exacerbated to some degree by human activities. When an event occurs that actually does result in impacts to humans, the hazard becomes a disas- ter.
Geomorphic hazards Richard A. Marston Kansas State University, USA
William D. Butler Texas State University, USA
Nicholas L. Patch Kansas State University, USA
A hazard, generally speaking, is an element of the environment that is potentially harmful to humans in the form of loss of life, property damage, and socioeconomic disruption. Hazards can be “natural” in origin and only become a hazard because of the juxtaposition of geomorphic forms, processes, and materials with human activities. Hazards can also be caused or exacerbated to some degree by human activities. When an event occurs that actually does result in impacts to humans, the hazard becomes a disaster. The degree to which humans contribute to a hazard/disaster is often difficult to discern. A new field of inquiry termed “attribute science” has developed to focus on this challenge. Geomorphic hazards/disasters are those that originate from exogenic processes – processes that originate at or near the Earth’s surface from the action of water, wind, ice, and gravity. Geomorphic hazards are distinguished from geological hazards that originate deep within the Earth from the endogenic processes of volcanism or earthquakes. The distinction is not always clear between the two main subdivisions of geohazards. For instance, volcanic eruptions and earthquakes can trigger slope failures, tsunamis, and floods. Moreover, the distinction is not
always clear between geomorphic hazards and other main categories of hazards: meteorological, hydrological, biological, technological, and social. For instance, floods are usually classified as hydrological hazards but river channel changes are considered a geomorphic hazard. Geomorphic hazards considered here (Table 1) include: • • • • • • • • •
expansive soils, soil erosion, including dust storms, slope failures, ground subsidence and karst, periglacial, river channel changes, glaciers, coastal erosion, climate change.
Burton, Kates, and White (1978) listed the critical information needs regarding hazards: • • • • • • •
magnitude: high to low, frequency: often to rare, duration: long to short, areal extent: widespread to limited, speed of onset: rapid to slow, spatial dispersion: diffuse to concentrated, temporal interval: regular (e.g., annual, seasonal, diurnal) to random.
Expansive soils cause billions of dollars of damage each year to homes in the United States alone (Alexander 1993). The swelling is caused by chemical attraction of water to plates in certain clay minerals. Pure montmorillonite (or smectite) clay, for example, can expand 15 times when water is added. Soils with high gypsum
The International Encyclopedia of Geography. Edited by Douglas Richardson, Noel Castree, Michael F. Goodchild, Audrey Kobayashi, Weidong Liu, and Richard A. Marston. © 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd. DOI: 10.1002/9781118786352.wbieg1117
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Table 1 General characteristics of geomorphic hazards. Type of hazard/disaster
Frequency
Duration
Speed of onset
Expansive soils
Seasonal/irregular
Months to years
Months to years
Soil erosion Wind/dust storms Slope failures
Progressive Seasonal Seasonal/irregular
Hours to millennia Hours Seconds to decades
Minutes to years Hours Seconds to years
Subsidence Periglacial
Sudden/progressive Seasonal/progressive
Minutes to decades Months to millennia
Seconds to years Hours
River channel change Glaciers
Seasonal/irregular
Months to centuries
Hours to years
Calving ice Ice advances Surging glaciers
Seasonal Seasonal/progressive Seasonal to years
Months to decades Decades to millennia Years
Seasonal Decades Months
Ice avalanches GLOFs
Seasonal/irregular Seasonal/irregular
Seconds to years Hours to days
Seconds to years Seconds to years
Seasonal to years
Days/seasonal
Hours to decades
Coastal erosion
Source: Modified from Alexander (1993, 10 & 22).
