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Coastal cliffs
Rates and processes of coastal cliff retreat. Natural and human-induced hazard
By Stefano FURLANI, Sara BIOLCHI, Stefano DEVOTO
Global features of sea cliffs
The term coastal cliff, or sea cliffs, refers to a steeply sloping surface where elevated land meets the shoreline (Hampton et al., 2004). Sea cliffs are a geomorphic feature occurring along about 80 percent of the world’s shorelines (Emery & Kuhn, 1982).
Most cliffs are developed on coasts that are tectonically stable, while others can be modified by tectonic uplift or subsidence (Bird, 2016). In these environments, cliffs formed as a consequence of uplift along fault can also be the result of repeated tectonic coseismic events (Bird, 2016). Usually, tectonic sea cliffs are the result of differential erosion that removes soft rocks overlapping hard rocks along the fault plane.
Sea cliff can be also originated as the outcome of volcanic eruption, such as the island of Santorini, Ustica in the Tyrrenian Sea or Krakatau, in Indonesia.
Similar to virtually all world landforms, present-day coastal cliffs can be considered a “work in progress,” continually acted upon by a broad assortment of offshore (marine or lacustrine) and terrestrial processes that cause them to change form and location through time. An important consequence is that coastal cliffs retreat (that is, move landward), and the adjacent coastal land is permanently removed as they do so. Retreat can be slow and persistent, but on many occasions it is rapid and episodic.
Coastal cliff is a general term that refers to steep slopes along the shorelines of both the oceans (where they are commonly called sea cliffs) and lakes (where they are commonly called “lake bluffs”). The term bluff also can refer to escarpments eroded into unlithified material, such as glacial till, along the shore of either an ocean or a lake. Often, the terms cliff and bluff are used interchangeably.
Fig. 1: Distribution of sea cliffs around the world (from Emery & Kuhn, 1982).
Coastal cliffs typically originate by marine or lacustrine erosional processes, in particular when the sea level rise. However, some sea cliffs start as scarps of large landslides or faults (e.g. Moore and others, 1989; Kershaw and Guo, 2001) or by glacial erosion (Shipman, 2004). These types of features are recognized as coastal cliffs, because they usually evolve similarly to other coastal cliffs, despite their origin is different. The aforementioned definition of sea cliffs defines no bounds on the materials, height, or the slope of the eroded surface, but the limits are defined by their utility. Erosional processes can carve a cliff face into any geologic material, slowly into hard rocks such as granite or basalts, rapidly into soft sedimentary rocks, such as sandstones, soft limestones and even more rapidly into unlithified materials such as glacial till (Sunamura, 1983). An empirical lower bound of bluff or cliff height is a few meters, below which there are few hazard concerns, but above which the serious engineering and land-use issues associated with coastal cliff retreat become significant (Hampton et al., 2004). Some coastal cliffs are more than 100 m high. Cliffs higher than 500 m are also called megacliffs (Bird, 2016). Typical inclination of surfaces that are recognized as true coastal cliffs ranges from about 40° to 90°, but it can be as low as 20° in soft sediment such as clay. In some places, overhanging rock faces can exist.
Some types of marine cliffs are more characteristics of some parts of the world, although it is difficult to classify them on the basis of climate, wave energy, etc (Trenhaile, 1987).
Variations in local factors can produce greater differences in cliff profiles and morphology within the same region. Anyway, some generalizations can be made
Fig. 2: Distribution of rocky coasts along the Mediterranean and Black Sea coasts (from
Furlani et al., 2014).
starting from differences in climate and waves (Davies, 1964, 1972). The profiles of cliffs are generally the result of the interplay of:
• geology, lithology and integrity of the rock mass, bedding, dipping of rock beds
• climate, • wave regime, in particular the frequency of waves under storm conditions • tides, • vegetation • nearshore water depth • type and amount of beach material at their base • topography of the cliff-top area and role of subaerial erosion • changing in sea level
Some examples have been provided in Fig. 3.
Classification of rocky coasts
Sunamura (1991) recognized three morphological types usually developing on the rocky coasts, or Type-A platforms, Type-B platforms and plunging cliffs. Usually, cliffs develop at the onshore limit of the shore platforms.
Shore platforms Type A (sloping shore platforms)
Fig. 3: Classification of coastal cliffs (from Hill, 2004).
This type of platforms are gently sloping platforms without a significant topographic break, extending from the base of a cliff to the nearshore sea floor below low tide level (Fig. 3).
