Analisi Geomorfologica dei sistemi bacino-conoide dell’Appennino campano: scenari di suscettibilità alluvionale
Scorpio, Vittoria (2011) Analisi Geomorfologica dei sistemi bacino-conoide dell’Appennino campano: scenari di suscettibilità alluvionale. [Tesi di dottorato] (Inedito)
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Basin-fan systems are very dynamic environments in which erosion, transfer and depositional processes are strictly interconnected. In 1996, NRC introduced the term “alluvial fan flooding” to indicate a particular type of flood that occurs only on alluvial fans. The intensity of the flood depends on the flow processes, which can range from water-flood to debris flow (Wells & Harvey, 1987; Costa, 1988). Differentiation between these processes is essentially dependent on the relative concentration of the water and solid components (Costa, 1988; Blair & McPherson, 1994a,b). The increase in the relative proportion of sediments induces an increase in viscosity and shear strength of the flowing mixture. As a consequence, flows on fans may be characterized by different velocities, types of transported material and depositional behavior. This leads to deposits with different size and sedimentary facies and to fans with different slope and morphology (Blair & McPherson, 1994a). Debris flow fans may be subject to violent surges with high sediment concentration, often carrying large boulders and tree trunks, which can be very destructive and affect any part of a fan surface. They are mainly related to massive depositional processes, resulting in weak stratification, lack of sorting, and to the presence of matrix-supported angular to sub-angular clasts (Blair, 1999a; Sorriso-Valvo et al., 1998). Generally these fans are fed by small and step catchments and are characterized by small dimension and step gradients. Geomorphological evidence of debris flow transport is often represented by the occurrence depositional lobes (Marchi et al., 1993; Wilford et al., 2004; De Scally & Owens, 2004, De Scally et al., 2010). Water flood fans are usually bigger than previous fans and characterized by lower gradients. Their facies features include a crude horizontal stratification with alternating beds of cobble, gravel and sand to fine gravel (Blair & Pherson, 1994a,b, Blair, 1999a; Sorriso Valvo et al., 1998; Sohn et al., 1999; Wilford et al., 2004; De Scally et al., 2010). They presents magnitudes and peak discharges from 5 to 40 times lower than debris flow (Kellerhalls & Church, 1990; Hungr et al., 2001; Wilford et al., 2004). This explains why identification of fans that can be reached by debris flows is important for hazard evaluation because they are a much more destructive phenomenon than fluvial floods. Mitigation of the hazard in debris flow fans requires a very peculiar approach (Jackson, 1987; Jackson et al., 1987; Kellerhals & Church, 1990; Wilford et al., 2004). In last few decades several studies pointed out on the possibility to define statistically significant relationships between morphometrical parameters of the basin-fan system and the main depositional process (Melton, 1965; Pasuto et al., 1992, Marchi et al., 1993; Calvache et al., 1997; Guzzetti et al., 1997; Crosta & Frattini, 2004; De Scally & Owens, et al., 2004; Saito & Oguchi, 2005; De Scally et al., 2010). The present study refers to the geomorphological and morphometric analysis of 102 basin/fan systems, located along the border slopes of the carbonate massifs of Southern Apennines (Matese Mts, Taburno Mts, Caserta Mts, Picentini Mts and Maddalena Mts). These areas are prime spots for urban development and are generally considered to be safer than the valley floors. As a result, villages and towns have been built on alluvial fans which, during intense storms, may be affected by flooding and/or debris flow processes. The main goal is to define the most significant morphometric features of these systems that can be used to classify the fans in terms of transport process (debris flow or water flood). This discrimination may be useful to analyze the susceptibility distribution to alluvial fan flooding in the region. The investigation was carried out by means of a multidisciplinary approach, integrating stratigraphic, morphologic, morphometric, historical and statistical analysis. Particularly, field survey was aimed to find stratigraphic logs and morphological evidences that may be useful to evaluate the main depositional process. In order to investigate the relation between depositional process and the morphometry of the basin/fan sytems, 10 variables were determined for each basin-fan system (basin area, basin length, mean basin inclination, feeder channel length, mean feeder channel inclination, basin relief, Melton index, fan area, fan length and main fan inclination). The values were calculated by building up a DTM (resolution 0f 5m) from the contour lines and the point elevation derived from 1:5,000 topographic maps. It was interpolated with the Regularized Spline method by means of ArcGIS Spatial Analyst. The statistical analysis of these variables was carried out by means of Erre software and elaboration of multivariate techniques (Fisher linear discriminant analysis; Logistic Binomial Regression model; Principal Components Analysis; Cluster analysis). Fisher linear discriminant analysis was applied, using as training data the set represented by the 46 systems whose main type of transport was determined by field survey. In addition, the Logistic Binomial Regression model was analyzed in order to find the simplest and most powerful set of predictor variables. The main result was that the best discrimination between debris flow and water flood processes is achieved by means of only two variables, one for the basin (feeder channel inclination) and one for the fan (fan length). Among the 102 systems studied, 68 were classified as debris flow dominated and 34 as water flood dominated, with a probability of a correct classification higher than 90 %. The results obtained were validated by comparison with field data and the descriptions of historical floods. They allowed to detect where alluvial fan flooding might occur and give information on the different degrees of danger, thus contributing to the knowledge of the susceptibility distribution at a regional scale. Regrettably, urban development in recent decades has failed to take the presence of such alluvial fans into account due to the relatively long return time between floods, with consequent loss of historical memory by the local community. This has served only to increase the territorial vulnerability. The results of this thesis may represent a useful tool for further studies aiming at hazard mapping and civil protection interventions.
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