content can also experience shrink-and-swell to the point where structural damage occurs. Most soils have a portion of clay that would cause 25–50% expansion. Damage to structures is likely if the soil expands more than 3%. In the US Department of Agriculture (USDA) Soil Taxonomy, expansive soils fall into the order of Vertisols, which cover about 2% of the ice-free land surface on Earth. It is critical to consult soil surveys when trying to avoid expansive soils or to plan for mitigation. Soil erosion has been increasing steadily since the advent of cropland agriculture about 3500 years ago, primarily from fluvial processes of rainsplash, sheetwash, rills, gullies, and piping. Wind erosion is important in flat terrain, and slope failures become important in mountainous terrain. Perhaps as much as one-half of the topsoil on Earth has been lost in the last 150 years through erosion or degraded soils quality. Soil erosion becomes a hazard when rates of
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erosion exceed rates of new soil formation, progressively destroying soils structure, nutrient content, and fertility. Rates of soil erosion are naturally high in semiarid climates, where vegetation cover is sporadic and storm-period rainfall intensity is high. Land-use/land-cover change (from deforestation, agriculture, overgrazing, climate change, desertification, wildfires, etc.) can greatly accelerate soil erosion by water and wind. Soil erosion causes millions of hectares of land to be removed from cultivation each year. Soil erosion differentially affects poor countries such as Ethiopia and El Salvador, where arable land is limited, and farmers lack modern farming technology and training in soil conservation methods. Trimble’s (2012) award-winning narrative about historical agriculture and soil erosion in the upper Mississippi Valley hill country documented how agriculture initially triggered landscape destruction, impoverishment and instability. However, farmers eventually adapted
GEOMOR PHIC H A ZA R D S
their land use and settlement practices to recover and enrich the landscape. Dust storms pose a hazard to health and property by blocking roads and causing highway accidents, filling irrigation ditches and canals, burying crops and ruining machinery, dispersing weeds and insects, depleting soils, and sandblasting plant roots. Dust storms play a role in human health, animal health, climate change, and ocean sedimentation. Dust storms can reduce visibility to less than 1 km and can reach altitudes greater than 10 km. They can be traced for thousands of kilometers on remotely sensed imagery. Dust from World War II tank battlegrounds in North Africa has been found in the Caribbean. The magnitude, frequency, and duration of wind exert key controls on dust production, especially when wind acts on a surface with an abundance of silt-sized sediment that is dry and low in organic material. The generation of dust is further enhanced on a surface with low vegetation density, low litter cover, low surface roughness, and a long fetch. In a geospatial analysis of dust storm potential in the Chihuahuan Desert, Marston (1986) found these variables combined to create highest dust storm potential in the broad basins of northern Mexico, where irrigated cropland had been abandoned or is seasonal, and in deteriorated rangeland and regions of desert shrub vegetation (Figure 1). Slope failures cause an annual average of 25 deaths and losses of US$ 1 billion in the United States. One of the worst natural disasters occurred in Hsian, China, because of widespread slope failures triggered by an earthquake in 1556. A 1963 landslide of 238 million cubic meters into the reservoir behind Vaiont Dam in Italy created a wave 90 m high over the dam. Amazingly, the dam survived but 2600 people lost their lives downstream. The landslide was composed of fractured limestone blocks that slid on beds of clay and soft marl. The landslide had been
Figure 1 Dust storm from SE winds entraining dust from a broad basin in northern Chihuahua. Note that the borders of Texas, New Mexico, and Mexico have been added. Source: Aqua MODIS, 2003. Reproduced from NASA. Image dated April 4, 2003.
