VALUTAZIONE DELLA PERICOLOSITÀ ALLUVIONALE DELLE CONOIDI DEL VALLO DI DIANO (SALERNO, ITALIA MERIDIONALE)

Abstract

Santangelo et al., Alluvial fan flooding hazard assessment in the Vallo di Diano area (Salerno, Southern Italy). (IT ISSN
0394-3356, 2006).
This paper deals with the problem of alluvial fan flooding in the piedmont areas of the Campania region trying to apply a new method
to evaluate the effective alluvial hazard conditions. Piedmont areas are widespread all over the region and often present a high development of urban and/or agricultural settlements.
The selected area is located at the foothill of the Maddalena mountains, a NW-SE trending elongated ridge which bounds to the east
with the wide intramountain basin of “Vallo di Diano”. Here a wide detrital talus made up by coalescent alluvial fans, ranging in age
from Middle Pleistocene to Holocene, is present.
The geomorhological analysis firstly allowed the recognition of active and inactive alluvial fans by means of the individuation of their
hydrographic apex; in this way 27 active fans have been selected for morphometrical analysis. Then the main morphometric parameters (area, gradient, Melton index) of each fan and hydrographic basin have been calculated in order to obtain information related to the main processes (stream or debris flow) prevailing in the basin/fan system and to calculate the concentration time. Such data show that 24 out of 27 studied fans can be considered as “debris flow” dominated and that the concentration time of the hydrographic basins is always very short (lower than 1 hour). At the same time an historical analysis has been carried out to point out the state of activity of the fans during the last century. The collected data indicated that the “return time” of the main alluvial fan flooding events is more than 50 years.
Finally to define the relative hazard conditions among the studied fans, a recent method proposed in the scientific literature for the alluvial fans of the Po River National Authority has been applied, partly modifying it. Four different classes of relative alluvial hazard have been distinguished from very high degree (H4) to low degree (H1). Moreover, a hazard zonation inside the same alluvial fan has been proposed taking into account the main gradient variations. More than half among the studied fans belong to H4 and H3 classes and for at least 8 of them high risk conditions are determined by the interaction with urban centres and important roads.