As part of a project finalized to evaluate landslide dams hazard in Umbria (central Italy), an analysis of recent and historical events occurred in the Umbria-Marche Apennines and neighboring (on the boundaries with Tuscany, Emilia-Romagna, Lazio and Abruzzo) was carried out. The aim of this analysis is to characterize the phenomenon of landslide dam, identifying the parameters (geometrical, geological, hydrological and hydrogeomorphological) that could, if properly combined, make it possible or not the occurrence of a landslide dam in the study area. Available data are related to 49 cases of landslides that have interfered with the river dynamics. 35 of these leaded to the formation of a dam lake, totally occluding the riverbed – floodplain system (landslide dams s.s.). The analysis indicates that over half of those 35 events (60%) involved the turbiditic sediments in facies of flysch (alternation of marls and sandstones) outcropping in the area; prevailing landslide types are slumps, slides, flows and combination between the previous (complex or composite landslides); 83% has a volume of material involved less than 5 million cubic meters; the total width of the floodplain affected by the damming does not exceed 200 meters in 86% of examined cases. In 29% of the total cases, the formation of a dam lake did not occurred; in the remaining cases, 49% of them was naturally drained by overflow and progressive erosion of the threshold; in 46% of cases the dam has resisted, so as to cause the filling of the lake with sediments or its current persistence (in the most recent cases). For the evaluation of the landslide dams hazard, two representative parameters for the examined cases were chosen: the landslide volume (which provides informations on magnitude of the landslide) and the area of the watershed outlet at the section of blockage (related to the stream power of the river, to its flow discharge and then to its ability to erode the natural barrier). The data indicate that most of the landslides (97%) that have interfered with the stream, producing a complete occlusion of the riverbed – floodplain system and the formation of a dam lake are characterized by a high index ratio: landslide volume / subtended watershed area. However, this condition appears to be necessary, but not sufficient to make certain the total occlusion: the values of this index, in cases of partial occlusion (i.e. without that the landslide has produced the formation of a dam lake), are extremely dispersed, which does not allow to use the same index to determine with certainty the possibility of total occlusion. Other parameters (the most important: velocity of the landslide and width of the involved riverbed - floodplain system) play, obviously, an important role that has to be properly evaluate.
Analysis and characterization of historical and recent landslide dams in the Umbria-Marche Apennine area.
CENCETTI, Corrado
Writing – Review & Editing
;DE ROSA, PIERLUIGIWriting – Original Draft Preparation
;FREDDUZZI, ANDREAWriting – Original Draft Preparation
2016
Abstract
As part of a project finalized to evaluate landslide dams hazard in Umbria (central Italy), an analysis of recent and historical events occurred in the Umbria-Marche Apennines and neighboring (on the boundaries with Tuscany, Emilia-Romagna, Lazio and Abruzzo) was carried out. The aim of this analysis is to characterize the phenomenon of landslide dam, identifying the parameters (geometrical, geological, hydrological and hydrogeomorphological) that could, if properly combined, make it possible or not the occurrence of a landslide dam in the study area. Available data are related to 49 cases of landslides that have interfered with the river dynamics. 35 of these leaded to the formation of a dam lake, totally occluding the riverbed – floodplain system (landslide dams s.s.). The analysis indicates that over half of those 35 events (60%) involved the turbiditic sediments in facies of flysch (alternation of marls and sandstones) outcropping in the area; prevailing landslide types are slumps, slides, flows and combination between the previous (complex or composite landslides); 83% has a volume of material involved less than 5 million cubic meters; the total width of the floodplain affected by the damming does not exceed 200 meters in 86% of examined cases. In 29% of the total cases, the formation of a dam lake did not occurred; in the remaining cases, 49% of them was naturally drained by overflow and progressive erosion of the threshold; in 46% of cases the dam has resisted, so as to cause the filling of the lake with sediments or its current persistence (in the most recent cases). For the evaluation of the landslide dams hazard, two representative parameters for the examined cases were chosen: the landslide volume (which provides informations on magnitude of the landslide) and the area of the watershed outlet at the section of blockage (related to the stream power of the river, to its flow discharge and then to its ability to erode the natural barrier). The data indicate that most of the landslides (97%) that have interfered with the stream, producing a complete occlusion of the riverbed – floodplain system and the formation of a dam lake are characterized by a high index ratio: landslide volume / subtended watershed area. However, this condition appears to be necessary, but not sufficient to make certain the total occlusion: the values of this index, in cases of partial occlusion (i.e. without that the landslide has produced the formation of a dam lake), are extremely dispersed, which does not allow to use the same index to determine with certainty the possibility of total occlusion. Other parameters (the most important: velocity of the landslide and width of the involved riverbed - floodplain system) play, obviously, an important role that has to be properly evaluate.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