monitored for three years prior to the disaster; it was moving between 1 and 30 cm per week. Rising groundwater and heavy rains caused a threshold to be exceeded for the massive movement. Slope failures are pervasive in the Nepal Himalaya but linking cause and effect has been made difficult by the multiplicity effect in geomorphology – any one variable can explain some of the variation but the interactions between variables remain poorly understood. Marston, Miller, and Devkota (1998) showed that forest cover did not affect the occurrence of slope failures, but position above and below the main central thrust was shown to be important, as was slope aspect. Dahal and Hasegawa (2008) demonstrated that a threshold of rainfall intensity and duration initiated landslides. In April 2015, a magnitude 7.8 earthquake in Gorkha, Nepal, caused thousands of fatalities, destroyed entire villages, and triggered thousands of landslides, some of which blocked rivers to create yet another hazard (landslide dam outburst floods) downstream (Collins and Jibson 2015). Variables
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that control slope failures in the Himalayas change from one time to another and contrast with the controlling variables in other mountainous areas of the world. The direct hazard posed by slope failures on humans is illustrated by the work of Butler (2013). Slope failures (rockfalls, debris flows, rockslides, snow avalanche tracks) were mapped in relation to hiking/skiing trails and campgrounds that are heavily used each year by backcountry visitors to the national park (Figure 2). The National Park Service could use this information to educate backcountry tourists and to modify their location of trails and campgrounds. As defined by Alexander (1993, 276), “subsidence is a type of mass movement that involves the downward displacement of surface material caused by natural or artificial removal of underlying support (collapse) or by compression of soils (consolidation).” The extraction of oil, gas, and groundwater has been demonstrated to cause subsidence in Long Beach, CA, in Venice, Italy, and in Niigata, Japan, as just three examples. The construction of the tower in Pisa, Italy, on soils of contrasting compressibility caused the tower to lean over the course of centuries. Soils on deltas consolidate over time as organic matter is oxidized and soils are compressed under their own weight. This enhances flooding hazards, in the Mississippi River delta region, for example. An interesting example is provided by the Dead Sea, which has been affected by river inflow being diverted for irrigation agriculture and other purposes. The water level has dropped over 7 m in the last decade and the lake shore has receded. Fresh groundwater has replaced the saline groundwater, dissolving the buried salt deposits and causing the ground to collapse. Sudden collapse of the ground, forming sinkholes, is more common in karst terrain. The Florida peninsula is a structural arch of fractured
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limestone. Groundwater has dissolved the limestone, particularly where fractures intersect, to create caverns. When groundwater drops during extended droughts, the support provided by the water in caverns is lost and sinkholes appear. In the rapidly urbanizing Rio Grande Valley downstream from El Paso, TX, former agricultural fields are covered by impermeable asphalt and concrete. Gophers abandon the urban area for the few remaining fields that are surrounded by pavement. When farmers dump a meter of water on their field to leach salts from the soils, the gopher tunnels experience erosion by piping, sometimes leading to formation of a sinkhole in alluvium and ruining the farm. Periglacial environments are dominated by the action of ground ice (ephemeral, seasonal, permanent) and mass movement. The active zone is affected by freeze–thaw cycles over diurnal and seasonal intervals of time. Permafrost underlies about 26% of the Earth’s land surface. Freeze–thaw action displaces larger particles more than smaller sediment, resulting in an upfreezing of stones and sorting the mix of sizes. Mass movement occurs by the process of frost creep on slopes, whereby repeated freeze–thaw cycles lead to a slow movement of material downslope. Solifluction involves the slow downslope movement of saturated soils during the summer thaw. An intriguing variety of periglacial landforms is created by these processes, including elongated thaw lakes, ice wedges, pingos, patterned ground, blockfields, and rock glaciers. Hazards are created when human activities and settlements encroach on periglacial landscapes. During thaw periods, the bearing strength of soils is lost, which leads to differential settling and frost heaving, and structural damage to buildings, roads, airfields, railroads, and utilities. In the cases where felsenmeer and rock glaciers are situated on a cliff, the melting of interstitial ice can lead to rockfalls, which
GEOMOR PHIC H A ZA R D S
Figure 2 Map of existing slope failures (rockfalls, rockslides, debris flows, snow avalanches) for Death Canyon in the Grand Teton National Park compared to locations of campgrounds and popular trails for hiking and cross-country skiing. Source: Butler (2013). Reproduced by permission of William Butler.
