692 SEDIMENTARY PROCESSES/Landslides Prevention of Landslides and Mitigating Their Impact Prevention of landsliding or repairing the damage occurring as the result of a landslide necessitates understanding the basic relationship of driving forces and resisting forces The need is to reduce the driving forces and/or increase the resisting forces on the slope Decreasing the slope angle to less than the angle of repose for the slope will significantly increase stability Removing water from the slope through drains or temporary water diversion by the use of tarps in full will decrease the driving forces and increase the resisting forces Building a buttress at the bottom of the slope by erecting a wall of boulders, building a concrete wall with weep holes (to prevent build-up of water pressure behind it), or building a gabion basket wall will increase the resisting forces of the slope Recognizing and mapping old landslides is important Planning regulations recommend not building on ancient landslides, so preventing disasters like Kelso, Washington Replacing slope vegetation aids the slope stability and increases the resisting forces through the removal of water In rockfall-prone areas, the use of screens or sprayed-on shotcrete will keep the rock fragments on the slope, as will rock bolts for the larger boulders Avalanche shelters will provide additional protection, aided by electronic fences to give warning of rockfall events A landslide killed 2600 people in Longarone, Italy, on October 1963 The second highest concrete dam in the world had just been built to a height of 265 m— the Vaiont Dam at the base of Mt Toc The reservoir area was prone to landslides, and it was known that Mt Toc had sedimentary beds that dipped steeply into the valley below and that the slopes had been oversteepened by the river at the base Filling of the reservoir aggravated the situation, raising the groundwater level (and water pressure) at the base, aggravated by persistent heavy rains that raised the reservoir level 20 m higher than expected within a short period The slope began to creep at a rate reaching 400 mm dayÀ1 the day before failure On the day of failure, 240 million m3 of rock broke away on the south side of the valley and slid down into the reservoir, rapidly displacing the water and creating a huge wave 100 m high, which topped over the dam and flooded the Piave River Valley A total of 2600 people were drowned Today, the dam still stands, intact Its foundations were adequate, but the location with respect to the reservoir impounded behind was quite inappropriate The landslide could have been prevented if the planners had paid attention to the broader context of the location and, in particular, the effect of impounding on slope instability of the reservoir margins, and the movements that had been noted from the past See Also Engineering Geology: Overview; Aspects of Earthquakes; Geological Maps; Natural and Anthropogenic Geohazards; Site and Ground Investigation; Site Classification Geological Engineering Geomorphology Sedimentary Environments: Alluvial Fans, Alluvial Sediments and Settings Sedimentary Processes: Particle-Driven Subaqueous Gravity Processes Solar System: Mars Tectonics: Earthquakes Urban Geology Volcanoes Further Reading Brabb EE and Harrod BL (eds.) (1989) Landslides: Extent and Economic Significance Rotterdam: AA Balkema Carson MA and Kirkby MJ (1972) Hillslope Form and Process Cambridge, UK: Cambridge University Press Chacon J, Irigaray C, and Fernandez T (eds.) (1996) Landslides Rotterdam: AA Balkema Chorley RJ, Schumm SA, and Sugden DE (1984) Geomorphology London: Methuen Cruden DM and Fell R (1997) Landslide Risk Assessment Rotterdam: AA Balkema Easterbrook DJ (1999) Surface Processes and Landforms, 2nd edn Upper Saddle River NJ: Prentice Hall Ritter DF, Kochel RC, and Miller JR (2002) Process Geomorphology, 4th edn Dubuque IA: William C Brown Selby MJ (1982) Hillslope Materials and Processes Oxford: Oxford University Press Slosson JE, Keene AG, and Johnson JA (eds.) (1992) Landslides/Landslide Mitigation, Reviews in Engineering Geology, vol IX Boulder: Geological Society of America Summerfield MA (1991) Global Geomorphology London: Longman Scientific and Technical Turner AK and Schuster RL (eds.) (1996) Landslides: Investigation and Mitigation Special Report 247 Washington DC: Transportation Research Board, National Research Council Walker BF and Fell R (eds.) (1987) Soil Slope Instability and Stabilisation Rotterdam: AA Balkema