thumb|300px|right|Свлачище
'''Свлачището''' е геоложки дадености в определени райони.

Свлачищните процеси нямат внезапен характер и е възможно да бъдат регулирани с технически средства. Във времето те имат периоди на затихване и усилване. След активизирането на свлачището може да се стигне до възникване на бедствена ситуация. 

На територията на България свлачищата са около 1000 и засягат площ от над 200&nbsp;000 декара. 

== Елементи и класификация на свлачищата ==
Основите елементи на свлачищата са<ref name="dnsk-nar12">ДНСК - [http://www.dnsk.mrrb.government.bg/default.asp?r=47&m=279 Наредба № 12 от 2001 г. за проектиране на геозащитни строежи, сгради и съоръжения в свлачищни райони], Приложение 3 към чл.12</ref>:
* повърхнина - повърхнината (повърхнините) на плъзгане, по която се свличат земните маси. Тя може да е най-различна в зависимост от типа на свлачището. При някои видове свлачища вместо повърхнина се формира слой (зона), в който настъпва разрушаването;
* дълбочина - разстоянието от горната повърхност на свлачището до повърхнината на плъзгане. Разстоянието се измерва перпендикулярно на горната повърхност на свлачището;
* тяло - целият обем на свличащите се земни маси;
* глава (вежда) - най-горната част на свлачищното тяло;
* пета (език) - най-долната част на свлачищното тяло;
* дължина - разстоянието от главата до петата на свлачищното тяло;
* широчина - най-голямото разстояние напречно на посоката на свлачището.

Според Наредба 12<ref name="dnsk-nar12"/>, свлачищата се разделят в зависимост от:
# геоложките и тектонските си характеристики:
#* асеквентни - свлачища, образувани в еднородни почви;
#* консеквентни - свлачища, при които свличането е по разделителните повърхнини между разнородни пластове, по прослойки със или без наличие на пукнатини;
#* инсеквентни - свлачища, при които повърхнината на плъзгане пресича повърхнините на напластяване.
# механизма на процесите:
#* делапсивни - свлачища, които се зараждат в долната част на склоновете, след което свлачищният процес постепенно се придвижва нагоре. Обикновено тези свлачища се предизвикват от морска и речна железопътни линии, неправилна технология при планировка на терена, линейни изкопи за водопроводни и канализационни мрежи и др.
#* детрузивни - свлачища, които започват в горната част на склоновете, като свличащите се земни маси вследствие натиска отгоре образуват т. нар. "свлачищен купол". Най-често причините за възникване на детрузивни свлачища са: претоварване в горния край на откоса; преовлажняване в горния край на откоса, причинено от извори и водопроводни и канализационни съоръжения; натиск, който се упражнява при пропадането на льосови почви (образуване на почвени клинове) и др.;
#* консистентни - свлачища, които възникват вследствие на преовлажняване и влошаване консистенцията на глинести почви, от които са изградени склоновете. Те са сравнително плитки, често повърхностни, вкл. при незначителни наклони на откосите;
#* срутища - внезапно нарушаване на устойчивостта на стръмни склонове от сравнително твърди почви или скали, което често се наблюдава по пукнатини. При срутища повърхнината на плъзгане обикновено е по-стръмна от 45градуса.
#* пълзящи склонове - свлачища с движение на повърхността до 0,05 mm в денонощие.
# дълбочината на повърхнината на плъзгане:
#* дълбоки - свлачища с криволинейна, нефиксирана повърхнина на плъзгане, която се образува на дълбочина от повърхността на терена, по-голяма от 4 m;
#* плитки - свлачища с дълбочина до 4 m;
#* повърхностни - свлачища, които са силно зависими от атмосферните води. При тях фиксираната повърхнина (плоскост) на плъзгане следва приблизително наклона на терена. Такива свлачища обикновено са от делапсивен или консеквентен тип.

thumb|right|350px|Landslide of soil and [[regolith in Pakistan]] 
A '''landslide''' is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.

==Causes of landslides==
250px|thumb|right|[[Tropical Storm Isabel (1985)#Puerto Rico|The Mameyes Landslide, which buried more than 100 homes, was caused by extensive accumulation of rains and, according to some sources, lightning.]]

