff strengthening and stabilisation techniques: 

    • Coastal works addressing erosion which are capable of altering the coast fall under Annex II of the EU Environmental Impact Assessment (EIA) Directive. Member States decide whether these projects should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria. However, this requirement does not affect the maintenance and reconstruction of these works. If not subject to impact assessment, the implementation of these techniques may require prior declaration or authorisation. 
    • Any project likely to have a significant impact on valuable and threatened species and habitats protected under the EU Natura 2000 network must undertake an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site, as per Art. 6(3) of the EU Habitat Directive. Moreover, certain types of cliffs are considered ‘habitats of community interest’ under Annex 1 of the Habitat Directive. In some cases, cliff stabilisation projects could be part of the management plans for Natura 2000 sites including such habitat types. 
    • Additional national legislation may apply, such as permitting requirements. 

    Implementation Time

    Implementation times vary from months to a few years, depending on the combination of measures selected. Implementation may require more planning time if the measures are conceived as part of a ICZM plan and require an active and broad stakeholder engagement. 

    Life Time

    With regular maintenance, most cliff strengthening methods generally have a relatively long lifetime. Cliff stabilisation techniques, in particular littoral strip reloading, require repeated regular actions as they do not actually lead to stopping ongoing erosion but rather buffer its effects. 

    Reference information

    Published in Climate-ADAPT Jun 07 2016   -   Last Modified in Climate-ADAPT Aug 17 2023

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