Shore platforms Type B (horizontal shore platforms)
This type of platforms are nearly horizontal platforms with a cliff developing at their seaward edge. Bird (1976) subdivided the subhorizontal platforms type into high-tide and low-tide shore platforms (Fig. 3).
Plunging cliffs
A cliff or steep sloping coast that descend vertically into the deep water without any shore platform, rocky shore or beach at the sea level (Fig. 4).
Fig. 4: Classification of rocky coasts (from Sunamura, 1992).
Cliff processes
Even if the main involving cliff retreat is wave erosion, other processes contribute to the total amount of cliff recession (Bird, 2016). Subaerial processes, biological weathering and other marine processes can significantly increase the recession rates. Weathering processes is more active on the top of the cliffs, while erosion processes dominate the cliff foot (Hill, 2004). Coastal landslides involve large masses of rocks, earth or debris at the foot of a coastal slope. The instability of a cliff can be due to the weight of a massive caprock and develops with an increase of shear stress or a decrease in shear strength (Bird, 2016).
Coastal landslides (Fig. 7) can be divided in falls, slides, topples, spreads and flows (Cruden and Varnes, 1996). Landslides are the rapid movement of cliff materials downslope under the effect of gravity. It is not possible to predict the extent and the time of coastal landslide events. Anyway, they have occurred frequently enough that
Fig. 5: Cliff and shore platform at Debeli Rtic/Punta Grossa (Slovenia).
Fig. 6: Plunging cliffs at Sistiana-Duino (Gulf of Trieste, Italy)
geologic analysis and informed land use can help to risk reduction and improved response.
This rise in the sea level allows waves to erode beaches and flats at the base of coastal bluffs, increasing the risk of coastal landslides. Erosion removes material from the base of coastal cliffs and steepens their face. Sediments at the base of the cliffs stabilize it, and when they are removed, the cliff is no longer in equilibrium. Continued erosion or lubrication of the cliffs and bluffs by groundwater may overcome this internal resistance, in particular in clay materials, and produce a landslide. A landslide event restores the equilibrium of the system, and the slumped materials at the foot of the cliff support a new cliff face with a gentler slope. Erosion is a continuing process because the level of the sea is rising, and coastal waves and currents immediately begin to remove the edges of the displaced sediment. At the end, erosion destroys the equilibrium of the bluff and leads to another landslide, repeating the cycle (Fig. 5).
Cliffs and other landforms of rocky shores can be eroded by many different interrelated processes, such as hydraulic action, corrosion, attrition, solution and quarrying and cavitation (Hill, 2004).
Runoff processes also involve sea cliffs, since rain and melting can generate water flowing down a cliff slope. On soft rock outcrops it washes away sediments producing rills and gullies. The materials accumulated at the cliff foot are subsequently removed by wave action and in some cases can protect the cliff from wave attack (Furlani et al., 2011). In this case, rilling processes continue until when the cliff foot is cut back by marine processes.
Fig. 7: Classification of coastal landslides (from Devoto, 2013).
Sea spray can also generate runoff down the cliff. It contributes to weathering promoting processes of wetting and drying, when crystals of salt pluck the rock surface forming pits, honeycombs (Bird, 2016) and tafoni.
Winds blowing against a cliff can remove fine-grained particles from the slope, scouring hollows and clefts, up to create small caves (Bird, 2016).
The water coming from rainfall or melting snow percolates into the rock mass through fractures, joints and cavities. Groundwater seepage from a cliff face can wash out finer particles leaving cracks and crevices up to create an apron at the cliff base. Vertical ridges or speleothems can be formed because of local precipitation of carbonates. The accumulation of groundwater in permeable rocks can increase the instability of rock masses. The increase shear stress due to the additional loading of groundwater can result in cliff collapse.
Soluble rocks, such as gypsum or limestone, can be weathered by producing coastal karst landforms, particularly significant along the littoral zone and notably in the mid-tidal zone (De Waele and Furlani, 2013).
The effects of thermal changes on the cliffs can lead to expansion and contraction of the rock mass. These processes lead to flacking, fracturing, and spalling cliff faces, mainly in correspondence of lines of weakness. Slumping can be a result of modification in rock volume related to thermal changes, insomuch as rockfalls can occur as a consequence of significant cold weather.
Wetting and drying result in disintegration of the rocks outcropping on the cliff face, while drying processes of saline spray increase the plucking effects of salt crystallization.
