Cliff stabilisation
Coastal cliffs can be unstable due to the combined effect of several factors, discussed in the article Rocky shore morphology. The need for stabilization concerns mainly eroding soft rock cliffs, which are the primary focus of this article. Soft cliffs consist of weakly consolidated sediments or weak and highly weathered rocks that offer relatively little resistance to erosion or slope failure. They include clay, glacial deposits, weak shale and mudstone, poorly cemented sandstone and mixtures of sediment and rock fragments. They occur on many coasts worldwide, but their behavior differs greatly according to cliff material, hydrodynamic conditions, climate and human interventions. This article discusses management options for controlling cliff recession, and the effectiveness of measures in relation to local conditions and prevailing retreat processes.
Contents
Cliff retreat processes
Three important processes responsible for cliff retreat are (see Rocky shore morphology):
- Wave attack at the cliff base. This can undermine the cliff by forming a notch or cave at its foot, oversteepening the cliff face, removing debris that supports a pre-existing landslide, or lowering the shore platform or beach. The presence of debris at the cliff base can offer protection but also enhance the abrasion power of waves.
- Water infiltration into the cliff face and cliff top. Prolonged or intense rainfall can recharge groundwater and raise pore-water pressure, reducing cliff stability and eventually triggering slides or slumps. Water infiltration can be facilitated by tension cracks associated with slope deformation and by desiccation cracks that develop during dry periods in clay-rich material. Freeze–thaw cycles can further weaken susceptible cliff material.
- Weathering of the cliff face by salt crystallization, dissolution and freeze-thaw processes, which can cause surface material to fall or ravel downslope and can strongly increase susceptibility to wave quarrying (wave-induced detachment of blocks and fragments).
Buildings, roads, fill or stored material near the cliff edge can add load to an unstable slope and promote failure.
Cliff recession is typically episodic and therefore difficult to predict. A cliff may remain apparently stable for years and then retreat substantially in a single failure; short monitoring records can consequently give misleading estimates of long-term retreat.
Planning of remediation measures
Remediation planning should be based on:
- Knowledge of cliff characteristics (cliff material, vegetation cover, cliff slope, talus, beach, shore platform), hydrodynamic conditions (wave climate, tides, surges) and weather conditions (rainfall).
- Available evidence of past recession and failure events.
- An indication of the dominant processes responsible for cliff recession at the site.
- The assets and people at risk, and the consequences of continued cliff retreat and failure.
- The likely effects of intervention on beach sediment supply, neighboring shores, coastal habitats, landscape value and public access.
An investigation should determine how material moves through the complete cliff–foreshore system: how and when the cliff fails, whether the debris temporarily protects the toe, how rapidly waves remove it, whether beach or platform levels are changing, and whether intervention would affect sediment transport along the neighboring coast[1].
The intervention options should then be compared with:
- no active intervention;
- land-use restrictions or setback;
- relocation or managed retreat;
- drainage or slope modification;
- toe protection or combined measures.
This assessment establishes whether intervention is necessary and which management options are technically sound and environmentally acceptable.
Cliff stabilization measures
Depending on the dominant causes of recession, management may involve stabilization of the cliff face and top, protection of the cliff toe, or land-use measures that reduce exposure to continuing retreat.
Cliff face and slope stabilization
Measures aim to reduce instability of the cliff face and top, particularly instability caused by excessive pore-water pressure. Such measures include:
- Reprofiling: reducing the cliff slope angle, removing unstable or overhanging material and, where appropriate, creating benches. Reprofiling requires sufficient land at the foot of the cliff and may itself expose erodible material or disturb valuable cliff habitat.
- Surface-water management: diverting runoff away from the cliff, sealing or filling major cracks and holes and preventing concentrated discharge over the cliff face.
- Drainage: maintenance and installation of drains (vertical, horizontal) and ditches to evacuate groundwater and relieve pore-water pressure. Drainage systems require regular inspection and maintenance, and their outlets must be arranged so that concentrated discharge does not initiate erosion elsewhere.
