SCENARI D'EVENTO E PREVISIONE DELLA RISPOSTA DEL SISTEMA VERSANTE-RETICOLO-OPERE DI DIFESA PER LA PREVENZIONE DEL DISSESTO IDROGEOLOGICO NEI BACINI MONTANI

Abstract

Every year Italy is affected by several major hydro-geological events occurred in mountain or hilly basins. These phenomena can lead to dangerous situations on the human activity near streams. These can be summarized, as follows, in four types of phenomena: a) Shallow and deep landslides b) Debris/mud flows c) Debris-flood, bedload transport d) Flash – floods This project will focus especially on the second and third phenomenon. Sometimes it appears that events of great magnitude occur due to special situations (basin characteristics, rainfall patterns, ecc.) and not in relation to extreme hydrological input (return periode Tr = 100 - 200 years). One of the main aims of this study is to foresee the hazard concerning the whole of occurrence of alluvial events, when meteorological scenarios (extreme event or unfavorable succession of moderate events) are combined with possible scenarios of efficiency/vulnerability of defensive measures. Besides, the project aims to investigate the event influence on the system ‘hillslope-network-defense works’ with the purpose to identify interventions and prevention of natural disasters. It’s important to identify specific cases (small-medium basin, <= 100km2) and critical events which have showed set of hydrogeological problems in association with defense systems: - different storm patterns; climate change events; combined rainfall events; - snowmelt and antecedent soil moisture condition; - hydrogeological unfavorable conditions (karst, ecc..) These situations were used to analyze typical tools or methods to define design discharge (corrivation time and others) and call into question the influence of this parameters. Several post event back-analyses suggest scenarios of hazard of an intense event. Field observations were carried out in some cases at outlet of the basin and/or at subasin scale. The work of the thesis allowed to define quick empirical methods to measure the maximum discharge; these methods can be applied in other streams where hydraulic structures can obstruct the passage of unforeseen exceptional discharge. In addition, the project analyzes, for several case studies, if soil bioengineering techniques are functional and efficient during major alluvial events, combined with sediment. Literature already shows outcomes regarding the applicability of these methods in Alpine area; instead, a serious lack of information affects Prealpine areas, in particular concerning bankfull discharge and debris flood, ecc.. The aim, in this case, is to analyze the shear stress resistance of consolidation works (e.g. vegetated double-fence timber walls) in order to improve the design phase. The shear stresses, which were imposed by severe floods in six mountain streams, have been assessed through post event back analysis and field surveys. After the calculation of maximum shear stress at the bottom and at the banks, a dedicated database was created. This one was used to verify the efficiency of the protection works and to estimate their vulnerability accounting for contour shear stresses and form of sediment transport. The shear stress values were analyzed also to research a correlation with some morphological variables of stream reaches (local slope, channel width, stream power). Hydrodynamic modeling, such as FLO-2D, has been carried out to achieve useful information about the hazard conditions and to improve the design of defensive measures.The hazard mapping produced and the information identified by this thesis work will be useful tools for the assessment of Civil Protection acts and actions in organizing prevention and rescue activities. The study areas are located in the North-East Prealpine area (Veneto Region) and Bolzano autonomous Province, precisely in: - Alto Vicentino (Basins: Leogra e tributari); - Rio Secco basin (Brentino-Belluno, VR) - Corin creek basin (Follina, TV) - Rio Gadria basin (Lasa, BZ) - Rio Tulve basin(Val di Vizze, BZ