Biological weathering strongly affects coastal erosion. The growth of plants, mainly through the shear strength produced by roots, on the cliffs can widen joints and produce rock falls. In the mid-tidal zone, bioerosion is particularly effective because of the erosion produced by algae, cyanobacteria, and animals.
Human impact is very significant in as much as it can modify, directly and indirectly, sea cliffs and bluffs (Bird, 2016), such as quarrying sand and rocks, loading the cliff-top with buildings, building roads or bridges, railways or removing and weakening rock masses for fossils searching, etc.
Methods to study cliff retreat
A wide variety of methods for measuring the retreat of sea cliffs and bluffs have been developed and used in the past. Some of the earliest documentation of cliff retreat interested U.S. coasts, where field survey techniques were used since the late nineteenth century, as cited by Andrews (1870), Chamberlain (1877), and Leverett (1899). Field surveys of the cliffs along the northeastern U.S. Cape Cod coast were also carried out in the late 1800’s by Marindin (1889).
The traditional methods of measuring cliff retreat, such as field surveying, profiling, and standard aerial photographic techniques, have been supplemented, and in some cases also replaced, with new approaches, such as digital photogrammetry, and LiDAR (Light detection and ranging), satellite interferometry and GPS measures (Fig. 9).
Usually, the techniques applied to the measurement of sea cliff and bluff recession have developed from methods developed to measure shoreline change along low-lying coasts, where erosion or accretion is measured by comparison of the changes in the horizontal position of a line on the beach, such as the wet/dry line (Dolan and others, 1980; Anders and Byrnes, 1991, Furlani et al., 2011). Anyway, along rocky or bluffed coasts, the coastline proxy is well defined by the geomorphological features of
Fig. 8: Processes affecting cliff retreat and shore platforms lowering (from Furlani et al.,
2014).
a particular area rather than a linear datum. In regions of uplifted marine terraces, the recession of the top edge of the cliff may best describe the long-term trends in shoreline changes. Along very steep coastal slopes, the feature that best captures coastline change may be the cliff foot, the first significant slope break, or some other geomorphic feature specific to a particular geographic location. Moreover, the identification of the best feature to measure can represent an additional problem together with the description of its morphometric features, such as vegetation obscuring the top edge of a cliff, rock or rubble obscuring the base of the cliff, rounding effects of the cliff edge due to weathering or overwash processes, and the discontinuity of distinct features (Hapke, 2004). In general, techniques developed for shoreline change measurement on low-relief coasts may not be readily applicable because of the complexities associated with identifying and measuring distinct geomorphic features along cliffed coastlines.
As suggested by Hapke (2004), in addition to errors due to measurement and identification and ambiguities associated with accuracy in measuring long-term cliff erosion, there are many problems related to interpretation of the data and understanding data interpretation and how they can be applied for process studies and community planning.
Fig. 9: Example of GPS network installed for monitoring a landslide affecting coastal cliff
stability at Anchor Bay (Malta)
Table 1: Advantages and disadvantages of measurement methods used in sea cliff erosion
studies (from Hapke, 2004)
Rates of cliff retreat
Sunamura (1992) suggested that many factors are involved in wave erosion at the base of a cliff. These factors can be summarized in assailing forces of waves and resisting forces of the material forming the lower cliffs.
The resulting cliff retreat are summarized in the following tables for the Mediterranean (Table 2) and the Black Sea (Table 3).
Table 2. Rates of sea cliff retreat in the Mediterranean Sea (modified from Furlani et al. 2014) A
Location B
Lithology C
Erosion rate
(m/yr)
D Interval
(yr)
E Method
F Reference
Catalunya (Spain)
Fluviatile Quaternary
deposits 0.2 / Aerial analytical
photogrammetry Montoya (2008)
Mallorca (Balearic Is.) Calcarenite 0.74x10-3 235 k Field survey Fornos et al.
(2005) Mallorca
(Balearic Is.) Calcarenite 40 m3 1 Field survey Balaguer et al. (2002)
Mallorca (Balearic Is.) Calcarenite 143 m3 1 Field survey Balaguer et al.
(2002)
Mallorca (Balearic Is.)
Fluviatile – Eolianite
Quaternary deposits
0.09-0.56 46 Aeria photos
survey Balaguer et al.