become a hazard for downslope human activities and settlements. River channels change location by meander cutoffs, meander migration, and by avulsion. These changes can occur in the absence of human activities but can be exacerbated by humans through direct manipulation of the channel or through land-use/land-cover changes upstream. When rivers change course, land on the floodplain that would otherwise be available for farming and other human activities can be
lost. Stream channel change is driven by the frequency and magnitude of peak flows on the one hand, and the resistance of stream banks (controlled by alluvial materials, large woody debris, and streamside vegetation) on the other hand. A landowner situated on the outside (cut bank) of a meander bend stands to lose a significant amount of land if erosion progresses as is normal. Often a river marks a property boundary or a political border. According to the law in many jurisdictions, if the river moves
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by progressive meander migration, the border moves with it, causing the landowner on one side to gain land by accretion while the land owner on the other side loses land. The land owner who gained land may further rub salt into the wound of his or her neighbor by acquiring a permit to mine the river for sand and gravel. In the case where a river changes course by a sudden avulsion, the property line does not move with it, creating another set of problems and disputes. Glaciers can create hazards for humans in at least five ways. First, glaciers and ice sheets that extend to the coast will experience calving of ice at the terminus. This generates icebergs, which then become a hazard for shipping. The Greenland and Antarctic ice sheets are notorious for this hazard, as are coastal glaciers along the southern coast of Alaska. Second, ice advances, although slow, can overrun villages and farms. This was known to have occurred in the Alps, Norway, and Greenland during the Little Ice Age. Third, surging glaciers can block water bodies, leading to potential outburst floods. This has occurred multiple times with the Hubbard Glacier in Yakatat Bay, AK, blocking the entrance to Russell Fjord. Fourth, ice avalanches from tall peaks can create a rock-ice avalanche and demolish towns that are located down valley. The collapse of the ice summit of Mount Huascaran in Peru in 1970 resulted in the deaths of 25 000 people. A smaller scale event in 1962 claimed 4000 lives. Fifth, glacier changes can lead to a glacial lake outburst flood (GLOF), or jokulhlaup. When valley glaciers retreat, they leave behind a moraine, which may dam water between the retreating glacier and the moraine. If the moraine experiences piping, it may collapse. If a rock-ice avalanche lands in the lake, it can create a wave that overtops the moraine dam, causing it to collapse. If an advancing valley glacier cuts off a tributary, it
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can create a lake behind the ice. The ice dam may collapse or a subglacial tunnel may enlarge to suddenly release the water, endangering life and property downstream. GLOFs are common in glaciated mountains and cases of imminent danger have been identified in the Himalayas, central Asia, the Andes, and the Alps. Coastal hazards include erosion and sedimentation by waves and longshore currents. Coastlines are dramatically affected by tropical cyclones, hurricanes, and tidal flooding, but these are usually considered meteorological hazards. Tsunamis are triggered by earthquakes or large slope failures into a water body, so are not considered here. Alexander (1993) stated that five factors influence coastal erosion: exposure of the rocks and sediments to waves and currents; the supply of sediments (beach starvation or nourishment); the topography of the coast and neighboring continental shelf; the tidal range and intensity of currents; and the climate of the coastal area. Once again, it is worth pointing out that natural processes along the coast become hazards because of the intersection with human activities. The hazards depend on the geomorphic type of coastline. Sandy beaches experience seasonal transfer of sand, offshore in winter, onshore in summer. Thus, in winter, beachfront homes lose a buffer against attack by storm-period waves, especially during high tides. Littoral movement of sand, another important process affecting the beach sand budget, is responsible for a variety of coastal landforms and can close the mouths to rivers, estuaries, and harbors if left unchecked. Sand spits and barrier islands are commonly breached by storm-period events, leading to a call for artificial beach sand nourishment. On the Arctic coast, the longer fetch for wind over ice-free ocean already appears to be creating higher waves and greater coastal erosion, especially where permafrost is melting and sea level rising.