Landslides are caused when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone:

'''Natural causes:'''
*groundwater pressure acting to destabilize the slope
*Loss or absence of vertical vegetative structure, soil nutrients, and soil structure.
*erosion of the toe of a slope by rivers or ocean waves
*weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains 
*earthquakes adding loads to barely-stable slopes 
*earthquake-caused liquefaction destabilizing slopes (see Hope Slide)
*volcanic eruptions

'''Human causes:'''
*vibrations from machinery or traffic
*blasting
*earthwork which alters the shape of a slope, or which imposes new loads on an existing slope
*in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock
*Construction, agricultural, or forestry activities which change the amount of water which infiltrates into the soil.

==Types of landslide==

===Debris flow===
right|200px|thumb| [[Amboori debris flow, occurred on 09<sup>th</sup> November 2001 in Kerala, India. The event killed 39 people<ref>Kuriakose, S.L., Sankar, G., Muraleedharan, C. (2008) History of landslide susceptibility and a chorology of landslide prone areas in the Western Ghats of Kerala, India. Environmental Geology. DOI: 10.1007/s00254-008-1431-9. URL: http://www.springerlink.com/content/c1q6465428351032/</ref>]]
Slope material that becomes mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Debris flow is often mistaken for flash flood, but they are entirely different processes.

Muddy-debris flows in alpine areas cause severe damage to structures and infrastructure and often claim human lives.
Muddy-debris flows can start as a result of slope-related factors, and shallow landslides can dam stream beds, provoking temporary water blockage. As the impoundments fail, a "domino effect" may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid-liquid mixture can reach densities of up to 2&nbsp;tons/m³ and velocities of up to 14&nbsp;m/s (Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derive from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin in the Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and Luino (1998) estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s, a value about 40 times lower than that calculated for the debris flow that occurred.

===Earth flow=== 
thumb|right|200px|A rock slide in [[Guerrero, Mexico.]]
Earthflows are downslope, viscous flows of saturated, fine-grained materials, that move at any speed from slow to fast. Typically, they can move at speeds from .17 to 20 km/h. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that they are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be.

These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are usually derived from the slumping at the source.

Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.<ref>Easterbrook, Don J. ''Surface Processes and Landforms.'' Upper Saddle River, NJ: Prentice-Hall, Inc, 1999.</ref>

===Sturzstrom===
A sturzstrom is a rare, poorly understood type of landslide, typically with a long run-out. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain. They are suspected of "riding" on a blanket of pressurized air, thus reducing friction with the current underlying surface.

===Shallow landslide=== 
thumb|right|200px|Block glide at Mile Marker 23 along [[I-99 in Blair County, Pennsylvania.  Part of a hill of Devonian shale was removed to make the road, forming a dip-slope. The upper block detached along a bedding plane and is sliding down the hill, forming a jumbled pile of rock at the toe of the slide.]]

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some metres). They usually include debris slides, debris flow, and failures of road cut-slopes.  Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.

Shallow landslides can often happen in areas that have slopes with high permeable soils on top of low permeable bottom soils.  The low permeable, bottom soils trap the water in the shallower, high permeable soils creating high water pressure in the top soils. As the top soils are filled with water and become heavy, slopes can become very unstable and slide over the low permeable bottom soils. Say there is a slope with silt and sand as its top soil and bedrock as its bottom soil.  During an intense rainstorm, the bedrock will keep the rain trapped in the top soils of silt and sand.  As the topsoil becomes saturated and heavy, it can start to slide over the bedrock and become a shallow landslide. 
R. H. Campbell did a study on shallow landslides on Santa Cruz Island California.  He notes that if permeability decreases with depth, a perched water table may develop in soils at intense precipitation. When pore water pressures are sufficent to reduce effective normal stress to a critical level, failure occurs. <ref>  Renwick,W., Brumbaugh,R. & Loeher,L. 1982. Landslide Morphology and Processes on Santa Cruz Island California. Geografiska Annaler. Series A, Physical Geography, Vol. 64, No. 3/4, pp. 149-159 </ref>