- Structural support of the cliff face: rock bolts, anchors, retaining structures, mesh or sprayed concrete may locally restrain unstable rock or protect weak surfaces. These measures require geotechnical design, drainage and continuing inspection. Shotcrete (sprayed concrete) is generally unsuitable as a stand-alone erosion-control measure where it is directly exposed to repeated wave impact and toe scour[2].
- Unloading: removing or relocating heavy structures, fill or stored material near the cliff edge. Unloading requires land-use setback regulations, preventing new construction near a retreating cliff.
At many sites, effective stabilization requires a combination of slope, drainage and toe measures.
Cliff toe stabilization
Measures aim to prevent cliff notching and undermining. Such measures include[3]:
- Revetments: rubble mound revetments with a rock armor layer, a finer-grained underlayer and a geotextile filter, see Stability of rubble mound breakwaters and shore revetments. The toe must be designed against undermining caused by scour and fluctuating beach levels. Depending on the foundation and hydraulic conditions, this may require embedment, a flexible launching apron, additional toe rock or another site-specific foundation measure. Sand-filled containers and gabions may provide temporary or emergency protection in relatively sheltered conditions, but their durability is limited and failure can release fill or structural material. They are generally unsuitable for highly exposed coasts.
- Protection or retention of cliff-foot debris: where landslide debris forms a persistent protective apron, an armor layer or retaining structure may impede its removal by waves. Covering or immobilizing mobile debris may also reduce abrasion of the cliff foot by rock fragments carried by waves[4].
- Wave attenuation and beach retention: offshore or shore-connected breakwaters can reduce wave energy at the cliff toe. Groynes can help retain a protective beach by reducing longshore sediment transport, but may trap sediment updrift and cause serious downdrift erosion. These structures should be used only after assessing their effects on waves, currents and the sediment budget of the wider coast.
- Sediment management: regular beach nourishment can maintain a protective beach at the cliff toe where a suitable sediment source is available. Offshore sediment deposits near headlands (headland-associated sandbanks[5][6]) can provide a suitable sediment source, provided that sediment extraction does not otherwise affect the coastal sediment balance or the wave-sheltering function of the sandbank.
Need for cliff stabilization
The need for cliff stabilization should be assessed in relation to the assets at risk, the dominant retreat processes, the feasibility and long-term maintenance requirements of possible measures, and their environmental and geomorphological consequences. Where preservation of a dynamic coastal landscape is important, stabilization should be restricted to locations where important assets are at stake. Materials and designs should be compatible with the character of the site, while also providing adequate stability and durability.
Where sufficient space is available, allowing natural retreat or relocating exposed assets may be preferable, because stabilization generally requires continuing maintenance and may interfere with natural cliff habitats and geomorphological processes. Where cliffs supply substantial amounts of beach-compatible sand or gravel, stabilization can reduce sediment supply to adjacent and downdrift beaches.
Related articles
References
- ↑ Lee, E.M. and Clark, A.R. 2002. Prediction of Recession Rates and Erosion Control Techniques. Manual issued by DEFRA/Environment Agency (UK), Project FD2403/1302
- ↑ California Coastal Commission, 2023. Staff Report: Coastal Development Permit Application No. 3-23-0014, 74 pp.
- ↑ Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, S.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
- ↑ Sunamura, T. 2015. Rocky coast processes: with special reference to the recession of soft rock cliffs. Proceedings of the Japan Academy, Series B 91: 481–500
- ↑ Berthot, A. and Pattiaratchi, C. 2006. Mechanisms for the formation of headland-associated linear sandbanks. Continental Shelf Res. 26: 987–1004
- ↑ McCarroll, R.J., Masselink, G., Valiente, N.G., Wiggins, M., Scott, T., Conley, D.C. and King, E.V. 2020. Impact of a headland-associated sandbank on shoreline dynamics. Geomorphology 355, 107065
Please note that others may also have edited the contents of this article.
|