GEOMOR PHIC H A ZA R D S
Cliff coastlines experience direct wave attack throughout the year, with progressive cliff erosion and slope failures as a hazard. Wave-cut abrasion platforms, if present, serve to dissipate wave energy. The rate of cliff retreat needs to be studied by coastal geomorphologists, so coastal planning authorities can establish appropriate setback regulations. Along the central Oregon coast, cliff retreat from 1935 to 1975 was measured at 23 cm per year in marine sandstones/shales versus 5.3 cm per year in basalt over the same period. Earlier mention was made about consolidation of delta deposits. Deltas will be subject to erosion if the rivers that originally deposited the sediment are not allowed to deposit new material on top of the consolidating sediment. This is the situation being faced along the Gulf coast shores of Louisiana, where about 4900 km2 of coastal land was lost between 1970 and 2000. Some of this loss is attributed to saline water intruding into freshwater wetlands, killing the freshwater marsh grasses that hold the sediment in place. Coral reefs protect the shore from the direct impact of waves, but coral are threatened in several ways. They are being attacked by a crown-of-thorns starfish, the population of which is exploding because their predators have been overfished. The coral polyps that create reefs are supported by algae. The coral die within weeks if the algae die, and algae are dying from warmer ocean temperatures and sedimentation. Climate change will have an impact on all of the geomorphic hazards discussed here by affecting one or more of the seven dimensions of hazards defined by Burton, Kates, and White (1978) listed previously. However, global circulation models must continue to be refined to the point where predicting change is possible for geomorphic hazards. Geospatial analysis provides many tools for understanding and visualization of the results. It is clear that the geomorphic hazards discussed here are sensitive to climate,
so the degree to which these controlling factors change deserves further study. Critical research is underway to identify the key indicator variables in geomorphic systems and their response to external driving forces. SEE ALSO: Anthropogeomorphology; Applied geomorphology; Coastal depositional processes and landforms; Coastal erosion processes and landforms; Eolian erosional processes and landforms; Fluvial depositional processes and landforms; Fluvial erosional processes and landforms; Geomorphic thresholds; Geomorphological mapping and geospatial technology; Glacial depositional processes and landforms; Glacial erosional processes and landforms; Glacier lake outburst floods; Global climate change; Karst processes and landforms; Land-use/cover change and climate; Mass movement processes and landforms; Natural hazards and disasters; Periglacial processes and landforms; Soil erosion and conservation
References Alexander, David. 1993. Natural Disasters. New York: Chapman & Hall. Burton, Ian, Robert Kates, and Gilbert White. 1978. The Environment as Hazard. Oxford: Oxford University Press. Butler, William. 2013. “Spatial Patterns and Impacts of Slope Failures in Five Canyons of the Teton Mountains, Grand Teton National Park, Wyoming.” MA thesis, Kansas State University. Collins, Brian, and Randall Jibson. 2015. Assessment of Existing and Potential Landslide Hazards Resulting from the April 25, 2015 Gorkha, Nepal Earthquake Sequence. Open-File Report 2015-1142. Reston, VA: US Geological Survey.
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Dahal, Ranjan, and Shuichi Hasegawa. 2008. “Representative Rainfall Thresholds for Landslides in the Nepal Himalaya.” Geomorphology, 100: 429–443. Marston, Richard. 1986. “Dust Storm Potential in the Chihuahuan Desert.” Second Symposium on Resources of the Chihuahuan Desert Region, United States and Mexico. Alpine, TX: Chihuahuan Desert Research Institute. Marston, Richard, Maynard Miller, and Lochan Devkota. 1998. “Geoecology and Mass Movement in the Manaslu-Ganesh and Langtang-Jugal Himals, Nepal.” Geomorphology, 26: 139–150. Trimble, Stanley. 2012. Historical Agriculture and Soil Erosion in the Upper Mississippi Valley Hill Country. Boca Raton, FL: CRC Press.
Further reading Alcántara-Ayala, Irasema, and Andrew Goudie, eds. 2010. Geomorphological Hazards and Disaster Preven-
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tion. Cambridge: Cambridge University Press. Gares, Paul, Douglas Sherman, and Karl Nordstrom. 1994. “Geomorphology and Natural Hazards.” Geomorphology, 10: 1–18. Paul, Bimal. 2011. Environmental Hazards and Disasters. Chichester, UK: John Wiley & Sons. Sidle, Roy, and Hirotaka Ochiai. 2006. Landslides: Processes, Prediction, and Land Use. Washington, DC: American Geophysical Union. Slaymaker, Olav, ed. 2000. Geomorphology, Human Activity and Global Environmental Change. Chichester, UK: John Wiley & Sons.