2008
Cilento, Campania (Italy) Flysch 0.5-0.8 30-45 Aerial analytical
photogrammetry Budetta et al.
(2000)
Apulia (Italy) Limestone 0.06-0.8 100 Field survey Mastronuzzi & Sansò (1998)
Murgia, Apulia (Italy) Limestone 0.01-0.1 1997-
2003 Field survey Andriani & Walsh (2007)
Ortona-Vasto, Abruzzo (Italy)
Conglomerates, sands and
pelites 0.3-0.9 109
Comparison of aerial
photographs and maps
D’Alessandro et al. 2001
Monte S. Bortolo (Pesaro), Italy
Sandstone and marls
0.05-0.16 6 k Archaeological
remains Colantoni et al.
(2004)
Slovenian coast Flysch 0.01-0.02
1999-2010
Comparison of terrestrial
photographs
Furlani et al. (2011b)
North of Khan Yunis (Israel) / 0.41 1956-
1984 Aerial analytical photogrammetry
Golik & Goldsmith
(1984, 1985), from Zviely & Klein (2004)
North Gaza (Israel) / 0.3-0.9 1956-
1984 Aerial analytical photogrammetry
Golik & Goldsmith
(1984, 1985), from Zviely & Klein (2004)
North of Ashkelon (Katza,
Israel) / 1.07 1976-
1984 Aerial analytical photogrammetry
Golik & Goldsmith
(1984, 1985), from Zviely & Klein (2004)
Jaffa (Israel) / 0.11-0.25
1945-1987
Aerial analytical photogrammetry
Ron (1982), from Zviely & Klein (2004)
Table 3. Rates of sea cliff retreat in the Black Sea (modified from Furlani et al. 2014)
A Location
B Lithology
C Erosion rate
(m/yr)
D Interval
(yr)
E Method
F Reference
Cape Sabla (Bulgaria) Limestone 0.01 / / Simeonova (1985)
Jaffa (Israel) / 0.03-0.29
1987-1996
GPS measurements
Greenstein (1997), from
Zviely & Klein (2004)
Herzliya (Israel) / 0.2-0.4 1942-1996
Comparison of aerial
photographs
Nir(1992), from Zviely & Klein
(2004)
Apolonia (Israel) / 0.09-0.36
1944-2000
Comparison of aerial
photographs
Ben-David (2001), from
Zviely & Klein (2004)
Netanya (Israel) / 0.3-0.4 1945-1978
Aerial analytical photogrammetry
Ron (1982), from Zviely & Klein (2004)
Netanya (Israel) / 0.5 1962-1994
Comparison of aerial
photographs and maps
Ben-David (1995), from
Zviely & Klein (2004)
Shoshanat Hamakim
(Israel) / 0.05-
0.58 1939-1991
Comparison of aerial
photographs and maps
Nir (1992), from Zviely & Klein
(2004)
Neurim (Israel) / 0.8-3.2 1992-1995
Field measurements
Perath & Almagor (1996), from Zviely & Klein (2004)
Tel-Aviv and Beit-Yannay
(Israel)
Eolianite (kurkars)
0.15-0.22 1982 Field
measurements
Perath (1982), from Zviely & Klein (2004)
Beit-Yannay (Israel)
Eolianite (kurkars) 0.15-0.3 1939-
1991
Comparison of aerial
photographs and maps
Nir (1992), from Zviely & Klein
(2004)
Michmoret and Givat Olga
(Israel) / 0.24 1991-
1996 Field
measurements
Schwartz (1997), from Zviely & Klein (2004)
Beit-Yannay (Israel)
Eolianite (kurkars) 0.2 1918-
2000
Comparison of aerial
photographs
Zviely & Klein (2004), from
Zviely & Klein (2004)
Cape Sabla (Bulgaria)
Limestone with
underlying clays
8 / / Simeonova (1985)
Tauk-Liman, Cape Sabla (Bulgaria)
Limestone with
underlying clays
0.01 / Survey Koštjak & Avramova
(1977)
Balčik-Varna (Bulgaria) / 1 / / Milev &
Cencov (1977)
Kavarna (Bulgaria) / 15 / / Simeonova (1976)
Crimean Peninsula (Russia) Limestones 0.3 / / Shuisky (1985)
Crimean Peninsula (Russia) Clays, silt 9 / / Shuisky (1985)
Primorsko-Atchtarsk, Azov Sea Coast Clay 12 / /
Sunamura 1992 (from
Zenkovich (1967)
Black Sea Flysch 0.02-0.03 20 yr Surveys, photos
Sunamura1992 (from
Zenkovich 1965)
Black Sea Flysch, shale 0.01-0.02 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Coquinite 0.002-0.005 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Crystallized limestone 0.003 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Massive limestone 0.3-0.5 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Limestone with loess 2-3 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Limestone with loess 0.61 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Quaternary loess 0.5-1.0 / / Sunamura 1992
(from
Zenkovich (1965)
Black Sea Quaternary conglomerate 12 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Quaternary brown loam
and clay 1 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Quaternary clay 2-3 / /
Sunamura 1992 (from
Zenkovich (1965)
Black Sea Diluvial deposits 0.11 / /
Sunamura 1992 (from
Zenkovich (1965)
Cliff geo-hazard
Sea cliffs can be very dangerous because of their morphological features, such as steepness and height, and of the processes acting on the cliffs, such as rock falls and landslides.
