ABSTRACT
SOURCES OF INPUT WATER Rainstorms: High rainfalls from individual storms cause numerous shallow slides where high water pressures can rapidly reach slip surfaces. Hundreds of slides in Jordan, in early 1992, due to rare heavy snowfall and rapid melt in a normally semi-desert terrain; soils, rocks and fills all equally affected. Destructive 1988 slide at Catak, Turkey, failed during the first period of high rainfall since road widening had steepened the slope four years previously. Shallow earth slides and debris flows are annual events during rainstorms on steep slopes of the shanty town favellas in Rio de Janeiro. Rainfall seasons: Deep-seated slides are more affected by annual fluctuations of water table. Winter groundwater maxima create a landslide season lasting from November to March in Britain. Monsoons trigger most of the landslides in SE Asia. Spring snowmelt is the main factor in alpine regions, after slopes have been stable during winter freeze. Numerous sets of data show correlation between rainfall and slide movement; mostly on small scale with rapid response, or on large scale with response delayed 1-10 weeks; Portuguese Bend and Vaiont are examples. Artificial inputs: Impounding water in a reservoir raises regional water tables, as at Vaiont. De-vegetation of a slope allows increased infiltration. Irrigation of farmland or gardens has caused many terrace edge failures in dry regions such as California. Secondary effects: Opening of tension fissures in head zone, as slide starts to move, captures runoff and in creases infiltration.
