Landslide volumes and landslide mobilization rates in Umbria, central Italy.

A catalogue of 677 landslides of the slide type was selected from a global database of geometrical measurements of individual landslides, including landslide area (A(L)) and volume (V(L)). The measurements were used to establish an empirical relationship to link A(L) (in m(2)) to V(L) (in m(3)). The relationship takes the form of a power law with a scaling exponent alpha=1.450, covers eight orders of magnitude ofAL and twelve orders of magnitude of V(L), and is in general agreement with existing relationships published in the literature. The reduced scatter of the experiential data around the dependency line, and the fact that the considered landslides occurred in multiple physiographic and climatic environments and were caused by different triggers, indicate that the relationship between V(L) and A(L) is largely independent of the physiographical setting. The new relationship was used to determine the volume of individual landslides of the slide type in the Collazzone area, central Italy, a 78.9 km(2) area for which a multi-temporal landslide inventory covering the 69-year period from 1937 to 2005 is available. In the observation period, the total volume of landslide material was V(LT)=4.78x10(7) m(3), corresponding to an average rate of landslide mobilization phi(L)= 8.8 mm, yr(-1). Exploiting the temporal information in the landslide inventory, the volume of material produced during different periods by new and reactivated landslides was singled out The wet period from 1937 to 1941 was recognized as an episode of accelerated landslide production. During this 5-year period, approximately 45% of the total landslide material inventoried in the Collazzone area was produced, corresponding to an average rate of landslide mobilization phi(L)= 54 mm yr(-1), six times higher than the long term rate. The volume of landslide material in an event or period was used as a proxy for the magnitude of the event or period, defined as the logarithm (base 10) of the total landslide volume produced during the event, or period. With this respect, the new relationship to link A(L) and V(L) is a starting point for the adoption of a quantitative, process based landslide magnitude scale for landslide events.