Rock falls are very common. They usually involve small volume of rock and are associated to cracks generated by structural setting and/or enlarged by marine or subaerial processes. They can be very dangerous because they can cause casualties.
Rock slides are characterized by the presence of a slip surface, which can be circular or not. Rotational landslide usually involves large volumes of materials.
Rock spreads involve clayey terrains capped by resistant materials. They are extensive and are characterized by slow speeds. They generate persistent cracks, which can assist geomorphologists to identify them. Rock spreads can be coupled to other types of landslides such as rock falls and block slides.
In coastal environments, flows are not common processes. They usually involve small quantities of material and are associated to clayey terrains. Usually they are triggered by intense rainfalls, therefore, they can be very dangerous for human artifacts or to human activities, such as cliff paths, etc. Landslides can injure or kill people mainly because of rock falls or slumpings.
Williams and Williams (1988) reported that the risk vary significantly both between individuals and social groups. Coastal cliffs can be very attractive sites for suicides (Birds, 2016). Considering the number of cliff footpaths, the number of people killed
or injured by falling from cliffs is fairly small. Accidents from cliff-top breakaways are very rare, but trekkers should be aware of the hazard in particular conditions.
The impact of hammering and excavation of cliff faces by people searching for fossils or minerals also injured unfortunate people.
Geologists, such as earth scientists, botanists, ecologists, etc, need to join training, research and safety (Birds, 2016).
References
Bird, E., 2016. Coastal cliffs: Morphology and Management. Switzerland: Springer.
Cruden, D.M., Varnes, D.J., 1996. Landslide Types and Processes. In: Turner, A.K., Shuster, R.L. (Eds.), Landslides: Investigations and Mitigation, pp. 36-75. National Research Council, Special Report 247.
De Waele, J., Furlani, S., 2013. Seawater and biokarst effects on coastal karst. In: Shroeder, J.F. (Ed.), Treatise on Geomorphology, Vol. 6, pp. 341-350. Amsterdam: Elsevier.
Devoto, S., 2013. Geomorphological map of the NW coast of the Island of Malta (Mediterranean Sea). Unpublished PhD thesis, University of Modena and Reggio Emilia.
Emery, K.O., Kuhn, G.G., 1982. Sea cliffs: their processes, profiles, and classifications. Geological Society of American Bulletin 93, 644-654.
Furlani, S., Devoto, S., Biolchi, S., Cucchi, F., 2011. Factors triggering sea cliff instability along the Slovenian coasts. In: Micallef, A. (Ed.), MCRR3-2010 Conference Proceedings, pp. 387-393. Journal of Coastal Research, Special Issue 61.
Hampton, M.A., Griggs, G.B., 2004. Formation, Evolution, and Stability of Coastal Cliffs–Status and Trends. USGS Professional Paper 1693, 1-4.
Hapke, C.J., 2004. The Measurement and Interpretation of Coastal Cliff and Bluff Retreat. USGS Professional Paper 1693, 39-50.
Hill, M., 2004. Coasts and Coastal Management. Hodder Murray.
Sunamura, T., 1992. Geomorphology of Rocky Coasts. Chichester: Wiley.
Trenhaile, A.S., 1987. The Geomorphology of Rock Coasts. Oxford: Oxford University Press.
Williams, M.J., Williams, A.T., 1988. The perception of, and adjustment to, rockfall hazards along the Glamorgan Heritage Coast, Wales. Ocean and Shoreline Management 11, 319-338.
