Relationships between mass wastings and rainfall in the Parma Valley (Northern Apennines)

Abstract

In the ambit of a historic research, 31 landslide reactivations and 79 soil slips were dated. The correspondence between rainfall and the development of mass wastings, distributed over periods with annual rainfall above the average and especially in periods with monthly rainfall clearly above the average, was evident. The highest concentration of mass wastings was found during the hydrological year 1959-’60. An examination of cumulative rainfall over a period of 60 days prior to the landslide revealed values which, in the majority of cases, were between 300 and 450 mm, with at least 200 mm 30 days before reactivation. The landslide phenomena are distributed over the Spring and Autumn-Winter periods, peaking in April and December. The Spring landslides are reactived in correspondence with the secondary maximum annual rainfall, while the Autumn-Winter landslides are approx. one month in delay with respect to the principal maximum rainfall. Moreover, the cumulative rainfall measured over the 60-day period necessary for the Spring landslides appears to be lower than that necessary for the Autumn-Winter landslides, when the initial degree of saturation of the soil is lower, following the lack of rain over the Summer. The soil slip phenomena are concentrated in the months of March, April, October and December, with a distribution which does not entirely correspond with monthly rainfall peaks. In relation to these instability events, we consider the presence of intense precipitation events which, in these conditions of greater permeability, can rapidly increase the saturation level and trigger instability, to be a determining factor.