Mass Wasting

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Prevention of Mass Wasting

 

Discussion      Stopping mass wasting of a slope, or at least reducing the effects of mass wasting, is important for people living on or near a slope.   Gravity is the primary force causing mass wasting to occur, but since we cannot reduce the pull of gravity on a slope, humans attempt to deal with the other causes of mass wasting as best we can.   Listed below are some of the causes of mass wasting, described in detail on the causes of mass wasting page, and methods employed to stop or prevent each cause.

 

Mass Wasting Causes and Preventions

earthquake - Violent shaking due to an earthquake can cause unstable slopes to collapse.   To reduce earthquake damage to roads, buildings or other human-made structures on a slope or at the base of a slope, it is important to try to stabilize the slope as much as possible.   This can be done by planting trees on the slope, constructing a retaining wall along the bottom of the slope, or perhaps draping a heavy metal net over the slope to trap loosened rocks.

 LagunaScreenFarS.JPG (195786 bytes) 

Screen mesh draped over a steep slope keeps rocks loosened by an earthquake from bouncing onto the road at the base of the slope.   This is a relatively cheap and effective method to protect the roadway.   The screen mesh is also referred to as "slope revetment" (covers the face of the slope) or as "rock-fall barrier".   This revetment/barrier is along Laguna Canyon Road, California.

 

 

LagunaScreenBaseS.JPG (184526 bytes)

Close view of the base of the screen mesh slope revetment from the photograph above.   Note the collection of rocks trapped by the barrier. 

 

 

UnstableSlopeSanGabMtnsFeb2004S.JPG (118854 bytes)

A slope in the San Gabriel Mountains that would benefit from a slope revetment/rock-fall barrier, trees with their binding roots, or a retaining wall to protect the road.

 

 

over-steepening of a slope - With time, even moderately steep slopes can become somewhat stable as sediment becomes more compacted or cemented together, or as plants grow and their roots bind slope material tightly together.   Or, if a slope is composed of solid rock, it may be extremely stable.   Human activities such as road or home construction, or natural processes such as stream or wave erosion can remove some of the base of a slope, making the remaining upper part of the slope less stable and so more prone to mass wasting.   Construction of a retaining wall, illustrated below, can support the upper part of a slope, stopping or at least slowing mass wasting.

          LaConchitaRetainingWallClose1S.JPG (121566 bytes)  road, La Conchita, CA               RetainingWallTrail1S.JPG (104006 bytes) trail, San Gabriel Mountains

Some communities, such as Laguna Beach, California, are using more flexible approaches to reducing the impact of mass wasting.   Rocks routinely fall or bounce down the slope shown in the photos below.   To prevent them from moving onto the road and into traffic, steel mesh fencing and screen catch and trap rocks before they can do any harm.   This is a very effective and relatively inexpensive method of reducing the effect of mass wasting from an over-steepened slope.

           LagunaFenceScreenFarS.JPG (98038 bytes) Laguna Canyon Road                   LagunaScreenFenceSideView1S.JPG (121830 bytes) fence and screen from side view

 

 

removal of slope vegetation - The natural removal of slope vegetation by fire can be difficult to prevent, but human activities which lead to the removal of slope vegetation, such as clear-cutting of a forest, should not be allowed.   Selective cutting can thin the forest, but still leave enough mature trees to anchor the slope and protect sediment from rainfall and runoff.   The pictures below show a portion of the Angeles National Forest in California that was selectively harvested many years ago.   Enough large trees remained to anchor and protect the slope from severe mass wasting.

                   OpenForestSlopeS.JPG (148450 bytes)                    OpenForestSanGabMtnS.JPG (138391 bytes)                    OpenForestTrailS.JPG (150007 bytes)

 

 

introduction of water into slope material - Excessive water in a slope adds weight and reduces shear strength within the slope itself.   To counter these effects you can either prevent rain or snowmelt from percolating into slope sediment with plastic sheets or tarps, or improve surface drainage so that water flows rapidly off the slope, reducing the potential for water to percolate underground into slope sediment and rock.   

The pictures below were taken at Palos Verdes Peninsula, California.   This coastal area, which includes the Portuguese Bend Landslide, is prone to mass wasting, with one of the contributing factors being excess groundwater which can lubricate a subsurface clay-rich layer.   The large pipes visible in the photographs are part of a system which readily drains away surface runoff from rainfall before it can percolate underground.

                    drainpipePortBendfarS.jpg (70673 bytes)                     PortBendSlideDrainagePipesS.JPG (165996 bytes)                       PVPortPointdrainpipeS.jpg (150695 bytes)

Plastic sheets, shown in the photographs below, can be deployed on top of a slope to prevent water from flowing on the slope and from percolating into the slope material.   The sheets are staked, strapped or weighted down to keep them in position.

landslPlasticRustCanFeb03S.JPG (105182 bytes) Plastic protects steep slope in Rust Canyon, Santa Monica Mountains, California.  

SlopeProtectionPlasticNewpEstS.JPG (161439 bytes) Plastic along slope of Upper Newport Estuary, California.

 

 

ice wedging - As liquid water freezes to its solid form, ice, it increases in volume by 9%, wedging rocks apart.   This is a common way that rock falls and rock avalanches form in mountains and canyons.   With this cause of mass wasting you can either prevent water from flowing into slope rocks and sediment, or keep the slope warm enough so that ice will not form.   Neither approach is practical except over a very small area.   Ice wedging is prominent in high-elevation, mountainous regions as well as in canyons during the wintertime.

IceWedgingOurayCanyonDanScottS.JPG (356348 bytes) Winter ice in a canyon near Ouray, Colorado, high in the San Juan Mountains.

ArchesRockFallS.JPG (404798 bytes) Result of ice wedging in Arches National Park, Utah.

RockFallTalusColoNatMonumentS.JPG (178334 bytes) 

In Colorado National Monument, Colorado, the bases of the cliffs are surrounded by rock debris (talus) produced, in part, by ice wedging.

 

biological activity - On a farm or ranch one could erect a fence to keep animals away from unstable slopes and ridges, but in the wild there is not much that can be done to stop animals from dislodging rocks from steep slopes.   But, signs can be posted to keep humans from straying too close to the edges of cliffs where they might trigger a rock fall, or become part of the fall as shown in the picture below.

CliffAreaDangerSignS.JPG (106840 bytes)

 

freeze/thaw and wet/dry cycles - Both of these gradual, long-term processes require water, so reducing their effects would require keeping water from entering the slope material via percolation of rain or snowmelt into slope material.   Improved water drainage and/or covering an area with plastic would help slow both processes.

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