To reduce cliff erosion and its consequences – landslide, collapse, falling of rocks – cliff strengthening techniques aim at increasing the strength and overall stability of the slope by minimizing landside pressures.
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 and are more characterized by rockslides or block fall. Cliff erosion in coastal areas is almost always the result of structural erosion, resulting in a gradual retreat of the coastline because the amount of sediment that gets eroded (rocks, cobbles or sand) exceeds the amount of deposited sediment.
To reduce cliff erosion and its consequences – landslide, collapse, falling of rocks – cliff strengthening techniques aim at increasing the strength and overall stability of the slope by minimizing landside pressures. Some techniques also protect the foot of the cliff against marine erosion, a key factor in strengthening cliffs. Techniques include:
- 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 adapted to 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 bolting unstable rock faces to increase cohesion and stability and prevent slippage, using metallic bolts, tie-rods, steel soil nails driven horizontally into the cliff. It reduces mass movements and thus reduces net erosion rates. It is particularly suited to prevent rockslides and collapses.
- 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: stabilizing 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.
Generally, cliff strengthening projects use several of these methods and often combine them with ‘green’ cliff stabilization measures (described in another sheet) such as revegetation. Decisions on the methods to be applied are based on the natural characteristics of the cliff (nature of the cliff, cliff geometry, hydraulic behavior and mechanical forces), the type of instability, socio-economic stakes and access conditions.
If a project could have a significant impact on a Natural 2000 site, its ‘appropriate assessment’ could include a public participation process, but this is not mandatory. Moreover, public participation may be required under national procedures. If a planning permission is needed from a municipal authority for the placing of materials on a cliff face, public participation may be required.
Success and Limiting Factors
Most of these techniques (bock bolting, geogrid, concrete buttress, reshaping, drainage) allow significant reduction of cliff erosion. However, the specific type of cliff needs to be considered in the choice of technique. Foreshore and cliff base erosion is a key factor that needs to be assessed and addressed where necessary. Some of these techniques, such as rock bolting and pinned nets, can only be applied for localised stabilisation and cannot remedy overall instability.
Cliff reshaping can disturb biodiversity by destroying habitats, although in some cases combination with revegetation can limit the disturb or even improve local habitats. Concrete buttress and rip-raps have a strong impact on landscape. Pinned nets also negatively impact the landscape despite localised intervention. Reshaping work can have a strong visual impact depending on the scale of the works. These techniques can negatively influence tourism. Some works will require regular maintenance and inspections to ensure they remain effective.
Costs and Benefits
Some of these 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. In nearly all cases, however,, specialised civil engineering contractors must be hired.
Although most of these techniques provide long-term solutions, regular maintenance costs will be needed for cliff reshaping, concrete buttress and rock bolting. This is also true for techniques aiming at preventing collapses and rock falling – geogrid and pinned nets – and these need regular inspections and surveillance for safety reasons.
In general, implementation and maintenance costs are high. The benefits of cliff stabilisation techniques must be balanced with the costs of the measures. Letting the cliff erode has been considered in some area as more cost-efficient than stabilisation or reshaping measures. In Norfolk and East Anglia (UK), a policy of ‘no active intervention’ in some small communities has been adopted in the Shoreline Management Plan, after cost-benefit analyses showed that compensation costs for residents were lower than active management measures (see the separate fact sheet on shoreline retreat).
Cliff reshaping does not fall under Annex II of the EIA Directive (codified as Directive 2011/92/EU) and, therefore, does not require an environmental impact assessment. Any project likely to have a significant impact on a Natura 2000 site must be subjected to an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site. Certain types of cliffs are considered habitats of community interest under Annex 1 of the Habitat Directive.
Implementation times of several years may be required to adequately plan actions.
With regular maintenance, most cliff strengthening methods should have a relatively long lifetime, stretching into techniques.