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|Authors: ||Polemio, M.*|
|Title: ||La ricerca internazionale sulle relazioni tra piogge e frane|
|Other Titles: ||The international research on rainfall triggering landslides|
|Issue Date: ||2001|
|Abstract: ||The literature contains many examples, which demonstrates the importance of climatic conditions, particularly of rainfall but also temperature, on landslide activation. Normally landslides occur during specific hydrologic conditions; the relative importance of each factor will however vary with the landslide type.
Heavy rains are one of the main natural factors producing landslides as they may cause pore pressure to increase on slopes or can create adverse seepage conditions, modify the slope's geometry as a result of erosion and originate swelling and softening processes in clay soils.
According to their different behaviour, it is necessary to distinguish shallow or deep-seated landslides, first-time or reactivated landslides, the nature of landslide body (terrain or rock) as well as the hydrogeological nature of soils.
Shallow landslides in soils and weathered rocks are often generated on steep slopes during the most intense part of a storm (lasting from minutes to several hours). The higher permeability of the outermost portion of the slope, compared to the bedrock below, allows for the creation of a temporary perched water-table, inducing the progressive increase of pore-water pressure.
On the other hand, deep-seated landslides can be mobilised as a consequence of moderate intensity rainfalls lasting several days, owing to the longer recharging time required for ground-water level to rise and cause a significant build up in pore-water pressure.
Pioneer studies, concerning landslides triggered by rainfalls, go back to '30s. Were selected 138 papers and key information was collected in a database. Nearly 82 % of all records are local investigations carried out in 23 countries. Italy provides the largest sample as for author’s nationality and widespread proneness to landslides; followed by United States (15 %), Hong Kong (8 %), Japan and United Kingdom.
Two main approaches have been used to investigate rainfall-induced landslides:
-empirical or semi-empirical relationships, using statistical correlation and forecasting techniques;
-deterministic modelling of groundwater recharge and pore-water pressure changes.
Studies concerning the first type of approach have reached interesting results about combination of antecedent rainfall and rainstorms intensity able to trigger widespread soilslip activation in specific geological, morphological and climatic settings.
Relationships between rainfall intensities and duration associated with shallow landslides and debris flows activation has also been found and they are often used for warning purposes.
For the activation of widespread mass movements, some authors underline the role of an effective rainfall period ended by a pronounced increased of rainfall intensity; in this situations hazard precursors may be considered as a combination of cumulate rainfall and rainfall intensity.
The second approach requires the development of a clear picture of the modes of water flow on and below the ground surface: where groundwater flow is concerned, attention is focused on both the saturated and the unsaturated zones and complete hydrological models of slopes must be defined.
The present paper is a critical overview of recent international research activity on this subject. Methods and experiences are compared and classified. Some author's direct experiences are utilised for these purposes.|
|Appears in Collections:||Conference materials|
05.08.02. Hydrogeological risk
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